Densities, Sound Speed, and UV Absorption Studies of Trisodium

Nov 26, 2014 - Department of Chemistry, Dr B R Ambedkar National Institute of Technology, Jalandhar, 144011 Punjab India. •S Supporting Information...
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Densities, Sound Speed, and UV Absorption Studies of Trisodium Citrate and Tripotassium Citrate in Aqueous Solutions of 1‑Hexyl-3methylimidazolium Chloride [C6mim][Cl] Harsh Kumar* and Chanda Chadha Department of Chemistry, Dr B R Ambedkar National Institute of Technology, Jalandhar, 144011 Punjab India S Supporting Information *

ABSTRACT: Densities, ρ, and speeds of sound, u, for trisodium citrate and tripotassium citrate in (0.01 and 0.03) mol·kg−1 in aqueous solutions of 1-hexyl-3-methylimidazolium chloride ([C6mim][Cl]) over a range of temperatures T = (288.15, 293.15, 298.15, 303.15, 308.15) K have been measured at atmospheric pressure. The experimental densities are used to calculate the apparent molar volume, limiting apparent molar volumes, and transfer volumes. Experimental values of the speed of sound were used to estimate apparent molar isentropic compression, limiting apparent molar isentropic compression, and transfer parameter. The structure making or breaking ability of 1-hexyl-3-methylimidazolium chloride has been discussed in terms of the sign of (∂2V0ϕ/∂T2). The UV absorption studies have also been made in this study. The results have been discussed in terms of competing patterns of interactions prevailing in the solute and solvent, that is, citrate salt + [C6mim][Cl] + water mixtures.

1. INTRODUCTION

It is well-known that both trisodium citrate and tripotassium citrate find uses in the medicinal field. The citrate ions of trisodium citrate bind the calcium ions in the blood through the formation of calcium citrate complexes and act as anticoagulant. Tripotassium citrate can be used to control kidney stones derived from either uric acid or cystine. Both these citrate salts are also used in foods and beverages and also find technical applications as buffering and emulsifying agent. Many studies have been done on the thermodynamic, viscometric, two-phase equilibria, surface phenomenon etc. of binary mixtures of ionic liquids and of ionic liquids with water and aqueous solutions of various additives3−18 and also of citrate salts with additives,19−25 but studies on mixtures of ionic liquids with organic salts of biological importance like citrate salts have not been made. As a part of our ongoing research program on aqueous solutions of citrate salts, in the present study we have shifted our focus on aqueous citrate salt-ionic liquid mixtures. Studies on volumetric and acoustical properties of aqueous ionic liquidsalt solutions provide useful information on the interactions,

Room temperature ionic liquids are salts made up of organic cations and smaller inorganic or organic anions that are fluid around or below 373.15 K. The main aim of considering ionic liquids is to replace the volatile organic compounds (VOCs) with nonvolatile ionic liquids in industrial applications to prevent the emission of VOCs, which are the major source of environmental pollution. Some ILs are toxic but they can be designed to be environmentally friendly with a large number of benefits for green chemistry.1 Furthermore, ILs are known as “designer solvents” since their properties can be adjusted by the choice of the cation and/or the anion.2 Ionic liquids have certain unique properties such as low melting temperature, negligible vapor pressure, and nonflammability, and they pollute less. All these properties make them green solvents. Ionic liquids are gaining great attention because of many physicochemical properties that can be modified by the addition of a cosolvent allowing a wide range of application. Therefore, investigation of thermodynamic properties, mixing behavior, and various interactions in the binary mixtures of room temperature ILs in various organic solvents are of immense importance. © XXXX American Chemical Society

Received: July 23, 2014 Accepted: November 16, 2014

A

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m−3 and ± 5·10−1 m·s−1, respectively. The UV absorption values were also recorded for mixtures to analyze solute− solvent interactions using a UV spectrophotometer (Eppendorf Biospectrometer Kinetic, USA).

association, and solvation behavior of ions in these solutions. Therefore, in this work, we studied volumetric, acoustic, and spectroscopic behavior of trisodium citrate and tripotassium citrate in (0.0, 0.01, and 0.03) mol·kg−1 aqueous solutions of 1hexyl-3-methylimidazolium chloride [C6mim][Cl] at temperatures T = (288.15, 293.15, 298.15, 303.15 and 308.15) K. As per our knowledge no studies on densities, speeds of sound of citrate salt with 1-hexyl-3-methylimidazolium chloride have been reported so far. The effect of temperature and concentration on the properties of citrate salts in aqueous [C6mim][Cl] has been studied.

3. RESULTS AND DISCUSSION The densities and speeds of sound of trisodium citrate and tripotassium citrate in binary aqueous solutions of [C6mim][Cl] were measured in order to provide important information about solute−solute and solute−solvent interactions occurring in these type of mixtures at different temperatures. 3.1. Density Measurements. Experimental densities, ρ, of trisodium citrate and tripotassium citrate in (0.0, 0.01 and 0.03) mol·kg−1 aqueous solutions of [C6mim][Cl] at temperatures T = (288.15, 293.15, 298.15, 303.15 and 308.15) K are reported in Table 2. The experimental density values for trisodium citrate + water and tripotassium citrate + water mixtures have been compared with literature values.19−22 The values are graphically represented in Figures S1 and S2 of Supporting Information file and are found to be in good agreement with literature values. The density values have also been analyzed by calculating average absolute deviations (AAD), bias, and maximum deviation (MD) values. The AAD have been calculated using the formula (1/nΣ|x1 − x|), ̅ that is, the average of the absolute values of difference of the density or speeds of sound values xi from their mean density or speeds of sound values x.̅ The bias values have been calculated by taking the difference of the mean of the experimental density or speeds of sound values and literature density or speeds of sound values, whereas MD values have been determined by taking the difference of the maximum and minimum values of the density and speeds of sound. These values are reported in Table 3. It is observed from Table 3 that for the mixtures of trisodium citrate + water, the bias of −0.03624 and −0.01945 is obtained at T = 298.15 K with reference to densities values reported in citations 19 and 20, respectively. In the case of tripotassium citrate + water mixtures the positive bias of 0.031687, 0.031223, 0.030695, 0.030226, and 0.029629 is obtained at T = (288.15, 293.15, 298.15, 303.15 and 308.15) K with reference to density values reported in citation 21. Also for tripotassium citrate + water mixtures the negative bias of −0.02636, −0.02661, −0.02661, and −0.02734 is obtained at T = (293.15, 298.15, 303.15 and 308.15) K with reference to densities values reported in citation 22. Average absolute deviations in the case of experimental densities for aqueous solutions of trisodium citrate are in the range of 0.04194 to 0.04410 while for literature data19,20 at T = 298.15 K the values are 0.07765 and 0.06499. Further, for aqueous tripotassium solutions the average absolute deviations are in the range 0.04727 to 0.04928 for experimental densities and for literature densities of refs 21 and 22 the values are in the range 0.03455 to 0.03529 and 0.07651 to 0.07736, respectively. Maximum deviations for aqueous solutions of both citrate salts have also been calculated for experimental as well as literature densities. The values obtained are in the range 0.13924 to 0.14648 for experimental densities of trisodium citrate + water mixtures. The corresponding values of maximum deviations for literature densities19,20 at T = 298.15 K are 0.27214 and 0.22879, respectively. Further, for aqueous tripotassium solutions the maximum deviations are in the range 0.0.15632 to 0.16279 for experimental densities and for literature densities21,22 the values are in the range 0.12615 to 0.12875 and 0.26174 to 0.26447. From the data reported in Table 2, it is observed that the

