STRUCTURE OF PHOSPHOLIPID MONOLAYERS CONTAINING POLY(ETHYLENE GLYCOL)LIPIDS AT THE AIR-WATER-INTERFACE

The density distribution of a lipid monolayer al the air-water interfa ce mixed with varying amounts of lipid with poly(ethylene glycol) poly mer headgroups (polymer-lipid or PEG-lipid) was measured using neutron reflectometry. The structure of the monolayer at the interface was gr eatly perturbed by the presence of the bulky polymer-lipid headgroups resulting in a large increase in the thickness of the headgroup region normal to the interface and a systematic roughening of the interface with increasing polymer-lipid content. These results show how bulky hy drophilic moieties cause significant deformations and out-of-plane pro trusions of phospholipid monolayers and presumably bilayers, vesicles, and biological membranes. In terms of polymer physics, very short pol ymer chains tethered to the air-water interface follow scaling behavio r with a mushroom to brush transition with increasing polymer grafting density.