Thermophysical properties of aqueous solutions of tetraalkylphosphonium based ionic liquids at different temperatures and atmospheric pressure

Accepted Manuscript Thermophysical Properties of Aqueous Solutions of Tetraalkylphosphonium Based Ionic Liquids at Different Temperatures and Atmospheric Pressure Ila J. Warke, Kesharsingh J. Patil, Santosh S. Terdale PII: DOI: Reference:

S0021-9614(15)00359-6 http://dx.doi.org/10.1016/j.jct.2015.09.029 YJCHT 4415

To appear in:

J. Chem. Thermodynamics

Received Date: Revised Date: Accepted Date:

22 April 2015 26 August 2015 23 September 2015

Please cite this article as: I.J. Warke, K.J. Patil, S.S. Terdale, Thermophysical Properties of Aqueous Solutions of Tetraalkylphosphonium Based Ionic Liquids at Different Temperatures and Atmospheric Pressure, J. Chem. Thermodynamics (2015), doi: http://dx.doi.org/10.1016/j.jct.2015.09.029

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Thermophysical Properties of Aqueous Solutions of Tetraalkylphosphonium Based Ionic Liquids at Different Temperatures and Atmospheric Pressure Ila J. Warke,a Kesharsingh J. Patil,b Santosh S. Terdale a ,* a

Department of Chemistry,

Savitribai Phule Pune University, Pune (MS)-411 007, India b

School of Chemical Sciences,

North Maharashtra University, Jalgaon (MS)-425 001, India *

Corresponding author: Tel: +91 (020) 25691395 Extn. 565, Email address:

[email protected]

ABSTRACT Densities, speed of sound and viscosities of the aqueous solutions of three tetraalkylphosphonium based ionic liquids namely Tributylmethylphosphonium methyl sulphate (0 ─ ~0.28) mol.kg-1, Triisobutylmehtylphosphonium tosylate (0 ─ ~0.52) mol.kg-1 and Tetrabutylphosphonium chloride (0 ─ ~0.57) mol.kg-1 have been measured at the temperatures (298.15, 303.15, 308.15 and 313.15) K and at atmospheric pressure. The apparent molar volume, limiting apparent molar volume and coefficient of thermal expansion values have been obtained from the experimental density data whereas relative viscosities and viscosity A and B coefficients have been determined from experimental viscosity data. The density values have been combined with speed of sound values to evaluate the isothermal and isentropic compressibilities. The hydration number has also been calculated from both viscosity and compressibility data. The data obtained are used to interpret the ion-ion and ion-solvent interactions in the studied systems and also we

1

tried to explain the effect of temperature and concentration on it. The volumetric data, negative values of BV (the deviation parameter from Debye-Hückel theory) and the large positive values of viscosity B coefficient for the studied ionic liquids indicate strong solute-solute interaction with water structure making effect of ionic liquids. All these are discussed in terms of cation-cation, cation-anion interactions and probable alterations in water structural effects.

KEYWORDS: Phosphonium based ionic liquid; Density; Apparent molar volume; Speed of sound; Apparent molar compressibility; Viscosity

1. Introduction Ionic Liquids (ILs) are very well known today because of their properties. The nature of ILs helps in designing ILs in terms of their physicochemical properties and hence it is possible to have properties suitable for particular application. Many ILs have been proved useful for their potential use in many ways [1]. The literature available on the physicochemical properties of ILs is found to be mainly focused on ammonium, imidazolium, pyridinium and pyrrolidinium-based ILs [1-4]. On the other hand there is scarcity of the physicochemical properties of phosphonium based ILs either in pure form or in form of their solutions [3,5-7]. This is because less number of phosphonium compounds fit in the broad definition of ILs. However compared with other families of ILs, phosphonium based ILs are more thermally stable, less toxic, less dense than water and less expensive [3,8]. Among phosphonium based ILs, the most attention in the literature has been given to those

containing

trihexyltetradecylphosphonium

cation

[P6,6,6,14]+,

trioctylalkylphosphonium cation [P8,8,8,n]+ (n = 1-10, 12, 14) and tributylalkylphosphonium

2

cation [P4,4,4,n]+ (n = 1-8, 10, 12, 14) [3,6,9]. As per our literature survey there is no published data on the binary mixtures of Tributylmethylphosphonium methyl sulphate [P4,4,4,1][CH3SO4],

Triisobutylmehtylphosphonium

tosylate

[Pi(444)1][Tos]

and

Tetrabutylphosphonium chloride [P4,4,4,4][Cl] with water. This enabled us to study these systems in terms of physicochemical properties in dilute concentration range. In the present work, the densities (ρ), viscosities (η) and speed of sound (u) values of binary solutions of [P4,4,4,1][CH3SO4], [Pi(444)1][Tos] and [P4,4,4,4][Cl] in water at atmospheric pressure and at temperatures (298.15, 303.15, 308.15 and 313.15) K have been reported. The experimental data were further used to obtain the isentropic and isothermal compressibility (βs, βT), apparent molar volume (φV) and apparent molar isentropic and isothermal compressibility ( φ K S and φ KT ) values. The hydration numbers (n h) of studied ILs were obtained from viscosity B coefficient and from isentropic compressibility data. In short the objective of this study is to understand the effect of temperature and concentration on ion-ion interactions and hydration of ILs as well as the effects due to anion-water interactions in dilute solutions.

2. Experimental 2.1 Chemicals The purity and other information of chemicals used in this work are given in table 1. TABLE 1 List of chemicals, supplier, purity and abbreviation of studied ILs. Chemical

Supplier

Puritya

Abbrevation

Tributylmethylphosphonium methyl sulphate

Io-li-tec

> 0.95

[P4,4,4,1][CH3SO4]

Triisobutylmethylphosphoni um tosylate

Io-li-tec

> 0.95

[Pi(444)1][Tos]

3

Tetrabutylphosphonium chloride 1-Butyl-3methylimidazolium chloride

Io-li-tec

> 0.95

[P4,4,4,4][Cl]

Otto Chemicals

0.98

[C4mim][Cl]

AR

NaCl

S D Fine Chem. Limited a Mass percentage as mentioned by respective suppliers Sodium Chloride

The structures of studied ILs are given in figure 1. The ILs were used without any further purification. The ILs used were thought to be moisture sensitive and hence were handled in the glove box to minimize the possibility of absorption of atmospheric moisture. The reliability of glove box constructed was checked by keeping preheated silica gel for 48 hrs and then by observing the color of it. The thermogravimetric analysis was also done [In Supporting information figures S1−S3]. The water content in all three ILs was measured using Karl-Fischer Titrator, Veego Company, India (Model- Matic-1). The water content was found to be 50 ppm in [P4,4,4,1][CH3SO4], 720 ppm in [P4,4,4,4][Cl] and 150 ppm in [Pi(444)1][Tos]. The samples were collected by handling them in activated glove box like the same way followed for solution preparation. The corresponding changes in the concentrations of [P4,4,4,1][CH3SO4] and [Pi(444)1][Tos] were found to be within the uncertainty limits observed for molality and hence were not considered. But for [P4,4,4,4][Cl] the changes were found to be appreciable at high concentrations studied and hence the corrected concentration values were used for [P4,4,4,4][Cl] at all temperatures selected for study.

4

P

FIGURE 1. Molecular structures of studied ILs

The aqueous solutions of ILs were prepared on molality basis by the dilution using an analytical balance (WENSAR make DAB 220 model) having a precision of ± 0.0002 g and by using Millipore grade water (having conductivity of 0.0549 µS.cm-1 at 25o C) obtained by using Synergy® UV instrument. The solubility of [P4,4,4,1][CH3SO4] in water was found to be rather low and dissolution is a slow process in contrast to behavior expected from its hydrophilic nature [10]. The slight turbidity obtained while preparing the stock solution was removed by keeping the solution for at least 15 hrs to become clear, with intermittent shaking and then used for further dilutions. The possibility of vaporization of solvent during this time was avoided by maintaining temperature to about 298 K. The change in weights of the stock solution measured initially and after keeping for 15 hrs was found to be very small. The concentrations calculated from these weights were found to differ by less than ± 5 × 10 −4 mol.kg−1.

2.2 Density measurement The density measurements of aqueous solutions of all ILs were made at temperatures (298.15, 303.15, 308.15 and 313.15) K using a Rudolph Research Analytical 5

DDM 2911 automatic density meter. The instrument works on the principle of vibrating tube flow density meter. The piezo-electric system causes the hollow U-tube inside the instrument to vibrate at a resonance frequency depending upon the experimental temperature selected. At a constant temperature the harmonic frequency of the tube is a constant and also reproducible. After filling the U-tube with standard/sample the change in oscillation frequency caused will be indirectly proportional to the mass of the standard/sample. Since the volume of tube is constant the oscillation period is directly related to the density and the instrument is programmed to display the density as per the experimental temperature set up. Before each series of measurement, the apparatus was calibrated using Millipore grade water and dry air at each of the experimental temperature and the values were compared with those reported in density meter instruction manual. The measurements were repeated ten times for each solution and the average values were used for further calculations. The uncertainty of density measurement and temperature was found to be ± 5 x 10-2 kg.m−3 and ± 0.03 K, respectively. The reliability of the measurement was checked by measuring the density of aqueous sodium chloride (NaCl) solutions at (298.15, 303.15, 308.15 and 313.15) K [See figure S4 in Supporting Information]. The values obtained were in good agreement with the reported data [11].

