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 interface mixed with varying amounts of lipid with poly(ethylene glycol) polymer headgroups (polymer-lipid or PEG-lipid) was measured using neutron reflectometry. The structure of the monolayer at the interface was greatly 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 hydrophilic moieties cause significant deformations and out-of-plane protrusions of phospholipid monolayers and presumably bilayers, vesicles, and biological membranes. In terms of polymer physics, very short polymer chains tethered to the air-water interface follow scaling behavior with a mushroom to brush transition with increasing polymer grafting density.