The order-disorder transition in model lipid bilayers is a first-order hexatic to liquid phase transition

TL;DR

This study uses molecular dynamics simulations to analyze the first-order transition from a hexatic to a liquid phase in model lipid bilayers, revealing detailed structural and interfacial properties.

Contribution

It provides the first detailed characterization of the order-disorder transition in lipid bilayers as a first-order hexatic to liquid phase transition.

Findings

The ordered phase is hexatic with long-range orientational order.

The transition exhibits hysteresis and phase coexistence with capillary scaling.

Finite interfacial tension influences membrane solute interactions.

Abstract

We characterize the order-disorder transition in a model lipid bilayer using molecular dynamics simulations. We find that the ordered phase is hexatic. In particular, in-plane structures possess a finite concentration of 5-7 disclination pairs that diffuse throughout the plane of the bilayer, and further, in-plane structures exhibit long-range orientational order and short-range translational order. In contrast, the disordered phase is liquid. The transition between the two phases is first order. Specifically, it exhibits hysteresis, and coexistence exhibits an interface with capillary scaling. The location of the interface and its spatial fluctuations are analyzed with a spatial field constructed from a rotational-invariant for local 6-fold orientational order. As a result of finite interfacial tension, there necessarily exist associated forces of assembly between membrane-bound solutes that pre-melt the ordered phase.