Characterisation of water-in-oil microemulsions and organo-gels based on sulphonate surfactants

Abstract

Microemulsion systems containing two different anionic sulphonate surfactants have been prepared and characterised. The surfactants — a di-alkylsulphosuccinate and a tri-alkylsulphocarballylate — are analogous to the better-known Aerosol-OT (AOT) molecule and show similar phase-stability behaviour with respect to temperature and water content (expressed as [water]/[surfactant] or w o). Small-angle neutron scattering (SANS) was used to determine the structure of the microemulsion droplets as a function of w o. All the systems studied consisted of spherical, polydisperse droplets whose radius was proportional to w o. Adding gelatin to AOT microemulsions can cause gelation under certain conditions. Gelation by gelatin is also observed for some of the microemulsion systems stabilised by the surfactants introduced here. A generally-applicable mechanism for gelation of oil-continuous microemulsion systems involving droplet percolation coinciding with the gelatin helix-coil transition is proposed. SANS was used to characterise the structures of the gelled systems and a variety of configurations were found depending on the composition of the parent microemulsions. Structures range from a fractal-like aggregate of droplets to a network of gelatin + water rods coexisting with microemulsion droplets, all stabilised by a monolayer of surfactant molecules.