LiquidâLiquid Equilibria in Aqueous Mixtures of ... - ACS Publications

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Liquid−Liquid Equilibria in Aqueous Mixtures of Alkylmethylimidazolium Glutamate with Potassium Carbonate and Some Physicochemical Properties of Aqueous [Cnmim][Glu] (n = 4, 6, 8) Solutions Elena V. Alopina, Evgenia A. Safonova,* Igor B. Pukinsky, and Natalia A. Smirnova St. Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg, 199034 Russia S Supporting Information *

ABSTRACT: Aqueous biphasic systems containing ionic liquids (ILs) attract rather high attention; in particular their application in liquid−liquid extraction is very attractive due to the specificity of the systems and great possibilities in the modification of IL chemical structures. In the present work, a series of amino acid ionic liquids (AAILs) based on the 1-alkyl-3-methylimidazolium cation and L-glutamic acid anion [Cnmim][Glu] (n = 4, 6, 8) were synthesized. Densities, refractive indices, and specific electrical conductivities of aqueous solutions of these AAILs in the concentration range 0−70 wt % (for [C4mim][Glu] solutions up to 87 wt %) were measured at temperature 298.15 K and ambient pressure 0.1 MPa. The liquid−liquid equilibria for ternary solutions [Cnmim][Glu] + salt (K2CO3) + water at 298.15 K were under study. In aqueous solutions of [C8mim][Glu] micellization is observed, whereas [C4mim][Glu] and [C6mim][Glu] in aqueous solutions form small ordinary aggregates. The critical micellar concentration for [C8mim][Glu] aqueous solutions at 298.15 K was estimated from the experimental data on the concentration dependence of the specific electrical conductivities of the solutions.

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INTRODUCTION During the last few years, aqueous solutions of amino acid ionic liquids (AAILs) have been paid special attention due to various possible applications of these ionic liquids in chemistry, medicine, techniques, and so forth.1−4 Like conventional ILs, AAILs are good solvents with the negligible volatility, high ionic conductivity, and nonflammability; in addition they are more biodegradable and biocompatible than traditional ILs.5 It has been shown experimentally6 that AAILs can form aqueous biphasic systems (ABS) in the presence of inorganic salts above some certain concentration. ILs (alkylmethylimidazolium ILs among them) in such systems have demonstrated their good qualities as cosolvents or additives to water for different extraction processes. The limited solubility observed in many ternary systems water−hydrophilic IL−inorganic salt is of special interest, in particular, for the liquid−liquid extraction of various solutes, including biomolecules.2,7 The potential of the AAILs and the advantages of their usage can be understood only if the detailed information about their structure and physicochemical properties in the pure state as well as the information about their interactions in aqueous medium are available. After their first synthesis by Fukumoto et al.,8 AAILs were specially investigated in order to evaluate the influence of cations (imidazolium,9−14 phosphonium,15 cholinium,16 or ammonium17 and their derivatives) on the physicochemical and thermal properties of ILs such as their viscosity, conductivity, density, surface tension, heat capacity, © XXXX American Chemical Society

osmotic pressure, and so forth. The effects of AAILs anions on the properties under study were also discussed in many publications, most of them being presented as reports.13−17 In particular, it was stated that the thermal decomposition temperatures of the AAILs based on 1-ethyl-3-methylimidazolium change in the order [Emim][Gly] < [Emim][Ala] < [Emim][Ser] < [Emim][Pro] (where [Gly], [Ala], [Ser], [Pro] are glycine, L-alanine, L-serine, and L-proline, respectively).18 Such studies are also of high importance for AAIL design. The binary systems containing AAILs have been only poorly investigated. 1-Butyl-3-methylimidazolium aspartate [C4mim][Asp] + methanol solutions were studied at different temperatures by Wei et al.19 The density data obtained at different temperatures for aqueous solutions of [C4mim][AA] based on [Gly], [Ala], [Val] (L-valine), [Leu] (L-leucine), and L-isoleucine in the molality range from 0.05 to 0.5 have given the grounds for the researchers to conclude that hydrophobic interactions play an important role in hydration of amino acid anions of AAIL.14,20 Moreover, the analysis of the data on the volumetric characteristics and osmotic coefficients obtained at 298.15 K for aqueous solutions containing AAILs with 1-ethyl3-methylimidazolium ([Emim]) cations and amino acid anions (the same as above) shows that cooperative H-bonding of Received: November 10, 2015 Accepted: April 22, 2016

