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

Abstract

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.The study of lipid monolayers at the air-water interface provides direct information about the free energy of surface phases as a function of lipid molecular area, temperature, and other intensive parameters. From such studies monolayers are known to exhibit physical behavior-e.g., thermotropic phase transitions, similar to that of bilayers and other surface phases. Hence, in theory, it should be possible to use data from the monolayer experiment to better understand surface structure in general. This has not been possible, however, because of the lack of an adequate thermodynamic description of lipid-water interfaces and because of uncertainties about the point at which monolayer and other surface phases correspond thermodynamically (e.g., ref. 1).Early descriptions of lipid monolayer properties concentrated on the behavior of the lipids themselves, ignoring the presence of water in the surface phase (e.g., refs. 2-4). Subsequently, the importance of water was realized (5-9), and application of such models to lipid films at the point of collapse has suggested a two-dimensional structural model based on the presence of water molecules associated with each lipid species (10,11). Studies of bilayer structure have also focused on the role of water as a determinant of interfacial structure. These studies have led to the recognition that at small distances water-water interaction is the principal mediator of the approach of two lipid-covered surfaces through an aqueous medium (12)(13)(14).Prompted by the apparent similarities between monolayer and bilayer systems and the importance of water in determining their structures, we have derived a water-oriented, quasi-two-dimensional equation of state that accurately describes the properties of liquid-type (15,16) (17) is useful because it can be directly applied to data describing interfacial tension as a function of system composition. However, it is incomplete in that it fails to account for changes in the activity coefficients of the components as a ...

Surface dipole moments of lipids at the argon-water interface. Similarities among glycerol-ester-based lipids

“…where wo is the totally dehydrated area of the lipid, w1 is the cross-sectional area of water, f, is the rationalized activity coefficient of interfacial water, and q is a cumulative interaction parameter (28). Application of this model to -r-A data in the range 1.0 c 7r c7d yields the values of w0, fl, and q given in Tables 1 and 2.…”

Thermodynamic and kinetic investigations of the release of oxidized phospholipids from lipid membranes and its effect on vascular integrity

“…A theoretical fit is generated using an osmotic two-dimensional equation of state: $$\Pi =\frac{q.kT}{a_w}\times \ln true[\frac{1+a_w∕(A-a_{\mathrm{normale}})}{f}true]$$ where f and q are effective surface activity coefficients (for most lipids f ~ and q ~ 1–4 (Wolfe and Brockman, 1988)), a e is the excluded area per lipid molecule (~0.4 nm 2 for phosphatidylcholine headgroups), and a w is the partial area per water molecule (~0.09 nm 2 ) (Feng et al, 1994; Wolfe and Brockman, 1988; Marsh, 1996). …”

Phosphatidylcholine acyl unsaturation modulates the decrease in interfacial elasticity induced by cholesterol

“…where k is Boltzmann's constant, w, is the cross-sectional area of an interfacial water molecule (9.65 A2), and wo is the cross-sectional area of dehydrated lipid, f1 is the activity coefficient of interfacial water, and A7, is the total surface area divided by the number of lipid molecules present at a given surface pressure (Wolfe and Brockman, 1988;Smaby and Brockman, 1992;Feng et al, 1994). The scaling parameter q correlates to f, and is not unique, but does provide a better fit of the data because it allows for higher-order terms involving the activity coefficient (Smaby and Brockman, 1991b;Feng et al, 1994).…”