2.3 Speed of sound The speed of sound measurements were carried out for aqueous solutions of all ILs at (298.15, 303.15, 308.15 and 313.15) K using ultrasonic interferometer (M/s Mittal Enterprises, F-81) at fixed frequency of 2 MHz. In ultrasonic interferometer ultrasonic waves of known frequency are produced by quartz crystal fixed at the bottom of the cell. These waves are reflected back by a piston which can be moved in vertical direction inside the cell. If the separation between the plate and the crystal is a whole number multiple of

6

the sound wavelength, the standing waves are formed in the medium. At resonance condition the current from the cell becomes maximum and the distance of separation between the plate and the crystal is recorded. By moving the piston upwards or downwards, depending upon initial position of plate with respect to the crystal, number of consecutive maximas (minimas can also be obtained) can be obtained. From the average value of the distance of separation and the fixed frequency, the sound velocity of the medium inside the cell can be calculated. The temperature inside the cell was maintained to ± 0.05 K by circulating water through Equibath Refrigerated circulating water bath (Make: Medica, Model-8506) having a temperature stability of ± 0.01 K around the cell. The reliability of the measurements was checked by obtaining sound velocity data for water at all studied temperatures and comparing with the literature data [11]. The expanded uncertainty in speed of sound was deduced to be ± 1 m.s-1 at 0.95 level of confidence.

2.4 Viscosity measurement The dynamic viscosity values of aqueous solutions of all ILs were measured at (298.15, 303.15, 308.15 and 313.15) K using suspended level Ubbelhode type viscometer. For this, specially constructed glass vessel was used to hold the viscometer and the water was circulated from the Equibath refrigerated circulating water bath (Make: Medica, Model-8506)

maintained with an accuracy of ± 0.01 K. Inside the glass vessel the

temperature constancy was observed to be ± 0.05 K or better than that some times. The efflux time was measured using a digital stopwatch with accuracy of ± 0.1 sec and for water it was 258.6 sec at 298.15 K. The reliability of measurement was checked by performing similar measurements on aqueous solutions of NaCl and [C4mim][Cl] [See table S5 in Supporting Information] at 298.15 K and comparing with reported data [12,13].

7

The measurements were repeated five times for each solution and the average values were used for further calculations. The viscometer was calibrated using Millipore grade water at each of the temperature. The expanded uncertainty of viscosity measurement was found to be ≤ ± 1% at 0.95 level of confidence.

3. RESULTS 3.1 Volumetric properties The density of aqueous solutions of [P4,4,4,1][CH3 SO4] in the concentration range of (0 ─ ̴ 0.28) mol.kg-1 and aqueous solutions of [Pi(444)1][Tos] and [P4,4,4,4][Cl] in the concentration range (0 ─ ̴ 0.57) mol.kg-1 are reported in tables 2─4 respectively, at (298.15, 303.15, 308.15 and 313.15) K with temperature accuracy of ± 0.03 K and at atmospheric pressure. [See figures S6─S8 in supporting information] The experimental density data obtained were further used to calculate apparent molar volume (φV) of the solutions using the Eq. (1) [14].  M   ρ − ρ    φV / m 3 .mol −1 =  2  +  0  ρ   mρρ 0 

(1)

where, m is the molality in mol.kg-1 of solution, ρ0 and ρ are the densities of water and solution in kg.m-3, respectively, M2 is the molar mass of ILs in kg.mol-1. The resulting values of φV for the selected ILs at studied temperatures are given in tables 2─4. The error in φV

8

TABLE 2 The experimental values of density (ρ)a, apparent (φv) and partial molal volumes ( V1 , V2 ), excess molar volume (VE), coefficient of thermal expansion (α) and dynamic viscosity (η)b for H2O (1) + [P4,4,4,1][CH3SO4] (2) at temperatures (298.15, 303.15, 308.15 and 313.15) K and at P = 0.1MPa. m/ ρ/ (mol.kg-1) (kg. m3)

106.φv/ (m3.mol-1)

0.0000 0.0282 0.0438 0.0564 0.0658 0.0847 0.1130 0.1740 0.2336 0.2813

997.04 997.65 997.98 998.25 998.45 998.86 999.46 1000.77 1002.05 1003.07

307.67 ± 2.5 307.57 ± 1.6 307.48 ± 1.3 307.42 ± 1.1 307.30 ± 0.8 307.11 ± 0.6 306.71 ± 0.4 306.32 ± 0.3 306.01 ± 0.2

0.0000 0.0282 0.0438 0.0564 0.0658 0.0847 0.1130 0.1740 0.2336 0.2813

995.64 996.22 996.54 996.80 996.99 997.38 997.96 999.21 1000.44 1001.42

309.00 ± 2.5 308.90 ± 1.6 308.82 ± 1.3 308.76 ± 1.1 308.64 ± 0.8 308.46 ± 0.6 308.07 ± 0.4 307.70 ± 0.3 307.39 ± 0.2

0.0000 0.0282 0.0438 0.0564 0.0658 0.0847 0.1130 0.1740 0.2336 0.2813

994.03 994.58 994.89 995.14 995.32 995.70 996.25 997.45 998.62 999.56

310.35 ± 2.5 310.26 ± 1.6 310.18 ± 1.3 310.12 ± 1.1 310.01 ± 0.8 309.83 ± 0.6 309.46 ± 0.4 309.10 ± 0.3 308.81 ± 0.2

0.0000 0.0282 0.0438

992.21 992.74 993.04

311.69 ± 2.5 311.60 ± 1.6

106. V1 / 106. V2 / (m3.mol-1) (m3.mol-1) 298.15 K 18.068 18.069 307.48 18.069 307.28 18.069 307.11 18.069 306.99 18.069 306.74 18.070 306.37 18.072 305.56 18.075 304.79 18.077 304.17 303.15 K 18.094 18.094 308.82 18.094 308.62 18.094 308.46 18.094 308.34 18.095 308.10 18.095 307.74 18.097 306.97 18.100 306.22 18.102 305.63 308.15 K 18.123 18.123 310.18 18.123 309.99 18.124 309.83 18.124 309.72 18.124 309.49 18.125 309.14 18.127 308.40 18.129 307.68 18.131 307.11 313.15 K 18.156 18.157 311.52 18.157 311.34 9

10 6. VE/ (m3mol-1)

10 4.α / K-1

106.η / Pa.s

-0.0013 -0.0021 -0.0028 -0.0034 -0.0045 -0.0064 -0.0112 -0.0166 -0.0215

2.552 2.672 2.652 2.732 2.672 2.751 2.811 2.890 3.069 3.108

890.5 919.9 936.2 949.4 959.2 978.9 1008.5 1072.6 1135.2 1185.5

-0.0011 -0.0018 -0.0024 -0.0029 -0.0039 -0.0055 -0.0097 -0.0146 -0.0191

2.978 3.094 3.072 3.150 3.089 3.167 3.222 3.294 3.465 3.498

797.7 822.5 836.2 847.3 855.5 872.2 897.1 951.0 1003.8 1046.1

-0.0009 -0.0014 -0.0019 -0.0023 -0.0032 -0.0046 -0.0082 -0.0125 -0.0165

3.405 3.519 3.494 3.571 3.508 3.584 3.635 3.700 3.864 3.891

719.5 739.5 750.5 759.5 766.2 779.6 799.7 843.2 885.7 919.9

-0.0007 -0.0011

3.834 3.945 3.919

653.2 672.5 683.2

0.0564 0.0658 0.0847 0.1130 0.1740 0.2336 0.2813

993.27 993.45 993.81 994.34 995.49 996.62 997.52

311.53 ± 1.3 311.47 ± 1.1 311.36 ± 0.8 311.19 ± 0.6 310.83 ± 0.4 310.48 ± 0.3 310.20 ± 0.2

18.157 18.157 18.157 18.158 18.160 18.162 18.164

311.19 311.08 310.86 310.53 309.81 309.11 308.56

-0.0015 -0.0018 -0.0025 -0.0037 -0.0067 -0.0105 -0.0141

3.993 3.929 4.003 4.051 4.108 4.265 4.286

691.9 698.3 711.3 730.8 772.8 814.0 847.0

a

Standard uncertainties are: u(T) = ± 0.03 K, u(P) = ± 0.01MPa, u(m) = ± 0.0002 mol.kg-1 and the expanded uncertainty is U (ρ) = ± 5 . 10-2 kg.m-3 at 0.95 level of confidence. b Standard uncertainties u are: u(T) = ± 0.05 K, u(P) = ± 0.01MPa, u(m) = ± 0.0002 mol.kg1 and the expanded uncertainty for dynamic viscosity is U(η ) ≤ ±1% at 0.95 level of confidence.

