Study of Bivalent-Metal Dodecyl Sulfates by Electrodes of Liquid Membrane Selective to Surfactant Ions

Abstract

Liquid membrane electrodes selective to surfactant ions were devised, and employed to thestudy of micellar solutions. In this study, a nitrobenzene solution of hexadecyltrimethyl-ammonium-dodecyl sulfate complex was used as the liquid membrane. The liquid membrane was much more permselective to dodecyl sulfate ions than to inorganic anions (cell B and Table 1). With the liquid membrane electrode, the membrane potentials for the bivalent-metal dodecyl sulfate solutions were measured in the absence as well as in the presence of inorganic salts (cell A and Figs. 2-6). The membrane potential vs. the logarithmic concent-ration of surfactants plots showed that the potential had a maximum at the critical micelle concentration, (CMC). Below the CMC, the membrane potential exhibited the ideal Nern-stian behavior, while above the CMC, it decreased with the increase in the concentration of surfactants. A simple thermodynamic consideration of a micellar solution based on the pseudo-phase model of micelle gave the degree of dissociation of micelle, α for Cu-, Co-, Mn-, and Mg dodecyl sulfates (Table 2). No appreciable difference was found among those values of α. The values were, however, about a half of that of the NaDS micelle. These results were corresponding to the facts that CMC's values and micellar association numbers in the various bivalent-metal dodecyl sulfate solutions were almost the same within experimental error. It has been deduced that the concentration of the singly dispersed surfactant ions above the CMC decreases with the increase of the total concentration of the surfactants unless a large excess of the electrolytes, such as CuSO₄, CoC1₂, MnSO₄, and MgC1₂, is added to the surfactant solution, and that the degree of dissociation of the micelle is unchanged in that case even when the excess electrolyte is present.