An Experimental Investigation on the Impact of Divalent Ions on the Emulsification of Oil: Comparison of Biodegradable (Green) and Petroleum-Based Surfactants

Emulsification of oil with surfactants often yields in complex colloidal systems which require understanding their physicochemical properties prior to their use in industry. This study aims to investigate the effects of divalent ions (calcium and magnesium) on the emulsification performance of biodegradable (green) surfactants of Triton™ CG-110 (alkyl polyglycoside, APG) and GreenZyme (GZ) at two temperature conditions (25°C and 70°C). For this purpose, the phase behaviour, interfacial tension (IFT), stability, droplet size distribution and rheological properties of APG and GZ microemulsion were evaluated. The outcomes were then compared with the emulsification performance of petroleum-based (synthetic) surfactants of alpha-olefin sulfonate (AOS) and cetyltrimethylammonium bromide (CTAB). Furthermore, the impacts of alkalis (ethanolamine and sodium carbonate) on microemulsions and the demulsification performance of a zwitterionic demulsifier (cocobetaine) were explored. The results show that only APG and CTAB could produce water in oil (W/O) microemulsions in the presence of divalent ions. The stability of APG and CTAB microemulsions was affected by divalent ions since the water segregation rate (WSR) index of these microemulsions went up from ≈ 21% to ≈ 85% as salinity increased up to 15 wt%. Also, the average droplet size of these microemulsions went up from 50-64 nm to 220-255 nm as salinity increased to 9 wt%. This is because the negative charge density on the microemulsion droplet surface decreases as the salinity increases, thus the electrostatic repulsive force between the droplets becomes weaker. In addition, APG and CTAB microemulsions showed a shear thinning behaviour, and their viscosity value increased with the increase in salinity. It is speculated that the presence of
divalent ions generates more friction between the molecules, which causes an increase in the viscosity of the microemulsions.