Interaction between colloidal silica and a nonionic surfactant hexagonal liquid crystalline phase

The hexagonal-micelle transition was investigated for Synperonic A7 at 40% w/w using rheology, microscopy, differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS). The pure phase showed signs of a two-step process indicated by inflections in SAXS peak height and interlayer spacing plots. There was also a small discrepancy between the transition temperature determined by rheology and that determined by microscopy. It was postulated that the two processes were the partial break up of the hexagonal phase leading to a loosening of the structure followed by the full conversion of the hexagonal phase to isotropic micelles. This idea was supported by the fact that the addition of silica considerably lowered the rheological transition temperature while having only a small effect on the microscopy value. The lowering in the rheology transition temperature was found to be much smaller at high ionic strength indicating an electrostatic mechanism involving the overlap of electrical double layers. A model was devised which explained the concentration of the silica on the formation of the hexagonal phase: a phenomenon observed experimentally by SAXS. (C) 2001 Elsevier Science B.V. All rights reserved.