TABLE 3 The experimental values of density (ρ)a, apparent (φv) and partial molal volumes ( V1 ,

V2 ),

excess molar volume (VE), coefficient of thermal expansion (α) and dynamic viscosity (η)b for H2O (1) + [Pi(444)1][Tos] (2) at temperatures (298.15, 303.15, 308.15 and 313.15) K and at P = 0.1MPa. m/ (mol.kg-1)

ρ/

0.0000 0.0237 0.0468 0.0663 0.0901 0.1848 0.2883 0.3948 0.4278 0.4780 0.5156

997.04 997.74 998.42 999.00 999.70 1002.50 1005.48 1008.42 1009.28 1010.56 1011.48

0.0000 0.0237 0.0468 0.0663 0.0901 0.1848 0.2883 0.3948 0.4278 0.4780 0.5156

995.64 996.31 996.97 997.53 998.21 1000.89 1003.76 1006.58 1007.42 1008.64 1009.52

.

-3

(kg m )

10 6 .φv/ (m3.mol-1)

359.82 ± 3.0 359.53 ± 1.5 359.30 ± 1.1 359.02 ± 0.8 358.05 ± 0.4 357.22 ± 0.3 356.64 ± 0.3 356.52 ± 0.2 356.40 ± 0.2 356.36 ± 0.1

361.35 ± 3.0 361.08 ± 1.5 360.86 ± 1.1 360.59 ± 0.8 359.67 ± 0.4 358.88 ± 0.3 358.34 ± 0.2 358.24 ± 0.2 358.13 ± 0.1 358.09 ± 0.1

10 6. V1 / (m3.mol-1) 298.15 K 18.068 18.069 18.069 18.070 18.070 18.071 18.074 18.078 18.080 18.083 18.086 303.15 K 18.094 18.094 18.095 18.095 18.095 18.097 18.099 18.103 18.104 18.107 18.110 308.15 K 10

10 6. V2 / 106. VE/ 3 -1 3. -1 (m mol ) (m mol )

10 4.α / K-1

106.η / Pa.s

359.28 358.77 358.40 357.98 356.61 355.49 354.71 354.54 354.35 354.26

-0.0034 -0.0069 -0.0100 -0.0140 -0.0313 -0.0521 -0.0740 -0.0807 -0.0904 -0.0974

2.552 2.598 2.702 2.764 2.845 3.015 3.175 3.480 3.521 3.628 3.720

890.5 919.1 947.1 970.7 999.6 1114.8 1241.3 1372.5 1413.2 1475.3 1522.0

-0.0031 -0.0064 -0.0093 -0.0130 -0.0291 -0.0485 -0.0689 -0.0751 -0.0841 -0.0905

2.978 3.002 3.091 3.144 3.218 3.391 3.578 3.890 3.920 3.992 4.042

797.7 822.4 846.6 866.9 891.9 991.4 1100.6 1213.8 1248.9 1302.5 1342.8

360.83 360.35 360.00 359.61 358.31 357.26 356.53 356.37 356.20 356.12

0.0000 0.0253 0.0530 0.0791 0.1003 0.2056 0.2689 0.3193 0.3643 0.4345 0.5175

994.03 994.73 995.49 996.21 996.79 999.66 1001.35 1002.66 1003.80 1005.51 1007.42

362.69 ± 2.8 362.41 ± 1.4 362.16 ± 0.9 361.96 ± 0.7 361.09 ± 0.4 360.68 ± 0.3 360.40 ± 0.2 360.20 ± 0.2 359.97 ± 0.2 359.85 ± 0.1

0.0000 0.0253 0.0530 0.0791 0.1003 0.2056 0.2689 0.3193 0.3643 0.4345 0.5175

992.21 992.90 993.66 994.36 994.92 997.66 999.26 1000.50 1001.59 1003.25 1005.14

363.49 ± 2.8 363.35 ± 1.4 363.23 ± 0.9 363.13 ± 0.7 362.65 ± 0.4 362.39 ± 0.3 362.19 ± 0.2 362.02 ± 0.2 361.77 ± 0.2 361.50 ± 0.1

18.123 18.124 18.124 18.124 18.125 18.126 18.127 18.128 18.130 18.132 18.137 313.15 K 18.156 18.157 18.157 18.157 18.157 18.160 18.161 18.163 18.165 18.168 18.172

362.21 361.72 361.32 361.03 359.84 359.29 358.93 358.66 358.35 358.15

-0.0032 -0.0069 -0.0106 -0.0137 -0.0306 -0.0415 -0.0503 -0.0583 -0.0704 -0.0838

3.405 3.447 3.497 3.578 3.568 3.774 3.977 4.111 4.191 4.274 4.381

719.5 741.5 765.6 788.3 806.8 898.9 954.6 999.0 1038.8 1101.0 1174.9

363.35 363.08 362.84 362.65 361.79 361.32 360.96 360.67 360.23 359.77

-0.0033 -0.0069 -0.0105 -0.0134 -0.0285 -0.0381 -0.0459 -0.0530 -0.0643 -0.0779

3.834 3.854 3.887 3.958 3.943 4.159 4.379 4.526 4.608 4.673 4.704

653.2 671.7 692.0 711.1 726.6 804.2 851.0 888.3 921.8 974.1 1036.2

a

Standard uncertainties are: u(T) = ± 0.03 K, u(P) = ± 0.01MPa, u(m) = ± 0.0002 mol.kg-1 and the expanded uncertainty is U (ρ) = ± 5 . 10-2 kg.m-3 at 0.95 level of confidence. b Standard uncertainties u are: u(T) = ± 0.05 K, u(P) = ± 0.01MPa, u(m) = ± 0.0002 mol.kg1 and the expanded uncertainty for dynamic viscosity is U(η ) ≤ ±1% at 0.95 level of confidence.

TABLE 4 The experimental values of density (ρ)a, apparent (φv) and partial molal volumes ( V1 , V2 ), coefficient of thermal expansion (α) and dynamic viscosity (η)b for H2O (1) + [P4,4,4,4][Cl] (2) at temperatures (298.15, 303.15, 308.15 and 313.15) K and at P = 0.1MPa. m/ (mol.kg-1)

ρ/

0.0000 0.0249 0.0773 0.1592 0.2659 0.3842 0.4509 0.5040 0.5771

997.04 996.83 996.44 995.93 995.41 994.99 994.80 994.67 994.50

.

-3

(kg m )

10 6.φv/ (m3.mol-1)

304.12 ± 2.9 303.72 ± 0.9 303.12 ± 0.5 302.42 ± 0.2 301.75 ± 0.2 301.43 ± 0.2 301.21 ± 0.1 300.96 ± 0.1

10 6. V1 / (m3.mol-1) 298.15 K 18.068 18.069 18.069 18.072 18.076 18.082 18.084 18.086 18.089 303.15 K 11

106. V2 / (m3.mol-1)

10 4.α / K-1

106.η / Pa.s

303.91 303.15 302.01 300.77 299.80 299.45 299.26 299.14

2.552 2.676 2.783 2.871 3.199 3.405 3.609 3.752 3.857

890.5 923.4 992.5 1100.6 1241.4 1397.4 1485.5 1555.7 1652.2

0.0000 0.0249 0.0773 0.1592 0.2659 0.3842 0.4509 0.5040 0.5771

995.64 995.41 994.97 994.37 993.73 993.17 992.90 992.70 992.42

305.52 305.15 304.61 303.98 303.40 303.14 302.96 302.77

± 2.9 ± 0.9 ± 0.5 ± 0.3 ± 0.2 ± 0.2 ± 0.1 ± 0.1

0.0000 0.0295 0.0842 0.1722 0.2979 0.3554 0.4266 0.4508 0.5619

994.03 993.74 993.23 992.52 991.68 991.35 990.98 990.86 990.34

306.79 306.46 305.97 305.35 305.10 304.83 304.75 304.43

± 2.4 ± 0.9 ± 0.4 ± 0.2 ± 0.2 ± 0.2 ± 0.1 ± 0.1

0.0000 0.0295 0.0842 0.1722 0.2979 0.3554 0.4266 0.4508 0.5619

992.21 991.90 991.36 990.57 989.61 989.21 988.76 988.61 987.95

308.00 307.74 307.36 306.88 306.69 306.49 306.43 306.21

± 2.4 ± 0.9 ± 0.4 ± 0.2 ± 0.2 ± 0.2 ± 0.1 ± 0.1

18.094 18.094 18.095 18.097 18.101 18.105 18.107 18.108 18.109

308.15 K 18.123 18.123 18.124 18.126 18.130 18.132 18.135 18.135 18.138 313.15 K 18.156 18.156 18.157 18.159 18.162 18.163 18.165 18.165 18.166

305.32 304.63 303.60 302.53 301.75 301.52 301.43 301.45

2.978 3.094 3.186 3.258 3.574 3.771 3.970 4.108 4.204

797.7 825.6 884.3 976.1 1095.5 1227.9 1302.5 1362.0 1443.8

306.60 305.98 305.05 304.00 303.65 303.33 303.25 303.06

3.405 3.490 3.568 3.786 4.028 4.122 4.318 4.396 4.599

719.5 747.6 799.8 883.6 1003.6 1058.3 1126.2 1149.4 1255.3

307.85 307.36 306.64 305.85 305.60 305.40 305.35 305.30

3.834 3.910 3.973 4.176 4.406 4.495 4.686 4.763 4.954

653.2 677.3 722.0 793.7 896.4 943.2 1001.4 1021.2 1111.9

a

Standard uncertainties are: u(T) = ± 0.03 K, u(P) = ± 0.01MPa, u(m) = ± 0.0002 mol.kg-1 and the expanded uncertainty is U (ρ) = ± 5 . 10-2 kg.m-3 at 0.95 level of confidence. b Standard uncertainties u are: u(T) = ± 0.05 K, u(P) = ± 0.01MPa, u(m) = ± 0.0002 mol.kg1 and the expanded uncertainty for dynamic viscosity is U(η ) ≤ ±1% at 0.95 level of confidence.

