The compressibility of alkyltrimethylammonium bromide micelles

Ultrasonic measurements have been carried out for solutions of octyl-, decyl-, dodecyl-, tetradecyl-, and hexadecyltrimethylammonium bromides (C(n)TAB) over a wide range of concentrations above and below their critical micelle concentrations (CMC) at 25 degrees C. The apparent molar adiabatic compressibilities of the surfactants in monomer, (Phi(ks))(f), and in micellar forms, (Phi(ks))(mic), have been determined. The values for (Phi(ks))(f) decrease with increasing chain length of the surfactant by an increment of -0.87 x 10(-4) cm(3) mol(-1) bar(-1) per -CH2- group. The opposite trend has been found for the value of (Phi(ks))(mic), the compressibility of surfactants in the micelle, which is positive, and increases with increasing chain length of the surfactant by 9.5 x 10(-4) cm(3) mol(-1) bar(-1) per -CH2- group. The coefficient of adiabatic compressibility of the internal cores of micelles is estimated to be 59.5 x 10(-6) bar(-1), which is close to, but slightly lower than the coefficient of adiabatic compressibility of pure hydrocarbon liquids having the same number of -CH2- groups. (C) 1998 Academic Press.