* Corresponding author : E-mail: [email protected] KUET @ JES, ISSN 2075-4914 / 04(1), 2013
TEMPERATURE AND COMPOSITION DEPENDENCE OF VISCOMETRIC BEHAVIOR OF METHANOL, ETHANOL, 1-PROPANOL, 1-BUTANOL AND
1-PENTANOL IN SULFOLANE MIXTURES
Md. Abdul Motin1, M. Azhar Ali2, M. Nazrul Islam2 and M. A. Hafiz Mia1
1Department of Chemistry, Khulna University of Engineering & Technology (KUET), Khulna 9203, Bangladesh
2Department of Chemistry, University of Rajshahi, Rajshahi-6205, Bangladesh Received: 30 October 2012 Accepted: 27 April 2013 ABSTRACT
Viscosity of the binary mixtures of sulfolane, +methanol, +ethanol, + 1-propanol, + 1-butanol, and + 1- pentanol were measured at 298.15, 303.15, 308.15, 313.15 and 318.15K respectively covering the whole composition range except methanol and ethanol. The methanol and ethanol systems with sulfolane were studied at 303.15 to 323.15K owing to its lower solubility at 298.15K. The viscosities decrease gradually with increase in the mole fraction of alkanols for all mixtures. There is a marked decrease in viscosity with increase of temperature for all the alcohol systems. Excess viscosities, E for the systems were found to be negative. For all alcohols, excess viscosities show minima in sulfolane rich region. The minima occurring between 0.15 and 0.25 mole fraction of alcohols are in the order: sulfolane + 1-pentanol >sulfolane + 1-butanol> sulfolane+ 1- propanol> sulfolane+ ethanol> sulfolane + methanol. Excess viscosity data have been fitted by the least squares method to the four parameters Redlich-Kister equation. The values of E for the mixtures have been explained in terms of (i) the differences in chain length of the alcohols, (ii) dipole-dipole interactions between the polar molecules, (iii) interstitial incorporation, (iv) breakdown of the structure of pure liquids, (v) hydrophobic interaction and (vi) association between dissimilar liquids.
Keywords:Viscosity, Excess viscosity, Sulfolane, Methanol, Ethanol, 1-Propanol, 1-Butanol 1-Pentanol.
1. INTRODUCTION
The physical properties and the thermodynamic behavior of binary mixtures have been studied for many reasons, one of the most important of which is that these properties may provide information about molecular interactions.
These properties are extremely useful for design of many types of transport and process equipment in chemical industries. Sulfolane is an important aprotic industrial solvent bearing several advantageous physicochemical properties and the ability to extract monocyclic aromatic hydrocarbons from petroleum products. Studied alcohols are protic self- associated liquids through H- bonding. Alcohols possess hydrophilic -OH group as well as hydrophobic group. On the other hand, sulfolane is a globular molecule in which only the negative end of its exposed large dipole moment and that can not act as proton acceptor/donar. Therefore, the mode of interactions of alcohols and sulfolane is of vital importance in the field of solution chemistry as it can provide with important information regarding hydrophilic and hydrophobic interactions (Motin 2007).
To explain the interaction mode, it is necessary to have complete thermodynamic data for these mixtures.
Research on their thermophysical properties has been reported by several authors (Al-Azzawi 1990; Al-Dujaili 2006; Aminabhavi 1996; Awwad 2000, 2001, 2002; Chen 1995, Sacco 1975; Karvo 1980, 1982; Motin 2007).
Jannelli et al (1980, 1984, 1985) have studied the density, dielectric constant and solid liquid phase diagram of sulfolane mixtures. Several other researchers (Al-Dujaili 2006; Domanska 1996; Gonzalez 2001; Letcher 2000, 2000; Redlich) have studied the relative Permittivities, refractive index, excess molar volume, viscosities, vapor - liquid and liquid –liquid equilibrium for sulfolane with binary and ternary mixtures.
Here we report viscosity and excess viscosity of five binary mixtures, viz, sulfolane, + methanol, +ethanol + 1- propanol, + 1-butanol, + and 1-pentanol at 298.15 to 318.15K except methanol and ethanol at 303.15K to 323.15K.