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DOI: 10.1021/acs.jced.5b00948 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Specification of Chemicalsa

amino acid anions leads to the stabilization of water structure (cosmotropic effect).21 Ternary aqueous−salt solutions containing AAILs are of special interest due to possible applications of aqueous biphasic systems for the liquid−liquid extraction. The studies of the phase behavior of such solutions are still at the very beginning. Wu et al.22,23 have determined the binodal curves for a number of ABSs of AAILs at 298.15 K. In particular, these are biphasic aqueous solutions of tetraalkylammonium (methyl-, ethyl-, nbutyl-, n-pentyl) or tetra-n-butylphosphonium glycine in the presence of dipotassium hydrophosphate, K2HPO4. For [C4mim][Ser], [C4mim][Gly], [C4mim][Ala], or [C4mim][Leu] aqueous solutions the formation of ABSs due to the salting-out effects was studied in the presence of K3PO4. The thermophysical properties (density, viscosity, surface tension, and heat capacity) of AAILs based on 1-octyl-3-methylimidazolium with glycinate, alaninate, serinate, prolinate, and asparaginate anions were measured at different temperatures.24 In the present work, several AAILs based on L-glutamic acid (Glu), [Cnmim][Glu], with the different lengths of AAILs hydrophobic tails (n = 4, 6, 8) were synthesized (Figure 1). The

chemical name

source

1-butyl-3-methylimidazolium chloride [C4mim]Cl 1-hexyl-3-methylimidazolium chloride [C6mim]Cl 1-octyl-3-methylimidazolium chloride [C8mim]Cl L-glutamic acid

Sigma-Aldrich, U.S.A. Sigma-Aldrich, U.S.A. Sigma-Aldrich, U.S.A. AppliChem Panreac, Germany Vekton, Russia Vekton, Russia Sigma-Aldrich, U.S.A. Vekton, Saint-Petersburg, Russia

acetonitrile methanol Amberlite IR-400 Cl potassium carbonate K2CO3 a

mass fraction purity

CAS (Chemical Abstracts Service)

≥0.98

79917-90-1

≥0.97

171058-17-6

≥0.97

64697-40-1

≥0.985

56-86-0

≥0.998 >0.995 60177-39-1 ≥0.99

All of the chemicals were used without further purification.

8) were prepared from [Cnmim]Cl using an anion-exchange resin over a 100 cm column. However, the obtained hydroxides [Cnmim]OH are not quite stable, and they should be used immediately after preparation. The excess amount of the prepared aqueous solution of [Cnmim]OH was added dropwise to the L-glutamic acid aqueous solution cooled to 0 °C. The mixture was stirred under cooling and then heated up to 96 °C. The excess residual water was evaporated under reduced pressure. The mixed solvent acetonitrile/methanol (volumetric ratio 4/1) was added, and the mixture was stirred vigorously. Then the mixture was filtered to remove excess glutamic acid. The solvents were removed from the filtrate by evaporation. The products, [Cnmim][Glu] (n = 4, 6, 8), have been dried for 2 days in the vacuum desiccator at the room temperature. The structures of the AAILs based on glutamic acid were confirmed by 1H NMR and 13C NMR spectroscopies (see Supporting Information, Figure S1). The results of 1H NMR and 13C NMR analysis of [C4mim][Glu] are in a good agreement with the literature data.9,10 The total peak integral in the 1H NMR spectrum was found to correspond for all three AAILs to a nominal purity higher than 99% (Table 2). The water content in the AAILs determined by a volumetric Karl Fisher titration (V20 Mettler Toledo) was less than 0.1 wt %.

