Microemulsion-templated nickel hydroxide nanoparticles as efficient stabilizers for twofold phase inversion Pickering emulsions

Twofold phase inversion Pickering emulsions were successfully prepared using reverse micellar-templated nickel hydroxide nanoparticles in combination with two different anionic surfactants, sodium dodecyl sulfate (SDS) or sodium dodecylbenzene sulfonate (SDBS). The nickel hydroxide nanoparticles were synthesized within the reverse micellar core of a Span 80/1-butanol/hexane/water system. TEM and FESEM analyses confirmed the spherical morphology of the synthesized nanoparticles. The obtained nanoparticles exhibited a positive surface charge of +24.67 mV at near-neutral pH (∼6.76). Due to their highly hydrophilic nature, the nanoparticles alone produced only unstable oil-in-water (O/W) Pickering emulsions. However, in the presence of an anionic surfactant (SDS or SDBS), the nickel hydroxide nanoparticles were in situ hydrophobized through electrostatic adsorption of negatively charged surfactant head groups onto the positively charged nanoparticle surfaces. The first phase inversion from oil-in-water (O/W) to water-in-oil (W/O) occurred as a consequence of the enhanced hydrophobicity of the modified nanoparticles, where SDS (≤10 mM) or SDBS (≤6 mM) formed a surfactant monolayer on the nanoparticle surface. At higher surfactant concentrations, bilayer adsorption arising from tail-to-tail interactions of SDS (>10 mM) or SDBS (>6 mM) restored the hydrophilic character of the nickel hydroxide nanocomposites, leading to a second phase inversion from W/O back to O/W. The observed twofold phase inversion behavior originates from reversible wettability transitions of SDS@Ni(OH)₂ and SDBS@Ni(OH)₂ nanocomposites, which undergo hydrophilic-to-hydrophobic transformation followed by recovery of hydrophilicity at higher surfactant concentrations.