Choline Chloride + Urea - ACS Publications - American Chemical

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Densities and Viscosities of (Choline Chloride + Urea) Deep Eutectic Solvent and Its Aqueous Mixtures in the Temperature Range 293.15 K to 363.15 K Anita Yadav and Siddharth Pandey* Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India ABSTRACT: Deep eutectic solvents (DESs) have been regarded as one of the most promising environmentally benign and cost-effective alternatives to conventional ionic liquids and volatile organic solvents. Aqueous mixtures of DESs have the potential to afford modified properties for specific applications. Densities and dynamic viscosities of a common and popular DES composed of choline chloride and urea in 1:2 molar ratio, named reline, and its aqueous mixtures in the temperature range 293.15 K to 363.15 K are reported. A decrease in density with increasing temperature is found to follow a quadratic expression. Excess molar volumes of the aqueous mixtures of reline are found to be negative at all temperatures and compositions. The absolute excess molar volume is found to decrease, in general, as the temperature is increased from 293.15 K to 323.15 K. For temperatures above 323.15 K, the excess molar volume does not change much with further increase in temperature to 363.15 K. The temperature dependence of dynamic viscosity of aqueous mixtures of reline in the temperature range 293.15 K to 363.15 K at all compositions is found to be better described by a Vogel−Fulcher−Tamman (VFT) model as opposed to an Arrhenius expression. Excess logarithmic viscosities for aqueous mixtures of reline are found to be negative at most temperatures and compositions; however, they become positive at 353.15 K and 363.15 K. The excess logarithmic viscosities of aqueous reline mixtures are in stark contrast to that reported for aqueous mixtures of DES glyceline, composed of choline chloride and glycerol in the same mole ratio, where the excess logarithmic viscosities are positive. Facile interstitial accommodation of water within H-bonded reline network as opposed to formation of extensive H-bonding is proposed to be the reason for this experimental observation. The important role of the H-bond donor as a constituent of DES is amply highlighted as it controls the interactions present in a DES and its aqueous mixtures.

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INTRODUCTION During the last couple of decades several greener alternatives to volatile organic compounds (VOCs) have been proposed by various research groups. Among the greener alternatives, water, supercritical CO2, ionic liquids, renewable solvents, liquid polymers, and so forth are becoming the obvious choices for researchers.1−8 However, in many of these common environmentally benign alternate media, a lot of interest has been generated in ionic liquids due to their unique and tunable physicochemical properties. Among the key properties of ionic liquids, negligible vapor pressure, good thermal stability, high solubility, and nonflammability are the most noteworthy.9−12 However, several disadvantages are also observed with common ionic liquids during their investigations, such as limited solute solubility, high viscosity, poor biodegradability, unfavorable toxicity, and high cost.13−16 Recently, deep eutectic solvents (DESs) have emerged as attractive alternatives to ionic liquids showing several advantages over the latter. They are constituted from natural and renewable nontoxic bioresources, a starting material which renders their substance of paramount importance from a green point-of-view. A common and simple DES is a mixture of quaternary ammonium salt complexed with various H-bond © 2014 American Chemical Society

donors, such as poly(carboxylic acid)s, polyamides, and polyalcohols.17,18 The purity of the resulting DES will simply depend on the purity of its individual components; therefore, these liquids are easy to prepare in a pure state as compared to ionic liquids. The preparation of DES is easy and cost-effective and does not involve any postpurification or disposal problem; therefore, DESs aptly fulfill many requirements of alternative environmentally benign media due to their enormous potential as solvents in the field of electrochemistry (i.e., metal and alloy electrodeposition,19 separations, electropolishing,20 and preparation of electrolytes21), synthesis (organic, ionothermal, and polymer),22,23 nanomaterials,24 material preparation,25 biochemistry (biocatalysis, enzymatic reactions, biomaterials, or other biotransformations),26,27 and so forth. The first DES system, investigated by Abbott and co-workers and published in 2003, contained quaternary ammonium salt choline chloride (ChCl) and urea in a molar ratio of 1:2.17 Both choline chloride and urea are biodegradable, nontoxic, and readily available components. The choline chloride is an essential nutrient, Received: February 21, 2014 Accepted: June 4, 2014 Published: June 13, 2014 2221

