An investigation of the size and structure of the micelles in sodium octanoate solutions by small-angle X-ray scattering

Abstract

Micellar radii and the electron densities of the polar and nonpolar regions in micelles have been determined by small-angle X-ray scattering. This was done in aqueous solutions of sodium octanoate (NaC 8), in 1.2 M (=mole/liter) NaC 8 with added NaCl and in 1.2 M solutions of NaC 8 to which were added the solubilizates p-xylene, N,N-dimethyl aniline (DMA) and decanol. The micellar model developed by Luzzati, which describes the micelle in terms of different electron densities at different radii, was employed. Interpretation of the results according to this model results in a thick polar layer, indicating considerable hydration of the micelles and penetration of water between the hydrocarbon chains. Support for this interpretation is also given by the relatively low electron density in the polar layer. Solubilized decanol and DMA appears to be located in the polar region of the micelle, while p-xylene is mainly solubilized in the inner, nonpolar part. It was also shown that there is a slightly increased adsorption of counterions as their concentration in the solution increases.