Thermodynamic properties and conductivities of some dodecylsurfactants in water

Densities, heat capacities, enthalpies of dilution, osmotic coefficients and conductivities are reported for dodecylamine hydrochloride, dodecyldimethylammonium and dodecyltrimethylammonium chloride in water over a wide range of concentration. The last two properties were also measured for dodecyltrimethylammonium bromide. From the thermodynamic data partial molar volumes, heat capacities and relative enthalpies and nonideal free energies and entropies were derived as a function of the surfactant concentration. The cmc's and degree of counterion dissociation were also calculated from the transport properties. It is shown that the trends of volumes, enthalpies, free energies and entropies are quite regular whereas heat capacities present maxima and minima at concentrations which depend on the nature of surfactants. Corresponding changes were observed in the osmotic coefficients and specific conductivities. The thermodynamic functions of micellization were evaluated on the basis of the pseudo-phase transition model. Finally, the effects of the introduction of methyl groups in the hydrophilic moiety of the surfactant and of the nature of the counterion on the thermodynamic properties of monomers and micelles are examined.