dx.doi.org/10.1021/je5001796 | J. Chem. Eng. Data 2014, 59, 2221−2229

Journal of Chemical & Engineering Data

Article

which can be extracted from biomass, and it is regarded as a part of B-complex vitamins.28 It is being widely used as a nutritional additive for livestock.29,30 Urea plays an vital role in the metabolism of nitrogen-containing compounds in living beings and is the most familiarly traded nitrogenous fertilizer. This approach has potential to increase the overall utility and applications of DES many-fold. Recognizing this, researchers, including us, have started exploring properties and application potentials of DES systems. Although DESs share several advantages over the conventional ionic liquids, knowledge regarding their fundamental physicochemical properties is still lacking in literature. Among the important physicochemical properties, density and viscosity play key roles in academics and industries. In this paper, we report the temperature-dependent densities and dynamic viscosities of the DES reline prepared using choline chloride and urea in a 1:2 mole ratio, and its aqueous mixtures over the entire composition regime in the temperature range (293.15 K to 363.15 K) at 10 K intervals. Our choice of DES is governed by the fact that reline is perhaps the most investigated DES to date as evident from the chemical literature.31−34 Both of the components constituting reline, choline, chloride, and urea, are solids at room temperature with a significantly high melting point thus representing a “true DES”. Further, reline has a melting point of 285.15 K which allows the measurement of their properties such as densities and viscosities at 293.15 K and above. Densities of aqueous mixtures of reline have been measured by other researchers. In a recent study, Li et al.35 reported the high-pressure densities of reline and their aqueous mixtures under pressures up to 50 MPa and temperatures of 298.15 K to 323.15 K. However, the temperature range of this report is significantly narrower. Various pharmaceutical and biotechnological industries and chemical engineering processes require knowledge of viscosity and its temperature dependence as an essential piece of information. Viscosity is one of the routinely used properties in both industrial and academic research as well. Furthermore, viscosity is an essential parameter that helps develop a solubilizing milieu and plays a major part in chemical transformations in solution.36,37 Few attempts were made by Mantle and co-workers38 to study the viscosity of neat reline and other DESs at different temperatures. However, we could not find any report on temperature-dependent viscosities of aqueous mixtures of reline.

Table 1. Description of the Chemicals Used in This Work chemical

source

reline 202/3 (1 mol choline chloride + 2 mol Scionix Ltd. urea) doubly distilled deionized water (HPLC Merck grade)

purification done none none

measured using a Mettler Toledo, DE45 delta range density meter. The density measurement with the above-mentioned density meter was based on electromagnetically induced oscillations of a U-shaped glass tube. The standard deviations associated with the density measurement are ≤ 0.00005 g·cm−3. The measurements were performed at 10° intervals in the temperature range of 293.15 K to 363.15 K. Table 2 presents comparison of densities of pure reline and their aqueous mixtures measured with our instrumentation with those available in literature.35 Our values are in good agreement with those reported in the literature. The dynamic viscosities (η) were measured with a Peltier-based (resolution of 0.01 K and accuracy < 0.05 K) automated Anton Paar microviscometer (model AMVn) having calibrated glass capillaries of different diameters (1.6, 1.8, 3.0, and 4.0 mm). This instrument is based on the rolling-ball principle, where the steel ball rolls down the inside of inclined, sample-filled calibrated glass capillaries. The deviation in η was ≤ 0.5 %.

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RESULTS AND DISCUSSION Density and Its Temperature Dependence. Experimentally measured densities of reline and (reline + water) mixtures as a function of temperature in the range 293.15 K to 363.15 K over the entire composition range are reported in Table 3. As expected, densities of reline, water, and their mixture are found to decrease with the increase in temperature. Thermal expansion usually results in decreased density of a substance as the temperature is increased. The experimentally measured densities are found to vary quadratically with the absolute temperature and were fitted to the following equation: ρ = ρ0 + aT + bT 2

(1)

−3

where ρ/g·cm is the density of reline and its aqueous mixtures. The values of the parameters ρ0, a, and b along with the standard deviation of the fits are listed in Table 4. Measured densities of (reline + water) mixtures along with the fits to eq 1 are presented in Figure 1. It is clear from the recovered values of r2 listed in Table 4 as well as from careful examination of Figure 1, where the fits of the measured density against the temperature according to eq 1 for (reline + water) mixtures over the entire composition range are presented as solid curves, that the temperature dependence of the densities of reline and its aqueous mixtures can be conveniently expressed by a simple quadratic relation in the temperature range 293.15 K to 363.15 K. It is important to mention that Li group has reported a linear decrease in density with temperature for reline and its aqueous mixtures.35 It is attributed to the fact that the temperature range for density data of Li group was much narrower (298.15 K to 323.15 K only). It is clear that measurements at higher temperatures (363.15 K ≥ T > 333.15 K) reveal a more complex dependence of density on temperature for reline and its aqueous mixtures. This is manifested in changed molecular interactions between reline and water in this temperature region (vide infra). It is important to mention that for DES