2. EXPERIMENTAL SECTION 2.1. Materials. 1-Hexyl-3-methylimidazolium chloride [C6mim][Cl], with mass fraction purity ≥ 0.985 was obtained from Sigma-Aldrich, USA. Tripotassium citrate and trisodium citrate with mass fraction purities > 0.99 were obtained from SD Fine Chem Ltd., India. The ionic liquid [C6mim][Cl] was dried under high vacuum at 343.15 K overnight to remove the moisture content. The water content was found to be less than 350 ppm upon Karl Fischer analysis. All the chemicals were vacuum dried and stored in desiccators over P2O5 for at least 2 days before their use. The specifications of the chemicals used in this study are also given in Table 1. Table 1. Specification of Chemicals chemical name 1-hexyl-3methylimidazolium chloride trisodium citrate tripotassium citrate

source

purification method

mass fraction purity

Sigma-Aldrich

vacuum drying

0.985

SD Fine Chem. Ltd. India SD Fine Chem. Ltd. India

vacuum drying vacuum drying

> 0.99 > 0.99

2.2. Apparatus and Procedures. Freshly prepared triply distilled and degassed water (specific conductance < 10−6 S· cm−1) was used for the preparation of solutions. The solutions were prepared by weighing on a Sartorius CPA 225 D balance having a precision of ± 0.00001 g. The uncertainties in the molality of solutions are within ± 2·10−5 mol·kg−1. Density measurements were made on an Anton Paar DSA 5000 M densimeter. The speed of sound is measured using a propagation time technique. The sample is sandwiched between two piezoelectric ultrasound transducers. One transducer emits sound waves through the sample-filled cavity at a frequency of approximately 3 MHz; the second transducer receives the waves.26 Thus, the speed of sound is obtained by dividing the known distance between transmitter and receiver by the measured propagation time of the sound wave. A density check or an air/water adjustment was performed at 293.15 K with triply distilled, degassed water, and with dry air at atmospheric pressure. Before each series of measurements, the densimeter was calibrated with triple distilled and degassed water, in the experimental temperature range. The density and speeds of sound values are extremely sensitive to temperature, so it was controlled to ± 1·10−3 K by a built-in Peltier device. The sensitivity of the instrument corresponds to a precision in density and speeds of sound measurements of 1·10−2 kg·m−3 and 1·10−1 m·s−1. The standard uncertainty of the density and speeds of sound estimates was found to be within ± 5·10−2 kg· B

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Table 2. Densities ρ and Apparent Molar Volumes Vϕ of Trisodium Citrate and Tripotassium Citrate in Aqueous Solutions of [C6mim][Cl] at Different Temperatures and Experimental Pressure p = 0.1 MPa ρ·10−3/(kg·m−3) mA mol·kg

−1a

Vϕ·106/(m3·mol−1)

T/K

T/K

T/K

T/K

T/K

T/K

T/K

T/K

T/K

T/K

288.15

293.15

298.15

303.15

308.15

288.15

293.15

298.15

303.15

308.15

105.69 105.97 106.45 107.08 109.77 112.08 114.24 116.36 118.79 121.01 122.69 124.74 126.93

107.76 108.53 108.81 109.55 111.83 114.16 116.34 118.51 120.75 122.86 124.72 126.72 128.98

109.40 110.28 110.56 111.36 113.53 115.65 117.79 120.02 122.31 124.35 126.11 128.24 130.60

111.40 111.99 112.35 113.20 115.27 117.41 119.56 121.67 123.89 126.09 127.96 129.91 132.25

107.35 107.88 108.30 108.64 110.66 112.68 114.53 116.36 118.10 120.14 121.89 123.99 125.65

109.17 109.61 109.97 110.45 112.39 114.20 116.05 117.91 119.59 121.45 123.21 125.49 127.15

110.35 110.87 111.30 111.77 113.68 115.42 117.41 119.22 120.97 122.63 124.39 126.52 128.38

111.85 112.30 112.82 113.20 115.05 116.79 118.66 120.41 122.14 124.02 125.79 127.69 129.49

109.78 110.48 110.99 111.66 113.72 115.78 117.83 120.13 122.05 124.12 125.86 128.23 130.14

112.29 112.88 113.36 113.80 115.87 117.96 119.96 122.35 124.29 126.37 128.12 130.36 132.29

113.87 114.65 114.98 115.70 117.64 119.76 121.78 123.90 125.76 127.85 129.69 132.08 133.81

115.65 116.50 116.92 117.50 119.55 121.49 123.67 125.74 127.54 129.62 131.38 133.69 135.39

117.08 117.99 118.44 119.26 121.04 123.16 124.84 126.66 127.98 130.07 131.85 133.53 135.47

119.57 120.22 120.64 121.12 122.80 124.84 126.55 128.45 129.84 131.80 133.64 135.26 137.17

121.07 121.61 122.09 122.50 124.62 126.35 128.26 129.90 131.21 133.27 135.25 136.88 138.85

123.30 123.54 123.99 124.53 126.23 128.18 129.98 131.79 133.28 135.29 136.96 138.55 140.30

0.01959 0.04954 0.06743 0.09908 0.19983 0.29918 0.39514 0.48759 0.59704 0.69512 0.78612 0.88436 0.99939

1.002846 1.008487 1.011813 1.017640 1.035594 1.052468 1.068031 1.082255 1.098255 1.111625 1.123747 1.135935 1.149327

1.001892 1.007475 1.010764 1.016533 1.034231 1.050868 1.066171 1.080177 1.095905 1.109148 1.121059 1.133063 1.146434

0.02120 0.04992 0.07188 0.09994 0.19994 0.29742 0.40087 0.49740 0.59183 0.69326 0.79125 0.90310 1.00250

1.003894 1.009237 1.013272 1.018372 1.036051 1.052565 1.069286 1.084138 1.098054 1.112225 1.125250 1.139272 1.151076

1.002955 1.008249 1.012253 1.017335 1.034877 1.051199 1.067811 1.082552 1.096339 1.110312 1.123254 1.137135 1.148963

0.02052 0.04988 0.06975 0.10121 0.20079 0.29924 0.40117 0.50683 0.60004 0.70061 0.78538 0.89874 0.99230

1.004000 1.009414 1.013029 1.018710 1.036116 1.052541 1.068735 1.084563 1.097912 1.111438 1.122377 1.136091 1.146722

1.003019 1.008352 1.011916 1.017499 1.034646 1.050809 1.066755 1.082341 1.095423 1.108794 1.119475 1.132906 1.143326

0.02026 0.04910 0.06926 0.09937 0.19724 0.29804 0.39472 0.49947 0.57886 0.69593 0.79601 0.88725 0.99316

1.003334 1.009283 1.013385 1.019455 1.038677 1.057592 1.074963 1.092896 1.105980 1.124407 1.139132 1.152050 1.166127

1.002398 1.008280 1.012350 1.018344 1.037362 1.056030 1.073241 1.091055 1.104022 1.122178 1.136880 1.149581 1.163554

Trisodium Citrate + 0.0 mol·kg−1 [C6mim][Cl] 1.000685 0.999260 0.997602 102.92 1.006180 1.004698 1.002990 103.77 1.009435 1.007919 1.006175 104.31 1.015115 1.013535 1.011725 105.06 1.032635 1.030885 1.028900 107.53 1.049051 1.047186 1.045014 109.83 1.064141 1.062141 1.059787 111.91 1.077922 1.075751 1.073290 114.05 1.093520 1.091150 1.088543 116.45 1.106644 1.104176 1.101303 118.83 1.118249 1.115718 1.112610 120.50 1.130100 1.127321 1.124190 122.58 1.143182 1.140137 1.136843 124.99 Trisodium Citrate + 0.01 mol·kg−1 [C6mim][Cl] 1.001749 1.000317 0.998665 105.79 1.006993 1.005520 1.003825 106.27 1.010961 1.009454 1.007719 106.79 1.015979 1.014433 1.012661 107.39 1.033346 1.031663 1.029752 109.39 1.049552 1.047755 1.045700 111.20 1.065998 1.064009 1.061853 113.18 1.080573 1.078473 1.076221 115.07 1.094245 1.091975 1.089616 116.80 1.108183 1.105911 1.103283 118.75 1.120992 1.118611 1.115838 120.58 1.134571 1.132188 1.129475 122.69 1.146242 1.143564 1.140803 124.49 Trisodium Citrate + 0.03 mol·kg−1 [C6mim][Cl] 1.001771 1.000308 0.998651 106.96 1.007034 1.005512 1.003797 107.76 1.010551 1.009002 1.007242 108.33 1.016080 1.014450 1.012639 108.89 1.033002 1.031201 1.029181 111.09 1.048934 1.046941 1.044777 113.17 1.064671 1.062477 1.060071 115.29 1.079979 1.077730 1.075141 117.66 1.092834 1.090486 1.087763 119.53 1.105975 1.103469 1.100577 121.74 1.116463 1.113766 1.110784 123.42 1.129750 1.126749 1.123669 125.77 1.139963 1.137012 1.133819 127.70 Tripotassium Citrate + 0.0 mol·kg−1 [C6mim][Cl] 1.001182 0.999758 0.998094 115.35 1.007002 1.005537 1.003820 115.96 1.011026 1.009528 1.007771 116.57 1.016985 1.015445 1.013615 117.21 1.035830 1.034057 1.032102 119.03 1.054336 1.052462 1.050265 121.04 1.071362 1.069255 1.066915 122.86 1.088945 1.086780 1.084168 124.87 1.101723 1.099481 1.096612 126.25 1.119746 1.117270 1.114182 128.25 1.134217 1.131481 1.128445 130.23 1.146808 1.143923 1.140757 131.84 1.160643 1.157519 1.154417 133.86

