Evaluation of reverse micellar templated nickel carbonate nanospheres: exploration of its role in double-phase inversion Pickering emulsion

Reverse micellar templated nickel carbonate nanospheres are employed to prepare a double-phase inversion Pickering emulsion. Nickel carbonate nanospheres are synthesized within the core of reverse micelles, composed of Span 80/1-butanol/toluene/water. The evaluation of nanospheres within the core of reverse micelles was conducted using TEM and FESEM instrumentations. Micromorphological analysis of the results indicated a two-phase formation process for the nanospheres: nucleation phase (5–15 min) followed by a growth phase (15–60 min). At neutral pH (⁓ 6.16), the synthesized nanospheres are positively charged (+ 5.342 mV). Due to their extreme hydrophilic properties, they alone cannot provide the desired stability to O/W Pickering emulsions. With the assistance of negatively charged head groups, anionic surfactants like SDS and nickel carbonate nanospheres are involved in in situ hydrophobization through the electrostatic adsorption of surfactant molecules onto their surfaces. It eventually provides stability to the toluene–water Pickering emulsions, and an intriguing double-phase inversion is detected. The first-phase inversion (O/W to W/O) occurred due to the increased hydrophobicity of modified nickel carbonate nanospheres where SDS (≤ 7 mM) molecules formed a monolayer on the nickel carbonate surface. The second-phase inversion (W/O to O/W) is detected due to the bilayer adsorption (through tail-to-tail interaction) of SDS (> 7 mM) on nickel carbonate nanospheres, thereby converting the hydrophobic nickel carbonate nanocomposites to hydrophilic again. Consequently, the wettability of nickel carbonate-SDS nanocomposites can shift from hydrophilic to hydrophobic and back again to hydrophilic, triggering the evaluation of double-phase inversion Pickering emulsions.