Effects of Salinity and pH on the Stability of Clay-Stabilized Oil-in-Water Pickering Emulsions

Abstract

Summary Most emulsions are thermodynamically unstable and will lose their effectiveness once the emulsion droplets experience creaming or coalescence, which constrains potential further and wide application. Therefore, finding a stable emulsion system is important. In this work, a systematic study was carried out on the use of an oil-in-water (O/W) Pickering emulsion stabilized by sodium (Na)-montmorillonite particles. The particle-size distribution and zeta potential of Na-montmorillonite particle solutions at 0 to 20 wt% sodium chloride (NaCl) and pH of 3.3 to 11 were characterized first. The emulsifying ability of the clay particles and stability of O/W emulsions against creaming and coalescence at a set of salinities and pH were measured to provide suitable salinity and pH range for the emulsions. Results show that the Na-montmorillonite particles can be used as an effective stabilizer for O/W emulsion. Because of the decreased zeta potential, the clay particles are more likely to adsorb on the oil/water interface to form emulsions with smaller droplet size in an acidic and high-salinity environment. At a fixed pH, the emulsion tends to be unstable against creaming at less than a certain critical salinity, which is usually 0.25 wt% NaCl. At the critical salinity, the clay particles form a three-dimensional (3D) clay network to effectively stabilize the emulsion from creaming. This clay network interconnects the emulsion droplets, thus hindering the upward movement of the droplets because of buoyancy. At salinities higher than the critical salinity, the emulsion stability against creaming gradually decreases with salinity because of the compaction of the 3D clay network. At a fixed salinity, the creaming stability also has a similar critical pH, which increases with salinity. In addition, the O/W emulsion droplets are more stable against coalescence under more acidic and saline conditions. This results from denser clay particle adsorption on the droplet surface and a greater restriction on droplet motion and collision by the clay structure. Three 2D diagrams showing clear trend of clay emulsifying ability, long-term emulsion stability against creaming and coalescence at a range of salinity from 0 to 20 wt% NaCl, and at a range of pH from 3.3 to 11 were achieved to provide appropriate emulsions.