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EXPERIMENTAL SECTION Materials. Reline 202/3 (mol wt 86.69 g·mol−1), a mixture of choline chloride + urea (1:2 mole ratio), was purchased from Scionix Ltd. and used as received. Alternatively, reline is also prepared by mixing choline chloride (w ≥ 0.99 from SigmaAldrich) and urea (w ≥ 0.99 from Sigma-Aldrich) in a mole ratio of 1:2 and stirred under heating (∼353.15 K) until a homogeneous, colorless liquid has been formed.17 The density and dynamic viscosity measurements of reline purchased from Scionix Ltd. and that prepared using choline chloride (w ≥ 0.99 from Sigma-Aldrich) and urea (w ≥ 0.99 from Sigma-Aldrich) were found to be statistically similar. Doubly distilled deionized water of HPLC grade was obtained from Merck. The description of the chemicals used together with their sources is provided in Table 1. Methods. The aqueous mixtures of reline were prepared by mass using a Denver Instrument balance having a precision of ± 0.1 mg. Densities (ρ) of the aqueous mixtures of reline were 2222



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Table 2. Comparison of Experimental Densities (ρb) of Reline (1) + Water (2) Mixtures with the Literature Values at Pressure p = 0.1 MPa for Different Temperatures (x1 Is the Mole Fraction of Reline)a Td/K x1c

a

303.15

313.15

323.15

exptl

lit.

exptl

lit.

exptl

lit.

exptl

lit.

0.1000 0.2000 0.2992 0.4000 0.4945 0.5996 0.6988 0.8001 0.8976 1.0000

(0.1001) (0.2000) (0.3000) (0.3997) (0.5000) (0.5995) (0.6998) (0.7998) (0.9000) (0.1000)

1.0608 1.0996 1.1254 1.1439 1.1568 1.1678 1.1762 1.1832 1.1891 1.1945

1.0613 1.1005 1.1266 1.1448 1.1584 1.1689 1.1775 1.1844 1.1902 1.1951

1.0560 1.0944 1.1201 1.1385 1.1514 1.1624 1.1707 1.1777 1.1834 1.1887

1.0565 1.0954 1.1212 1.1394 1.1529 1.1633 1.1718 1.1786 1.1844 1.1893

1.0509 1.0891 1.1147 1.1331 1.1460 1.1570 1.1654 1.1723 1.1779 1.1831

1.0514 1.0899 1.1156 1.1337 1.1473 1.1576 1.1660 1.1728 1.1786 1.1835

All literature values are taken from ref 35. bUncertainties: u(ρ) = ± 0.00005 g·cm−3. cu(x1) = ± 10−4. du(T) = ± 0.05 K.

Table 3. Densities (ρa/g·cm−3) of Reline (1) + Water (2) Mixtures at Pressure p = 0.1 MPa and T = 293.15 K to 363.15 K as a Function of Mole Fraction of Reline (x1) Tc/K

a

x1b

wt % (reline)

293.15

303.15

313.15

323.15

333.15

343.15

353.15

363.15

0.0000 0.1000 0.2000 0.2992 0.4000 0.4945 0.5996 0.6988 0.8001 0.8976 1.0000

0.00 (water) 34.86 54.63 67.37 76.26 82.81 87.82 91.84 95.07 97.75 100 (reline)

0.9982 1.0652 1.1047 1.1307 1.1492 1.1621 1.1732 1.1817 1.1889 1.1948 1.2001

0.9957 1.0608 1.0996 1.1254 1.1439 1.1568 1.1678 1.1762 1.1832 1.1891 1.1945

0.9922 1.0560 1.0944 1.1201 1.1385 1.1514 1.1624 1.1707 1.1777 1.1834 1.1887

0.9881 1.0509 1.0891 1.1147 1.1331 1.1460 1.1570 1.1654 1.1723 1.1779 1.1831

0.9833 1.0458 1.0838 1.1093 1.1276 1.1404 1.1514 1.1597 1.1666 1.1722 1.1773

0.9778 1.0400 1.0778 1.1031 1.1213 1.1341 1.1450 1.1533 1.1601 1.1657 1.1708

0.9717 1.0335 1.0711 1.0963 1.1144 1.1270 1.1379 1.1461 1.1529 1.1584 1.1635

0.9650 1.0263 1.0637 1.0887 1.1066 1.1192 1.1300 1.1381 1.1449 1.1504 1.1554

Uncertainties: u(ρ) = ± 0.00005 g·cm−3. bu(x1) = ± 10−4. cu(T) = ± 0.05 K.

