Dependence of Phase Boundary and Interfacial Characteristics of Liquid Crystalline Structures on Surfactant–Cosurfactant Composition

This study explores the effect of surfactant–cosurfactant (S_mix) ratio on the characteristics of liquid crystalline (LC) structures formed in a nonionic microemulsion system. Pseudoternary phase diagrams were constructed using TPGS-Span 80 (TS) in 1:1, 1:3, and 3:1 ratios. Oleic acid was used as the oil phase. Characterizations were performed using polarized microscopy, small-angle X-ray scattering (SAXS), and rheology. It was found that the S_mix ratio influences the monophasic area in the order: TS 1:1 > TS 3:1 > TS 1:3. Isotropic-to-anisotropic phase change occurred during gradual water addition, which was confirmed by the birefringence patterns observed under polarized light microscopy. SAXS data revealed the conversion of microemulsion into lamellar liquid crystals. Characteristic peaks observed at wave-vector transfer in the ratio of 1:2:3 were suggestive of a lamellar configuration. Transition points differed in accordance with the composition of the samples. While analyzing the interlayer distance of specific lamellae, more profoundly swollen lamellae with TS 1:3 composition were noticed. This was accompanied by an increase in the viscosity of the formulation. Contrary to the low viscosity and Newtonian behavior of microemulsion, LC structures exhibited non-Newtonian and viscoelastic characteristics during oscillatory tests. The composition TS 1:3 facilitated the formation of mechanically robust and flexible lamellae. Altogether, these findings highlight the importance of optimizing surfactant mixtures for developing viscoelastic LC matrices, which can be used as long-standing depot formulations.