Regulation of the surface pressure of lipid monolayers and bilayers by the activity of water: derivation and application of an equation of state.

A quasi-two-dimensional equation of state for liquid-type lipid monolayers has been derived and successfully applied to surface pressure-area isotherms obtained with a variety of lipids. For lipids with acyl moieties of similar length, the surface pressure and area at monolayer collapse can be accurately predicted from data obtained at lower surface pressure. Consideration of the rationalized activity coefficient as a linear scaler in an expression for surface pressure as a function of depth in the surface phase permits comparison of surface pressure-area data for monolayers with force-distance data for bilayers. This analysis shows the thermodynamic equivalence of monolayers at collapse and fully hydrated bilayers. It also supports the interpretation of the activity coefficient as a scaler and allows its determination solely from bilayer-derived data. Overall, the results show the common assumption that partial specific volume of water equals its bulk value to be inappropriate for the analysis of surface structure.