Journal of Molecular Liquids 177 (2013) 11–18

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Journal of Molecular Liquids

journal homepage: www.elsevier.com/locate/molliq

Volumetric, viscosimetric and surface properties of aqueous solutions of triethylene

glycol, tetraethylene glycol, and tetraethylene glycol dimethyl ether
Syeda K. Begum a,⁎, Ronald J. Clarke b, M. Shamsuddin Ahmed a, Shahanara Begum a, Mohammad A. Saleh a, 1
a
Professor M. A. Saleh Research Laboratory, Department of Chemistry, University of Chittagong, Chittagong-4331, Bangladesh
b
School of Chemistry, University of Sydney, Sydney, NSW 2006, Australia

a r t i c l e i n f o a b s t r a c t

Article history: Densities (ρ) and viscosities (η) for water (W) + triethylene glycol (TrEG), W+ tetraethylene glycol (TeEG), and
Received 24 August 2012 W+ tetraethylene glycol dimethyl ether (TeEGDME) were measured for the whole range of composition at five
Received in revised form 18 September 2012 different temperatures ranging from 303.15 K to 323.15 K. Surface tensions for these systems were measured at
Accepted 19 September 2012 E
303.15 K for different mole fractions. The excess molar volumes, Vm , and excess viscosities, (ηE), were calculated
Available online 3 October 2012
from measured parameters. Derived volumetric and viscosimetric properties were fitted to Redlich–Kister type
Keywords:
equation. The properties were found to change significantly with increasing the number of glycol units and to be
Volumetric greatly affected by methyl substitution within the glycol unit. For unsubstituted glycols a gradual increase in den-
Viscosimetric sity and viscosity was observed on increasing the concentration, whereas for the methyl-substituted glycol
Surface property TeEGDME sharp maxima were apparent in the density–composition and viscosity–composition curves. The
Aqueous-glycol surface tensions of aqueous solutions of methyl-substituted glycol TeEGDME were found to be significantly
Methyl-substitution lower than other aqueous glycols.
© 2012 Elsevier B.V. All rights reserved.

1. Introduction methyl-substitution within the glycol on their volumetric, viscosi-

metric and surface properties.
Recently we reported both volumetric and viscosimetric proper-
ties of aqueous solutions of alcohols [1], diols [2], dimethoxyethane 2. Experimental section
[3], and diamines [4]. Considering the versatile applications of glycols
and their aqueous solutions, very recently we reported density, vis- Triethylene glycol, tetraethylene glycol and tetraethylene glycol
cosity and surface tensions of aqueous solutions of diethylene glycol dimethyl ether were obtained from Merck Schuchardt with a purity of
[5]. From a similar point of view, in this work we report the properties 0.99 mass fraction. Prior to use the samples were kept over molecular
for aqueous solutions of triethylene glycol (TrEG), tetraethylene gly- sieves (0.4 nm) to reduce water content and to protect from moisture
col (TeEG) and tetraethylene glycol dimethyl ether (TeEGDME). A and CO2. The purity of the glycols and ether was confirmed by com-
literature survey reveals that although volumetric and viscosimetric paring the experimental densities and viscosities of the samples with
data on these systems are available [6–10], most of the works has corresponding literature values in the temperature range (298.15 to
been done at limited mole fractions or temperatures. For instance, 308.15) K. There was a good agreement between our measured values
densities and viscosities for aqueous solutions of TrEG were studied and the literature values [9,10,12–18], as shown in Table 1. Doubly
only at three mole fractions by Sun et al. [6]. The same properties distilled water was used for all solution preparation.
for aqueous TrEG and TeEG were studied by Pal et al. [7] only at Solutions were prepared by mixing known weights of pure com-
two temperatures. Furthermore, in spite of the importance of surface ponents. An electronic analytical balance (Mettler Toledo) with an ac-
tension data in mass transfer processes [11], no data are available in curacy of ± 1 × 10 −5 g was used for weighing. To avoid evaporation
the literature on the surface tensions of these systems. and contamination, solutions were kept in air-tight glass stoppered
In this study we aim to extend the experimental information on bottles. The accuracy of the mole fraction of each mixture was calcu-
volumetric and viscosimetric properties of aqueous TrEG, TeEG and lated from the measured masses of the pure liquids and was found
TeEGDME, provide new data on surface tensions for these systems to be ± 2 × 10 −5. For the measurement of density and viscosity at tem-
and determine the effect of temperature, number of glycol units and peratures 293.15 to 323.15 K, a thermostatic water bath with an accu-
racy of ±0.05 K was used. The densities of pure components and
mixtures were measured by a 10 cm 3 bicapillary pycnometer which
⁎ Corresponding author. Tel.: +880 1830034350.
had been calibrated using redistilled water. Our density values for
E-mail address: syedacu@gmail.com (S.K. Begum). pure TrEG are within 0.008 % of available literature values [12,13,15]
1
Deceased. at 298.15 K and 0.06% of literature values [7,13–16] at 303.15 K and

0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molliq.2012.09.015
12 S.K. Begum et al. / Journal of Molecular Liquids 177 (2013) 11–18

