CHARACTERIZATION OF WATER-IN-OIL MICROEMULSIONS AND ORGANO-GELS BASED ON SULFONATE SURFACTANTS

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(0)). Small-angle neutron scattering (SANS) was used to determine the structure of the microemulsion droplets as a function of w(0). All the systems studied consisted of spherical, polydisperse droplets whose radius was proportional to w(0). 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 ranged 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.