Ionic Liquids: Physico-Chemical, Solvent Properties and Their Applications in Chemical Processes

Indian Journal of Chemistry Vol. 47A, April 2008, pp. 495-503

Advances in Contemporary Research

Ionic liquids: Physico-chemical, solvent properties and their applications in chemical processes

Geetanjali Singh & Anil Kumar* Physical Chemistry Division, National Chemical Laboratory, Pune 411 008, India Email: [email protected] Received 27 February 2008

A new class of compounds, called as ionic liquids, has recently emerged as potential substitute for volatile organic solvents. Ionic liquids possess certain special physico-chemical characteristics that render them very useful in different areas of research and development. In this review, the physico-chemical properties and the applications of ionic liquids in chemical processes are critically discussed and recommendations made for probing further the poorly understood aspect of this newly emerging class of compounds.

Ms Geetanjali Singh holds a M.Sc. developed. However, their role in minimizing degree in Physical Chemistry from environmental pollution cannot be appreciated due to Lucknow University, Lucknow, and is lack of sufficient database on interactions between currently working with Dr Anil Kumar on a project concerning thermodynamics substrate and ionic liquid in the modified situations. of ionic systems. Though it will not be possible to discuss all the alternate methodologies outlined above, an effort will be made in this article to critically evaluate the research work carried out in the area of ionic liquids. The potential research problems in this area of Dr Anil Kumar is a scientist at the contemporary research will also be discussed with a National Chemical Laboratory, Pune. His view to enhancing the research activities in different research interests are in the areas of laboratories. In this article, no efforts have been made thermodynamics, ionic liquids, physical- to cover exhaustive literature survey. For the benefits organic chemistry and green chemistry. of the readers interested in data on ionic liquids, a His contribution has been recognized by several awards such as FICCI award, the free-access web database for thermodynamic Fellowships of National Academies, etc. properties has now been made available because of He is the member of the Editorial Boards concerted efforts of the National Institute of Standard of Indian Journal of Chemistry. Sec A and Technology (NIST), Gaithersburg, USA, and and Journal of Solution Chemistry. University of Canterbury, Christchurch New 1 Introduction Zealand . The role of ionic liquids as substitutes to 2 The last decade has witnessed an upsurge in the VOC has been discussed in several reports . research activities related to green chemistry. Most of In general, an ionic liquid is a liquid wholly the efforts in this direction have been focused on composed of ions. Conventionally, an ionic liquid is replacing the abundantly used volatile organic in liquid state below 100°C. Several other solvents (VOC) by suitable alternate solvent systems terminologies are used to describe ionic liquids. These for easy chemical transformations with the minimum include room temperature ionic liquids, non-aqueous chemical waste and environmental pollution. Several ionic liquids, molten salts and fused salts. This alternate methodologies based on water reaction nomenclature will be useful for workers who are medium, ionic liquids, supercritical fluids, required to search chemical literature using electronic microwave, sonochemical treatment, etc., have been means. Normally, molten salt is used for the salts that 496 INDIAN J CHEM, SEC A, APRIL 2008

are molten at very high temperatures such as NaCl points and are stable in water and the medium with its melting point as 803°C. Ionic liquids, containing Lewis acids. Some ionic liquids under this however, refer to the molten phase below 100 oC. category are noted to act as moderately coordinating From historical point of view, it is interesting to solvents. mention here that the ethyl ammonium nitrate Contrary to the massive investigations made in the

