Action of hydrotropes in water-in-CO₂ microemulsions

The effects of a homologous series of sodium p-n-alkylbenzoate hydrotropes in water-in-supercritical CO₂ (w/c) microemulsions have been investigated, by comparing the phase behaviour and droplet structures obtained using small-angle neutron scattering (SANS). The w/c microemulsions appeared to be generally stable upon addition of hydrotropes, however, on increasing the alkyl chain length of the hydrocarbon and fluorocarbon moieties of the surfactants, different effects on stability were observed. Using high-pressure SANS (HP-SANS), the effects of hydrotrope type on the structures of microemulsion droplets were studied. Interestingly, evidence was found for multiple shell structures with a hydrotrope rich layer between the water cores and the surfactant films. These findings are relevant to the understanding of self-assembly of co-adsorbed species in supercritical CO₂ (scCO₂), as the hydrotrope layers potentially have significant effects on surfactant packing, and can modify the physico-chemical properties of scCO₂ through formation of worm-like micellar assemblies.