Table 1 The apparatus used to measure the surface tension, γ, of the solutions
Experimental densities, ρ (g·cm−3), and viscosities, η (mPa s), of TrEG, TeEG and was a Langmuir trough (type 601) from Nima Technology (Coventry,
TeEGDME at T = (303.15, 308.15, 313.15, 318.15, 323.15) K and literature values at
T = (298.15, 303.15, 308.15) K.
UK). The surface tension was determined via the Wilhelmy paper plate
method as discussed in our earlier report [5]. At each temperature
ρ η measurement was repeated for six times and then average was taken.
T/K this work lit. this work lit. Based on the measurements and comparing the values of water with
TrEG literature value [22] the uncertainty in surface tension measurement
298.15 1.1199 1.119813 3.682 was estimated to be ±0.51 mN.m−1.
1.119812
1.119815 3. Results and discussion
303.15 1.1158 1.11647 2.919 2.9267
308.15 1.1119 1.112613 2.336 2.2847
1.112014 The densities and excess molar volumes for aqueous solutions of
1.112615 TrEG, TeEG and TeEGDME in the whole range of composition at differ-
1.11277 ent temperatures ranges from 303.15 to 323.15 K are listed in Table 2.
1.112016
The densities for the systems are plotted in Fig. 1. The excess molar
313.15 1.1084 1.898
318.15 1.1040 1.559 volumes are plotted in Fig. 2. The viscosities and excess viscosities
323.15 1.1000 1.299 of the systems at the same compositions and temperatures are listed
in Table 3. The viscosity values are plotted in Fig. 3 and excess viscos-
TeEG ities are plotted in Fig. 4. All the values are plotted at two different
298.15 1.1211 1.120113 4.463
1.120112
temperatures, 303.15 K and 323.15 K, to see the effect of tempera-
1.120115 ture. Densities, excess molar volumes, viscosities and excess viscosi-
303.15 1.1166 1.11707 3.497 3.4587 ties of aqueous solutions of diethylene glycol (DEG), at 323.15 K,
1.116816 3.56915 from the literature [5] are shown in Figs. 1, 2, 3 and 4 respectively
308.15 1.1126 1.112313 2.789 2.6927
for comparison purposes.
1.11317 2.79315 E
1.112815 The excess molar volumes, Vm , and excess viscosities, η E, were
313.15 1.1087 2.248 fitted to the following form of the Redlich–Kister equation:
318.15 1.1045 1.842
323.15 1.1005 1.532

E
Xn i
TeEGDME Y ¼ x2 ð1−x2 Þ Ai ð2x2  1Þ ð3Þ
298.15 1.0070 1.006618 3.334 3.38010 i¼0
1.006012 3.2949
3.39415
3.31317
where x2 is the mole fraction of the organic solute and Ai is the ith co-
303.15 1.0020 2.955
308.15 0.9972 2.646 efficient of the equation. The coefficients of this equation and the
313.15 0.99256 2.388 standard deviations, σ, for excess molar volumes and excess viscosi-
318.15 0.9878 2.169 ties are listed in Tables 4 and 5 respectively.
323.15 0.9830 1.976 It has been found that, at 303.15 K, the densities of the pure
glycols, DEG (1.1095 g cm −3) , TrEG (1.1150 g cm −3) and TeEG
(1.1166 g cm −3) are considerably higher than those of the methyl-
308.15 K. For TeEG it is within 0.09% of literature values [12,13,15] substituted glycol TeEGDME (1.002 g cm −3). This observation clearly
at 298.15 K. In the case of tetraethylene glycol dimethyl ether it suggests that glycols, owing to their terminal ―OH groups, are highly
was found to be within 0.09% of literature values [12,18] at 298.15 K. associated through intermolecular H-bonds but, due to the lack of a
The uncertainty in density measurements was estimated to be terminal ―OH group, such strong association is not possible in the
± 1.5 × 10 − 4 g cm − 3. methyl-substituted glycol TeEGDME.
The coefficient of viscosity, η, of pure glycols and their aqueous Fig. 1 shows that for the systems W + TrEG and W + TeEG, the den-
solutions were measured using an A-type Ostwald U-tube viscometer sity rises sharply in the water-rich region, followed by a consistently
(British Standard Institution) with sufficiently long efflux time, which slower increase as the solution becomes richer and richer in glycol.
had been calibrated with redistilled water. The flow time was deter- This is similar to the behavior observed in the system W + DEG [5].
mined using a digital stopwatch with an accuracy of ±0.01 s. The un- The sharpness of the initial increase of density with composition in-
certainty in viscosity measurements was estimated to be ±0.04 mPa s. creases with an increase in the number of glycol units. The magnitude
The excess molar volumes were calculated using the following of the initial sharp increase of density with composition also increases
relation: with increasing number of glycol units. The densities for these systems
follow the order W + TeEG > W + TrEG > W + DEG. The density profile
E
V m ¼ ðx1 M 1 þ x2 M2 Þ=ρ−ðx1 M 1 =ρ1 þ x2 M2 =ρ2 Þ ð1Þ of the system consisting of water and TeEGDME is completely dif-
ferent from those of other aqueous glycols. Although initially the den-
where ρ, ρ1, and ρ2 represent the densities of the solution, water (1) and sity of the W + TeEGDME system rises very rapidly (as also observed
organic solutes (2) respectively. M1, M2, and x1, x2 represent the molar for three other aqueous glycols), after showing a well-defined maxima
masses and mole fractions of the corresponding components. Here the at ~ 0.08 mole fraction of TeEGDME the density then declines sharply
word ‘excess’ means ‘deviation from additivity’. with increasing mole fraction of TeEGDME. Although this density–
The excess viscosity η E was calculated from the following equa- composition curve for the glycol ether TeEGDME is different from
tion [19–21]: those of the unsubstituted glycols, it is similar to those found for

 some other glycol ethers [23,24]. Similar observations were also
E
η ¼ η – exp x1 ln η1þ x2 lnη2 ð2Þ reported by Li et al. [25] for aqueous diethylene glycol monomethyl
ether (DEGMME), triethylene glycol monomethyl ether (TrEGMME),
where η is the experimental viscosity of the solution. η1 and η2 are the diethylene glycol monoethyl ether (DEGMEE), and triethylene glycol
viscosities of water and the organic solute, respectively. monoethyl ether (TrEGMEE).
 S.K. Begum et al. / Journal of Molecular Liquids 177 (2013) 11–18 13

Table 2
Densities, ρ (g cm−3), and excess molar volumes, Vm
E
(cm3·mol−1) for water (1) + TrEG (2), water (1) + TeEG (2), water (1) + TeEGDME (2) at T = (303.15, 308.15, 313.15,318.15,
323.15) K.

