Demulsification of Bacteria-Stabilized Pickering Emulsions Using Modified Silica Nanoparticles

Abstract

Pickering emulsions stabilized by bacteria acting as particle emulsifiers are new platforms for microbial transformations of hydrophobic chemicals. However, their high stability often hampers demulsification during downstream processing. Since the existing methods (like addition of surfactants) to demulsify bacteria-stabilized Pickering emulsions have negative effects, new practical methods need to be developed. Here, using chemically modified fumed silica particles with different hydrophobicity, the demulsification of W/O Pickering emulsions stabilized by Mycobacterium neoaurum whole cells was first studied. The binary particle-stabilized emulsions exhibited phase inversion and dewatering induced by the coalescence of W/O emulsions or creaming of O/W emulsions. The silica particle hydrophobicity and concentration were the important parameters influencing the emulsion type, droplet morphology, and dewatering rate. The highest dewatering rate and largest droplet size were obtained at the inversion point from W/O to O/W. Confocal microscopy showed that no interaction between the bacteria and silica particles existed and the silica particle adsorption at the interface induced the detachment of bacteria from the interface, revealing that there was competitive adsorption between the binary particles at the interface. Based on these results, we suggested that the average hydrophobicity of the binary particles at the interface would determine the emulsion type and stability. Finally, this strategy was successfully applied to the demulsification of the Pickering emulsion formed during microbial transformation of sterols. Overall, this study provides a new strategy to demulsify Pickering emulsions by addition of another particle emulsifier. This is also the first example of separation of products as well as organic phases after microbial transformation in Pickering emulsions.

Correlation Relationship between Phase Inversion of Pickering Emulsions and Biocatalytic Activity of Microbial Transformation of Phytosterols

“…Around the phase inversion point, Pickering emulsions usually present as multiple unstable emulsions with good flowability. Methods of adjusting phase inversion range from the co-stabilization of Pickering emulsions by adding other soft or hard particles [25,27], to the modification of solid particles' wettability by physical ad-sorption [16,28,29], chemical graft modification [30,31], or even genetic manipulation of bacterial cells [32]. In addition to wettability, the concentration of solid emulsifiers is also a major factor influencing the phase inversion of Pickering emulsions [23].…”

Surfactants Influencing the Biocatalytic Performance of Natural Alkane-Degrading Bacteria via Interfacial Biocatalysis in Pickering Emulsions

“…In our previous studies, fumed SiO 2 particles have also been applied for regulating the stability of bacteria-stabilized Pickering emulsion. 11,39 In an alkane-water two-phase bioreactor, it is usually believed that the surfaces of bacterial cells are hydrophilic, and the bacterial cells disperse very well in the aqueous phase where microbial growth takes place. The chemicals dissolved in the oil phase or the oil phase itself can only be accessible to microbes in the aqueous phase by strongly stirring or the addition of dispersants.…”

Coalescence of immiscible sessile droplets on a partial wetting surface

“…Thus, these different characteristics might have significant influences on the short-time dynamics of coalescing immiscible droplets, as well as on the subsequent long-time behaviours of the coalesced droplets. However, despite wide applications in multiphase processing (Xie et al 2022), material synthesis (Winkelmann et al 2013) and biological interaction of cells (Kusumaatmaja, May & Knorr 2021), a clear understanding of the coalescence of immiscible sessile droplets is still missing.…”

“…However, despite wide applications in multiphase processing (Xie et al. 2022), material synthesis (Winkelmann et al. 2013) and biological interaction of cells (Kusumaatmaja, May & Knorr 2021), a clear understanding of the coalescence of immiscible sessile droplets is still missing.…”