lubricants

Review Liquid Crystals as Lubricants

Nadezhda V. Usol’tseva * and Antonina I. Smirnova

Nanomaterials Research Institute, Ivanovo State University, Ermak str., 39, 153025 Ivanovo, Russia * Correspondence: [email protected]



Received: 31 October 2019; Accepted: 3 December 2019; Published: 9 December 2019


Abstract: The review summarizes the literature data and the authors’ own research results on the application of liquid crystals in tribology. It has been shown that both thermotropic (calamitic, discotic, cholesteric) and lyotropic (surfactants, chromonics) mesogens as tribological additives are able to optimize the properties of lubricating compositions when introduced even at low concentrations to oils and greases. A wide possibility of varying the chemical structure of mesogens and studying the relationship between their structure and tribological properties can be used for the desired (programmed) change of the quality of tribotechnical processes. The synergism of the combined use of mesogenic esters of cholesterol and carbon nanostructures as additives in improving tribological properties has been established. The use of synthetic lubricants in biological systems still requires further research as the experimental results obtained on models of joint prostheses in vitro conditions are significantly worse than the results obtained in vivo. Considering the annual loss of billions of US dollars worldwide due to the low efficiency of friction processes in the industry and the resulting wear, liquid crystals and the systems based on them can be the most effective way to optimize these processes. The present review will be useful for researchers and industrialists.

Keywords: tribology; thermotropic and lyotropic liquid crystals; calamitic and discotic mesogens; biological fluids

1. Introduction Long before the introduction of the term “tribology”, which for the first time was used in 1966 [1,2] and combines a set of scientific and technological ideas about interacting surfaces when they move relative to each other, a number of phenomena displaying in this area were discovered. For example, in order to reduce friction, various oils and fats were empirically used. On the copy of the Egyptian kingdom fresco, one can see that in order to move the huge pharaoh statue, the surface in front of the sleigh on which the statue is placed is poured with liquid. Leonardo da Vinci (1452–1519) was the first to begin to formulate the basic laws of friction. The first reliable friction wear test was performed on gold coins by Hatchett (1760–1820), who compared the wear during the direct friction of coins and the friction with sand between them. For the first time, the graphic characteristics of the friction regime were obtained by Professor Stribeck (1861–1950) of the Royal Prussian Institute of Control (Berlin) [3]. It can be said that centuries passed before the field of science that is now called “tribology” was developed. It should be noted that the growth of interest in these issues occurred in the mid-1970s, when specialized centers were opened in different countries, and scientific societies such as the Society for Tribologists and Lubrication Engineers (STLE) in the USA and the Gesellschaft für Tribologie in Germany were established. At present, practically all technical universities have groups dealing with tribology problems. The theory and practice of implementing tribological processes have attracted increasing attention as objects of interdisciplinary research that have a serious impact on the economies of developed countries. It was proven that the corresponding application of tribological principles in the industry

Lubricants 2019, 7, 111; doi:10.3390/lubricants7120111 www.mdpi.com/journal/lubricants Lubricants 2019, 7, 111 2 of 25 and their practical realization can save from 1.0% to 1.4% of the total national product of the state due to wear reduction [4]. Obtaining the most efficient lubricants is not only a technological problem, it is also an important economic one, since half of the consumed energy is spent on friction [5]. In this paper, a critical analysis of the literature data and the authors’ own results regarding the use of thermotropic and lyotropic mesogens in technical and biological systems as ‘smart’ materials that allow the development of new lubricants with programmable properties is presented.

2. Surface Orientation in Tribology as a Special Case of Mesomorphism In recent years, researchers have been especially keen on boundary lubrication problems. This is due to the tightening of the operating conditions of friction lubrication units both in terms of loads and temperature conditions, energy and resource-saving needs, a rapid development of nanotribology [6], a complication of lubricating compositions [7], etc. Up to now, the generally accepted criteria that allow predicting the relationship between the chemical structure of lubricants (or tribological additives to lubricants) and their physicochemical properties have not been developed. The main functions of lubricants are reducing the friction between contacting surfaces, distributing the temperature and pressure between surfaces, removing heat and wear products from the friction zone, and preventing corrosion of the surface layer of friction pairs. New requirements for the optimization of friction processes caused due to technological progress, such as functioning at certain temperatures, reversible lubricating properties, thermal stability of mesophases, etc. demand to create new ‘smart’ lubricant materials and compositions with programmable properties. Liquid crystals are among these materials [8]. At present, there is no doubt that the presence of adsorbed ordered layers on interacting surfaces prevents their direct contact and leads to a significant reduction in friction and wear, while the chemical feature of the surface controls wear resistance. The near-surface boundary/interfacial layer of lubricant, oriented by the force field of the solid surface, forms an epitropic mesophase. It was shown that the wear during the frictional interaction of solids decreases with the increasing of the optical anisotropy of the boundary layer formed by anisometric surfactant molecules [9]. If fatty acids are used as surfactants, then the orientation effect is most pronounced. The fact that mesogens, i.e., compounds that are capable of forming liquid crystal phase(s), can form surface-oriented layers attracted attention to them. They began to be considered as new lubricant materials or additives to lubricants, lubricoolants, and mineral oils. Already in 1936, V.K. Fredericks, an outstanding Russian researcher of liquid crystals, expressed the idea of the need for studies of the lubricating properties of liquid crystalline materials [10]. The first generalization of work in this direction occurred at a special 198th symposium of the American Chemical Society “Tribology and liquid crystal state” about 30 years ago [11]. The results of the work regarding the use of thermotropic and lyotropic mesogens or their additives in tribological processes were presented there. In fairness, it should be noted that in the USSR, such works at that time were also successfully carried out. The leader in this direction was Ivanovo State University, where in 1964 at the Department of Physics, the Laboratory of Liquid Crystals and Polymers was opened for the first time in the USSR after the Second World War. The presence of two scientific schools led by Professor V.N. Latyshev (who worked in the field of tribosystems) and Professor I.G. Chistyakov (who was the pioneer in the field of liquid crystals in USSR) predetermined the success of the Ivanovo scientific school in this direction [12–15]. The liquid crystalline (mesomorphic) state is a state of matter in which the degree of molecular order is intermediate between the long-range three-coordinate translational order (solid crystal) and the absence of the translational order (isotropic liquid) [16]. According to the formation conditions, there are thermotropic liquid crystal (LC) phases and lyotropic LC phases. The basic units of thermotropic LCs are organic molecules—mesogens, which satisfy special requirements [17]. Usually, they have an anisometric shape and combine flexible and rigid fragments. Lubricants 2019, 7, 111 3 of 25 Lubricants 2019, 7, x 3 of 26

Lyotropic LCs are formed in binary or multicomponent systems containing water or other Lyotropic LCs are formed in binary or multicomponent systems containing water or other solvents. solvents. The building blocks of lyotropic mesophases are not individual molecules, but rather The building blocks of lyotropic mesophases are not individual molecules, but rather supramolecular supramolecular ensembles, for example, micelles [18]. ensembles, for example, micelles [18]. The characteristic structures and chemical properties of a large number of mesogens, i.e., The characteristic structures and chemical properties of a large number of mesogens, i.e., substances substances exhibiting a liquid crystalline state, are collected in a special database named LiqCryst, exhibiting a liquid crystalline state, are collected in a special database named LiqCryst, which was which was created by Vill (Germany) [19]. created by Vill (Germany) [19]. The main classes of mesogens united by the form of their molecules are calamitic (rod-shaped) The main classes of mesogens united by the form of their molecules are calamitic (rod-shaped) and discotic (disc-like) mesogens. Some of the LC phases formed by calamitic and discotic and discotic (disc-like) mesogens. Some of the LC phases formed by calamitic and discotic compounds compounds are shown in Figure 1. are shown in Figure1.

Figure 1. Mesophases formed by calamitic and discotic mesogens: N—nematic phase, SmA—smectic A phase, SmC—smectic C phase, Ch—cholesteric phase, N —discotic nematic phase, Col—columnar Figure 1. Mesophases formed by calamitic and discotic mesogens:D N—nematic phase, SmA—smectic hexagonal ordered phase. А phase, SmC—smectic C phase, Ch—cholesteric phase, ND—discotic nematic phase, Col—columnar Thehexagonal study ordered of liquid phase. crystals at the empirical level occurred at the end of the 19th and the beginning of the 20th century, but later, a significant impetus to its further development gave the needs of The study of liquid crystals at the empirical level occurred at the end of the 19th and the electronics: the creation of information display screens, including computers, light emitting diodes, beginning of the 20th century, but later, a significant impetus to its further development gave the solar panels, etc. needs of electronics: the creation of information display screens, including computers, light emitting Below, we review the main achievements associated with the established patterns on the influence diodes, solar panels, etc. of the chemical structure of mesogenic compounds on tribological processes. Below, we review the main achievements associated with the established patterns on the 3.influence Thermotropic of the chemical Mesogens structure in Tribology of mesogenic compounds on tribological processes.

3.1.3. Thermotropic Calamitic Mesogens Mesogens in Tribology To create new effective lubricants, it is important to understand the relationship between 3.1. Calamitic Mesogens the chemical, supramolecular structure of thermotropic mesogens and the realization of their tribologicalTo create properties. new effective lubricants, it is important to understand the relationship between the chemical,A typical supramolecular example of solving structure this of task thermotropic is the study mesogens of calamitic and mesogens the realization by varying of their their tribological chemical structure.properties. Examples of the compounds investigated in that work are presented in Table1[20]. A typical example of solving this task is the study of calamitic mesogens by varying their chemical structure. Examples of the compounds investigated in that work are presented in Table 1 [20].

LubricantsLubricants 2019 2019, 7, x7 , x 4 of4 of26 26 LubricantsLubricants 2019 2019, 7, x, 7, x 4 of 426 of 26 LubricantsLubricants 2019 2019, 7, ,7 x, x 4 4of of 26 26 LubricantsLubricants 2019 2019, 7, ,7 x, x 4 4of of 26 26 LubricantsLubricants 2019 2019, 7, ,7 x, x 4 4of of 26 26 LubricantsLubricants 2019 2019, 7, ,7 x, x TableTable 1. 1.Calamitic Calamitic mesogens mesogens studied studied by by Cognard Cognard [20]. [20]. 4 4of of 26 26 Lubricants 2019, 7, x TableTable 1. Calamitic 1. Calamitic mesogens mesogens studied studied by Cognard by Cognard [20]. [20]. 4 of 26 LubricantsLubricants 2019 2019, 7, ,7 x, x TableTable 1. 1. Calamitic Calamitic mesogens mesogens studied studied by by Cognard Cognard [20]. [20]. 4 4of of 26 26 LubricantsLubricants 2019 2019, 7, ,7 x, x Table 1. Calamitic mesogens studied by Cognard [20]. 4 4of of 26 26 Lubricants Lubricants 2019 2019, 7,, 7x, x Table Table 1. 1. Calamitic Calamitic mesogens mesogens studied studied by by Cognard Cognard [20]. [20]. 4 4of of 26 26 Lubricants Lubricants 2019 2019, 7, ,7 x, x Table Table 1. 1. Calamitic Calamitic mesogens mesogens studied studied by by Cognard Cognard [20]. [20]. 4 4of of 26 26 Lubricants Lubricants 2019 2019, 7, x, 7, x Table 1. Calamitic mesogens studied by Cognard [20]. 4 of4 26of 26 Lubricants Lubricants 2019 2019, 7, ,x 7 , x Table Table 1. 1. Calamitic Calamitic mesogens mesogens studied studied by by Cognard Cognard [20]. [20]. 4 of4 26of 26 LubricantsLubricants Lubricants2019 2019, 20197,, 7 111, ,x 7 , x Table Table 1. 1. Calamitic Calamitic mesogens mesogens studied studied by by Cognard Cognard [20]. [20]. 44 ofof4 25 26of 26 Lubricants Lubricants 2019 2019, 7,, x7 , x Table Table 1. 1.Calamitic Calamitic mesogens mesogens studied studied by by Cognard Cognard [20]. [20]. 4 of4 of26 26 1 1 TableTable 1. 1. Calamitic Calamitic mesogens mesogens2 studied studied2 by by Cognard Cognard [20]. [20]. 1 1 Table Table 1. Calamitic 1. Calamitic mesogens mesogens2 studied 2studied by Cognardby Cognard [20]. [20]. 1 1 Table Table 1. Calamitic 1. Calamitic mesogens mesogens2 studied 2 studied by byCognard Cognard [20]. [20]. 1 Table Table 1. Calamitic 1. Calamitic mesogens mesogens 2 studied studied by byCognard Cognard [20]. [20]. 1 1 Table Table 1. Calamitic1. Calamitic mesogens mesogens2 studied 2 studied by by Cognard Cognard [20]. [20]. 1 1 Table 1. Calamitic mesogens studied2 2 by Cognard [20]. 1 1 2 2 13 1 3 24 2 4 31 1 3 42 2 4 Notation31 131 Chemical Structure Notation42 242 Chemical Structure 13 24 31 3 1 42 4 2 13 3 1 24 4 2 13 MBBA31 1 PEPN424 42 2 3 3 4 4 35 3 35 46 4 46 53 3 5 64 4 6 53 5 3 64 6 4 35 5 46 6 53 5EBBA 3 PEPN564 6 4 35 53 46 64 5 5 6 6 5 5 6 6 5 55 6 66 57 5 7 68 6 8 75 57 86 68 75 7PAA 5 PEPN686 8 6 57 5 68 6 75 75 86 86 7 7 8 8

