Mixing Behavior in Binary Anionic Gemini Surfactant–Perfluorinated Fatty Acid Langmuir Monolayers

The miscibility and film structure of mixed Langmuir monolayer films composed of an anionic gemini N,N,N′,N′-dialkyl-N,N′-diacetate ethylenediamine surfactant (Ace(12)-2-Ace(12)) with perfluoro­tetradecanoic acid (C₁₃F₂₇COOH; PF) have been investigated using a variety of thermodynamic and structural characterization methods. The two film components were found to be miscible in monolayers at the air–water interface over a range of compositions and at all but the lowest surface pressures, with attractive interactions occurring between the two components. While pure PF monolayers formed crystalline lattices with hexagonal symmetry and with the surfactant tails oriented normal to the underlying water subphase, the pure gemini surfactant formed amorphous films with little tendency to orient at the subphase. In mixed films with mole ratios of PF:Ace(12)-2-Ace(12) < 2.5, the miscibility of the two components resulted in a nearly complete loss of crystallinity of the PF, though films at higher mole fractions of PF showed some residual crystallinity, albeit with lattice structures that were significantly different from that of pure PF. Miscibility and film structure in this mixed system are discussed in comparison with other mixed gemini surfactant systems in the literature as well as binary mixtures of phospholipids or monomeric fatty acids with PF.