Capsules from Pickering emulsion templates

Following the resurgence of interest in particle-stabilised or Pickering emulsions recently, the preparation of capsules from such templates has become feasible. We review some of the recent activity in this area and focus on both the methods used to reinforce the particle shell and the applications of these novel capsules, which may be empty or liquid-filled. The methods reported include interfacial polymerisation or crystallisation, interfacial cross-linking, the use of a sacrificial template, formation of polymer layers, gelation and evaporation from multiple emulsions. Potential applications are in the areas of wastewater treatment, perfume encapsulation, drug/dye release and as self-healing coatings

Emulsion stabilisation by complexes of oppositely charged synthetic polyelectrolytes

We investigate the possibility of stabilising oil-water emulsions from the polyelectrolyte complexes (PEC) obtained in mixtures of a strong cationic polyelectrolyte (poly(diallyldimethylammonium chloride), PDADMAC) and a weak anionic one (poly(acrylic acid) sodium salt, PAANa). Unlike other previous work however, both polyelectrolytes (PEL) are chosen as they are completely water-soluble and possess no surface activity when present alone over nearly all the pH range. In water, the effects of PEL concentration, PEL mixing ratio and pH on the formation of PEC are studied in detail. At low pH where the anionic PEL is uncharged, complex coacervation occurs in which droplets rich in both polymers are dispersed in water. At intermediate pH, the PEC comprise a mixture of coacervate droplets and solid particles. At high pH where the anionic PEL is significantly charged, only complex coacervation is observed. On addition of dodecane followed by homogenisation, no stable emulsions arose from dispersions containing solid particle PEC due to either the large precursor particle aggregates or their inherent hydrophilicity. By contrast, oil-in-water emulsions stable to coalescence could be prepared from coacervate dispersions. We discuss the feasibility of the coacervate phase spreading at the oil-water interface in terms of the relevant spreading coefficients and compare the predictions with experiment for a range of oils. We encounter oils whose drops become engulfed by the coacervate phase as well as oils where no engulfing occurs

Catalysis in Pickering emulsions

Particle-stabilised or Pickering emulsions are versatile systems. In the past 10 years a new application has emerged in the field of catalysis to use them as vehicles to carry out catalytic reactions, allowing a more environmentally friendly process with high conversions and selectivities and important advantages for catalyst recovery. As the area has advanced rapidly, the intention of this review is to summarize the latest innovations being reported. An overview is given regarding the kinds of liquid phases comprising the emulsion system, the different types of solid particle stabilizers (whether they contain catalyst or not) and the catalytic reactions studied. A section describing methods for recovering the catalyst is also included, in which various stimuli are discussed. Finally, the importance of using Pickering emulsions to carry out reactions in flow and in multi-step cascade systems is highlighted with various examples to support the benefits of transferring this technology to industrial processes

Emulsions stabilized with polyelectrolyte complexes prepared from a mixture of a weak and a strong polyelectrolyte

The possibility of stabilizing emulsions with polyelectrolyte complexes (PEC) obtained from the interaction of two non-surface-active oppositely charged polyelectrolytes (PEL) is described. Poly(allylamine hydrochloride) (PAH) and poly(4-styrene sulfonate) sodium salt are selected as the weak cationic and the strong anionic polyelectrolyte, respectively. Aqueous polymer mixtures are investigated by light scattering to determine the size of the complexes and whether precipitation or complex coacervation occurs. The effects of PEL mixing ratio, pH, and PEL concentration are studied in detail. By increasing the pH, the transition precipitate-precipitate/coacervate-coacervate-polymer solution is observed. At low pH, both PEL are fully ionized and therefore precipitates (soft particles) arise as a result of strong electrostatic interactions. By increasing the pH, the degree of ionization of PAH decreases and weak electrostatic interactions ensue, supporting the formation of coacervate droplets. The most stable oil-in-water emulsions are prepared from aqueous mixtures around charge neutralization. Although emulsions can be prepared from coacervate droplet dispersions, their coalescence stability is worse than those stabilized by soft PEC particles. By increasing the PEL concentration, the average droplet diameter decreases and the fraction of cream in the emulsion increases for emulsions prepared with PEC particles, following the limited coalescence model. However, at high concentrations, emulsion stability is slightly worse probably due to extensive aggregation of the particles. Viscous high internal phase emulsions can be prepared at low pH in which oil droplets are deformed. Here, PEC particles are detected only at the oil-water interface. At lower oil content, excess particles form a network in the aqueous phase aiding emulsion stability to coalescence

