THERMODYNAMICS OF N,N,N-OCTYLPENDYLDIMETHYLAMMONIUM CHLORIDE IN WATER-UREA MIXTURES

Specific conductivities, densities, heat capacities, and enthalpies of dilution at 25 degrees C were measured for N,N,N-octylpentyldimethyla mmonium chloride (OPAC) in water-urea mixtures at various urea concent rations m(u) as functions of the surfactant concentration m,. From con ductivity data, the cmc and the degree of the counterion dissociation beta of the OPAC micelles were calculated. The cmc increases linearly with increasing m(u) while beta vs. m(u) is a smooth concave curve. Fr om the experimental thermodynamic data, the apparent (Y-phi) and parti al (Y-2) molar properties (volumes, heat capacities, and relative enth alpies) are derived as functions of m(u) and m(2). The effect of urea on the dependences of the different properties on m(s) are discussed. From data in the premicellar region the standard partial molar volumes V-2(0) and heat capacities C-p2(0) were evaluated. It was observed th at V-2(0) increases linearly with m(u) while C-p2(0) decreases. The pr operties of OPAC in the dispersed and micellized forms at the cmc were obtained and, therefore, the thermodynamic functions of micellization were calculated on the basis of the pseudo-phase transition model.