7 7 8 8 79 7 79 108 8810 97 7 9 108 8 10 97 9PAP 7 PEPN710810 8 79 9 10810 9 9 7 1010 8 79 97 10810 8 9 9 1010 119 911 101210 12 119 9 11 121010 12 11911 9 9 1210101210 119 S3 HPEH231012 11911 9 121012 10 11911 9 101212 10 11911 99 101212 1010 111311 13 121412 14 131111 13 141212 14 1311111311 S6 HPEH331412121412 1113 1214 131113 11 141214 12 111313 11 121414 12 111313 1111 121414 1212 131513 15 141614 16 151313 K15 15 HPEH43161414 16 1513131513 1614141614 1315 1416 151315 13 161416 14 131515 13 141616 14 131515 1313 141616 1414 151715 17 K21 PCH3161816 18 171515 17 181616 18 1715151715 1816161816 1517 1618 171517 15 181618 16 151717 15 161818 16 151717 1515M9 PCH5161818 1616 171917 19 182018 20 191717 19 201818 20 1917171917 2018182018 1719 1820 191719 17 201820 18 171919 M1517 PCH7182020 18 171919 1717 182020 1818 192119 21 202220 22 21191919 21 22202020 22 21191921 22202022 211921 222022 2121 M2119 BCH52222 20 192121 19 202222 20 192121 1919 202222 2020 2121 2222 21232121 23 22242222 24 232121 23 242222 24 232123 242224 2323 M2421 P352424 22 232123 21 242224 22 212323 2121 222424 2222 212323 222424 2323 2424 23252323 25 24262424 26 252323 25 262424 26 252325 M2723 P37262426 24 232525 23 242626 24 252325 23 262426 24 232525 23 242626 24 2525 2626 2525 2626 25272525 27 26282626 28 272525 CB15 27 TP34282626 28 272527 282628 27 25 28 26 272527 25 282628 26 25 2525 26 2626 2727 2828 2727 2828 272927 29 T15 3HPPH3283028 30 292727 29 302828 30 29272729 30282830 2927 27 3028 28 292729 27 302830 28 272929 27 283030 28 272929 27 283030 28 2929 3030 292929 Thus, Thus,Thus, Mori MoriMori and Iwata andand [ Iwata21Iwata] considered [21][21] considered threeconsidered representatives 30 thre3030 thre e e representativesrepresentatives of calamitic mesogens: ofof calamiticcalamitic cyanobiphenyls, mesogens:mesogens: 2929 Thus,Thus, Mori Mori and and Iwata Iwata [21] [21] considered considered 30thre30 three representativese representatives of ofcalamitic calamitic mesogens: mesogens: alkoxycyanobiphenyls,29 29 Thus, Mori and and Iwata alkylcyanophenylcyclohexyls [21] considered 30thre 30 with e representatives a variation of the of length calamitic of the aliphaticmesogens: 29cyanobiphenyls, cyanobiphenyls,Thus, Mori alkoxycyanobiphenyls, andalkoxycyanobiphenyls, Iwata [21] considered and and alkylc alkylc 30threyanophenylcyclohexyls yanophenylcyclohexylse representatives of with calamiticwith a avariation variation mesogens: of of the the cyanobiphenyls,29 29cyanobiphenyls, Thus,Thus, MoriMori alkoxycyanobiphenyls, and andalkoxycyanobiphenyls, IwataIwata [21][21] consideredconsidered and and alkylc alkylc 30threyanophenylcyclohexylsthre 30e yanophenylcyclohexyls e representativesrepresentatives of ofwith calamitic withcalamitic a variation a variation mesogens:mesogens: of theof the chaincyanobiphenyls,29cyanobiphenyls, 29 CThus,Thus,nH n MoriMori(n = alkoxycyanobiphenyls, 5–12) alkoxycyanobiphenyls,andand (FigureIwataIwata 2 [21]).[21] consideredconsidered and and alkylc alkylc 30threthreyanophenylcyclohexyls 30yanophenylcyclohexylse e representativesrepresentatives of ofwith with calamiticcalamitic a avariation variation mesogens:mesogens: of of the the cyanobiphenyls,lengthlengthThus,Thus, of of2 the + the 1Mori aliphaticMori aliphatic alkoxycyanobiphenyls, andand chain chainIwataIwata C n CH n[21]H2[21]n+12n +1(n considered( =nconsidered 5–12)=and 5–12) alkylc(Figure (Figure threyanophenylcyclohexylsthre 2).e 2).e representatives representatives of ofwith calamiticcalamitic a variation mesogens:mesogens: of the cyanobiphenyls,lengthcyanobiphenyls,lengthThus,Thus, of the of Mori thealiphaticMori aliphatic alkoxycyanobiphenyls, alkoxycyanobiphenyls,andand chain Iwata chainIwata Cn H C[21]2n[21]+1H ( 2n +1considered = considered( n5–12) and = and 5–12) alkylc(Figure alkylc (Figure threyanophenylcyclohexylsthreyanophenylcyclohexyls 2).e e 2).representativesrepresentatives of ofwith with calamiticcalamitic a avariation variation mesogens:mesogens: of of the the lengthcyanobiphenyls,lengthcyanobiphenyls,Thus,Thus, of of the the Mori aliphaticMori aliphatic alkoxycyanobiphenyls, alkoxycyanobiphenyls,andand chain chain IwataIwata C Cn Hn H[21]2n[21]2+1n+1 ( n( considered n=considered =5–12) and5–12) and (Figurealkylc (Figurealkylc threthreyanophenylcyclohexyls yanophenylcyclohexyls2). e2). e representativesrepresentatives of ofwith with calamiticcalamitic a avariation variation mesogens:mesogens: of of the the lengthcyanobiphenyls,lengthThus,Thus, of of the the Mori aliphaticMori aliphatic alkoxycyanobiphenyls, andand chain chain IwataIwata C Cn Hn H[21]2n[21]2+1n+1 ( n( considered n= considered =5–12) 5–12) and (Figure alkylc(Figure threthreyanophenylcyclohexyls 2). e2). e representatives representatives ofof with calamiticcalamitic a variation mesogens:mesogens: of the lengthcyanobiphenyls,lengthcyanobiphenyls,Thus,Thus, of of the the Mori aliphaticMori aliphatic alkoxycyanobiphenyls, alkoxycyanobiphenyls,andand chain chain IwataIwata C Cn Hn H[21]2n[21]2+1n+1 ( n( consideredn=considered =5–12) and5–12) and (Figurealkylc (Figurealkylc threthreyanophenylcyclohexyls yanophenylcyclohexyls2). e2). e representativesrepresentatives of ofwith with calamiticcalamitic a avariation variation mesogens:mesogens: of of the the lengthcyanobiphenyls,cyanobiphenyls,lengthThus, of Thus,of the theMori aliphatic aliphaticMori alkoxycyanobiphenyls, alkoxycyanobiphenyls,and andchain chainIwata Iwata C Cn HnH[21]2 n2+1n[21]+1 ( n(considered n= =5–12) considered and5–12) and alkylc(Figure alkylc(Figure thre yanophenylcyclohexylsyanophenylcyclohexyls thre2). e2). e representatives representatives of with withofcalamitic calamitica avariation variation mesogens: mesogens: of of the the cyanobiphenyls,lengthlengthcyanobiphenyls,Thus, of of the theMori aliphatic aliphatic alkoxycyanobiphenyls, alkoxycyanobiphenyls,and chain chainIwata C Cn HnH[21]2n2+1n+1 ( n(consideredn = =5–12) and5–12) and alkylc(Figure (Figurealkylc threyanophenylcyclohexyls yanophenylcyclohexyls2). e2). representatives of with withcalamitic a avariation variation mesogens: of of the the lengthcyanobiphenyls,cyanobiphenyls,length ofThus, of the the aliphatic aliphaticMori alkoxycyanobiphenyls, alkoxycyanobiphenyls, and chain chain Iwata C CnHnH2 n2+1n[21]+1 ( n( n= =5–12) considered and5–12) and alkylc(Figure alkylc(Figure yanophenylcyclohexylsyanophenylcyclohexylsthre 2). 2). e representatives with ofwith calamitica avariation variation mesogens: of of the the cyanobiphenyls,lengthlengthcyanobiphenyls,Thus, Thus,of of the theMori aliphatic aliphaticMori alkoxycyanobiphenyls,and alkoxycyanobiphenyls,and chain chainIwata Iwata C Cn HnH[21] 2n2+1[21]n+1 ( n(consideredn = =considered5–12) and5–12) and alkylc(Figure (Figure alkylcthre yanophenylcyclohexylsthre 2). eyanophenylcyclohexyls2). e representatives representatives of with of calamitic withcalamitic a variationa variation mesogens: mesogens: of ofthe the lengthcyanobiphenyls, length of of the the aliphatic aliphatic alkoxycyanobiphenyls, chain chain C CnHnH2n+12n +1(n ( n= =5–12) and5–12) alkylc(Figure (Figureyanophenylcyclohexyls 2). 2). with a variation of the cyanobiphenyls, lengthlengthcyanobiphenyls, of of the the aliphatic aliphatic alkoxycyanobiphenyls, alkoxycyanobiphenyls, chain chain C CnHnH2n2+1n+1 ( n(n = =5–12) and5–12) and alkylc(Figure (Figure alkylcyanophenylcyclohexyls 2). yanophenylcyclohexyls2). with with a variationa variation of ofthe the cyanobiphenyls,length cyanobiphenyls,length of theof the aliphatic aliphatic alkoxycyanobiphenyls, alkoxycyanobiphenyls, chain chain Cn HC2nnH+1 2(nn+1 =(n 5–12) and= 5–12)and alkylc(Figure alkylc (Figureyanophenylcyclohexyls 2).yanophenylcyclohexyls 2). with with a variationa variation of ofthe the length length of theof the aliphatic aliphatic chain chain Cn HCn2nH+12 n(+1n =(n 5–12) = 5–12) (Figure (Figure 2). 2). length length of theof the aliphatic aliphatic chain chain Cn HC2nnH+12 (nn+1 =(n 5–12) = 5–12) (Figure (Figure 2). 2). length length of ofthe the aliphatic aliphatic chain chain Cn CHn2Hn+12 n(+1n (=n 5–12)= 5–12) (Figure (Figure 2). 2).

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Table 1. Calamitic mesogens studied by Cognard [20].

Notation Chemical Structure Notation Chemical Structure

MBBA PEPN4

EBBA PEPN5

PAA PEPN6

PAP PEPN7

S3 HPEH23

S6 HPEH33

K15 HPEH43

K21 PCH3

M9 PCH5

M15 PCH7

M21 BCH5

M24 P35

M27 P37

CB15 TP34

T15 3HPPH3

Thus, Mori and Iwata [21] considered three representatives of calamitic mesogens: cyanobiphenyls, alkoxycyanobiphenyls, and alkylcyanophenylcyclohexyls with a variation of the Lubricants 2019, 7, 111 5 of 25 length of the aliphatic chain CnH2n+1 (n = 5–12) (Figure 2).

Figure 2. Calamitic mesogens reported in [21]: I—alkylcyanobiphenyls, II—alkyloxycyanobiphenyls, Figure 2. Calamitic mesogens reported in [21]: I—alkylcyanobiphenyls, II—alkyloxycyanobiphenyls, and III—alkylcyanophenylcyclohexyls; R—alkyl group (CnH2n+1), n = 5–12. and III—alkylcyanophenylcyclohexyls; R—alkyl group (CnH2n+1), n = 5–12.

Tribological tests were carried out in the temperature range from 20 to 60 ◦C on a device with two Tribological tests were carried out in the temperature range from 20 to 60 °C on a device with rotating cylinders at a load of 735 N. The rotation speed of 220 rpm was constant for one cylinder and two rotating cylinders at a load of 735 N. The rotation speed of 220 rpm was constant for one cylinder varied from 220 to 350 rpm for the other one. The data obtained were compared with each other and and varied from 220 to 350 rpm for the other one. The data obtained were compared with each other with the results of similar studies obtained for mineral oils. It was found that at the same rotation and with the results of similar studies obtained for mineral oils. It was found that at the same rotation speeds, the mesogens form thicker layers compared to synthetic oils, but their friction coefficients are lower than those of synthetic oils. When studying the dependence of the friction coefficient on the chemical structure of mesogens, it was shown that this parameter strongly depends on the structure of the rigid fragment of the molecules. The lowest friction coefficient among the studied compounds was detected for cyanobiphenyls; it equals 0.035. If one of the phenyl groups is replaced by a cyclohexyl fragment (compound III), then the friction coefficient increases to 0.045. The authors concluded that the flatter molecular structure of compound I (compared with compound III) positively affects the friction coefficient. Replacing the alkyl group (compound I) with an alkoxy group (compound II) also leads to a slight increase in the friction coefficient (up to 0.04). At the same time, the value of the friction coefficient practically did not depend on the number of carbon atoms in the aliphatic chain in the corresponding homological series. The authors drew attention to the fact that the coefficient remained practically unchanged, although the viscosity of mesogens increased with an increase in the number of carbon atoms in the alkyl chain. There was also no pronounced dependence of this parameter on temperature: the study affected only those temperature regions at which thin layers without shear conditions showed both mesomorphic and isotropic liquid states. Apparently, the ability of mesogenic material to form special “epitropic” structures under shear conditions plays the leading role in tribological effects in these cases, in particular, to orient themselves in a special way. The data of FT-IR spectroscopy under shear conditions prove that the rigid cyanobiphenyl fragments of molecules are oriented parallel to the shear plane [21]. Nakano’s studies [22] on the variation of the friction coefficient by using alkylcyanobiphenyls, alkoxycyanobiphenyls, and other calamitic mesogens are also in the same line. The main conclusions drawn for the compounds indicated in Table1 can be represented as follows: (1) the higher the nematic – isotropic phase transition temperature (TNI), the lower the friction coefficient; (2) the lowest friction coefficient is observed in a mixture of nematogens with high TNI values and a high orientation degree [20]. These data were in good agreement with previous studies on the tribological properties of the eutectic mixture of N-(4-Methoxybenzylidene)-4-butylaniline (MBBA) and N-(4-Ethoxybenzylidene)-4-butylaniline (EBBA) in comparison with the tribological properties of both aviation oil (MS 20 aviation oil) and vaseline oil (Vaseline oil) [20]. Amann and Kailer [23] synthesized a number of calamitic compounds with intramolecular hydrogen bonds and studied them as potential materials for lubrication processes. They investigated their viscosity properties at different temperatures, taking into account the important role of this parameter in tribology. It was found that an increase in molecular weight correlates with an increase in viscosity. For all compounds, elongation of the alkyl chain also increases viscosity. Reducing the size of the molecules reduces the friction coefficient, but the running-in period becomes longer. Wa´zy´nska and Okowiak [24] devoted their study of calamitic mesogens as lubricants to the relationship between the type of the formed LC phase and tribological properties. They studied compounds exhibiting Lubricants 2019, 7, 111 6 of 25 smectic A, B, and E phases, as well as the nematic phase (Figure3). Pure individual mesogens as well as Lubricantsmixtures 2019 of smectics, 7, x with nematic mesogens were used. 6 of 26

Figure 3.3. MolecularMolecular structures, structures, phase phase transition transi temperaturestion temperatures (given in(given◦C), and in the°C), reference and the designations reference designationsof mesogens describedof mesogens in [ 24described]. in [24].

The main objective of the work was to determine the orientation of mesogens on the surface of steel samples. In In the the ‘ball–disk’ ‘ball–disk’ ge geometryometry using using the the tribolog tribologicalical tester tester T-11, T-11, the the friction friction forces, forces, wear, wear, and friction coecoefficientfficient were were studied studied at at the the following following conditions: conditions: load load 20, 20, 30, 30, 40, 40, 50 N,50 shearN, shear rate rate 0.1 m 0.1/s, m/s,and testand timetest time 900 s. 900 An s. important An important result result obtained obtain ined this in workthis work was thatwas thethat smallest the smallest weight weight loss wasloss wasobserved observed when when mesogenic mesogenic mixtures mixtures exhibiting exhibiting a nematic a nematic phase phase (compared (compared with smecticwith smectic phases) phases) were wereused atused all appliedat all applied loads. loads. The frictionfriction coe coefficientsfficients values values were were obtained obtained from from 900 points 900 points depending depending on the load on forthe mesogensload for mesogensin different in phases different and phases for para andffin oil.for Theparaffin obtained oil. The data obtained showed thatdata all showed the studied that nematicall the studied phases nematicshowed aphases decrease showed in the a frictiondecrease coe inffi thecient friction with increasingcoefficient load.with increasing load. In this case, the nematic phase in comparison with all studied smectic phases showed the lowest friction coefficient. coefficient. It It is is interesting interesting to to note note th thatat the mesogen mixture being in the smectic B phase initially also also had had a a rather rather low low friction friction coefficient, coefficient, but but it itgrew grew with with the the load load increase, increase, and and at at50 50N, N,it becameit became the the largest largest among among all all the the st studiedudied smectics. smectics. The The dependence dependence of of the the friction friction coefficient coefficient on the load was also noted for mixtures with smectic A andand smecticsmectic EE phases.phases. The authors tried to analyze the obtainedobtained datadata fromfrom the the point point of of view view of of the the chemical chemical structure structure of theof the mesogenic mesogenic compound. compound. It was It wasconcluded concluded that that the typethe type of mesophase of mesophase does does not influencenot influence the tribologicalthe tribological properties properties as much as much as the as thechemical chemical structure structure of theof the calamitic calamitic mesogen mesogen and and especially especially the the length length of of its its rigid rigid fragment. fragment. In this case, thethe polarpolar groups groups of mesogenof mesogen molecules molecules are adsorbed are adsorbed on the on surface the ofsurfac triboconjugatione of triboconjugation elements, elements,the molecules the molecules obtain the obtain perpendicular the perpendicular orientation orientation on the steel on surface, the steel and surface, thus, the and resistance thus, the to resistancemovement to is movement reduced. It is has reduced. been shown It has that been this shown adsorption that this phenomenon adsorption dependsphenomenon mainly depends on the mainlychemical on structure the chemical of compounds structure of [24 compounds]. [24]. 3.2. Chiral Mesogens 3.2. Chiral Mesogens Cholesteric (chiral) mesogens represent another type of calamitic molecules. Cholesteric liquid Cholesteric (chiral) mesogens represent another type of calamitic molecules. Cholesteric liquid crystals (CLCs) have excellent tribological characteristics (exceeding those of conventional calamitic crystals (CLCs) have excellent tribological characteristics (exceeding those of conventional calamitic mesogens), due to which they are able to reduce the friction coefficient and wear during the friction process. From a physical point of view, such behavior can be explained by the fact that upon friction, the polymolecular layers of calamitic mesogens and surfactants are transformed into monomolecular ones. This transformation leads to an abrupt increase in shear resistance and has a negative effect on

Lubricants 2019, 7, 111 7 of 25 mesogens), due to which they are able to reduce the friction coefficient and wear during the friction process. From a physical point of view, such behavior can be explained by the fact that upon friction, the polymolecular layers of calamitic mesogens and surfactants are transformed into monomolecular ones. Lubricants 2019, 7, x 7 of 26 This transformation leads to an abrupt increase in shear resistance and has a negative effect on the wearthe wear of mating of mating parts. parts. In contrast, In contrast, cholesteric cholesteric interfacial interfacial layers dolayers not havedo not this have disadvantage, this disadvantage, because theybecause form they a stack form ofa stack nematic of nematic layers where layers thewhere upper the andupper lower and layerslower arelayers adhesively are adhesively bonded bonded to the frictionto the friction surfaces. surfaces. At a shift, At a theshift, molecules the molecules of nematic of nematic layers layers can freely can freely move move (slide) (slide) relative relative to each to other.each other. In addition, In addition, the cholesteric the cholesteric boundary boundary phase is characterizedphase is characterized by the possibility by the of possibility orientation of itsorientation molecules of byits microreliefmolecules by grooves microrelief [25]. grooves [25]. The influenceinfluence of the phase state ofof cholestericcholesteric mesogensmesogens (Figure4 ,4, theirtheir phasephase transitiontransition temperatures are presented in Table 2 2[ [26])26]) onon thethe frictionfriction coecoefficientfficient was studied by Syrbu and coauthors [[27].27]. The studiesstudies werewere performedperformed using using the the laboratory laboratory tribometer tribometer TLPF1 TLPF1 in in the the conditions conditions of boundaryof boundary friction friction during during reciprocating reciprocating movements. movements.

