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Densities and refractive indices of the deep eutectic solvents (choline chloride + ethylene glycol or glycerol) and their aqueous mixtures at the temperature ranging from 298.15 to 333.15 K
Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 551–557
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Densities and refractive indices of the deep eutectic solvents (choline chloride + ethylene glycol or glycerol) and their aqueous mixtures at the temperature ranging from 298.15 to 333.15 K Rhoda B. Leron a, Allan N. Soriano b, Meng-Hui Li a,* a b
R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan Christian University, Chung Li, 32023, Taiwan School of Chemical Engineering and Chemistry, Mapu´a Institute of Technology, Manila, 1002, Philippines
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 October 2011 Received in revised form 14 December 2011 Accepted 20 January 2012 Available online 23 February 2012
Deep eutectic solvents (DES) are new emerging alternatives to conventional ionic liquids that may find a number of interesting applications in industrial and chemical processes. In this study, the densities, r, and refractive indices, nD, of the DESs (choline chloride + ethylene glycol) and (choline chloride + glycerol) and their aqueous mixtures were investigated at atmospheric pressure over the temperature range 298.15–333.15 K and across a complete composition range. The excess molar volumes, VE, and refractive index deviations, DnD, were also calculated from experimental results. The calculated excess molar volumes were negative at all temperatures over the entire range of composition considered, suggesting the presence of strong interactions between water and the DES in the mixtures. The refractive index deviations, on the other hand, were found positive in the entire concentration range. The calculated properties were fitted to a Redlich–Kister type equation to correlate them to the temperature and composition. The correlations used satisfactorily represent the densities and refractive indices of the pure DESs and their aqueous binary mixtures as functions of temperature and composition as indicated by the low overall average absolute deviations obtained in the calculations. ß 2012 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Deep eutectic solvent Density Excess molar volume Refractive index Refractive index deviation Mixing rule
1. Introduction Room temperature ionic liquids (RTILs) have attracted significant research interest over the last decades owing to their unique physical and chemical properties i.e. extremely low vapor pressures, high thermal stability, high solvation capacity, nonflammability, high thermal conductivity and wide liquid range [1– 3]. RTILs, being designer solvents, have huge number of possible applications in electrochemical, analytical, synthetic and engineering processes [4,5]. They have been used as gas absorption media (i.e. CO2 removal), heat transfer fluids, solvents in electroplating and catalysts in organic and chemical synthesis [6–8]. However, most RTILs work at the disadvantage of high cost for bulk applications. They are relatively expensive and some of them even have very low tolerance to moisture [6]. Furthermore, their toxicology has yet to be examined and some studies suggest for further assessment of their applicability as green media [9]. Recently emerging alternatives to conventional RTILS are deep eutectic solvents (DESs). They belong to a new class of ionic liquids that are made by mixing a substituted quaternary ammonium salt
* Corresponding author. Tel.: +886 3 265 4109; fax: +886 3 265 4199. E-mail address: [email protected] (M.-H. Li).
(i.e. 2-hydroxy-N,N,N-trimethylethanaminium [choline] chloride) and a hydrogen bond donor (i.e. amide, carboxylic acid or alcohol), both of which having high melting points, to form a eutectic mixture with a substantially lower melting point [4,10]. They were found to have solvent properties similar to those of RTILs while possessing several advantages over the latter. DESs are easier to prepare in high purity and at a relatively cheaper cost. Many of them are biodegradable and the toxicology of their components is well characterized [9–11]. It is due to these unique features of DESs that they are considered as potential green solvents for a number of industrial applications. Recent researches reported the applicability of DESs as solvents in the bulk processing of metals [12–14], biodiesel purification [15,16], polymer synthesis [17,18], drug solubilization [11], biological transformations [19,20], carbon– carbon nanotube composite preparation [21], and even CO2 absorption [3,5]. DESs were also found to be viable solvents for the fabrication of novel metal surfaces and coatings (i.e. superhydrophobic Ni films) [22,23] and thermochromic PVDF composite films [24]. In order to establish the possible use of deep eutectic solvents in industrial and chemical processes, it is necessary to know their physical properties including the density and refractive index. These could also provide important information on the purity of the samples and the molecular interaction in the liquid [25,26].
1876-1070/$ – see front matter ß 2012 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jtice.2012.01.007
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R.B. Leron et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 551–557
To date most of these properties of DES have not yet been investigated extensively. Thus, in this work, the density, r, and the refractive index, nD, of the deep eutectic solvents ethaline (1 mol choline chloride: 2 mol ethylene glycol) and glyceline (1 mol choline chloride: 2 mol glycerol) were determined at atmospheric pressure within the temperature range 298.15–333.15 K. In addition, since the properties of the DESs maybe affected by the presence of water [19–21,27] the r and nD of their corresponding aqueous binary mixtures over the entire range of mole fraction (x1 = 0.1–0.9) were also determined. Furthermore, the excess molar volume, VE, and refractive index deviations, DnD, were calculated and correlated to the system’s temperature and composition by fitting the experimental data into a Redlich–Kister type equation. Consequently, the resulting correlations were used to predict the density and refractive index of the binary mixtures as functions of temperature and composition. Lastly, the refractive indices of the binary mixtures were predicted using the most important mixing rules such as the Arago–Biot, Gladstone–Dale, Lorentz–Lorenz, Eykman, Weiner, Heller, Newton and Eyring–John correlations which were then compared with the experimental data.
line wavelength. The temperature was controlled by a built-in internal solid state Peltier thermostat fitted with two internal Pt100 Platinum resistance temperature sensors that allowed temperature measurement to an accuracy of 0.03 K. For each measurement, at least 1.0 mL sample was placed on the measuring prism to minimize vaporization of the water at high temperatures. A cone-shaped yellow light beam illuminated the sample from its bottom side under different angles of reflection and then a microprocessor automatically calculates the refractive index of the sample from the obtained data. The refractometer was calibrated using deionized water and calibration was checked after every few measurements. The estimated overall uncertainty in the refractive index measurement was 5 105. All measurements were carried out in three to five replicate runs and the average values were reported. Prior to the analysis of the DES samples, the accuracy of the equipment and procedure used were validated first by measuring the density and refractive index of water and comparing the measurements obtained with the reference data reported by Spieweck and Bettin [28] and Schiebener et al. [29]. The average deviations found were 0.004% and 0.0003% for r and nD, respectively.