values were calculated using the method of propagation of errors and are reported in tables 2 ─4. The φV values can also be expressed as,

φV = φV0 + SV c1 / 2 + BV c

(2)

The values of limiting apparent molar volume ( φV0 ) were then obtained by plotting (φV – S vc1/2) against concentration (c) of ILs as shown in figures 2─4. Sv is the DebyeHückel limiting slope for the apparent molar volume of aqueous solutions of 1:1

12

electrolyte, calculated by F. J. Millero [15]. The values of φV0 and the deviation parameter (Bv) obtained at all the studied temperatures are collected in table 5. The temperature dependence of limiting apparent molar volume can be expressed as a second order polynomial of absolute temperature [16],

φV0 = A + BT + CT 2

(3)

The coefficients of Eq. (3) obtained for all the selected ILs are given in table 6.

TABLE 5 The limiting apparent molar volume ( φV0 ), deviation parameter (BV), apparent molar expansibility ( φ E0 ) values for H2O + [P4,4,4,1][CH3SO4], H2O + [Pi(444)1][Tos] and H2O (1) + [P4,4,4,4][Cl] (2) at temperatures (298.15, 303.15, 308.15 and 313.15) K and at P = 0.1MPa. T/ (K)

106. φv0 / (m3.mol-1)

298.15 303.15 308.15 313.15

307.60 308.90 310.24 311.56

298.15 303.15 308.15 313.15

359.97 361.47 362.76 363.34

298.15 303.15 308.15 313.15

304.12 305.49 306.73 307.89

10 6Bv / 10 6Dv/ 3. . -2 (m L mol ) (m3.L2.mol-3) [P4,4,4,1][CH3SO4] -9.91 -9.78 -9.67 -9.59 [Pi(444)1][Tos] -18.79 17.47 -18.22 17.06 -16.46 15.23 -9.33 4.27 [P4,4,4,4][Cl] -12.36 6.61 -11.95 7.34 -10.71 6.13 -9.51 6.08

13

10 6. φ E0 / (m3.mol-1.K-1) 0.2614 0.2634 0.2654 0.2674

0.3660 0.2740 0.1820 0.0900 0.2825 0.2615 0.2405 0.2195

Standard uncertainties are: u(T) = ± 0.03 K, u(P) = ± 0.01MPa

FIGURE 2. Variation of (φV – S V√c), for H2O (1) + [P4,4,4,1][CH3SO4] (2) plotted against concentration (c), ◊, 298.15 K; □, 303.15 K; ∆, 308.15 K; ×, 313.15 K.

FIGURE 3. Variation of (φV – SV√c), for H2O (1) + [Pi(444)1][Tos] (2) plotted against concentration (c), ◊, 298.15 K; □, 303.15 K; ∆, 308.15 K; ×, 313.15 K. 14

FIGURE 4. Variation of (φV – SV√c), for H2O (1) + [P4,4,4,4][Cl] (2) plotted against concentration (c), ◊, 298.15 K; □, 303.15 K; ∆, 308.15 K; ×, 313.15 K.

TABLE 6 The coefficients (A, B and C) of Eq. 3 and the regression coefficient (R2).

10 6.A 247.4393 -566.9970 33.2189

106.B 106.C [P4,4,4,1][CH3SO4] 0.1421 0.0002 [Pi(444)1][Tos] 5.8520 -0.0092 [P4,4,4,4][Cl] 1.5347 -0.0021

R2 1.00 1.00 1.00

The limiting apparent molar expansibility ( φ E0 ) were calculated using Eq. 4,

 ∂φ 0  φ E0 =  v   ∂T  P

(4)

The values of limiting apparent molar expansibility of selected ILs at studied temperatures are reported in table 5.

15

The density data was further combined with apparent molar volume data to calculate partial molar volume of solvent ( V 1 ) [17] and solute ( V 2 ) [18] by using the equations,

V 1 = M1 /  ρ − c ( ∂ρ / ∂c ) 

(

(5)

))

V2 = φV + (1000 − cφV ) / 2000 + c c ∂φV / ∂ c  c ∂φV / ∂ c  

(

(

)

(6)

The values of V 1 and V 2 for selected ILs at studied temperatures are given in tables 2─4. [See figures S9─S14 in supporting information]. The excess molar volume (VE) was then calculated for aqueous solutions of [P4,4,4,1][CH3SO4] and [Pi(444)1][Tos] from the experimental density data at all the studied temperatures using Eq. 7 − 9 . As pure density data of [P4,4,4,4][Cl] is not available and it is solid in nature so it was not possible to measure its density. (7)

V E = Vm (expt) − Vm (ideal)

where

Vm (expt ) = [(mM 2 + 1000) / ρ (m + (1000/ M 1 ))] (8)

and

Vm (ideal) = ( x1 M 1 / ρ 0 ) + ( x2 M 2 / ρ1 )

(9)

where M1 and M2 are the molar masses of pure water and pure IL, respectively, x2 is the mole fraction of IL in the aqueous solution, ρ, ρ0 and ρ1 are the densities of solution, pure water and pure IL [10,19], respectively. The calculated values of VE and their concentration dependence for [P4,4,4,1][CH3SO4] and [Pi(444)1][Tos] are shown in tables 2,3 and figures 5, 6, respectively. Alternatively, the excess molar volume data can also be expressed in terms of the molality of solution at any temperature as,

VE =

3

∑Am

i

i

i =1

The coefficients of Eq. 10 at studied temperatures are given in table 7. 16

(10)

The different temperature density data of aqueous solutions of selected ILs were used to calculate isobaric expansivity and the calculated values are given in tables 2─4. The standard uncertainty in the measurement of α is found to be ± 5 × 10-6 K-1.  1  ∂ρ  α =  −    ρ  ∂T 

(11)

FIGURE 5. Excess molar volume (VE) for H2O (1) + [P4,4,4,1][CH3SO4] (2) plotted against molality (m), ◊, 298.15 K; □, 303.15 K; ∆, 308.15 K; ×, 313.15 K.

FIGURE 6. Excess molar volume (VE) for for H2O (1) + [Pi(444)1][Tos] (2) plotted against molality (m), ◊, 298.15 K; □, 303.15 K; ∆, 308.15 K; ×, 313.15 K. 17

TABLE 7 The coefficients (Ai) of excess molar volume (VE) according to equation (10) for H2O (1) + [P4,4,4,1][CH3SO4] (2) / Pi(444)1][Tos] (2) in the temperature range (298.15 to 313.15) K and at P = 0.1MPa with the regression (R2) of fitting.

T /K

10-6.A1

298.15 303.15 308.15 313.15

-0.0436 -0.0361 -0.0282 -0.0203

298.15 303.15 308.15 313.15

-0.1382 -0.1281 -0.1218 -0.1278

10 -6.A2

10 -6.A3 [P4,4,4,1][CH3SO4] -0.1185 0.0046 -0.1142 0.0044 -0.1101 0.0041 -0.1065 0.0039 [Pi(444)1][Tos] -0.2028 0.1803 -0.1919 0.1719 -0.1629 0.1472 -0.0590 0.0298

10-6.A4

10-6.A5

R2 1.00 1.00 1.00 1.00

0.0449 -0.0014 0.0429 -0.0013 0.0353 -0.0009 -0.0014

1.00 1.00 1.00 1.00

Standard uncertainties are: u(T) = ± 0.03 K, u(P) = ± 0.01MPa

3.2 Acoustic properties The speed of sound of aqueous solutions of all ILs at (298.15, 303.15, 308.15 and 313.15) K are reported in tables 8─10, respectively [see figures S15─S17 in supporting information]. The isentropic compressibility (βs) was obtained using Laplace equation,

βs = 1/ u 2 ρ

(12)

where, u and ρ are speed of sound and density of the aqueous solution of ILs [see figures S18─S20 in supporting information]. The uncertainty in βs values was obtained by method of propagation of errors using the following equation, 1

 2∂u  2  ∂ρ  2  2 ∂β s = β s   +     u   ρ   and it was found to be ± 1.1×10-13 Pa-1 for all the ILs. 18

(13)

TABLE 8 Speed of sound (u)a, compressibilities and hydration number (nh) for H2O (1) + [P4,4,4,1][CH3SO4] (2) at temperatures (298.15, 303.15, 308.15 and 313.15) K and at P = 0.1MPa.

m/ (mol.kg-1)

u/ (m.s-1)

10 11.βs/ (Pa-1)

