Microstructure of Concentrated Perfluoropolyether Surfactants in Water

• Published in: Langmuir Vol. 12; no. 25; pp. 6022 - 6027
• Main Authors: Caboi, F, Chittofrati, A, Monduzzi, M, Moriconi, C
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 11.12.1996
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Surface physical chemistry; Surface-active agents: properties; Ethylene oxide polymer; Organic perhalocompound; Phase diagram; Non ionic surfactant; NMR spectrometry; Aqueous solution; Experimental study; Microstructure
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la960582r

The phase diagram of some perfluoropolyether (PFPE) surfactants in water has been recently characterized by optical microscopy and 2H NMR. Here, the investigation focuses on the self-association structures formed at high surfactant concentration by the PFPE ammonium carboxylates, having short (S1, av MW = 434), intermediate (S2, av MW = 722), and long (S3, av MW = 923) PFPE chains. It has been previously pointed out that lamellar (Lα) liquid crystals (LC) mainly form in the case of S1 and S2, whereas inverse hexagonal (H2) LC occur in the S3/water system only. In this work the 14N and 13C NMR relaxation rates are used to get a deeper insight on the organization of the surfactant interface and on the molecular dynamics of the aqueous concentrated phases. The packing features of the surfactant are discussed in relation with the structural and dynamic parameters calculated from the NMR experiments. A spontaneous curvature around zero seems to account well for the packing molecular constrains of S1 and S2, whereas S3 only fullfills the packing requirements suitable to produce, at high surfactant concentration, and almost independently of the water content, a stable inverse curvature of the interface.