The effect of pressure on micelle formation and solubility of hexadecyltrimethylammonium bromide in water

Abstract

The conductivities of hexadecyltrimethylammonium bromide (HTAB) in water have been measured in the concentration range 2×10^-4～4×10^-3 mol-dm^-3 at pressures up to 2500kg/cm^2 at 25℃, and used to determine the critical micelle concentrations (c.m.c.) and the apparent solubilities. The limiting equivalent conductivities of hexadecyltrimethylammonium ion, λ°(HTA^+) decrease with increasing pressure. The c.m.c. increase with the increase in pressure up to about 1000kg/cm^2 and decrease at higher pressures. The fact that the maximum pressure of the c.m.c. of HTAB is not so much different from those of the lower homologues appears to indicate that there occurs no solidification at the maximum pressure. The partial molal volume change accompanying the micelle formation at 1 atm, ΔV^^-^(1) is 10cm^3・mol^-1, being calculated from the pressure dependence of the c.m.c. The rate of decrease in the apparent solubilities of HTAB with increasing pressure is very large up to about 1500kg/cm^2 and rather small at higher pressures.