Conformational Transitions of Mixed Monolayers of Phospholipids and Poly(ethylene oxide) Lipopolymers and Interaction Forces with Solid Surfaces

We studied conformational changes in monolayers of lipopolymers consisting of poly(ethylene oxide) head groups coupled to L-α-distearoylphosphatidylethanolamine (DSPE-EO_n; n = 45,110) and of mixtures of these lipopolymers with L-α-dimyristoylphosphatidylethanolamine (DMPE) with the aim to deposit composite polymer-lipid films on solid substrates. Film balance experiments performed with pure DSPE-EO„ over areas ranging from 0.5 to 60 nm² per molecule revealed two conformational changes which are interpreted as a transition from a pancake-like to a mushroom-like conformation and a mushroom-to-brush transition, as predicted by the Alexander-de Gennes theory of grafted polymers. The mixtures of DSPE-EO_n (< 10 mol %) with DMPE exhibit in general lateral phase separation as demonstrated by film-balance and microfluorescence studies. The very complex behavior is determined (1) by the well-known tendency of two-dimensional polymer solutions for phase segregation at very low polymer molar fractions, (2) by the interaction of the polymer head groups, and (3) by the chain melting transition of DMPE. A homogeneous phase was found for the case of 10 mol % DSPE-EO₄₅ in DMPE in the lateral pressure range between about 3 and 5 mN/m. This is interpreted in terms of a pancake-like state with the lipopolymer head groups starting to overlap. In this region the monolayer is below the DMPE main transition. Finally we studied the interaction of DSPE-EO₄₅ monolayers with Si/SiO₂ wafers by ellipsometry. The thickness of the lipopolymer monolayers was determined as a function of the relative humidity of the atmosphere surrounding the sample in a hydration chamber connected to a film balance. The measurements yield film thickness vs disjoining pressure curves. These distance vs disjoining pressure measurements show interaction regimes which are determined by steric and electrostatic forces, respectively, in analogy to recent force apparatus measurements.