C

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Table 2. continued ρ·10−3/(kg·m−3)

Vϕ·106/(m3·mol−1)

mA

T/K

T/K

T/K

T/K

T/K

T/K

T/K

T/K

T/K

T/K

mol·kg−1a

288.15

293.15

298.15

303.15

308.15

288.15

293.15

298.15

303.15

308.15

120.85 121.35 121.80 122.09 123.65 125.23 126.80 128.20 129.76 131.46 133.00 134.78

122.36 122.90 123.31 123.67 125.23 126.77 128.30 129.80 131.29 133.01 134.59 136.35

123.47 123.95 124.34 124.75 126.27 127.89 129.41 130.82 132.28 133.99 135.50 137.25

124.88 125.37 125.73 125.95 127.46 129.10 130.62 132.00 133.41 135.18 136.70 138.50

121.54 122.10 122.37 122.90 124.55 126.25 128.12 129.96 131.75 133.47 135.31 137.19 138.97

123.20 123.85 124.26 124.68 126.53 128.19 130.11 131.89 133.78 135.45 137.31 139.09 140.85

124.65 125.25 125.56 126.00 127.87 129.60 131.32 133.38 135.09 136.90 138.75 140.62 142.45

125.99 126.69 127.01 127.62 129.39 131.20 133.08 134.85 136.63 138.39 140.30 142.10 143.89

0.02128 0.05144 0.07903 0.10059 0.29917 0.39891 0.49366 0.58796 0.69758 0.80003 0.91320 0.99282

1.004264 1.010372 1.015899 1.020173 1.057798 1.075537 1.091667 1.107097 1.124143 1.139339 1.155277 1.165874

1.003326 1.009383 1.014861 1.019104 1.056425 1.074019 1.090080 1.105314 1.122274 1.137389 1.153172 1.163674

0.01981 0.05025 0.06898 0.09907 0.19782 0.29554 0.39831 0.49835 0.59973 0.69505 0.79627 0.89399 0.98838

1.004218 1.010376 1.014122 1.020077 1.039086 1.057133 1.075266 1.092054 1.108216 1.122747 1.137316 1.150739 1.162937

1.003254 1.009347 1.013062 1.018964 1.037822 1.055690 1.073618 1.090239 1.106308 1.120676 1.135154 1.148315 1.160379

Tripotassium Citrate +0.01 mol·kg−1 [C6mim][Cl] 1.002126 1.000695 0.999045 119.33 1.008133 1.006668 1.004972 119.79 1.013568 1.012073 1.010336 120.17 1.017770 1.016245 1.014495 120.53 1.054770 1.053000 1.050989 122.13 1.072216 1.070331 1.068190 123.75 1.088077 1.086120 1.083875 125.35 1.103194 1.101159 1.098828 126.89 1.119961 1.117823 1.115317 128.35 1.134887 1.132700 1.130061 130.15 1.150510 1.148228 1.145421 131.76 1.161078 1.158570 1.155863 133.53 Tripotassium Citrate + 0.03 mol·kg−1 [C6mim][Cl] 1.002025 1.000565 0.998918 119.50 1.008060 1.006556 1.004859 120.05 1.011731 1.010205 1.008479 120.45 1.017584 1.016014 1.014224 121.06 1.036216 1.034501 1.032561 122.91 1.053893 1.052019 1.049897 124.58 1.071592 1.069643 1.067286 126.39 1.088036 1.085819 1.083422 128.26 1.103842 1.101579 1.098988 130.19 1.118040 1.115557 1.112852 131.89 1.132302 1.129682 1.126784 133.83 1.145361 1.142520 1.139535 135.58 1.157275 1.154221 1.151148 137.37

mA is the molality of citrate salts in aqueous [C6mim][Cl] solutions. Standard uncertainties u (ρ) in densities measurements are ± 5·10−2 kg·m−3; standard uncertainties u(m) in molality are ± 2·10−5 mol·kg−1; standard uncertainties u(T) in temperature are ± 0.001 K.

a

Table 3. Analysis of Experimental (expt) and Theoretical (theor) Density Values for Aqueous Solutions of Trisodium Citrate and Tripotassium Citrate AAD/(kg·m−3) T/K

theor

MD/(kg·m−3) −3

expt

bias/(kg·m )

theor

expt

Trisodium Citrate + Water 288.15 293.15 298.15

0.0776519 0.0649920

303.15 308.15 288.15 293.15 298.15

303.15 308.15 a

0.04410 0.04347 0.04289

−0.03624a

−0.01945b

0.2721419 0.2287920

0.04244 0.04194 0.0352921 0.0350521 0.0773622 0.0736320 0.0348621 0.0770222 0.0346921 0.0767822 0.0345521 0.0765122

0.04928 0.04879

0.14648 0.14454 0.14249 0.14088 0.13924

Tripotassium Citrate + Water 0.03169c 0.03122c

−0.02636

d

0.04826

0.03069c

−0.02661d

0.04779

0.03027c

−0.02696d

0.04727

0.02963c

−0.02734d

0.1287521 0.1278621 0.2644722

0.16279 0.16116

0.1272421 0.2633722 0.1266521 0.2627822 0.1261521 0.2617422

0.15946 0.15776 0.15632

From ref 19. bFrom ref 20. cFrom ref 21. dFrom ref 22.

magnitude of average absolute deviation, bias and maximum deviations decrease with an increase in temperature for both citrate salts. It is also observed that the values of AAD and MD

obtained from experimental densities are less than the AAD and MD values obtained from literature densities in the case of trisodium citrate + water mixtures. On the other hand for D

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Figure 1. Plots of apparent molar volume Vϕ for trisodium citrate in (a) 0.01 mol·kg−1 aqueous [C6mim][Cl] and (b) 0.03 mol·kg−1 aqueous [C6mim][Cl] and for tripotassium citrate in (c) 0.01 mol·kg−1 aqueous [C6mim][Cl] and (d) 0.03 mol·kg−1 aqueous [C6mim][Cl] solutions at different temperatures: ⧫, 288.15 K; ■, 293.15 K; ●, 298.15 K; ▲, 303.15 K; +, 308.15 K].

Table 4. Limiting Apparent Molar Volumes V0ϕ and Experimental Slopes Sv* of Trisodium Citrate and Tripotassium Citrate in Aqueous Solutions of [C6mim][Cl] at Different Temperatures V0ϕ·106/(m3·mol−1)

SV*·106/(m3·mol−2·kg)

mB

T/K

T/K

T/K

T/K

T/K

T/K

T/K

T/K

T/K

T/K

mol·kg−1a

288.15

293.15

298.15

303.15

308.15

288.15

293.15

298.15

303.15

308.15

22.37 (±0.16) 19.11 (±0.07) 21.23 (±0.09)

22.29 (±0.25) 18.85 (±0.08) 20.86 (±0.09)

21.90 (±0.19) 18.46 (±0.05) 20.68 (±0.06)

21.63 (±0.13) 18.34 (±0.06) 20.48 (±0.07)

21.47 (±0.10) 18.04 (±0.04) 20.23 (±0.06)

18.84 (±0.14) 16.01 (±0.02) 18.37 (±0.05)

18.48 (±0.22) 15.63 (±0.03) 17.93 (±0.1)

17.99 (±0.08) 15.59 (±0.03) 18.10 (±0.04)

18.15 (±0.12) 15.46 (±0.04) 18.28 (±0.08)

17.77 (±0.10) 15.24 (±0.05) 18.29 (±0.05)

Trisodium Citrate 0.00 0.01 0.03

0.00 0.01 0.03 a

102.90 (±0.09) 105.46 (±0.04) 106.76 (±0.01)

105.19 (±0.14) 106.94 (±0.04) 109.49 (±0.02)

107.50 (±0.10) 108.68 (±0.03) 111.81 (±0.02)

115.23 (±0.07) 118.95 (±0.01) 119.17 (±0.03)

117.25 (±0.01) 120.54 (±0.02) 121.08 (±0.05)

119.35 (±0.04) 122.09 (±0.02) 122.91 (±0.02)

109.19 111.01 (±0.07) (±0.05) 109.99 111.45 (±0.03) (±0.02) 113.56 115.45 (±0.02) (±0.03) Tripotassium Citrate 120.83 122.83 (±0.06) (±0.05) 123.18 124.51 (±0.02) (±0.03) 124.23 125.74 (±0.04) (±0.03)

mB is the molality of aqueous [C6mim][Cl] solutions.