[C 2H5NH 3] [NO 3] with m.pt. ∼12°C was the first ionic area of structure of liquids, one finds relatively very liquid synthesized about 90 years ago 3. However, this few studies made on ionic liquids. Neutron diffraction ionic liquid did not attract further application-based and X-ray diffraction techniques have been applied to research due to its explosive nature. There are several elucidate structure of ionic mixtures in water medium. reasons that ionic liquids are considered as “green Information on solute-solvent interactions in AlCl 3- solvents”. The most important reason is that ionic BPC ( N-1-butylpyridinium chloride), AlCl 3-[EMIM] liquids possess negligible vapor pressure and hence (1-ethyl, 3-methylimidazolium), [C 2MIM Cl] they do not evaporate to environment. Secondly, they (methylimidazolium)-[EMIM]Cl has recently been are good solvents for a whole range of inorganic and obtained using neutron and X-ray diffraction methods. organic materials. Thirdly, some of them are For example, first order differences using immiscible with organic solvents and therefore they hydrogen/deuterium replacement on both the provide a polar alternative with non-aqueous nature imidazolium ring and HCl indicates the presence of - 5,6 for two-phase systems. Lastly, ionic liquids can be [HCl 2] as an asymmetric species . EXAFS has also recycled to offer comparable performance in chemical been employed 7 to study the manganese and bromine transformations. coordination within the solid and molten states of [Bu N] [MnBr ]. This technique has also been Structural features 4 3 explored to determine the ionic character of ionic An ionic liquid consists of a cation, which is liquids. General information about ionic liquids has normally a bulk organic structure with low symmetry. also been extracted by neutron scattering studies of The widely used cations in ionic liquids are based on 1,3-dimethyl-imidazolium based ionic liquids 8. The ammonium, sulfonium, phosphonium, imidazolium, reverse Monte-Carlo procedure in conjugation with pyridinium, picolinium, pyrrolidinium, etc. with the empirical potential structural refinement process different substitutions. The anion of ionic liquid may when used on the neutron scattering data provided be organic or inorganic. For example, anions include - - - - - probability distribution of chloride around a central [BF 4] , [SbF 6] , [PF 6] , [CF 3SO 3] , [(CF 3SO 2)2N] ≡ 9 - imidazolium cation . These studies concluded that [Tf 2N] , alkyl sulfates, etc. The forces operating between cation and anion of an ionic liquid are overwhelmingly columbic in nature. In Scheme 1 are displayed some typical cations and anions that are commonly employed in synthesizing ionic liquids. The first generation ionic liquids include a mixture of organic species with AlCl 3 in which one can vary the nature of medium by changing the amount of AlCl 3. These ionic liquids are known as chloroaluminates 4. One can thus obtain the acidic, basic and neutral ionic liquids. Though one can have a more efficient control over the nature of these ionic liquids, their extremely hygroscopic nature restricts their use in organic processes or one needs to take strict precautions for using them under a dry atmosphere. In spite of their hygroscopic nature these ionic liquids have however been used as electrolytes in batteries. Ionic liquids - - with [BF 4] , [PF 6] anions etc. are air stable and neutral. The ionic liquids can react exothermically with Lewis acids and water. Ionic liquids containing - - [CF 3SO 3] or [Tf 2N] anions possess low melting SINGH & KUMAR: PHYSICO-CHEMICAL PROPERTIES OF IONIC LIQUIDS 497

strong charge ordering exists in ionic liquids with the zolium cation offers ionic liquids with low melting anions and cations alternating in the radial distribution points. However, the most difficult point in connection function. with ionic liquids is that a majority of them are The neutron and X-ray reflectivity methods used hygroscopic in nature and therefore pose a problem in for analyzing the surface structure of long and short their handling and storage. Air-instability is another alkyl chain length ionic liquids revealed a lamellar demerit of some of the ionic liquids. structure 10 . The simulation studies conducted on ionic liquids however, require extensive experimental Physico-chemical properties verifications. Physico-chemical properties of ionic liquids can be Very recently, the structure of aqueous solutions of dramatically altered by impurities such as water, [BMIM] [BF 4] has been investigated by small-angle organic solvents and chloride ions. Hence, as discussed neutron scattering 11 . It has been observed that the large above one needs to take precautions during the concentration fluctuations and Kirkwood-Buff integral synthesis of ionic liquids in order to avoid the presence values point towards phase separation. The vicinity of of such impurities. In Table 1 are listed important phase separation occurs at 278 K and at a mole fraction physico-chemical and solvent properties for some ionic of ionic liquid of 0.075. liquids. This compilation is provided for prospective research workers to obtain an idea about the Synthesis and handling of ionic liquids characteristics of ionic liquids. Below are summarized Besides brief discussion on the types of ionic some pertinent physical and chemical properties of liquids, one should note that it is possible to synthesize ionic liquids. numerous ionic liquids with varying properties. Excellent reviews are available on their synthesis and Melting points handling techniques 12,13 . Several permutations and This is the most significant characteristic property of combinations of cations and anions are possible. For ionic liquids that can be correlated with the structure instance, ionic liquids based on the bis and composition of ionic liquids. Selection of both the - - (methanesulfonyl) amide ([Ms 2N] ) and [Tf 2N] anions cation and anion determine the melting point of an have provided deeper insights into anion fluorination, ionic liquid. An ionic liquid with a cation with low hydrogen bonding and conductivity in ionic liquids. symmetry possesses low melting point than the one Some other anions like carboranes and orthoborates are with high symmetry. Weak intermolecular interactions inert, low coordinating and non-fluorinated ionic and a good distribution of charge in the cation favor liquids. One can also synthesize ionic liquids with low melting point of ionic liquid. Chloroaluminate functional groups to impart certain physical properties ionic liquid comprising of [EMIM] Cl/AlCl 3, for like self-aggregation. In general, N-N-dialkylimida- example, shows interesting trend in their melting

