MICROSTRUCTURE OF CONCENTRATED PERFLUOROPOLYETHER SURFACTANTS IN WATER

The phase diagram of some perfluoropolyether (PFPE:) surfactants in wa ter has)seen recently characterized by optical microscopy and H-2 NMR. Here, the investigation focuses on the self-association structures Fo rmed at high surfactant concentration by the PFPE ammonium carboxylate s, having short (S1, av MW = 434), intermediate (S2, av MW = 722), and long (S3, av MW = 923) PFPE chains. It has been previously pointed ou t that lamellar (L(alpha)) liquid crystals (LC) mainly form in the cas e of S1 and S2, whereas inverse hexagonal (H-2) LC occur in the S3/wat er system only. In this work the N-14 and C-13 NMR relaxation rates ar e used to get a deeper insight on the organization of the surfactant i nterface and on the molecular dynamics of the aqueous concentrated pha ses. The packing features of the surfactant are discussed in relation with the structural and dynamic parameters calculated from the NMR exp eriments. A spontaneous curvature around zero seems to account well fo r the packing molecular constrains of S1 and S2, whereas S3 only fullf ills the packing requirements suitable to produce, at high surfactant concentration, and almost independently of the water content, a stable inverse curvature of the interface.