Freeze/thaw induced demulsification of water-in-oil emulsions with loosely packed droplets

Freeze/thaw treatment has been widely investigated for phase separation of oil-in-water (O/W) emulsions. However, it is a new application for destroying the inverted emulsions, water-in-oil (W/O) emulsions. In this study, freeze/thaw treatment was used to break the W/O emulsions with loosely packed droplets that were produced from the oils generally adopted as membrane phase in emulsion liquid membrane (ELM) process. The effects of emulsion system parameters and freezing conditions on demulsification performance were investigated. A near linear relationship was observed between demulsification performance and water content (30–65%). Demulsification performance greatly increased with the increase of droplet size (2.7–7.3 μm), while it was slightly affected by oil type. Four freezing methods were employed including freezing in refrigerator, cryogenic bath, dry ice and liquid nitrogen. The best freezing method for water removal was freezing in cryogenic or dry ice, and its efficiency was over 70% for all experimental systems with 60% water content regardless of droplet size and oil phase type. Furthermore, microscopic process of demulsification was monitored using optical microscope and the coalescence of droplets was simulated. From the experiments, the gradual demulsification process induced by freeze/thaw was proven and a collision mechanism was proposed. Meanwhile, the volume expansion of water turning to ice and interfacial tension of oil–water interface were determined as main driving forces of demulsification. The proposed mechanism and driving forces can explain the influences of various parameters on demulsification performance well.