303.15 K 308.15 K 313.15 K 318.15 K 323.15 K

E E E E E
x2 ρ Vm ρ Vm ρ Vm ρ Vm ρ Vm

Water (1) + TrEG (2)

0.00000 0.9956 0.0000 0.9941 0.0000 0.9922 0.0000 0.9903 0.0000 0.9881 0.0000
0.05023 1.0412 −0.2697 1.0385 −0.2573 1.0357 −0.2481 1.0328 −0.2407 1.0296 −0.2298
0.10041 1.0681 −0.5013 1.0648 −0.4834 1.0616 −0.4693 1.0582 −0.4589 1.0547 −0.4478
0.15179 1.0843 −0.6614 1.0805 −0.6338 1.0769 −0.6117 1.0733 −0.6035 1.0694 −0.5848
0.20059 1.0934 −0.7428 1.0897 −0.7212 1.0860 −0.6938 1.0823 −0.6875 1.0782 −0.6677
0.25283 1.0997 −0.7834 1.0958 −0.7551 1.0920 −0.7260 1.0881 −0.7188 1.0841 −0.7004
0.29951 1.1042 −0.8259 1.1001 −0.7892 1.0962 −0.7575 1.0924 −0.7582 1.0884 −0.7410
0.39283 1.1087 −0.7835 1.1045 −0.7469 1.1007 −0.7132 1.0971 −0.7374 1.0931 −0.7257
0.50127 1.1112 −0.6615 1.1072 −0.6358 1.1034 −0.6053 1.0995 −0.6229 1.0954 −0.6050
0.59952 1.1127 −0.5406 1.1088 −0.5233 1.1049 −0.4863 1.1013 −0.5319 1.0972 −0.5214
0.66479 1.1139 −0.4928 1.1098 −0.4587 1.1059 −0.4224 1.1020 −0.4493 1.0980 −0.4501
0.79810 1.1147 −0.2865 1.1108 −0.2749 1.1068 −0.2254 1.1030 −0.2721 1.0990 −0.2765
0.84448 1.1152 −0.2429 1.1112 −0.2274 1.1073 −0.1789 1.1035 −0.2331 1.0994 −0.2274
0.89980 1.1156 −0.1677 1.1115 −0.1424 1.1075 −0.0935 1.1039 −0.1731 1.0997 −0.1540
0.95404 1.1162 −0.1301 1.1120 −0.0929 1.1080 −0.0370 1.1041 −0.0885 1.1000 −0.0852
1.00000 1.1158 0.0000 1.1119 0.0000 1.1084 0.0000 1.1040 0.0000 1.1000 0.0000

Water (1) + TeEG (2)

0.00000 0.9956 0.0000 0.9941 0.0000 0.9922 0.0000 0.9903 0.0000 0.9881 0.0000
0.02061 1.0226 −0.1373 1.0204 −0.1325 1.0181 −0.1300 1.0157 −0.1271 1.0132 −0.1280
0.03953 1.0433 −0.3108 1.0407 −0.3025 1.0379 −0.2957 1.0350 −0.2889 1.0319 −0.2809
0.05990 1.0591 −0.4540 1.0560 −0.4383 1.0529 −0.4280 1.0498 −0.4195 1.0464 −0.4072
0.07592 1.0704 −0.5979 1.0671 −0.5800 1.0637 −0.5649 1.0602 −0.5508 1.0568 −0.5393
0.10155 1.0797 −0.6630 1.0760 −0.6360 1.0723 −0.6157 1.0688 −0.6046 1.0650 −0.5840
0.15131 1.0940 −0.8369 1.0890 −0.7998 1.0860 −0.7790 1.0824 −0.7719 1.0783 −0.7442
0.20173 1.1017 −0.9166 1.0976 −0.8819 1.0936 −0.8560 1.0896 −0.8360 1.0856 −0.8159
0.25162 1.1060 −0.9398 1.1024 −0.9260 1.0983 −0.8936 1.0942 −0.8768 1.0901 −0.8494
0.30060 1.1091 −0.9396 1.1049 −0.9065 1.1009 −0.8817 1.0970 −0.8726 1.0928 −0.8497
0.40194 1.1123 −0.8610 1.1081 −0.8296 1.1042 −0.8160 1.1005 −0.8314 1.0961 −0.7939
0.50486 1.1142 −0.7669 1.1100 −0.7278 1.1060 −0.7124 1.1020 −0.7089 1.0980 −0.6985
0.59699 1.1151 −0.6384 1.1109 −0.6001 1.1068 −0.5777 1.1029 −0.5908 1.0988 −0.5736
0.69775 1.1159 −0.5150 1.1117 −0.4802 1.1076 −0.4531 1.1037 −0.4684 1.0993 −0.4291
0.75145 1.1160 −0.4153 1.1117 −0.3646 1.1077 −0.3484 1.1030 −0.3704 1.0997 −0.3587
0.80710 1.1163 −0.3300 1.1120 −0.2844 1.1079 −0.2625 1.1039 −0.2810 1.0997 −0.2600
0.85096 1.1165 −0.2776 1.1122 −0.2309 1.1081 −0.1984 1.1045 −0.2750 1.1002 −0.2346
0.90434 1.1168 −0.2132 1.1126 −0.1718 1.1084 −0.1392 1.1040 −0.1814 1.1004 −0.1649
0.95600 1.1169 −0.1341 1.1127 −0.0864 1.1087 −0.0779 1.1047 −0.1118 1.1004 −0.0711
1.00000 1.1166 0.0000 1.1126 0.0000 1.1087 0.0000 1.1045 0.0000 1.1005 0.0000

