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APTEFF, 48, 1-323 (2017) UDC: 678.1:677.46:546.226 https://doi.org/10.2298/APT1748221M BIBLID: 1450-7188 (2017) 48, 221-229 Original scientific paper

INTERFACIAL PROPERTIES OF CHITOSAN/ DODECYL COMPLEXES

Jelena R. Milinković*, Milijana V. Aleksić, Lidija B. Petrović, Jadranka L. Fraj, Sandra Đ. Bučko, Jaroslav M. Katona, Ljiljana M. Spasojević

University of Novi Sad, Faculty of Technology Novi Sad, Bulevar Cara Lazara 1, 21000 Novi Sad, Serbia

Contemporary formulations of cosmetic and pharmaceutical emulsions may be achieved by using combined polymer/ system, which can form complexes with different structure and physicochemical properties. Such complexation can lead to additional stabilization of the emulsion products. For these reasons, the main goal of this study was to investigate the interfacial properties of chitosan/sodium dodecyl sulfate complexes. In order to understand the stabilization mechanism, the interface of the /water systems that contained mixtures of chitosan and sodium dodecyl sulfate, was studied by measuring the interfacial tension. Considering the fact that the properties of the oil phase has influence on the adsorption process, three different types of oil were investigated: medium-chain triglycerides (semi-synthetic oil), paraffin oil () and natural oil obtained from the grape seed. The surface tension measurements at the oil/water interface, for chitosan water solutions, indicate a poor surface activity of this biopolymer. Addition of sodium dodecyl sulfate to chitosan solution causes a significant decrease in the interfacial tension for all investigated . The results of this study are important for understanding the influence of polymer-surfactant interactions on the properties of the solution and stability of dispersed systems.

KEY WORDS: interfacial properties, chitosan, sodium dodecyl sulfate

INTRODUCTION

The interactions between polymers and in aqueous media give rise to the formation of association structures, thereby modifying functional and interfacial pro- perties of the solution. The morphologies of association complexes depend on the molecular properties of the polymer and the surfactant. Polymer/surfactant assembly has received a constant interest over several decades since polymers may modify the proper- ties of surfactant solutions. Especially, oppositely charged polymer/surfactant systems were widely studied, as they bind strongly to each other and offer drastic changes in the properties of surfactant in the bulk and at the interface (1-4). The existence of a pheno- menon of the interaction in the polymer-surfactant mixtures enabled the formation of

* Corresponding author: Jelena R. Milinković, University of Novi Sad, Faculty of Technology Novi Sad, Bule- var Cara Lazara 1, 21000 Novi Sad, Serbia, e-mail: [email protected] 221 APTEFF, 48, 1-323 (2017) UDC: 678.1:677.46:546.226 https://doi.org/10.2298/APT1748221M BIBLID: 1450-7188 (2017) 48, 221-229 Original scientific paper complexes, which has been widely used in the pharmaceutical and cosmetic industries, as it may greatly influence the characteristics of the product. In that manner the adsorption and orientation of polymer/surfactant complexes at various interfaces and their effect on the stability of emulsion is a subject of great importance in the area of formulation of pharmaceuticals and (5, 6). Chitin, the second most abundant natural biopolymer (next to cellulose), can be widely found in exoskeleton of shellfish, shrimps and lobsters. Chitosan (Ch), a cationic polysaccharide, is obtained by alkaline deacetylation of chitin. Solubility, rheological properties and surface adsorption of aqueous Ch solutions depend on the average molecu- lar weight, degree of deacetylation, distribution of acetyl and amino groups along the po- lymer chain, as well as on the pH and ionic strength of the solution (7, 8). At the pH <6.5, chitosan is soluble in water and has a positive charge due to the presence of protonated amino groups. Due to the possessing the combination of properties such as biocompatibi- lity, biodegradability, nontoxicity, bioactivity and antifungal activity, chitosan has many applications in medicine, cosmetics and controlled drug delivery systems (9-13). As mentioned above, polymer may interact with some other polymer or surfactant, and it is possible to expect various phenomena caused by their physicochemical pro- perties. Since chitosan is positively charged at low pH values, it spontaneously associates with oppositely charged polymers in the solution and forms polyelectrolyte complexes (10). Also, anionic surfactants may associate with chitosan, forming chitosan/surfactant complexes. In the last decade, the interactions between chitosan and sodium dodecyl sulfate have become an increasingly interesting research area (14-18). These specific interactions and the systems elaborated on their basis are important for many practical applications, such as colloidal stabilization, wettability, and adhesion. The emulsion stabilized by the polymer/surfactant complex is generally more stable than the one stabilized by surfactant only. This is because it is stabilized not only by the electrostatic repulsion of the droplets, but also by the steric hindrance of the bulky oppo- sitely charged polymer/surfactant complexes (19). In addition, for environmental or safety reasons, the use of a large quantity of low molecular mass surfactants is not recom- mended. Furthermore, the bulk physicochemical characteristics are also important for the formation and stability of the emulsion. Also, chemical structure of the oil, such as fatty acids chains length, degree of unsaturation and molecule configuration, influence the interfacial properties as well as emulsion stability and its use (20). For these reasons, our studies have focused on the investigation of the interfacial behavior of selected surfactant, sodium dodecyl sulfate (SDS), and its mixture with chi- tosan at the oil/water interface. In this study, three different types of oils were used: me- dium-chain triglycerides (semi-synthetic oil), paraffin oil (mineral oil), and grape seed oil (natural origin).