fraction of water is found to be nonlinear. It is noteworthy that the variation of density with concentration of water at a given temperature is reported to be linear for (urea + water)40 and nonlinear for (choline chloride + water)41 systems. Further, the variation in density of (choline chloride + glycerol) DES with water concentration is also reported to be nonlinear.39 To afford the extent of molecular-level interactions within (reline + water) mixtures, we estimated excess molar volume (VE) from experimental density data using the relationship

constituted of (choline chloride + glycerol) and its aqueous mixtures a similar quadratic dependence of density on temperature in the same temperature range is recently reported.39 As reline is constituted of choline chloride and urea, it is imperative to compare the temperature dependence of density of aqueous mixtures of reline with those of aqueous mixtures of urea and aqueous mixtures of choline chloride, respectively. The difference in the temperature dependence of the density of aqueous mixtures of reline from that of aqueous mixtures of urea is revealed by the measurements of Huque and coworkers.40 They report an almost linear decrease in density with temperature for the (urea + water) system; however, the temperature range of their investigation was only from 308.15 K to 328.15 K. It is noteworthy that, for the (choline chloride + water) system, on the other hand, the dependence of density on temperature in the temperature range 278.15 K to 318.15 K is found to be quadratic.41 It appears that the temperature dependence of density of (reline + water) resembles more that of (choline chloride + water) than (urea + water) although reline is constituted of 2 mol of urea and 1 mol of choline chloride. Composition Dependence of Density. The density of reline decreases monotonically as water is added as the density of neat reline is higher than that of water at a given temperature (Table 3). The decrease in density with increasing mole

VE =

⎛x M (x1M1 + x 2M 2) xM ⎞ − ⎜⎜ 1 1 + 2 2 ⎟⎟ ρm ρ2 ⎠ ⎝ ρ1

(2)

Here, x1, x2, and ρ1, ρ2 refer to the mole fractions and densities, respectively, of reline and water at a given temperature, and ρm is the density of the mixture. M1 and M2 are the molecular weights of components 1 (reline) and 2 (water), respectively. The molecular weight of the pure reline was calculated from their individual components according to the equation:42 M1 = xChClMChCl + x ureaM urea

(3)

E

The V at each temperature for (reline + water) mixtures are presented as a function of x1, the mole fraction of reline, in Figure 2. It is clear that VE are negative and are significant at each temperature throughout the entire composition range for all (reline + water) mixtures. Interestingly, the maximum absolute VE for the (reline + water) mixture is observed in the 2223



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Table 4. Result of the Regression Analysis of Density (ρ/g· cm−3) versus Temperature (T/K) Data According to Equation: ρ/(g·cm−3) = ρo/(g·cm−3) + a(T/K) + b(T/K)2 for [Reline + Water] over the Temperature Range 293.15 K to 363.15 Ka x1 0.0000

a

wt % (reline)

0.1000

0.00 (water) 34.86

0.2000

54.63

0.2992

67.37

0.4000

76.26

0.4945

82.81

0.5996

87.82

0.6988

91.84

0.8001

95.07

0.8976

97.75

1.0000

100 (reline)

ρ0/(g·cm−3)

a·10−3

b·10−6

r2

0.777 (± 0.006) 0.997 (± 0.021) 1.070 (± 0.028) 1.096 (± 0.030) 1.108 (± 0.032) 1.114 (± 0.032) 1.121 (± 0.033) 1.130 (± 0.035) 1.143 (± 0.037) 1.163 (± 0.039) 1.171 (± 0.035)

1.749 (± 0.037) 0.867 (± 0.125) 0.678 (± 0.169) 0.688 (± 0.186) 0.735 (± 0.194) 0.779 (± 0.196) 0.809 (± 0.203) 0.810 (± 0.211) 0.775 (± 0.225) 0.697 (± 0.238) 0.686 (± 0.216)

−3.391 (± 0.056) −2.157 (± 0.191) −1.915 (± 0.258) −1.948 (± 0.283) −2.032 (± 0.295) −2.105 (± 0.299) −2.157 (± 0.309) −2.166 (± 0.322) −2.120 (± 0.342) −2.010 (± 0.363) −2.001 (± 0.328)

0.9999 0.9997 0.9996 0.9995 0.9995 0.9995 0.9995 0.9994

Figure 2. Variation of excess molar volume (VE/cm3·mol−1) with the mole fraction of reline (x1) for (reline + water) mixtures as the temperature is increased from (A) 293.15 K to 323.15 K [●, 293.15 K; red ▼, 303.15 K; green ■, 313.15 K; yellow ⧫, 323.15 K] to (B) 333.15 K to 363.15 K [⧫, 333.15 K; blue ▲, 343.15 K; ●, 353.15 K; red ▼, 363.15 K]. Solid curves show fits according to the Redlich− Kister equation (eq 4) with parameters (Aj) reported in Table 5.

0.9994 0.9993 0.9995

Standard deviations are given as ± in parentheses.