Water (1) + TeEGDME (2)

0.0000 0.9956 0.0000 0.9941 0.0000 0.9922 0.0000 0.9903 0.0000 0.9881 0.0000
0.0056 1.0040 −0.1534 1.0018 −0.1451 0.9992 −0.1362 0.9962 −0.1188 0.9929 −0.1004
0.0290 1.0179 −0.4856 1.0150 −0.4767 1.0121 −0.4732 1.0090 −0.4651 1.0057 −0.4595
0.0457 1.0265 −0.7640 1.0230 −0.7467 1.0194 −0.7335 1.0157 −0.7162 1.0119 −0.7057
0.0765 1.0325 −1.1004 1.0283 −1.0744 1.0241 −1.0532 1.0198 −1.0257 1.0153 −1.0034
0.1008 1.0340 −1.2992 1.0296 −1.2703 1.0252 −1.2453 1.0204 −1.2087 1.0161 −1.1934
0.1271 1.0333 −1.4309 1.0286 −1.3979 1.0241 −1.3734 1.0195 −1.3432 1.0147 −1.3158
0.1512 1.0320 −1.5195 1.0273 −1.4861 1.0227 −1.4584 1.0179 −1.4257 1.0132 −1.4023
0.2009 1.0281 −1.5898 1.0234 −1.5590 1.0187 −1.5344 1.0140 −1.5055 1.0092 −1.4815
0.2493 1.0247 −1.6136 1.0198 −1.5762 1.0152 −1.5593 1.0106 −1.5375 1.0056 −1.5059
0.2985 1.0212 −1.5643 1.0164 −1.5390 1.0118 −1.5198 1.0069 −1.4865 1.0021 −1.4670
0.3508 1.0184 −1.5232 1.0137 −1.5025 1.0091 −1.4860 1.0043 −1.4561 0.9995 −1.4452
0.4015 1.0159 −1.4371 1.0111 −1.4172 1.0065 −1.4018 1.0016 −1.3644 0.9968 −1.3519
0.5033 1.0119 −1.2454 1.0073 −1.2396 1.0026 −1.2259 0.9977 −1.1853 0.9929 −1.1803
0.5975 1.0094 −1.0754 1.0047 −1.0645 1.0000 −1.0539 0.9950 −1.0051 0.9902 −0.9969
0.7028 1.0069 −0.8255 1.0023 −0.8350 0.9975 −0.8263 0.9928 −0.7973 0.9880 −0.7924
0.8017 1.0049 −0.5554 1.0002 −0.5635 0.9956 −0.5623 0.9909 −0.5594 0.9861 −0.5646
0.8532 1.0041 −0.4336 0.9995 −0.4482 0.9950 −0.4780 0.9900 −0.4220 0.9853 −0.4330
0.9025 1.0034 −0.2990 0.9988 −0.3204 0.9940 −0.3082 0.9893 −0.2958 0.9846 −0.3107
0.9447 1.0026 −0.1450 0.9979 −0.1600 0.9934 −0.1801 0.9885 −0.1509 0.9838 −0.1562
1.0000 1.0020 0.0000 0.9972 0.0000 0.9925 0.0000 0.9878 0.0000 0.9830 0.0000

The excess molar volumes are negative for all the studied systems additive behavior, which could be due to a decrease in association be-
at all compositions and temperatures, which is an indication of tween water and glycols/glycol ether with an increase of temperature.
volume contraction in these systems due to negative deviation from Fig. 2 shows that as the number of glycol units increases, the magnitude
E E
additive behavior. Vm –composition curves in Fig. 2 show well-defined of the negative Vm increases i.e. the volume contraction increases with
minima in the water-rich region. At and around the composition corre- an increase of the number of (―CH2―O―CH2―) units in the glycol
sponding to the occurrence of minima, the temperature effect has been series, indicating more deviation from ideal behavior. The minima
found to be significant. With an increase in temperature the magnitude in excess molar volume–composition curves have been found to shift
E
of the negative Vm decreases, indicating a decrease in deviation from towards a more water-rich region as the chain length of the glycol
14 S.K. Begum et al. / Journal of Molecular Liquids 177 (2013) 11–18

1.13 showing corresponding excess molar volumes of about −0.77, −0.83,

−0.94, and −1.59 cm3 mol−1. Such a large volume contraction in
1.11 W + TeEGDME was also observed by Henni et al. [10] where, in agree-
ment with our result, at a lower temperature 298.15 K the excess
molar volume was found to be about −1.7 cm3 mol −1.
1.09 The factors which are thought to play the most vital roles for con-
E
traction of volume in solution, i.e., to make Vm negative, are the hydro-
philic effect, hydrophobic hydration in the water-rich region, weak
1.07
physical forces, such as dipole-dipole and dipole-induced dipole inter-
ρ / gm . cm–3

