Equilibrium swelling behavior of solid supported poly(ethylene glycol) lipid monolayers. Effects of short chain lengths

In this study phase transitions and thermodynamic properties of monolayers of short poly(ethylene glycol) chains (abbreviated as EG) covalently attached to lipids were determined by analyzing pressure-area isotherms at three different temperatures by using a film balance. The EG chain lengths were varied systematically between N = 3 and N = 15 repeating EG units. For the two longest EG chains (N = 12 and N = 15) a new synthesis is described. For short chains (N<9) the monolayer phase transition is determined by the alkyl chain moiety of the headgroup, while for N≥9 the typical behavior of lipopolymers is observed. For the fluid-gel phase transition the entropy and the corresponding latent heat were determined for 3, 6, and 9 EG lipids. In the second part the lipids were transferred to hydrophilic silicon oxide substrates by the Langmuir-Blodgett technique and characterized by their equilibrium swelling behavior under controlled humidity by using ellipsometry. In agreement with the monolayer experiments, we find a `polymer brush'-like behavior already at chain lengths of N≥12 despite the fact that the `statistical' limit N≫1 is hardly fulfilled. For degrees of polymerization of N = 3 and N = 6 EG units, relative small swelling ratios ρ are observed due to a `rigid rod'-like behavior. Between N = 6 and N = 9 repeating units an intermediate swelling behavior is found.