(mixture of water + [C6mim][Cl]), M is the molar mass of the solute (kg·mol−1), ρ0 and ρ are the densities (kg·m−3) of the solvent and solution. The values of Vϕ along with densities are reported in Table 2. The values of Vϕ for trisodium citrate and tripotassium citrate in (0.01 and 0.03) mol·kg−1 aqueous solutions of [C6mim][Cl] are also represented graphically in Figure 1. The values of Vϕ increase with increase in temperature and concentration of ionic liquid [C6mim][Cl]. Furthermore, the values of Vϕ also increase with an increase in the molar mass of the citrate salt, that is, higher values of Vϕ are obtained for tripotassium citrate as compared to trisodium citrate. This may occur because the increase in molar mass

tripotassium citrate + water mixtures the values of AAD and MD obtained from experimental densities are more than AAD and MD values obtained from densities reported in ref 21 and less than the AAD and MD values obtained from densities reported in ref 22. 3.1.1. Apparent Molar Volume. These values of densities were used to calculate apparent molar volumes Vϕ using the following equation: Vϕ = M /ρ − (ρ − ρ0 )/mA ρρ0

(1)

where mA is the molality (mol·kg−1) of the citrate salts, that is, amount of solute (citrate salt) per one kilogram of solvent E

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interactions in solutions of citrate salts in [C6mim][Cl]. Though not a very regular trend has been observed, this clearly indicates that several effects35 influence the solute− solute interactions. The partial molar volume of transfer ΔV0ϕ of each citrate salt from water to aqueous [C6mim][Cl] solutions at infinite dilution calculated by using the eq E1 are reported in Table S1 of the Supporting Information. The calculated values of ΔV0ϕ are reported in Table S1 at different temperatures for citrate salts in aqueous [C6mim][Cl] solutions. The results may be explained by the cosphere overlap model.31,32 This model predicts that, for polar species, the volume of water molecules is smaller in the solvation sphere due to the effect of electrostriction and the decrease of hydrogen-bonded network with water molecules in the solvate spheres being transferred to the bulk solution. Because of this, the overlap of hydrophilic hydration cospheres releases some water molecules to the bulk solution that give rise to a positive change in the volume. Moreover, the positive transfer volumes according to the cosphere overlap model are due to solute− solvent interactions that in the present study are between citrate salts and [C6mim][Cl]. These interactions can be classified as (i) an hydrophilic−ionic group interaction between the −Cl¯ group of [C6mim][Cl] and Na+ and K+ ions of citrate salts, (ii) interaction of K+ ion with the N atom in the heterocyclic ring of [C6mim][Cl], (iii) hydrophilic−hydrophobic group interactions between the polar headgroup of citrate salts and the nonpolar group of [C6mim][Cl], or (iv) hydrophobic−hydrophobic group interactions between the nonpolar group of citrate salt and nonpolar group of [C6mim][Cl]. According to the cosphere model, hydrophilic−hydrophobic group interactions contribute negatively, whereas hydrophilic-ionic group interactions contribute positively to the ΔV0ϕ values. The ΔV0ϕ values are all positive and decrease with increase in temperature. The addition of citrate salts to aqueous [C6mim][Cl] solution will coordinate the hydration sphere of the Na+ and K+ with those of the chloride (Cl¯) ions and those of Cit3− ions with the hydration spheres of the 1-hexyl-3-methylimidazolium ([C6mim]+) ions. So the water molecules are allowed to relax to the bulk state, and they account for the positive transfer volume of citrate salts thereby leading to a reduction in the electrostriction of the solvent. The positive partial molar volume at infinite dilution in salts of citrate mixed solvent suggests that the ion−hydrophilic and hydrophilic−hydrophilic group interactions are predominant over hydrophilic−hydrophobic group interactions. It is observed from Supporting Information, Table S1 that the magnitude of ΔV0ϕ increases from a lower concentration of [C6mim][Cl] to a higher concentration of [C6mim][Cl]. The explanation of this observation lies in the fact that ions from [C6mim][Cl] interact with salts and electrostriction is reduced which leads to the positive values of ΔV0ϕ. The values of ΔV0ϕ decrease with an increase in temperature. The possible reason for a decrease in ΔV0ϕ with an increase in temperature is that Vϕ of citrate salt is increased in some water and aqueous solutions of [C6mim][Cl] with increasing temperature, but in the aqueous solution of [C6mim][Cl] the hydrophobic hydration gives more negative volume contribution to the whole system than in water, so it induces the V0ϕ of citrate salt to increase more in water than in the aqueous solutions of [C6mim][Cl]. This leads to a decrease in ΔV0ϕ values with the increase in temperature. The same behavior has also been observed for tripotassium phosphate from water to aqueous [C2mim][Br] solutions.6

causes greater affinity for the solvent and therefore enhances greater ion−solvent interactions. The higher values in the case of tripotassium citrate are because potassium is placed above sodium in the Hofmeister series.27,28 As we move from sodium to potassium in the Hofmeister series, the kosmotropic nature or structure making ability of the cation increases, therefore the solute−solvent interaction increases, which is further responsible for the higher values of the apparent molar volume in the case of tripotassium citrate as compared to trisodium citrate. 3.1.2. Partial Molar Volume. Partial molar volume V0ϕ, which is the limiting value of apparent molar volume, is calculated by least-squares fitting of the apparent molar volume Vϕ by the following equation: Vϕ = V ϕ0 + SV*mA

(2)

where SV* the experimental slope is the volumetric pair-wise interaction coefficient and mA is the molality of the citrate salt in aqueous [C6mim][Cl] solutions. The values of V0ϕ and S*V together with standard errors derived by least-squares fitting of the Vϕ values to eq 2 are reported in Table 4. The positive V0ϕ values for citrate salts in aqueous [C6mim][Cl] solutions indicate the presence of solute−solvent interactions, and these values increase with an increase in the ionic liquid concentration and temperature. The V0ϕ values for citrate salts in aqueous [C6mim][Cl] solutions also increase with an increase in the molality of [C6mim][Cl] solutions. Valuable information regarding solute−solvent interactions may be obtained from the temperature dependence of a standard partial molar property because solute−solute interactions are negligible at infinite dilution.29,30 At infinite dilution, solvent molecules surround each ion which are infinitely distant with other ions; it follows therefore that V0ϕ is free from ion−ion interaction thereby showing the presence of strong ion−solvent interactions. According to the cosphere overlap model,31,32 an overlap of cospheres of two ionic species causes an increase in volume, whereas an overlap of hydrophobic−hydrophobic groups and ion-hydrophobic groups results in a decrease in volume. The observed positive V0ϕ values are due to ion− hydrophilic interactions, that dominate over ion−hydrophobic interactions and hydrophobic−hydrophobic interactions. The increase in V0ϕ for citrate salts in aqueous [C6mim][Cl] solutions may be attributed to the increase in solvation of citrate salts at higher temperature as well as at higher region of salt, that is, release of some solvent molecules from loose salvation layers of the solute in solution. Further, the increase in V0ϕ values with an increase in the concentration of salt is due to strong attractive interactions due to the hydration of ions. Partial molal volumes are known to be sensitive to solute salvation; it provides information about structural volume of solute in solvent and volume change of solvent in the process of shell formation around the ion.33,34 In the present case, V0ϕ increases by increasing molar mass, that is, tripotassium citrate having a higher molar mass among both citrate salts has the highest value of V0ϕ. In general V0ϕ values increase with an increase in molar mass of citrate salts; that is, the values are higher for tripotassium citrate as compared to those for trisodium citrate because there is more hydration of ions in tripotassium citrate as compared to trisodium citrate, causing tripotassium citrate to undergo greater interactions. From Table 4, it is also observed that the magnitude of S*V is positive for all concentrations of [C6mim][Cl] at all temperatures. The positive values of SV* indicate the presence of solute−solute F

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3.1.3. Temperature-Dependent Partial Molar Volume. Further, the variation of apparent molar volumes at infinite dilution V0ϕ with the temperature can be expressed by the general polynomial equation as follows V ϕ0 = a + b(T − Tref ) + c(T − Tref )2

decrease with an increase in temperature for all the mixtures. The positive values of ϕ0E may occur due to phenomenon of packing effect or caging,36,37 which further suggests interactions between citrate salts and [C6mim][Cl] molecules. The values of ϕ0E increase with an increase in the molality of [C6mim][Cl] aqueous solutions. Further, the temperature derivative of limiting apparent molar expansivity was calculated using the relation

(3)

where T is temperature in Kelvin. Tref = 298.15 K. a, b, and c are empirical constants. The values of these constants for citrate salts in aqueous [C6mim][Cl] solutions along with standard deviation values are reported in Table 5.

