Enthalpies of micellization of quaternary tetradecyl- and cetyltrimethylammonium

Abstract

Direct enthalpies of micellization (AHm°) of cetyltrimethylammonium salts (CTAX, X = OH", F", Cl", Br", and N03") and tetradecyltrimethylammonium hydroxide (TTAOH) have been measured by using a microcalorimeter method. The dilution of the detergent solution was carried out in the calorimeter chamber in where heats of dilution were detected. The chamber was fed with two peristaltic pumps, one pump delivered the solvent and the other the detergent solution whose concentration varied continuously. The concentration gradient was obtained by inserting a flask containing solvent in the detergent pump path followed by a flask containing the detergent solution at a concentration about ten times its critical micelle concentration (cmc). When the dilution in the calorimeter chamber is done with water and the resulting solution is above the cmc of the given detergent, the heat detected arises from the heat of demicellization and the heat of micellar dilution. On the other hand, when the dilution is done with a solution just above the cmc, the heat detected would only correspond to the heat of micellar dilution. The standard enthalpies of micellization (AHm°) obtained in this manner are discussed in terms of hydrophobic and electrostatic contributions. The AHm° values and the standard free energies of micellization obtained from the cmcs were used to calculate the standard entropies of micellization (AS^0). A linear trend between ASm° and AHm° was obtained which was interpreted in terms of dehydration of ions when they are transfered from the water to the micellar phase. It is concluded that the binding of small ions to micelles would be promoted mainly by entropic factors. It is also concluded that the thermodynamic behavior of micellar solutions is esentially determined by the nature and properties of the counterions.