Effect of Particle Wettability and Particle Concentration on the Enzymatic Dehydration of n-Octanaloxime in Pickering Emulsions

© 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Pickering emulsion systems have emerged as platforms for the synthesis of organic molecules in biphasic biocatalysis. Herein, the catalytic performance was evaluated for biotransformation using whole cells exemplified for the dehydration of n-octanaloxime to n-octanenitrile catalysed by an aldoxime dehydratase (OxdB) overexpressed in E. coli. This study was carried out in Pickering emulsions stabilised solely with silica particles of different hydrophobicity. We correlate, for the first time, the properties of the emulsions with the conversion of the reaction, thus gaining an insight into the impact of the particle wettability and particle concentration. When comparing two emulsions of different type with similar stability and droplet diameter, the oil-in-water (o/w) system displayed a higher conversion than the water-in-oil (w/o) system, despite the conversion in both cases being higher than that in a “classic” two-phase system. Furthermore, an increase in particle concentration prior to emulsification resulted in an increase of the interfacial area and hence a higher conversion

Novel stabilisation of emulsions by soft particles: polyelectrolyte complexes

We put forward the concept of a novel particle stabiliser of oil-water emulsions being the polyelectrolyte complex (PEC) formed between oppositely charged water-soluble polymers, in cases where either polymer alone is incapable of stabilising an emulsion. Using poly(4-styrene sulfonate) sodium salt, PSSNa and poly(diallyldimethylammonium chloride), PDADMAC, of low polydispersity and similar molecular mass, we correlate the behaviour of their mixtures in water with that of emulsions after addition of oil. In aqueous mixtures, spherical particles of diameter between 100 and 150 nm are formed through electrostatic interactions between charged polymer chains. Around equal mole fractions of the two polymers, the zeta potential of particles reverses in sign and emulsions of oil-in-water (o/w) for a range of oils can be prepared which are the most stable to coalescence and creaming. Effects of PEC concentration and oil:water ratio have been examined. All emulsions are o/w and stability is achieved by close-packed particle layers at drop interfaces and particle aggregation in the continuous phase. Increasing the salt concentration initially causes destabilisation of the aqueous particle dispersion due to particle aggregation followed by dissolution of particles at high concentrations; the corresponding emulsions change from being stable to completely unstable and are then re-stabilised due to adsorption of uncharged individual polymer molecules

Effect of Particle Wettability and Particle Concentration on the Enzymatic Dehydration of n-Octanaloxime in Pickering Emulsions

Binks BP, Bago Rodriguez AM, Schober L, Hinzmann A, Gröger H. Effect of Particle Wettability and Particle Concentration on the Enzymatic Dehydration of n-Octanaloxime in Pickering Emulsions. Angewandte Chemie International Edition. 2021;60(3):1450-1457.Pickering emulsion systems have emerged as platforms for the synthesis of organic molecules in biphasic biocatalysis. Herein, the catalytic performance was evaluated for biotransformation using whole cells exemplified for the dehydration of n -octanaloxime to n -octanenitrile catalysed by an aldoxime dehydratase (OxdB) overexpressed in E. coli . This study was carried out in Pickering emulsions stabilised solely with silica particles of different hydrophobicity. We correlate, for the first time, the properties of the emulsions with the conversion of the reaction, thus gaining an insight into the impact of the particle wettability and particle concentration. When comparing two emulsions of different type with similar stability and droplet diameter, the oil-in-water (o/w) system displayed a higher conversion than the water-in-oil (w/o) system, despite the conversion in both cases being higher than that in a "classic" two-phase system. Furthermore, an increase in particle concentration prior to emulsification resulted in an increase of the interfacial area and hence a higher conversion. © 2020 Wiley-VCH GmbH

Janus and patchy nanoparticles: general discussion

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