0.0000 0.0282 0.0438 0.0564 0.0658 0.0847 0.1130 0.1740 0.2336 0.2813

1497.6 1504.7 1507.9 1510.3 1512.1 1515.5 1520.9 1533.3 1545.1 1554.3

44.72 44.27 44.07 43.92 43.81 43.59 43.25 42.50 41.80 41.26

0.0000 0.0282 0.0438 0.0564 0.0658 0.0847 0.1130 0.1740 0.2336 0.2813

1509.6 1517.7 1521.6 1524.6 1526.7 1530.7 1536.2 1547.8 1558.6 1566.7

44.07 43.58 43.34 43.16 43.03 42.79 42.46 41.77 41.15 40.68

0.0000 0.0282 0.0438 0.0564 0.0658 0.0847 0.1130 0.1740 0.2336 0.2813

1519.9 1527.2 1530.9 1533.7 1535.9 1539.9 1545.9 1557.5 1568.4 1576.3

43.55 43.11 42.89 42.72 42.59 42.35 42.00 41.33 40.71 40.26

0.0000 0.0282 0.0438 0.0564 0.0658 0.0847

1528.5 1536.3 1540.0 1542.7 1544.8 1548.8

43.14 42.68 42.46 42.30 42.18 41.95

1015. φ K / S

(m3. mol-1 Pa-1) 298.15 K -23.77 ± 4.3 -13.93 ± 2.8 -8.11 ± 2.1 -4.84 ± 1.8 -0.42 ± 1.4 2.38 ± 1.0 2.29 ± 0.7 2.79 ± 0.5 3.01 ± 0.4 303.15 K -41.36 ± 4.2 -34.14 ± 2.7 -29.29 ± 2.1 -25.67 ± 1.8 -19.89 ± 1.4 -12.35 ± 1.0 -3.98 ± 0.6 0.80 ± 0.5 3.93 ± 0.4 308.15 K -22.85 ± 4.2 -18.39 ± 2.7 -15.10 ± 2.1 -14.09 ± 1.8 -10.62 ± 1.4 -7.58 ± 1.0 -0.42 ± 0.6 3.53 ± 0.5 6.86 ± 0.4 313.15 K -31.21 ± 4.1 -23.12 ± 2.6 -17.55 ± 2.0 -15.29 ± 1.7 -11.12 ± 1.3 19

15 1011.β T/ 10 . φ K / (Pa-1) (m3. mol-1 Pa-1) T

nh

45.19 44.78 44.57 44.45 44.32 44.13 43.82 43.10 42.47 41.95

-6.38 ± 8.1 -3.99 ± 5.1 5.51 ± 4.1 3.43 ± 3.4 10.08 ± 2.7 12.82 ± 2.1 11.57 ± 1.4 13.62 ± 1.0 12.98 ± 0.8

19.8 18.5 17.7 17.3 16.6 16.1 15.8 15.5 15.3

44.72 44.28 44.03 43.88 43.73 43.52 43.22 42.56 42.02 41.57

-20.95 ± 9.1 -22.60 ± 5.8 -13.48 ± 4.6 -16.16 ± 3.9 -7.76 ± 3.1 -0.35 ± 2.3 6.63 ± 1.5 13.15 ± 1.9 15.24 ± 0.9

22.1 21.1 20.4 19.9 19.1 18.0 16.6 15.8 15.2

44.41 44.03 43.79 43.67 43.50 43.30 42.98 42.34 41.81 41.38

0.66 ± 10.2 -5.24 ± 6.5 2.92 ± 5.1 -3.36 ± 4.3 3.15 ± 3.4 5.98 ± 2.6 11.49 ± 1.7 17.39 ± 1.3 19.49 ± 1.1

19.8 19.2 18.7 18.5 18.0 17.4 16.3 15.5 14.9

44.25 43.85 43.62 43.51 43.35 43.16

-4.50 ± 11.4 -8.37 ± 7.3 2.72 ± 5.7 -3.37 ± 4.8 4.29 ± 3.8

21.0 19.8 19.1 18.7 18.1

0.1130 0.1740 0.2336 0.2813

1554.2 1564.7 1572.0 1578.0

41.63 41.03 40.60 40.26

-4.65 ± 0.9 5.39 ± 0.6 16.70 ± 0.5 21.79 ± 0.4

42.87 42.30 41.97 41.64

10.45 ± 2.9 18.60 ± 1.9 32.06 ± 1.5 35.73 ± 1.2

17.1 15.6 14.0 13.2

Standard uncertainties u are: u(T) = ± 0.05 K, u(P) = ± 0.01MPa, u(m) = ± 0.0002 mol.kg-1 and the expanded uncertainty is U(u) = ± 1.0 m.s-1 at 0.95 level of confidence. a

TABLE 9 Speed of sound (u)a, compressibilities and hydration number (nh) for H2O + [Pi(444)1][Tos] at temperatures (298.15, 303.15, 308.15 and 313.15) K and at P = 0.1MPa.

m/ (mol.kg-1)

u/ (m.s-1)

1011.β s/ (Pa-1)

0.0000 0.0237 0.0468 0.0663 0.0901 0.1848 0.2883 0.3948 0.4278 0.4780 0.5156

1497.6 1507.2 1513.7 1518.8 1524.8 1546.4 1566.5 1586.1 1592.4 1601.7 1608.3

44.72 44.12 43.72 43.40 43.02 41.72 40.53 39.42 39.07 38.57 38.22

0.0000 0.0237 0.0468 0.0663 0.0901 0.1848 0.2883 0.3948 0.4278 0.4780 0.5156

1509.6 1518.8 1525.3 1530.8 1536.6 1555.9 1574.5 1590.7 1596.2 1604.0 1609.4

44.07 43.51 43.11 42.78 42.43 41.27 40.19 39.26 38.96 38.54 38.24

0.0000 0.0253 0.0530 0.0791 0.1003 0.2056 0.2689 0.3193

1519.9 1528.8 1536.4 1541.7 1545.6 1566.2 1577.9 1587.2

43.55 43.01 42.56 42.23 42.00 40.78 40.11 39.59

10 15. φ K / S

(m3. mol-1 Pa-1) 298.15 K -95.70 ± 5.1 -57.81 ± 2.6 -44.16 ± 1.8 -34.33 ± 1.3 -13.64 ± 0.6 -1.02 ± 0.4 5.93 ± 0.3 6.88 ± 0.2 8.51 ± 0.2 9.82 ± 0.2 303.15 K -80.90 ± 5.0 -50.26 ± 2.5 -41.56 ± 1.7 -30.32 ± 1.3 -3.81 ± 0.6 8.82 ± 0.4 18.31 ± 0.3 19.48 ± 0.2 21.69 ± 0.2 23.39 ± 0.2 308.15 K

-56.74 ± 4.6 -34.33 ± 2.2 -14.40 ± 1.4 -3.84 ± 1.1 11.81 ± 0.5 16.01 ± 0.4 17.96 ± 0.3 20

15 1011.β T/ 10 . φ K / (Pa-1) (m3. mol-1 Pa-1) T

nh

45.19 44.60 44.24 43.94 43.60 42.36 41.24 40.28 39.95 39.50 39.20

-86.92 ± 9.4 -44.01 ± 4.9 -30.17 ± 3.5 -19.85 ± 2.6 -1.51 ± 1.3 10.20 ± 0.8 18.91 ± 0.7 19.63 ± 0.6 21.55 ± 0.6 23.21 ± 0.5

31.5 26.6 24.8 23.4 20.2 18.0 16.7 16.4 16.0 15.6

44.72 44.17 43.81 43.50 43.18 42.10 41.11 40.35 40.06 39.68 39.42

-74.12 ± 10.5 -37.17 ± 5.4 -27.93 ± 3.9 -15.75 ± 2.9 9.37 ± 1.4 21.86 ± 0.9 33.53 ± 0.7 34.25 ± 0.7 36.31 ± 0.6 37.88 ± 0.6

29.9 25.8 24.6 23.0 19.1 17.0 15.3 15.1 14.6 14.2

44.41 43.89 43.46 43.18 42.94 41.83 41.27 40.83

-45.32 ± 11.1 -22.37 ± 5.4 0.19 ± 3.7 7.78 ± 2.8 24.93 ± 1.5 31.61 ± 1.2 34.50 ± 1.0

27.0 23.9 21.2 19.7 17.2 16.3 15.8

0.3643 0.4345 0.5175

1592.7 1602.0 1612.8

39.27 38.75 38.16

0.0000 0.0253 0.0530 0.0791 0.1003 0.2056 0.2689 0.3193 0.3643 0.4345 0.5175

1528.5 1538.7 1545.1 1551.5 1556.3 1575.1 1584.7 1592.3 1598.1 1607.3 1617.1

43.14 42.54 42.16 41.78 41.50 40.40 39.85 39.42 39.09 38.58 38.05

23.34 ± 0.3 28.44 ± 0.2 32.58 ± 0.2 313.15 K -83.97 ± 4.6 -33.91 ± 2.1 -21.56 ± 1.4 -14.26 ± 1.1 12.34 ± 0.5 21.05 ± 0.4 25.44 ± 0.3 29.58 ± 0.3 33.92 ± 0.2 38.36 ± 0.1