actions. The shifting of minima to more water-rich regions for higher

E
1.05 glycols and the largest negative Vm , for methyl-substituted glycol
TeEGDME can not be explained by hydrophilic hydration between
water and glycols alone or by weak water–TeEGDME association.
1.03 Such large volume contractions found in earlier studies [3,26] were
thought to be mainly due to hydrophobic hydration in the water-
rich region where water molecules restructured around the hydro-
1.01
phobic part of the organic solutes forming a cage-like structure.
More structured water molecules around the terminal hydrophobic
0.99 groups ―CH3 of TeEGDME might explain the severe volume con-
traction in the system W + TeEGDME. Due to the presence of the
(―CH2―O―CH2―) unit and ―OH groups in the same molecule in
0.97 DEG, TrEG, TrEG and the (―CH2―O―CH2―) unit and ―OCH3 groups
0 0.2 0.4 0.6 0.8 1
x2 in the same molecules in TeEGDME, the glycol–water or TeEGDME–
water interaction becomes complex. H-bonding between glycol
Fig. 1. Densities for aqueous solutions of TrEG (■), TeEG (♦), and TeEGDME (●) at and water might be through the ―OH oxygen and through the
303.15 K. Densities at 323.15 K for the same solutions of TrEG (□), TeEG (◊), TeEGDME (―CH2―O―CH2―) unit oxygen to water molecules. In the case of
(○). Densities of aqueous DEG at 303.15 K (▲) and 323.15 K (Δ) from literature [5]. TeEGDME, this H-bonding might be through the (―CH2―O―CH2―)
unit oxygen and the ―OCH3 oxygen to water. There is another possi-
increases, i.e. as the number of (―CH2―O―CH2―) unit increases in the bility for the formation of H-bonding of TrEG and TeEG with water
E
glycols. The magnitude of the negative Vm value is far greater for through alcoholic hydrogen of the glycols. In an earlier study on
methyl- substituted glycol TeEGDME than those of unsubstituted gly- 2-butoxyethanol in water, infrared spectroscopy (IR) combined with
cols DEG, TrEG, TeEG, which is an indication of maximum volume con- quantum chemical calculations confirmed the formation of H-bonds be-
traction for the system W + TeEGDME in the studied series. Moreover, tween the ether oxygen atom and water molecules [27]. In TeEGDME
methyl-substitution was found to cause a shifting of the minimum in the presence of two terminal ―CH3 groups makes this compound the
the excess molar volume–composition curve towards a more water- most hydrophobic one in the studied series, as Andini et al. [28] demon-
rich region. At 303.15 K, the minima correspond to about 0.30, 0.28, strated that ―CH3 group is the most potent hydrophobic group,
0.24, 0.20 mole fractions of DEG, TrEG, TeEG, TeEGDME, respectively, followed by ―CH2 and ―CH groups. The highest degree of hydropho-
bicity and the maximum size difference between components of the
mixture together probably make major contributions to the maximum
-0.1 volume contraction for W+ TeEGDME. It is assumed that, in aqueous
solutions, the minima in excess molar volume–composition curves cor-
respond apparently to the composition at which the maximum number
-0.3
of cages is formed. Following the minima, hydrophobic interaction
becomes prominent. The hydrophobic interaction is supposed to occur
-0.5
by the overlap of the hydrated co-spheres, with the consequent release
of water molecules from these co-spheres to the bulk. The process con-
-0.7 tinues until the pure state of the organic solutes is reached.
VmE/cm3.•.mol–1

Comparing the viscosities of pure glycols and glycol ether at

-0.9 303.15 K it is seen that the viscosities of the unsubstituted glycols
(ranging from 221.56 to 349.71 mPa.s) are far greater than the viscos-
-1.1 ities of the methyl-substituted glycol TeEGDME (29.55 mPa.s), de-
spite the fact that TeEGDME has a greater molar mass than that of
any of the unsubstituted glycols. This is because the unsubstituted
-1.3
glycols are self-associated very strongly through H-bonding via the
terminal ―OH group and thus experience strong resistance to flow.
-1.5 Such a strong association is not expected in TeEGDME, as the H
from the terminal ―OH groups is replaced by a methyl group ―CH3.
-1.7 Fig. 3 shows that on addition of the unsubstituted glycols to water,
there is a gradual rise of η initially, followed by a sharper rise and
-1.9 then the rise again becomes more gradual as the concentration ap-
0 0.2 0.4 0.6 0.8 1 proaches that of pure glycol. This behavior is similar to that found in
x2 the viscosity–composition curves of W + DEG [5]. The effect of tem-
perature on viscosity is significant. Viscosity values measured by
Fig. 2. Excess molar volumes for aqueous solutions of TrEG(■), TeEG (♦), and TeEGDME
(●) at 303.15 K. Excess molar volumes at 323.15 K for the same solutions of TrEG (□),
Pal et al. [7] for W + TrEG and W + TeEG agree reasonably well with
TeEG (◊), TeEGDME (○). Excess molar volumes of aqueous DEG at 303.15 K (▲) and our experimental values, as shown in the graph. The viscosity for gly-
323.15 K (Δ) from literature [5]. col solutions follows an order (W + TeEG) > (W + TrEG) > (W + DEG)
 S.K. Begum et al. / Journal of Molecular Liquids 177 (2013) 11–18 15

Table 3
Experimental viscosities, η (mPa·s), and excess viscosities, ηE (mPa·s) for water (1) + TrEG (2), water (1) + TeEG (2), water (1) + TeEGDME (2) at T = (303.15, 308.15,
313.15,318.15, 323.15) K.