2. Experimental 3. Results and discussion 2.1. Chemicals The pure deep eutectic solvents ethaline and glyceline (purity >98%) were obtained from Scionix Ltd1. They are mixtures of (choline chloride + ethylene glycol) and (choline chloride + glycerol) at 1:2 mol ratios. Each pure sample was dried under vacuum at 333 K for at least 48 h to remove any volatile impurities and stored in a dry box for further use. The water contents of the samples after drying were measured using a Mettler Toledo Karl-Fischer (model DL31) titrator and were 0.002 (mass fraction). The deionized water (Type I reagent-grade; resistivity = 18.3 M( cm; total organic carbon content (TOC) <15 ppb) used in all the experiments was processed in a Barnstead Thermolyne (model Easy Pure 1052) water purification system and degassed under vacuum prior to use. Aqueous solutions were prepared using a Mettler Toledo (model AL204) digital balance with an accuracy of 1 104 g. 2.2. Measurement of specific density The densities of the investigated systems were measured using an Anton Paar (model DMA 5000 M) vibrating tube density meter. It has a measurement cell that is made of a U-shaped borosilicate glass tube and that is equipped with two integrated Pt 100 platinum thermometers together with Peltier elements which controls and measures the temperature. To obtain maximum accuracy for the measurements done at higher temperatures, a temperature range adjustment was done using air and degassed deionized water at 20, 40 and 60 8C. The accuracy of the temperature measurement was 0.01 K. The instrument is also equipped with an integrated reference oscillator that provides longterm stability. As for viscosity-related errors, automatic correction was done using the instrument by measuring the damping effect of the sample followed by a mathematical correction of the density. Prior to the measurements, the instrument was calibrated using air and degassed deionized water as standard fluids. The estimated overall experimental uncertainty in the density measurements was 5 105 g/cm3. 2.3. Measurement of refractive index The refractive indices of the investigated systems were measured using an Anton Parr (model Abbemat WR) automatic refractometer utilizing a yellow light beam of 589.3 nm sodium D
The densities and refractive indices of pure ethaline and glyceline and their aqueous binary mixtures were measured over the temperature range 293.15–323.15 K at atmospheric pressure. The experimental values are tabulated in Tables 1 and 2 for ethaline and glyceline systems, respectively. For the pure DESs, both the r and nD were found to decrease linearly with temperature. Such results are expected since generally, as temperature increases substances become less dense due to thermal expansion which also results in decreased refractive index. The experimental r values were also compared with those reported by Shahbaz et al. [30,31] and were found to be in good agreement with those data having a maximum relative deviation of 0.01% and a minimum of 0.18%. To the best of our knowledge, no data has been reported for the nD of the investigated DESs that no comparison was made here. To obtain correlations for the r and nD of the pure DESs as functions of temperature, the experimental values were fitted by least-squares method using a linear equation of the form Y ¼ a0 þ a1 ðT=KÞ
(1)
where Y represents r, in g/cm3 or nD; a0 and a1 are empirical constants; and T is the absolute temperature in K. The determined empirical constants together with the corresponding AAD% are presented in Table 3. Based on the AAD% of 0.001 and 0.003 for r and nD, respectively, it can be said that, with only two fitting parameters the proposed linear equation successfully represents the measured properties as a function of temperature. For the binary aqueous mixtures, results showed that both the r and nD of the investigated systems increased with increasing DES mole fraction. This is expected since densities and refractive indices of the pure DESs are higher than those of water. A decreasing trend in r and nD with temperature was also observed which is similar to that of the corresponding pure DESs. This behavior is consistent with those observed in other aqueous ionic liquid (IL) mixtures reported in literature [32–35]. The excess molar volumes, VE, were calculated from the experimental molar volumes of the binary mixtures using the following equation VE ¼
N X xi M i ðr1 r1 i Þ i¼1
(2)
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Table 1 Densities and refractive indices of ethaline (1) + H2O (2) solutions. x1
r (g/cm3)
nD
x1
r (g/cm3)
nD
T = 298.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.99707 1.04214 1.06289 1.08444 1.09487 1.10202 1.10697 1.11051 1.11326 1.11538 1.11704
1.33246 1.37761 1.39909 1.42264 1.43529 1.44455 1.45158 1.45691 1.46140 1.46512 1.46823
T = 318.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.99027 1.03286 1.05264 1.07344 1.08365 1.09069 1.09558 1.09911 1.10182 1.10391 1.10557
1.32973 1.37395 1.39500 1.41818 1.43064 1.43981 1.44674 1.45198 1.45641 1.46009 1.46320
T = 303.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.99568 1.03997 1.06043 1.08174 1.09210 1.09921 1.10413 1.10766 1.11040 1.11250 1.11416
1.33189 1.37677 1.39813 1.42156 1.43416 1.44340 1.45039 1.45571 1.46019 1.46389 1.46699
T = 323.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.98808 1.03029 1.04992 1.07060 1.08078 1.08782 1.09271 1.09624 1.09896 1.10106 1.10271
1.32888 1.37293 1.39390 1.41702 1.42947 1.43861 1.44554 1.45076 1.45519 1.45886 1.46197
T = 308.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.99407 1.03770 1.05790 1.07901 1.08931 1.09639 1.10129 1.10481 1.10754 1.10963 1.11129
1.33126 1.37589 1.39713 1.42044 1.43301 1.44223 1.44919 1.45450 1.45896 1.46264 1.46575
T = 328.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.98573 1.02764 1.04713 1.06773 1.07789 1.08493 1.08982 1.09337 1.09610 1.09820 1.09986
1.32797 1.37193 1.39283 1.41589 1.42829 1.43745 1.44434 1.44958 1.45399 1.45766 1.46078
T = 313.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.99226 1.03533 1.05530 1.07624 1.08649 1.09355 1.09844 1.10197 1.10469 1.10677 1.10842
1.33052 1.37493 1.39607 1.41933 1.43185 1.44102 1.44795 1.45322 1.45765 1.46134 1.46445
T = 333.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.98323 1.02473 1.04428 1.06481 1.07497 1.08203 1.08693 1.09049 1.09323 1.09535 1.09702