222 APTEFF, 48, 1-323 (2017) UDC: 678.1:677.46:546.226 https://doi.org/10.2298/APT1748221M BIBLID: 1450-7188 (2017) 48, 221-229 Original scientific paper

EXPERIMENTAL

Materials

Low molecular weight Ch (product number: 448869) was obtained from Sigma- Aldrich (Shanghai, China). The chitosan's degree of deacetylation, determined by poten- tiometric titration according to the procedure described by Yuan at al. (21), is found to be 81.8%. SDS, purity >99%, was purchased from Merck (Darmstadt, Germany). Different oils were used as oil phase: medium-chain triglycerides or caprylic/capric triglycerides (Saboderm TCC, Comcen, Zemun, Serbia), paraffin oil (Centrohem, Stara Pazova, Serbia), and grape seed oil (Olitalia, Forlì (FC), Italy). In all experiments, buffered water was used as a solvent and the pH was adjusted using 0.2 M water solution of acetic acid (Zorka-Pharma, Šabac, Serbia) and 0.2 M water solution of sodium acetate (Centrohem, Stara Pazova, Serbia).

Methods

Preparation of solutions

The experiments were carried out at pH 4. The pH measurements were performed by using an 827 lab pH-meter (Metrohm, Herisau, Switzerland). Stock solution of 1% (w/w) Ch was prepared by dissolving a given mass of the polymer in the buffered water, while stirring and after relaxation at room temperature during 24 h, the pH value of the solution was checked. The SDS stock solution of 2% (w/w) was prepared by using the same procedure, while the Ch/SDS mixtures were prepared by mixing required volumes of Ch and SDS stock solutions. Concentrations of SDS were varied from 0.00001% (w/w) up to 1% (w/w), while the Ch concentration was kept constant at 0.01% (w/w). The mixtures were left for 24 h at room temperature. Before measurements, pH value of the solutions was checked.

Interfacial tension

Measurements of the interfacial tension between the oil and water phase containing chitosan and various surfactant concentrations, were carried out on a Sigma 703D tensiometer (KSV Instruments, Helsinki, Finland) using the Du Noüy ring method (21). Prior to the measurement, the ring was immersed in deionized water (below the surface), then the oil phase was slowly added a top, and the system was left for 15 min, allow equlibriation of the interface. The interfacial tension of each oil/water system was measu- red by pulling the ring out of heavier, aqueous phase into lighter, oil phase. In all mea- surements, the temperature was kept constant at 30 °C. The reported values of the interfa- cial tension were average of three measurements, at least.

223 APTEFF, 48, 1-323 (2017) UDC: 678.1:677.46:546.226 https://doi.org/10.2298/APT1748221M BIBLID: 1450-7188 (2017) 48, 221-229 Original scientific paper

RESULTS AND DISCUSSION

Interfacial tension measurements

The formation of interfacial tension at the interface of two liquids which do not interfere with each other is due to different effects of their molecules at the interface. The stability of the system can be influenced by a variety of factors, especially by the pre- sence of various soluble solids, which, depending on the type and degree of solubility in the phases, can result in the changes of interfacial tension, and thereby influence the characteristics of the given system. In order to investigate the adsorption properties of chitosan, surface tension was measured at the oil/water interface. Besides the physicochemical properties of the adsor- bent, the adsorption process is influenced by the properties of the oil phase. For that reason, different types of oils were selected: medium-chain triglycerides, grape seed oil, and paraffin oil. Chitosan solution (0.01%) in buffered water (pH 4) was used as the aqueous phase. The obtained results are shown in Table 1.

Table 1. Water/oil interfacial tension at 30 °C and pH 4.

