THE COMPRESSIBILITY OF ALKYLTRIMETHYLAMMONIUM BROMIDE MICELLES

Ultrasonic measurements have been carried out for solutions of octyl-, decyl-, dodecyl-, tetradecyl-, and hexadecyltrimethylammonium bromide s (C(n)TAB) over a wide range of concentrations above and below their critical micelle concentrations (CMC) at 25 degrees C. The apparent mo lar adiabatic compressibilities of the surfactants in monomer, (Phi(ks ))(f), and in micellar forms, (Phi(ks))(mic), have been determined. Th e 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) pe r -CH2- group. The opposite trend has been found for the value of (Phi (ks))(mic), the compressibility of surfactants in the micelle, which i s positive, and increases with increasing chain length of the surfacta nt by 9.5 x 10(-4) cm(3) mol(-1) bar(-1) per -CH2- group. The coeffici ent 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 hydro carbon liquids having the same number of -CH2- groups. (C) 1998 Academ ic Press.