2. EXPERIMENTAL 2.1 Materials
The chemicals used were purchased from Aldrich chemical co. with the quoted purities: methanol (99.5%), ethanol (99%), 1- propanol (99.5%), 1-butanol (99.5%) and 1-pentanol (99.0%) and sulfolane (99%) These chemicals were used without any further purification, except that they were allowed to stand over molecular sieves (4A) about two weeks before measurements. As measures of purity check, the densities and viscosities of pure liquids were compared with the available literature values shown in Table 1 (Awwad 2002; Aminabhavi 1996; Chen 1995; Indraswati 2001; Kabir 2004; Lopez 1982, 1983; Motin 2005; Nikam 2000; Pansinl 1986;
Rodringuez 2001; Riddick 1986; Timmermans 1950; Yu 1988, Yang 2005, 2006). Our measured values of viscosities of pure liquids have been found to be very satisfactory with literature (Table 1).
2.2 Density Measurements
Densities were measured by using 5 mL bicapillary pycnometers. The volumes of the pycnometers were calibrated with deionized and doubly distilled water at 298.15, 303.15, 308.15, 313.15, 318.15 and 323.15 K. The densities of solutions in water and 1- alkanols + sulfolane solutions were determined from the mass of the solution in the pycnometer after reaching thermal equilibrium with a water bath at the studied temperatures. A HR-200 electronic balance with an accuracy of 0.0001g was used for the mass determination. The uncertainty in the experimental data for density was found to be 0.0004 g. cm-3. Reproducibility of the results was checked by taking each measurement three times.
2.3 Viscosity Measurements
The viscosities were measured by calibrated U-type Ostwald viscometer of the British standard institution with sufficiently long efflux time to avoid kinetic energy correction. The provided calibration constants were checked with water, ethanol, and 1-hexane. The flow time of liquids was recorded by a timer to 0.1sec. Temperatures were controlled by a thermostatic water bath fluctuating to 0.05K. The viscosity, of the solutions was calculated by
= At (1)
Where t is the flow time, is the density of the solution, and A is the viscometer constant. The accuracy in the measurement of viscosity was 0.8%.
3. RESULTS AND DISCUSSION
The viscosity, of sulfolane and alkanols mixture was measured by using the density, and flow time, t of the solution. Firstly densities of sulfolane, +methanol, +ethanol, + 1-propanol, + 1-butanol and + 1-pentanol were measured at 298.15, 303.15, 308.15, 313.15 and 318.15 K respectively covering the whole composition range.
The sulfolane, + methanol, + ethanol mixture temperature were at 303.15 to 323.15 K owing to its lower solubility at 298.15K (Motin 2007).
Excess molar volume, VE for the systems of methanol, ethanol, 1-propanol, 1-butanol and 1-pentanol in sulfolane systems have been plotted in Figure 1 at 303.15K. The VE values for the sulfolane, + methanol, + ethanol, and + 1-propanol mixtures are negative over the whole range of mole fractions and for the sulfolane + 1-butanol and sulfolane + 1-pentanol mixtures are being slightly positive at lower and higher mole fractions (x2). The VE vs. x2
plots are showed asymmetrical minima around 0.4- 0.65 mole fractions for the systems. The minima shift toward the alkanol rich region to sulfolane rich region from methanol to 1-pentanol (Motin 2007). The magnitude of the minima are in the order: sulfolane + methanol> sulfolane + ethanol> sulfolane+ 1-propanol> sulfolane + 1- butanol > sulfolane + 1-pentanol.
Figure 1: Plots of excess molar volume, (VE) versus X2 for , methanol; ,ethanol; ,1-propanol; ,1-butanol ; O1-pentanol in sulfolane systems at 303.15K
Viscosities of sulfolane, +methanol, +ethanol, + 1-propanol, + 1-butanol, and + 1-pentanol solutions were determined at 298.15, 303.15, 308.15, 313.15, and 318.15 K over the entire composition range. The sulfolane, + methanol, + ethanol mixture temperature were at 303.15 to 323.15 K. The viscosities of the pure components are shown in Table 1 together with the literature values at different temperatures. The agreement between the measured values and literature values of viscosity of the pure components has been found to be almost satisfactory except 1-pentanol. In pure state the viscosity of alcohols has been found to be in the order of,
1-pentanol>1-butanol> 1- propanol > ethanol > methanol
Figure 2: Plots of viscosity (E) versus X2 for Ethanol at , 303.15 K; , 308.15 K; , 313.15K; O, 318.15 K;
K.
The viscosities of the binary systems of 1-butanol and sulfolane have been shown in Table 2 at different temperatures. The viscosities decrease gradually with increase in the mole fraction of ethanol (shown in Figure 2).
Because of a similar behaviour no figures of viscosities of methanol, 1-propanol, 1-butanol and 1-pentanol in sulfolane systems are shown.