(∂ϕE0 /∂T)p = (∂ 2V ϕ0 /∂T 2)p = 2c

The values of temperature derivative of partial molar expansivity (∂ϕ0E/∂T)p as reported in Table 5 are negative for all mixtures. The sign of (∂ϕ0E/∂T)p predicts38,39 whether the particular solute when dissolved in a solvent will act as a structure maker or a structure breaker. The positive and small negative (∂ϕ0E/∂T)p values are observed for solutes having structure making capacity, whereas negative (∂ϕ0E/∂T)p values are for structure breaking solutes. The small negative values of (∂ϕ0E/∂T)p as reported in Table 5 show the structure-making ability of citrate salts in aqueous [C6mim][Cl] solutions. 3.2. Speed of Sound Measurements. The speed of sound may be considered as a thermodynamic property, provided that a negligible amount of ultrasonic absorption of the acoustic waves of low frequency and of low amplitude is observed. The experimentally measured values of speed of sound are reported in Table 7. The experimental values of speeds of sound for tripotassium citrate + water mixtures have been compared with literature values.21,22 The values graphically represented in Figure S3 of the Supporting Information file and are found to be in good agreement with literature values. We have also analyzed the experimental speeds of sound values with the literature values21,22 by calculating average absolute deviations (AAD), bias, and maximum deviation (MD) values. These values are reported in Table 8. It is observed from Table 8 that for the mixtures of tripotassium citrate + water, the positive bias of 32.31, 33.37, 30.23, 48.90, and 26.37 is obtained at T = (288.15, 293.15, 298.15, 303.15 and 308.15) K with reference to speeds of sound values reported in citation 21. Also for tripotassium citrate + water mixtures the negative bias of −29.56, −30.78, 30.22, and −31.26 is obtained at T = (293.15, 298.15, 303.15 and 308.15) K with reference to speeds of sound values reported in citation 22. Average absolute deviations in the case of experimental speeds of sound for aqueous solutions of tripotassium citrate are in the range of 44.19 to 55.97 at different temperatures while for the literature data21,22 the values are in the range of 32.72 to 37.09 and 77.04 to 83.95, at different temperatures.

Table 5. Values of Empirical Parameters of eq 3 for Trisodium Citrate and Tripotassium Citrate in Aqueous Solutions of [C6mim][Cl] along with R2 Values and Standard Deviations σ mBa mol·kg

a

a·106 −1

m ·mol 3

b·106 −1

0.00 0.01 0.03

107.38 108.57 111.74

0.00 0.01 0.03

119.19 122.00 122.81

−1

m ·mol ·K 3

c·106 −1

m ·mol−1·K−2

R2

σ

0.9999 0.9999 0.9999

2.88 2.12 3.04

0.9999 0.9999 0.9999

2.65 1.95 2.31

3

Trisodium Citrate 0.405 −0.0045 0.300 −0.0014 0.429 −0.0065 Tripotassium Citrate 0.375 −0.0018 0.275 −0.0028 0.326 −0.0037

mB is the molality of aqueous [C6mim][Cl] solutions.

The limiting apparent molar expansivities are calculated as follows ϕE0 = (∂V ϕ0 /∂T )p = b + 2c(T − Tref )

(4)

These calculated values are included in Table 6. The limiting apparent molar expansivity arises because of two major components in the case of an electrolyte: ϕE0 = ϕE0(elect) + ϕE0(str)

(6)

(5)

ϕ0E(elect)

where is the expansivity due to electrostriction changes (contribution of hydration around the solute) and ϕ0E(str) is the expansivity due to changes in structure of solvent. At low temperature, the structural component ϕ0E(str) is the predominant element, whereas the electrostriction component ϕ0E(elect) is predominant at higher temperatures. The data reported in Table 6 reveals that at each temperature, ϕ0E values for citrate salts in pure water and [C6mim][Cl] aqueous solutions are positive and these values

Table 6. Limiting Apparent Molar Expansibilities (ϕ0E) for Trisodium Citrate and Tripotassium Citrate in Aqueous Solutions of [C6mim][Cl] at Different Temperatures ϕ0E·106/(m3·mol−1·K−1)

mBa T = 288.15 K

T = 293.15 K

0.00 0.01 0.03

0.494 0.327 0.558

0.449 0.314 0.493

0.00 0.01 0.03

0.411 0.331 0.401

0.393 0.303 0.363

mol·kg

a

−1

T = 298.15 K Trisodium Citrate 0.405 0.300 0.429 Tripotassium Citrate 0.375 0.275 0.326

(∂ϕ0E/∂T)p T = 303.15 K

T = 308.15 K

m3·mol−1·K−2

0.360 0.287 0.364

0.315 0.273 0.299

−0.0089 −0.0027 −0.0129

0.357 0.247 0.288

0.339 0.219 0.251

−0.0036 −0.0056 −0.0075

mB is the molality of aqueous [C6mim][Cl] solutions. G

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Table 7. Speeds of Sound, c and Apparent Molar Isentropic Compression κϕ,s for Trisodium Citrate and Tripotassium Citrate in Aqueous Solutions of [C6mim][Cl] at Different Temperatures and Experimental Pressure p = 0.1 MPa c/(m·sec−1) mA mol·kg−1a

T/K 288.15

T/K 293.15

0.01959 0.04954 0.06743 0.09908 0.19983 0.29918 0.39514 0.48759 0.59704 0.69512 0.78612 0.88436 0.99939

1467.21 1473.47 1477.25 1483.88 1505.26 1525.93 1546.04 1565.06 1586.88 1604.46 1618.77 1632.44 1646.15

1485.18 1491.36 1495.04 1501.61 1522.71 1542.77 1561.82 1579.77 1599.75 1615.98 1629.94 1642.70 1654.58

0.02120 0.04992 0.07188 0.09994 0.19994 0.29742 0.40087 0.49740 0.59183 0.69326 0.79125 0.90310 1.00250

1472.99 1478.95 1483.45 1489.21 1509.89 1529.53 1549.85 1568.38 1585.67 1602.73 1618.19 1634.67 1648.04

1488.99 1494.94 1499.37 1505.05 1525.72 1545.13 1564.69 1582.38 1598.34 1614.89 1628.96 1646.06 1658.68

0.02052 0.04988 0.06975 0.10121 0.20079 0.29924 0.40117 0.50683 0.60004 0.70061 0.78538 0.89874 0.99230

1476.18 1481.75 1485.58 1491.63 1510.72 1529.84 1549.58 1569.22 1586.81 1605.51 1620.08 1639.65 1655.26

1492.62 1498.19 1501.89 1507.85 1526.76 1545.45 1564.73 1584.53 1600.94 1618.58 1632.81 1651.74 1666.82

0.02026 0.04910 0.06926 0.09937 0.19724 0.29804 0.39472 0.49947 0.57886 0.69593 0.79601 0.88725 0.99316

1467.22 1473.04 1477.18 1483.37 1503.69 1525.09 1545.64 1566.79 1582.54 1604.16 1619.83 1633.83 1647.52

1485.13 1490.85 1494.91 1501.09 1520.95 1541.37 1560.89 1581.25 1596.15 1616.26 1630.37 1642.41 1655.47

κϕ,s·106/(m3·mol−1·GPa−1)