40.56 40.09 39.57

39.87 ± 0.9 44.36 ± 0.8 48.23 ± 0.7

15.0 14.1 13.3

44.25 43.66 43.29 42.96 42.67 41.70 41.29 40.96 40.68 40.21 39.70

-75.68 ± 12.4 -24.29 ± 5.9 -8.33 ± 4.0 -3.95 ± 3.2 26.29 ± 1.6 38.55 ± 1.3 44.37 ± 1.1 48.57 ± 1.0 51.90 ± 0.8 54.83 ± 0.7

30.5 23.9 22.1 21.1 17.1 15.7 15.0 14.3 13.5 12.7

Standard uncertainties u are: u(T) = ± 0.05 K, u(P) = ± 0.01MPa, u(m) = ± 0.0002 mol.kg-1 and the expanded uncertainty is U(u) = ± 1.0 m.s-1 at 0.95 level of confidence. a

TABLE 10 Speed of Sound (u )a, Compressibilities and hydration number (n h) for H2O + [P4,4,4,4][Cl] at temperatures (298.15, 303.15, 308.15 and 313.15) K and at P = 0.1MPa.

m/ (mol.kg-1)

u/ (m.s-1)

10 11.βs/ (Pa-1)

0.0000 0.0249 0.0773 0.1592 0.2659 0.3842 0.4509 0.5040 0.5771

1497.6 1506.4 1520.0 1541.6 1567.7 1593.7 1607.4 1617.5 1630.3

44.72 44.21 43.44 42.25 40.88 39.57 38.91 38.43 37.83

0.0000 0.0249 0.0773 0.1592 0.2659 0.3842 0.4509 0.5040 0.5771

1509.6 1518.2 1532.4 1551.9 1575.2 1598.2 1610.1 1619.5 1633.2

44.07 43.59 42.80 41.76 40.56 39.42 38.85 38.41 37.78

0.0000 0.0295

1519.9 1529.8

43.55 43.00

1015. φ K / S

(m3. mol-1 Pa-1) 298.15 K -71.51 ± 4.9 -34.47 ± 1.5 -27.45 ± 0.7 -21.33 ± 0.4 -15.03 ± 0.3 -12.00 ± 0.2 -9.47 ± 0.2 -5.85 ± 0.1 303.15 K -63.43 ± 4.7 -34.80 ± 1.5 -18.93 ± 0.7 -9.55 ± 0.4 -2.06 ± 0.3 1.35 ± 0.2 3.47 ± 0.2 4.78 ± 0.1 308.15 K -55.26 ± 3.9 21

15 10 11.βT/ 10 . φ K / (Pa-1) (m3. mol-1 Pa-1) T

nh

45.19 44.71 43.99 42.84 41.61 40.40 39.84 39.44 38.90

-51.26 ± 9.1 -21.30 ± 3.0 -17.83 ± 1.5 -9.03 ± 0.9 -2.99 ± 0.7 1.22 ± 0.6 4.39 ± 0.6 7.81 ± 0.5

25.5 20.6 19.2 17.9 16.6 16.0 15.5 14.8

44.72 44.28 43.54 42.53 41.49 40.46 40.00 39.64 39.07

-40.52 ± 10.3 -20.30 ± 3.4 -8.47 ± 1.7 4.10 ± 1.0 11.35 ± 0.8 16.11 ± 0.7 18.91 ± 0.6 19.93 ± 0.6

24.6 20.7 18.3 16.6 15.2 14.6 14.1 13.7

44.41 43.90

-37.53 ± 9.7

23.7

0.0842 0.1722 0.2979 0.3554 0.4266 0.4508 0.5619

1541.2 1559.9 1585.8 1595.6 1607.8 1611.9 1628.9

42.39 41.41 40.10 39.62 39.04 38.84 38.06

0.0000 0.0295 0.0842 0.1722 0.2979 0.3554 0.4266 0.4508 0.5619

1528.5 1537.3 1547.8 1566.5 1588.4 1597.3 1607.8 1611.4 1627.3

43.14 42.66 42.11 41.14 40.05 39.62 39.12 38.96 38.22

-8.81 ± 1.3 1.52 ± 0.6 5.97 ± 0.4 9.65 ± 0.3 12.59 ± 0.2 13.34 ± 0.2 17.52 ± 0.1 313.15 K -32.21 ± 3.9 5.99 ± 1.3 9.38 ± 0.6 18.48 ± 0.4 21.81 ± 0.3 25.07 ± 0.2 25.84 ± 0.2 28.88 ± 0.1

43.33 42.47 41.31 40.88 40.42 40.28 39.63

4.25 ± 3.4 16.76 ± 1.7 21.36 ± 1.0 24.95 ± 0.9 29.26 ± 0.8 30.62 ± 0.8 35.11 ± 0.6

17.6 15.8 14.7 14.1 13.5 13.3 12.4

44.25 43.81 43.30 42.46 41.52 41.15 40.79 40.67 40.08

-13.34 ± 10.8 19.57 ± 3.8 25.66 ± 1.9 35.15 ± 1.1 38.43 ± 1.0 43.26 ± 0.8 44.72 ± 0.8 48.05 ± 0.7

20.9 15.8 14.9 13.3 12.7 12.1 11.9 11.3

a Standard uncertainties u are: u(T) = ± 0.05 K, u(P) = ± 0.01MPa, u(m) = ± 0.0002 mol.kg-1 and the expanded uncertainty is U(u) = ± 1.0 m.s-1 at 0.95 level of confidence.

The isothermal compressibility for ILs at studied temperatures was calculated by using β s, α and Cp and using Eq. 14,

βT - βS = δ = α2T/σ = α2T/Cpρ

(14)

where α is the coefficient of thermal expansion, σ is the volumetric specific heat capacity and δ is the difference between the compressibilities. The Cp values for water at different temperatures were taken from the literature [11] and were assumed to be constant for studied concentration range. The uncertainty in βT values was obtained by method of propagation of errors using the following equation and the error was found to be ± 2.5×10 13

Pa-1 [see figures S21─S23 in supporting information]. 2 2     2αT   α 2T 2 ∂β T =  ∂β s +  ∂α  +  ∂ρ   2   CP ρ   CP ρ  

1

2

(15)

The apparent molar isentropic compressibility ( φ K ) and apparent molar isothermal s

compressibility ( φ KT ) of aqueous solutions of [P4,4,4,1][CH3SO4], [Pi(444)1] [Tos] and [P4,4,4,4][Cl] at (298.15, 303.15, 308.15 and 313.15) K were calculated using the equation, 22

(

)

 M 2 β   βρ 0 − β 0 ρ  φ k =    +   ρ   mρρ 0 

(16)

where, M2 is molar mass of ILs, m is molality while β, ρ, β0, ρ0 are the (isentropic or isothermal) compressibility and density values of solution and solvent, respectively. The values of φ K and φ K are given in tables 8 − 10. The variation of ( φ K − S K s

T

s

m ) as a

function of molality of ILs at different temperatures are shown in figures 7 ─ 9. [For variation of φ K see figures S24─S26 in supporting information]. To evaluate the T

uncertainty in φ K and φ K values the Eq. 17 was used. s

T

 1   ∂β ∂φ K =  M 12 + 2   m   ρ 

2

  β ∂ρ   +  2    ρ 

2

   

1

2

(17)

In this, M1 is the molecular weight of ILs, m, ρ and β s are the molality, density and isentropic or isothermal compressibility values for aqueous solutions of ILs. The uncertainties obtained for φ K and φ K are reported in tables 8−10. The uncertainty values s

T

for apparent molar compressibility were found to be large in magnitude for low concentrations. The hydration number (nh) of studied ILs at selected temperatures were calculated by using Passynski equation [20],

nh =

n1 n2

 β  1 − s0   βs 

(18)

where, βs and β s0 are the isentropic compressibilities of solution and solvent, respectively and n 1 and n2 are the number of moles of solute and solvent, respectively. The values of hydration number are given in tables 8─10.

23

FIGURE 7. The dependence of apparent molar isentropic compressibility ( φKS −SK m) for

H2O (1) +[P4,4,4,1][CH3SO4] (2) plotted against molality (m), ◊, 298.15 K; □, 303.15 K; ∆, 308.15 K; ×, 313.15 K

FIGURE 8. The dependence of apparent molar isentropic compressibility ( φKS −SK m) for

H2O (1) + [Pi(444)1][Tos] (2) plotted against molality (m), ◊, 298.15 K; □, 303.15 K; ∆, 308.15K; ×, 313.15 K.

24

FIGURE 9. The dependence of apparent molar isentropic compressibility ( φKS −SK m) for

H2O (1) +[P4,4,4,4][Cl] (2) plotted against molality (m), ◊, 298.15 K; □, 303.15 K; ∆, 308.15 K; ×, 313.15 K.

