  Lipid monolayers and bilayers have been used as experimental models for the
investigation of membrane thermal transitions. The main transition takes place
near ambient temperatures for several lipids and reflects the order-disorder
transition of lipid hydrocarbonic chains, which is accompanied by a small
density gap. Equivalence between the transitions in the two systems has been
argued by several authors. The two-state statistical model adopted by numerous
authors for different properties of the membrane, such as permeability,
diffusion, mixture or insertion of cholesterol or protein, is inadequate for
the description of charged membranes, since it lacks a proper description of
surface density. We propose a lattice solution model which adds interactions
with water molecules to lipid-lipid interactions and obtain its thermal
properties under a mean-field approach. Density variations, although
concomitant with chain order variations, are independent of the latter. The
model presents both chain order and gas-liquid transitions, and extends the
range of applicability of previous models, yielding Langmuir isotherms in the
full range of pressures and areas.
