Phospholipid monolayers at water∣oil interfaces: theoretical modelling of surface pressure–molecular area isotherms

Abstract

Abstract The phospholipid adsorption and surface pressure–molecular area isotherms at interfaces are interpreted theoretically from two-dimensional (2D) lattice and real gas models that incorporate a minimum number of adjustable parameters. The first model is based on the lattice statistics of binary solutions and the molecular parameters introduced are the energy changes involved in the mixing process of the phospholipid and organic solvent molecules and the effective phospholipid head area. The surface pressure is interpreted in terms of the difference between the two liquid surface tensions. The second model makes use of (i) a non-localised adsorption model with a square-well potential energy term for the calculation of the surface concentration of the phospholipids at the interface as a function of the volume concentration of the phospholipids in the organic solvent phase, and (ii) a 2D hard disc gas model with a mean-field term accounting for the attractive interactions between the tails of the adsorbed phospholipids. The molecular parameters introduced in this model are the interfacial phospholipid adsorption energy, the effective hard disc diameter of the phospholipid head and the interaction energy between the phospholipid tails. The surface pressure is interpreted in terms of a 2D gas pressure in this model. The theoretical results obtained are compared with experimental data for the water ∣ 1,2-dichloroethane and water ∣ air interfaces. The two models predict correctly the typical order of magnitude for surface concentration and pressure values, as well as some qualitative features of the experimental isotherms, for low phospholipid surface concentrations.