Double phase inversion of emulsions stabilized by a mixture of CaCO3 nanoparticles and sodium dodecyl sulphate

The interaction between unmodified CaCO3 nanoparticles and sodium dodecyl sulphate (SDS) in aqueous solution and the subsequent emulsification of the mixed dispersions are studied by means of zeta potential measurements, adsorption and emulsion characterization. Using toluene or octane as oil phase, hydrophilic CaCO3 nanoparticles stabilize only temporarily oil-in-water, O/W, emulsions. Upon the addition of trace amounts of SDS in the aqueous phase, the stability of the emulsions is greatly improved. Further increasing the SDS concentration results in a double phase inversion, O/W(1) to water-in-oil, W/O to O/W(2). It is suggested that the adsorption of SDS molecules on particle surfaces rendering them hydrophobic induces the first inversion. When the adsorption of SDS reaches a maximum, the surface of the particles are re-converted to hydrophilic due to a surfactant bilayer, and the equilibrium concentration of SDS in the continuous phase is high enough to stabilize a water continuous emulsion, promoting the second phase inversion. The O/W(2) emulsions however, are in general unstable, due to flocculation and sedimentation of the particles.