1.32687 1.37087 1.39171 1.41470 1.42708 1.43623 1.44312 1.44835 1.45277 1.45642 1.45954
where VE is in cm3/mol; r and ri are the density of the mixture and the density of the pure components in g/cm3, respectively; xi is the mole fraction of component i; and Mi is the molecular weight of component i. The molecular weights of the pure DESs were calculated from their individual components according to the equation [36] M ¼ x1 M 1 þ x2 M 2
(3)
where M is the molecular weight of the DES; x1, x2 and M1, M2 are the mole fractions and the molecular weights of pure components 1 and 2 in the DES, respectively. The experimental excess molar volumes for ethaline (1) + H2O (2) and glyceline (1) + H2O (2) mixtures are shown in Figs. 1 and 2, respectively. It can be observed that both systems exhibit negative excess molar volumes throughout the studied temperatures and composition range considered with minima occurring at intermediate compositions (x1 0.4). Such results imply increased densities of the mixture compared with the ideal solution which
has been observed for many IL-solvent systems. Increase in excess molar volumes (more negative) as the temperature decreased were also observed for the two binary systems considered which is indicative of the dependence of strength of the hydrogen bonds in the solutions on temperature [33,35]. Also, slightly larger (more negative) excess molar volumes were observed for glyceline (1) + H2O (2) mixtures than for ethaline (1) + H2O (2). Similarly, the refractive index deviations, DnD, for the binary systems were calculated. According to Brocos et al. [37], DnD is the deviation of n from ideality that is expected to correlate well with VE when defined on a volume fraction basis. From such definition, the DnD was calculated as
DnD ¼ nD ðf1 nD1 þ f2 nD2 Þ
(4)
where nD is the refractive index of the mixture; nD1, nD2 and f1, f2 are the refractive indices and the volume fractions of pure components 1 and 2, respectively. The experimental DnD values are shown in Fig. 3 for ethaline (1) + H2O (2) and Fig. 4 for glyceline
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Table 2 Densities and refractive indices of glyceline (1) + H2O (2) solutions. x1
r (g/cm3)
nD
x1
r (g/cm3)
nD
T = 298.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.99707 1.07409 1.10675 1.13924 1.15507 1.16611 1.17400 1.17985 1.18468 1.18829 1.19123
1.33246 1.38892 1.41370 1.43972 1.45303 1.46274 1.46946 1.47538 1.48000 1.48368 1.48675
T = 318.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.99027 1.06471 1.09655 1.12851 1.14418 1.15514 1.16300 1.16881 1.17362 1.17725 1.18022
1.32973 1.38519 1.40964 1.43542 1.44863 1.45817 1.46490 1.47071 1.47538 1.47903 1.48211
T = 303.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.99568 1.07188 1.10429 1.13660 1.15248 1.16339 1.17127 1.17710 1.18192 1.18555 1.18850
1.33189 1.38802 1.41278 1.43866 1.45197 1.46163 1.46837 1.47423 1.47883 1.48251 1.48558
T = 323.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.98808 1.06214 1.09386 1.12574 1.14140 1.15231 1.16022 1.16603 1.17084 1.17448 1.17746
1.32888 1.38417 1.40854 1.43427 1.44746 1.45702 1.46373 1.46951 1.47421 1.47784 1.48093
T = 308.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.99407 1.06959 1.10177 1.13394 1.14978 1.16066 1.16852 1.17435 1.17916 1.18279 1.18574
1.33126 1.38713 1.41178 1.43760 1.45084 1.46049 1.46724 1.47308 1.47769 1.48135 1.48443
T = 328.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.98573 1.05949 1.09111 1.12294 1.13859 1.14956 1.15742 1.16325 1.16801 1.17169 1.17468
1.32797 1.38315 1.40747 1.43317 1.44632 1.45588 1.46259 1.46837 1.47302 1.47668 1.47978
T = 313.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.99226 1.06720 1.09919 1.13124 1.14704 1.15791 1.16576 1.17158 1.17639 1.18002 1.18230
1.33052 1.38617 1.41072 1.43652 1.44974 1.45936 1.46613 1.47189 1.47653 1.48019 1.48326
T = 333.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.98323 1.05670 1.08820 1.12010 1.13576 1.14674 1.15462 1.16046 1.16523 1.16890 1.17193
1.32687 1.38209 1.40634 1.43197 1.44511 1.45465 1.46139 1.46712 1.47180 1.47545 1.47856
(1) + H2O (2). It can be seen that opposite to VE, the refractive index deviations for both systems were all positive over the whole composition range at all temperatures considered. It can also be observed that for both systems DnD decreased as the temperature increased. It is also worthy to note that the DnD maxima were virtually present at the same regions where the VE minima were found. To obtain correlations for the r and nD of the binary mixtures as functions of temperature and composition, the calculated values of
the VE and DnD were fitted to a Redlich–Kister type equation of the form Y E ¼ x1 x2
N X Bi ðx1 x2 Þi1
(5)
i¼1
where YE is the VE or DnD; x1 and x2 are the mole fractions of components 1 and 2, respectively; and Bi is an empirical constant that is assumed to be temperature dependent and follows the
Table 3 Fitting parameters of Eq. (1)a for pure ethaline and glyceline. Substance
Ethaline Glyceline a b
r (g/cm3)
nD
a0
104 a1
AADb (%)
a0
104 a1
AADb (%)
1.28760 1.35583
5.72112 5.51990
0.001 0.001 0.001
1.54238 1.55720
2.48738 2.36167
0.002 0.003 0.003
Y ¼ a0 þ a1 ðT=KÞ. PN AADð%Þ ¼ 100 i¼1 jðecal eexpt Þ=eexpt ji where ecal and eexpt are the calculated and experimental values, respectively. N
R.B. Leron et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 551–557
Fig. 1. Plot of excess molar volume against x1 for ethaline (1) + H2O (2) solutions at different temperatures: 298.15 K (&); 303.15 K (*); 308.15 K (~); 313.15 K (!); 318.15 K (b); 323.15 K ("); 328.15 K (^); 333.15 K ($); lines are predicted values.
equation Bi ¼ bi;0 þ bi;1 ðT=KÞ
(6)
where bi,0 and bi,1 are fitting parameters. The experimental data obtained from this work were fitted to Eqs. (5) and (6) by the leastsquares method to determine the parameters bi,0 and bi,1. The fitting parameters are tabulated in Table 4 together with the corresponding AAD%. The number of terms (Bi), in Eq. (5), used to represent VE and DnD depends on the degree of complexity of the binary systems. Here, three terms for VE and four terms for DnD were found to satisfactory correlate the present measurements to the temperature and DES mole fraction. The AAD% of the fits were 0.006 and 1.25 for r and VE and 0.002 and 2.5 for nD and DnD, respectively. Such agreements between the experimental and calculated values are also shown via the smooth fitted curves in Figs. 1–4.