Figure 1. Structure of AAILs studied in this work. These are [Cnmim][Glu] molecules where Cn denotes R = C4H9, C6H13, and C8H17; Glu denotes a glutamic acid anion.

basic physicochemical properties (densities, refractive indices, specific electrical conductivities) of their aqueous solutions were measured at 298.15 K in the concentration range 0−70 wt % (for [C4mim][Glu] solutions up to 87 wt %). The AAILs aggregation is a phenomenon paid a special attention. The binodal curves and the tie lines were determined for ternary systems [Cnmim][Glu] + water + salt (K2CO3) at a temperature of 298.15 K.

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EXPERIMENTAL METHODS Materials. Table 1 includes the information on the source and purity of chemical reagents used in this work. Commercially available 1-butyl-3-methylimidazolium chloride [C4mim]Cl (98%), 1-hexyl-3-methylimidazolium chloride [C6mim]Cl (97%), and 1-octyl-3-methylimidazolium chloride [C8mim]Cl (97%) were purchased from Sigma-Aldrich Company and used without any additional purification. LGlutamic acid was purchased from AppliChem Panreac ITW Company, and it was used also without any additional purification. All other reagents were of commercially available analytical grade, and they were used as received. Anionexchange resin (type Amberlite IRA-400 chloride form) was purchased from Sigma-Aldrich Company and activated by the regular method before usage. Potassium carbonate K2CO3 of analytical grade (Vekton, Saint-Petersburg, Russia) and bidistilled water were used in the experiments. Synthesis of [Cnmim][Glu]. The molecular structure10−12 of the investigated AAILs is shown in Figure 1. The AAILs based on glutamic acid were prepared by the modified Fukumoto’s method.8,9 First, aqueous 1-alkyl-3methylimidazolium hydroxides ([Cnmim]OH with n = 4, 6,

Table 2. Specification of the Synthesized Compounds molar mass (g·mol )

purification method

purity, mole fraction

[C4mim] [Glu]

285.3

reprecipitation

0.993

[C6mim] [Glu]

313.3

reprecipitation

0.994

[C8mim] [Glu]

341.3

reprecipitation

0.991

chemical name

−1

analysis method 1

H NMR, 13 C NMR 1 H NMR, 13 C NMR 1 H NMR, 13 C NMR

Densities and Refractive Indices Measurements. The densities were measured using Mettler Toledo DM 45 Delta Range densimeter; the standard uncertainty of the data obtained was 0.0005 g·cm−3 at 298.15 K. Each registered value is the average of the results of three measurements. The refractive indices were determined at 298.15 K with standard uncertainty 0.0001 using RL-1 refractometer (Poland). The temperature was regulated by the water thermostat U7 with standard uncertainty 0.05 K. B



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Specific Electrical Conductivities Measurements. The measurements were performed using a PP-50 conductometer (Sartorius) equipped with the conductivity cell Py-CO2. The conductivity measurement is carried out applying an alternating current of the frequency 50 Hz. The cell constant was determined using aqueous solutions of potassium chloride. The cell with the sample was placed into the water bath thermostated with standard uncertainty 0.05 K. Determination of Liquid−Liquid Equilibrium. The binodal curves were determined by the isothermal titration method. The vessel with the solution under study was immersed in the thermostated glass jacket, and the temperature of the system was set at 298.15 K by the Julabo F32 liquid thermostat. The weighted amount of the homogeneous aqueous solution of AAIL of the known concentration was titrated by an aqueous solution of the chosen salt (the concentration of the latter being known) until the mixture became cloudy. The additions of a few drops of water made the mixture clear again. The mass and the composition of the saturated mixture in each of the titration experiments were determined by weight with the standard uncertainty of 0.0001 g, the analytical balance OHAUS PA214C being used. The tie lines were determined in the following experiments. The appropriate weighted amounts of [Cnmim][Glu], K2CO3, and water were mixed in the vessel, which was placed into a thermostated at 298.15 K jacket. The heterogeneous mixture was vigorously stirred for at least 1 h. Further, the mixture was kept at rest until the complete separation of the phases. The samples of the top and bottom phases were taken to determine the content of water using the volumetric Karl Fisher method, the standard uncertainty being 0.4 wt %. The combination of the data on the water content in each of the phases and the data on the binodal curve gives the positions of two points in the triangle diagram relating to the coexisting liquid phases. The binodal curve was fitted by the least-squares regression to the empirical relationship proposed by Merchuk:25 w1 = a exp(bw20.5 − cw2 3)

Figure 2. Density values versus [Cnmim][Glu] concentration in aqueous solutions at 298.15 K: ■, [C4mim][Glu]; ●, [C6mim][Glu]; ▲, [C8mim][Glu].