3.3 Viscometric properties The dynamic viscosity (η) values of aqueous solutions of [P4,4,4,1][CH3SO4], [Pi(444)1][Tos] and [P4,4,4,4][Cl] at all studied temperatures were calculated using experimental densities and efflux time (t) of viscometer using equation,

η tρ = η0 t 0 ρ 0

(19)

where η and η0 are the dynamic viscosities of solution and solvent, respectively, ρ and ρ0 represents the densities of solution and solvent, respectively, t and t0 are the efflux times of solution and solvent, respectively. The dynamic viscosity values of selected ILs are reported in tables 2 – 4 [see figures S27─S29 in supporting information]. The viscosity data was also tested by using Jones-Dole equation [21, 22],

25

1 η = 1 + Ac 2 + Bc + Dc 2 η0

(20)

where, η and η0 are the viscosities of solution and solvent, respectively, and c represents the solute concentration. As Jones-Dole equation is applicable to low concentration for c > 0.5 M the additional parameters are required. The coefficients obtained according to Eq. 20 are given in table 11. The variation of viscosity B-coefficient with temperature is given in figure 10.

TABLE 11

Viscosity A, B and D coefficients, temperature coefficient of B (dB/d T) and limiting effective volume of solution ( V e0 ) for H2O (1) + [P4,4,4,1][CH3SO4] (2), H2O (1) + [Pi(444)1][Tos] (2) and H2O (1) + [P4,4,4,4][Cl] (2) at temperatures (298.15, 303.15, 308.15 and 313.15) K and at P = 0.1MPa.

T/K

A/ (dm3/2mol-1/2)

298.15 303.15 308.15 313.15

-0.0045 -0.0051 -0.0046 -0.0048

298.15 303.15 308.15 313.15

-0.0102 -0.0097 -0.0097 -0.0089

298.15 303.15 308.15 313.15

-0.0095 -0.0092 -0.0096 -0.0092

B/ D/ (dm3mol-1) (dm6.mol-2) [P4,4,4,1][CH3SO4] 1.2078 (1.14)a 0.6465 1.1409 (1.07)a 0.5907 1.0223 (0.97)a 0.5285 1.0908 (1.03)a 0.5703 [Pi(444)1][Tos] 1.4261 (1.33)a 0.7783 1.3762 (1.29)a 0.7554 1.2751 (1.19)a 0.6955 1.1838 (1.11)a 0.6490 [P4,4,4,4][Cl] 1.5489(1.45)a 0.6122 1.4709(1.38)a 0.5804 1.3929 (1.30)a 0.5534 1.3161 (1.23)a 0.5273

d B/d T

(dm3.mol-1)

nh

-0.0094

0.46 (0.47)b 0.43 (0.44)b 0.39 (0.40)b 0.41 (0.42)b

8.16 6.67 4.22 5.52

-0.0166

0.53 (0.54)b 0.52 (0.52)b 0.48 (0.48)b 0.44 (0.45)b

9.61 8.53 6.33 4.45

-0.0155

0.58 (0.59)b 0.55 (0.56)b 0.52 (0.53)b 0.49 (0.50)b

15.22 13.57 11.80 10.20

Standard uncertainties are: u(T) = ± 0.03 K, u(P) = ± 0.01MPa a The values of Viscosity B- coefficient are calculated by using equation 25. b The values of V e0 are calculated using equation 24.

26

V e0 /

FIGURE 10. The Viscosity B-coefficient plotted against Temperature (K), ◊,

[P4441][CH3SO4]; □, [Pi(444)1][Tos]; ∆, [P4444][Cl]. The hydration number (nh) were calculated using following equation [23], nh =

Ve0 − φ V0 V H 2O

(21)

where, φV0 is the limiting apparent molar volume, Ve0 is the limiting value of effective flowing volume of solution, V H 2O is the molar volume of water. The values of hydration number (nh) are given in table 11. The value of Ve0 is obtained by extrapolating the graph

Ve vs c to zero concentration. The variation of Ve against c is given in figures 11 ─ 13.

27

FIGURE 11. Variation of effective flowing volume Ve for H2O (1) + [P4,4,4,1][CH3SO4] (2)

solutions plotted against concentration (c), ◊, 298.15 K; □, 303.15 K; ∆, 308.15 K; ×, 313.15 K.

FIGURE 12. Variation of effective flowing volume Ve for H2O (1) + [Pi(444)1][Tos] (2)

solutions plotted against concentration (c), ◊, 298.15 K; □, 303.15 K; ∆, 308.15 K; ×, 313.15 K.

28

FIGURE 13. Variation of effective flowing volume Ve for H2O (1) + [P4,4,4,4][Cl] (2)

solutions plotted against concentration (c), ◊, 298.15 K; □, 303.15 K; ∆, 308.15 K; ×, 313.15 K. The value of Ve was obtained using Eq. 22 [28]. − 2.5c + Ve =



  η 0 

(2.5c )2 − 4(10.05c 2 )1 − η 

2(10.05)c

2

(22)

and also by using the Vand theory of viscosity [25,26], η  2.5φ ln   =  η 0  1 − Kφ

(23)

Taking log of both side and substituting φ = cVe we get, c η log  η0

  

=

2.303 2.303 K − c 2.5V e 2.5

(24)

The plot of c/log(η/η0) vs c was found to be linear for all ILs at all studied temperature as mentioned by Eagland and Pilling [31]. So, from the slope of this plot Ve0

29

can be calculated. The values of Ve0 obtained by Eq. 22 and 24 are listed in table 11 and they are almost comparable. Also the viscosity B coefficient can be related with Ve0 by the relation,

B = 2.5 Ve0

(25)

4. Discussion

The density data obtained for aqueous solutions of [Pi(444)1][Tos] agree well with the data obtained with the use of pycnometer from the same laboratory [28]. The density values [See figures S6─S8 in supporting information] of aqueous solutions of [P4,4,4,1][CH3SO4] and [Pi(444)1][Tos] increase with increase in concentration whereas those of [P4,4,4,4][Cl] decrease with increase in concentration, at all the selected temperatures. The trend observed for the density of [P4,4,4,4][Cl] solutions indicates the possibility of densities passing through a minimum and then increasing at higher concentrations of [P4,4,4,4][Cl]. The increase in temperature was found to cause the decrease in the density for all the studied systems. Unlike the different variations of density with concentration, the apparent molar volume (φV) values were found to decrease with increase in concentration for all the three ILs at the selected temperatures and are shown in figures 2─4. These observations indicate increase in ion-ion interaction with increase in concentration. Similarly increase in φV0 values with increase in temperature indicates increase in ionsolvent interaction. The nature of figures 2 ─4 show negative deviation from Debye-Hückel limiting law for all the three ILs as evident from negative deviation parameter (BV) (see table 5) obtained at all the selected temperatures. BV is generally negative as observed for monofunctional alcohols, amines and tetra-alkyl ammonium salts due to cation-cation sphere overlap [29]. Negative BV indicate presence of water induced IL co-sphere overlap i.e. increase in ion-ion interactions showing water structure making effect and 30

simultaneous release of water molecules to bulk solvent. The water structure making effect is due to increased H-bonding in the water cluster formed around solute molecule i.e. hydrophobic hydration. The negative value of Bv has been observed for large structure making cations [30─32]. The BV values of [Pi(444)1][Tos] at all the temperatures are more negative than [P4,4,4,1][CH3SO4] and [P4,4,4,4][Cl]. The relative magnitude of BV values indicate ion-ion interactions is in the sequence [Pi(444)1][Tos] > [P4,4,4,4][Cl] > [P4,4,4,1][CH3SO4] at all temperatures except at 313.15 K, where they are comparable. The negative BV values are decreasing with increase in temperature means decrease in ion-ion interaction with temperature or increase in ion-solvent interaction. The same can be concluded from the behavior of partial molar volume of solvent ( V 1 ) and solute ( V 2 ) as a function of concentration (see figures S8−S13 in supporting information). Here the V 1 for the studied systems increases with IL concentration. This behavior is similar to that observed for hydrophobic neutral solutes like alcohols, ethers and tetra-alkyl ammonium salts [33]. In the case of [Pi(444)1][Tos] the increase in V 1 is more at higher concentration than [P4,4,4,1][CH3SO4] and [P4,4,4,4][Cl]. These results indicate that the selected ILs are structure makers even though one of the IL have [Cl]− which marginally behaves as structure breaker [22,34]. As reported in table 5 the φ E0 values for all the three ILs at all the selected temperatures are positive but in case of [Pi(444)1][Tos] and [P4,4,4,4][Cl] these are decreasing with temperature while for [P4,4,4,1][CH3SO4] the values are almost comparable or only marginally increase. The decrease in φ E0 with temperature for [Pi(444)1][Tos] and [P4,4,4,4][Cl] indicates more expansion of solvent than corresponding solutions. This again indicates water structure making effect at all the studied temperatures but with decrease in extent as temperature increases in case of [Pi(444)1][Tos] and [P4,4,4,4][Cl]. This is because increase in temperature counterbalances partly the increased H-bonding in the water cluster due to the presence of (hydrophobic) solute molecule. The 31

marginal increase in φE0 with temperature for [P4,4,4,1][CH3SO4] indicates more expansion of solution than pure solvent but to very small extent. As observed from figures 5 and 6 the excess molar volumes are negative for both [P4,4,4,1][CH3SO4] and [Pi(444)1][Tos]. The VE values could not be calculated for [P4,4,4,4][Cl] system as values of density of pure [P4,4,4,4][Cl] were not available at any temperature. The VE values become more negative with increase in concentration. The small negative values of VE can be related to quasi–ideal behavior similar to that reported for imidazolium based ILs in water and ethanol [35]. The negative VE values can be explained according to Varela, L. M. et al. [36] as due to the solvent molecules occupying the free holes of the pseudolattice formed by IL cations. Such occupation of free holes of the [P4,4,4,1]+ [CH3SO4]− and [Pi(444)1] + [Tos] − along with ion-dipole interaction between ILs and water results in negative values of VE. This may be due to the large difference between the molar volumes of ILs and water. The figures 5 and 6 show that [Pi(444)1][Tos] have large negative