303.15 K 308.15 K 313.15 K 318.15 K 323.15 K

x2 η ηE η ηE η ηE η ηE η ηE

Water (1) + TrEG(2)

0.00000 0.801 0.000 0.722 0.000 0.656 0.000 0.598 0.000 0.549 0.000
0.05023 2.114 1.155 1.848 0.988 1.618 0.841 1.455 0.749 1.295 0.651
0.10041 4.102 2.953 3.491 2.467 3.012 2.093 2.621 1.796 2.296 1.541
0.15179 6.604 5.222 5.543 4.318 4.689 3.595 4.011 3.028 3.473 2.585
0.20059 9.270 7.623 7.668 6.217 6.418 5.130 5.421 4.267 4.630 3.594
0.25283 11.908 9.920 9.722 7.988 8.113 6.576 6.753 5.388 5.724 4.501
0.29951 14.768 12.417 11.987 9.938 9.888 8.091 8.235 6.645 6.927 5.511
0.39283 18.527 15.239 14.942 12.112 12.223 9.762 10.067 7.912 8.441 6.538
0.50127 21.835 16.979 17.511 13.384 14.270 10.726 11.789 8.721 9.827 7.145
0.59952 24.343 17.429 19.479 13.673 15.835 10.902 13.017 8.790 10.836 7.171
0.66479 25.694 16.951 20.579 13.293 16.750 10.606 13.730 8.501 11.450 6.950
0.79810 27.131 13.010 21.801 10.220 17.668 8.045 14.527 6.452 12.094 5.234
0.84448 27.859 11.175 22.317 8.711 18.119 6.871 14.917 5.524 12.412 4.468
0.89980 28.369 8.018 22.654 6.166 18.440 4.893 15.197 3.949 12.643 3.182
0.95404 28.738 3.999 23.059 3.146 18.687 2.423 15.405 1.979 12.803 1.568
1.00000 29.185 0.000 23.362 0.000 18.983 0.000 15.595 0.000 12.992 0.000

Water (1) + TeEG (2)

0.00000 0.801 0,000 0.722 0.000 0.656 0.000 0.598 0.000 0.549 0.000
0.02061 1.451 0.585 1.287 0.508 1.150 0.444 1.033 0.391 0.934 0.345
0.03953 2.341 1.412 2.041 1.206 1.792 1.037 1.590 0.904 1.415 0.789
0.05990 3.478 2.474 2.972 2.073 2.580 1.769 2.255 1.520 1.991 1.321
0.07592 4.756 3.689 4.032 3.079 3.462 2.604 3.000 2.223 2.611 1.903
0.10155 6.252 5.077 5.242 4.195 4.462 3.523 3.826 2.978 3.31 2.540
0.15131 10.332 8.914 8.453 7.197 7.029 5.909 5.949 4.944 5.054 4.145
0.20173 14.417 12.702 11.671 10.161 9.591 8.253 8.033 6.837 6.728 5.653
0.25162 18.171 16.099 14.561 12.749 11.861 10.264 9.792 8.373 8.187 6.918
0.30060 21.407 18.915 17.027 14.860 13.785 11.887 11.369 9.692 9.052 7.911
0.40194 26.382 22.728 21.32 18.182 16.992 14.275 13.737 11.363 11.783 9.285
0.50486 29.879 24.489 23.517 18.947 18.932 15.024 15.466 12.088 12.753 9.805
0.59699 31.864 24.232 25.093 18.695 20.184 14.772 16.437 11.806 13.626 9.620
0.69775 33.272 22.104 26.314 17.069 21.157 13.430 17.272 10.730 14.27 8.668
0.75145 33.840 20.162 26.698 15.448 21.499 12.157 17.585 9.721 14.505 7.808
0.80716 34.279 17.397 27.149 13.610 21.874 10.499 17.889 8.371 14.764 6.702
0.85096 34.462 14.544 27.386 11.205 22.038 8.758 18.063 7.005 14.937 5.611
0.90434 34.626 10.254 27.53 7.861 22.204 6.164 18.188 4.908 15.075 3.934
0.95600 34.900 5.282 27.696 3.944 22.427 3.176 18.328 2.477 15.201 1.972
1.00000 34.971 0.000 27.892 0.000 22.489 0.000 18.429 0.000 15.349 0.000

Water (1) + TeEGDME (2)

0.0000 0.801 0.000 0.722 0.000 0.656 0.000 0.598 0.000 0.549 0.000
0.00562 1.177 0.370 1.049 0.321 0.937 0.276 0.839 0.236 0.767 0.213
0.02904 2.119 1.288 1.844 1.093 1.618 0.937 1.431 0.810 1.275 0.705
0.04576 3.140 2.290 2.677 1.910 2.309 1.613 1.992 1.357 1.766 1.183
0.07654 4.235 3.350 3.566 2.768 3.043 2.319 2.621 1.961 2.273 1.667
0.10064 4.915 4.002 4.126 3.303 3.487 2.740 2.994 2.313 2.593 1.968
0.12718 5.245 4.300 4.392 3.540 3.729 2.956 3.181 2.476 2.758 2.112
0.15125 5.331 4.356 4.475 3.595 3.802 3.004 3.273 2.546 2.838 2.171
0.20093 5.151 4.110 4.371 3.433 3.744 2.894 3.243 2.467 2.839 2.128
0.24930 4.865 3.756 4.154 3.155 3.600 2.694 3.128 2.303 2.760 2.004
0.29859 4.539 3.356 3.909 2.845 3.397 2.432 2.982 2.103 2.630 1.825
0.35081 4.269 3.003 3.695 2.556 3.228 2.196 2.845 1.904 2.528 1.667
0.40155 4.026 2.673 3.508 2.291 3.089 1.986 2.735 1.731 2.433 1.515
0.50333 3.660 2.115 3.220 1.831 2.853 1.596 2.543 1.398 2.281 1.235
0.59751 3.428 1.681 3.025 1.456 2.698 1.278 2.420 1.128 2.183 1.003
0.70288 3.236 1.231 2.883 1.084 2.577 0.950 2.323 0.843 2.102 0.751
0.80178 3.113 0.832 2.780 0.734 2.497 0.648 2.257 0.577 2.047 0.514
0.85325 3.066 0.627 2.744 0.557 2.467 0.491 2.231 0.435 2.029 0.391
0.90250 3.019 0.417 2.704 0.372 2.436 0.330 2.203 0.290 2.003 0.259
0.94473 2.982 0.233 2.679 0.216 2.414 0.190 2.182 0.162 1.990 0.149
1.00000 2.955 0.000 2.646 0.000 2.388 0.000 2.168 0.000 1.975 0.000