The following characteristic features of viscosity are observed:
-1 -0.8 -0.6 -0.4 -0.2 0 0.2
0 0.2 0.4 0.6 0.8 1
X2 VE /(cm3 .mole-1 )
0 2 4 6 8 10 12
0 0.2 0.4 0.6 0.8 1
x2
/(mPa.s)
i) Viscosities decrease slowly with alcohol mole fraction.
ii) Viscosities decrease with rise of temperature.
iii) The value of mixture of viscosity follow the order:
1-pentanol + sulfolane>1-butanol+ sulfolane > 1- propanol+ sulfolane > ethanol+sulfolane > methanol+ sulfolane For concentrated solution of sulfolane it is believed that sulfolane which are known to exist in associated forms through strong interaction. This explains the high viscosity of the solution in the sulfolane rich regions. In alcohol rich region the slow decrease of viscosity is thought to be due to the continuous decrease of self association of sulfolane and new H-bonds between alcohol and sulfolane will be formed.
Table 1: Comparison of experimental and literature values of density, (g.cm-3) and viscosity, (mPa.s) of pure components at different temperatures
Component Temperature (K) Viscosity(mPa.s)
lit exp
Sulfolane 298.15 - 11.7580
303.15 10.0700
10.3000 10.0742
10.0097
308.15 - 8.4314
313.15 7.8080 7.4522
318.15 - 6.5998
Methanol 303.15 0.5146
0.5140
0.5143
308.15 0.4837
0.4800
0.4832
313.15 0.4542 0.4557
318.15 0.4256 0.4304
323.15 0.3983 0.4072
Ethanol 303.15 1.1808
0.9760
1.1829
308.15 1.0638 1.0659
313.15 0.9646 0.9492
318.15 0.8714 0.8448
323.15 0.8010 0.7669
1-Propanol 298.15 - 1.9154
303.15 1.7466
1.7050
1.7027
308.15 1.5422
1.5260
1.5138
313.15 1.3897
1.3720
1.3523
318.15 1.2440 1.2130
1-Butanol 298.15 - 2.4751
303.15 2.2460 2.1867
308.15 1.9820 1.9327
313.15 1.7640 1.7159
318.15 - 1.5233
1-Pentanol 298.15 - 3.0967
303.15 2.9730 2.6904
308.15 2.5920 2.3449
313.15 2.2620 2.0569
318.15 - 1.8108
Table 2: Densities (), viscosities () and excess Viscosity (E) for1- butanol + sulfolane systems at 298.15, 303.15, 308.15, 313.15, 318.15 and 323.15 K.
x2 / g. cm-3 / mPa.s E/mPa.s x2 / g. cm-3 / mPa.s E/mPa.s (1)Sulfolane + (2) 1-Butanol
T= 298.15K
0.0000 1.26581 11.7580 0 0.6029 0.99797 2.8749 -1.5444
0.0989 1.22082 7.1487 -2.0800 0.7033 0.94964 2.5636 -1.2539
0.2060 1.17610 5.4991 -2.4438 0.7851 0.90991 2.4036 -0.9839
0.3028 1.13493 4.4409 -2.4109 0.9090 0.85437 2.2889 -0.6035
0.3550 1.11290 4.0982 -2.3278 1 0.80737 2.4750 0
0.5090 1.04394 3.2284 -1.9227
T= 303.15K
0.0000 1.26141 9.6166 0 0.6029 0.99355 2.5270 -1.4095
0.0989 1.21617 6.5080 -1.7981 0.7033 0.94512 2.2570 -1.1363
0.2060 1.17131 4.8664 -2.2648 0.7851 0.90545 2.1223 -0.8838
0.3028 1.13030 3.9262 -2.2149 0.9090 0.85022 2.0259 -0.5318
0.3550 1.10832 3.6271 -2.1231 1 0.80349 2.1866 0
0.5090 1.03930 2.8280 -1.7660
T= 308.15K
0.0000 1.25701 8.4539 0 0.6029 0.98889 2.2453 -1.2265
0.0989 1.21159 5.7592 -1.5466 0.7033 0.94043 2.0042 -0.9902
0.2060 1.16639 4.3448 -1.9291 0.7851 0.90082 1.8887 -0.7652
0.3028 1.12563 3.5146 -1.8928 0.9090 0.84585 1.8074 -0.4493
0.3550 1.10363 3.2409 -1.8204 1 0.79959 1.9327 0
0.5090 1.03495 2.5171 -1.5292
T= 313.15K
0.0000 1.25261 7.4879 0 0.6029 0.98465 2.0139 -1.0656
0.0989 1.20716 5.1263 -1.3462 0.7033 0.93621 1.7959 -0.8606
0.2060 1.16197 3.8926 -1.6652 0.7851 0.89666 1.7008 -0.6541
0.3028 1.12100 3.1537 -1.6392 0.9090 0.84181 1.6166 -0.3841
0.3550 1.09906 2.9137 -1.5703 1 0.79565 1.7158 0
0.5090 1.03057 2.2582 -1.3270
T= 318.15K