VE value than [P4,4,4,1][CH3SO4]. This indicates that there is a strong ion-ion interaction and more packing effect in [Pi(444)1][Tos] than in [P4,4,4,1][CH3SO4]. The trend in VE with temperature can be explained with the help of competing effects of ion-dipole interaction between IL and water and the temperature dependent kinetic energy of molecules. In both cases the increase in VE suggests that the kinetic energy effect (which tends to reduce ion-ion interaction) dominates interaction effect thereby making system more ideal and hence volume contraction decreases with increasing temperature [37, 38]. The sound velocity increases with both concentration and temperature for all the three ILs [see figures S15−S17 in supporting information]. For [Pi(444)1][Tos] and [P4,4,4,4][Cl] the difference in sound velocity due to change in temperature at a fixed concentration goes on decreasing after which the sound velocity may become temperature 32

independent or may show inverse effect with temperature. As the measurements in [Pi(444)1][Tos] and [P4,4,4,4][Cl] systems were done only upto ~ 0.57 mol kg-1 the further variation could not be observed. For [P4,4,4,1][CH3SO4] system similar intersection point in sound velocity with temperature was not observed in the studied concentration range. All the three ILs show decrease in βS and βT with respect to concentration and temperature [see figures S18–S23 in supporting information]. The decrease in βS values with increasing concentration is due to combined effect of solvation of ions and breaking of solvent structure as solute intrinsic contribution is dominating than solvent intrinsic contribution to compressibility. The decrease in βT values with increase in concentration for all the three systems indicates decrease in compressibility with increase in concentration. This is because with increase in concentration more number of bulk water molecules get incorporated in hydration layer thereby decreasing the number of bulk water molecules which are compressible. The decrease in βS with increase in temperature indicates decrease in thermal agitation giving rise to more ion-solvent interactions with temperature, resulting into less solution volume making solution less compressible. Like sound velocity the isentropic compressibility isotherms [see figures S18 ─S20 in supporting information] intersect approximately at ~0.4 mol.kg-1 in case of [P4,4,4,4][Cl] and for [Pi(444)1][Tos] approximately above ~0.5 mol.kg-1. This means that above this intersection point all the water in solution is in hydration layer of ions and according to Onori [37] the β S is temperature independent above this point. As IL is an organic electrolyte so there is possibility of formation of clathrate like structure in aqueous solutions [40,41].

The βT decreases with increase in temperature up to certain

concentration and then increases. This is similar to the behavior exhibited by alcohol-water solutions, where compressibility goes through a minimum at a certain low concentration [33,42] and also similar to the behavior of aqueous solutions of salts like NaCl, Na2SO4, 33

MgCl2, and MgSO4 [43]. It can be attributed to dominance of more structured form of water over less structured form, in the studied temperature range, as suggested by F. J. Millero et al. [43]. The β T decreases with an intersection at a fixed concentration, like sound velocity and isentropic compressibility, for [P4,4,4,4][Cl] and [Pi(444)1][Tos] and may be for [P4,4,4,1][CH3SO4] at higher concentrations. Again the decrease in β T with temperature for solutions is more than or comparable to that for water which indicates the increase in water structure breaking effect of ions with temperature. The variation of apparent molar isentropic ( φ K ) data with concentration of s

[P4,4,4,1][CH3SO4], [Pi(444)1][Tos] and [P4,4,4,4][Cl] at studied temperatures are shown in figures 7─9 [For isothermal compressibility ( φ K ) see figures S24─S26 in supporting T

information]. Figures shows that for dilute region the values of ( φ K s and

φK

T

) are

negative and increases with concentration and temperature for all ILs. Negative

φ K indicates strong attractive solute-solvent interactions due to solvation of solute making s

the solution less compressible. The

φ K values are in the sequence [Pi(444)1][Tos] < s

[P4,4,4,4][Cl] < [P4,4,4,1][CH3SO4] indicating ion-solvent interactions to be present in the sequence [P4,4,4,1][CH3SO4] > [P4,4,4,4][Cl] > [Pi(444)1][Tos]. From figures 7-9 we can see that the values of limiting apparent molar isentropic compressibilities φ K0 are negative for s

all the temperatures and are increasing with temperature. The negative values of

φ K0 indicate that the hydrophobically hydrated phosphonium cations are not compressible s

and the water structure around the P+ centre is less compressible i.e. it is attributed to hydration of cation. The

φ K0 values increase in order [Pi(444)1][Tos] < [P4,4,4,4][Cl] < s

0

[P4,4,4,1][CH3SO4] i.e. [Pi(444)1][Tos] has more negative φ K s values. This may be due to the bulky cation and anion of this IL. As the cation contains three isobutyl chains and anion 34

has one hydrophobic –CH3 group, the water molecules around this group are less compressible and the negative delocalized charge on oxygen causes electrostriction of water molecules and increase in the H-bonding around the water molecules in water cluster. The trend in φ K0 is similar to that observed for φV0 values [44,45]. The negative s

values are generally observed for electrolytes in aqueous solutions and large negative values are observed for metal halides [43,46]. The values of coefficient A, which signifies the electrostatic interaction between the ions, are negative as reported in table 11. This shows that there exist strong cation-anion interactions. The decrease in viscosity A coefficient with temperature indicates decrease in ion-ion interaction with temperature. The second term of Jones-Dole equation signifies the effect of ion on hydrogen bond. For large hydrophobic ions in aqueous solutions the value of viscosity B coefficient are expected large and positive. So, the large positive values of viscosity B coefficient denote a water structure making nature of IL [34]. The viscosity D coefficient is the higher term for the columbic forces and signifies about the solute-solute interactions. Presence of positive value of viscosity D coefficient indicates the presence of solute-solute interaction [21]. The variation of viscosity B coefficient with temperature can be explained from the sign of dB/dT. From table 11, it is seen that B > 0 and dB/dT < 0 i.e. the phosphonium cations are water structure making (i.e. hydrophobic hydration) [23]. Even the slope of Eq. 24 is positive for all ILs at all the temperatures. This also supports water structure making effect of these ILs [27]. The negative d B/dT decreases in the sequence [Pi(444)1][Tos] > [P4,4,4,4][Cl] > [P4,4,4,1][CH3SO4]. This indicates the decrease in ion-ion interaction in the same sequence as evident from BV parameter also. The hydration number (nh) measures the number of water molecules surrounding the solute molecule. The hydration numbers are obtained from Passynski’s equation [20] and from viscosity B coefficient using Eq. 21 35

(The Ve0 was obtained from the variation of Ve against c, see figures 11 ─ 13). The hydration number values decrease with concentration for all three ILs and at all the studied temperatures. The decrease in hydration number is an evidence of presence of solvent induced co-sphere overlap i.e. increase in ion-ion interactions with simultaneous release of water molecules from hydration. This results into increase in water structure making effect with concentration thereby decreasing the compressibility. This result is in accordance with the decrease in βT values with increase in concentration for all the three systems.

5. Conclusion

In this paper, we have discussed the results of our study about the effect of temperature and concentration on the volumetric, acoustic and viscometric properties of phosphonium based ILs namely [P4,4,4,1][CH3SO4], [Pi(444)1][Tos] and [P4,4,4,4][Cl] at atmospheric pressure. The measured and derived properties of these ILs at different temperatures indicate the presence of solvent induced ion-ion interactions which increases with concentration and decreases with temperature. This makes the systems water structure forming as a function of concentration and the effect decreases with increase in temperature. The increase in ion-ion interactions makes the systems less compressible as the IL concentration increases. The results indicate the dominance of phosphonium containing cation over the anions present in the ILs. The alkyl groups and their carbon framework affect the hydrophobic effect and hence cation-cation interactions to the large extent.

Acknowledgement

The authors are grateful towards the DST-SERB for using the facility of automatic density meter, ultrasonic interferometer and refrigerated circulating water bath purchased 36

through the funding received under DST FAST-TRACK Scheme for Young scientists and also for financial assistance provided to Ms. Ila J. Warke to work as a project assistant for the same. The authors would like to acknowledge Dr. (Mrs.) Ayesha Khan, Department of Chemistry, Savitribai Phule Pune University, Pune and Mr. Garge S. for the Karl fischer analysis.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/

37

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40

TOC Graphics:

◊, [P4441 ][CH3SO4 ] □, [Pi(444)1][Tos] ∆,[P4444 ][Cl]

41

Research Highlights

•

Solvent induced ion-ion interactions bas been studied.

•

Water structure forming effect decreases with increase in temperature.

•

Increase in ion-ion interactions makes the systems less compressible as concentration increases.

42