Intrinsic parameters for structural variation of reverse micelles in nonionic surfactant (glycerol α-monolaurate)/oil systems: a SAXS study

Abstract

Using Small-Angle X-ray scattering (SAXS), we have studied self-assembled structures of glycerol α-monolaurate (C₁₂G₁) in different nonpolar solvents, such as cyclohexane, n-octane, n-decane, n-tetradecane, and n-hexadecane, as a function of composition, temperature, and added polar additives. The dilute mixtures of C₁₂G₁ with the aforementioned oils are essentially a solid dispersion at lower temperatures without forming any liquid crystalline phases, which are transformed into isotropic reverse micellar solutions upon heating. The generalized indirect Fourier transformation (GIFT) analysis of the SAXS data has drawn a clear picture of the structural variations of the reverse micellar aggregates in the isotropic solution phase. A notable effect on the micellar growth emerges when cyclohexane is replaced with the linear-chain hydrocarbon oils; a gradual increase of the carbon number of the hydrocarbon skeleton from octane to hexadecane, ellipsoidal prolate-like micelles are eventually transformed to long rod-like ones. Far more pronounced structural enhancement could be achieved by increasing the hydrophilicity of the surfactant, as short-rod micelles observed in the C₁₂G₁/decane system are replaced with long cylindrical ones when a more hydrophilic diglycerol monolaurate (C₁₂G₂) is used. This fact also supports a scenario that regulates the ability to form elongated to reverse structures. Lowering the temperature and increasing the surfactant concentration similarly lead to micellar growth while the cross-sectional structure remains essentially unchanged. In contrast, polar additive-induced growth is accompanied by the rapid swelling of the micellar core. Our data demonstrate that optimization of the oil properties and/or other outer conditions provide possible routes to flexibly controlling the size, shape, and internal structure of the reverse micelles despite lower universality for a shorter hydrophilic chain surfactant.