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 attach ed to lipids were determined by analyzing pressure area isotherms at three different temperatures by using a film balance. The EG chain lengths were v aried systematically between N = 3 and N = 15 repeating EG units. For the t wo 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 al kyl chain moiety of the headgroup, while for N less than or equal to 9 the typical behavior of lipopolymers is observed. For the fluid-gel phase trans ition 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 hydro philic silicon oxide substrates by the Langmuir-Blodgett technique and char acterized by their equilibrium swelling behavior under controlled humidity by using ellipsometry. In agreement with the monolayer experiments, we fmd a "polymer brush "-like behavior already at chain lengths of N greater than or equal to 12 despite the fact that the "statistical" limit N >> 1 is har dly fulfilled. For degrees of polymerization of N = 3 and N = 6 EG units, r elative small swelling ratios rho are observed due to a "rigid rod"-like be havior. Between N = 6 and N = 9 repeating units an intermediate swelling be havior is found.