Figure 4. Structural formula of ch cholesterololesterol derivatives.

Table 2. Cholesterol derivatives used as additives in [27] and their mesomorphic properties [26]. Table 2. Cholesterol derivatives used as additives in [27] and their mesomorphic properties [26]. Notation Compound Molecular Weight Phase Transitions ( C) Notation Compound Molecular Weight Phase Transitions (°C)◦ X-19X-19 Chol–COOHChol–COOH 414 414 Cr 97.5 Cr 97.5 Iso Iso X-26X-26 Chol–ClChol–Cl 405 405 Cr 97.0 Cr 97.0 Iso Iso X-4 Chol–COOC4H9 470 Cr 93 Ch 101.5 Iso X-4 Chol–COOC4H9 470 Cr 93 Ch 101.5 Iso X-6 Chol–COOC7H15 512 Cr 110 Iso X-6 Chol–COOC H 512 Cr 110 Iso X-5 Chol–COOC7 9H1519 540 Cr 85.5 Ch 92.5 Iso X-5X-18 Chol–COOCChol–COOC910HH1921 554540 Cr Cr 85.592.5 ChIso 92.5 Iso X-20 Chol–COOC12H25 582 Cr 63.5 Sm 78.8 Ch 84.8 Iso X-18 Chol–COOC10H21 554 Cr 92.5 Iso X-28 Chol–COOC14H29 610 Cr 70 Sm 78.3 Ch 82.9 Iso X-20 Chol–COOC12H25 582 Cr 63.5 Sm 78.8 Ch 84.8 Iso X-37 Chol–COO–(C6H4)–C8H17 602 Cr 138 Sm 171.5 Ch 220.5 Iso X-28 Chol–COOC H 610 Cr 70 Sm 78.3 Ch 82.9 Iso X-68 Chol–COO–(C6H144)–C2912H25 658 Cr 128.5 Sm 179.5 Ch 200.5 Iso X-37Chol– Chol–COO–(CC27H645H 4)–C8H17 369602 Cr 138 Sm 171.5 Ch 220.5 Iso

X-68 Chol–COO–(C6H4)–C12H25 658 Cr 128.5 Sm 179.5 Ch 200.5 Iso It was found that during friction within the mesophase existence temperature range, the friction Chol– C27H45 369 coefficient decreases with increasing temperature. Such behavior was not observed for non- mesogenic cholesterol esters, i.e., with the temperature increase, the decrease of the friction coefficient was Itabsent. was found In the that isotropic during frictionliquid region within theof mesogenic mesophase compounds, existence temperature the mesogens range, with the frictionhigher coemolecularfficient decreasesweight have with a lower increasing friction temperature. coefficient. Such The best behavior lubricating was not properties observed (the for non-mesogeniclowest friction cholesterolcoefficient µ esters, = 0.06–0.08) i.e., with were the shown temperature by cholesterol increase, ester the X-68 decrease, which of thehas frictionthe widest coe ffimesophasecient was absent.range. In the isotropic liquid region of mesogenic compounds, the mesogens with higher molecular weightNon-mesogenic have a lower frictioncholesterol coeffi derivativescient. The best X-19 lubricating and X-26, properties which do (the not lowestform mesophases, friction coeffi havecient µsimilar= 0.06–0.08) melting were points shown and bymolecular cholesterol weights. ester X-68Howeve, whichr, their has influence the widest on mesophase the friction range. coefficient is different:Non-mesogenic for X-26, the cholesterolfriction coefficient derivatives does X-19not dependand X-26 on, temperature, which do not while form for mesophases, X-19, it gradually have similarincreases melting with increasing points and temperature molecular weights. from 0.08 However, to 0.13. The their authors influence assume on the that friction this difference coefficient can is be explained by the chemical activity of the substituents that comprised the compound X-26. Analysis of the molecular weight influence on the properties of the studied CLCs was performed. It was revealed that at the temperature of 150 °C, the friction coefficient of compounds with a molecular weight less than 550 a.m.u. does not depend on their molecular weight value. Above 550 a.m.u., an increase in the molecular weight of the compound results in a decrease of the friction coefficient proportionally to the molecular weight. This dependence was also described by Hardy [28] for boundary friction. The data obtained confirm the importance of the viscosity of systems in

Lubricants 2019, 7, 111 8 of 25 different: for X-26, the friction coefficient does not depend on temperature, while for X-19, it gradually increases with increasing temperature from 0.08 to 0.13. The authors assume that this difference can be explained by the chemical activity of the substituents that comprised the compound X-26. Analysis of the molecular weight influence on the properties of the studied CLCs was performed. It was revealed that at the temperature of 150 ◦C, the friction coefficient of compounds with a molecular weight less than 550 a.m.u. does not depend on their molecular weight value. Above 550 a.m.u., an increase in the molecular weight of the compound results in a decrease of the friction coefficient proportionally to the molecular weight. This dependence was also described by Hardy [28] for boundary friction. The data obtained confirm the importance of the viscosity of systems in tribological processes. In the mesophase, viscosity increases, while in the isotropic liquid phase, it decreases.

3.3. Mesogens as Additives to Lubricants and Greases Since liquid crystal materials are quite expensive to produce, the attention of researchers was attracted to the possibility of their use as additives (lubricant additives) to lubricants (oils and greases) [5]. It should be at once noted as the expected result that the friction coefficient of oil systems with mesogen is higher than that of individual mesogens. The use of CLCs as additives in tribological processes raised the question of their compatibility with oils. The analysis of solubility of cholesterol esters in medical Vaseline oil, polymethylsiloxane fluid (PMS-200), and engine oil M-12G1 revealed three groups of systems: (1) a true solution of CLC in oil, (2) the heterogeneous system ‘CLC crystals in oil’, and (3) a thickened suspension of small crystals in oil. Therefore, the second group is characterized by the partial solubility of mesogens in oil, and in the third group, the solubility of mesogens in oil below their melting point is absent. The authors [25] attribute this to the dimensional value of the intermolecular interaction between ‘mesogen–mesogen’ and ‘mesogen–oil’. The rheological properties of oils change with the introduction of CLCs: viscosity increases over the entire temperature range. At the same time, the viscosity of the synthetic fluid PMS-300 increases to a greater extent than that of Vaseline oil. As for the tribological properties of oils, a decrease in the friction coefficient is observed when CLCs are introduced into the oil. It should be noted that the minimum wear does not always correspond to the lowest friction coefficient. Nevertheless, there is a tendency to improve the tribotechnical characteristics of oil solutions containing mesogens with higher molecular weight (X-7, X-5, and X-16), which have lower transition temperatures from crystalline phase to the liquid crystalline phase. At the same time, with an increase of the alkyl radical length of mesogens, an increase in the wear value is observed in comparison with Vaseline oil [25]. The kinetics of the friction coefficient and the surface topography were studied by Kupchinov B.I. et al. [29] when the content of mesogen X-16 in Vaseline oil was in the concentration range of 0.5–9.0 wt % (slip velocity was 0.5 m/s, load was 3.5 MPa). It was found that in general, the introduction of X-16 leads to a decrease in the friction coefficient. However, the additive concentration affects the time that it takes to reach this value from the onset of dynamic contact: an increase in concentration leads to a decrease in this time. Moreover, when lubricant contains high mesogen content, there are probably enough mesogen molecules to form a layer that can hide the microroughness of the friction surface. Due to that, Bermudez et al. [30] carried out studies on the influence of the molecular structure features of calamitic mesogens: homologues of 4,40-dialkyl- and dialkyloxybenzenes, 4,40-dialkylazoxybenzenes, and cyanoaryls, which were used as additives to commercial oils. The ‘finger-disk’ geometry of the tribometer was used at various temperatures and additive concentrations. In this case, neither the temperature variation of the phase transitions of mesogens, nor the difference in viscosity had a significant effect on the magnitude of friction. It is important to note that for all 14 compounds studied, the addition of only 1% of mesogen to two different base oils significantly reduced the friction coefficient of steel. The wear degree in the contacts of ‘steel–steel’ and ‘steel–aluminum’ pairs also decreased with the addition of these mesogens to base oils. Lubricants 2019, 7, 111 9 of 25

A wide range of tribological studies was carried out using as additives the cholesterol esters (CLC), which exhibit a chiral nematic (N*) phase [25,31]. Vekteris and Mokshin [31] studied boundary lubrication processes using industrial mineral oil I–40A with the additives of four homologues of cholesterol esters (2 vol. %): valerianic (C4H9), caproic (C5H11), myristic (C13H27), and stearic (C17H35). Data obtained were compared with parameters of pure mineral oil without additives. The tests were conducted in conditions of boundary lubrication. The roughness of the cylinder before each experiment was Ra = 2.5 µm. The cylinder constantly rotated at a speed of 545 rpm (with a peripheral speed of ν = 1.4 m/s). The load on a segment was 1000 N. Firstly, it was found that only the three studied homologues are completely soluble in mineral oil at a concentration of 2 vol. % (stearyl ether of cholesterol crystallizes in industrial oil). Therefore, additional melting of the mixture was necessary before use. Tests have shown that all the LC additives of this series positively influenced the antifriction properties of mineral oil. Chiral mesogens with the highest molecular weight (stearate and myristate of cholesterol) are the most effective: when they were used as additives to industrial oils, the friction coefficient for the ‘steel–steel’ tribological system decreases by 1.12 times compared to pure industrial oil (namely, from 0.105 to 0.094) [31]. The works of Latyshev et al. [32] are logically related to the above-mentioned data. They concern the rheological properties of lubricants with CLC additives. Based on the view that studies of flow curves can serve as a means of revealing the internal structure of anisotropic liquid and its change with varying external conditions [25], the authors studied the rheological properties of a number of cholesteryl esters of alkyl, alkyloxy, and alkyloxybenzoic acids, as well as cholesterol chloride as an additive to two types of base oils. They investigated the solubility of these additives and then the rheological properties of compositions. It was shown that cholesteryl esters of undecyl and oleic acids dissolve well in base oils, while cholesteryl esters of benzoic acids have limited solubility. It was also found that the introduction of the CLC additives into low-viscous oils influences their viscosity by an average of 10%. The higher the degree of the influence, the less the amount of other additives inserted into the original oil. For more viscous gear oils, the introduction of new additives has practically no effect on rheological properties [32]. The study results of the temperature effect on the friction coefficient of 1% solutions of cholesteric liquid crystals in industrial oil I-20 are presented in [27]. It was established that at temperatures below 100 ◦C, the friction coefficient of solutions is practically independent on temperature, and it is approximately the same for the different studied compounds (µ = 0.13 ... 0.14). However, with a further temperature increase, a linear growth in the friction coefficient begins, and the values of µ reach 0.17 ... 0.18. The authors explain the obtained pattern by the formation and temperature destruction of the adsorbed boundary-lubricating layer formed by CLC molecules on rubbing surfaces. At low temperatures, a chemisorption bond occurs between the molecules of nanomaterial and the surface, which gives stability to the film. At a certain critical temperature, the adsorption bond between the CLC molecules and the surface breaks down, which results in an increase in the friction coefficient. The critical temperature value depends on the structural composition of the nanomaterial. For all compounds with the –COO– group, this temperature is in the range between 125 and 130 ◦C, and for cholesterol chloride, it is between 110 and 115 ◦C. Unlike isotropic lubricating oils, plastic lubricants (PLs) are structured colloidal systems. The rheological characteristics of the ‘PL-CLC’ systems were studied by Berezina et al. [33]. In this work, Litol-24 was used as the PL material, and CLCs were represented by five cholesterol esters having aliphatic substituent with a varying number of carbon atoms (n = 8–17). Additive concentrations were varied from 0.5 to 5.0 wt %. The introduction of CLC compounds in all cases led to a decrease in yield strengths. The relationship between shear stress and shear rate was well described by the Shvedov–Bingham model [33,34]. It was found that the introduction of CLC additives changes the rheological properties of the system, which the authors attribute to the formation of a less durable spatial structure. The authors note as characteristic rheological data that at low shear rates, kinks in the course of the curves are observed, indicating structural changes in the system. When additives are added, the Lubricants 2019, 7, 111 10 of 25 viscosity of systems is significantly reduced. Different flow mechanisms were presented at different sections of the rheological curve, which corresponded to the Rebinder concept [35]. According to the flow curves, the studied samples obeyed the Casson model and regression coefficients decreased with the increasing shear rate. A change in the rheological properties should have a beneficial effect on the tribological properties of LSs. It was found that the introduction of even a very small amount of additive leads to a decrease in the effective viscosity of systems by several tens of times. The authors concluded that by introducing small amounts of additives (cholesterol esters), it is possible to achieve a sharp change in the properties of the lubricating composition. With the aim to optimize the antifriction and antiwear properties, the structure of classical cholesterol esters was modified by fluorination [8]. The introduction of fluorinated (perfluoroalkyl) chainsLubricants enhanced 2019, 7, the x polarity of compounds due to the C–F bond. This should have led to10 of a change26 in the mesophase existence temperature and better adsorption of the material on the metal surface. in the mesophase existence temperature and better adsorption of the material on the metal surface. FourFour cholesterol cholesterol esters esters were were synthesized synthesized (Figure (Figure5 5).).

FigureFigure 5. T 5. Fluorinated T Fluorinated cholesterol cholesterol esters esters ((LC-BLC-B,, LC-DLC-D) )and and their their non-fluorinated non-fluorinated analogues analogues (LC-A (LC-A, , LC-CLC-C) studied) studied by Gaoby Gao [8]. [8].

It wasIt was found found that that among among the the four four synthesizedsynthesized compounds, compounds, three three showed showed mesomorphism mesomorphism monotropically,monotropically, and onlyand oneonly compoundone compound (LC-D) (LC-D) showed showed enantiotropically. enantiotropically. The regionThe region of the of mesophase the mesophase existence was expanded. The tribological data obtained for the systems of these mesogens existence was expanded. The tribological data obtained for the systems of these mesogens with mineral with mineral oil using BMI Tribolab indicate fairly good tribological properties for all the oil using BMI Tribolab indicate fairly good tribological properties for all the compounds. However, the compounds. However, the LC-D compound, which has the largest range of mesophase existence, LC-Dshowed compound, the best which tribological has the proper largestties. rangeThe authors of mesophase associate it existence, with an additional showed buffer the best layer tribological in the properties.oil film Thecomposition, authors associatewhich is formed it with by an fluorinated additional substituents. buffer layer Itin is thevery oil important film composition, that within whichthe is formedmesophase by fluorinated existence substituents. range, the dispersion It is very importantof LC in mineral that within oil significantly the mesophase reduces existence the friction range, the dispersioncoefficient. of LC The in authors mineral believe oil significantly that this featur reducese can the be friction applied coeforffi thecient. development The authors of medical believe that this featureequipment, can equipment be applied for for bionics, the development etc. of medical equipment, equipment for bionics, etc. A largeA large cycle cycle of works of works on the on tribological the tribological properties properties of cholesteric of cholesteric mesogens mesogens and their and application their was carriedapplication out was in Belarus carried out and in summarized Belarus and summarized in a monograph in a monograph [36]. [36]. Our own studies on the rheological and tribotechnical properties of industrial oil systems with Our own studies on the rheological and tribotechnical properties of industrial oil systems with CLC CLC additives (cholesteric esters of carbonic acid: stearic X-7, propionic X-10, oleic X-16, and additives (cholesteric esters of carbonic acid: stearic X-7, propionic X-10, oleic X-16, and tridecanoic tridecanoic X-20, the concentrations of mesogens are shown in Table 3) revealed that X-7 and X-20 X-20,are the good concentrations antiwear additives of mesogens [37]. are shown in Table3) revealed that X-7 and X-20 are good antiwearThe additives search [for37 ].smart materials led us to the idea of considering the possibility of an additional introductionThe search forto CLCs smart of materials carbon nanostructures, led us to the idea which of consideringare also known the to possibility affect the oftribological an additional introductionprocesses to[38], CLCs and then of carbonuse the ‘CLC–carbon nanostructures, nanost whichructure’ are system also as known a complex to a lubricantffect the additive tribological processesto industrial [38], and oil. thenIt turned use theout ‘CLC–carbonthat the introduction nanostructure’ of multi-walled system carbon as a nanotubes complex lubricant(MWCNT) additive in to industrialmicro quantities oil. It turned in relation out to that the thetotal introduction volume of lubricant of multi-walled (0.0001 wt %) carbon in compositions nanotubes with (MWCNT) X-7 or in microX-20 quantities CLC improved in relation the antifriction to the total qualities volume of of the lubricant whole (oil) (0.0001 system wt [37]. %) in compositions with X-7 or X-20 CLC improved the antifriction qualities of the whole (oil) system [37]. Table 3. Compositions of two- and three-component systems1 [39]. MWCNT: multi-walled carbon nanotubes.

