Structure of Nonionic Surfactant (Glycerol α-Monomyristate) Micelles in Organic Solvents: A SAXS Study

Abstract

In this paper we present the study of reverse micellar aggregates formed by glycerol alpha-monomyristate (C(14)G(1)) in different organic solvents. We have investigated the structure (mainly shape and size) of the monomyristin reverse micelles depending on alkyl chain length of the oils, temperature, and the surfactant concentration. Moreover, we present how addition of trace polar additives such as water or glycerol and the hydrophilic size of the surfactant affect the structure of the micelles. Monomyristin could not form any self-organized structure in nonpolar media at room temperature because of its smaller headgroup size and tends to separate from the nonpolar media. In dilute regions this surfactant forms a solid dispersion, which upon heating transforms into an isotropic reverse micellar solution. For the structural characterization of the reverse micelles formed at elevated temperatures, we have performed small-angle X-ray scattering (SAXS) measurements, and the data were analyzed utilizing the generalized indirect Fourier transformation (GIFT) method. The SAXS data have shown that the size of the reverse micelles increases with the alkyl chain length of oils and surfactant concentration but decreases with temperature. A dramatic growth to the micelles could be achieved when the hydrophilic size of the surfactant is increased. Furthermore, addition of trace water or glycerol induces a significant change in the structure of micelles in terms of a cross-section and maximum length. The added water tends to form a water pool in the micellar core and results in swollen micelles. Model-free cross-section analysis of the SAXS data provided complementary information on the structural parameters. Thus, the present study has highlighted the possible ways of controlling the structure, mainly the shape and the size of the inverse micelles.