Langmuir films of dipalmitoyl phosphatidylethanolamine grafted poly(ethylene glycol): In-situ evidence of surface aggregation at the air-water interface

Abstract

The molecular packing-dependent interfacial organization of polyethylene glycol grafted dipalmitoylphosphatidylethanolamine (PE-PEGs) Langmuir films was studied. The PEG chains covered a wide molecular mass range (350, 1000 and 5000 Da).In surface pressure-area (π-A), isotherms PE-PEG1000 and PE-PEG5000 showed transitions (midpoints at πm,t1 ∼ 11 mN/m, "t1"), which appeared as a long non-horizontal line region. Thus, t1 cannot be considered a first-order phase transition but may reflect a transition within the polymer, comprising its desorption from the air-water interface and compaction upon compression. This is supported by the increase in the νs(C-O-C) PM-IRRAS signal intensity and the increasing surface potentials at maximal compression, which reflect thicker polymeric layers. Furthermore, changes in hydrocarbon chain (HC) packing and tilt with respect to the surface led to reorientation in the PO2− group upon compression, indicated by the inversion of the νasym(PO2−) PM-IRRAS signal around t1. The absence of a t1 in PE-PEG350 supports the requisite of a critical polymer chain length for this transition to occur. In-situ epifluorescence microscopy revealed 2D-domain-like structures in PE-PEG1000 and PE-PEG5000 around t1, possibly associated with gelation/dehydration of the polymeric layer and appearing at decreasing π as the polymeric tail became longer.Another transition, t2, appearing in PE-PEG350 and PE-PEG1000 at πm,t2 = 29.4 and 34.8 mN/m, respectively, was associated with HC condensation and was impaired in PE-PEG5000 due to steric hindrance imposed by the large size of its polymer moiety.Two critical lengths of polymer chains were found, one of which allowed the onset of polymeric-tail gelation and the other limited HC compaction.