Two-Component Systems Three-Component Systems Industrial oil I-20A (base) with additives: X-7 (2%) X-7 (2%) + 0.0001% of MWCNT X-10 (2%) X-10 (2%) + 0.0001% of MWCNT X-16 (2%) X-16 (2%) + 0.0001% of MWCNT X-20 (2%) X-20 (2%) + 0.0001% of MWCNT 1 Concentrations are given in wt %, names of CLCs are given in the text.

Lubricants 2019, 7, 111 11 of 25

Table 3. Compositions of two- and three-component systems 1 [39]. MWCNT: multi-walled carbon nanotubes.

Two-Component Systems Three-Component Systems Industrial oil I-20A (base) with additives: X-7 (2%) X-7 (2%) + 0.0001% of MWCNT X-10 (2%) X-10 (2%) + 0.0001% of MWCNT X-16 (2%) X-16 (2%) + 0.0001% of MWCNT X-20 (2%) X-20 (2%) + 0.0001% of MWCNT 1 Lubricants 2019, 7, x Concentrations are given in wt %, names of CLCs are given in the text. 11 of 26

In addition to the thermotropic mesomorphism andand temperaturetemperature rangerange ofof thethe LCLC phase, the most important physical parameters for the use of cholestericcholesteric mesogens in tribologytribology are theirtheir rheologicalrheological characteristics. The The flow flow curves curves of of pure pure CLCs CLCs and and ‘CLC–MWCNT’ ‘CLC–MWCNT’ dispersions dispersions were were obtained obtained at the at thetemperatures temperatures of smectic of smectic А phase A phase (SmA), (SmA), chiral chiral nematic nematic (N*), (N*), and and isotropic isotropic (Iso) (Iso) phases phases with thethe 1 variation ofof shearshear ratesrates betweenbetween 0.010.01 andand 30003000 ss−−1 (Figure(Figure 66).). The rheological propertiesproperties ofof thethe samplessamples under the conditions of constant shear rate and inin thethe low-amplitudelow-amplitude oscillatoryoscillatory deformationdeformation mode were studiedstudied usingusing aa PhysicaPhysica MCR301MCR301 rotational rheometer (Anton Paar, Austria).Austria). With the aim of creating smartsmart materialsmaterials for for tribological tribological processes, processes, three-component three-component systems systems containing containing not not only only an oilan base,oil base, but but also also cholesteric cholesteric LCs LCs with with the additionthe addition of nanoparticles of nanoparticles were were studied. studied. Performing Performing this task,this wetask, considered we considered the possibility the possibility of a positive of a positive (synergetic) (synergetic) effect ineffect the processin the process of friction, of friction, since carbon since nanostructurescarbon nanostructures (CNSs) contribute(CNSs) contribute to an increase to an increase in the thermal in the conductivitythermal conductivity of systems. of Insystems. addition, In theyaddition, are resistant they are to resistant high mechanical to high mechanical loads and load in somes and cases in some reduce cases the reduce friction the coe frictionfficient coefficient and wear rateand ofwear friction rate of pairs friction [38,39 pairs]. [38,39].

3 0.00 % 10 0.01 % 3 102 2 0.02 % 10 SmA N* Iso

10 G" (Па) 1

s) 10 2 ⋅ 10 100 101 SmA N* Iso SmA (Pa (Па*с) -10 -5 0 5 (Pa) (Pa) o 1 T-T ( C) ∗ 10 c η 100 G' (Па) 0.00 % Iso 0.01 % N* 100 0.02 % 10-1 -10 -5 0 5 -10 -5 0 5 T-T (oC) T-T (oC) c c (a) (b)

Figure 6.6. Temperature dependencesdependences of of complex complex viscosity viscosity (a ()a and) and dynamic dynamic modules modules (b )(b of) X-15of X-15and and its compositionsits compositions with with MWCNTs MWCNTs [39 ].[39].

The eeffectffect of dispersed multi-walled carbon nanotubes (MWCNTs) (material “Taunit-M”, OOO NanoTekhTsentr, Tambov, Tambov, Russia) Russia) on theon mesomorphicthe mesomo andrphic rheological and rheological properties properties of three mesogenic of three cholesterolmesogenic cholesterol derivatives, derivatives, R = CH3(CH R =2 CH)7CH3(CH=CH(CH2)7CH=CH(CH2)7– (Tekon-20),2)7– (Tekon-20), R = CH R3 =(CH CH23)(CH11–(2X-20)11– ()X-20 and) CHand3 (CHCH32(CH)12–(2)12X-15– (X-15), was), was studied studied in [39 in]. [39]. It was It was found found that that the introductionthe introduction of MWCNTs of MWCNTs into theinto CLC the matrixCLC matrix (at concentrations (at concentrations of 0.01–0.04 of 0.01–0.04 wt %) wt extends %) extends the temperature the temperature range range of the of mesophase. the mesophase. The textures of the ‘CLC–MWCNT’‘CLC–MWCNT’ dispersions are identical to the individual CLCs, but when cooled toto the the crystalline crystalline phase, phase, the polygonal the polygonal texture oftexture the dispersion of the dispersion becomes striated becomes as “fingerprints” striated as (Figure“fingerprints”7)[39] (i.e., (Figure there 7) is [39] the (i.e., preservation there is the of chiralitypreservation by the of “memory chirality by e ff theect” “memory type [40]). effect” type [40]).

Lubricants 2019, 7, 111 12 of 25 Lubricants 2019, 7, x 12 of 26

(a) (b)

FigureFigure 7. Micrographs 7. Micrographs of of the the polygonal polygonal texturetexture of the cr crystallineystalline phase phase of ofcholesteric cholesteric liquid liquid crystal crystal (CLC)(CLC) samples: samples: (a) individual(a) individual compound compoundX-20 X-20,(, (b) composition composition X-20X-20/MWCNT/MWCNT (0.01 (0.01 wt %), wt on %), cooling, on cooling, crossedcrossed polarizers, polarizers,250 ×250 [39 [39].]. × The introductionThe introduction of MWCNTs of MWCNTs (with (with a concentrationa concentration of of ≤0.02 wt wt %) %) into into the the studied studied CLC CLC (Figure (Figure 6) ≤ does6) not does aff ectnot theaffect complex the complex viscosity viscosity of the of the resulted resulted system system in in the the isotropic isotropic phase, but but increases increases η*η * by 25% inby the25% N* in the phase N* phase and increases and increases the the complex complex viscosity viscosity approximately approximately twofold twofold in in the the SmA SmA phase. phase. The dynamicThe dynamic moduli moduli of theof the composition composition respond respond to to the the introduction introduction ofof MWCNTs as as follows: follows: the the value of the elastic modulus G’ increases and the viscosity modulus G’’ decreases in the isotropic value of the elastic modulus G’ increases and the viscosity modulus G” decreases in the isotropic phase. phase. Probably, in the isotropic phase, the additive particles form a percolation network. Probably, in the isotropic phase, the additive particles form a percolation network. Analysis of the temperature dependence of dynamic modules showed an increase of G’ and G’’ Analysisvalues by ~25% of the in temperature the N* phase dependenceand an increase of by dynamic ~2.5 times modules in the SmA showed phase. an With increase an increase of G’ andin G” valuesthe by deformation ~25% in the frequency N* phase from and 0.01 an to increase 20 Hz, the by ~2.5complex times viscosity in the SmAη* of CLC phase. decreases With an by increase more in the deformationthan two orders frequency of magnitude, from 0.01 and to the 20 storage Hz, the an complexd loss moduli viscosity increaseη* of approximately CLC decreases by by twofold more than two orders[41]. of magnitude, and the storage and loss moduli increase approximately by twofold [41]. The tribologicalThe tribological characteristics characteristics of of two-component two-component systems systems ‘industrial ‘industrial oil oilI-20A-CLC’, I-20A-CLC’, as well as wellas as three-componentthree-component systems systems containing containing the industrialthe industrial oil I-20Aoil I-20A and theand complex the complex additive additive ‘CLC-MWCNT’, ‘CLC- wereMWCNT’, presented were in [39 presented]. The studied in [39]. CLCs The werestudied cholesteryl CLCs were stearat cholesteryl (X-7), cholesteryl stearat (X-7 propionate), cholesteryl (X-10 ), propionate (X-10), cholesteryl oleate (X-16), and cholesteryl tridecylate (X-20); they are listed in Table cholesteryl oleate (X-16), and cholesteryl tridecylate (X-20); they are listed in Table4 along with 4 along with concentrations of compositions. concentrationsThe wear of compositions.parameters (the area of the ball wear spot and the width of the wear of the roller track) Thewere wear obtained. parameters The experimental (the area results of the are ball presented wear spot in andTable the 4. width of the wear of the roller track) were obtained. The experimental results are presented in Table4. Table 4. Experimental conditions and data of tribological tests [39]. Table 4. Experimental conditions and data of tribological tests [39]. Temperature Friction Wear of the Roller Area of the Ball Wear Spot Sample (°C) Coefficient Temperature (◦C) Wear(mm) of the Area(mm) of the Ball Friction Sample Start End (f) Roller (mm) Wear Spot (mm) Coefficient (f) Industrial Oil I-20A 19Start 50 End 0.35 0.110 0.051 IndustrialI-20A Oil + X-7 I-20A 22 19 55 50 0.300.35 0.106 0.110 0.088 0.051 I-20A + X-7 + 19 51 0.525 0.280 0.046 I-20AMWCNT+ X-7 22 55 0.30 0.106 0.088 I-20A + I-20AX-7 + +MWCNT X-20 30 19 61 510.275 0.525 0.102 0.280 0.064 0.046 I-20A + X-20 + I-20A + X-20 1930 49 61 0.50 0.275 0.165 0.102 0.046 0.064 MWCNT I-20A + X-20I-20A+ + MWCNTX-10 19 19 48 490.625 0.50 0.298 0.165 0.051 0.046 I-20A + X-10 + I-20A + X-10 2119 56 48 0.60 0.625 0.306 0.298 0.088 0.051 MWCNT I-20A + X-10I-20A+ + MWCNTX-16 24 21 54 56 0.60 0.60 0.158 0.306 0.088 0.088 I-20A + X-16 + I-20A + X-16 2424 54 54 0.575 0.60 0.108 0.158 0.076 0.088 MWCNT I-20A + X-16 + MWCNT 24 54 0.575 0.108 0.076

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The performed studies of the rheological and tribotechnical properties of the industrial oil with the addition of a number of CLC (see Table4) showed that X-7 and X-20 are good antiwear additives. Three-component systems of the industrial oil with ‘X-7-MWCNT’ and ‘X-20-MWCNT’ additives showed good antifriction properties [39]. The influence of structural features of carbon nanoparticles (CNPs) (namely, shungite nanocarbon, Sh) on the rheological properties of cholesteric mesogen (cholesterol tridecylate, X-20) were analyzed after studying the rheological characteristics of ‘X-20-Sh’ binary systems (concentrations of Sh were 0.005, 0.01, and 0.02 wt %) [42]. Based on the experimental data on the thermodynamic and the rheological characteristics of the CLC–Sh over the temperature range of the existence of SmA and N* phases and isotropic liquid, as well as a comparison of the results with those previously published for the CLC–MWCNT systems [38], we revealed the general tendency of the carbon nanoparticle concentration to have an insufficient effect on the temperature range of the N* phase existence and on the expansion of the SmA phase temperature range (by ~10 ◦C at 0.02 wt % of Sh). Furthermore, both composites exhibited an increase in the elastic modulus G’ by at least 25% in the N* phase. With regard to viscosity, the CLC–Sh composites demonstrate a decrease in value compared to the CLC–MWCNT systems [39]. The elasticity of the system in the SmA phase in the presence of Sh was also decreased twofold with the increasing filler content, while the introduction of MWCNTs led to an approximately 2.5-fold increase in the elastic modulus. The results of tests of the CLC system with Sh were quite interesting and revealed a decrease in the viscosity of the composites over the ranges of high strain rates at which the lubricants are used. The later evidences that one can vary the properties of the lubricant system by adding various types of carbon nanostructures. This is in good agreement with the data of [43].

3.4. Discotic Mesogens in Tribology Along with calamitic mesogens, in recent years, discotic (disc-like) mesogens [44] have been studied as lubricants. Discotic mesogens form three main types of thermotropic mesophases: columnar (Col), discotic nematic (ND), and columnar lamellar (ColL). The least ordered of them is the ND phase. Discotic molecules in this phase, assimilate to calamitic nematic mesogens, have only orientational ordering relative to the director (→n) of the mesophase, without any positional order. Columnar mesophases are characterized by the feature that their structural unit is a supramolecular ensemble, resembling a stack of coins. They have significant polymorphism associated with the type of two-dimensionally ordered lattice formed by the columns, as well as the degree of ordering of the molecules inside the columns. According to their chemical structure, they can be conditionally divided into four groups: (1) derivatives of six-membered rings with a saturated or unsaturated core, (2) multiring homo- or heterocyclic compounds-super-discs, (3) coordination compounds—discotic metallomesogens and their metal-free analogues, and (4) metallorganyls. The typical representatives of groups 1–3, which are known to be used as lubricants, are shown in Figure8. Lubricants 2019, 7, 111 14 of 25 Lubricants 2019, 7, x 14 of 26

(a) (b) (c)

FigureFigure 8. 8. TheThe typical typical representatives representatives of of discotic discotic mesogens: mesogens: ( (aa)) Hexasubstituted Hexasubstituted benzenes benzenes (group (group 1); 1); ((bb)) Triphenylene Triphenylene derivatives (group 2); ( c) Phthalocyanine derivatives (group 3)3) [[44].44].

