Abstract
The formation of the different phases of lyotropic liquid crystals is considered as a function of the relative importance of cohesive forces and affinity forces for water. The elementary monolayer being the building stone of these phases, the conditions governing the state of these monolayers, and particularly of mixed monolayers, are analyzed. This shows how the composition of lipidic mixture determines its physical state in the presence of water: from the solid to the liquid crystalline state, and finally to complete solubilization in water. As biological applications, the case of he bile with formation of gallstones and that of the lipids in the arterial wall wit hformation of atheroma are given. Finally, the state of teh lipids in the model of the cell membrane is considered from a purely physical and cryatallographic point of view which fits with the biologica concepts.

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