Shape, Size, and Structural Control of Reverse Micelles in Diglycerol Monomyristate Nonionic Surfactant System

Abstract

We have investigated the self-organization structures of diglycerol monomyristate (DGM) in different types of organic solvents. Study of phase behavior shows the presence of solid at lower temperature, and upon increasing temperature, the solid phase transforms to lamellar liquid crystal (L(alpha)) phase in the surfactant axis in all the DGM/oil systems. In the dilute regions, however, the dispersion of the solid or L(alpha) phase is observed, depending on the solvent and temperature. At higher temperatures, the L(alpha) phase melts to the isotropic reverse micellar solution phase. GIFT analysis of small-angle X-ray scattering data supported by a complementary modeling method have unambiguously shown that the structure of the DGM reverse aggregates can flexibly be controlled by optimizing the fundamental properties of solvent oils. In aromatic oils, the observed moderate micellar elongation is almost solely governed by the polarity of the aromatic ring, the length of the hydrocarbon side chain group showing no drastic effect. In contrast, when the solvent is replaced with linear-chain hydrocarbon oils, the drastic effects depending on the chain length emerge; by gradually increasing the length from decane to hexadecane, the long cylindrical particles in decane are finally transformed into planar aggregates, whose mechanism may be explained in terms of the transfer free energy of the diglycerol moiety from the hydrophilic environment to the hydrocarbon oils with a different chain length. We have also systematically examined the effects of temperature, the surfactant concentration, and added water.