Surface tension (mN/m) Medium-chain Oil Paraffin oil Grape seed oil triglycerides Buffered water 19.56 43.20 26.28 Buffered water with 0.01% Ch 15.05 43.88 26.01

The results suggest that the highest value of the interfacial tension was obtained for the paraffin oil/water system, and the lowest one for the medium-chain triglyceri- des/water system, which is in accordance with the different polarity of the oils. The addition of chitosan to buffered water decreased slightly the value of the interfacial ten- sion in the medium-chain triglycerides/water system, while practically did not have any influence on the other two systems. These results indicate a weak surface activity of the biopolymer, and are in agreement with previous studies (14). The introduction of surfactant molecules to the oil/water system leads to their adsorp- tion at the interface, which results in a reduction of the interfacial tension. The extent of changes in the interfacial tension depends on the characteristics of surfactant molecules, as well as of the oil and water phases. The results of the interfacial tension measurements in different oil/water system containing SDS are presented in Figure 1.

224 APTEFF, 48, 1-323 (2017) UDC: 678.1:677.46:546.226 https://doi.org/10.2298/APT1748221M BIBLID: 1450-7188 (2017) 48, 221-229 Original scientific paper

Figure 1. Interfacial tension in the oil/water system of pure SDS as a function of SDS concentration, at 30 °C and pH 4

As expected, the increase in the SDS concentration resulted in a rapid decrease, to reach a plateau at a certain SDS concentration. It seems that the plateau value shows no dependence on the oil nature, since it was reached at 0.1% (w/w) SDS for all investigated oils. It can be thought that at this concentration the surface is completely saturated with SDS molecules and further increase of SDS concentration leads to the formation of in the aqueous phase that do not have adsorption ability at the interface. The addition of the polyelectrolyte to the solution of the opposite charged surfactant most often leads to the changes in the system charge that are caused by their electrostatic and hydrophobic interactions. In that manner, the interfacial tension of the Ch/SDS mixtures in water and different oils were measured. The results are presented in Figure 2.

Figure 2. Interfacial tension in the oil/water system of Ch/SDS mixtures as a function of SDS concentration at 30 °C and pH 4. The chitosan concentration was 0.01%. 225 APTEFF, 48, 1-323 (2017) UDC: 678.1:677.46:546.226 https://doi.org/10.2298/APT1748221M BIBLID: 1450-7188 (2017) 48, 221-229 Original scientific paper

As observed from Figure 2, the introduction of Ch into the system influenced signi- ficantly the oil/water interfacial tension. Namely, the changes in the interfacial tension of the Ch/SDS mixtures showed characteristic brake points at a certain SDS concentrations. At the beginning, the interfacial tension decreases gradually until reaching the first break point T1 at 0.001% SDS (Ch/SDS mass ratio 10:1) for the system medium-chain ’ triglycerides/water and T 1 at 0.005% SDS (Ch/SDS mass ratio 5:1) for paraffin oil/water and grape seed oil/water systems. The value of the interfacial tension of the medium- chain triglycerides/water system at T1 was 8.62 mN/m. This value for the paraffin ’ ’ oil/water system at T 1 was 16.99 mN/m, and for the grape seed oil/water system at T 1 it was 10.46 mN/m. After reaching T1, the interfacial tension approximate a plateau till second break point T2 at 0.02% (w/w) SDS (Ch/SDS mass ratio 1:2) for all investigated systems. After T2, the interfacial tension decreased significantly until reaching the third break point T3 at 0.1% (w/w) of SDS concentration (Ch/SDS mass ratio 1:10), which is the critical concentration for the pure SDS solution. Since the changes in all investigated systems indicate the presence of Ch/SDS interactions, they will be explained on the medium-chain triglycerides/water system (see Figure 3).

Figure 3. Interfacial tension in the medium-chain triglycerides/water system of pure SDS and Ch/SDS mixtures as a function of SDS concentration at 30 °C and pH 4

As can be seen from Figure 3, at low SDS concentrations surface, active Ch/SDS complexes were formed and the interfacial tension of the mixture had a lower value than the solution of pure SDS of the corresponding concentration. It can be assumed that in such conditions electrostatic binding of single SDS molecules to the amino groups of chi- tosan chain took place resulting in formation of the complexes that have synergistic effect on the interfacial tension. Upon reaching an SDS concentration of 0.001% (w/w), at T1, turbidity in the mixtures occurs (Fig. 4), which is in accordance with our previously published results (14).

