RESULTS AND DISCUSSION
4.3 Viscometric Properties
Results and discussion Cliapt er IV
In the case of aqueous SDS solution, for prc-micellar or post-mice11ar region there is no appreciable change of V observed.
Results (711(1 discuisswn Cii apter IV
increases similarly of water systems, indicating that the miceile formation of SDS is not occurred in alcohol solutions. in SDS systems, viscosity increases in comparison to corresponding systems without SDS. This indicate that the SDS solution are reorganized the alcohol structure again so that the viscosity increasing are observed. The viscosity of alcohol in Water and SDS systems has been found to be in the order of
Alcohol - 0.01M SDS > Alcohol - 0.005M SI)S> Alcohol - Water.
Excess viscosities. 17 were calculated by using equation 3.11. The
i/
values are shown in Table 4.1 6-4.1 9. The excess viscosities were fitted by least squares method to a polynomial equation 3.13. The values of the fitting parameters along with the standard deviation are presented in Table 4.56-4.59. The variation of if
against mole fraction of alcohol (x2) is shown in Figure 4.39-4.42. The i/ values are found to be positive and large in magnitude, indicating that the aqueous solutions of alcohols are highly non ideal. All the curves pass through maxima in water—rich region. The height of the maxima are in the order:iso-Propanol + Water> n-Propanol + Water> Ethanol + Water> Methanol + Water
The viscosities and excess viscosities are accounted for mainly by the following factors:
.41
Strong Alcohol-water and Alcohol-Alcohol interactions, ilydrophobic hydration of Alcohols.
The rapidly ascending part of viscosity curves (Figure 4.27-4.30) in the dilute region of alcohols can be explained primarily in terms of the phenomenon called hydrophobic hydration, which assumes that, in water rich region, the water molecules form highly ordered structures through hydrogen bonding around the hydrocarbon moieties of alcohols.
These are variously known as ice-bergs, clusters or cages. There is a large body of experimental evidences which suggest the existence of such cages. On addition of alcohol to water, cages are formed continuously till the water molecules necessary to form these cages are available. Simultaneously, the hydroxyl groups of alcohol form hydrogen bonds with the surrounding water molecules. The increase in viscosity with the mole fraction of alcohol in water rich region may be attributed to these two effects collectively. This evidence suggest that, at least in the case of t-Butanol and iso-propanol , the hydroxyl group is involved in I-I-
56
Results (117(1 discussioii Cli apter IV
bonding with water solvent (52). After attaining the state of maximum viscosity further addition of alcohol continuously breaks down both cages and alcohol-water associates, and instead, alcohol-alcohol associates are preferentially formed, which result in the regular decrease in viscosity. The appearance of viscosity maxima is therefore expected as a result of these competing processes. The hydrophobic effect obviously increases with the size of the hydrocarbon chain of alcohols, while the hydrophilic effect is expected to be the same for all the studied alcohols.
The difference in maxima of viscosity over the temperature range (Aiiinax) of the different systems can be explained in terms of the thermal fragility of the cages formed. In comparison with alcohol-water association, the water-water association in the cage structure is assumed to be more fragile to heat. Examination of viscosity curves of different alcohol solutions (Figure 4.27-4.30) shows that AiII1,Ix varies in the order,
iso-Propanol (1.8 mPa.$) > n-Propanol (1.3 mPa.S) > Ethanol (1.1 mPa.S) > Methanol (0.75 mPa.S).
The values, therefore, indicate the extent of the destruction of the cages structures by thermal effect which, in turn, reflects the extent of cage formation. The cages formed by the water-water association around hydrocarbon tails of alcohols are also assumed to be thermally unstable than water-water association in normal water (53,54). The shallow minima occurring at 0.9 mole fraction of alcohol as showed in the iso-Propanol (Figure 4.30) seen to be somewhat prominent at lower temperatures. This observation is in agreement with that made by Tanaka et al (51). In the study of the viscosity of aqueous solutions of isomeric butanols. Scnanayake ct al (55) noticed similar minima. A work by Kipkcmboi et al (56) on the viscosity aqueous mixtures of t-Butanol in the temperature range 288-318K also confirmed this phenomenon. The aqueous solutions of methanol as studied by Tanaka et at (51), however, do not show this effect. Incidentally, minima of static dielectric constants of alcohol-water mixtures occur at about the same composition where the shallow minima of viscosity are observed (57,58). Franks and Ives (58) explained these minima in terms of the formation of so called "centrosymmctric" associates which are thought to be composed of one water and four alcohol molecules.
