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

Aqueous solutions of the two hydrotropic agents sodium benzoate and so dium salicylate exhibit moderate surface activity as surface tension m easurements indicated. The surface cross sectional area of the salicyl ate ion is reasonably higher than that of benzoate. The area occupied per ion increases with the rise in temperature for both hydrotropes. T he critical aggregate concentration (CAC) obtained from viscosity and conductivity measurements substantiates those obtained from surface te nsion measurements. Analysis of the temperature dependence of CAC show ed that aggregation of benzoate and salicylate ions in aqueous solutio n is driven by enthalpy and entropy factors. The entropy contribution is larger especially for salicylate ion. This indicates that the entro py gain associated with freeing water molecules structured around sali cylate ions is the primary driving force for aggregation. The enthalpy contribution to aggregate formation of benzoate ion is relatively lar ger than that of salicylate, and this agrees well with more hydrophobi c nature of the former species.