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Structural Investigation of Diglycerol Polyisostearate Reverse Micelles in Organic Solvents

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journal contribution
posted on 24.09.2009 by Lok Kumar Shrestha, Rekha Goswami Shrestha, Keiichi Oyama, Makoto Matsuzawa, Kenji Aramaki
The structure of glycerol-based reverse micelles in the surfactant/oil binary system without external water addition has been investigated using a small-angle X-ray scattering technique, and different tunable parameters for the structure control of reverse micelles are determined. The scattering data were evaluated by the generalized indirect Fourier transformation (GIFT) method and complemented by model fitting. It was found that diglycerol polyisostearates (abbreviated as (iso-C18)nG2, n = 2−4, where n represents the number of isosterate chains per surfactant molecule) form reverse micelles in a variety of organic solvents such as cyclohexane, n-decane, and n-hexadecane without the addition of water from outside, and their structure (shape and size) depends on solvent properties (alkyl chain length), tail architecture of the surfactant, temperature, and added water. Small globular types of micelles were observed in the (iso-C18)2G2/cyclohexane system at 25 °C. The micellar size and the aggregation number were increased with increasing the alkyl chain length of the oils resulting in elongated ellipsoidal prolate or rodlike type micelles in the (iso-C18)2G2/hexadecane system. This structural evolution is caused by the different penetration tendency depending on the chain length of oils to the lipophilic chain of the surfactant. At fixed oil, composition, and temperature, the tail architecture of the surfactant played a crucial role in the micellar structure. The micellar size and, hence, the aggregation number decreased monotonically with increasing number of isostearate chain per surfactant molecule due to the voluminous lipophilic part of the surfactant. Composition could not modulate the structure of micelles but led to strong repulsive interactions among the micelles due to reduced osmotic compressibility of the system at higher concentrations. Increasing temperature decreased the micellar size, while the cross-section structure remains essentially the same. The structure was modified significantly in terms of micellar size and cross-section diameter upon solubilization of traces water in the surfactant/oil/water system. Both the maximum size and the cross-section diameter of the micelles increase with water; i.e., reverse micelles swell with water forming a water pool in the micellar core. Furthermore, from the results of model fittings, it was found that the aggregation number increases with water concentration.