Figure 3. Refractive index values versus [Cnmim][Glu] concentration in aqueous solutions at 298.15 K: ■, [C4mim][Glu]; ●, [C6mim][Glu]; ▲, [C8mim][Glu].

Table 3. Extrapolated Values of Density (ρ) and Molecular Volume (Vm) of [Cnmim][Glu] (n = 4, 6, 8) at 298.15 Ka [C4mim][Glu] [C6mim][Glu] [C8mim][Glu]

where w1 and w2 are, respectively, the weight fractions of AAILs and K2CO3, and a, b, and c are the parameters obtained by the least-squares data regression procedure (applying the program Origin Lab). The tie line length, TLL, and the slope of the tie line, S, at different compositions were calculated using eqs 2 and 3:26,27 TLL = [(w1t − w1b)2 + (w2t − w2b)2 ]0.5

(2)

S = (w1t − w1b)/(w2t − w2b)

(3)

ρ/g·cm−3

AAIL

(1)

1.215 1.153 1.132

Vm/nm3

1.1933

10

0.390 0.451 0.501

0.397210

Standard uncertainties: u(ρ) = 0.001 g·cm−3, u(T) = 0.05 K, u(Vm) = 0.001 nm3.

a

where wt1, wb1, wt2, and wb2 are the equilibrium mass fractions of the AAIL (1) and the salt (2) in the top (t) and bottom (b) phases.

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RESULTS AND DISCUSSION Densities and Refractive Indices of Aqueous Solutions of [Cnmim][Glu]. The values of the density, ρ, and the refractive index, nD, of aqueous solutions of [C4mim][Glu], [C6mim][Glu], and [C8mim][Glu] with various contents of AAILs (0−70 wt %, for [C4mim][Glu] solutions up to 87 wt %) at temperature 298.15 K are presented in Figures 2, 3, and in Table S1 (see Supporting Information). The intercept of the fitted lines is the value for pure water. The correlation coefficients, R2, of all linear regressions were larger

Figure 4. Specific conductivity values (κ) for aqueous solutions of [C4mim][Glu] (■), [C6mim][Glu] (●), and [C8mim][Glu] (▲) at various AAIL molalities; T = 298.15 K.

C



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Table 4. Values of the Critical Aggregation Concentration (cac) or Critical Micellar Concentration (cmc*) of AAILs in the Aqueous Solutions at 298.15 Ka

a

IL

cac or cmc (mmol·kg−1)

[C4mim][Glu] [C6mim][Glu] [C8mim][Glu]* [C8mim]Cl*

425 375 245 240 (23429)

Standard uncertainties: u(cac or cmc) = 5 mmol kg−1, u(T) = 0.05 K.

Figure 6. Binodal curves data plotted in weight fractions (w) of the solutes for the systems AAIL (1) + K2CO3 (2) + water at 298.15 K: ■, for [C4mim][Glu]; ●, for [C6mim][Glu]; ▲, for [C8mim][Glu], lines, from the fitting of the experimental data by Merchuk’s relationship (eq 1).

