PHYSICOCHEMICAL PROPERTIES OF AQUEOUS MIXTURES OF TETRABUTYLAMMONIUM BROMIDE AND ANIONIC SURFACTANTS .3. EFFECTS OF SURFACTANT CHAIN-LENGTH AND SALINITY

The solution behaviors of equimolar mixtures of tetrabutylammonium bromide and sodium alkyl sulfate (B4NCnS, n = 8-18) have been studied. The surface tension measurement shows that the ratios of tetrabutylammonium to alkyl sulfate both in micelles at the cmc and in surface adsorption layers obtained by using the Gibbs-Duhem equation are less than unity and decrease with increasing surfactant chain length. The Krafft point of B4NCnS in 0.2 M NaBr decreases with increasing surfactant concentration and is considerably lower than that of the corresponding alkyl sulfate salt without tetrabutylammonium. The cloud point phenomenon is observed in all surfactant systems. The cloud point increases with increasing surfactant chain length except for n > 16 below 56 mM surfactant concentration. This behavior is interpreted in terms of the variation of tetrabutylammonium content in the micelles by using our previously proposed model for the cloud point phenomenon. The addition of NaBr increases the cloud point initially, passing through a maximum, and then decreases again. The possible causes for the phenomenon of a maximum in the cloud point are discussed. B4NC18S may form a stable homogeneous micellar solution at room temperature, the micellar size of which increases with temperature in the presence of a comparable amount of NaBr to the surfactant or in the absence of NaBr and decreases in excess NaBr. An Arrhenius type of equation cannot be used in calculating the activation energy of viscous flow of micellar solutions without considering the effect of micellar size change with temperature. © 1990 American Chemical Society.