Table 1—A summary of some physico-chemical properties of ioni c liquids at 25 oC

Ionic liquid Density Viscosity Conductivity (g cm -3) (cP) (mS cm -1)

[BMIM][BF 4] 1.12 1.7 233 [BMIM][PF 6] 1.368 450 n.a. [BMIM][Tf 2N] 1.436 52 n.a. [BMIM] I 1.44 1110 n.a. [BMIM][TfO] 1.29 90 3.7

[BMIM][CF 3CO 2H] 1.21 73 3.2 [OMIM][PF 6] 1.237 682 n.a. [EMIM][Tf 2N] 1.519 28 n.a. [EMIM][NMs 2] 1.343 787 1.7 [EM2IM][Tf 2N] 1.51 88 3.2 [OMIM][PF 6] 1.237 682 n.a. [OMIM][Tf 2N] 1.320 n.a. [OMIM]Cl 1.00 337 n.a. Fig. 1—Experimental phase diagram for [EMIM] Cl / AlCl 3 n.a. not available system.Plot of t versus xAlCl3 . 498 INDIAN J CHEM, SEC A, APRIL 2008

points. The experimental phase diagram for [EMIM] with the counterion used to constitute an ionic liquid. Cl/AlCl 3 system is shown in Fig. 1. One can see that The diffusion coefficients of ionic liquids have been the melting point of [EMIM] Cl/AlCl 3 is very observed to be strongly influenced by ion-pairs and sensitive to the composition of ionic liquid 14 . It seems ion-aggregates present in ionic liquids. The issue that it is possible to combine two species to form a pertaining to the contrast between the information suitable ionic liquid that will have low melting point. obtained from the analysis of conductivity and This is an important characteristic of ionic liquid, diffusion data merits investigation. The relationship which can be helpful in designing ionic liquids between fluidity and conductance of ionic liquids suitable for a given application. indicated that ionic liquids could be characterized with ideal quasi lattice structure 19 . It is now observed that Densities the viscosity alone does not account for the In general, ionic liquids are denser than water. The conductivity of ionic liquids. Other factors like ionic magnitude of density, ρ, in the case of ionic liquid size and ion pairs also play significant role in depends upon the constituent cation and anion. For governing the conductivity of ionic liquids 20 . instance, the ρ, value of ionic liquid varies with the length of the N-alkyl chain on the imidazolium cation. Viscosities As a thumb rule, the density of comparable ionic Viscosity of ionic liquids is of immense importance liquids decreases with the increase in the bulkiness of if ionic liquids need to used as solvent media during a the organic cation. Further, the density of ionic liquid chemical reaction or for any other applications. For also depends upon the choice of the anion. Normally, example, if the viscosity of ionic liquid is very high, it will be detrimental to the progress of a reaction. ρ of an ionic liquid varies in the range of 1.05 to 1.36 Several ionic liquids possess very high viscosities and g cm -3 at ambient temperature 15,16 . therefore have been noted to be less suitable as

