MICELLE FORMATION AND PHASE-BEHAVIOR OF ALKYLSULFONIC ACIDS

The critical micelle concentrations (CMC) of tetradecyl-, hexadecyl-, and octadecylsulfonic acids in aqueous solution were precisely determined at varying temperatures by an electrical conductivity method. As to hexadecyl- and octadecylsulfonic acids, a jump in the electrical conductivity reversibly appeared above the CMC at 21.5 and 35.0°C, respectively, while the jump appeared at ca. 15°C on increasing temperature for tetradecylsulfonic acid only after storage in a refrigerator for a long period of time. This jump results from dissolution of solid-like precipitates of the acids, and the precipitates give rise to an electric conductivity increase due to their electrical unneutrality. Enthalpy change of the precipitate dissolution is 21.7 and 57.0 kJ mol-1 for hexadecyl- and octadecylsulfonic acid, respectively. Aqueous solution of these acids showed a gel-sol transition at higher concentrations: ca. 6, 33, and 48°C of the transition temperature for tetradecyl-, hexadecyl-, and octadecylsulfonic acid, respectively, and 21.9 and 19.9 kJ mol-1 of the transition enthalpy for hexadecyl- and octadecylsulfonic acid, respectively. Phase diagrams were drawn using this information for hexadecyl- and octadecylsulfonic acids. © 1992.