Table 6. Values of Parameters of eq 1 and the Correlation Coefficients (R2) for the Systems [Cnmim][Glu] (n = 4, 6, 8) + K2CO3 + Water at T = 298.15 Ka a

b

c

R2

5.61 ± 0.36

1.52 ± 0.07

0.082 ± 0.005

0.9991

4.11 ± 0.35

1.21 ± 0.09

0.136 ± 0.007

0.9992

12.51 ± 1.57

2.27 ± 0.13

0.078 ± 0.009

0.9994

5.98 ± 0.20

1.36 ± 0.04

0.132 ± 0.005

0.9995

IL [C4mim] [Glu] [C6mim] [Glu] [C8mim] [Glu] [C6mim]Cl

Figure 5. Binodal curves plotted in weight fractions (w) of the solutes for the systems AAIL (1) + K2CO3 (2) + water at 298.15 K: ■, ▲, data for [C6mim]Cl from refs 31 and 26, respectively; ▼, ●, data of the present work for [C6mim]Cl and [C6mim][Glu], respectively.

Table 5. The Binodal Curve Data (in Weight Fractions) for the Systems [Cnmim][Glu] (n = 4, 6, 8) + K2CO3 + Water at T = 298.15 K and Ambient Pressure (p = 0.1 MPa)a [C4mim] [Glu]

K2CO3

0.533 0.483 0.437 0.408 0.365 0.324 0.266 0.224 0.187 0.155 0.124 0.083 0.068 0.052 0.035 0.024 0.014 0.009 0.008

0.092 0.104 0.117 0.126 0.137 0.155 0.179 0.199 0.215 0.234 0.256 0.284 0.298 0.319 0.340 0.371 0.404 0.452 0.489

[C6mim] [Glu]

K2CO3

[C8mim] [Glu]

K2CO3

0.524 0.467 0.435 0.389 0.320 0.271 0.239 0.211 0.151 0.081 0.069 0.057 0.037 0.025 0.011 0.006

0.081 0.092 0.102 0.116 0.142 0.163 0.181 0.194 0.226 0.274 0.284 0.295 0.317 0.335 0.366 0.397

0.457 0.416 0.388 0.361 0.328 0.293 0.258 0.214 0.189 0.145 0.123 0.105 0.085 0.055 0.036 0.030

0.127 0.136 0.143 0.150 0.162 0.172 0.186 0.203 0.215 0.237 0.251 0.263 0.278 0.307 0.334 0.353

a

Standard uncertainty: u(T) = 0.05 K.

The experimental density value of the aqueous solution containing 86.31 wt % [C4mim][Glu] is 1.1857 g·cm−3. The linear extrapolation to pure AAIL gives the limiting density value equal to 1.215 g·cm−3. The density value obtained in the present work by the linear extrapolation (Table 3) is in some disagreement with the density value of pure [C4mim][Glu] at 298.15 K, ρ = 1.1933 g·cm−3 presented in literature.10 Taking into account experimental difficulties arising in the density studies for such viscous substances as [C4mim][Glu], one can suppose that the possible reason for the discrepancies between the presented values and the literature data are the errors of extrapolation. We have assumed the linear dependence but at low concentrations of water in water + [Cnmim][Glu] mixture the density may vary in a nonlinear manner.28 The limiting density values for pure [C6mim][Glu] and [C8mim][Glu] at 298.15 K, obtained in the present work by the linear extrapolation to pure AAIL, are 1.153 and 1.132 g·cm−3, respectively (Table 3). The molecular volume of AAILs at 298.15 K, Vm, was calculated from the experimental value of the density of the AAIL under consideration, the following expression being applied:

a

Standard uncertainties: u(w) = 0.001, u(T) = 0.05 K, and u(p) = 10 kPa.

Vm = M /(Nρ)

(4)

where M is the molar mass, N is Avogadro’s constant, and ρ is the density of [Cnmim][Glu] at 298.15 K. The difference between [C4mim][Glu] and [C6mim][Glu] molecular volumes can be considered as the contribution of methylene (−CH2−) groups, which is in good agreement with the estimations presented in literature.10,11 The values of Vm are also presented

than 0.99 and all values of the standard deviation, s, were within the experimental error. Figures 2 and 3 show that the densities and refractive indices of the solutions with the same concentration of AAIL decrease in the following sequence: [C4mim][Glu] > [C6mim][Glu] > [C8mim][Glu]. D



