SURFACE-ADSORPTION AND MICELLE FORMATION OF SODIUM-CHLORIDE DODECYLAMMONIUM CHLORIDE MIXTURES

The surface tension of an aqueous solution of sodium chloride (NaCl) a nd dodecylammonium chloride (DAC) mixture was measured as a function o f the total molality and composition of mixture at constant temperatur e under atmospheric pressure. The surface densities of the solutes and the DAC compositions of the adsorbed film and micelle were then calcu lated. The miscibility of NaCl and DAC in the adsorbed film and micell e was discussed by drawing the composition diagrams of adsorption (CDA ) and micelle formation (CDM). Because of the negative value of the su rface density of sodium ion due to the electrostatic repulsive interac tion with the adsorbed dodecylammonium ions, the surface DAC compositi on became larger than 1. In a similar way, the micelles also had a DAC composition value larger than 1, probably because of the negative val ue of the number of sodium ions in the micelles. Furthermore, at the c ritical micelle concentration, the shapes of the CDM and CDA were simi lar. It was finally shown that the miscibility of NaCl and DAC in the micelle is similar to that in the adsorbed film and that our thermodyn amic approach to elucidate the miscibility in adsorbed films and in mi celles by use of the CDA and CDM is also applicable to mixed systems c onsisting of an inorganic salt and an ionic surfactant.