TheThe first first such such material material was was a a liquid crystalline derivative derivative of of hexaoxytru hexaoxytruxen,xen, which which displays displays a a columnarcolumnar phase phase and and was was used used as as a a lubricant lubricant in in a a ma machinechine element element [45]. [45]. Later, Later, a a new new antifriction antifriction and and antiwearantiwear additive additive to to lubricating lubricating compositions compositions was was proposed proposed [46], [46], it it has has a a columnar columnar supramolecular supramolecular structure.structure. Then, Then, a aseries series of of studies studies were were carried carried out out on onthe the influence influence of ofthe the molecular molecular structure structure of variousof various discotic discotic mesogens. mesogens. Among Among them, them,columnar columnar mesogens mesogens (hexaoxytr (hexaoxytriphenyleneiphenylene esters as esters well as as theirwell analogues as their analogues with nitro with and nitroamino and groups) amino and groups) discotic and nematic discotic mesogens nematic (derivatives mesogens (derivativesof biphenyl andof biphenyl hexa(cyclohexanebenzoate)triphenylene) and hexa(cyclohexanebenzoate)triphenylene) were reported. were As reported.a result, it Aswas a result,found itthat was discotic found mesogensthat discotic displaying mesogens columnar displaying mesophases columnar are mesophases better used areas antifriction better used additives as antifriction in heavily additives loaded in frictionheavily units, loaded while friction the units,mesogens while with the mesogensnematic discotic with nematic phase were discotic used phase as antiwear were used additives. as antiwear The effectivenessadditives. The of the eff ectivenessadditive is ofdetermined the additive by the is determined molecular structure by the molecular of discotic structuremesogen. of The discotic most mesogen. The most favorable features are: (1) the presence of a large number≥ of ester groups ( 6) in favorable features are: (1) the presence of a large number of ester groups ( 6) in the central fragment≥ ofthe the central discotic fragment mesogen, of the and discotic (2) a mesogen,large cross-sectional and (2) a large area cross-sectional of the central area fragment of the central of the fragmentdiscotic of the discotic mesogen molecule per one≥ heteroatom ( 30) [47–50]. mesogen molecule per one heteroatom ( 30) [47–50]. ≥ DiscoticDiscotic mesogens mesogens (crown-containing (crown-containing phthalocyanines) phthalocyanines) have have been been used used as as additives additives to to plastic plastic lubricantslubricants [51]. [51]. It It was was found found that that the the effectiveness effectiveness of of their their use use is is determined determined by by the the following following peculiarities:peculiarities: high high strength strength characteristics, characteristics, increase increasedd thermal thermal stability, stability, the the ability ability to to structure structure on on the the frictionfriction surface intointo columnarcolumnar ensembles, ensembles, and and the th abilitye ability to formto form chemisorbed chemisorbed films. films. Adsorption-active Adsorption- activeadditives additives of this typeof this are type effective are effective under various under frictionvarious conditions friction conditions and are promising and are aspromising tribological as tribologicaladditives to additives oil and plastic to oil lubricants.and plastic lubricants. OtherOther columnar columnar mesogens mesogens (copper (copper carboxylates) carboxylates) ha haveve been been investigated investigated as as additives additives to to Solidol Solidol grease.grease. The The dependence dependence of of the the friction friction coefficients coefficients on on the the chemical chemical structure structure of of copper copper carboxylates carboxylates waswas studied.studied. AllAll the the homologues homologues studied studied increased increased the upperthe upper temperature temperature limit of limit the stable of the operation stable operationof the lubricant, of the lubricant, being successful being successful thickeners thickeners [52]. [52]. AA large large range range of of works works on on the the use of discotic compounds in in tribological processes processes was was carried carried outout using using phthalocyanine phthalocyanine (Pc) (Pc) deri derivatives.vatives. This This class class of of compounds compounds has has attracted attracted the the attention attention of of researchersresearchers because because they they possesses possesses a a number number of of nece necessaryssary features features of triboa triboactivective additives: (1) (1) the the possibilitypossibility of of structure structure formation formation (supramolecular (supramolecular columnar columnar organization) organization) in in the the bulk bulk of of the the solution solution andand at at the the interface; interface; and and (2) (2) the the ability ability to to ther thermalmal decomposition with with the the formation formation of of chemical chemical lubricatinglubricating filmsfilms at at the the corresponding corresponding contact contact temperature temperature in the frictionin the units.friction Therefore, units. Therefore, Pc derivatives Pc derivativesas solid lubricating as solid lubricating components components can be used can both be toused provide both to dry provide friction dry and friction as additives and as to additives oils [53]. toIn oils the region[53]. In ofthe low region additive of low concentrations, additive concentr an increaseations, an in theincrease number in the of molecules number of of molecules heterocyclic of heterocycliccompounds compounds and their associates and their adsorbed associates by metaladsorb frictioned by metal surfaces friction occurs. surfaces It was occurs. suggested It was that suggestedduring the that initial during accumulation the initial of theaccumulation additive in of the the boundary additive layer, in the adsorption boundary probably layer, adsorption occurs due probablyto lateral substituentsoccurs due to of Pc.lateral On thesubstituents one hand, of the Pс substituents. On the one hold hand, the molecularthe substituents associates hold on the the molecularsurface; on associates the other on hand, the surface; they create on the a flexible other hand, “pile” they that create provides a flexible easy gliding, “pile” that similar provides to a classic easy gliding, similar to a classic surfactant. When the Pc concentration is increased, supramolecular

Lubricants 2019, 7, 111 15 of 25 Lubricants 2019, 7, x 15 of 26 surfactant.columns are When likely the to Pcreorient concentration in such a is way increased, that the supramolecular long axes of the columns molecules are are likely perpendicular to reorient into suchthe metal a way plane, that thewhich long increases axes of the the molecules friction coefficient are perpendicular [53]. to the metal plane, which increases the frictionResults coe relatedfficient to [the53]. influence of thermotropic discotic mesogens on the tribological properties of lubricatingResults related compositions to the influence are summarized of thermotropic in several discotic monographs mesogens and on thereviews tribological [7,54,55]. properties of lubricating compositions are summarized in several monographs and reviews [7,54,55]. 4. Lyotropic Mesogens in Tribology 4. Lyotropic Mesogens in Tribology Lyotropic mesogens (lyomesogens) are amphiphilic molecules or long, rod-shaped molecular and Lyotropicsupramolecular mesogens structures (lyomesogens) that form are liquid amphiphilic crystalline molecules phases or in long, water rod-shaped and some molecular organic andsolvents. supramolecular Molecules of structures a compound, that form which liquid is capable crystalline of forming phases a in lyotropic water and mesophase some organic under solvents. certain Molecules of a compound, which is capable of forming a lyotropic mesophase under certain conditions, conditions, consist of hydrophilic (–COOH, –OH, –CO2Na, SO3K, –O(CH2CH2O)nH, etc.) and consist of hydrophilic (–COOH, –OH, –CO Na, SO K, –O(CH CH O) H, etc.) and lipophilic (–C H ) lipophilic (–CnH2n+1) fragments. Rod-shaped2 lyomesogens3 include2 2 ntobacco mosaic virus particlesn 2n+ or1 fragments.elongated polypeptide Rod-shaped chains, lyomesogens which, include being in tobacco an order mosaiced state virus (due particles to the anisotropy or elongated of their polypeptide shape), chains,impart anisotropic which, being properties in an ordered to the stateentire (due system tothe [56]. anisotropy of their shape), impart anisotropic propertiesA very to detailed the entire review system regarding [56]. lyotropic mesogens and the phases that they form was made by EkwallA very [57]. detailed review regarding lyotropic mesogens and the phases that they form was made by EkwallThe [57 lyotropic]. state (as well as the thermotropic one) is characterized by polymorphism (Figure 9), i.e.,The the lyotropic ability to state form (as various well as liquid the thermotropic crystalline phases one) is under characterized changing by conditions polymorphism (concentration, (Figure9), i.e.,temperature, the ability toand form other various environmental liquid crystalline factor phasess). The under formation changing of conditions lyotropic (concentration, mesophases temperature,(lyomesophases) and otherby environmentalamphiphilic compounds factors). The formationoccurs due of lyotropicto electrostatic mesophases (hydrophilic) (lyomesophases) and byelectrokinetic amphiphilic (lipophilic) compounds interactions. occurs due The to electrostaticfirst type of (hydrophilic)such organization and electrokineticis micelles that (lipophilic) appear in interactions.solution at a certain The first concentration type of such of organizationamphiphile, which is micelles precedes that the appear formation in solution of lyomesophases. at a certain concentrationThen, with an ofincrease amphiphile, in the whichconcentration precedes of the am formationphiphile, various of lyomesophases. types of lyotropic Then, withmesophases an increase are informed the concentration [56]. of amphiphile, various types of lyotropic mesophases are formed [56].

(a) (b) (с)

Figure 9. Different types of lyotropic mesophases: (a) lamellar phases (Lam), (b) hexagonal phases Figure 9. Different types of lyotropic mesophases: (a) lamellar phases (Lam), (b) hexagonal phases (Hex), and (c) cubic phases (Cub) [56]. (Hex), and (c) cubic phases (Cub) [56]. 4.1. Ionogenic Lyotropic Mesogens 4.1. Ionogenic Lyotropic Mesogens The favorite objects of experimenters are ionogenic lyotropic mesogens, including salts of fatty acids,The anionic favorite and objects cationic of surfactants,experimenters and are alkyl ionogenic polyglucosides lyotropic [ 58mesogens,]. Even small including concentrations salts of fatty of surfactantacids, anionic additives and cationic (less than surfactants, 0.1 wt %) and lead alkyl to the polyglucosides formation of boundary[58]. Even orderedsmall concentrations layers on shear of surfaces.surfactant Therefore, additives the(less good than tribological 0.1 wt %) lead properties to the formation were attributed of boundary by the authorsordered of layers [59–63 on] toshear the formationsurfaces. Therefore, of epitropic the mesophases. good tribological properties were attributed by the authors of [59–63] to the formationThe lamellar of epitropic (layered) mesophases. phase, an analogue of thermotropic smectic phases, attracted the special attentionThe lamellar of tribologists. (layered) However, phase, an its analogue use was restrainedof thermotropic by the smectic negative phases, impact attracted of water the on special metal conjugation.attention of tribologists. Friberg [64] However, showed that its theuse lamellar was restrained phase can by bethe formed negative by impact surface-active of water substances on metal (surfactants)conjugation. alsoFriberg in binary [64] showed systems that with the glycerol lamellar as phase the polar can be solvent. formed This by surface-active gave rise to a numbersubstances of studies(surfactants) of the also lamellar in binary phases systems of binary with surfactant glycerol as systems the polar with solvent. non-aqueous This gave solvents, rise to a and number then toof theirstudies use of in the tribology. lamellar On phases the example of binary of ‘glycerol–triethanolaminesurfactant systems with non-aqueous (TEA) – oleic solvents, acid (OLA)’ and systems then to withtheir ause di ffinerent tribology. ratio ofOn OLA the example and TEA of components, ‘glycerol–triethanolamine it was shown that(TEA) if the– oleic load acid is su(OLA)’fficiently systems low andwith allows a different saving ratio the of LC OLA structure and TEA without components, deformation, it was then shown extremely that if lowthe frictionload is sufficiently coefficients low are and allows saving the 1LC structure without deformation, then extremely low2 friction coefficients are achieved (a steel ball ( 2 “diameter) on steel with a sliding rates of 1.7 10− inch/s were used). achieved (a steel ball (½“diameter) on steel with a sliding rates of 1.7× × 10−2 inch/s were used). In 1991, Pennzoil Products Co. received the first patent for the use of lyotropic phases (in a non- aqueous system) as a lubricant composition [65].

Lubricants 2019, 7, 111 16 of 25

In 1991, Pennzoil Products Co. received the first patent for the use of lyotropic phases (in a non-aqueous system) as a lubricant composition [65]. Thus, Friberg [64] made a great contribution to the practical application of lyotropic liquid crystals in tribological processes. However, the lubricating properties of lyotropic systems such as aqueous solutions of soaps and biological fluids (e.g., saliva) were known long before they were recognized as lyotropic mesogens. An interesting effect was discovered when the friction and wear properties of lamellar phases and their dispersions in water were compared [60]. The authors established that a decrease in viscosity in the latter case leads to rapid relaxation of the system for the formation of new films after the destructive action of two sliding surfaces. It was shown that mixtures of cationic and anionic amphiphiles exhibit the best lubricating properties with a slight excess of any of the surfactants. Interest in the use of aqueous lyotropic systems for certain friction pairs has persisted all these years, since water-based tribosystems are environmentally friendly. A good example of such lyotropic mesogens are alkyl polyglucosides (APGs). They are biodegradable, exhibit surface activity, and form micellar and lyotropic LC phases in aqueous systems. In [58], alkyl polyglucosides with different lengths of aliphatic substituents were used as lubricants. Tribological tests were carried out on 1% solutions. The friction couple elements were balls 6.35 mm in diameter made of 100 Cr6 and discs 25 mm in diameter and 8 mm in thickness made of steel, aluminum oxide (Al2O3), zirconium oxide (ZrO2), polyamide 6 (PA6), and poly(methyl methacrylate) (PMMA). It was found that the friction coefficient values decreased 2-, 5-, 2-, 1.5-, and 2.5-fold for the ‘steel–steel’,

‘steel–Al2O30 , ‘steel–ZrO20 , ‘steel–PA60, and ‘steel–PMMA’ couples, correspondingly, with respect to water. When a steel ball was used, no wear of discs was observed (except for PA6 and PMMA). The search for similar lubricants led the authors of [66] to use aqueous solutions of maracuja oil, ethoxylated with 60 mol of ethylene oxide (MEO60). It was found that when the concentration of this compound in water is about 60%, a fan-shaped texture is formed that is characteristic of the lyotropic hexagonal phase. Rheological studies performed in this concentration field testified that the flow of this system has the non-Newtonian nature. Tribological studies have shown that this system has good wear resistance in the four-ball tester. Summing up their results, the authors concluded that the studied LLCs could be used because they are characterized by low resistance to motion, low wear, and high load-carrying capacity. In addition, they are safe to use. Their application in the food, pharmaceutical, and cosmetic industries is proposed.

4.2. Nonionic Lyotropic Mesogens In the development of the above-mentioned works, tribological studies of nonionic surfactant additives and their mixtures with ionic surfactants were carried out [67,68]. The additives were selected in such a way as to reveal the relationship between tribological characteristics and the molecular weight of the surfactant. Neonol 9/6, Neonol 9/10, and Phenoxol were used as nonionic additives, and the disodium salt of sulfosuccinic acid (DSSA) was used as an ionic surfactant (Table5). The SMC-2 friction machine was utilized in the study. Materials of friction pairs were Steel 45, Caprolon-B, and RMF-4 [67,68]. Lubricants 2019, 7, x 17 of 26 Lubricants 2019, 7, x 17 of 26 Lubricants 2019,LubricantsLubricants7, 111 20192019,, 77,, xx 17 of 25 1717 ofof 2626 Lubricants 2019, 7, x 17 of 26 Table 5. Surfactants studied in [67]. DSSA: disodium salt of sulfosuccinic acid. Table 5. Surfactants studied in [67]. DSSA: disodium salt of sulfosuccinic acid. Table 5. Surfactants studied in [67]. DSSA: disodium salt of sulfosuccinic acid. Table 5. Surfactants studied in [67]. DSSA: disodium salt of sulfosuccinic acid.