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Table 7. Tie-Line Data for the ABSs Containing [Cnmim][Glu] (n = 4, 6, 8) and K2CO3 at T = 298.15 K and Ambient Pressure (p = 0.1 MPa)b,a total composition [C4mim][Glu]

[C6mim][Glu]

[C8mim][Glu]

a b

top phase

bottom phase

w1

w2

w1

w2

w1

w2

TLL

S

0.176 0.242 0.241 0.265 0.185 0.207 0.225 0.285 0.195 0.194 0.235 0.214

0.265 0.257 0.290 0.294 0.273 0.266 0.260 0.219 0.223 0.231 0.218 0.257

0.303 0.416 0.492 0.525 0.405 0.468 0.504 0.524 0.259 0.299 0.332 0.379

0.164 0.124 0.102 0.094 0.111 0.092 0.086 0.081 0.186 0.169 0.160 0.144

0.020 0.012 0.009 0.008 0.037 0.018 0.011 0.009 0.068 0.051 0.043 0.034

0.388 0.432 0.453 0.473 0.317 0.348 0.366 0.376 0.297 0.316 0.328 0.342

0.360 0.509 0.597 0.642 0.421 0.518 0.567 0.593 0.221 0.288 0.334 0.398

−1.27 −1.31 −1.38 −1.36 −1.78 −1.76 −1.75 −1.76 −1.73 −1.70 −1.73 −1.75

wi, weight fraction of the i-component: (1) [Cnmim][Glu], (2) K2CO3; TLL is the tie-line length (eq 2), and S is the slope of the tie line (eq 3). Standard uncertainties: u(wi) = 0.001, u(T) = 0.05 K, u(p) = 10 kPa.

good agreement with those reported in literature.29,30 The comparison of the cmc values for [C8mim]Cl and [C8mim][Glu] solutions shows the effect of [Glu]− anions on the IL micellization. The higher cmc value of [C8mim][Glu] is a result of the lower electrostatic interactions (attraction) between the IL head groups and [Glu]− counterions in comparison with the chloride anions. The latter anions are smaller, and expectedly they are more tightly bounded with [C8mim]+ cations in the micelle crown. The effect of the hydrocarbon tail length of AAIL on its aggregation in the aqueous solutions is highly pronounced (see Table 4). This effect is well-known and can be explained in terms of the hydrophobic interactions in solutions of amphiphiles. Liquid−Liquid Equilibrium (LLE) Data. The binodal curve and the tie-line data obtained for the system [C6mim]Cl + K2CO3 + H2O are in a satisfactory agreement with the literature data (Figures 5, Table S4 in the SI).26,31 The binodal curves data determined for the ternary systems AAILs + K2CO3 + H2O are presented in Table 5 and in Figures 5 and 6. The data obtained show that the heterogeneous region is noticeably wider for the systems with glutamates than for similar systems with chlorides (Figure 5). The length of the hydrocarbon tail of AAIL weakly affects the heterogeneous region extension (Figure 6). However, this area is slightly wider for the system with [C6mim][Glu]. Pei et al. reported that the phase-forming ability of the ILs with different alkyl chain lengths was not in accordance with the order of their hydrophobicity and [C6mim]+ based ILs had the best phase-forming ability.26 This anomalous behavior of [C6mim]+ has been observed by other researchers32,33 in their studies of the polarities and the melting points of ILs. The values of parameters of Merchuk eq 1 with the corresponding correlation coefficient values (R2) are given in Table 6. The results have indicated that the empirical eq 1 gives a good fit to the data for descriptive purposes. Tie-Line Data. For the most of ABSs based on ILs, the IL concentration in one phase (mostly in the bottom aqueous saltrich phase) is low in comparison with the IL concentration in the second (top, IL-rich) phase. The similar behavior was observed for the ABSs with [Cnmim][Glu] investigated in the present work at 298.15 K. The coexisting phases compositions

Figure 7. Tie lines at 298.15 K for the ABSs containing [Cnmim][Glu]: ■, [C4mim][Glu]; ●, [C6mim][Glu]; ▲, [C8mim][Glu]; wi, weight fraction of the i-component: (1) [Cnmim][Glu], (2) K2CO3.

