CHARACTERIZATION OF LIPID MISCIBILITY IN LIQUID-EXPANDED MONOLAYERS AT THE GAS-LIQUID INTERFACE

A surface solution model for liquid-expanded monolayers at the gas-liquid interface (Smaby, J. M.; Brockman, H. L. Langmuir 1991, 7, 1031-1034) has been extended to include both ideal and experimental surface pressure-area isotherms. Surface potential-area isotherms for liquid-expanded, mixed monolayers have been analyzed by an extension of the Helmholz equation modified to include an area-independent potential term (Smaby, J. M.; Brockman, H. L. Biophys. J. 1990, 58, 195). For experimental mixtures which exhibit ideal surface pressure-area behavior, the constant potential term is shown to apportion among species on a fractional area basis. Application of the equations derived to experimental data is computationally straightforward, requiring no a priori assumptions about the size or shape of the lipid molecules. Comparison of the four unique parameters obtained for each mixed monolayer provides insight into the origins of mixing nonideality. For example, the parameters reasonably indicate that nonideality in mixtures of 1-palmitoyl-2-oleoylphosphatidylserine, an anionic lipid, and sphingosine, a cationic lipid, arises from head group interactions/dehydration rather than steric accommodation in the aliphatic region. Conversely, in the liquid-disordered state, nonideal mixing of cholesterol and 1,2-dimyristoyl-phosphatidylcholine appears driven by steric factors which are not evident in the liquid-ordered state. Thus, the analysis of monolayer-derived data by the surface solution model provides a useful adjunct to more classical methods of assessing and characterizing lipid miscibility in fluid monolayers.