C H Notation Surfactant StructuralC9H19 Formula C 9H 19 C99H1919 C9H19 31 3131 Surf 1 Neonol31 9/6 O(CH2CH2O)6H O(CH2CH2O)6H O(CH2CH2O)6H O(CH2CH2O)6H O(CH2CH2O)6H

32 3232 Surf 2 Neonol32 9/10

33 Surf 3 Syntanol3333 33 33

34 Surf 4 Phenoxol34 9/10 3434 34

Surf 535 DSSA 3535 35

It was shown that the maximum positive effect was observed when the ‘ionic surfactant–nonionic ItIt waswas shownshown thatthat thethe maximummaximum positivepositive effecteffect waswas observedobserved whenwhen thethe ‘ionic‘ionic surfactant–surfactant– surfactant’ mixtureItIt waswas was shownshown introduced thatthat inthethe a maximummaximum concentration positivepositive of 0.5 effecteffect wt %. waswas In observedobserved contrast to whenwhen the individual thethe ‘ionic‘ionic surfactant–surfactant– nonionicIt was surfactant’ shown that mixture the maximum was introduced positive in effecta concentration was observed of 0.5when wt %.the In‘ionic contrast surfactant– to the surfactants ofnonionic nonionic surfactant’ nature, a significantmixture was improvement introduced ofin antifrictiona concentrationconcentration properties ofof 0.50.5 of wtwt the %.%. aqueous InIn contrastcontrast toto thethe individualnonionicindividual surfactant’ surfactantssurfactants mixture ofof nonionicnonionic was nature,nature,introduced aa significsignific in aant concentration improvement of of 0.5 antifric wt %.tiontion In propertiesproperties contrast toofof thethe system wasindividual observedindividual using surfactantssurfactants the mixtures. ofof nonionicnonionic According nature,nature, to aa thesignificsignific microscopyant improvement data of surface of antifric layers,tiontion when propertiesproperties ofof thethe aqueousindividual system surfactants was observed of nonionic using nature, the mixtures. a signific Accoant rdingimprovement to the microscopy of antifric datation ofproperties surface layers, of the using surfactantaqueous additives, system there was isobserved no transfer using of the body mixtures. matter (RMF-4)Accordingrding to toto the thethe counter-body microscopymicroscopy data (steeldata ofof 45), surfacesurface layers,layers, whenaqueous using system surfactant was observed additives, using there the mixtures.is no transfer Acco ofrding body to thematter microscopy (RMF-4) data to the of surfacecounter-body layers, which confirmswhen the using presence surfactant of contact additives, with thethere boundary is no transfer lubricant. of body The matter influence (RMF-4) of mesogenic to the counter-body (steelwhen(steel 45),45),using whichwhich surfactant confirmsconfirms additives, thethe presencepresence there is ofof no contaccontac transfertt withwith of bodythethe boundaryboundary matter (RMF-4) lubricant.lubricant. to theTheThe counter-body influenceinfluence ofof additives on(steel(steel the wear 45),45), characteristicswhichwhich confirmsconfirms of polymericthethe presencepresence materials ofof contaccontac is unambiguous.tt withwith thethe boundaryboundary The greatest lubricant.lubricant. effect TheThe was influenceinfluence ofof mesogenic(steel 45), whichadditives confirms on the thewear presence characteristics of contac of polymerict with the materials boundary is lubricant.unambiguous. The Theinfluence greatest of achieved whenmesogenic the additive additives ‘Surf on 4 +theSurf wear 50 characteristicswas added to water.of polymeric Depending materials on the is unambiguous. concentration, The greatest effectmesogenic was additivesachieved onwhen the wearthe additive characteristics ‘Surf 4 of + polymericSurf 5′′ waswas materials addedadded istoto unambiguous. water.water. DependingDepending The greatest onon thethe the wear valueeffect was was decreased achieved by when 2.5–3 times.the additive ‘Surf 4 + Surf 5′′ waswas addedadded toto water.water. DependingDepending onon thethe concentration,effect was achieved the wear when value the was additive decreased ‘Surf by 4 2.5–3+ Surf times. 5′ was added to water. Depending on the Lyotropicconcentration,concentration, mesomorphism thethe waswearwear obtained valuevalue waswas not decreaseddecreased only in mixtures byby 2.5–32.5–3 of nonionictimes.times. and ionogenic surfactants, concentration,Lyotropic themesomorphism wear value was was decreased obtained by not2.5–3 only times. inin mixturesmixtures ofof nonionicnonionic andand ionogenicionogenic but in mixtures ofLyotropic nonionic surfactantsmesomorphism with ionic was liquids, obtained in particularnot only ethylammonium inin mixturesmixtures ofof nitratenonionicnonionic (EAN). andand ionogenicionogenic surfactants,Lyotropic but mesomorphismin mixtures of nonionic was obtained surfactants not withonly ionic in mixturesliquids, in of particular nonionic ethylammonium and ionogenic Araos and Warrsurfactants,surfactants, [69] found butbut inin cubic, mixturesmixtures hexagonal, ofof nonionicnonionic and surfactantssurfactants lamellar phases withwith ionicionic in these liquids,liquids, systems. inin particularparticular Such a ethylammoniumethylammonium rich nitratesurfactants, (EAN). but Araos in mixtures and Warr of nonionic [69] found surfactants cubic, he withxagonal, ionic andliquids, lamellar in particular phases inethylammonium these systems. polymorphismnitrate can also(EAN). be usedAraos in and tribological Warr [69] processes. found cubic, hexagonal, and lamellar phases in these systems. Suchnitrate a rich(EAN). polymorphism Araos and Warr can also [69] be found used cubic,in tribological hexagonal, processes. and lamellar phases in these systems. Such a rich polymorphism can also be used in tribological processes. 4.3. Discotic HeterocyclicSuch a rich polymorphism Compounds can also be used in tribological processes. 4.3. Discotic Heterocyclic Compounds A new4.3. approach Discotic to Heterocyclic the use of lubricantsCompounds with lyotropic mesogens is the application of discotic 4.3. DiscoticA new Heterocyclicapproach to Compounds the use of lubricants with lyotropic mesogens is the application of discotic heterocyclic compoundsAA newnew (or approachapproach their systems) toto thethe as useuse additives ofof lubricantslubricants to classical withwith surfactants lyotropiclyotropic in mesogensmesogens aqueous systemsisis thethe applicationapplication [7,70–72]. ofof discoticdiscotic heterocyclicA new approachcompounds to the(or usetheir of systems) lubricants as withadditives lyotropic to classical mesogens surfactants is the application in aqueous of systemsdiscotic Earlier,heterocyclicheterocyclic it was suggested compoundscompounds that the (or(or complex theirtheir systems)systems) and anisometric asas additivesadditives structure toto classicalclassical of some surfactantssurfactants phthalocyanine inin aqueousaqueous systemssystems [7,70–72].heterocyclic compounds (or their systems) as additives to classical surfactants in aqueous systems derivatives can[7,70–72].[7,70–72]. contribute to the formation of a latent mesomorphic state in solutions. Moreover, [7,70–72].Earlier, it was suggested that the complex and anisometric structure of some phthalocyanine in non-aqueous solutions,Earlier,Earlier, itit a waswas number suggestedsuggested of phthalocyanine thatthat thethe complexcomplex derivatives andand anisometricanisometric showed columnar structurestructure lyomesophases ofof somesome phthalocyaninephthalocyanine derivativesEarlier, can it was contribute suggested to the that formation the complex of a laandtent anisometric mesomorphic structure state in of solutions. some phthalocyanine Moreover, in (Figure 10). derivativesderivatives cancan contributecontribute toto thethe formationformation ofof aa lalatenttent mesomorphicmesomorphic statestate inin solutions.solutions. Moreover,Moreover, inin non-aqueousderivatives can solutions, contribute a number to the formation of phthalocyani of a latentne derivativesmesomorphic showed state incolumnar solutions. lyomesophases Moreover, in non-aqueousnon-aqueous solutions,solutions, aa numbernumber ofof phthalocyaniphthalocyaninene derivativesderivatives showedshowed columnarcolumnar lyomesophaseslyomesophases (Figurenon-aqueous 10). solutions, a number of phthalocyanine derivatives showed columnar lyomesophases (Figure(Figure 10).10). (Figure 10).

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(a(a) ) (b(b) ) (a) (b) FigureFigure 10. 10. Microphotographs Microphotographs of of mesophase mesophase textures textures of of octa(octyloxy)phthalocyanine octa(octyloxy)phthalocyanine derivative derivative in in binaryFigurebinaryFigure systems 10.systemsMicrophotographs 10. Microphotographswith with linear linear alkanes: alkanes: of mesophase of mesophase ( a()a )with with textures texturespentadecane, pentadecane, of octa(octyloxy)phthalocyanineof octa(octyloxy)phthalocyanine fan-like fan-like texture texture of of two-dimensionally two-dimensionally derivative derivative in in binary orderedsystemsorderedbinary columnar withcolumnar systems linear lyotropic alkanes: lyotropicwith linear (amesophase, )mesophase, withalkanes: pentadecane, ( aat ) at withheat heat ing,pentadecane, fan-likeing, T T = texture =112 112 fan-like°C, °C, of two-dimensionallycrossed crossed texture polarizers, polarizers, of two-dimensionally ordered ×320; ×320; ( columnarb(b) )with with lyotropicdecane,ordered Schlieren mesophase, columnar texture at lyotropic heating, of columnar Tmesophase,= 112 ◦ C,nematic crossedat heat ing,lyomesophase, polarizers, T = 112 °C,320; crossedon (b )cooling, with polarizers, decane, T = Schlieren×320;50 °C, (b )polarizers texturewith of decane, Schlieren texture of columnar nematic lyomesophase,× on cooling, T = 50 °C, polarizers columnardecane, nematic Schlieren lyomesophase, texture of columnar on cooling, nematic T = 50 lyomesophase,C, polarizers on crossed, cooling,320 T = [ 1750]. °C, polarizers crossed,crossed, ×320 ×320 [17]. [17]. ◦ × crossed, ×320 [17]. PhthalocyaninePhthalocyanine derivatives derivatives on on solid solid friction friction surfaces surfaces can can form form columnar columnar structures structures similar similar to to Phthalocyanine derivatives on solid friction surfaces can form columnar structures similar to discoticdiscotic liquid liquid liquid crystals crystals crystals (Figure (Figure (Figure 1) 1)1. )..The The The friction-sliding friction-sliding friction-sliding process process process at at a a atconstant constant a constant speed speed speed and and changing and changing changing load load loaddiscotic was studied liquid withcrystals the (Figure help of1). theThe frictionfriction-sliding machine process SMC-2. at a constant The structural speed and formulas changing of load studied waswas studiedwasstudied studied withwith with thethe the helphelp help ofof theofthe the frictionfriction friction machine machinemachine SMC-2.SMC-2.SMC-2. The The structural structuralstructural formulas formulasformulas of ofstudiedof studiedstudied heterocyclic compounds are shown in Figure 11. The texture of lyotropic mesophase formed by the heterocyclicheterocyclicheterocyclic compounds compounds compounds are are areshown shown shown in in inFigure Figure Figure 11. 11.11. Th Thee texture texturetexture of of lyotropiclyotropic lyotropic mesophase mesophase mesophase formed formed formed by theby by the the aqueousaqueousaqueous solution solution solution of of Hc3, Hc3, of Hc3, which which which is is a isa mixture a mixture mixture of ofof Hc1 Hc1Hc1 and and Hc2 Hc2 Hc2 (1:1), (1:1), isis isshown shown shown in in Figure in Figure Figure 12. 12. As1212. .Ascan As can be can be be seenseen in inseen Figure Figure in Figure 13, 1313, ,the the13, composition the composition composition Hc3 Hc3Hc3 Hc3 most mostmost most of ofof of all allallall decreases decreasesdecreases the thethe frictionfriction frictionfriction coefficient. coefficient. coecoefficient.fficient.

(a) (b) Figure 11. Formulas( aof(a) heterocyclic) compounds used as triboactive additives in [7]:( b(ab)) )phthalocyanine derivative—active turquoise dye 2 “3” T (Hc1); (b) chromonic lyomesogen—active bright yellow 5 FigureFigure 11. 11. Formulas FormulasFormulas of of of heterocyclic heterocyclic heterocyclic compounds compounds used used as as triboactive triboactive additives additives in in [7]: [[7]:7]: (a ()a )phthalocyanine) phthalocyanine phthalocyanine “3” (Hc2) [7]. derivative—activederivative—active turquoise turquoiseturquoise dye dyedye 2 2 “3” “3” TT T (Hc1);(Hc1); (Hc1); (b( b()b) chromonic )chromonic chromonic lyomesogen—active lyomesogen—active lyomesogen—active bright bright bright yellow yellow yellow 5 “3”5 5 “3”(Hc2)“3” (Hc2) (Hc2) [7]. [7]. [7].

Figure 12. Schlieren-texture of the aqueous solution of the Hc3 mixture after shear deformation in crossed polarizers [7]. Figure 12. Schlieren-texture of the aqueous solution of the Hc3 mixture after shear deformation in FigurecrossedFigure 12. 12. polarizers Schlieren-texture Schlieren-texture [7]. of of the the aqueous aqueous solution solution of of the the Hc3 Hc3 mixture mixture after after shear shear deformation deformation in in crossedcrossed polarizers polarizers [7]. [7].

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0,30

0,24

Hc2Гц 2 0,18 Коэффициент трения

Friction coefficient Friction coefficient 0,12 ГцHc1 1 ГцHc3 3 0,06 0 20406080 Load,Нагрузка, ×10 mN x 10 мН FigureFigure 13. 13.Dependence Dependence of theof frictionthe friction coeffi cientcoefficient on the on contact the contact load of aqueousload of solutionsaqueous ofsolutions heterocyclic of compounds:heterocyclic Hc1—Active compounds: turquoiseHc1—Active dye turquoise 2 “3” T; Hc2—Activedye 2 “3” T; Hc2—Active bright yellow bright 5 “3”; yellow Hc3—a 5 “3”; mixture Hc3— of Hc1a mixture and Hc2 of (1:1) Hc1[ 7and]. Hc2 (1:1) [7].

InIn general, general, based based on on the the performed performed studies,studies, it was concluded concluded that that the the introduction introduction of ofheterocyclic heterocyclic additivesadditives into into aqueous aqueous surfactant surfactant solutions solutions decreasesdecreases thethe friction coefficient coefficient [7]. [7].

4.4.4.4. Biological Biological Fluids Fluids DuringDuring the the friction friction investigation investigation ofof biologicalbiological fluids, fluids, it it was was found found that that they they are are able able to toform form lyotropiclyotropic and and epitropic epitropic mesophases mesophases [ 73[73].]. Currently,Currently, therethere is no no doubt doubt that that the the lyotropic lyotropic liquid liquid crystal crystal statestate plays plays an an important important role role in thein the functioning functioning of theof the living living systems. systems. The The attention attention of of researchers researchers in in this directionthis direction is mainly is mainly concentrated concentrated on the studyon the of study the structure of the structure and functioning and functioning mechanisms mechanisms of biological of membranesbiological andmembranes their role and in thetheir development role in the ofdevelo pathologicalpment of processes.pathological However, processes. it turnedHowever, out thatit theturned lyotropic out that liquid the crystalline lyotropic liquid state alsocrystalline plays state an important also plays role an important in physiology role in and physiology biomechanics, and providingbiomechanics, the lubrication providing function the lubrication [56,73– function75]. [56,73–75]. AA preliminary preliminary study study ofof the lubricating lubricating properties properties of ofsynovial synovial fluid, fluid, mucous mucous membranes membranes of fish of fishscales, scales, and and blood blood showed showed that that these these biological biological flui fluidsds acquire acquire the the ability ability to toorientate orientate structural structural elementselements under under the the influence influence of the of activethe active solid solid surface. surface. This orientationThis orientation leads toleads a significant to a significant decrease decrease in the friction coefficient and wear [74]. The preferred orientation direction of molecules and in the friction coefficient and wear [74]. The preferred orientation direction of molecules and its its homogeneity in the boundary layer affects macroscopic phenomena as friction, boundary homogeneity in the boundary layer affects macroscopic phenomena as friction, boundary lubricating lubricating effect, wedging pressure value, etc. The orientational ordering of polar molecules in the effect, wedging pressure value, etc. The orientational ordering of polar molecules in the boundary boundary layer caused by the “image forces” action leads to the fact that these layers transform into layer caused by the “image forces” action leads to the fact that these layers transform into a metastable a metastable liquid crystalline state. liquid crystallineKupchinov state. et al. [75] studied human and animal synovial fluid, as well as its lipid extracts and driedKupchinov preparations. et al. [The75] studiedpresence human of mesogenic and animal cholesterol synovial derivatives fluid, as wellwas detected, as its lipid and extracts the dry and driedpreparations preparations. showed The birefringence presence of during mesogenic thermopo cholesterollarization derivatives studies. These was and detected, other studies and thehave dry preparationsallowed the showed authors birefringence to conclude that during the synovial thermopolarization fluid is structurally studies. ordered These andat a othertemperature studies not have allowedexceeding the authors45 °C. Using to conclude the Pendulum that the tribometer, synovial it fluid was isshown structurally that the orderedsynovial atfluid a temperature with a higher not exceedingconcentration 45 ◦C. of Using cholesterol the Pendulum esters has tribometer,better antifriction it was properties. shown that the synovial fluid with a higher concentrationThese works of cholesterol were further esters developed has better in antifriction[25,76,77] bo properties.th in the direction of the concept of synovial fluidThese influence works and were the further creation developed of artificial in compositions–synovial [25,76,77] both in the direction fluid substitutes. of the concept of synovial fluid influenceAlong with and this, the creationattempts ofhave artificial been made compositions–synovial to a theoretical description fluid substitutes. of joint lubrication. The ideaAlong of Kupchinov with this, about attempts the liquid have been crystal made state to of a synovial theoretical fluid description was supported. of joint However, lubrication. the Themodel idea of Kupchinovof liquid crystalline about the structures liquid crystalhas also state been of advanced synovial with fluid regard was supported. to articular However,cartilage and the other model of liquidconnective crystalline tissues [78,79]. structures has also been advanced with regard to articular cartilage and other connective tissues [78,79].

