2025/08/06 by Chari, S. Siva Nasarayya, Kumar, Bharat
#FOS: Physical sciences #Soft Condensed Matter (cond-mat.soft)
paper · doi:10.48550/arxiv.2508.04330
A two-dimensional system consisting a mixture of highly coarse-grained saturated (S-type), unsaturated (U-type) lipid molecules, and cholesterol (C-type) molecules is considered to form a model lipid monolayer. All the S-, U- and C-type particles are spherical in shape, with distinct interaction strengths. The phase behavior of the system is studied for various compositions (x) of the C-type particles, ranging from x = 0.1 to 0.9. The results show that a structurally ordered complex is formed with the S- and C-types in the fluid-like environment of U-type particles, for x ∈ \lbrace 0.5 - 0.6\rbrace. The time-averaged hexatic order parameter ⟨ Ψ6 ⟩ indicates that the dynamical segregation of S- and C-types exhibits a positional order, that is found to be maximum for x in the range of 0.5 - 0.6. The mean change in the free energy (ΔG(x)) obtained from the mean change in enthalpy (ΔH) and entropy (ΔS) calculations suggests that ΔG is minimum for x ∼ 0.6. A phenomenological expression for the Gibbs free energy is formulated by explicitly accounting for the individual free energies of S-,U- and C-type particles and the mutual interactions between them. Minimizing this phenomenological G with respect to the C-type composition results in the optimal value, x^* = 0.564 ± 0.001 for stable coexistence of phases; consistent with the simulation results and also the previous experimental observations \citeraghavendraeffect2023. All these observations signify the optimal C-type composition, x ∼ 0.5 - 0.6.