The osmotic coeffs. of N-nonyl and N-decyl nicotinamide chlorides (synthesized in the author's lab. were detd. by using the OSMOMAT vapor pressure osmometer. Within the precision limits of the method, they agree as a trend with those previously measured for octyl and dodecyl compds. If interpreted by using a pseudo-phase transition model, they suggest the presence of a phase transition involving a change in the no. of particles present in soln., as already suggested in the case of octyl and dodecyl compds. from measurements of heat capacities vs concn. The exptl. trends can be fitted by using different theor. approaches, allowing for the detn. of the ionic interaction parameters. The detn. of osmotic coeffs. allows the evaluation of the excess free energies of the surfactant solns. and of the entropic contributions once the enthalpic ones are known. By 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.
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