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The replacement of large joints with artificial prostheses is a great achievement of orthopedics, but itThe raised replacement the issue of of large the joints friction with and artificial wear prostheses processes isin a greatthese achievementsystems. Under of orthopedics, experimental but conditionsit raised the (in issue vitro), of the mostly friction excellent and wear tribological processes data in these were systems. obtained, Under but they experimental were not confirmed conditions by(in practice vitro), mostly (in vivo) excellent [80]. tribological data were obtained, but they were not confirmed by practice (in vivo)Recently, [80]. attempts have been made to create liquid crystalline compositions as lubricants for artificialRecently, joints. attempts Thus, have Ruiz-Fernández been made to create et liquidal. crystalline[81] proposed compositions a composition as lubricants forcontaining artificial tetradecyltrimethylammoniumjoints. Thus, Ruiz-Fernández et al. bromide [81] proposed (TTAB), a composition decanol (DeOH), containing sodium tetradecyltrimethylammonium bromide (NaBr), and a naturalbromide mixture (TTAB), of decanol lipids (DeOH),in water. sodiumThis system bromide had (NaBr), a viscosity and arange natural two mixture times wider of lipids than in water.other similarThis system systems. had A a viscositywide viscosity range range two times does widernot affect than the other presence similar of systems. a nematic A discotic wide viscosity phase, whichrange doespromotes not affect lubrication the presence processes. of a nematicIn the experiments, discotic phase, ceramic-on-ceramic which promotes lubrication (CoC) and processes.ceramic- on-polyethyleneIn the experiments, (COP) ceramic-on-ceramic implants were used. (CoC) Since and ceramic-on-polyethylene four biological components (COP) are implants most represented were used. inSince the foursynovial biological fluid—hyaluronic components acid are most(HA), represented albumin mucinous in the synovial glycoproteins fluid—hyaluronic (mainly lubricin), acid (HA), and globulin—therefore,albumin mucinous glycoproteins the variation (mainly of these lubricin), comp andonents globulin—therefore, was the subject the of variationthe tribological of these experiments.components was It was the found subject in of experiments the tribological that experiments. the lubricant It wasfluid found composition in experiments LO2 (manufactured that the lubricant by BSF)fluid shows composition the highest LO2 (manufacturedviscosity and the by least BSF) wear shows on thethe highestsurfaces viscosity of friction and pairs. the This least composition wear on the containssurfaces ofvarious friction types pairs. of Thisproteins composition and lipids, contains among various which types HA plays of proteins a major and role lipids, in the among viscosity which of fluids.HA plays a major role in the viscosity of fluids. Another important biological biological fluid fluid is is saliva, saliva, which which also also performs performs the the function function of of lubricant. lubricant. Mucin glycoproteins glycoproteins in in saliva saliva make make up up the the bulk bulk of ofits its dry dry residue. residue. Due Due to the to thenegative negative charge, charge, the moleculesthe molecules of the of theglycoproteins glycoproteins have have a arigid rigid un unfoldedfolded comb-like comb-like structure, structure, which which significantly significantly increasesincreases the viscosity viscosity of saliva. saliva. It It has has a a high high film-f film-formingorming ability, ability, and and the the ions ions contained contained in in the the saliva, saliva, inin particular the thiocyanate ion, contribu contributete to the crosslinking of biopolymers.biopolymers. For aa numbernumber of of polypeptides polypeptides and and enzymes enzymes (which (whi arech proteins are proteins by their nature),by their it nature), was established it was establishedthat they are that liquid they crystalline are liquid [82crystalline]. The liquid [82]. crystallineThe liquid statecrystalline leads tostate a significant leads to a orderingsignificant of orderingstructural of elements structural of elements a multicomponent of a multicomponent system. Since system. saliva Since moistens saliva food, moistens it is naturalfood, it tois assumenatural tothat assume the movement that the movement of food in of the food mouth in the and mout esophagush and esophagus is accompanied is accompanied by friction, by andfriction, a mixture and a mixtureof saliva of and saliva the and mucous the mucous membrane membrane secretion secretio of then esophagusof the esophagus surface surface acts as acts a lubricant.as a lubricant. The Themicroscopic microscopic observations observations showed showed [73,83 ,[73,83,84]84] that in that the processin the process of saliva of drying, saliva thedrying, tree-like the structuretree-like structureis formed is (Figure formed 14 (Figure). It is made14). It ofis made aggregates of aggregates that are that close are in close shape in to shape a square, to a square, with a sidewith lengtha side lengthof 16.2 of 2.716.2 ±10 2.73 cm× 10.− As3 cm. a rule,As a therule, aggregates the aggregates merge merge when when they approachthey approach each othereach other and form and ± × − formcontinuous continuous filaments filaments with dendriticwith dendritic growth. growth.

Figure 14. Saliva preparation after drying. Polarizers are parallel. ×200.200. × InIn crossed polarizers, on on the dark field field of drieddried saliva, luminous do dotsts and rod-shaped small formationsformations are visible. They They chan changege their their light light transmission transmission intensity intensity and and color color when when the the microscope microscope stage is rotated.

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NativeNative saliva saliva does does not not exhibitexhibit birefringence.birefringence. However, However, after after its itsfriction friction between between two twoglasses, glasses, a a didifferentfferent polarization polarization microscopic microscopi picturec picture is observed. is observed. In this In case, this characteristic case, characteristic elongated elongated birefringent structuresbirefringent are formed,structures indicating are formed, an indicating ordered liquid an ordered crystalline liquid state crystalline (Figure state 15). (Figure 15).

Figure 15. Saliva preparation after friction with twisting. Polarizers are crossed. 200. Figure 15. Saliva preparation after friction with twisting. Polarizers are crossed. ×200.× These structures are preserved for 18–36 h, and then, in the process of drying, the birefringent lines These structures are preserved for 18–36 h, and then, in the process of drying, the birefringent begin to crush into separate point areas, which are reminiscent of the shape of the tree-like structure lines begin to crush into separate point areas, which are reminiscent of the shape of the tree-like of drystructure saliva. of Obviously,dry saliva. Obviously, in this case in ofthis saliva, case of there saliva, is there an interesting is an interesting example example of the of lyotropic the lyotropic liquid crystallineliquid crystalline state formation state formation in the process in the of friction.process of The friction. friction The determines friction determines the uniform the orientation uniform of salivaorientation biopolymers. of saliva biopolymers. SuchSuch a labilea labile mechanism mechanism of of thethe lubricantlubricant transitiontransition from from an an unoriented unoriented state state to to a auniformly uniformly orientedoriented state state can can be be used used in in the the development development ofof technicaltechnical lubricants. These These lubricants lubricants can can be be applied applied to reduceto reduce the frictionthe friction of interacting of interacting elastomers, elastome reducers, thereduce hydrodynamic the hydrodynamic resistance resistance when transporting when substancestransporting through substances pipes madethrough of polymericpipes made materials, of polymeric and alsomaterials, to study and the also eff toect study of salivary the effect fluid ofon thesalivary friction fluid and on wear the of friction tooth and enamel, wear the of tooth surfaces enam ofel, dentures, the surfaces and of joint dentures, prostheses. and joint prostheses. Thus,Thus, biological biological lubricants lubricants containing containing glycoproteinsglycoproteins and and proteins, proteins, even even with with a asmall small amount amount of of drydry residue, residue, provide provide the the formation formation ofof epitropicepitropic lyomesophases,lyomesophases, which which give give a ahigh high efficiency efficiency to tothe the lubricationlubrication process. process. Glycoproteins, Glycoproteins, which which are are the the main main dry dry residue residue of saliva, of saliva, make make up 1–2% up 1–2% by weight. by Theweight. metastable The metastable liquid crystalline liquid crystalline state that wasstate foundthat was in saliva,found in combinationsaliva, in combination with the previouslywith the obtainedpreviously information obtained information [74] on the induced[74] on the liquid induced crystalline liquid crystalline state of the state synovial of the synovial fluid and fluid mucous and membranemucous membrane of fish scales, of fish gave scales, one gave reason one to reason conclude to conclude that the that mechanism the mechanism of sliding of sliding friction friction of the of the biological media containing polymers and large percentage of water is of the same type. This biological media containing polymers and large percentage of water is of the same type. This conclusion conclusion was confirmed in the works of Vazina et al. [85], who established that the secretion of the was confirmed in the works of Vazina et al. [85], who established that the secretion of the glands of the glands of the gastrointestinal tract also has liquid crystalline ordering. The above-mentioned data gastrointestinal tract also has liquid crystalline ordering. The above-mentioned data allowed us to allowed us to obtain a new lubricant-cooling technological agent [86]. obtain a new lubricant-cooling technological agent [86]. 5. Conclusions 5. Conclusions This review of the use of thermotropic and lyotropic mesogens as smart materials in tribological This review of the use of thermotropic and lyotropic mesogens as smart materials in tribological processes shows the great promise of these materials. The outlined data do not exhaust the entire processespalette of shows modern the research great promise directions of these in the materials. field of application The outlined of liquid data docrystals not exhaustin tribology. the entire At palettepresent, of modern the desire/tendency research directions to miniaturize in the field friction of application units and the of liquid task of crystals developing in tribology. environmentally At present, thefriendly desire/ tendencylubricants—as to miniaturize well as highly friction effective units andantifriction, the task antiwear, of developing antiseize environmentally additives to them— friendly lubricants—asstimulate the well development as highly of eff theoretiective antifriction,cal studies. antiwear,In particular, antiseize this review additives includes to them—stimulate investigations in the developmentthe field of boundary of theoretical lubrication, studies. studying In particular, the relationship this review between includes the investigationsstructure of molecules, in the field the of boundaryrheological lubrication, and tribological studying properties the relationship of me betweensogens, and the structurethe development of molecules, of materials the rheological with and tribological properties of mesogens, and the development of materials with programmable

Lubricants 2019, 7, 111 22 of 25 properties. Experiments are being performed on LC compositions with non-mesogens, nanoparticles, carbon nanostructures, and ionic liquids. The supramolecular structuring underlying the formation of mesophases, including epitropic ones, becomes a key mechanism for creating effective lubricant compositions in the 21st century. The establishment of the important role of the liquid crystalline state in tribological processes has led to the use of new compositions of lubricating and cutting fluids in both metal processing and living systems. Further research on biolubricants exhibiting extremely low values of friction and wear can lead to the creation of fundamentally new lubricants for industrial use.

Funding: This research was funded by the RFBF, grant number 18-29-19150_mk and by the Ministry of Education and Science of the Russian Federation in the framework of the state task for Ivanovo State University, grant number 16.1037.2017/4.6. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Usol’tseva, N.V.; Akopova, O.B. Tribology and Mesomorphism. In Physics, Chemistry and Mechanics of Tribosystems, Interuniversity Collection of Scientific Papers; Latyshev, V.N., Ed.; Ivanovo State University: Ivanovo, Russia, 2011; pp. 14–23. (In Russian) 2. Myshkin, N.K.; Petrokovec, M.I. Friction, Lubrication, Wear. Physical Fundamentals and Technical Applications of Tribology; FIZMATLIT: Moscow, Russia, 2007; p. 368. ISBN 978-5-9221-0824-9. (In Russian) 3. Stribeck, R. Die wesentlichen Eigenschaften der Gleit-und Rollenlager. Z. Ver. Dtsch. Ing. 2002, 36, 1341–1348. 4. Minami, J. Ionic Liquids in Tribology. Molecules 2009, 14, 2286–2305. [CrossRef][PubMed] 5. Carrion, F.-J.; Martinez-Nicolas, G.; Iglesias, P.; Sanes, J.; Bermudez, M.-D. Liquid Crystals in Tribology. Int. J. Mol. Sci. 2009, 10, 4102–4115. [CrossRef][PubMed] 6. Shen, M.; Luo, J.; Wen, S.; Yao, J. Nanotribological properties and mechanisms of the liquid crystal as an additive. Chin. Sci. Bull. 2001, 46, 1227–1232. [CrossRef] 7. Berezina, E.V. Phthalocyanine Derivatives as Additives to Lubricating Compositions; Ivanovo State University: Ivanovo, Russia, 2007; p. 240. ISBN 5-7807-0614-X. (In Russian) 8. Gao, Y.; Jiang, Y.; Hu, W.; Jiang, H.; Li, J. Cholesteric liquid crystals as oil based lubricant additives: Effect of mesogenic phases and structures on tribological characteristics. Langmuir 2019, 35, 6981–6992. [CrossRef] 9. Deryagin, B.V. Research in Surface Forces: Surface Forces in Thin Films and Disperse Systems; Springer: Moscow, Russia, 1972; p. 326. (In Russian) 10. Sonin, A.S.; Frenkel, V.Y. Vsevolod Konstantinovich Frederiksz: 1885–1944; FIZMATLIT: Moscow, Russia, 1995; p. 176. ISBN 5-02-015179-3. (In Russian) 11. Comstock, M.J. Tribology and the Liquid Crystalline State; ACS Symposium Series; Biresaw, G., Ed.; American Chemical Society: Washington, DC, USA, 1990; Volume 441, p. 133. 12. Latyshev, V.N.; Karabanov, V.B.; Chaikovsky, V.M.; Chistyakov, I. Cutting Fluid for Machining of Metals. SU No. 601304, 27 April 1978. (In Russian). 13. Latyshev, V.N.; Korotkov, V.B.; Godlevskiy, V.A.; Volkov, V.F.; Usol’tseva, N.V. Antifriction Additive for Oils. SU No. 1086009, 15 April 1984. (In Russian). 14. Latyshev, V.N.; Korotkov, V.B.; Godlevskiy, V.A.; Alexandrov, A.I.; Usol’tseva, N.V.; Volkov, V.F. Cutting Fluid for Machining of Metals. SU No. 1149622, 22 December 1983. (In Russian). 15. Latyshev, V.N.; Lazuk Yu, N.; Usol’tseva, N.V. Liquid Crystals and Their Practical Application. In Proceedings of the Republican Conference, Baku, Azerbaijan, 20 January 1990; pp. 37–38. (In Russian) 16. Goodby, J.W.; Gray, G.W. Guide to the Nomenclature and Classification of Liquid Crystals. In Handbook of Liquid Crystals, 1st ed.; Demus, D., Goodby, G., Gray, G.W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, Germany; New York, NY, USA; Chichester, UK; Singapore, 1998; Volume 1, pp. 17–23. 17. Usol’tseva, N.V.; Akopova, O.B.; Bykova, V.V.; Smirnova, A.I.; Pikin, S.A. Liquid Crystals: Discotic Mesogens, 1st ed.; Usol’tseva, N.V., Ed.; Ivanovo State University: Ivanovo, Russia, 2004; p. 546. ISBN 5-7807-0458-9. (In Russian) 18. Usol’tseva, N.V. Lyotropic Liquid Crystals: Chemical and Supramolecular Structure; Ivanovo State University: Ivanovo, Russia, 1994; p. 220. ISBN 5-230-02212-4. (In Russian) Lubricants 2019, 7, 111 23 of 25