Thermal stability solvent media. Further, high viscosities of ionic Most of the ionic liquids are stable at and above liquids also reduce the diffusion rates of redox 400 oC. The thermal decomposition depends on the reactions. The viscosity of [EMIM][Tf 2N], for nature of anions rather than on that of cations. Further instance is 28 cP, while that of [BMPYRR][NMs 2] thermal decomposition also decreases with increase in 1680 cP at 298 K. [BMPYRR] stands for 1-butyl-1- hydrophilicity of anions. Due to contrasting reports on methylpyrrolidinium, whereas [NMs 2] for bis- thermal stability of various ionic liquids, it is (methanesulfonyl)amide species. It is important to recommended that high decomposition temperature of note here that viscosities of ionic liquids show non- ionic liquids be calculated from fast Arrhenius behabiour. Efforts have, however, been thermogravimetric analysis (TGA) scans under a made to fit viscosity data with the Vogel-Tammann- protective environment 17 . Fulcher (VFT) equation. Viscosity of a majority of ionic liquids remains constant with increasing shear Diffusion and conductivity rate; hence these substances are termed as Newtonian In order to effectively use ionic liquids for different fluids 21-23 . applications, it is necessary to study diffusion and Viscosity data of ionic liquids have also been conductivity behaviour of ionic liquids. From the investigated at high pressures. Important contribution pulse-gradient spin-echo NMR data, it has been shown in this regard has been made by Harris and that both [BMIM] and [BPy] cations diffuse almost at coworkers 24 . In Fig. 2 the pressure dependence of - o the same rate as [BF 4] . The sum of cationic and viscosity of [BMIM][BF 4] at 25 C is shown. As is anionic diffusion coefficients for each ionic liquid clear from the illustration, the viscosity-pressure plot follows the order: [EMIM][Tf 2N] > [EMIM] [BF 4] > for the ionic liquid shows normal variation. The high [BP] [Tf 2N] > [BP] [BF 4]. One should note that the pressure viscosity data have been analysed in terms of relationship between chemical structures and transport Litovitz and VFT equations. properties of ionic liquids is not properly investigated The viscosity of an ionic liquid changes drastically so far. The Stokes-Einstein and Nernst-Einstein upon addition of a solvent. Several years ago, it was equations have been employed to analyse the ionic shown that viscosity of a high melting point tetra-n- diffusivity of ionic liquids 18 . The analysis of the data butylammonium picrate (m.pt. ∼89 oC) reduced showed that diffusion of an ion was strongly correlated dramatically upon addition of anisole, nitrobenzene and SINGH & KUMAR: PHYSICO-CHEMICAL PROPERTIES OF IONIC LIQUIDS 499

Fig. 2—Pressure dependence of viscosity, η, for [BMIM]BF 4 at Fig. 3—The changes in viscosity, η, of [OMIM] I upon addition o 25 C. of dichloromethane at 25 oC. n-butanol 25 . This phenomenon is also noted in ionic liquids and their mixtures with solvents at or around room temperature. Accordingly, the viscosities of these ionic liquids can be lowered by adding a solvent possessing low viscosity. This observation bears great significance in carrying out chemical reactions in those ionic liquids the viscosities of which are very high. For instance, The viscosity of [OMIM]I can decrease exponentially upon addition of dichloromethane in the ionic liquid-rich mixture as shown in Fig. 3 (ref. 26). The authors are not aware of any explanation to describe this behaviour, however, a possibility exists to employ the concept of hole formation to examine the composition dependence of viscosity of ionic liquids. More data on a variety of ionic liquids and their mixtures with solvents of different polarity are required E to ascertain this behaviour. Fig. 4—Excess partial molar enthalpies, Hi versus mole fraction, o xi in binary mixtures of [MEIM][(CF 3SO 2)2N] at 25 C in (○) t-butanol, ( ●) methanol, ( σ) 1-hexanol, ( □) ethylene glycol, Phase equilibrium studies Phase equilibrium data for ionic liquids in water (◆ ) toluene ( ◇◇◇ ) chloroform. and other organic solvents have been recently studied 27,28 Enthalpy of ionic liquids by Domanska and her co-workers . They The measurement of enthalpies of ionic liquids is investigated phase equilibria from melting point of still at the state of infancy. The solution enthalpies ionic liquid to boiling point of organic solvent. The have been measured for some ionic liquids based on (solid + liquid) and (liquid + liquid) phase equlibria o Ex imidazolium cation in different solvents at 25 C by curves were correlated by means of different G using isothermal titration calorimeter. In Fig. 4, the models derived from basic parameters. Though the mole fraction, xi dependence of the excess partial prediction of phases seem to be good using the E 29 molar enthalpies, Hi for the ionic liquid is shown. models, one needs to examine the application of non E One finds a strong exponential dependence of Hi in random two liquid (NRTL) and other methods to this very dilute solutions for 1-hexanol, ethylene glycol, system. This is an area of future investigation. 500 INDIAN J CHEM, SEC A, APRIL 2008

while for t-butanol, methanol, toluene and Table 2—Solvent parameters for some ionic liquids chloroform, the dependence is linear. No modeling N work of enthalpies in ionic liquids has so far been Ionic liquid ET π* α β initiated. [BMIM][BF 4] 0.67 1.047 0.627 0.376 [BMIM][PF 6] 0.669 1.032 0.634 0.207 Solvent properties [BMIM][TfO] 0.656 1.006 0.625 0.464

