The structure of the ripple phase of phospholipid membranes remains poorly
understood in spite of a large number of theoretical studies, with many
experimentally established structural features of this phase unaccounted for.
In this article we present a phenomenological theory of phase transitions in
single- and two-component achiral lipid membranes in terms of two coupled order
parameters -- a scalar order parameter describing {\it lipid chain melting},
and a vector order parameter describing the {\it tilt of the hydrocarbon
chains} below the chain-melting transition. This model reproduces all the
salient structural features of the ripple phase, providing a unified
description of the phase diagram and microstructure. In addition, it predicts a
variant of this phase which does not seem to have been experimentally observed.
Using this model we have calculated generic phase diagrams of two-component
membranes. We have also determined the phase diagram of a two-component lipid
membrane from x-ray diffraction studies on aligned multilayers. This phase
diagram is found to be in good agreement with that calculated from the model.Comment: 10 pages, 10 figure
