Role of Confined Water on the Compressibility Modulus of Lipid Monolayers

Abstract

The studies on lipid monolayer properties have been done by compressing the lipid spread in a large water surface up to its collapse. In this procedure, a surface pressure vs area per molecule isotherm can be obtained. In this methodology, it is assumed that the lipids spread at huge areas are considered as being in a gas phase and when they are compressed at a temperature below the critical one (the transition temperature) a condensed phase appears. The thermodynamic treatment is compared with a real gas in which the condensation is analyzed as a consequence of the manifestation of the intermolecular forces between the lipids. In other words, lipid interacts between them as gas particles in vacuum.A more realistic approach to account on monolayer behavior is that lipids, even at large areas, are in contact with the water phase. Upon compression, the energy input is not merely used to make work of compression but to overcome the friction of hydrated lipid molecules with the water (stationary) phase. Thus, the lipids drag water during its compression, force its reorganization and/or a displacement work can occur. With this picture, the whole thermodynamic of lipid membrane is reevaluated. For this purpose, we analyzed the compression of the monolayer from a state identified as a free-force monolayer state up to the collapse for lipids with different hydration degrees (mainly PCs and PEs). The comparison of these compression curves with the branch of the isotherm obtained by compressing the lipids from the gaseous state allow to conclude that the water arrangements, mainly confined water and hydration water, determines the hysteresis and synergistic effects of membrane response.