Synergism and formation of vesicle gels in salt-free catanionic hydrocarbon/fluorocarbon surfactant mixtures

Abstract

The surface and bulk properties of salt-free catanionic surfactant mixtures of a zwitterionic surfactant (tetradecyldimethylaminoxide, C₁₄DMAO), and two partially fluorinated carboxylic acids (C₆F₁₃C₂H₄OC₂H₄COOH, TFOPA and C₈F₁₇C₂H₄OC₂H₄COOH, HFDPA) in aqueous solutions were studied. Surface tension measurements showed that strong synergism existed when C₁₄DMAO was mixed with TFOPA or HFDPA, which induces a lower critical micellar concentration (cmc) and a lower surface tension at cmc (γ_cmc) compared to the individual surfactants. The phase behavior of both C₁₄DMAO/TFOPA and C₁₄DMAO/HFDPA systems in bulk aqueous solution is rich. When the molar ratio of TFOPA (or HFDPA) to C₁₄DMAO was increased, an isotropic L₁ phase, an L₁/L_α two-phase region, a single L_α phase, and an L₁/precipitate two-phase region, were successively observed. At a C₁₄DMAO concentration of 300 mmol L⁻¹, the formation of gels that contain densely packed vesicles occurred. Rheological measurements showed that the viscoelasticity of the vesicle gels depends on the mixing molar ratio of the anionic surfactant to C₁₄DMAO and total surfactant concentration, but is almost independent of temperature. IR spectra indicate that the hydrocarbon chains adopt a highly ordered all-trans conformation while the fluorinated chains adopt a disordered gauche conformation. Differential scanning calorimetry (DSC) measurements showed that the alkyl chains are mainly in a liquid crystalline state at room temperature.