Nematic ordering drives the phase separation of mixed monolayers containing phospholipids modified with poly(ethylene glycol) at aqueous–liquid crystal interfaces

Abstract

We report that homogeneous Langmuir monolayers formed from mixtures of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylethanolamine conjugated to poly(ethylene glycol) (M_w = 2000 g mol⁻¹, DPPE-PEG2000), when transferred onto interfaces formed between aqueous phases and nematic liquid crystals (LCs), phase separate in a manner that leads to patterned orientations of the LCs. Quantitative fluorescence microscopy establishes that the patterned orientations of the LCs correspond to coexisting interfacial phases that differ in total lipid density as well as lipid composition. Subsequent investigation determined that the interfacial phase separation of lipid is driven by the combined influence of the nematic ordering of the LC and the interactions between the PEG chains of DPPE-PEG2000. The latter effects impact the packing of the lipid monolayer and thus its interaction with the LC. In contrast to the phase separation of PEG-lipids within Langmuir films, where the repulsive interactions between PEG chains suppress phase separation at high interfacial densities of lipid, the results of our study indicate that repulsive interactions between PEG chains promote the phase separation of lipid at the aqueous–LC interface. Our results reveal a fundamentally new mechanism leading to control of the interfacial phase behavior of PEG-lipids, and suggest new principles for manipulation of the organization of multi-component lipid assemblies at interfaces.