In order to use ionic liquids as solvents in chemical [BMIM][Tf 2N] 0.644 0.984 0.617 0.243 a processes, it is necessary to determine their solvent [BM 2IM][BF 4] 0.576 1.083 0.402 0.363 properties. The literature data have shown that the [BMP][Tf 2N] 0.544 0.954 0.427 0.252 determination of polarity of ionic liquids is a difficult [BM 2IM][Tf 2N] 0.541 1.010 0.381 0.239 task by using a single solvatochromic probe. The [OMIM][PF 6] 0.633 polarity of ionic liquid has been measured by [OMIM][Tf 2N] 0.629 absorbance and fluorescence methods. However, both [OM 2IM][Tf 2N] 0.525 these methods do not yield similar results. About 20 [OM 2IM][BF 4] 0.543 a years ago, a simple method was proposed to separate [BM 2IM]= 1-butyl-2,3-dimethylimidazolium non-specific effects of the local electrical fields from [OM 2IM]= 1-octyl-2,3-dimethylimidazolium hydrogen-bonding effects. Several parameters like dipolarity/polarizability ( π*), H-bond basicity ( β), and important role in reaction kinetics. The rates of Diels- H-bond acidity ( α) are employed in this method 30 . In Alder reaction increase with an increase in viscosity Table 2 are listed values of solvent parameters for up to ∼1 cP. A sharp downfall in the rates of these some ionic liquids. Though measurements have not reactions is witnessed in solvents possessing been made for a large number of ionic liquids, the viscosities above ∼1 cP. The increase in the rates was general trend suggests that the π* values for ionic attributed to the gain of vibrational mode at the liquids are higher than that of alkyl chain alcohols. expense of translational ones up to ∼1 cP thus 34 The anion of an ionic liquid determines the magnitude facilitating the bond formation . However, the of the β parameter. The α values for ionic liquids are reactions considerably slow down in viscous solvents less than those of water and alkyl chain alcohols. owing to the diffusion problems. This suggests that There are two main issues that need to be reactants cannot see each other in highly viscous addressed in this case. First, the solvent parameters environment thus causing retardation in the reaction for ionic liquids should be determined. Secondly, no rates. No exception to these observations has so far knowledge exists on how these parameters are come to light. There is however three significant modified under mixed conditions. reports on the effect of viscosity of ionic liquids on the reaction rates. As evident from the measured Applications in chemical processes pseudo-first order rate constants, kobs , for the reaction Several organic reactions have been successfully of cyclopentadiene with methyl acrylate, the reaction carried out in ionic liquids. These include Diels- was faster in the ionic liquids possessing higher Alder, Baylis-Hillman, Henry, Suzuki, esterification, viscosities than in the ones with lower viscosities 35 . isomerization reactions 31,32 . Ionic liquids have also This means that the high viscosities favour the above been used as solvents in transition metal catalysed reaction. It appears that the rate of reaction is not reactions, like hydrogenation, oxidation, limited by the migration of the reagents through the hydroformylation, telomerization, Trost-Tsuji coupl- ionic liquid investigated. In another study, the rate ings, oligomerization to name a few 2f . In an constants for a diffusion-controlled reaction involving interesting and trend-setting research work, it has neutral reactants were measured in ionic liquids with been demonstrated that cellulose can be dissolved differing viscosities at many temperatures 36 . The without activation or pre-treatment in the imidazolium overall bimolecular rate constant, k2, was noted to based ionic liquids 33 . This work from the school of increase with an increase in the viscosities of ionic Rogers is a step forward towards achieving green liquids. However, the data reported in the work process for the bio-renewable material. appeared to be insufficient for conclusive evidence of Pressure, temperature and concentrations of the role of viscosity in Diels-Alder reactions. Also, reactants govern the progress of reaction. However, it the viscosities of the ionic liquids did not correspond has been observed that viscosity can also play an to the temperatures at which the kinetic studies were SINGH & KUMAR: PHYSICO-CHEMICAL PROPERTIES OF IONIC LIQUIDS 501

