An investigation of the solubilization mechanism of sodium octanoate micelles by small-angle X-ray scattering

The micellar radii and the electron density of the polar and non polar regions in micelles have been determined by a small-angle X-ray scattering (SAXS) technique originally designed by Luzzati et al. and subsequently modified by Larsson. The model employed shows the micellar structure as consisting of a liquid hydrocarbon core and a polar shell comprising the ionic polar head groups plus the diffuse double layer of bound counterions. The shortcomings of the model are discussed in the light of a recent paper by Hayter & Penfold providing general support for the parameters obtained and the conclusions drawn.The results found for the binary aqueous systems of potassium hexanoate, sodium octanoate and sodium decanoate describes the micelles as having quite an open structure with a rather small truly anhydrous hydrocarbon core. The low electron density contrast found between the polar layer and the surrounding media prevents us from drawing conclusions as to the absolute value of the polar (total) micellar radius, although the changes in this radius seem relevant. The octane-1, 8-diol is found to partly penetrate the hydrocarbon core but mostly to reside in the polar shell or to remain in the bulk solvent. Both the cyclohexane and carbon tetrachloride seem to be solubilized in the interior of the micelle, the former causing a considerable swelling of the micelle.