VISCOSITY B COEFFICIENTS
OF ALKYL CARBOXYLATES
S. K. SANYAL and S. K. MANDAL Department of Chemistry, Presidency College, Calcutta 700 073, India (Received
Abstract-Viscosity B coefficients of aqueous solutions of sodium carboxylates namely sodium formate, acetate, propionate, n-butyrate, n-pentanoate, and trichloroacetate have been determined at 30,35 and WC by studying the relative viscosity of these solutions and the subsequent use of Jones-Dole’s equation. The temperature coefficient, dB/dT, for each solution has also been obtained graphically from these data. The results have been interpreted in terms of the effect on “local” structure of water of these largely hydrophobic solutes. An attempt has been made, in this context, to correlate the B coefficientwith the corresponding heat of transport, a thermal diffusion parameter.
In an attempt to elucidate the effect of alkyl carboxylates on structure of water, the viscosity B coefficients of the former in aqueous solutions have been measured at a number of temperatures between 30and WC, and the sign and magnitude of the temperature coefficient, dB/dT, is also ascertained. The latter, it may be noted in passing, is often regarded as a better (or, at least a supplementary) index of structure-making or breaking ability of a large solute than the corresponding B coefficient itself owing to the “obstruction effect” of the solute[ 13. It hasbeen recently pointed out[Z] that the viscosity Bcoefficient, as an index of solutesolvent interaction?, is analogous to the corresponding heat of transport, Q, a quantity introduced by thermodynamics of thermal diffusion. The latter is directly related to the “local’ changes in entropy density of the solvent, brought about by the presence of a solute in its neighbourhood as compared to that at i_nEnite distance. Hence, the analogy between B and Q.
The solutes of the present study have been the organic electrolytes such as the sodium salts of formic, acetic, propionic, n-butyric, n-pcntanoic and trichloroacetic acids. Except for the acetate, which is readily available commercially, the other salts have been prepared in the laboratory from the pure acids and anhydrous Na,CO, and then purified by crystallization (twice) from water. The B coefficients at 30, 35 and 40°C (5 0.05”C in each case) were determined by having recourse to a graphical technique based on Jones-Dole’s equation, ie by plotting q,/,,/C against ,/C, and noting B from the slope of such linear plots. The intercept on the Y axis gives the corresponding A coefficients (not recorded in Table 1). A suspended level Ubbelhode type (three necked, all glass) viscometer was used for viscosity experiments. It was fitted in the thermostatic bath in reproducible positions with metallic holders to ensure verticality. The viscometer was exhaustively cleaned and rinsed
Table 1 B coefficient (slope of best fit line for the Plot of rl,/JC us JC; 9,P = 9 - tlolrla)
&/kJ mol. 1 (in 0.01m aqueous solution taken from literature at 25°C)
dBfdT (slope of best fit line for the plot of B us T)
+0.0018 (k4.5 x 10-y
( + 0.023)
-0.0048 (+6.4x lo+)
( * 0.042)
-0.0106 (&3.0X 10-S)
0.606 ( f0.048)
0.397 f*O.Q%) 0.455
(* [email protected]
-0.0151 (f3.3 x 10-q
+0.0117 (k4.3 x lo-“)
The value is for the 5-carbon carboxylate, sodium neopentanoate. I875
with filtered solvents. The average flow-time for water (10 ml) through the capillary ofthe viscometer was 502 +O.l s (at 30°C). The accuracy of the viscosity and relative viscosity determinations has also been ascertained and the proportional error was found to lie within f 0.154.20 7”. The densities of the solutions were measured by means of specific gravity bottle, taking care to fill the bottle at the appropriate temperature (of the experiment), by placing the same in a water thermostat and allowing sufficient time for attainment of thermal equilibrium, and then taking the weight quickly.
III. RESULTS Table 1 records the viscosity B coefficients at different temperatures as well as the corresponding temperature coefficients, dB/dT. The B coeficients are obtained, as mentioned above, from the slo es of the (least-square) best fit linear plots of q,,/ 4 C us JC. The corresponding dB/dTvalues are also calculated in a similar way, ie from the slope of the best fit line for the plot of B usTassuming a linear variation of B with temperature. With the appropriate treatment oferrors, the standard deviations (not recorded in Table 1) for each B and dB/dT were calculated and the confidence limits at 95 % confidence were found out, using these standard deviations, and given in parenthesis below each B and dB/dT. The heat of transport (0) values for the present solutes in aqueous solutions, wherever available from literature, have also been recorded in the last column of Table 1 for the purpose of correlation between viscosity and thermal diffusion parameters of the solutes studied.
IV. DISCUSSION Among the sodium salts of the simple carboxylic acids, a regular increase in viscosity B coefficient, especially at lower temperatures such as 3O”C, is observed with molecular weight and hence size of the solute (uide column 2 of Table 1). This is consistent with the structure-forming effect of the hydrophobic methylene group, and the general observation that the structure-promotion in water is directly correlated with the length of thecarbon chain of the solute[5,6]. Some irregularity exhibited by the acetate (B being less than B of both formate and propionate) may well be due to a partial overlapping of the spheres of influence on water structure of the hydrophobic methyl and the hydrophilic carboxylate groups, situated in close proximity in acetate. This gets a qualitative support from similar observations reported for the enthalpy of
transfer of methanol from water to heavy water (as compared to that for higher alcobols), as well as the reported heats of transport of a-alanine and fl-alanine in aqueous medium. On inspection of dB/dT values (column 5 of Table l), this trend becomes even clearer. Whereas it is positive for the formate (a net structure-breaker), the acetate has almost a zero temperature coefficient in the temperature range 30-35°C (beyond which a small fall with temperature is observed, such rather erratic variations in B with temperature for the acetate prompted us not to quote the corresponding dB/dTin Table 1), and dB/dT becomes progressively more negative (which is indicative of a greater structurepromoting ability) as one moves on to n-pentanoate through propionate and butyrate. The earlier contention that dB/dT is a better criterion than B to characterize the effect of solute on the solvent structure, gets a support from the results obtained for the trichloroacetate. Thus, the very considerable increase in B from acetate to trichloroacetate at all temperatures investigated may well have arisen from a concomitant increase in the size of the solute, ie from the “obstruction effect” of the solute as mentioned above, even though the corresponding dB/dT is seen to be positive (column 5 of Table 1); the trichloroacetate, therefore, appears to be a net structure-breaker in water. This is in agreement with the known structuredisruptive influence of the chIorine groups in aqueous solutions. A direct correlation between the heat of transport, & values (at 25”C), and the Bcoefficients, obtained at any given temperature, say 3O”C, of the sodium carboxylates (uide columns 2 and 6 of Table 1) corroborates what has been said above regarding these parameters as measures of the nature of solutePsolvent (“local”) interactions. The results obtained in this study, and explained as above, add to the evidence in favour of the theory of hydrophobic hydration in aqueous solutions of these largely hydrophobic (mixed) solutes[3, S].
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