Osmotic coefficients of N-nonyl- and N-decyl-nicotinamide chloride surfactant aqueous solutions

Abstract

The osmotic coefficients of N-nonyl and N-decyl nicotinamide chlorides, synthesized in our laboratory, have been determined by means of a OSMOMAT vapor pressure osometer. In the precision limits of the method used, they agree as a trend with those previously measured for octyl and dodecyl compounds. If interpreted by means of a pseudo-phase transition model, they could suggest the presence of a phase transition involving a change in the number of particles present in solution, as was already supposed in the case of octyl and dodecyl compounds from measurements of heat capacities vs concentration. — The experimental trends can be fitted by using different theoretical approaches, allowing for the determination of the ionic interaction parameters. — The determination of osmotic coefficients allows the evaluation of the excess free energies of the surfactant solutions and of the entropic contributions once the enthalpic ones are known. Analyzing the behavior of the excess free energies for the homologous series of alkyl-nicotinamide surfactants, a group contribution for each −CH₂ group of −2.9 kJ mol⁻¹ can be evaluated in the postmicellar region.