T/K T/K T/K T/K 298.15 303.15 308.15 288.15 Trisodium Citrate + 0.0 mol·kg−1 [C6mim][Cl] 1499.69 1512.58 1522.76 −17.38 1505.59 1518.34 1528.31 −17.14 1509.07 1521.74 1531.54 −17.04 1515.26 1527.87 1537.42 −16.79 1534.94 1546.94 1555.44 −16.14 1553.94 1565.08 1573.01 −15.42 1571.84 1582.54 1589.44 −14.82 1588.71 1598.51 1604.26 −14.24 1606.72 1615.56 1619.92 −13.55 1621.10 1628.73 1632.43 −12.85 1632.59 1639.72 1640.94 −12.20 1643.73 1648.36 1649.13 −11.48 1652.29 1655.76 1655.94 −10.65 Trisodium Citrate + 0.01 mol·kg−1 [C6mim][Cl] 1502.95 1515.15 1525.36 −16.91 1508.65 1520.75 1530.88 −16.66 1513.09 1525.11 1535.16 −16.43 1518.57 1530.54 1540.49 −16.20 1537.89 1549.74 1559.25 −15.55 1556.45 1567.84 1576.95 −14.86 1574.92 1586.57 1594.65 −14.15 1591.63 1602.42 1609.93 −13.53 1606.79 1617.48 1623.99 −12.92 1622.56 1631.45 1638.32 −12.25 1636.82 1645.45 1650.12 −11.62 1651.52 1657.54 1662.34 −10.95 1662.52 1667.25 1670.71 −10.37 Trisodium Citrate + 0.03 mol·kg−1 [C6mim][Cl] 1507.22 1520.19 1531.48 −15.41 1512.48 1525.29 1536.35 −15.24 1516.05 1528.78 1539.68 −15.16 1521.79 1534.35 1544.94 −15.00 1539.68 1551.75 1561.77 −14.40 1557.43 1568.82 1578.23 −13.88 1575.81 1586.18 1594.85 −13.34 1594.32 1603.82 1611.77 −12.72 1610.24 1618.97 1625.91 −12.26 1626.81 1635.17 1640.87 −11.76 1640.36 1647.79 1652.78 −11.31 1657.27 1664.17 1668.05 −10.77 1671.02 1676.13 1679.44 −10.34 Tripotassium Citrate + 0.00 mol·kg−1 [C6mim][Cl] 1499.69 1512.60 1522.81 −17.22 1505.19 1517.98 1528.04 −16.98 1509.04 1521.75 1531.67 −16.87 1514.85 1527.41 1537.13 −16.69 1533.77 1545.87 1554.77 −16.12 1552.91 1564.44 1572.58 −15.57 1571.29 1581.87 1589.01 −15.03 1590.02 1599.58 1605.45 −14.34 1603.25 1612.14 1617.43 −13.86 1622.05 1628.76 1632.58 −13.10 1635.25 1641.49 1644.04 −12.36 1645.62 1650.75 1651.81 −11.77 1656.11 1658.95 1659.96 −11.05

H

T/K 293.15

T/K 298.15

T/K 303.15

T/K 308.15

−16.26 −16.08 −15.94 −15.75 −15.13 −14.40 −13.75 −13.15 −12.43 −11.75 −11.17 −10.48 −9.68

−15.25 −15.07 −14.94 −14.72 −14.08 −13.42 −12.81 −12.24 −11.51 −10.84 −10.19 −9.54 −8.69

−14.50 −14.34 −14.22 −14.06 −13.39 −12.72 −12.15 −11.57 −10.85 −10.17 −9.57 −8.84 −8.01

−13.77 −13.61 −13.44 −13.28 −12.58 −11.99 −11.42 −10.84 −10.12 −9.48 −8.80 −8.13 −7.35

−16.10 −15.96 −15.73 −15.52 −14.95 −14.25 −13.50 −12.86 −12.21 −11.56 −10.92 −10.34 −9.78

−15.21 −15.00 −14.93 −14.68 −13.94 −13.32 −12.60 −11.98 −11.38 −10.78 −10.23 −9.60 −9.03

−14.72 −14.45 −14.36 −14.15 −13.48 −12.83 −12.19 −11.53 −10.95 −10.29 −9.77 −9.06 −8.47

−14.13 −13.93 −13.83 −13.63 −12.97 −12.34 −11.66 −11.01 −10.42 −9.83 −9.23 −8.59 −8.00

−14.62 −14.54 −14.38 −14.22 −13.68 −13.15 −12.63 −12.09 −11.57 −11.06 −10.64 −10.12 −9.70

−13.67 −13.49 −13.38 −13.28 −12.71 −12.21 −11.73 −11.19 −10.73 −10.25 −9.85 −9.32 −8.91

−12.88 −12.76 −12.70 −12.58 −12.07 −11.57 −11.06 −10.54 −10.11 −9.66 −9.26 −8.75 −8.32

−12.17 −12.02 −11.95 −11.81 −11.39 −10.94 −10.43 −9.95 −9.50 −9.06 −8.66 −8.17 −7.77

−16.20 −16.01 −15.93 −15.82 −15.24 −14.61 −14.06 −13.42 −12.95 −12.19 −11.46 −10.84 −10.18

−15.16 −15.00 −14.88 −14.74 −14.21 −13.58 −13.05 −12.40 −11.89 −11.21 −10.53 −9.91 −9.23

−14.50 −14.35 −14.23 −14.08 −13.54 −12.94 −12.36 −11.73 −11.23 −10.48 −9.84 −9.23 −8.51

−13.84 −13.70 −13.56 −13.39 −12.84 −12.24 −11.67 −11.00 −10.53 −9.78 −9.16 −8.54 −7.90

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Table 7. continued c/(m·sec−1) 0.02128 0.05144 0.07903 0.10059 0.19841 0.29917 0.39891 0.49366 0.58796 0.69758 0.80003 0.91320 0.99282

1472.95 1479.11 1484.79 1489.24 1509.02 1529.39 1548.91 1566.81 1583.52 1601.56 1617.78 1632.21 1642.04

1488.94 1494.98 1500.63 1504.83 1524.14 1543.98 1562.89 1579.61 1595.73 1612.69 1628.25 1643.29 1653.22

0.01981 0.05025 0.06898 0.09907 0.19782 0.29554 0.39831 0.49835 0.59973 0.69505 0.79627 0.89399 0.98838

1476.41 1482.64 1486.54 1492.68 1512.72 1532.48 1552.59 1570.95 1588.82 1604.55 1620.21 1634.65 1645.76

1492.75 1498.82 1502.48 1508.31 1527.54 1545.85 1564.45 1582.47 1597.95 1614.06 1625.79 1639.48 1651.08

κϕ,s·106/(m3·mol−1·GPa−1)

Tripotassium Citrate + 0.01 mol·kg−1 [C6mim][Cl] 1502.82 1515.03 1525.24 −16.88 1508.59 1520.81 1530.88 −16.60 1513.88 1525.98 1535.96 −16.43 1517.96 1530.18 1539.91 −16.29 1536.67 1548.45 1557.54 −15.55 1555.58 1566.86 1575.11 −14.91 1573.55 1584.01 1591.64 −14.24 1589.52 1599.51 1606.77 −13.61 1604.83 1613.85 1620.21 −12.98 1620.69 1629.35 1635.27 −12.25 1634.40 1642.44 1647.80 −11.63 1648.84 1655.40 1660.13 −10.86 1656.37 1663.22 1666.83 −10.38 Tripotassium Citrate + 0.03 mol·kg−1 [C6mim][Cl] 1507.17 1520.15 1531.35 −16.79 1512.78 1525.46 1536.40 −16.48 1516.28 1528.77 1539.48 −16.39 1521.75 1534.05 1544.52 −16.13 1539.91 1551.05 1560.42 −15.39 1556.99 1567.72 1575.81 −14.76 1574.95 1584.19 1591.71 −14.06 1591.19 1600.06 1606.19 −13.34 1606.26 1614.42 1619.49 −12.65 1620.15 1626.74 1630.26 −12.02 1630.69 1637.65 1639.85 −11.37 1642.20 1647.41 1647.97 −10.80 1651.47 1656.94 1656.26 −10.18

−16.06 −15.77 −15.67 −15.43 −14.74 −14.12 −13.47 −12.82 −12.22 −11.50 −10.92 −10.25 −9.81

