Apparent and Partial Molar enthalpies of some Potassium p-n-Alkyl-Benzoate Aqueous Solutions

Dilution heats at 298 K of aqueous solutions of potassium salts of p-n-hexylbenzoic acid (KHB), p-n-heptylbenzoic acid (KHEB), p-n-octylbenzoic acid (KOB) in KOH 0.1 m have been measured as a function of concentration by using the flow mixing cell of a Thermometric TAM microcalorimeter. From the experimental data, apparent and partial molar enthalpies versus concentration have been obtained. The group contribution of the -CH2- group in the plateau region results -1.6 kJ mol-1 per - CH2-. This value is comparable with that obtained in the case of cationic surfactants. The enthalpy changes upon micellization (DHm) have been obtained by using a pseudo phase transition approach and assuming as cmc the abscissa of the first inflection point of the curves of FL versus m. Micellization enthalpies result additive with a group contribution for the methylene group of -1.5 kJ mol-1 per group, comparable with that obtained for similar anionic compounds in the same experimental conditions. The behavior of the compounds under study is compared with that of the potassium salts of 4-((alkylcarbonyl)amino) benzoic acids in order to understand the role played by the amido group bridging the benzoic moiety and the alkyl chain. The amido group appears to behave as a part of the polar head, strongly increasing the hydrophilicity of the molecule.