A Study of Surfactant Solutions Using the Liquid Membrane Electrode Selective to Alkyl Sulfate Ions

Liquid membrane electrodes selective to alkyl sulfate ions were devised for octyl, decyl, and dodecyl sulfate ions, and employed in the study of micellar solutions. A nitrobenzene solution of hexadecyltrimethylammonium—alkyl sulfate complex was used as the liquid membrane. The membrane potential-logarithmic concentration showed a maximum at the critical micelle concentration (CMC), an increase with the Nernstian slope at the lower concentrations and a decrease at the higher concentrations. Below CMC, the presence of excess inorganic salt did not alter the Nernstian slope, i.e., the liquid membrane was more highly permselective to alkyl sulfate ions than to inorganic anions. From a simple thermodynamic consideration of a micellar solution based on the pseudo phase model of the micelle, the change in the activity of alkyl sulfate ions and the dissociation of the micelle were discussed. It has been deduced that the concentration of the singly dispersed surfactant ions above CMC is reduced with concentration, except for the presence of a large excess of added electrolyte, and that the degree of dissociation of the micelle is unaltered even when the excess electrolyte is added.