Structure of Phospholipid Monolayers Containing Poly(ethylene glycol) Lipids at the Air−Water Interface

Abstract

The density distribution of a lipid monolayer at 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.