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

Abstract

Abstract: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-I, 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.

Phase equilibriums, self-assembly and interactions in two-, three- and four medium-chain length component systems

“…Considering the micelle radius (1.39 nm) it shows that the hydration layer protrudes into the rough micelle surface and constitutes a part of the semi-spherical micelle entity. These results support early small-angle X-ray investigations of the sodium octanoate micelle structure [79][80][81], although illustrated as radial structure parameters. A statistical analysis of water penetration in radially oriented monomers in micelle [104,106] resulted in an average micelle radius of 1.2 nm and an average micelle core of 1.16 nm in agreement with small-angle neutron scattering results [107,108].…”

“…For sodium octanoate micelles equation (77a) predicted an aggregation number of 22. Assuming N = 11 the number of dry methylene groups are 4-5 which corresponds to a core radius of about 0.7 nm which is not far from the simulated one (0.5 nm, [22,23]) and that found with small-angle M A N U S C R I P T ACCEPTED MANUSCRIPT 39 X-ray scattering (0.45-0.60 nm, [79][80][81]) at cmc. Prolate (rod shaped micelle) growth may be considered a first step towards hexagonal liquid separation (cf.…”

Molecular dynamics simulation of the sodium octanoate micelle in aqueous solution

“…Zemb et al [44] have reported a radius of 1.08 nm in a light scattering experiment. Friman and Rosenholm [45] obtained a larger micellar radius using an X-ray scattering experiment, 1.8 nm. The agreement is quantitative considering that parameters derived from scattering experiments are obtained assuming particular features of the micellar system structure.…”

Neutron scattering study of pentanol solubilization in sodium octanoate micelles

“…The methods used to estimate this quantity will be discussed below. We note that small-angle X-ray scattering (SAXS) has previously been used to study a similar system, NaCd/Octane-l,8-diol/H20 [7]. However, the analysis used in that study relied on using data only on the high-Q side of the interference peak in the scattered intensity and it was assumed that this part of the data was wholly due to the single-micelle form factor.…”