which is in accordance with their increased molecular weight and hy- ethane–water [3] and alkoxyethanol–water solutions [24]. According
drophobicity. The viscosity–composition curves of W + TeEGDME are to those studies the interaction forces between the component mole-
entirely different from those of the three other glycol systems. On ad- cules are complicated and thought to be affected by molecular structure.
dition of TeEGDME to water, the viscosities rise very sharply, pass Owing to two terminal ―OCH3 groups and three (―CH2―O―CH2―)
through a maximum at ~0.12 mole fraction of TeEGDME, and then units, TeEGDME also shows complicated interactions with water and
decline gradually and until finally the curves tend to flatten as the shows a maximum in its viscosity–composition curve at around ~0.12
concentration of TeEGDME approaches the pure state. Such maxima in mole fraction of TeEGDME. This maximum in viscosity suggests the
viscosity–composition curves were also observed for 1,2-dimethoxy highest resistance to flow at this point for this system.
16 S.K. Begum et al. / Journal of Molecular Liquids 177 (2013) 11–18

45 Table 4
Coefficients, Ai, of Redlich–Kister Equation, (Eq. (3)), expressing excess molar volumes,
E
Vm (cm3·mol−1), and standard deviation, (σ) for water (1) + TrEG (2), water
40 (1) + TeEG (2), water (1) + TeEGDME (2).

T/K A0 A1 A2 A3 A4 A5 σ
35
Water (1) + TrEG (2)
303.15 −2.7009 2.0120 −1.428 0.718 0.2478 −2.6117 0.0176
30 308.15 −2.5698 2.0811 −1.6050 0.419 0.0913 −1.5947 0.0109
313.15 −2.4301 2.1390 −1.5996 0.9424 −0.28 −0.5571 0.0102
31.15 −2.5495 1.9804 −1.871 0.9316 0.1954 −1.5057 0.0119
η / mPa.•.s

25 323.15 −2.4927 1.8953 −1.3625 0.7627 0.1954 −1.1056 0.0133

Water (1) + TeEG (2)

20 303.15 −3.0665 2.0576 −1.9101 3.8515 −1.5807 −3.4530 0.0196
308.15 −2.9213 2.2063 −1.8769 3.3066 −0.9548 −2.5115 0.0209
313.15 −2.8604 2.2528 −1.4757 2.7507 −1.1003 −1.6753 0.0210
15 318.15 −2.8838 2.2510 −1.4354 2.1079 −1.6572 −1.7323 0.0217
323.15 −2.7935 2.2355 −1.3485 1.8812 −1.3185 −1.0808 0.0197

10 Water (1) + TeEGDME (2)

303.15 5.0496 3.7074 3.7497 4.2369 −3.1077 1.0556 0.01927
308.15 −5.0102 3.5736 −3.5894 3.9722 −3.3099 0.9912 0.01807
5
313.15 −4.9545 3.5866 −3.5723 3.4188 −3.3096 1.2470 0.01681
318.15 −4.7757 3.6771 −3.9913 2.8102 −2.2870 1.9501 0.01160
0 323.15 −4.7454 3.6595 −3.8018 2.2878 −2.5225 2.2452 0.01205
0 0.2 0.4 0.6 0.8 1
x2
Fig. 3. Viscosities for aqueous solutions of TrEG(■), TeEG(♦), and TeEGDME(●) at the more water-rich region with increasing chain length of glycols,
303.15 K. Viscosities at 323.15 K for the same solutions of TrEG (□), TeEG (◊), TeEGDME and it was found to shift even more for the methyl-substituted glycol
(○). Viscosities of aqueous DEG at 303.15 K (▲) and 323.15 K (Δ) from literature [5]. TeEGDME. With an increase in temperature, the excess viscosity de-
Viscosities of aqueous solutions of TrEG (+) and TeEG (x) at 303.15 K from literature
creases and the sharpness of the curves decreases. The position of
[7].
the maxima along the composition axis is virtually independent of
temperature. The large positive excess viscosities indicate that the so-
Fig. 4 shows that with an increase in concentration of the un- lutions experience more resistance to flow than if they were in an
substituted glycols, the excess viscosity increases, passes through a additive state, which is an indication of strong interaction between
maximum, and then declines smoothly. The sharpness of these maxi- the components of the studied solutions. This is consistent with the
ma increases with increasing of number of (―CH2―O―CH2―) units. large negative excess molar volumes found for the studied series.
The excess viscosities, η E, have been found to be positive throughout The surface tensions, γ, for W + TrEG, W + TeEG and W +
the whole range of composition, the magnitude of η E being quite TeEGDME at 303.15 K are listed in Table 6. Fig. 5 shows the variation
large. As was found in the case of excess molar volume, the positions of γ with concentration of the organic solutes at 303.15 K. The surface
of the maxima in the excess viscosity–composition curves shifted to tensions of W + DEG system are plotted on the same graph for com-
parison. It has been found that the surface tensions of DEG, TrEG
and TeEG are very similar to each other. The surface tension of the
25 methyl-substituted glycol TeEGDME is very low compared to the
three other glycols, which is due to weak molecular association in
TeEGDME because of the absence of the terminal ―OH group. With
the addition of glycols to water, initially a sharp decrease in surface
20 tension was observed, which is typical for aqueous solutions of hydro-
phobic solutes [29,30]. Most of the total decrease of surface tension
occurs within a very narrow range of composition, within ~ 0.04
mole fraction of glycols; in the remaining major part of the composi-
η Ε / mPa.•.s