−14.98 −14.74 −14.57 −14.41 −13.85 −13.25 −12.63 −12.01 −11.44 −10.74 −10.13 −9.52 −9.03

−14.52 −14.35 −14.11 −14.04 −13.38 −12.75 −12.09 −11.50 −10.90 −10.25 −9.65 −9.02 −8.57

−13.95 −13.78 −13.57 −13.43 −12.78 −12.13 −11.51 −10.96 −10.36 −9.74 −9.17 −8.55 −8.10

−15.72 −15.51 −15.32 −15.05 −14.39 −13.68 −12.97 −12.37 −11.63 −11.14 −10.36 −9.85 −9.33

−14.27 −14.12 −14.05 −13.83 −13.27 −12.60 −12.00 −11.39 −10.73 −10.20 −9.46 −8.92 −8.39

−13.50 −13.26 −13.18 −13.02 −12.40 −11.86 −11.23 −10.68 −10.07 −9.50 −8.86 −8.29 −7.81

−12.56 −12.43 −12.32 −12.19 −11.57 −11.01 −10.48 −9.93 −9.35 −8.76 −8.13 −7.55 −7.09

mA is the molality of citrate salts in aqueous [C6mim][Cl] solutions. Standard uncertainties u(c) in speed of sound measurements are ± 5·10−1 m· s1; standard uncertainties u(m) in molality are ± 2·10−5 mol·kg−1; standard uncertainties u(T) in temperature are ± 0.001 K.

a

Table 8. Analysis of Experimental (expt) and Theoretical (theor) Speeds of Sound Values for Aqueous Solutions of Trisodium Citrate and Tripotassium Citrate AAD/(m·sec−1) T/K

theor

MD/(m·sec−1) −1

expt

bias/(m·sec )

theor

expt

Trisodium Citrate + Water 288.15 293.15 298.15 303.15 308.15 288.15 293.15 298.15 303.15 308.15 a

55.99 52.89 48.45 45.97 42.93 37.0921 35.8421 83.9522 34.6821 81.4422 33.6621 79.3422 32.7221 77.0422

55.97 53.46

179.16 167.20 152.87 141.71 132.18 Tripotassium Citrate + Water 32.31a 33.37a

−29.56b

49.46

30.23a

−30.78b

46.79

48.90a

−30.22b

44.19

26.37a

−31.26b

137.3021 132.7421 292.1622 128.4821 283.6322 124.6821 276.6322 121.2421 268.6422

178.77 170.59 156.63 144.47 134.33

From ref 21. bFrom ref 22.

in Table 8, it is observed that the magnitude of average absolute deviation and maximum deviations decrease with increase in temperature. It is also observed that the values of AAD and MD obtained from experimental speeds of sound are more than AAD and MD values obtained from speeds of sound reported in ref 21 and less than the AAD and MD values obtained from

Maximum deviations calculated for aqueous solutions of tripotassium citrate are also reported in Table 8. The values obtained are in the range 134.33 to 178.77 for experimental speeds of sound. The corresponding values of maximum deviations for literature speeds of sound21,22 are in the range 121.24 to 137.30 and 26.64 to 292.16. From the data reported I

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Figure 2. Plot of κs of trisodium citrate in (a) 0.01 mol·kg−1 aqueous [C6mim][Cl] and (b) 0.03 mol·kg−1 aqueous [C6mim][Cl] and for tripotassium citrate in (c) 0.01 mol·kg−1 aqueous [C6mim][Cl] and (d) 0.03 mol·kg−1 aqueous [C6mim][Cl] solutions against molality of salts at different temperatures: ⧫, 288.15 K; ■, 293.15 K; ●, 298.15 K; ▲, 303.15 K; +, 308.15 K.

aqueous and mixed aqueous solutions of [C6mim][Cl] at different temperatures was determined by using following equation

densities reported in ref 22. The same results have been obtained from analysis of the density data as discussed above. 3.2.1. Isentropic Compressibility. On the basis of the speed of sound and density values, the isentropic compressibility, κS (Pa−1) values were calculated for the investigated mixtures from the Laplace−Newton’s equation: 2

κS = 1/(c ρ)

κϕ ,s = (MκS/ρ) − {(κS,0ρ − κSρ0 )/mρρ0 }

(8)

where κS,0 and κS are the isentropic compressibilities of pure solvent and solution, respectively, calculated using eq 7. The other symbols have their usual meanings defined earlier. The calculated values of κϕ,s along with speed of sound values for molal concentrations (m) of trisodium citrate and tripotassium citrate in (0.01 and 0.03) mol·kg−1 aqueous solutions of [C6mim][Cl] at different temperatures are reported in Table 8. The magnitude of negative values of κϕ,s decreases with an increase in temperature at each salt concentration. If we assume that the size of the ions is not pressure dependent and the electrostricted water is already compressed to maximum extent by the charge on the ions,30 we can assume that for a low concentration of citrate salts, the compressibility is due to the effect of pressure on the bulk water molecules. As the concentration of the citrate salts (or concentration of [C6mim][Cl]) increases, a large portion of the water molecule is electrostricted, the amount of bulk water decreases causing the compressibility to decrease. Also, over the temperature range investigated in this work, the compressibility decreases with temperature and therefore, the values of κϕ,s for solutions investigated also decrease with temperature. The negative values of κϕ,s indicate that the water molecules surrounding the solute are less compressible than the water molecules in the bulk, which is attributed to strong attractive interactions. Therefore, the compressibility of a solution is mainly due to the effect of pressure on the bulk water molecules. Previous studies of κϕ,s show that negative κϕ,s values come for ionic compounds in water, whereas positive values are

(7)

where c is the speed of sound and ρ is the density of the solution. The experimental isentropic compressibility values are plotted against the molality of salt in aqueous ionic liquid solution in Figure 2. From Figure 2, it is observed that the κS values decrease with the increases of concentration of IL at each temperature and decrease with the increase of temperature at the same concentration. From Figure 2, we note that at each working temperature as the concentration of IL is increased, κS of solution is decreased due to the combined effect of hydration of ions and breaking of three-dimensional network structure of water. It is assumed that40 in aqueous electrolyte solutions the isentropic compressibility is the sum of two contributions, κS (solvent intrinsic) and κS (solute intrinsic). Here, κS (solvent intrinsic) is the isentropic compressibility due to the compression of the three-dimensional network structure of water and κS (solute intrinsic) is the isentropic compressibility due to the compression of the hydration (hydrophobic and ionic) shell of ions and interionic distance. Figure 2 shows that the κS (solvent intrinsic) is the dominant contributor to the total value of κS from pure solvent up to the converging concentration, and beyond that κS (solute intrinsic) is the substantial contributor. 3.2.2. Apparent Molar Isentropic Compression. The apparent molar isentropic compression for citrate salts in J

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Table 9. Limiting Apparent Molar Adiabatic Compressibility,κ0ϕ,s, and Experimental Slopes, SK*, for Trisodium Citrate and Tripotassium Citrate in Aqueous Solutions of [C6mim][Cl] at Different Temperatures κ0ϕ,s·106/(m3·mol−1·GPa−1) mB mol·kg−1a 0.00 0.01 0.03

0.00 0.01 0.03 a

T/K 288.15

T/K 293.15

T/K 298.15

−17.50 (±0.02) −16.90 (±0.04) −15.49 (±0.02)

−16.42 (±0.01) −16.20 (±0.04) −14.72 (±0.02)

−15.40 (±0.02) −15.27 (±0.04) −13.73 (±0.02)

−17.36 (±0.04) −16.94 (±0.02) −16.80 (±0.03)

−16.41 (±0.04) −16.09 (±0.03) −15.73 (±0.04)

−15.35 (±0.03) −15.05 (±0.02) −14.44 (±0.02)

SK*·106/(kg·m3·mol−2·GPa−1)

T/K T/K 303.15 308.15 Trisodium Citrate −14.70 −13.93 (±0.03) (±0.03) −14.77 −14.23 (±0.02) (±0.02) −13.00 −12.27 (±0.02) (±0.01) Tripotassium Citrate −14.70 −14.02 (±0.03) (±0.02) −14.61 −14.01 (±0.03) (±0.03) −13.59 −12.72 (±0.01) (±0.02)

T/K 288.15

T/K 293.15

T/K 298.15

T/K 303.15

T/K 308.15

6.77 (±0.04) 6.65 (±0.07) 5.29 (±0.04)

