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 consis ting of poly(ethylene oxide) head groups coupled to L-alpha-distearoyl phosphatidylethanolamine (DSPE-EO(n); n = 45, 110) and of mixtures of these lipopolymers with L-alpha-dimyristoylphosphatidylethanolamine (D MPE) with the aim to deposit composite polymer-lipid films on solid su bstrates. Film balance experiments performed with pure DSPE-EO(n) over areas ranging from 0.5 to 60 nm(2) per molecule revealed two conforma tional changes which are interpreted as a transition from a pancake-li ke to a mushroom-like conformation and a mushroom-to-brush transition, as predicted by the Alexander-de Gennes theory of grafted polymers. T he mixtures of DSPE-EO(n) (less than or equal to 10 mol %) with DMPE e xhibit in general lateral phase separation as demonstrated by film-bal ance and microfluorescence studies. The very complex behavior is deter mined (I) by the well-known tendency of two-dimensional polymer soluti ons for phase segregation at very low polymer molar fractions, (2) by the interaction of the polymer head groups, and (3) by the chain melti ng transition of DMPE. A homogeneous phase was found for the case of 1 0 mol % DSPE-EO(45) in DMPE in the lateral; pressure range between abo ut 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 t he monolayer is below the DMPE main transition. Finally we studied the interaction of DSPE-EO(45) monolayers with Si/SiO2 wafers by ellipsom etry. The thickness of the lipopolymer monolayers was determined as a function of the relative humidity of the atmosphere surrounding the sa mple in a hydration chamber connected to a film balance. The measureme nts yield film thickness vs disjoining pressure curves. These distance vs disjoining pressure measurements show interaction regimes which ar e determined by steric and electrostatic forces, respectively, in anal ogy to recent force apparatus measurements.