| Abstract
| - Nonaqueous phase behavior and reverse micellar structures of diglycerol monolaurate (DGL) in different nonpolarorganic solvents, such as n-decane, n-tetradecane, and n-hexadecane, have been studied over a wide range of compositionsand temperatures. The equilibrium phases are identified by means of visual observation and small-angle X-ray scattering(SAXS). A solid phase present at lower temperature swells small amount of oils and transforms into a lamellar liquidcrystalline structure at higher temperature. The melting temperature of the solid phase is virtually constant at all mixingratios of the surfactant and oil. With the further increase of temperature, the liquid crystal transforms into an isotropicsingle-liquid phase near the surfactant axis, whereas there is a coexistence region of two isotropic phases near thesolvent axis. The area of the two-liquid (II) phase region depends largely on the hydrocarbon chain length of the oils,the longer chain leading to the wider II area. Accordingly, the DGL surfactant is most miscible with decane, exhibitinga reduced miscibility with increasing solvent hydrocarbon chain length. Increasing temperature enhances the dissolutiontendency of the surfactant in oil, where the two-liquid phase transforms into an isotropic single phase. SAXS analysisbased on the GIFT technique is used to characterize the structure of the reverse micellar aggregates in the isotropicsingle-phase liquids. We have demonstrated that instead of changing polarity or a functional group of the solventmolecules, if we optimize the hydrophilic nature of the surfactant head group, the alkyl chain length of the solventoils can serve as a tunable parameter of the micellar geometry. The hydrophilic surfactant DGL interestingly formscylindrical micelles in nonpolar oils, decane, and tetradecane in the dilute region above the II phase region. The micellarsize shows temperature dependence behavior, and the micellar length goes on increasing with decreasing temperature;eventually we found a signature of the onset of critical fluctuations in the deduced pair-distance distribution functionnear the phase separation line. The signature of the attractive interaction between the cylindrical reverse aggregateswhen a phase separation line is approached is likely to be a precursor of critical phenomenon. Doping with a traceof water results in a similar but more pronounced structural enhancement. The transfer free energy of diglycerol moietyfrom a hydrophilic environment to different hydrocarbon oils may account for these phenomena.
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