Behavior of DSPC/DSPE-PEG2000 mixed monolayers at the air/water interface

Lateral interactions in l-α-distearoyl phosphatidylcholine (DSPC)/distearoylphosphatidyl-ethanolamine-poly(ethylene glycol)2000 (DSPE-PEG2000) mixed monolayers were studied by measuring lateral film pressure–mean molecular area (Π–A) isotherms. Application of thermodynamic analysis to these isotherms reveals fundamental behavior of these mixed films on the water subphase. The results show that DSPC and DSPE-PEG2000 are miscible and exhibit nonideal mixed behavior at the interface. The mixed film are energetically stable if the molar fraction of DSPE-PEG2000 (XDSPE-PEG2000) is greater than 5 mol% and the most stable composition was found at XDSPE-PEG2000=7 mol%. Relaxation phenomenon was observed at constant film pressure. Compressibility for this system was estimated as well. It is concluded that mixed film are rather stable even at high surface pressure and the addition of DSPE-PEG2000 into DSPC monolayer make the film become more expanded and slight more compressible, except at XDSPE-PEG2000=5 mol%. The findings of this study corroborates well with those of the lipid–PEG2000 liposomes.