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 (⊺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 ⊺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 at. (1976), when comparison was possible. In a wide concentration range above c. m. c. ⊺2-1 of SLS sharply decreased with increasing concentration, increased with temperature, and decreased with addition of foreign salts. The ⊺2-1 of sodium tetradecyl sulphate (STS) and sodium hexadecyl sulphate (SCS) showed similar trends. The magnitudes of the ⊺2-1 were in the order SLS > STS > SCS. The ⊺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.