19. Volkmar, V.; Galewski, Z. LiqCryst 5.0. Database of Liquid Crystals; LCI Publisher: Hamburg, Germany, 2010. 20. Cognard, J. Lubrication with Liquid Crystals. In Tribology and the Liquid Crystalline State; ACS Symposium Series; Biresow, G., Ed.; American Chemical Society: Washington, DC, USA, 1990; Volume 441, pp. 1–47. 21. Mori, S.; Iwata, H. Relationship between tribological performance of liquid crystals and their molecular structure. Tribol. Intern. 1996, 29, 35–39. [CrossRef] 22. Nakano, K. Tribology of Liquid Crystals. Chem. Ind. 2004, 55, 460–465. 23. Amann, T.; Kailer, A. Relationship between ultralow friction of mesogenic-like fluids and their lateral chain length. Tribol. Lett. 2011, 41, 121–129. [CrossRef] 24. Wažy´nska,D.; Okowiak, J.A. Tribological properties of nematic and smectic liquid crystalline mixtures used as lubricants. Tribol. Lett. 2006, 24, 1–5. [CrossRef] 25. Ermakov, S.F.; Rodnenkov, V.G.; Beloenko, E.D.; Kupchinov, B.I. Liquid Crystals in Engineering and Medicine; Asap: Minsk, Belarus; ChePo: Moscow, Russia, 2002; p. 411. ISBN 985-6572-66-5, 5-88711-175-5. (In Russian) 26. Demus, D.; Demus, H.; Zaschke, H. Flussige Kristalle in Tabellen; VEB Deut.: Leipzig, Germany, 1974. 27. Syrbu, S.A.; Novikov, V.V.; Burchenkov, K.S.; Fedorov, M.S. New Functional Nanostructured Materials for Tribosystems Based on Nematogen Systems. In Organic and Hybrid Nanomaterials: Production, Research, Application: Monograph; Razumov, V.F., Kluev, M.V., Eds.; Ivanovo State University: Ivanovo, Russia, 2019; pp. 307–337. (In Russian) 28. Buyanovsky, A.I.; Zacharov, S.M. Lubrication. In Friction, Wear and Lubrication (Tribology and Tribotechnology); Chichivadze, A.V., Ed.; Mashinostroenie: Moscow, Russia, 2003; pp. 184–248. (In Russian) 29. Kupchinov, B.I.; Ermakov, S.F.; Parkalov, V.P.; Rodnenkov, V.G. Study of the influence of mesogenic lubricant additives on the kinetics of the friction coefficient. Dokl. Natl. Acad. Sci. Belarus 1989, 33, 340–343. (In Russian) 30. Bermúdez, M.D.; Martinez-Nicolás, G.; Carrion-Vilches, F.J. Tribological properties of liquid crystals as lubricant additives. Wear 1997, 212, 188–194. [CrossRef] 31. Vektris, V.; Mokshin, V. Tribological research of industrial oil with liquid-crystal additives. Mater. Sci. 2008, 44, 730–734. [CrossRef] 32. Latyshev, V.N.; Syrbu, S.A.; Novikov, V.V.; Kolbashov, M.A. Reological Properties of Lubricating Oils with the Additives of Cholesteric Liquid Crystals. Liq. Cryst. Appl. 2008, 3, 52–59. 33. Berezina, E.V.; Korsakov, M.N.; Pavlov, A.S.; Bykova, V.V.; Usol’tseva, N.V. Curve currents of viscous lubricants with liquid crystal additives. Liq. Cryst. Appl. 2010, 2, 85–96. 34. Berezina, E.V.; Korsakov, M.N.; Pavlov, A.S.; Usol’tseva, N.V. Viscometry of plastic lubricants with mesogenic additives. Fiz. Him. Mekhanika Tribosistem 2009, 8, 171–173. (In Russian) 35. Rebinder, P.A. Surface Phenomena in Disperse Systems. Physicochemical Mechanics; Nauka: Moscow, Russia, 1979; p. 428. (In Russian) 36. Ermakov, S.F.; Myshkin, N.K. Liquid-Crystal Nanomaterials. Tribology and Applications; Springer Intern Publishing AG: Basel, Switzerland, 2018; Volume 267. 37. Popova, M.N.; Zharova, M.A.; Usol’tseva, N.V.; Smirnova, A.I.; Bogdanov, V.S. Rheological and tribological properties of industrial oil with mesogenic additives and carbon nanotubes. Liq. Cryst. Appl. 2014, 14, 52–61. 38. Usol’tseva, N.V.; Smirnova, M.V.; Kazak, A.V.; Smirnova, A.I.; Bumbina, N.V.; Il’in, S.O.; Rozhkova, N.N. Reological Characteristics of Different Carbon Nanoparticles in Cholesteric Mesogen Dispersions as Lubricant Coolant Additives. J. Frict. Wear 2015, 36, 380–385. [CrossRef] 39. Usol’tseva, N.V.; Yakemseva, M.V. Mesogens and Polymers in Systems with Carbon Nanoparticles. In Organic and Hybrid Nanomaterials: Trends and Prospects: Monograph; Razumov, V.F., Kluev, M.V., Eds.; Ivanovo State University: Ivanovo, Russia, 2013; pp. 228–280. (In Russian) 40. De Jeu, W.H. Physical Properties of Liquid Crystalline Materials; Gordon and Breach Science Publishers: New York, NY, USA; London, UK; Paris, France, 1980; 133p. 41. Yakemseva, M.V.; Usol’tseva, N.V. Viscoelastic properties of cholesteric liquid crystal—Multiwall carbon nanotubes composite. Liq. Cryst. Appl. 2013, 2, 90–94. 42. Usol’tseva, N.V.; Smirnova, M.V.; Sotsky, V.V.; Smirnova, A.I. Physical properties of cholesteric liquid crystals—Carbon nanotube dispersions. J. Phys. Conf. Ser. 2014, 558, 012003. [CrossRef] 43. Ali, I.; Bashneer, A.A.; Kucherova, A.; Memetov, N.; Pasko, T.; Ovchinnikov, K.; Pershin, V.; Kuznetsov, D.; Galunin, E.; Grachev, V.; et al. Advances in carbon nanomaterials as lubricants modifiers. J. Mol. Liq. 2019, 279, 251–266. [CrossRef] Lubricants 2019, 7, 111 24 of 25

44. Usol’tseva, N.V.; Akopova, O.B. Molecular Structure of Organic Compounds and Their Thermotropic Mesomorphism. In Liquid Crystals: Discotic Mesogens, 1st ed.; Usol’tseva, N.V., Ed.; Ivanovo State University: Ivanovo, Russia, 2004; pp. 10–96. ISBN 5-7807-0458-9. (In Russian) 45. Eidenschink, R. Mechanical Component. Patent No 3821855.0 FRG, 29 June 1988. 46. Akopova, O.B.; Bobrov, V.I.; Tuneva, G.A. Anti-friction and Antiwear Lubricant Additive. SU No. 1664818, 22 March 1991. 47. Usol’tseva, N.V.; Akopova, O.B.; Bykova, V.V. Physico-Chemical Properties of Discotic Mesogens that Determine Their Use. In Liquid Crystals: Discotic Mesogens, 1st ed.; Usol’tseva, N.V., Ed.; Ivanovo State University: Ivanovo, Russia, 2004; pp. 490–538. ISBN 5-7807-0458-9. (In Russian) 48. Akopova, O.B.; Zemtsova, O.V.; Kotovich, L.N.; Lapshin, V.B. Tribological Characteristics of Triphenylene Derivatives with Columnar and ND Mesophases. In Vestnik IvGU; Ivanovo State University: Ivanovo, Russia, 2001; pp. 71–75. (In Russian) 49. Akopova, O.B.; Gribova, L.K.; Zemtsova, O.V.; Savchenko, V.E.; Usol’tseva, N.V. Application of quartz resonators for research of apolar and polar hexaalkoxytriphenylene derivatives phase transitions. In Proceedings of the XIV Conference on Liquid Crystals, Zakopane, Poland, 1 June 2001; p. 23. 50. Zemtsova, O.V.; Akopova, O.B.; Usol’tseva, N.V.; Erdelen, C.H. Columnar mesomorphism of the mixed substituted apolar triphenylenes. Prognosis and experimental data. Mol. Cryst. Liq. Cryst. 2001, 364, 625–634. [CrossRef] 51. Akopova, O.B.; Logacheva, N.M.; Baulin, V.E.; Tsivadze, A.Y.; Terentiev, V.V.; Subbotin, K.V. Investigation of tribological properties of discotic mesogens—Derivatives of phthalocyanine containing crown ether fragments. In Proceedings of the VII International Scientific Conference “Lyotropic Liquid Crystals and Nanomaterials” Together with the Symposium “Success in the Study of Thermotropic Liquid Crystals” (the Vth Chistyakov’s Readings), Ivanovo, Russia, 16–18 December 2009; p. 93. (In Russian). 52. Latyshev, V.N.; Terentiev, V.V.; Akopova, O.B.; Subbotin, K.V. The use of copper carboxylates as additives for solidol. In Proceedings of the International Scientific Conference “Actual Problems and Prospects for the Development of Agriculture”, Ivanovo, Russia, 26 August 2009; Volume 2, pp. 95–96. (In Russian). 53. Berezina, E.V.; Godlevskiy, V.A. Mesogenic Compounds as Triboactive Additives. In Achivements of Liquid Crystals Research, 1st ed.; Usol’tseva, N.V., Ed.; Ivanovo State University: Ivanovo, Russia, 2007; pp. 80–94. (In Russian) 54. Akopova, O.B.; Usol’tseva, N.V. Discotic Mesogens: From Monomers to Polymers and Dendrimers; Ivanovo State University: Ivanovo, Russia, 2010; p. 112. (In Russian) 55. Usol’tseva, N.V.; Bykova, V.V.; Akopova, O.B. Achivements of Liquid Crystals Research, 1st ed.; Usol’tseva, N.V., Ed.; Ivanovo State University: Ivanovo, Russia, 2007; 100p, ISBN 5-7807-0655-7. (In Russian) 56. Usol’tseva, N.V. Chemical characterization, biological and medical significance of lyotropic liquid crystals. Zhurnal Vsesoyuznogo Him. Obs. Mendeleeva 1983, 28, 36–45. (In Russian) 57. Ekwall, P. Composition, Properties of Liquid Crystalline Phases in Systems of Amphiphilic Compounds. In Advances in Liquid Crystals; Brown, G.H., Ed.; Academic Press: New York, NY, USA; San Francisco, CA, USA; London, UK, 1975; Volume 1, pp. 1–142. 58. Sułek, M.W.; Ogorzałek, M.; Wasilewski, T.; Klimaszewska, E. Alkyl Polyglucosides as components of water based lubricants. J. Surfactants Deterg. 2013, 16, 369–375. [CrossRef][PubMed] 59. Ma, C.S.; Li, G.Z.; Shen, Q.A. Study of lyotropic liquid crystal in lubrication on aluminium alloy surfaces. J. Dispers. Sci. Technol. 1999, 20, 1025–1030. [CrossRef] 60. Boschkova, K.; Elvesjo, J.; Kronberg, B. Frictional properties of lyotropic liquid crystalline mesophases at surfaces. Colloid Surf. A Physicochem. Eng. Asp. 2000, 166, 67–77. [CrossRef] 61. Latyshev, V.N.; Lazuk, Y.N.; Usol’tseva, N.V. Compositions based on lyotropic mesogens and their practical application in tribology. In Proceedings of the Republic Conference “Liquid Crystals and Their Application”; Union of Soviet Socialist Republics: Baku, USSR, 1990; pp. 37–38. (In Russian) 62. Bobrov, V.I.; Krasikov, N.N. Birefringence resulting from friction in thin surfactant layers in a solid state. Trenie Isnos 1981, 2, 545–548. (In Russian) 63. Levchenko, V.A. Epitropic liquid crystals—A new liquid phase. J. Mol. Liq. 2000, 85, 197–210. [CrossRef] 64. Frieberg, S.E.; Ward, A.J.; Gunsel, S.; Lockwood, F.E. Lyotropic Liquid Crystals in Lubrication. In Tribology and the Liquid-Crystalline State; ACS Symposium Series; Bireslaw, G., Ed.; American Chemical Society: Washington, DC, USA, 1990; Volume 441, pp. 101–110. Lubricants 2019, 7, 111 25 of 25

65. Pennzoil Products Company. Pennzoil Products Company. Non-Aqueous Lamellar Liquid Crystalline Lubricants. U.S. Patent No. 4,999,122, 30 December 1988. 66. Sułek, M.W.; B ˛ak-Sowinska,A. New Ecological Lubricants on the Basis of Lyotropic Liquid Crystals Formed by Solutions of Maracuja Oil Ethoxylate. Ind. Eng. Chem. Res. 2013, 52, 16169–16174. [CrossRef] 67. Shilov, M.A. Investigation of self-organization mechanism of non-ionic surface-active substances and their combinations with ionic ones in water-lubricating friction junctions. Liq. Cryst. Appl. 2011, 1, 57–64. 68. Berezina, E.V.; Shilov, M.A. Structured gels as lubricant and coolant mixtures in drilling. Russ. Eng. Res. 2011, 31, 91–93. [CrossRef] 69. Araos, M.U.; Warr, G.G. Self-Assembly of Nonionic Surfactants into Lyotropic Liquid Crystals in Ethylammonium Nitrate, a Room-Temperature Ionic Liquid. J. Phys. Chem. B 2005, 109, 14275–14277. [CrossRef][PubMed] 70. Berezina, E.V. Improving the Workability of Steels and Alloys Using Synthetic Aqueous Lubricant-Cooling Agent with New Triboactive Additives. Ph.D. Thesis, Ivanovo State University, Ivanovo, Russia, 1992. 71. Berezina, E.V.; Godlevskiy, V.A. On the use of aqueous solutions of phthalocyanine as triboactive additives to technological media for metal cutting. Bull. Acad. Sci. 1991, 55, 1757–1759. (In Russian) 72. Berezina, E.V.; Godlevskiy, V.A.; Usol’tseva, N.V.; Chrunov, A.A. The study of the viscosity concentration dependences of aqueous solutions of dyes. Liq. Cryst. Appl. 2004, 3, 63–71. 73. Usol’tseva, N.V.; Latyshev, V.N.; Bobrov, V.I.; Pischasova, G.K. On the Liquid Crystalline State of Saliva as a Lubricant Fluid. In Frictional Interaction of Solids Taking into Account the Medium; Ivanovo State University: Ivanovo, Russia, 1982; pp. 148–151. (In Russian) 74. Barchan, G.P. Liquid crystalline state of interfacial layers at sliding. Doklady Natl. Acad. Sci. Belarus 1981, 258, 86–88. (In Russian) 75. Kupchinov, B.I.; Ermakov, S.F.; Rodnenkov, V.G.; Bobrysheva, S.N.; Beloenko, E.D.; Kestelman, V.N. Role of liquid crystals in the lubrication of living joints. Smart Mater. Struct. 1993, 2, 7–12. [CrossRef] 76. Kupchinov, B.I. Tribological aspects of joint functioning. Trenie Isnos 1989, 10, 1013–1018. (In Russian) 77. Kupchinov, B.I.; Ermakov, S.F.; Beloenko, E.D. Biotribology in Synovial Joints; NANB–MZRB: Minsk, Belarus, 1997. 78. Szwajczak, E.; Kucaba-Pietal, A. Liquid crystalline properties of synovial fluid. Eng. Trans. 2001, 49, 315–358. 79. Dowson, D. Biotribology of Natural and Replacement Synovial Joints. In Biomechanics of Diarthrodial Joints; Mow, V.C., Woo, S.L.-Y., Ratcliffe, A., Eds.; Spribger: Berlin, Germany, 1990; Volume 2, pp. 305–345. 80. Ghosh, S.; Choudhury, D.; Roy, T.; Moradi, A.; Masjuki, H.H.; Pingguan-Murphy, B. Tribological performance of the biological components of synovial fluid in artificial joint implants. Sci. Technol. Adv. Mater. 2015, 16, 045002. [CrossRef] 81. Ruiz-Fernández, A.R.; Tejos, R.; Ahumada-Gutierrez, H.; Muñoz-Gacitúa, D.; Martínez-Cifuentes, M.; Araya-Maturana, R.; Weiss-López, B.E. Characterisation of a new nematic lyotropic liquid crystal with natural lipids from soybean. Mol. Phys. 2018, 117, 158–166. [CrossRef] 82. Papkov, S.P.; Kulichikhin, V.G. The Liquid Crystalline State of Polymers; Chimia: Moscow, Russia, 1977; p. 240. (In Russian) 83. Usol’tseva, N.V.; Bobrov,V.I.; Shabishev, L.S.; Pischasova, G.K. Polarization-Microscopic Analysis of Biological Fluid Containing Friction-Oriented Glycoproteins. In Abstract Book of the 1st All-Union Symposium on the Liquid Crystalline State of Polymers; USSR Academy of Sciences, Scientific Council on Macromolecular Compounds: Suzdal, Russia, 1982; pp. 96–97. (In Russian) 84. Usol’tseva, N.V. Molecular and Supramolecular Structure of Lyomesogens and Properties of Lyomesophases. Ph.D. Thesis, Leningrad State Univerisity, Leningrad, Russia, 1990. (In Russian). 85. Vazina, A.A.; Eliakova, L.A.; Skopinov, S.A.; Chernoborodova, S.V.; Yakovleva, S.V. Lyotropic Mesomorphism of Glucanes. In Abstract Book of the VIth All-Union Conference on Liquid Crystals and Their Application; Chernihiv National Pedagogical University named after T.G. Shevchenko: Chernigov, Russia, 1988; Volume 3, p. 427. (In Russian) 86. Lazuk, Y.N.; Latyshev, V.N.; Usol’tseva, N.V.; Kolesnikov, N.D. Lubricant-Cooling Agent. SU No 1692814A1, 11 October 1989. (In Russian).

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