6.71 (±0.02) 6.57 (±0.07) 5.16 (±0.04)

6.62 (±0.04) 6.37 (±0.08) 4.92 (±0.03)

6.57 (±0.05) 6.36 (±0.05) 4.76 (±0.03)

6.51 (±0.05) 6.31 (±0.05) 4.57 (±0.02)

6.23 (±0.07) 6.67 (±0.04) 6.80 (±0.06)

6.18 (±0.07) 6.45 (±0.06) 6.65 (±0.08)

6.06 (±0.06) 6.11 (±0.03) 6.16 (±0.03)

6.12 (±0.06) 6.18 (±0.05) 5.89 (±0.02)

6.11 (±0.04) 6.05 (±0.06) 5.71 (±0.03)

mB is the molality of aqueous [C6mim][Cl] solutions.

for hydrophobic solutes. The negative values of κϕ,s show the predominance of hydrophilic−ionic interactions which support Vϕ data. The variation of the apparent molar isentropic compression, κϕ,s, with the molal concentration can be adequately represented by the following equation: κϕ ,s = κϕ0,s + SK*m

liquid [C6mim][Cl] increase with increasing [C6mim][Cl] concentration. With an increase in concentration of [C6mim][Cl], electrostriction decreases and the structure-making tendency of ions increase. As a result, the electrostricted water is much less compressible than bulk water and leads to a large decrease in the compressibility with an increase in [C6mim][Cl] concentration. Thus, κ0ϕ,s values are negative and 0 values are positive for citrate salts with different Δκϕ,s concentrations of [C6mim][Cl]. This is because more water molecules were associated with the ion at lower temperature as well as at lower concentrations of [C6mim][Cl]; a greater number of water molecules of solute are associated with solvent molecules thereby increasing the interactions and thereby decreasing the compressibility. 3.3. Absorption Spectral Studies. Further, UV absorptions were recorded for different mixtures to analyze the solute−solvent interactions for the present study. The absorption values for different molalities of citrate salts in aqueous [C6mim][Cl] solutions have been reported in Table 10 and are also represented graphically in Figure 3. From the data, it is observed that the absorption maximum increases with an increase in molality of citrate salts in aqueous solutions of [C6mim][Cl] and also for a fixed composition of citrate salts in different aqueous solutions of [C6mim][Cl]. The values also increase with an increase in molality of [C6mim][Cl], that is, from 0.01 mol·kg−1 aqueous solutions of [C6mim][Cl] to 0.03 mol·kg−1 aqueous solutions of [C6mim][Cl] for fixed molality of citrate salt. The absorption values obtained for the mixtures are in accordance with volumetric and acoustic results. The shift observed in absorptions in the case of citrate salts with [C6mim][Cl] indicates the coordination of ions of citrate salts with [C6mim][Cl] by breaking the solvent layers of water that causes extended conjugation resonance.41 This effect is attributed to the increase in interactions between citrate salts and aqueous [C6mim][Cl] solutions that clearly supports and justifies our thermodynamic data.

(9)

where κ0ϕ,s is the limiting value of the isentropic compressibility and SK* is the experimental slope indicative of solute−solute interactions. The values of κ0ϕ,s and SK*, together with standard errors derived by least-squares fitting, are reported in Table 9. The citrate salts here in this study have negative κ0ϕ,s values in aqueous [C6mim][Cl] solutions at different temperatures thereby showing the presence of strong solute−solvent interactions. These values for both citrate salts are less negative than their corresponding values in water. With an increase in temperature the κ0ϕ,s values become less negative, which means that electrostriction reduces and some water molecules are released to bulk. The strong attractive interactions due to the hydration of ions produced from the dissociation of [C6mim][Cl] induce the dehydration of citrate salts and increase the water molecules in bulk. The electrostriction interaction between citrate salts and water molecules are suppressed due to formation of ion pairs between the ions of [C6mim][Cl] and citrate salts. Therefore, at high [C6mim][Cl] concentrations the water molecules around the citrate salts are more compressible than those at lower [C6mim][Cl] concentrations. The positive but less values of the slope S*K suggest the presence of weaker solute−solute interactions in the system. The values of S*K decrease with an increase in temperature and increase with an increase in [C6mim][Cl] concentration for both salts. The partial molar isentropic compression of transfer Δκ0ϕ,s of citrate salts from water to aqueous [C6mim][Cl] solutions at infinite dilution calculated by using Supporting Information eq E2 are reported in Table S2. The Δκ0ϕ,s values as reported in Table S2 in the Supporting Information are positive except for trisodium citrate in 0.01 mol·kg−1 aqueous solutions of [C6mim][Cl] at higher temperatures, where the negative values for Δκ0ϕ,s are observed. The positive values of Δκ0ϕ,s indicate the dominance of interactions between the ions of citrate salt and ionic liquid [C6mim][Cl] indicating the structure-making tendency of the ions. The interactions between the ions of citrate salts and ionic

4. CONCLUSION Results on density and speeds of sound measurements for trisodium citrate and tripotassium citrate aqueous solutions of [C6mim][Cl] at different temperatures have been reported in the present study. The apparent molar volume, partial molar K

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salts and aqueous [C6mim][Cl] solutions. The negative values of κϕ,s and κ0ϕ,s support our volumetric data. The absorption values recorded for different mixtures show an increase with increase in the molality of citrate salts and aqueous [C6mim][Cl] solutions, suggesting interactions between citrate salt + [C6mim][Cl] + water mixtures.

Table 10. UV Absorption Values for Trisodium Citrate and Tripotassium Citrate in Aqueous Solutions of [C6mim][Cl] mAa mol·kg

mAa −1

absorbance

Trisodium Citrate + 0.00 mol· kg−1 [C6mim][Cl] 0.01959 0.97 0.04954 1.18 0.06743 1.30 0.09908 1.51 0.19983 2.10 0.29918 2.66 0.39514 3.22 0.48759 3.76 0.59704 4.42 0.69512 5.06 0.78612 5.60 0.88436 6.16 0.99939 6.92 Tripotassium Citrate + 0.00 mol· kg−1 [C6mim][Cl] 0.02026 0.98 0.04910 1.25 0.06926 1.40 0.09937 1.69 0.19724 2.55 0.29804 3.45 0.39472 4.29 0.49947 5.19 0.57886 5.89 0.69593 6.97 0.79601 7.88 0.88725 8.77 0.99316 9.71 a

mol·kg

mAa −1

absorbance

Trisodium Citrate + 0.01 mol· kg−1 [C6mim][Cl] 0.02120 1.84 0.04992 2.15 0.07188 2.37 0.09994 2.74 0.19994 3.86 0.29742 4.82 0.40087 5.89 0.49740 6.93 0.59183 7.95 0.69326 8.99 0.79125 9.96 0.90310 11.05 1.00250 12.06 Tripotassium Citrate + 0.01 mol· kg−1 [C6mim][Cl] 0.02128 3.98 0.05144 4.27 0.07903 4.59 0.10059 4.82 0.19841 5.78 0.29917 6.68 0.39891 7.69 0.49366 8.65 0.58796 9.59 0.69758 10.68 0.80003 11.64 0.91320 12.75 0.99282 13.48

mol·kg−1

absorbance

■

Trisodium Citrate + 0.03 mol· kg−1 [C6mim][Cl] 0.02052 12.20 0.04988 12.50 0.06975 12.79 0.10121 13.09 0.20079 14.06 0.29924 15.07 0.40117 16.03 0.50683 17.02 0.60004 17.81 0.70061 18.72 0.78538 19.45 0.89874 20.47 0.99230 21.28 Tripotassium Citrate + 0.03 mol· kg−1 [C6mim][Cl] 0.01981 13.18 0.05025 13.84 0.06898 14.17 0.09907 14.78 0.19782 16.18 0.29554 17.65 0.39831 18.88 0.49835 20.28 0.59973 21.62 0.69505 22.92 0.79627 24.40 0.89399 25.82 0.98838 27.10

ASSOCIATED CONTENT

S Supporting Information *

Figures containing comparison of densities and speeds of sound of aqueous citrate salts with literature data, equations and tables for partial molar properties of transfer at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Funding

Chanda Chadha is thankful to Department of Science and Technology (DST), New Delhi, for providing DST Inspire Fellowship (IF120453) via sanction Order No. DST/Inspire FellowshiP/2012/428. Notes

The authors declare no competing financial interest.

■

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