Fig. 2. Plot of excess molar volume against x1 for glyceline (1) + H2O (2) solutions at different temperatures: 298.15 K (&); 303.15 K (*); 308.15 K (~); 313.15 K (!); 318.15 K (b); 323.15 K ("); 328.15 K (^); 333.15 K ($); lines are predicted values.
555
Fig. 3. Plot of refractive index deviation against x1 for ethaline (1) + + H2O (2) solutions at different temperatures: 298.15 K (&); 303.15 K (*); 308.15 K (~); 313.15 K (!); 318.15 K (b); 323.15 K ("); 328.15 K (^); 333.15 K ($); lines are predicted values.
Furthermore, the nD of the binary mixtures were predicted from the density data and the refractive indices of the pure DESs using the most important mixing rules such as the following AragoBiotðABÞ
nD ¼ nD1 f1 þ nD2 f2
GladstoneDaleðABÞ
nD 1 ¼ ðnD1 1Þf1 þ ðnD2 1Þf2
LorentzLorenzðLLÞ
n2D 1 ¼ n2D þ 2
Eykman
n2D 1 ¼ nD þ 0:4
! n2D1 1 f1 n2D1 þ 2 ! n2D2 1 þ f2 n2D2 þ 2
2 n2D1 1 nD2 1 f1 þ f nD1 þ 0:4 nD2 þ 0:4 2
(7) (8)
(9)
(10)
Fig. 4. Plot of refractive index deviation against x1 for glyceline (1) + H2O (2) solutions at different temperatures: 298.15 K (&); 303.15 K (*); 308.15 K (~); 313.15 K (!); 318.15 K (b); 323.15 K ("); 328.15 K (^); 333.15 K ($); lines are predicted values.
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Table 4 Fitting parameters of Eq. (5)a for the DES (1) + H2O (2) systems. i
System
r (g/cm3) bi,0
nD bi,1
AAD (%)
r
VE
bi,0
bi,1
AAD (%) nD
DnD
Ethaline (1) + H2O (2)
1 2 3 4
2.73824 1.84421 0.65280
0.00487 0.00484 0.00245
0.007
1.46
0.01799 0.02032 0.01781 0.00021
0.00004 0.00005 0.00006 4.02 106
0.001
2.25
Glyceline (1) + H2O (2)
1 2 3 4
2.61063 1.84942 1.68642
0.00435 0.00424 0.00481
0.005
1.04
0.01883 0.04163 0.02595 0.02616
0.00004 0.00003 0.00008 0.00008
0.002
2.76
0.006
1.25
0.002
2.50
Average a
Y E ¼ x1 x2
Heller
PN
i1 where Bi ¼ bi;0 þ bi;1 ðT=KÞ. i¼1 Bi ðx1 x2 Þ
" # nD nD1 3 ðnD2 =nD1 Þ2 1 ¼ f 2 ðnD2 =nD1 Þ2 þ 2 2 nD1 ! n2D2 n2D1 f2 n2D2 þ 2n2D1
Weiner
n2D n2D1 ¼ n2D þ 2n2D2
Newton
n2D 1 ¼ ðn2D1 1Þf1 þ ðn2D2 1Þf2
EyringJohn
2
(11)
[38,42]. Lastly, it can be noted that the AAD% obtained from Eqs. (5) and (6) were actually lower than those obtained from the above mixing rules which confirms that they can be used in predicting nD or r of the studied systems with sufficient accuracy.
(12)
4. Conclusions
(13) 2
nD ¼ nD1 f1 þ 2ðnD1 nD2 Þ0:5 f1 f2 þ nD2 f2
(14)
where nD is the refractive index of the mixture; nD1, nD2 and f1, f2 are the refractive indices and volume fractions of the pure components 1 and 2, respectively. The use of the above mixing rules would serve to test the accuracy of the refractive data obtained in this work since they have been found to be applicable in predicting the refractive index of a mixture from the densities and refractive indices of the pure components and vice versa [37– 41]. The predicted results were used to calculate the average absolute deviations in the refractive index for the different mixing rules which are then summarized in Table 5. It can be said that the correlations generally yield good predictions of the refractive index of the binary mixtures as indicated by the low values of the AAD%. It can be deduced further that among the mixing rules, the Newton correlation gave the most accurate predictions followed by the Arago–Biot and Gladstone–Dale, Eykman, Eyring–John, Heller and Lorentz–Lorenz correlations. The Weiner correlation, however, gave negative and relatively higher deviations. It is also obvious that the Arago–Biot and the Gladstone–Dale correlations yield exactly the same deviations for both systems at all temperatures. This is the case since volume additivity was assumed, which lead these equations to exhibit similarities in their functional forms Table 5 Average absolute deviations (%) in the refractive index from the different mixing rules for the studied binary systems. Mixing rule
Arago–Biot Gladstone–Dale Lorentz–Lorenz Eykman Weiner Heller Newton Eyring–John
AAD (%)
The densities and the refractive indices of the deep eutectic solvents ethaline and glyceline and their aqueous mixtures over the complete compositions range were determined within the temperature range 293.15–323.15 K. It was observed that the densities and refractive indices of the pure and aqueous mixtures decreased linearly with increasing temperature and increased with increasing DES mole fraction. The excess molar volumes and refractive index deviations were also determined from the experimental data and fitted to a Redlich–Kister type equation to correlate their dependences on temperature and composition. The calculated excess molar volumes were found negative over the entire range of composition considered at all temperatures. These corresponded to positive values of refractive index deviations. The accuracies of the reported data and of the applied correlations were also tested by comparing the experimental and calculated refractive indices with those predicted from the most important mixing rules and the results were found comparable. It can be realized that the applied correlations in this work satisfactorily predicted the densities and refractive indices of the pure DESs and their aqueous binary mixtures at different temperatures and mole fractions as indicated by the low average absolute deviations obtained in the calculations. Thus, it can be concluded that the proposed correlations can be conveniently used to accurately predict the r and nD of the studied systems as functions of temperature and composition. Acknowledgment This research was supported by Grant, NSC 99-2221-E-033044-MY3, of the National Science Council of the Republic of China. References
Ethaline (1) + H2O (2)
Glyceline (1) + H2O (2)
Average
0.071 0.071 0.136 0.092 0.200 0.103 0.027 0.104
0.073 0.073 0.154 0.099 0.243 0.110 0.024 0.114
0.072 0.072 0.141 0.096 0.222 0.107 0.026 0.109
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Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Densities and refractive indices of the deep eutectic solvents (choline chloride + ethylene glycol or glycerol) and their aqueous mixtures at the temperature ranging from 298.15 to 333.15 K Rhoda B. Leron a, Allan N. Soriano b, Meng-Hui Li a,* a b
R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan Christian University, Chung Li, 32023, Taiwan School of Chemical Engineering and Chemistry, Mapu´a Institute of Technology, Manila, 1002, Philippines
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 October 2011 Received in revised form 14 December 2011 Accepted 20 January 2012 Available online 23 February 2012
