Characterization of micellar solutions using surfactant ion electrodes

Surfactant ion electrodes were used to investigate the dimerization, aggregation, and micelle formation occurring at 25 °C in aqueous solutions of sodium dodecyl sulfate, decyltrimethylammonium bromide, tetradecyltrimethylammonium bromide (TTAB), and orange II (4-[(2-hydroxy-l-naphthalenyl)azo]benzenesulfonic acid monosodium salt) as a function of added electrolyte. The surfactant electrodes, which contained a liquid membrane ion exchanger, permitted a direct determination of the surfactant monomer activity. Analysis of these data gave an association constant of 1000 for orange II, an estimated fractional charge on the DTAB micelle of 0.78, a dimerization constant of 400 (L/M) for TTAB, and a second break above the commonly accepted critical micelle concentration for sodium dodecyl sulfate, a break which may result from an ordering of the micelles.