Conductance stopped-flow study on the micellar equilibria of ionic surfactants

Rapid ionic equilibria of solutions of cationic and anionic surfactants were studied by the concentration-jump method with the use of the conductance stopped-flow technique. The slow relaxation times $\tau _{2}$ assigned to the micellization-dissolution process were obtained from the traces of the conductance changes in a much wider concentration range than in the previous methods. The $\tau _{2}$ of sodium dodecyl sulphate (SLS) observed coincided fairly well with those obtained from the temperature-jump technique by Folger et al. (1974) and by Aniansson et al. (1976), when comparison was possible. In a wide concentration range above c.m.c. $\tau _{2}^{-1}$ of SLS sharply decreased with increasing concentration, increased with temperature, and decreased with addition of foreign salts. The $\tau _{2}^{-1}$ of sodium tetradecyl sulphate (STS) and sodium hexadecyl sulphate (SCS) showed similar trends. The magnitudes of the $\tau _{2}^{-1}$ were in the order SLS > STS > SCS. The $\tau _{2}^{-1}$ of hexadecyl trimethylammonium bromide solution, which was much smaller than those of the anionic micelles studied, also showed similar dependence on concentration (above c.m.c.) and temperature.