SURFACE-ACTIVITY AND ENERGETICS OF AGGREGATE FORMATION IN AQUEOUS SODIUM BENZOATE AND SALICYLATE SOLUTIONS

Aqueous solutions of the two hydrotropic agents sodium benzoate and sodium salicylate exhibit moderate surface activity as surface tension measurements indicated. The surface cross sectional area of the salicylate ion is reasonably higher than that of benzoate. The area occupied per ion increases with the rise in temperature for both hydrotropes. The critical aggregate concentration (CAC) obtained from viscosity and conductivity measurements substantiates those obtained from surface tension measurements. Analysis of the temperature dependence of CAC showed that aggregation of benzoate and salicylate ions in aqueous solution is driven by enthalpy and entropy factors. The entropy contribution is larger especially for salicylate ion. This indicates that the entropy gain associated with freeing water molecules structured around salicylate ions is the primary driving force for aggregation. The enthalpy contribution to aggregate formation of benzoate ion is relatively larger than that of salicylate, and this agrees well with more hydrophobic nature of the former species.