A nanosheet-based combination emulsifier system for bulk-scale production of emulsions with elongated droplets and long-term stability

In this study, we investigate the emulsion type, droplet morphology and emulsion stability that can be achieved using a novel combination emulsifier system which consists of water-soluble α-zirconium phosphate tetrabutyl ammonium (α-ZrP-TBA) nanosheets as the particulate emulsifier and Span 80 as the oil-soluble emulsifier, with the two emulsifiers present in distinct phases. We find that water-in-oil emulsions were formed for water volume fractions < 0.3, and oil-in-water (O/W) emulsions were formed for water volume fractions > 0.3. This phase inversion point was found to be the same irrespective of whether the emulsifier was only Span 80 or only nanosheet or their combination. Varying the concentrations of either surfactant or the nanosheet did not alter the phase inversion point. The emulsions with the greatest stability against creaming were formed using the combination emulsifier and were found to be the O/W type, that were stable for 3 months. Interestingly, we find that droplets in these stable emulsions were nonspherical, with the aspect ratio of droplets increasing with nanosheet and Span 80 concentration, suggesting a synergistic interaction between the two emulsifiers to create elongated droplets. We perform capillary neck thinning experiments to show that the combination emulsifier system offers unique interfacial properties that are different from either emulsifier alone, suggesting that interfacial jamming could occur to produce elongated droplets. Taken together, our results suggest a novel route to bulk-scale production of O/W emulsions that have long-term emulsion stability with unique droplet morphology.