Gel-Phase-like Ordered Membrane Properties Observed in Dispersed Oleic Acid/1-Oleoylglycerol Self-Assemblies: Systematic Characterization Using Raman Spectroscopy and a Laurdan Fluorescent Probe

Aqueous dispersions of oleic acid (OA) and those modified with 1-oleoylglycerol (monoolein, MO) form various kinds of self-assembled structures: micelles, vesicles, oil-in-water (O/W) emulsions, hexagonal phases, and dispersed cubic phases. Conventionally, these self-assembled structures have been characterized using cryogenic transmission electron microscopy or X-ray diffraction spectroscopy. However, these methodologies require specialized treatment before they can be used, which may lead to the self-assemblies not adopting their true equilibrium state. Herein, we systematically characterized the self-assemblies composed of OA and MO in aqueous solution using Raman spectroscopy and fluorescent probe 6-dodecanoyl-2-dimethylaminonaphthalene (Laurdan). The OA/MO dispersions at pH 5.0 showed increased chain packing in comparison to the OA micelle at pH 11 or OA vesicle at pH 9.0, which were characterized by the intensity ratio of the Raman peaks at 2850 and 2890 cm^-1, R = I₂₈₉₀/I₂₈₅₀. In the Laurdan fluorescence measurements, the obtained spectra were deconvoluted to two peak fractions (A₁: λ_em= 490 nm; A₂: λ_em = 440 nm), and the peak area ratio, A₁/(A₁ + A₂), was defined as the membrane hydrophilicity Ø_m. The Ø_m value of the OA/MO dispersion at pH 5.0 was similar to that of the OA O/W emulsion, indicating that the membrane surfaces of these self-assemblies were relatively dehydrated compared to the OA micelle or OA vesicle. To categorize the type of self-assembly dispersion, a Cartesian diagram plot was systematically drawn: R on the x axis and Ø_m on the y axis, with the cross point at x = 1, y = 0.5. By comparing the membrane properties of the OA-based micelles, O/W emulsions, and dispersed cubic phases, we determined that the OA/MO dispersion at pH 5.0 possessed higher chain packing (R > 1) and a dehydrated membrane surface (Ø_m < 0.5), which is similar to that of the ordered membranes in gel phases. This characterization method can be useful in evaluating the ordered membrane properties in dispersed self-assemblies in aqueous media.