Characterization of Polymer-Lipid Interfaces by Neutron Reflectivity Gregory S. Smith and Jaroslaw Majewski, MLNSC, Los Alamos National Laboratory, Los Alamos, NM 87545, Tonya Kuhl, and Jacob Israelachvili, Department of Chemical and Nuclear Engineering, University of California, Santa Barbara, CA 93106.

To gain insight into the physical properties of lipid-polymer liposomes, used in advanced drug delivery, we performed a series of experiments on the time-of-flight neutron reflectometer SPEAR at Manuel Lujan Jr. Neutron Scattering Center. We studied mixtures of deuterated or hydrogenated distearoyl phosphatidyl ethanolamine (DSPE) matrixed with 1-9% of the same lipid but modified by covalently linking polyethylene glycol (PEG, 2000-5000 MW) chains to its head group (DSPE-PEG). Both in-situ Langmuir monolayers and Langmuir-Blodgett monolayers deposited on a silane coated quartz substrate were studied. Three concentrations 1.3, 4.5, and 9% DSPE-PEG were investigated enabling the polymer surface coverage to be systematically varied from non-overlapping mushrooms to the polymer brush regime. The measured reflectivity profiles show that both the density and extension of the polymer segments increase with DSPE-PEG concentration, and can be well modeled with a parabolic profile which includes a polymer depletion layer at the lipid-water interface. Moreover, we find that the roughness of the water/polymer-lipid/air increases with the DSPE-PEG concentration, indicating that the bulky polymer disrupts the order in the lipid monolayer; whereas, this effect is less pronounced when the DSPE-PEG is deposited on a solid substrate.