Physical Chemistry of Gemini Surfactant Monolayers

Abstract

The objective of this Ph.D. thesis research is to explore the effect of cation (Na+, Ca2+, and Fe3+) binding on monolayers comprised of two newly synthesized anionic gemini surfactants (Ace(n)-2-Ace(n), n = 11, 17; Figure 1) at the air-water and air-solid interfaces. The study was further aimed at exploring the miscibility and interactions of the Ace(n)-2-Ace(n) surfactants with perfluorotetradecanoic acid (C13F27COOH; PF) in monolayers. A combination of monolayer characterization methods, including surface pressure-area isotherms, Brewster angle microscopy (BAM) and atomic force microscopy (AFM) were used to explore the effect of ion-binding and mixing on the thermodynamics of mixing and the morphology of the monolayer films.

Figure 1: Chemical structure of Ace(n)-2-Ace(n) (n = 11,17). Ace(n)-2-Ace(n) monolayers exhibited significant expansion with Na+ and Ca2+ in the sub-phase, while with Fe3+ in the sub-phase, compressed films were observed. Apparent association constants (Kapp) for Ace(n)-2-Ace(n) were smaller than the comparator phospholipids systems reported in the literature, and gemini-cation binding stoichiometries of 2:1 and 1:1 for Ca2+ and Na+ respectively were found. Multimolecular aggregates of Ace(n)-2-Ace(n) bonded to cations were observed in BAM and AFM images. Results were discussed in the context of grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XR) measurements performed by other members of our research group. For the Ace(n)-2-Ace(n)-PF mixed monolayers, thermodynamic, morphological and X-ray scattering studies indicated that PF-Ace(12)-2-Ace(12) mixed films were miscible over a wide range of compositions at lower surface pressures. For PF-Ace(18)-2-Ace(18) mixed films, phase separation was detected at all surface pressures. GIXD and XR studies revealed that Ace(n)-2-Ace(n) forms amorphous, disordered monolayers at the air-water interface while PF forms a crystalline lattice with hexagonal symmetry. Overall the research in this thesis provides insight into fundamental interactions of this new class of gemini surfactants with metal cations in monolayers and an understanding of the physical and chemical factors that control mixing behavior in PF-Ace(n)-2-Ace(n) mixed monolayers. Future areas of research that are suggested by this work include developing new approaches for quantifying the extent of surfactant binding to metal ions in the monolayers and clarifying the precise nature of binding between Ace(n)-2-Ace(n) and metal cations.