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We report shape, size, and internal cross-sectional structure of diglycerol monomyristate (C₁₄G₂) reverse micelles in n-hexadecane near the critical point using small-angle X-ray scattering (SAXS). Pair-distance distribution function, p(r), which gives structural information in real-space, was obtained by indirect Fourier transformation (IFT) method. The p(r) showed a clear picture of rodlike micelles at higher temperatures well above the critical point (micellar solution phase separates into two immiscible liquids at ~ 48°C). At a fixed surfactant concentration (5% C₁₄G₂), decrease in temperature increases the micellar size monotonously and surprisingly shape of the p(r) curve at 50°C; close to the critical point, mimics the shape of the two dimensional disk-like micelles indicating the onset of critical fluctuations (attractive interactions among rodlike micelles forming a weak network). A similar behavior has been observed with normal micelles in aqueous system near the critical point. When the system is heated to 60°C, shape of the p(r) curve regains rodlike structure. At fixed temperature of 60°C, increase in C₁₄G₂ concentration induced one dimensional micellar growth. Maximum length of micelles increases from ca. 23.5 to 46.0 nm upon increasing concentration from 1 to 12% keeping cross section diameter apparently unchanged at ca. 4.0 nm.

Water induced microstructure transitions of diglycerol monolaurate (C(12)G(2)) reverse micelles in aromatic liquid ethylbenzene have been investigated using small-angle X-ray scattering (SAXS) and rheometry. The SAXS data have been evaluated by generalized indirect Fourier transformation (GIFT) and geometrical model fittings. We found that the C(12)G(2) spontaneously self-assembles into ellipsoidal prolate micelles in ethylbenzene under ambient conditions. The maximum diameter of the micellar core is ca. 5.0 nm. We noted that 5% C(12)G(2)/ethylbenzene solubilize 0.8% water and the water causes micellar growth; micellar size is ~ 3 times bigger than the empty micelles. Rheology data showed that viscosity of 5% C(12)G(2)/ethylbenzene increases with increase in the water concentration sustaining the SAXS results that the viscosity increase is caused due to micellar growth. We found that increase in temperature of the water incorporated system decreases micellar size, which is equivalent to a rod-to-sphere type transition. The water induced microstructure transformations can be explained in terms of critical packing parameter, cpp. Water hydrates hydrophilic headgroup of surfactant so that effective cross-sectional area of surfactant increases. As a result the cpp decreases and micelles grow. On the other hand, increasing temperature promotes interpenetration of solvent and surfactant chain as a result cpp increases due to decrease in the effective cross-sectional area of surfactant. Moreover dehydration may occur and micelles with more curved interface are formed due to increases in the cpp.

We have investigated the structure and rheology of diglycerol monolaurate (C(12)G(2)) reverse micelles in styrene using small-angle X-ray scattering (SAXS) and rheometry techniques. The SAXS data have been evaluated by generalized indirect Fourier transformation (GIFT) method and further supported by geometrical model fittings. We found that the C(12)G(2) when added into styrene spontaneously self-assemble into ellipsoidal prolate type reverse micelles under ambient conditions. Micelles grew and micellar aggregation number (N(agg)) increased with the increase in surfactant concentration demonstrating concentration induced one dimensional micellar growth, which was further supported by rheology data; the relative viscosity, η(r), for the C(12)G(2)/styrene mixtures exhibit far steeper concentration dependence behavior than those predicted for a dispersion of spherical particles based on the Krieger-Dougherty relation, due to the elongated micellar structures. On the other hand, a distinct modulation in the shape and size of micelles favoring ellipsoidal prolate-to-sphere type transition was observed upon heating, whose scheme could be attributed to the enhanced penetration tendency of oil into the lipophilic shell of the surfactant at the higher temperatures. As anticipated reverse micelles swelled with water causing two dimensional micellar growth; both the maximum dimension and maximum core radius increase with water., (Copyright © 2011 by Japan Oil Chemists' Society)