15 tion the surface tension decreases only slowly to reach the value of
the pure components. The surface tension curves of W + TrEG, W +
TeEG, W + TeEGDME show some break points just below a mole frac-
tion of 0.15 of the glycols/glycol ether. Glinski et al. [31] observed such
10
points in the surface tension–composition curve for the system, W +
1,5-pentanediol and Manglik et al. [32] also observed such points in
the surface tension curve of the aqueous surfactant (Triton-100),
5 both in highly dilute region. In the case of 1,5-pentanediol the exis-
tence of two different structures, such as a micelle-like structure and
a typical monolayer-like structure (or “iceberg”-like) at the surface,
and gradual dissolution of the molecules forming the latter one in
0 the former was believed to be the reason of such points in the surface
0 0.2 0.4 0.6 0.8 1 tension–composition curve. The combined effect of hydrophilic hy-
x2 dration, hydrophobic hydration, hydrophobic interaction, and molec-
ular orientation, which are all significant in a highly water-rich region,
Fig. 4. Excess viscosities for aqueous solutions of TrEG(■), TeEG(♦), and TeEGDME(●) at
303.15 K. Excess viscosities at 323.15 K for the same solutions of TrEG (□), TeEG (◊),
might affect the surface tension of aqueous solutions of glycols and
TeEGDME (○). Excess viscosities of aqueous DEG at 303.15 K (▲) and 323.15 K (Δ) glycol ether in the very dilute region. It is found that though surface
from literature [5]. tension values of W+ DEG, W + TrEG, and W + TeEG are close to each
 S.K. Begum et al. / Journal of Molecular Liquids 177 (2013) 11–18 17

Table 5
Coefficients, Ai, of Redlich–Kister Equation, (Eq. (3)), expressing excess viscosity, ηE (mPa·s), and standard deviation, σ, for water (1) + TrEG (2), water (1) + TeEG (2), water
(1) + TeEGDME (2).

T/K A0 A1 A2 A3 A4 A5 σ

Water (1) + TrEG (2)

303.15 686.98 194.97 −75.085 279.01 −87.10 −62.00 0.8412
308.15 541.37 141.22 −41.472 213.74 −88.29 −48.52 0.9756
313.15 433.81 10262 −.04 156.23 −83.80 −24.16 0.6130
8.15 351.42 75.569 −24.6 117.36 −47.16 −6.107 0.9297
323.15 288.02 56.097 −11.985 92.524 −49.94 −7.687 0.7647

Water (1) + TeEG (2)

303.15 977.65 132.44 8.5550 369.67 −252.17 41.067 0.9005
308.15 762.79 55.504 8.6181 412.94 −196.21 −105.29 0.7769
313.15 602.39 49.546 17.541 227.92 −146.62 16.306 0.5864
318.15 481.62 38.834 40.129 135.66 −140.33 50.454 0.4740
323.15 392.01 29.788 28.614 75.129 −103.24 69.114 0.3497

Water (1) + TeEGDME (2)

303.15 83.480 −75.750 159.21 −333.45 124.41 130.46 0.5264
308.15 72.411 −65.911 127.56 −254.43 105.31 87.718 0.9854
313.15 63.335 −58.107 103.25 −195.56 89.291 56.615 0.7280
318.15 55.668 −5.8225 85.983 −153.45 73.300 35.832 0.5529
323.15 49.264 −44.926 71.618 −121.44 63.533 21.078 0.4141

Table 6 other, surface tension values for W + TeEGDME solutions are far less
Surface Tensions, γ (m N·m−1), for water (1) + TrEG (2), water (1) + TeEG (2) and
than that of unsubstituted glycols. This is in accordance with the highest
water (1) + TeEGDME (2) at 303.15 K.
hydrophobicity of the methyl-substituted glycol TeEGDME and such a
TrEG TeEG TeEGDME difference is consistent with the largest negative excess molar volume,
x2 γ x2 γ x2 γ maximum in density–composition and viscosity–composition curves
0.00000 70.90 0.00000 70.90 0.00000 70.90
or excess viscosity–composition curves in the water-rich region of
0.01995 55.35 0.03621 54.02 0.01360 44.45 W + TeEGDME.
0.02868 53.12 0.09713 51.88 0.03255 41.32
0.05871 51.15 0.13602 50.82 0.05443 39.06
0.09890 50.83 0.19526 48.45 0.08015 38.09 Acknowledgements
0.14753 51.28 0.26928 47.23 0.10236 38.72
0.19244 49.94 0.30435 46.82 0.15022 37.53 The authors acknowledge the financial support of the Ministry of
0.28051 48.85 0.38182 46.25 0.19654 36.54
0.39680 47.65 0.49809 46.15 0.25963 35.71
Science, Information and Communication Technology, Govt. of the
0.49535 47.20 0.60513 45.80 0.40941 35.15 People's Republic of Bangladesh, as a special allocation for the project,
0.60757 46.22 0.65034 45.71 0.50852 34.73 “Physical Properties and Molecular Interactions in Liquid Systems”.
0.70732 45.65 0.70245 45.53 0.60977 33.84
0.80409 45.63 0.80670 45.26 0.77824 33.82
1.00000 44.71 1.00000 44.72 1.00000 33.11 References

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