The VE were next fitted to the Redlich−Kister type polynomial expressions.43 According to combined nearly ideal binary solvent/Redlich−Kister (CNIBS/R-K) model, the VE in a binary solvent mixture at a constant temperature can be expressed as k

V E = xrelinex water ∑ Aj (xreline − x water) j (4)

j=0

where Aj and j are the equation coefficients and the degree of the polynomial expansion, respectively. The numerical values of j can be varied to find an accurate mathematical representation of the experimental data. Regression analysis was performed to fit the polynomials [i.e., eq 4] to our experimental density data, and the results of the fit are reported in Table 5. It is convenient to use a cross-validation method which is a practical and reliable to test the predictive significance when only little data are available.44 The solid lines connecting each of the VE in Figure 2 at all proportions are obtained from the CNIBS/R-K model fit (with j = 2) as reported in Table 5 which suggests a

Figure 1. Variation of density with temperature for reline (1) + water (2) mixtures at different mole fractions of reline (x1) (●, 0.0000; red ▼, 0.1000; green ■, 0.2000; yellow ⧫, 0.2992; blue ▲, 0.4000; purple ⬢, 0.4945; blue ●, 0.5996; gray ▼, 0.6988; brown ■, 0.8001; green ⧫, 0.8976; dark yellow ▲, 1.0000). The solid lines represents a fit to the equation ρ = ρ0 + aT + bT2. Parameters ρ0, a, and b along with the goodness-of-fit in terms of r2 are reported in Table 4.

water-rich region (xreline ∼ 0.25 to 0.35). The temperature dependence of VE, however, is not very straightforward. For xreline < 0.6, the absolute values of VE, in general, appear to decrease with increasing temperature in the temperature range 293.15 K to 323.15 K (Figure 2A); in reline-rich mixtures (for xreline ≥ 0.6), this is observed to be not true. Further, for T > 323.15 K, the change in VE with temperature becomes almost insignificant; in fact the variation is reversed at higher temperatures (333.15 K to 363.15 K; see Figure 2B). While, Li and co-workers also reported the VE of (reline + water) to decrease with increasing temperature in the range 298.15 K to 323.15 K; 323.15 K was the highest temperature in their investigation.35 Our VE clearly hints at complexity associated with aqueous mixtures of reline as far as interactions in the system are concerned (vide infra).

Table 5. Fit Parameters (Aj’s) and Correlation Coefficient (r2) in the Redlich-Kister Equation [eq 4] for Excess Molar Volume (VE) of Reline (1) + Water (2) Mixtures

2224

T/K

A0

A1

A2

r2

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15

−0.4443 −0.3961 −0.3837 −0.3800 −0.3815 −0.3837 −0.3868 −0.3890

0.6072 0.5266 0.4204 0.3049 0.3041 0.3078 0.3067 0.3113

−0.3312 −0.1306 −0.0356 −0.0143 −0.0240 −0.0189 −0.0199 −0.0198

0.9960 0.9953 0.9938 0.9952 0.9952 0.9954 0.9958 0.9958



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Table 6. Viscosities (ηa/mPa·s) of Reline (1) + Water (2) Mixtures at Pressure p = 0.1 MPa and T = 293.15 K to 363.15 K as a Function of Mole Fraction of Reline (x1) Tc/K

a

x1b

wt % (reline)

293.15

303.15

313.15

323.15

333.15

343.15

353.15

363.15

0.0000 0.1000 0.2000 0.2992 0.4000 0.4945 0.5996 0.6988 0.8001 0.8976 1.0000

0.00 (water) 34.86 54.63 67.37 76.26 82.81 87.82 91.84 95.07 97.75 100 (reline)

0.9968 1.6178 2.8436 5.0645 9.1829 17.5890 34.5047 73.3357 169.4133 436.1138 1371.9719

0.8012 1.3125 2.2654 3.9093 6.7782 12.2515 22.6048 43.4958 89.4403 200.6193 527.2786

0.6650 1.0858 1.8509 3.1117 5.1982 8.9032 15.8076 28.0056 52.3574 104.0292 238.0763

0.5518 0.9234 1.5403 2.5381 4.1152 6.5254 11.6217 19.2941 33.2125 61.1854 119.8049

0.4791 0.7980 1.3049 2.1172 3.3415 5.1376 8.2652 13.9586 22.5795 38.2495 68.6478

0.4246 0.7010 1.1287 1.7978 2.7739 4.1560 6.4745 10.323 16.2228 25.8876 41.9612

0.4042 0.6265 0.9911 1.5515 2.3461 3.4372 5.2030 17.742 12.1228 18.4986 28.1053

0.3821 0.5735 0.8788 1.3576 2.0167 2.8964 4.2758 6.1852 9.2950 13.7050 19.9490

Uncertainties: u(η) = ± 0.5 %. bu(x1) = ± 10−4. cu(T) = ± 0.05 K.

