Liquid−Liquid Domain Miscibility Driven by Composition and Domain Thickness Mismatch in Ternary Lipid Monolayers

This work describes how changes in surface pressure modulate the molecular organization of Langmuir monolayers formed by ternary mixtures of dlPC/pSM/Dchol that exhibit coexistence of liquid-expanded (LE) and liquid-ordered (Lo) phases. It provides a theoretical framework for understanding the pressure-induced critical miscibility point characteristic of monolayer systems with liquid−liquid phase coexistence. From compression isotherms and Brewster angle microscopy of Langmuir monolayers with a composition close to a tie line, we determined experimental values of mean molecular areas, surface potential, and monolayer thickness and could estimate the mean molecular area and composition of each coexisting phase. A surface-pressure-induced enrichment of the PC component in the Lo phase reduces both the compositional miscibility gap and the hydrophobic mismatch between phases. The liquid−liquid miscibility transition point observed at ≈25 mN/m can be explained by a competition between thermal energy and the line tension arising from the hydrophobic mismatch between the coexisting liquid phases.