Effect of alcohols on the properties of micellar systems: III. Elastic and quasielastic light scattering study

Abstract

The effect of 1-propanol, 1-butanol, and 1-pentanol on the micellar properties of aqueous solutions of tetradecyltrimethylammonium bromide (TTAB) at an ionic strength 0.1 M KBr, has been investigated by means of elastic and quasielastic light scattering. At surfactant concentration near the CMC, the results indicate that (i) in the absence of alcohol, the TTAB micelles have a molecular weight of 39,000 and a hydrodynamic radius of 26.7 A, consistent with quasispherical or slightly spheroidal micelles; (ii) the addition of 1.4 M propanol or 0.375 M butanol results in a decrease of the micelle hydrodynamic radius, even though some alcohol is dissolved into the palisade layer; (iii) upon addition of pentanol the micelle hydrodynamic radius goes through a maximum at about 0.1 M pentanol, then decreases. A large amount of pentanol appears to be dissolved into the micelles and located both in the micelle palisade layer and hydrophobic core, where it may constitute a pentanol core. As the surfactant concentration is increased, the diffusion coefficients and scattered intensity data suggest that the micelle size changes only little in water and water + 1.4 M propanol but increases in water + 0.375 M butanol and goes through a maximum in the three investigated water + pentanol mixtures. For these last systems, the apparent changes of hydrodynamic radius are shown to reflect changes of micelle shape and composition from a highly pentanol swollen, slightly spheroidal micelle at concentration close to the CMC, to an anisotropic micelle at high concentration, with the pentanol essentially solubilized in the palisade layer.