Deep eutectic solvents (DES) are new emerging alternatives to conventional ionic liquids that may find a number of interesting applications in industrial and chemical processes. In this study, the densities, r, and refractive indices, nD, of the DESs (choline chloride + ethylene glycol) and (choline chloride + glycerol) and their aqueous mixtures were investigated at atmospheric pressure over the temperature range 298.15–333.15 K and across a complete composition range. The excess molar volumes, VE, and refractive index deviations, DnD, were also calculated from experimental results. The calculated excess molar volumes were negative at all temperatures over the entire range of composition considered, suggesting the presence of strong interactions between water and the DES in the mixtures. The refractive index deviations, on the other hand, were found positive in the entire concentration range. The calculated properties were fitted to a Redlich–Kister type equation to correlate them to the temperature and composition. The correlations used satisfactorily represent the densities and refractive indices of the pure DESs and their aqueous binary mixtures as functions of temperature and composition as indicated by the low overall average absolute deviations obtained in the calculations. ß 2012 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Deep eutectic solvent Density Excess molar volume Refractive index Refractive index deviation Mixing rule
1. Introduction Room temperature ionic liquids (RTILs) have attracted significant research interest over the last decades owing to their unique physical and chemical properties i.e. extremely low vapor pressures, high thermal stability, high solvation capacity, nonflammability, high thermal conductivity and wide liquid range [1– 3]. RTILs, being designer solvents, have huge number of possible applications in electrochemical, analytical, synthetic and engineering processes [4,5]. They have been used as gas absorption media (i.e. CO2 removal), heat transfer fluids, solvents in electroplating and catalysts in organic and chemical synthesis [6–8]. However, most RTILs work at the disadvantage of high cost for bulk applications. They are relatively expensive and some of them even have very low tolerance to moisture [6]. Furthermore, their toxicology has yet to be examined and some studies suggest for further assessment of their applicability as green media [9]. Recently emerging alternatives to conventional RTILS are deep eutectic solvents (DESs). They belong to a new class of ionic liquids that are made by mixing a substituted quaternary ammonium salt
* Corresponding author. Tel.: +886 3 265 4109; fax: +886 3 265 4199. E-mail address: [email protected] (M.-H. Li).
(i.e. 2-hydroxy-N,N,N-trimethylethanaminium [choline] chloride) and a hydrogen bond donor (i.e. amide, carboxylic acid or alcohol), both of which having high melting points, to form a eutectic mixture with a substantially lower melting point [4,10]. They were found to have solvent properties similar to those of RTILs while possessing several advantages over the latter. DESs are easier to prepare in high purity and at a relatively cheaper cost. Many of them are biodegradable and the toxicology of their components is well characterized [9–11]. It is due to these unique features of DESs that they are considered as potential green solvents for a number of industrial applications. Recent researches reported the applicability of DESs as solvents in the bulk processing of metals [12–14], biodiesel purification [15,16], polymer synthesis [17,18], drug solubilization [11], biological transformations [19,20], carbon– carbon nanotube composite preparation [21], and even CO2 absorption [3,5]. DESs were also found to be viable solvents for the fabrication of novel metal surfaces and coatings (i.e. superhydrophobic Ni films) [22,23] and thermochromic PVDF composite films [24]. In order to establish the possible use of deep eutectic solvents in industrial and chemical processes, it is necessary to know their physical properties including the density and refractive index. These could also provide important information on the purity of the samples and the molecular interaction in the liquid [25,26].
1876-1070/$ – see front matter ß 2012 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jtice.2012.01.007
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R.B. Leron et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 551–557
To date most of these properties of DES have not yet been investigated extensively. Thus, in this work, the density, r, and the refractive index, nD, of the deep eutectic solvents ethaline (1 mol choline chloride: 2 mol ethylene glycol) and glyceline (1 mol choline chloride: 2 mol glycerol) were determined at atmospheric pressure within the temperature range 298.15–333.15 K. In addition, since the properties of the DESs maybe affected by the presence of water [19–21,27] the r and nD of their corresponding aqueous binary mixtures over the entire range of mole fraction (x1 = 0.1–0.9) were also determined. Furthermore, the excess molar volume, VE, and refractive index deviations, DnD, were calculated and correlated to the system’s temperature and composition by fitting the experimental data into a Redlich–Kister type equation. Consequently, the resulting correlations were used to predict the density and refractive index of the binary mixtures as functions of temperature and composition. Lastly, the refractive indices of the binary mixtures were predicted using the most important mixing rules such as the Arago–Biot, Gladstone–Dale, Lorentz–Lorenz, Eykman, Weiner, Heller, Newton and Eyring–John correlations which were then compared with the experimental data.
line wavelength. The temperature was controlled by a built-in internal solid state Peltier thermostat fitted with two internal Pt100 Platinum resistance temperature sensors that allowed temperature measurement to an accuracy of 0.03 K. For each measurement, at least 1.0 mL sample was placed on the measuring prism to minimize vaporization of the water at high temperatures. A cone-shaped yellow light beam illuminated the sample from its bottom side under different angles of reflection and then a microprocessor automatically calculates the refractive index of the sample from the obtained data. The refractometer was calibrated using deionized water and calibration was checked after every few measurements. The estimated overall uncertainty in the refractive index measurement was 5 105. All measurements were carried out in three to five replicate runs and the average values were reported. Prior to the analysis of the DES samples, the accuracy of the equipment and procedure used were validated first by measuring the density and refractive index of water and comparing the measurements obtained with the reference data reported by Spieweck and Bettin [28] and Schiebener et al. [29]. The average deviations found were 0.004% and 0.0003% for r and nD, respectively.
