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.