more efficient H-bonding between water and glycerol as compared to that between water and urea within the aqueous DES mixture as glycerol possesses three alkyl −OH groups as opposed to two −NH2 groups on CO functionality. This would lead to increased contraction in volume within aqueous mixtures of (choline chloride + glycerol) DES as opposed to that within aqueous mixtures of (choline chloride + urea) DES. Interstitial accommodation of water within (choline chloride + urea) DES appears to be a more important factor. Temperature and Composition Dependence of the Dynamic Viscosity. Experimentally measured dynamic viscosities (η) of (reline + water) mixtures over the complete composition range at 10° intervals in the temperature range 293.15 K to 363.15 K are reported in Table 6. It is evident from the entries in Table 6 that, as expected, monotonic decrease in dynamic viscosity is observed as the temperature is increased from 293.15 K to 363.15 K for a given composition of (reline + water) mixture. Plots of η versus x1 (mole fraction of reline) for aqueous mixtures of reline at different temperatures are shown in Figure 3. Within a (reline + water) mixture at fixed temperature, dynamic viscosity decreases monotonically upon increasing the mole fraction of water (x2). This is expected as η of water is significantly lower than that of neat reline at all investigated temperatures.

fair-to-good correlation between the predicted and the experimentally obtained values (the r2 also indicates the fits to be satisfactory). This is in line with the earlier observations of decrease in VE with increase in temperature until 323.15 K followed by a slight increase in VE with increase in temperature for 363.15 K ≥ T > 323.15 K (vide supra). The negative VE generally points to contraction in volume upon mixing. The interactions within the mixture are affected by the temperature and the composition of the mixture. In liquid mixtures, the absolute value of VE usually decreases with increasing temperature in systems where interactions are present. The negative VE of (reline + water) mixtures at all compositions in the temperature range 293.15 K to 363.15 K hints at facile interstitial accommodation of water within Hbonded reline network. The H-bonding involving the components of the DES and water may not be suggested to be the prominent interaction here as excess logarithmic viscosities are mostly negative (vide infra), although the presence of such H-bonding interactions may not be completely ruled out. Although, we observe a decrease in absolute value of VE in the temperature range 293.15 K to 323.15 K, a slight increase in the absolute value of VE as the temperature is increased from 333.15 K to 363.15 K could be attributed to the inherent complexity of the (reline + water) mixtures as far as interactions within the system are concerned. As mentioned earlier, interstitial accommodation of water within H-bonded reline network along with possible minor contribution from H-bonding and other interactions between the unlike components (i.e., reline and water) within the mixture would result in volume contraction and thus the negative VE for our system. Upon heating, further contraction in the mixture volume, albeit small, could be tentatively attributed to the weakening of the H-bond among urea and choline chloride (constituents of the reline) and/or among water molecules with subsequent strengthening of the Hbonding between reline and water. The interstitial exclusion of water molecules from (urea + choline chloride) H-bonded network in the temperature range 333.15 K to 363.15 K is the more important factor in increasing absolute value of VE with increasing temperature of the mixture. It is interesting to note that the maximum value of VE = −0.3372 cm3·mol−1 at xDES = 0.4022 at 293.15 K for aqueous mixtures of (choline chloride + glycerol) DES39 is more than two times the maximum value of VE = −0.1522 cm3·mol−1 at xDES = 0.2992 for (choline chloride + urea) DES at the same temperature. This is attributed to

Figure 3. Dynamic viscosity (η) as a function of mole fraction of reline (x1) for (reline + water) mixtures at different temperatures. [red ●, 293.15 K; orange ▼, 303.15 K; yellow ■, 313.15 K; green ⧫, 323.15 K; blue ▲, 333.15 K; blue ⬢, 343.15 K; purple ●, 353.15 K; red ▼, 363.15 K]. Solid curves at each temperature are obtained using the best fit parameters of the Redlich−Kister polynomial (eq 8) reported in Table 8. The error in measured η is ≤ 0.5 %. 2225



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Table 7. Summary of Parameters Associated with Viscosity According to the VFT Model

a

x1

wt % (reline)

A

B

T0/K

r2

Ea,ηa/(kJ·mol−1)

0.0000 0.1000 0.2000 0.2992 0.4000 0.4945 0.5996 0.6988 0.8001 0.8976 1.0000

00.00 34.86 54.63 67.37 76.26 82.81 87.82 91.84 95.07 97.75 100.00

−1.9795 −2.4081 −2.6970 −2.4927 −2.3219 −2.0614 −2.6645 −2.5958 −1.8879 −1.9308 −2.4093

138.3107 356.6059 571.3613 611.9273 633.7261 594.0347 848.2093 862.8576 698.1970 735.8715 854.0621

223.6754 169.8683 140.5283 144.4513 153.5365 172.8138 156.5463 167.8187 193.7058 201.3024 204.5833

0.9958 0.9998 0.9999 0.9999 0.9999 0.9998 0.9995 0.9999 0.9999 0.9999 0.9999

18.43 16.02 17.00 19.14 22.40 27.95 31.26 37.54 47.30 57.98 72.10

At 298.15 K, Ea,η is the activation energy for viscous flow.

