Thermodynamic Micellar Properties of n-Octyldimethylamine Oxide Hydrochloride in Water

Abstract

The apparent molar volumes and heat capacities of n-octyldimethylamine oxide hydrochloride (OAOHCl) were measured in water as a function of concentration between 2 and 55 °C and the apparent molar relative enthalpies of the same system at 25°C. The apparent molar expansibilities were calculated from the temperature dependence of the volumes. The volumes and heat capacities were also measured at 25°C for trimethylamine oxide hydrochloride in water in order to check the group additivity. The data for OAOHCl were fitted with a mass-action model (J. Phys. Chem. 1983, 87, 1397) and compared with those of n-octyldimethylamine oxide (OAO) and n-octylammonium bromide (OABr) in order to investigate the effect of the charge on the thermodynamic micellar properties. A comparison between the properties of OAOHCl and OABr shows that OAOHCl behaves as a typical cationic surfactant. The differences in the thermodynamic properties of micellization between OAO and OAOHCl are as expected from a stronger hydration in the premicellar region and counterion binding beyond the critical micellar concentration (cmc) of OAOHCl. The temperature dependence of most micellar properties is comparable for the two surfactants. As it is generally observed with ionic surfactants, the enthalpy of micellization of OAOHCl is endothermic at low temperature and exothermic at high temperature and the cmc goes through a minimum at about 35°C. However, the cmc of both surfactants at 25 °C are of the same magnitude. From the mass-action model, the thermodynamic properties of the surfactant can be predicted at any temperature and concentration in the range measured.