Results and discussion cli apter IV
The values of Excess viscosity, for the systems of Methanol. lthanol, n-Propanol and iso-Propanol in 0.005M and 0.01 M SDS in water systems are given in the Tables 4.20- 4.27. The plots of i/ of alcohol + SDS water systems against mole fraction of alcohols are shown in Figures 4.43-4.50, respectively. The lines are generated by the polynomial equation 3.13. For 77 1. the fitting coefficients (ai) are shown in Table 4.56-4.59 along with standard deviations. The i/ values are found to he positive and large in magnitude, indicating that the aqueous SDS solutions of alcohols are also highly non ideal. All the curves pass through maxima in water SDS-rich region.
Viscosities increase rapidly with alcohol concentration, showing maxima at 0.2-0.3 mole fraction of Alcohol in aqueous SDS solution. The position of maxima virtually does not change remarkably with the variation of temperature. At the alcohol rich region shallow minima are observed for iso-Propanol at 0.9 mole fraction of alcohol at 0.005M SDS solutions. The minima seem to disappear at the post micellar concentration (0.0IM SDS) of solutions.
In the present investigation at 298.1 5K, the maximum values of 77 have been found to he 0.7 (at X2 0.30). 1.25 (at X2 = 0.5), 1 .55 (at X2 = 0.25), 2.2 (at X2 = 0.25) for the, 0.01 M SDS -Water + Methanol, 0.0IM SDS -Water + Ethanol, 0.0IM SDS -Water + n-Propanol and 0.0IM SDS-Water + iso-Propanol mixtures. respectively. The height of the maxima are in the order has been found to be in the orders similar to water systems:
iso-Propanol + Water-SDS> n-Propanol + Water-SDS> Ethanol + Water-SDS> Methanol + Water-SDS.
In the case of aqueous SDS solution, for pre-micellar or post-micellar region, it is seen that there is no appreciable change of viscosity or excess viscosity after adding SDS to the systems observed.
Interaction parameter
The interaction parameters () have been calculated by using the equation 3.14. The values
-Y
of interaction parameters for different systems are shown in Table 4.16-4.27. The values
58
Results (111(1 discussion Cli apter IV
have been found to be positive and quite large in magnitude in water-rich region for all the systems. The c values are decrease with the increase of temperature.
From the studies of c and i/ values of a number of binary mixtures of different polar or non-polar liquids, Fort and Moore (59) indicated an approximate idea about the strength of interaction between liquids. They concluded that:
If c > 0 and
/>
0 and both are large in magnitudes, then strong specific interaction between the components would be anticipated.If c < 0 and /> 0 and both are not that much large in magnitudes, then weak interaction would be present between the components.
If c < 0 and
t
< 0 and the magnitude of both parameters are large, then specific interaction would be absent and dispersion force would be dominant.In our studied systems, both c and values are positive and large in magnitude. Therefore, the positive interaction parameters indicate that strong interactions between the components of the mixtures (59) are occurred. The interaction parameters, S increase rapidly with alcohol concentration, showing maxima at 0.1-0.2 mole fraction of alcohol and then decrease continuously and finally increases again at 0.8-0.9 mole fraction of alcohol solution. At the alcohol rich region small maxima are observed for all alcohol solutions. The position of maxima virtually does not change remarkably with the variation of temperature.
In the present investigation at 298.15K, the maximum values of c have been found to be 4.8 (at x2 = 0.1), 6.5 (at X2 = 0.1), 6.9 (at X2 = 0.1). 7.9 (at X2 = 0.1) for the Water + Methanol, Water + Ethanol, Water + n-Propanol and Water + iso-Propanol mixtures, respectively. The height of the maxima is in the order has been found:
iso-Propanol + Water> n-Propanol + Water> Ethanol + Water> Methanol + Water,
From these Figures 4.1 7-4.28, it is seen that the basic pattern of interaction parameter viscosity behavior of Methanol, Ethanol, n-Propanol and iso-Propanol in 0.005M and 0.01M SDS is very similar to pure water systems. For pre-micellar or post-micellar region, it is seen that there is no appreciable change of c after adding SDS to the systems observed.
Results (111(1 discussion Cli upter IV