Micellar properties of alkyldimethylphenylammonium bromides in water

The synthesis and methods applied for the purification of dodecyl-, tetradecyl-, and hexadecyldimethylphenylammonium bromides are described. The results of surface tension measurements of aqueous solutions of these surfactants show that slight amounts of strongly surface-active nonionic impurities are persistent in the crystalline materials presumably due to their low thermal stability. The Critical micelle concentration (cmc) and the degree of ionization (beta) of the micelles of the salts studied in aqueous solutions were determined at 25 degrees C from specific conductivity versus molality plots. The temperature dependence of the cmc and of beta of the tetradecyl homologue was measured in the range 4-34 degrees C. A minimum cmc amounting to 1.20 mmol/kg was determined at about 14 degrees C. The values of beta were found to grow linearly with temperature. From these results, the standard Gibbs energy, the enthalpy and the entropy of the process of micellization were obtained by application of the pseudo-phase-separation model. Enthalpy and entropy show a compensation effect in their contribution to the Gibbs energy. At low temperatures the process of micellization is driven mainly by the entropic term, whereas with increasing temperature the enthalpic term becomes predominant. At the temperature of the minimum cmc, the value of the enthalpy is far from being zero because of the important contribution of the (partial derivative beta/partial derivative T)R In X-cmc term.