Mesh phases in tetramethylammonium perfluorodecanoate–water ternary systems

The evolution of the phase behaviour and lyotropic phase structures formed in the tetramethylammonium perfluorodecanoate (C₁₀TMA)–water system upon the addition of a third component (tetramethylammonium chloride (TMACl) or 1H, 1H-perfluoro-1-decanol (C₁₀ol)) have been studied using optical polarizing microscope, ²H NMR and small angle X-ray scattering (SAXS) in order to understand the correlated mesh phase with Rm space group. With increasing concentration of both additives, the Mh₁(Rm) phase changes to a random mesh phase Mh₁(0) via a two phase region. The structure of the Mh₁(Rm) phase is essentially unaltered with addition of TMACl or C₁₀ol until the phase change to Mh₁(0). With higher additive concentration, the water fraction within a mesh layer of the Mh₁(0) phase is decreased. It is proposed that the transition from the Mh₁(0) to Mh₁(Rm) phases is driven by a reduction in the electrostatic interlayer interaction enabling non-electrostatic repulsive interactions to dominate. This is because of the strong association of the TMA counter ion with the head group region, the increase of water within the mesh layers and the decrease in separation of the mesh layers in the Mh₁(Rm) phase. It is also shown that the hydrophobic counter ion has to be small relative to the surfactant for both mesh and correlated mesh to occur.