N-ALKYLTRIMETHYLAMMONIUM BROMIDES IN A BUFFERED MEDIUM - A THERMODYNAMIC INVESTIGATION

The critical micellar concentrations (c(c.m)) of n-alkyltrimethylammonium bromides of alkyl-chain length 10, 12, and 14 carbon atoms in glycine buffer solutions at pH = 10.0 have been determined at the temperatures T = (288.15, 293.15, 298.15, 303.15, and 308.15) K by the electrical-conductivity method. From the conductivities the degrees beta of the counterion binding have been calculated. It is shown that ln(c(c.m)/c-degrees) decreases linearly with increase in the length of alkyl chain and that plots of the c(c.m) against temperature pass through minima between T = 293.15 K and 303.15 K. Beta increases with increasing alkyl-chain length and decrease with temperature. To derive thermodynamic quantities for micellization from temperature, the classical-equilibrium model has been applied. Only small differences in the standard molar Gibbs free energies of micellization over the temperature range studied have been observed. The enthalpy of micellization is strongly dependent on the temperature.