The freezing points, enthalpies of dilution, volumetric heat capacities, densities, and sound velocities of the homologous series R(CH3)2NO, for R = butyl, hexyl, octyl, and decyl, were measured in water at 25 °C and as a function of temperature in the case of octyl. The osmotic coefficients and the apparent and partial molar relative enthalpies, heat capacities, volumes, compressibilities, and expansibilities were calculated. Isochoric heat capacities and isothermal compressibilities can also be derived from these data. There is a gradual change in the trends of these functions when going from the lower homologue, which behaves like a medium-size alcohol, to the higher one, which is a typical nonionic surfactant. The osmotic coefficients are positive in the premicellar region at the freezing temperature but become negative at higher temperatures. The concentration dependence of the various functions can be accounted for quantitatively with a simple mass-action model. Aggregation numbers and thermodynamic functions of micellization can be derived with this model.
The apparent molar volumes and heat capacities of n-octyldimethylamine oxide hydrochloride (OAOHC1) 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 25OC 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 OAl3r 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 ofboth 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.
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