Deep Eutectic Solvents Based on Natural Ascorbic Acid Analogues and Choline Chloride.

Deep eutectic solvents (DES) are one of the most promising green technologies to emerge in recent years given their combination of environmentally friendly credentials and useful functionalities. Considering the continued search for new DES - especially those that exemplify the aforementioned characteristics, we report the preparation of DES based on natural analogues of l-ascorbic acid for the first time. The onset of eutectic melting occurred at temperatures far below the melting point of the individual components and resulted in the generation of glass forming fluids with glass transition temperatures, viscosities and flow behavior that are comparable to similar systems. This work expands the current array of DES that can be produced using naturally occurring components, which given their potential to be bio-derived, interesting physicochemical properties (e. g. propensity to supercool and vitrify) and apparent antibacterial nature, may provide utility within a range of applications.

Water-in-oil Pickering emulsions stabilized by an interfacial complex of water-insoluble polyphenol crystals and protein.

Long term stabilization of water-in-oil (W/O) emulsions remains a particularly challenging problem in colloid science. Recent studies have shown that polyphenols act as Pickering stabilizers at the water-oil interface. In this work we propose a novel way to stabilize water droplets via interfacial complex formation. It was observed that polyphenol crystals (curcumin or quercetin) absorb at the interface and provide stabilization of water droplets for several days; however formation of a polyphenol- whey protein (WPI) complex at the water-oil interface revealed a pronounced improvement in the stabilization. The mechanism of complex formation was tested by subjecting the systems to different environmental conditions, such as ionic strength and temperature. The evidence suggests that the complex is probably stabilized by electrostatic attraction between the oppositely-charged polyphenol particles and protein at the interface, although hydrogen bonding between the two components may also contribute. The resulting stable water droplets have a Sauter mean diameter (D3,2) of approximately ∼22 and ∼27 µm for curcumin and quercetin systems, respectively. Emulsions were more stable at pH 3 than at pH 7, due to either weaker complex formation at pH 7 and/or chemical degradation of the polyphenols at this more alkaline pH. Interfacial shear viscosity measurements confirmed that there was strong interfacial complex formation with aqueous WPI concentrations of ∼0.5 wt.%.

Stability and rheology of emulsions containing sodium caseinate: combined effects of ionic calcium and alcohol.

We have investigated the combined effect of ionic calcium and ethanol on the visual creaming behavior and rheology of sodium caseinate-stabilized emulsions (4 wt% protein, 30 vol% oil, pH 6.8, mean droplet diameter 0.4 microm). A range of ionic calcium concentrations, expressed as a calcium/caseinate molar ratio R, was adjusted prior to homogenization and varying concentrations of ethanol were added shortly after homogenization. A stability map was produced on the basis of visual creaming behavior over a minimum period of 8 h for different calcium/caseinate/ethanol emulsion compositions. A single narrow stable (noncreaming) region was identified, indicating limited cooperation between calcium ions and ethanol. The shear-thinning behavior of the caseinate-stabilized emulsions is typical of systems undergoing depletion flocculation. Addition of calcium ions and/or ethanol was found to lead to a pronounced reduction in viscosity and the onset of Newtonian flow. The state of aggregation was correlated with emulsion microstructure from confocal laser scanning microscopy. Time-dependent rheology (18 h) with a density-matched oil phase (1-bromohexadecane) revealed that the visually stable emulsions were time-independent low-viscosity fluids. Surface coverage data showed that increasing amounts of caseinate were associated with the oil-water interface with increasing R and ethanol content. A decrease in free calcium ions in the aqueous phase with moderate increases in R and ethanol content was observed, which is consistent with greater calcium-caseinate binding (aggregation). Ostwald ripening occurred at the high-ethanol emulsion compositions that were stable to depletion flocculation. While the coarsening rate was low, this can account for the cream plug formation observed during gravity creaming experiments. The caseinate emulsion with no ionic calcium or ethanol exhibits depletion flocculation from excess nonadsorbed caseinate submicelles. Addition of calcium ions reduces the submicelle number density via specific calcium-binding in the aqueous phase (fewer, larger calcium-caseinate aggregates) and at the droplet surface (increased surface coverage). Nonspecific ethanol-induced (calcium-dependent) caseinate submicelle aggregation in the bulk phase and on the droplet surface (increased surface coverage) culminates in a reduction in the number density of caseinate submicelles. A narrow window of inhibition of depletion flocculation occurs in systems containing both calcium ions and ethanol, both species combining to aggregate the protein and so reduce the density of free submicelles.