226 APTEFF, 48, 1-323 (2017) UDC: 678.1:677.46:546.226 https://doi.org/10.2298/APT1748221M BIBLID: 1450-7188 (2017) 48, 221-229 Original scientific paper

Figure 4. Phase separation in the mixtures of 0.01% chitosan and various SDS concentrations

The interaction continues mostly in the bulk, until reaching T2 at 0.02% SDS, by binding SDS in the form of aggregates to the Ch chains (1, 3, 14). Such phenomenon should not influence the oil/water interface. However, in the T1-T2 region, the surface tension slightly increases, indicating reorganization of the interfacial layer due to the hydrophobic SDS binding and stretching of the polymer chains, i.e. the interaction taking place simultaneously at the interface. At 0.02% SDS (Ch:SDS mass ratio 1:2) the Ch/SDS complexes fully precipitate as the coacervate phase (Figure 4). Further addition of SDS leads to desorption of Ch/SDS complexes and gradual allocation of SDS mole- cules at the interface, causing significant decrease in the interfacial tension until reaching the third break point T3 at 0.1% (w/w) SDS, which is critical micelle concentration for the pure SDS solution. After the concentration of 0.1% (w/w), the interface is saturated only with SDS molecules, and the interfacial tension remains unchanged. Once micelles of SDS abound in solution, the interfacial tension becomes essentially equal to that of a micelle, polymer-free surfactant solution.

CONCLUSION

The investigation of the interfacial behavior of Ch/SDS complexes showed that increasing concentration of SDS in a chitosan solution leads to characteristic changes in the interfacial tension. These changes indicate an electrostatic interaction between the po- sitively charged chitosan and the opposite charged SDS, i.e. formation of a chitosan/SDS complexes, which exhibit the surface activity. Surface active complexes begin to form at very low concentrations of SDS. As already demonstrated with various systems, the sur- face activity of these complexes is much higher than the one of the polymer or surfactant alone. At higher SDS concentrations, the interaction is more hydrophobic and SDS molecules bind to the polymer chains in the form of aggregates, leading gradually to pre- cipitation of the complexes as the coacervate phase. The lowest interfacial tension was achieved in the system medium-chain triglycerides/water. The results of this study show- ed that Ch/SDS complexes could be used for stabilization O/W emulsions with medium- chain triglycerides, paraffin and grape seed oils. In addition, this phenomenon suggests the possible application of Ch/SDS mixtures in the microencapsulation processes.

Acknowledgement

This work was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, Project Number III46010.

227 APTEFF, 48, 1-323 (2017) UDC: 678.1:677.46:546.226 https://doi.org/10.2298/APT1748221M BIBLID: 1450-7188 (2017) 48, 221-229 Original scientific paper

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MЕЂУФАЗНО ПОНАШАЊЕ КОМПЛЕКСА ХИТОЗАН/НАТРИЈУМ-ДОДЕЦИЛ-СУЛФАТ

Јелена Р. Милинковић, Милијана В. Алексић, Лидија Б. Петровић, Јадранка Л. Фрај, Сандра Ђ. Бучко, Јарослав М. Катона и Љиљана M. Спасојевић

Универзитет у Новом Саду, Технолошки факултет Нови Сад, Булевар цара Лазара 1, 21000 Нови Сад

Код савремених формулација козметичких и фармацеутских емулзија, стабили- зација се може постићи применом комбинованих полимер/површински активна ма- терија система, који у смеши могу да формирају комплексе различите структуре и физичко-хемијских особина, односно могу довести до додатне стабилизације емул- зионих производа. Из ових разлога, главни циљ ове студије био је испитивање осо- бина комплекса хитозан/натријум-додецил-сулфат. У циљу што бољег разумевања механизма стабилизације и међуфазних особина смеше хитозана и натријум-доде- цил-сулфата, мерен је међуповршински напон. Узимајући у обзир чињеницу да својства уљне фазе утичу на процес адсорпције, испитивана су три различита типа уља: триглицериди средње дужине ланаца (полусинтетско уље), парафинско (мине- рално) уље и уље коштица грожђа (природног порекла). Мерења површинског на- пона на граници фаза уље/вода за раствор хитозана указују на слабу површинску активност овог биополимера. Додатак натријум-додецил-сулфата у раствор са хи- тозаном доводи до значајног смањења међуповршинског напона за сва испитивана уља. Резултати ове студије су важни за разумевање утицаја интеракције поли- мер/површински активна материја на особине раствора и стабилност дисперзног система.

Кључне речи: међуфазне особине, хитозан, натријум-додецил-сулфат

Received: 26 July 2017. Accepted: 06 October 2017.

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