2. Experimental 3. Results and discussion 2.1. Chemicals The pure deep eutectic solvents ethaline and glyceline (purity >98%) were obtained from Scionix Ltd1. They are mixtures of (choline chloride + ethylene glycol) and (choline chloride + glycerol) at 1:2 mol ratios. Each pure sample was dried under vacuum at 333 K for at least 48 h to remove any volatile impurities and stored in a dry box for further use. The water contents of the samples after drying were measured using a Mettler Toledo Karl-Fischer (model DL31) titrator and were 0.002 (mass fraction). The deionized water (Type I reagent-grade; resistivity = 18.3 M( cm; total organic carbon content (TOC) <15 ppb) used in all the experiments was processed in a Barnstead Thermolyne (model Easy Pure 1052) water purification system and degassed under vacuum prior to use. Aqueous solutions were prepared using a Mettler Toledo (model AL204) digital balance with an accuracy of 1 104 g. 2.2. Measurement of specific density The densities of the investigated systems were measured using an Anton Paar (model DMA 5000 M) vibrating tube density meter. It has a measurement cell that is made of a U-shaped borosilicate glass tube and that is equipped with two integrated Pt 100 platinum thermometers together with Peltier elements which controls and measures the temperature. To obtain maximum accuracy for the measurements done at higher temperatures, a temperature range adjustment was done using air and degassed deionized water at 20, 40 and 60 8C. The accuracy of the temperature measurement was 0.01 K. The instrument is also equipped with an integrated reference oscillator that provides longterm stability. As for viscosity-related errors, automatic correction was done using the instrument by measuring the damping effect of the sample followed by a mathematical correction of the density. Prior to the measurements, the instrument was calibrated using air and degassed deionized water as standard fluids. The estimated overall experimental uncertainty in the density measurements was 5 105 g/cm3. 2.3. Measurement of refractive index The refractive indices of the investigated systems were measured using an Anton Parr (model Abbemat WR) automatic refractometer utilizing a yellow light beam of 589.3 nm sodium D
The densities and refractive indices of pure ethaline and glyceline and their aqueous binary mixtures were measured over the temperature range 293.15–323.15 K at atmospheric pressure. The experimental values are tabulated in Tables 1 and 2 for ethaline and glyceline systems, respectively. For the pure DESs, both the r and nD were found to decrease linearly with temperature. Such results are expected since generally, as temperature increases substances become less dense due to thermal expansion which also results in decreased refractive index. The experimental r values were also compared with those reported by Shahbaz et al. [30,31] and were found to be in good agreement with those data having a maximum relative deviation of 0.01% and a minimum of 0.18%. To the best of our knowledge, no data has been reported for the nD of the investigated DESs that no comparison was made here. To obtain correlations for the r and nD of the pure DESs as functions of temperature, the experimental values were fitted by least-squares method using a linear equation of the form Y ¼ a0 þ a1 ðT=KÞ
(1)
where Y represents r, in g/cm3 or nD; a0 and a1 are empirical constants; and T is the absolute temperature in K. The determined empirical constants together with the corresponding AAD% are presented in Table 3. Based on the AAD% of 0.001 and 0.003 for r and nD, respectively, it can be said that, with only two fitting parameters the proposed linear equation successfully represents the measured properties as a function of temperature. For the binary aqueous mixtures, results showed that both the r and nD of the investigated systems increased with increasing DES mole fraction. This is expected since densities and refractive indices of the pure DESs are higher than those of water. A decreasing trend in r and nD with temperature was also observed which is similar to that of the corresponding pure DESs. This behavior is consistent with those observed in other aqueous ionic liquid (IL) mixtures reported in literature [32–35]. The excess molar volumes, VE, were calculated from the experimental molar volumes of the binary mixtures using the following equation VE ¼
N X xi M i ðr1 r1 i Þ i¼1
(2)
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Table 1 Densities and refractive indices of ethaline (1) + H2O (2) solutions. x1
r (g/cm3)
nD
x1
r (g/cm3)
nD
T = 298.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.99707 1.04214 1.06289 1.08444 1.09487 1.10202 1.10697 1.11051 1.11326 1.11538 1.11704
1.33246 1.37761 1.39909 1.42264 1.43529 1.44455 1.45158 1.45691 1.46140 1.46512 1.46823
T = 318.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.99027 1.03286 1.05264 1.07344 1.08365 1.09069 1.09558 1.09911 1.10182 1.10391 1.10557
1.32973 1.37395 1.39500 1.41818 1.43064 1.43981 1.44674 1.45198 1.45641 1.46009 1.46320
T = 303.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.99568 1.03997 1.06043 1.08174 1.09210 1.09921 1.10413 1.10766 1.11040 1.11250 1.11416
1.33189 1.37677 1.39813 1.42156 1.43416 1.44340 1.45039 1.45571 1.46019 1.46389 1.46699
T = 323.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.98808 1.03029 1.04992 1.07060 1.08078 1.08782 1.09271 1.09624 1.09896 1.10106 1.10271
1.32888 1.37293 1.39390 1.41702 1.42947 1.43861 1.44554 1.45076 1.45519 1.45886 1.46197
T = 308.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.99407 1.03770 1.05790 1.07901 1.08931 1.09639 1.10129 1.10481 1.10754 1.10963 1.11129
1.33126 1.37589 1.39713 1.42044 1.43301 1.44223 1.44919 1.45450 1.45896 1.46264 1.46575
T = 328.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.98573 1.02764 1.04713 1.06773 1.07789 1.08493 1.08982 1.09337 1.09610 1.09820 1.09986
1.32797 1.37193 1.39283 1.41589 1.42829 1.43745 1.44434 1.44958 1.45399 1.45766 1.46078
T = 313.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.99226 1.03533 1.05530 1.07624 1.08649 1.09355 1.09844 1.10197 1.10469 1.10677 1.10842
1.33052 1.37493 1.39607 1.41933 1.43185 1.44102 1.44795 1.45322 1.45765 1.46134 1.46445
T = 333.15 K 0.0000 0.1000 0.1750 0.2999 0.4000 0.5000 0.6000 0.6970 0.7992 0.9002 1.0000
0.98323 1.02473 1.04428 1.06481 1.07497 1.08203 1.08693 1.09049 1.09323 1.09535 1.09702
