Phase separation and near-critical fluctuations in two-component lipid membranes: Monte Carlo simulations on experimentally relevant scales

TL;DR

This study uses large-scale Monte Carlo simulations to explore phase behavior, fluctuations, and domain dynamics in two-component lipid membranes, revealing near-critical phenomena and stability of membrane properties consistent with experimental observations.

Contribution

First large-scale simulation study of lipid membranes at micrometer scales showing near-critical fluctuations and phase transition behaviors.

Findings

Near-critical fluctuations occur during phase transition in certain lipid compositions.

Transient subdiffusion of lipids observed near critical points.

Line tension and correlation length vanish approaching the critical point.

Abstract

By means of lattice-based Monte Carlo simulations, we address properties of two-component lipid membranes on the experimentally relevant spatial scales of order of a micrometer and time intervals of order of a second, using DMPC/DSPC lipid mixtures as a model system. Our large-scale simulations allowed us to obtain important results previously not reported in simulation studies of lipid membranes. We find that, within a certain range of lipid compositions, the phase transition from the fluid phase to the fluid-gel phase coexistence proceeds via near-critical fluctuations, while for other lipid compositions this phase transition has a quasi-abrupt character. In the presence of near-critical fluctuations, transient subdiffusion of lipid molecules is observed. These features of the system are stable with respect to perturbations in lipid interaction parameters used in our simulations. The line tension characterizing lipid domains in the fluid-gel coexistence region is found to be in the pN range. When approaching the critical point, the line tension, the inverse correlation length of fluid-gel spatial fluctuations, and the corresponding inverse order parameter susceptibility of the membrane vanish. All these results are in agreement with recent experimental findings for model lipid membranes. Our analysis of the domain coarsening dynamics after an abrupt quench of the membrane to the fluid-gel coexistence region reveals that lateral diffusion of lipids plays an important role in the fluid-gel phase separation process.