Hexagonal and Lamellar Superstructure in DSPE-PEG1000 Monolayers at the Air/Water Interface

Inspired by diblock copolymer self-assembly, we study lipopolymer monolayers at the air/water interface. We investigated DSPE-PEG1000 (DSPE-EO₂₂) monolayers with alkyl chains in the liquid-condensed phase in dependence of the molecular area. Due to its conformational entropy, the moderately hydrophilic PEG has a larger area requirement than the alkyl chains in all-trans conformation. Small-angle grazing incidence X-ray diffraction (GID) measurements identified a hexagonal superstructure. The ordered alkyl chains form hydrophobic domains that are embedded in dissolved PEG. These domains consist of the alkyl chains of ≈200 PEGylated lipid molecules. During monolayer compression, the number of alkyl chains in a domain remains constant, while their area fraction increases. At an area fraction of 50%, a transition to a lamellar superstructure occurs. During this transition, the alkyl chain domains merge. This transition is attributed to the entropy loss of the laterally compressed PEG chains. Wide-angle GID reveals that the alkyl chains in the liquid-condensed phase possess the same small cross-sectional area (19.75 Ų) as those in DSPE monolayers, indicating that PEG has little influence on the liquid-condensed phase. The hexagonal superstructure was confirmed with AFM images.