Langmuir films of dipalmitoyl phosphatidylethanolamine grafted polyethylene glycol). In-situ evidence of surface aggregation at the air-water interface

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 (pi-A), isotherms PE-PEG(100) and PE-PEG(5000) showed transitions (midpoints at pi(m,t1) similar to 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 nu(s)(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 nu(asym)(PO2-) PM-IRRAS signal around ti. The absence of a ti in PE-PEG(350) 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-PEG(1000) and PE-PEG(5000) around ti, possibly associated with gelation/dehydration of the polymeric layer and appearing at decreasing pi as the polymeric tail became longer. Another transition, t2, appearing in PE-PEG(350) and PE-PEG(1000) at pi(m,t2) = 29.4 and 34.8 mN/m, respectively, was associated with HC condensation and was impaired in PE-PEG(5000) 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. (C) 2016 Elsevier B.V. All rights reserved.