Study of the Stability of Direct and Inverse Ordered Phases Within Surfactant–Water Systems: A Theoretical Perspective

ABSTRACT In this article, we have presented a simplified Ginzburg–Landau theory, based on a single scalar order‐parameter, for the study of mixtures comprising surfactants and water systems. The local concentration of surfactants in aqueous solutions represents the scalar order parameter, and the local free energy density is formulated as a linear combination of both the second and fourth powers of the scalar order parameter. The phase diagrams are derived by optimizing the free energy functional of the various phases with respect to the variational parameters. Our analysis has revealed several ordered lyotropic phases, including the body‐centered cubic () phase, present in both direct (I) and inverse (II) forms, the hexagonal () phase, also in both direct (I) and inverse (II) forms, and the lamellar phase denoted as . These phase diagrams illustrate the sequence of ordered phases: body‐centered cubic () – hexagonal () – lamellar () – hexagonal () – body‐centered cubic (), with an increasing concentration of surfactants, which is frequently observed in systems containing aqueous solutions of single‐chain surfactants. Furthermore, a three‐dimensional phase diagram has been developed, adding an additional dimension to our analysis.