Arrhenius-like behavior is usually considered the most simplistic as far as temperature dependence of dynamic viscosity is concerned. Although the measured viscosities of reline and its aqueous mixtures may be described by an Arrhenius model over the temperature range studied (results not shown), the Vogel−Fulcher−Tamman (VFT) model (a model which especially describes ionic liquids having small, symmetric cations)45−47 appears to more satisfactorily describe the viscosities of reline and its aqueous mixtures: ln η = A +

B T − T0

(5) Figure 4. Variation of ln η with 1/T for reline (1) + water (2) mixtures at different mole fractions of reline. (●, 0.0000; bright red ▼, 0.1000; light green ■, 0.2000; bright yellow ⧫, 0.2992; dark blue ▲, 0.4000; purple ⬢, 0.4945; light blue ●, 0.5996; gray ▼, 0.6988; dark red ■, 0.8001; dark green ⧫, 0.8976; dark yellow ▲, 1.0000). The solid lines represent the best fit for the VFT (Vogel−Fulcher−Tamman) model (eq 5).

Here A, B, and T0 are empirical constants (T0 generally corresponds to the temperature of divergence where the configurational entropy of the system vanishes). For some molten salts, whose ions are either associating to a network and eventually forming glass-like structures at lowered temperatures or suffer from hindered rotation in the melt are reported to follow such non-Arrhenius temperature dependence.48 The fits to this model returned correlation coefficients close to unity (Table 7). From the recovered parameters, Ea,η at 298 K was also determined using49 ⎛ T ⎞2 Ea, η = RB⎜ ⎟ ⎝ T − T0 ⎠

are presented in Figure 5. A careful examination of Figure 5 reveals that for T < 333.15 K, the (ln η)E are negative over the

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Plots of ln η versus 1/T along with the best fit lines according to VFT model for (reline + water) mixtures are presented in Figure 4. Goodness-of-fit in terms of r2 along with the recovered parameters are presented in Table 7. As expected, Ea,η decreases monotonically as the concentration of water is increased in the mixture. It is noteworthy that the temperature dependence of the dynamic viscosities of (urea + water) as well as (choline chloride + water) systems in the temperature range 273.15 K to 373.15 K is better described by an Arrhenius relationship.40,41 However, at lower concentrations of choline chloride, the VFT model appears to better describe the temperature variation of dynamic viscosity of the (choline chloride + water) mixtures.41 Excess Logarithmic Viscosity and Redlich−Kister Treatment. To assess the interactions within (reline + water) mixtures, excess logarithmic viscosities, (ln η)E, of (reline + water) mixtures were estimated from the equation:50 (ln η)E = ln η − [xreline ln ηreline + x water ln ηwater ]

Figure 5. Variation of (ln η)E with the mole fraction of reline (x1) for (reline + water) mixtures as the temperature is increased from 293.15 K to 363.15 K [●, 293.15 K; red ▼, 303.15 K; green ■, 313.15 K; yellow ⧫, 323.15 K; blue ▲, 333.15 K; purple ⬢, 343.15 K; light blue ●, 353.15 K; gray ▼, 363.15 K]. Solid curves show fits according to the Redlich−Kister equation (eq 8) with parameters (Aj) reported in Table 8.

entire composition range and are significant at lower temperatures. As the temperature is increased, the absolute value of (ln η)E decreases and becomes positive for xreline ≤ 0.2 at T = 333.15 K and for xreline ≤ 0.5 at T = 343.15 K. For T > 343.15 K, in general, the (ln η)E becomes positive. In contrast to what

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Plots of (ln η) versus x1 (mole fraction of reline) for (reline + water) mixtures in the temperature range 293.15 K to 363.15 K E