1.32687 1.37087 1.39171 1.41470 1.42708 1.43623 1.44312 1.44835 1.45277 1.45642 1.45954
where VE is in cm3/mol; r and ri are the density of the mixture and the density of the pure components in g/cm3, respectively; xi is the mole fraction of component i; and Mi is the molecular weight of component i. The molecular weights of the pure DESs were calculated from their individual components according to the equation [36] M ¼ x1 M 1 þ x2 M 2
(3)
where M is the molecular weight of the DES; x1, x2 and M1, M2 are the mole fractions and the molecular weights of pure components 1 and 2 in the DES, respectively. The experimental excess molar volumes for ethaline (1) + H2O (2) and glyceline (1) + H2O (2) mixtures are shown in Figs. 1 and 2, respectively. It can be observed that both systems exhibit negative excess molar volumes throughout the studied temperatures and composition range considered with minima occurring at intermediate compositions (x1 0.4). Such results imply increased densities of the mixture compared with the ideal solution which
has been observed for many IL-solvent systems. Increase in excess molar volumes (more negative) as the temperature decreased were also observed for the two binary systems considered which is indicative of the dependence of strength of the hydrogen bonds in the solutions on temperature [33,35]. Also, slightly larger (more negative) excess molar volumes were observed for glyceline (1) + H2O (2) mixtures than for ethaline (1) + H2O (2). Similarly, the refractive index deviations, DnD, for the binary systems were calculated. According to Brocos et al. [37], DnD is the deviation of n from ideality that is expected to correlate well with VE when defined on a volume fraction basis. From such definition, the DnD was calculated as
DnD ¼ nD ðf1 nD1 þ f2 nD2 Þ
(4)
where nD is the refractive index of the mixture; nD1, nD2 and f1, f2 are the refractive indices and the volume fractions of pure components 1 and 2, respectively. The experimental DnD values are shown in Fig. 3 for ethaline (1) + H2O (2) and Fig. 4 for glyceline
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Table 2 Densities and refractive indices of glyceline (1) + H2O (2) solutions. x1
r (g/cm3)
nD
x1
r (g/cm3)
nD
T = 298.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.99707 1.07409 1.10675 1.13924 1.15507 1.16611 1.17400 1.17985 1.18468 1.18829 1.19123
1.33246 1.38892 1.41370 1.43972 1.45303 1.46274 1.46946 1.47538 1.48000 1.48368 1.48675
T = 318.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.99027 1.06471 1.09655 1.12851 1.14418 1.15514 1.16300 1.16881 1.17362 1.17725 1.18022
1.32973 1.38519 1.40964 1.43542 1.44863 1.45817 1.46490 1.47071 1.47538 1.47903 1.48211
T = 303.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.99568 1.07188 1.10429 1.13660 1.15248 1.16339 1.17127 1.17710 1.18192 1.18555 1.18850
1.33189 1.38802 1.41278 1.43866 1.45197 1.46163 1.46837 1.47423 1.47883 1.48251 1.48558
T = 323.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.98808 1.06214 1.09386 1.12574 1.14140 1.15231 1.16022 1.16603 1.17084 1.17448 1.17746
1.32888 1.38417 1.40854 1.43427 1.44746 1.45702 1.46373 1.46951 1.47421 1.47784 1.48093
T = 308.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.99407 1.06959 1.10177 1.13394 1.14978 1.16066 1.16852 1.17435 1.17916 1.18279 1.18574
1.33126 1.38713 1.41178 1.43760 1.45084 1.46049 1.46724 1.47308 1.47769 1.48135 1.48443
T = 328.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.98573 1.05949 1.09111 1.12294 1.13859 1.14956 1.15742 1.16325 1.16801 1.17169 1.17468
1.32797 1.38315 1.40747 1.43317 1.44632 1.45588 1.46259 1.46837 1.47302 1.47668 1.47978
T = 313.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.99226 1.06720 1.09919 1.13124 1.14704 1.15791 1.16576 1.17158 1.17639 1.18002 1.18230
1.33052 1.38617 1.41072 1.43652 1.44974 1.45936 1.46613 1.47189 1.47653 1.48019 1.48326
T = 333.15 K 0.0000 0.1000 0.1750 0.3005 0.4000 0.5002 0.5997 0.6966 0.7997 0.9000 1.0000
0.98323 1.05670 1.08820 1.12010 1.13576 1.14674 1.15462 1.16046 1.16523 1.16890 1.17193
1.32687 1.38209 1.40634 1.43197 1.44511 1.45465 1.46139 1.46712 1.47180 1.47545 1.47856
(1) + H2O (2). It can be seen that opposite to VE, the refractive index deviations for both systems were all positive over the whole composition range at all temperatures considered. It can also be observed that for both systems DnD decreased as the temperature increased. It is also worthy to note that the DnD maxima were virtually present at the same regions where the VE minima were found. To obtain correlations for the r and nD of the binary mixtures as functions of temperature and composition, the calculated values of
the VE and DnD were fitted to a Redlich–Kister type equation of the form Y E ¼ x1 x2
N X Bi ðx1 x2 Þi1
(5)
i¼1
where YE is the VE or DnD; x1 and x2 are the mole fractions of components 1 and 2, respectively; and Bi is an empirical constant that is assumed to be temperature dependent and follows the
Table 3 Fitting parameters of Eq. (1)a for pure ethaline and glyceline. Substance
Ethaline Glyceline a b
r (g/cm3)
nD
a0
104 a1
AADb (%)
a0
104 a1
AADb (%)
1.28760 1.35583
5.72112 5.51990
0.001 0.001 0.001
1.54238 1.55720
2.48738 2.36167
0.002 0.003 0.003
Y ¼ a0 þ a1 ðT=KÞ. PN AADð%Þ ¼ 100 i¼1 jðecal eexpt Þ=eexpt ji where ecal and eexpt are the calculated and experimental values, respectively. N
R.B. Leron et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 551–557
Fig. 1. Plot of excess molar volume against x1 for ethaline (1) + H2O (2) solutions at different temperatures: 298.15 K (&); 303.15 K (*); 308.15 K (~); 313.15 K (!); 318.15 K (b); 323.15 K ("); 328.15 K (^); 333.15 K ($); lines are predicted values.
equation Bi ¼ bi;0 þ bi;1 ðT=KÞ
(6)
where bi,0 and bi,1 are fitting parameters. The experimental data obtained from this work were fitted to Eqs. (5) and (6) by the leastsquares method to determine the parameters bi,0 and bi,1. The fitting parameters are tabulated in Table 4 together with the corresponding AAD%. The number of terms (Bi), in Eq. (5), used to represent VE and DnD depends on the degree of complexity of the binary systems. Here, three terms for VE and four terms for DnD were found to satisfactory correlate the present measurements to the temperature and DES mole fraction. The AAD% of the fits were 0.006 and 1.25 for r and VE and 0.002 and 2.5 for nD and DnD, respectively. Such agreements between the experimental and calculated values are also shown via the smooth fitted curves in Figs. 1–4.
