Effect of aliphatic chain length on stability of poly(ethylene glycol)-grafted phospholipid monolayers at the air/water interface

The monolayer behavior of phosphatidylethanolamines bearing aliphatic chains of different length, C16 and C18, and grafted with PEG chains of molecular weight 2000, DPPE-PEG2000 and DSPE-PEG2000, respectively, was studied at the air/water interface. Comparative analysis of the surface pressure–molecular area (π–A) isotherms, Brewster angle microscopy (BAM) and atomic force microscopy (AFM) data has provided a new insight in the nature of the high-pressure transition observed in the isotherms of both DPPE-PEG2000 and DSPE-PEG2000 monolayers. This high-pressure transition appears in the low compressibility region of the two PEG2000–phospholipid isotherms at remarkably different surface pressures, at ∼26 and ∼19 mN/m for DPPE-PEG2000 and DSPE-PEG2000, respectively. The transition has been found to originate from the monolayer collapse. As visualized by BAM and AFM, fluid DPPE-PEG2000 and DSPE-PEG2000 monolayers are unable to sustain high pressures and collapse through the formation of micellar defects in the monolayer structure as a result of 2D to 3D relaxation of an overcompressed film.