in Table 3. As one can see the experimental value of the molecular volume of [C4mim][Glu] (Vm = 0.390 nm3) is quite close to the corresponding literature10 value. Conductivity Data. The results of the conductivity measurements for [C 4 mim][Glu], [C 6 mim][Glu], and [C8mim][Glu] aqueous solutions are shown in Figure 4 (and in Table S2 of the SI), where the specific conductivity (κ) in dependence on the AAIL concentration is presented. For the diluted solutions the κ value increases almost linearly with the concentrations of AAIL, but at some certain concentration (denoted by arrow in Figure 4) the slope for the curve “κ versus AAIL concentration” becomes lower. This steep change in the curve slope is due to structural changes (aggregation) in the solution; the supramolecular aggregates start to form intensively after the concentration corresponding to the break point (the so-called critical aggregation concentration, cac). The cac values determined for the [C4mim][Glu] and [C6mim][Glu] solutions (as the intersection points) are presented in Table 4. In case of [C8mim][Glu] the micellar self-organization in the aqueous solutions takes place. The critical micellar concentration (cmc) values obtained for [C8mim]Cl in the present work (see Table 4, Figure S2 and Table S3 in the SI) are in a E



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ACKNOWLEDGMENTS The NMR measurements were carried out at the Center for Magnetic Resonance, St. Petersburg State University.

determined for the systems with [Cnmim][Glu] are presented in Table 7 and in Figure 7. From Figure 7 one can see that the slope of the tie lines increases in case of AAILs with the longer alkyl chain. By this the salt content in the salt-rich phase decreases or the [Cnmim][Glu] concentration in the IL-rich phase increases, whereas in the salt-rich phase, it slightly decreases. Neves et al.34 have observed that an increase in the cation alkyl chain length of the IL results in a higher ability for the phase separation due to the increase of the fluids overall hydrophobicity and their lower affinity for water. The presented results are in an agreement with the recent LLE data for systems containing [Cnmim]Cl (n = 1−6).35 In the report35 describing ABS formation in systems with [Cnmim]Cl, where n = 8, 10, 12 and 14, it has been shown that the homogeneous region increases as the alkyl side-chain length increases.

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CONCLUSIONS The data obtained have shown that the density and refractive index for [C8mim][Glu] solutions are lower than those for solutions of [C4mim][Glu] and [C6mim][Glu]. The LLE data demonstrate that the ABS-forming ability of the [Cnmim][Glu] is higher than in case of similar systems with chlorides. The length of the hydrocarbon tail of AAIL weakly affects the extension of the heterogeneous region. However, this area is slightly wider for the systems with [C6mim][Glu]. The results of the electrical conductivity study clearly show that the general features of micellar aggregation of relatively long-chain imidazolium IL are analogous to those in the case of traditional surfactants. The concentration dependences of the specific conductivities of aqueous [Cnmim][Glu] solutions indicate that in the solutions the aggregation takes place. This tendency is the most pronounced for AAIL’s with molecules having longer alkyl chains (in our case at n = 8). Further studies of the structure of aggregates would be of interest. The present study has clarified some features of the aggregation behavior of [Cnmim][Glu] (n = 4, 6, 8) in dilute aqueous solutions, what is of interest for colloid science. The results obtained can be helpful for potential applications of [Cnmim][Glu] with various n as new surfactants. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b00948. 1 H NMR and 13C NMR spectra of the synthesized AAILs and the data on the densities, refractive indices and specific conductivities of [Cnmim][Glu] (n = 4, 6, 8) aqueous solutions at 298.15 K (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Address: Saint Petersburg State University, Institute of Chemistry, University pr., 26, St. Petersburg, 198504, Russian Federation. Tel.: +7(812) 4284065. Fax: +7(812) 4286939. Email: [email protected]. Funding

This work was supported by Saint Petersburg State University (grant 12.50.1192.2014) and by the Russian Foundation for Basic Research (grants 13-03-00981, 16-03-00723). Notes

The authors declare no competing financial interest. F



Article

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