0.0000 1.24821 6.6375 0 0.6029 0.98009 1.8139 -0.9183
0.0989 1.20250 4.6390 -1.0993 0.7033 0.93169 1.6171 -0.7402
0.2060 1.15725 3.5135 -1.4131 0.7851 0.89221 1.5262 -0.5637
0.3028 1.11626 2.8476 -1.4029 0.9090 0.83763 1.4527 -0.3211
0.3550 1.09424 2.6326 -1.3418 1 0.79166 1.5232 0
0.5090 1.02588 2.0351 -1.1428
The excess viscosities, E, have been calculated from viscosity data according to the equation:
E = obs - id (2)
Where, obs is the experimentally observed viscosity of the mixture and id is the ideal viscosity of the mixture and
) ln ln
exp( 1 1 2 2
id X X (3)
Where, X1 and 1 are the mole fraction and viscosity of component 1 (Sulfolane), X2 and 2 are the corresponding values of component 2 (Alkanols). The excess viscosities, E were fitted to a Redlich- Kister polynomial equation of the form (Redlich 1948),
n i i
i
E(mPa.s) X X a(1 2X1)
0 2
1
(4)
Where, Ai is the ith fitting coefficient. Using n=3, four Aiwere obtained through the least squares method. In each case, the optimum number of coefficients Ai was determined from an examination of the variation of the standard derivation
( exp)2/( )
21)
(Y
YcalY nm (5)
where, n is the total number of experimental values and m is the number of parameters.
The excess viscosities, E of 1-butanol +sulfolane systems are shown in Table 2. The values of the fitting parameters along with the standard deviation are presented in Table 3 for all studied alcohols +sulfolane systems.
Figure 3: Plots of excess viscosity (E) versus X2 for 1-Propanol at , 298.15 K; , 303.15 K; ,308.15 K; O,313.15 K; 318.15 K.
Figure 4: Plots of excess viscosity (E) versus X2 for 1-Pentanol at , 298.15 K; , 303.15 K; ,308.15 K;
O,313.15 K; 318.15 K.
Figures 3 and 4 represent the E vs. x2 curves at different temperatures for 1-propanol and 1-pentanol with sulfolane system. The excess viscosities are negative and decrease in absolute values as the temperature is increased. The E values are negative and large in magnitude, which demonstrate that the sulfolane solutions of alcohols are highly non- ideal. All the curves pass through minima in the sulfolane rich region.
Excess viscosity, E for the systems of methanol, ethanol, 1-propanol, 1-butanol and 1-pentanol in sulfolane systems have been plotted in Figure 5 at 303.15K. The magnitude of the minima is in the order:
1-pentanol>1-butanol> 1- propanol > ethanol > methanol. The magnitude of excess viscosity, E increases with chain length of alkanols, while it decreases with rise of temperature. This reveals that the strength of the intermolecular hydrogen bonding is not the only factor influencing the excess viscosity of liquid mixtures, but the molecular sizes and shapes of the components are also equally important. Larger the chain length of alkanols, greater is decrease in the average degree of association, as a result more negative excess viscosity vs. mole fraction curve is observed (Patwari 2009; Kelayeh 2009; Tilstam 2012). It has been reported that the E values of binary mixtures result from the chemical, physical, and structural characteristics of liquids (Motin 2005, Nikam 2000). Physical effects contribute to negative E; chemical and structural effects contribute to positive E.
Figure 5: Plots of excess viscosity (E) versus X2 for , methanol; ,ethanol; , 1-propanol; O,1-butanol ;
1-pentanol in sulfolane systems at 303.15K.