carried out. investigation of the isoviscosity kinetics resulted in the

In a very recent work from this laboratory, the k2 plot shown in Fig. 6. The “intrinsic” activation energy, -1 values for a number of Diels-Alder reactions in ionic Eo, was reported to be 51.8 kJ.mol implying a -1 liquids with varying viscosities were reported. These difference (E a-Eo) of 5.9 kJ.mol for [EMIM][BF 4] and -1 reactions were inhibited in the ionic liquids with high of 11.6 kJ.mol for [BMIM][PF 6]. These values were viscosities (Fig. 5). The observations were attributed to in agreement with the qualitative prediction that the the diffusion problems arising out of high viscosities. reactants would have to overcome a “higher barrier” in Efforts were also made to correlate k2 values with a more viscous medium, leading to a decrease in the different properties such as surface tension, solvent rate of the reaction. A detailed study of the isoviscosity properties, etc., of ionic liquids, but were not found to relationships, Arrhenius parameters and determination be successful. The results were interpreted in terms of of microviscosity are some of the most challenging the restricted diffusion of reactants in the encounter- problems one needs to address in this area. controlled regime. The rates of reaction of The reactions were also carried out in a mixture of cyclopentadiene with the three acrylates used follow the ionic liquids. It was noted that as the proportion of the order: methyl acrylate > ethyl acrylate > butyl [EMIM][BF 4] increased, the magnitude of the rate acrylate. The difference may arise due to the different constant also increased. This observation were microviscosity experienced by each of these acrylate explained by considering the fact that η of the binary molecules, which will be a complex function of the mixture decreased with an increasing fraction of the viscosity of the medium, the molecular volume of the less viscous component, [EMIM][BF 4], thus causing a reacting moiety and the viscosity of the acrylate itself. rise in the rates. This implied that any factor causing a The enhanced steric effect will also play an important reduction in the viscosity of ionic liquids would lead role in governing the reactions. to a rise in the rates of the reactions. The reaction of cyclopentadiene with methyl The effect of a “viscosity reducer” in the presence acrylate showed Arrhenius behaviour as evident from of ionic liquid on the rates of reactions was also the temperature dependence of rate constants in studied. For this purpose, the reaction of [BMIM][PF 6] and [EMIM][BF 4]. The activation cyclopentadiene with methyl acrylate was carried out -1 energy, Ea, was reported to be 63.4 kJ.mol for the at 298 K in a mixture of [BMIM][BF 4] with reaction of cyclopentadiene with methyl acrylate in dichloromethane (45 mol % of [BMIM][BF 4] in 55 -1 [BMIM][PF 6] and 57.7 kJ.mol for the same reaction mol % of dichloromethane). Here, dichloromethane in [EMIM][BF 4]. However, any change in temperature was used as a “viscosity reducer”, with η = ~18 cP as led to a change in the η values of the ionic liquids, compared to the value of η = 233 cP for which affected the rate of the reaction. A preliminary [BMIM][BF 4]. The resulting rate constant k2= 5.79 × 10 -5 dm 3.mol -1.s -1 was much higher than that in pure

Fig. 5—Viscosity dependence of second order rate constant, k2, for the reaction of cyclopentadiene with methyl acrylate in ionic Fig. 6—Eyring plots of Diels-Alder reaction of cyclopentadiene liquids of varying viscosities. with methyl acrylate in ( ■) [EMIM][BF 4] and (
) [BMIM][PF 6]. 502 INDIAN J CHEM, SEC A, APRIL 2008

[BMIM][BF 4] or dichloromethane alone. interested researchers in future. Although a simple correlation between a given Acknowledgement solvent property and the magnitude of a particular rate One of the authors (AK) thanks Department of constant does not conclusively imply a dependence of Science and Technology, New Delhi for supporting rate on that solvent property, the evidences do merit this work through Ramanna Award Fellowship via the further investigation. Grant-in-Aid No. SR/S1/RFPC-05/2006.

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