Fig. 2. Plot of excess molar volume against x1 for glyceline (1) + H2O (2) solutions at different temperatures: 298.15 K (&); 303.15 K (*); 308.15 K (~); 313.15 K (!); 318.15 K (b); 323.15 K ("); 328.15 K (^); 333.15 K ($); lines are predicted values.
555
Fig. 3. Plot of refractive index deviation against x1 for ethaline (1) + + H2O (2) solutions at different temperatures: 298.15 K (&); 303.15 K (*); 308.15 K (~); 313.15 K (!); 318.15 K (b); 323.15 K ("); 328.15 K (^); 333.15 K ($); lines are predicted values.
Furthermore, the nD of the binary mixtures were predicted from the density data and the refractive indices of the pure DESs using the most important mixing rules such as the following AragoBiotðABÞ
nD ¼ nD1 f1 þ nD2 f2
GladstoneDaleðABÞ
nD 1 ¼ ðnD1 1Þf1 þ ðnD2 1Þf2
LorentzLorenzðLLÞ
n2D 1 ¼ n2D þ 2
Eykman
n2D 1 ¼ nD þ 0:4
! n2D1 1 f1 n2D1 þ 2 ! n2D2 1 þ f2 n2D2 þ 2
2 n2D1 1 nD2 1 f1 þ f nD1 þ 0:4 nD2 þ 0:4 2
(7) (8)
(9)
(10)
Fig. 4. Plot of refractive index deviation against x1 for glyceline (1) + H2O (2) solutions at different temperatures: 298.15 K (&); 303.15 K (*); 308.15 K (~); 313.15 K (!); 318.15 K (b); 323.15 K ("); 328.15 K (^); 333.15 K ($); lines are predicted values.
R.B. Leron et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 551–557
556
Table 4 Fitting parameters of Eq. (5)a for the DES (1) + H2O (2) systems. i
System
r (g/cm3) bi,0
nD bi,1
AAD (%)
r
VE
bi,0
bi,1
AAD (%) nD
DnD
Ethaline (1) + H2O (2)
1 2 3 4
2.73824 1.84421 0.65280
0.00487 0.00484 0.00245
0.007
1.46
0.01799 0.02032 0.01781 0.00021
0.00004 0.00005 0.00006 4.02 106
0.001
2.25
Glyceline (1) + H2O (2)
1 2 3 4
2.61063 1.84942 1.68642
0.00435 0.00424 0.00481
0.005
1.04
0.01883 0.04163 0.02595 0.02616
0.00004 0.00003 0.00008 0.00008
0.002
2.76
0.006
1.25
0.002
2.50
Average a
Y E ¼ x1 x2
Heller
PN
i1 where Bi ¼ bi;0 þ bi;1 ðT=KÞ. i¼1 Bi ðx1 x2 Þ
" # nD nD1 3 ðnD2 =nD1 Þ2 1 ¼ f 2 ðnD2 =nD1 Þ2 þ 2 2 nD1 ! n2D2 n2D1 f2 n2D2 þ 2n2D1
Weiner
n2D n2D1 ¼ n2D þ 2n2D2
Newton
n2D 1 ¼ ðn2D1 1Þf1 þ ðn2D2 1Þf2
EyringJohn
2
(11)
[38,42]. Lastly, it can be noted that the AAD% obtained from Eqs. (5) and (6) were actually lower than those obtained from the above mixing rules which confirms that they can be used in predicting nD or r of the studied systems with sufficient accuracy.
(12)
4. Conclusions
(13) 2
nD ¼ nD1 f1 þ 2ðnD1 nD2 Þ0:5 f1 f2 þ nD2 f2
(14)
where nD is the refractive index of the mixture; nD1, nD2 and f1, f2 are the refractive indices and volume fractions of the pure components 1 and 2, respectively. The use of the above mixing rules would serve to test the accuracy of the refractive data obtained in this work since they have been found to be applicable in predicting the refractive index of a mixture from the densities and refractive indices of the pure components and vice versa [37– 41]. The predicted results were used to calculate the average absolute deviations in the refractive index for the different mixing rules which are then summarized in Table 5. It can be said that the correlations generally yield good predictions of the refractive index of the binary mixtures as indicated by the low values of the AAD%. It can be deduced further that among the mixing rules, the Newton correlation gave the most accurate predictions followed by the Arago–Biot and Gladstone–Dale, Eykman, Eyring–John, Heller and Lorentz–Lorenz correlations. The Weiner correlation, however, gave negative and relatively higher deviations. It is also obvious that the Arago–Biot and the Gladstone–Dale correlations yield exactly the same deviations for both systems at all temperatures. This is the case since volume additivity was assumed, which lead these equations to exhibit similarities in their functional forms Table 5 Average absolute deviations (%) in the refractive index from the different mixing rules for the studied binary systems. Mixing rule
Arago–Biot Gladstone–Dale Lorentz–Lorenz Eykman Weiner Heller Newton Eyring–John
AAD (%)
The densities and the refractive indices of the deep eutectic solvents ethaline and glyceline and their aqueous mixtures over the complete compositions range were determined within the temperature range 293.15–323.15 K. It was observed that the densities and refractive indices of the pure and aqueous mixtures decreased linearly with increasing temperature and increased with increasing DES mole fraction. The excess molar volumes and refractive index deviations were also determined from the experimental data and fitted to a Redlich–Kister type equation to correlate their dependences on temperature and composition. The calculated excess molar volumes were found negative over the entire range of composition considered at all temperatures. These corresponded to positive values of refractive index deviations. The accuracies of the reported data and of the applied correlations were also tested by comparing the experimental and calculated refractive indices with those predicted from the most important mixing rules and the results were found comparable. It can be realized that the applied correlations in this work satisfactorily predicted the densities and refractive indices of the pure DESs and their aqueous binary mixtures at different temperatures and mole fractions as indicated by the low average absolute deviations obtained in the calculations. Thus, it can be concluded that the proposed correlations can be conveniently used to accurately predict the r and nD of the studied systems as functions of temperature and composition. Acknowledgment This research was supported by Grant, NSC 99-2221-E-033044-MY3, of the National Science Council of the Republic of China. References
Ethaline (1) + H2O (2)
Glyceline (1) + H2O (2)
Average
0.071 0.071 0.136 0.092 0.200 0.103 0.027 0.104
0.073 0.073 0.154 0.099 0.243 0.110 0.024 0.114
0.072 0.072 0.141 0.096 0.222 0.107 0.026 0.109
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