Double Phase Inversion of Emulsions Containing Layered Double Hydroxide Particles Induced by Adsorption of Sodium Dodecyl Sulfate

Abstract

A liquid paraffin-water emulsion was investigated using layered double hydroxide (LDH) particles and sodium dodecyl sulfate (SDS) as emulsifiers. Both emulsifiers are well-known to stabilize oil-in-water (o/w) emulsions. Surprisingly, a double phase inversion of the emulsion containing LDH particles is induced by the adsorption of SDS. At a constant LDH concentration, the emulsion is o/w type when SDS concentrations are low. At intermediate SDS concentrations, the first emulsion inversion from o/w to w/o occurs, which is attributed to the enhanced hydrophobicity of LDH particles caused by the desorption of the second layer of surfactant, leaving a densely packed SDS monolayer on the LDH exterior surfaces. The second inversion from water-in-oil (w/o) to o/w occurs at higher SDS concentrations, which may be due to the competitive adsorption at the oil/water interfaces between the LDH particles modified by the SDS bilayers and the free SDS molecules in the bulk solution, but the free SDS molecules dominate and determine the emulsion type. Laser-induced fluorescent confocal micrographs clearly confirm the adsorption of LDH particles on the surfaces of the initial o/w and intermediate w/o emulsion droplets, whereas no LDH particles were adsorbed on the final o/w emulsion droplet surfaces. Also, transmission electron microscopy (TEM) observations indicate that the shape of the final o/w emulsions is similar to that of the monomeric SDS-stabilized emulsion but different from that of the initial o/w emulsions. The adsorption behavior of SDS on LDH particles in water was investigated to offer an explanation for the emulsion double phase inversion. The zeta potential results show that the particles will flocculate first and then redisperse following surfactant addition. Also, X-ray diffraction (XRD) measurements indicate that SDS adsorption on the LDH interior surfaces will be complete at intermediate concentrations.