-3 -2.5 -2 -1.5 -1 -0.5 0
0 0.2 0.4 0.6 0.8 1
x2
E /(mPa.s)
-4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0
0 0.2 0.4 0.6 0.8 1
x2
E /(mPa.s)
-3 -2.5 -2 -1.5 -1 -0.5 0 0.5
0 0.2 0.4 0.6 0.8 1
X2
E /(mPa.s)The excess viscosity of the binary mixtures under investigations may be considered to be the resultant of the above-mentioned competing interactions of the component molecules. All the components are polar compounds;
the value of dipole moment () being 4.8, 1.7, 1.69, 1.68, 1.66, 1.68 and 1.64 D for sulfolane, methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol, respectively (Riddick 1986). The dipole moment of sulfolane is higher than the rest of the studied alcohols. Therefore it has the possibility of the formation of H-bonding through the polar group of alcohols due to the hydrophilic effect. However if the steric hindrances by the bulky groups or geometrical mismatch of these groups are very strong, then the possibilities of the formation of H-bonding decrease. In the present systems, the negative E values are attributed to breaking of H-bonds in the self- associated alcohol and steric hindrance due to the bulky groups in alcohols. At the same time, segmental inclusion of sulfolane into the vacant spaces left in the structural network of alcohol may also occur.
As seen in Figure 3 and 4, the values of E for all studied systems are negative over the entire range of mole fractions at all temperatures and the curves are asymmetrical in nature and skewed to the sulfolane-rich range.
Similar to the excess molar volumes, viscosity is related to the molecular interaction between the components of mixtures as well as of the size and shape of molecules. Positive values of E are indicative of strong interactions whereas negative values indicate weaker interactions (Oswal 1999). The negative excess viscosity supports the main factor of breaking of the self-associated sulfolane or alcohols and weak interactions between unlike molecules.
Table 3: Coefficient, ai, of Redlich- Kister Eq 2 expressing E and standard deviation, for the Sulfolane +Methanol, +Ethanol, +1-Propanol, +1-Butanol, +1-Pentanol systems
Systems T/ K ao a1 a2 a3
Sulfolane+methanol
Systems 303.15 -1.1372 1.9065 -4.0426 3.7350 0.0203
308.15 -0.9739 1.4833 -3.5742 3.742 0.0263
313.15 -0.8454 1.2556 -2.9915 3.1179 0.0268
318.15 -0.7283 1.0524 -2.3315 2.4086 0.0209
323.15 -0.5625 0.9013 -1.3020 0.7798 0.0075
Sulfolane + ethanol
Systems 303.15 -4.4058 4.4367 -4.7790 3.0008 0.0215
308.15 -3.7828 3.6832 -4.1388 2.6013 0.0206
313.15 -3.1251 3.0475 -3.1908 1.8561 0.0198
318.15 -2.6654 2.4891 -2.5314 1.5257 0.0072
323.15 -2.3077 1.8426 -2.1732 1.8974 0.0199
Sulfolane +1-
propanol Systems 298.15 -6.9805 6.1058 -11.1679 9.9066 0.0852
303.15 -5.8546 5.1265 -8.8000 7.3239 0.0536
308.15 -4.8180 4.0134 -5.7183 4.0283 0.0238
313.15 -4.1798 3.4299 -4.3233 3.1025 0.0153
318.15 -3.5695 2.7668 -3.7597 2.6976 0.0178
Sulfolane +1- butanol Systems
298.15 -7.3400 5.8814 -10.3562 6.1364 0.1015
303.15 -5.8546 5.1265 -8.8196 7.3239 0.0536
308.15 -5.8657 4.7117 -7.4184 4.2524 0.0605
313.15 -5.0830 4.0121 -6.3723 3.9230 0.0549
318.15 -4.4032 3.4866 -5.0436 2.8437 0.0385
Sulfolane +1-
pentanol Systems 298.15 -8.6350 8.4145 -15.7717 11.5144 0.0144
303.15 -7.3674 6.7007 -11.7922 9.0616 0.0107
308.15 -6.1661 5.2169 -7.8753 5.7066 0.0082
313.15 -5.3902 4.3380 -6.0607 5.9194 0.0223
318.15 -4.0813 3.0083 -6.1961 6.4304 0.0233
4. CONCLUSION
The viscosities decrease gradually with increase in the mole fraction of alkanols for all mixtures. There is a marked decrease in viscosity with increase of temperature for all the alcohol systems. Excess viscosities of sulfolane + alkanol systems were found to be negative and large in magnitude indicating that the sulfolane solutions of alcohols are highly non ideal. Such viscometric behavior of these systems may be explained
qualitatively by the molecular interaction between unlike molecules through hydrogen bonding and the disruption of the molecular order existing in the pure alkanols by globular molecules of sulfolane. Since sulfolane are relatively weak proton acceptors and akanols are a relatively weak proton donor, a weak hydrogen bond is formed. This leads to the observed negative behavior of VE or E. Such behavior might also arise from interstitial accommodation of the globular molecules of sulfolane molecules within structure of alkanols.
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