Demulsification of Surfactant-Stabilized Water-in-Oil (Cyclohexane) Emulsions using Silica Nanoparticles

Abstract

Efficient phase separation of oil and water in emulsions is critical for water treatment and hydrocarbon processing. Our research aims at elucidating the separation of water-in-oil emulsions using silica nanoparticles (SNPs). By probing the surfactant–nanoparticle interactions, we show that surfactant-stabilized emulsions can be destabilized depending on the nanoparticle wettability and the mode of nanoparticle addition. The efficiency of nanoparticles to demulsify surfactant-stabilized emulsions depended on both the nanoparticle and surfactant concentration. Water-in-oil emulsions were destabilized when partially hydrophobic nanoparticles were added to the surfactant-stabilized emulsion after emulsion formation (post-mixing). Hydrophilic and partially hydrophobic nanoparticles adsorb the surfactants via hydrogen bonding which in turn leads to depletion of surfactants at the oil–water interface. Upon the addition of hydrophilic nanoparticles, the preferential distribution of nanoparticles in the water phase led to lower adsorption of surfactants from the oil phase, resulting in inefficient destabilization as compared to that of partially hydrophobic nanoparticles. Water-in-oil emulsions were not destabilized upon post-mixing hydrophobic nanoparticles due to weak hydrophobic interactions between surfactants and hydrophobic nanoparticles. For a fixed concentration of nanoparticles of specific wettability, changing the mode of nanoparticle addition altered the flow behavior and the network strength of surfactant-stabilized water-in-oil emulsions.