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was observed for VE, the maximum absolute value of (ln η)E is observed to be in the reline-rich region (xreline > 0.6) for T ≤ 333.15 K. The trend in (ln η)E again hints at the complexity inherent to the interactions within aqueous reline mixtures. It is worth mentioning that the negative (ln η)E for our aqueous DES mixture is found to be similar to the negative viscosity deviations (Δη) observed earlier for ionic liquid [bmim][BF4]51 mixtures with methyl formate, ethyl formate, methyl acetate, and acetone, respectively, as well as for ionic liquids [bmim][NO3]52 and [omim][NO3]53 mixtures with ethanol, 1propanol, and 1-butanol, respectively. There is a stark difference in (ln η)E for aqueous mixtures of (choline chloride + urea) DES as opposed to that for aqueous mixtures of (choline chloride + glycerol) DES.39 The (ln η)E are positive for aqueous mixtures of (choline chloride + glycerol) DES at all compositions in the same temperature range. Intensive H-bonding within a mixture between the components forming the mixture usually leads to positive (ln η)E as mixture viscosity comes out to be higher than that calculated using eq 7. The interstitial accommodation of one component with the other within the mixture may lead to negative (ln η)E as, in this case, the mixture viscosity would be less than that calculated using eq 7. The intraspecies interactions are usually disrupted in this case, and the interspecies interactions are usually not too significant. This re-emphasizes our hypothesis that interstitial accommodation of water within (choline chloride + urea) DES is perhaps the major factor that governs the properties of (reline + water) mixtures. The H-bonding between the components of the reline and water may not be ruled out completely (vide supra). At higher temperatures, the interstitial water may get expelled from the H-bonding network formed by the components of the DES reline thus reducing the contraction in excess molar volume and increasing the excess logarithmic viscosity in the process. At higher temperatures in our investigation, the expelled water may start to form H-bonds with the components of the DES reline thus giving rise to positive excess logarithmic viscosities. Finally, our (ln η)E were correlated by the Redlich−Kister polynomial equation.43 According to combined nearly ideal binary solvent/Redlich−Kister (CNIBS/R-K) model, the (ln η)E in a binary mixture at a constant temperature can be expressed as

Table 8. Parameters (Aj’s) and Correlation Coefficient (r2) Recovered from the Best Fit of (ln η)E According to Redlich−Kister Polynomial (eq 8) for Reline (1) + Water (2) Mixtures A0

A1

A2

A3

r2

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15

−2.9328 −2.0246 −1.2936 −0.6964 −0.3128 0.0378 0.1557 0.2612

−1.3813 −1.1593 −0.9232 −0.7498 −0.6895 −0.6145 −0.6147 −0.4650

−0.9410 −0.7597 −0.7323 −0.1690 −0.0332 0.1955 0.0256 −0.0765

0.3957 0.2513 0.0628 0.0851 0.1111 0.3764 0.9985 0.9484

0.9932 0.9929 0.9926 0.9724 0.9731 0.9820 0.9778 0.9510

■

CONCLUSIONS Both VE and (ln η)E of aqueous mixtures of reline and their temperature dependence hint at the presence of competing interactions within the mixtures. In general, both VE and (ln η)E are largely negative except at high temperatures of 353.15 K and 363.15 K where (ln η)E becomes positive. The large differences in the size and/or the shape of the component species of the mixture and the loss of Coulombic attractive interaction (or dipolar association) or rupture of H-bonding network within the mixture as compared to that in the neat components usually result in negative (ln η)E. Specific interactions between the two component species, e.g., formation of H-bonds and charge-transfer complexes, on the other hand, result in positive (ln η)E. Both size and shape of DES reline are very different from the size and shape of water, and there certainly is the loss of Coulombic attractive interaction present within DES reline upon forming the (reline + water) mixture. Both of these factors appear to contribute to the negative (ln η)E of the aqueous mixtures of reline. The smaller water molecules appear to be accommodated interstitially within H-bonded network of reline. This subsequently results in negative VE. The interactions present in aqueous reline mixture are in stark contrast to that present in aqueous glyceline mixture. In aqueous glyceline mixture, the major interaction between the components appear to be Hbonding which results in positive (ln η)E and negative VE. The role of H-bond donor (urea versus glycerol) as a constituent of DES is amply highlighted as it controls the interactions present in a DES and its aqueous mixtures.

■

k

(ln η)E = xrelinex water ∑ Aj (xreline − x water) j j=0

T/K

AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected]. Phone: +91-1126596503. Fax: +91-11-26581102.

where Aj and j are the equation coefficients and the degree of the polynomial expansion, respectively. The numerical values of j can be varied to find an accurate mathematical representation of the experimental data. Regression analysis was performed to fit the polynomials (eq 8) to our experimental data with j = 3, and the results of the fit are reported in Table 8. It is observed from the r2 values that the data show fair-to-good agreement for many (reline + water) mixtures at all temperatures. The solid curves connecting (ln η)E in Figure 5 are obtained from the CNIBS/R-K model fit (with j = 3) as reported in Table 8 which further highlights the correlation between the predicted and the experimentally obtained values. It is convenient to use a crossvalidation method which is a practical and reliable method to test the predictive significance when only little data are available.44

Funding

This work is generously supported by the Department of Science and Technology (DST), Government of India through a grant to S.P. (grant number SB/S1/PC-80/2012). Notes

The authors declare no competing financial interest.

■

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Choline Chloride + Urea - ACS Publications - American Chemical

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