Thermodynamics of the micellization of cationic surfactants. Part II: Effect of urea derivatives on the micellization of 1-hexadecyltrimethylammonium bromide and 1-hexadecylpyridinium bromide

The critical micelle concentrations (c.m.c.) and degrees of counterion binding of two ionic surfactants, 1-hexadecyltrimethylammonium bromide (HTMABr) and 1-hexadecylpyridinium bromide (HPBr), in the presence of urea and its derivatives (1-methylurea, 1,3-dimethylurea, 1-ethylurea and 1,1,3,3-tetramethylurea at a concentration of 1 mol dm -3 in aqueous solution) at various temperatures (20-50°C) were determined by the electrical conductivity method. The c.m.c. values of the surfactants increased with increasing temperature and concentration of the additives. The c.m.c.-raising activity became stronger as the number and length of the alkyl groups substituted in the urea increased. The values for the degree of counterion binding decreased with increasing temperature and increased with the molecular mass of the urea derivatives. The thermodynamic parameters of micellization - standard molar Gibbs energy, enthalpy, entropy, heat capacity of micellization and transfer values - were determined from the variation in c.m.c. with temperature by fitting these values to expressions derived on the basis of a micellization model. Only small differences in the standard molar Gibbs energies of micellization over the temperature range were observed. The molar enthalpy of micellization is highly dependent on temperature. In all temperature ranges studied, London-dispersion interactions are more predominant.