Abstract
Emulsions stabilized by solid nanoparticles, referred to as Pickering emulsions, are becoming increasingly important in applications as they are free of surfactants. However, the bulk properties and stability of Pickering emulsions are far from being well understood. In this work, the rheological behavior and catastrophic phase inversion of emulsions in the presence of starch nanoparticles were studied using in-situ measurements of viscosity and electrical conductivity. The aqueous phase consisting of starch nanoparticles was added sequentially in increments of 5% vol. to the oil phase under agitation condition to prepare the emulsions. The emulsions were water-in-oil (W/O) type at low to moderate concentrations of aqueous phase. At a certain critical volume fraction of aqueous phase, catastrophic phase inversion of W/O emulsion to oil-in-water (O/W) emulsion took place accompanied a sharp jump in the electrical conductivity and a sharp drop in the emulsion viscosity. The W/O emulsions were nearly Newtonian at low concentrations of aqueous phase. At high concentrations of aqueous phase, prior to phase inversion, the W/O emulsions exhibited a shear-thickening behavior. The O/W emulsions produced after phase inversion were shear-thinning in nature. The comparison of the experimental viscosity data with the predictions of emulsion viscosity model revealed only partial coverage of droplet surfaces with nanoparticles. With the increase in the concentration of starch nanoparticles (SNPs) in the aqueous phase of the emulsions, the phase inversion of W/O emulsion to O/W emulsion was delayed to higher volume fraction of aqueous phase. Thus SNPs imparted some stability to W/O emulsions against coalescence and phase inversion.
Highlights
Emulsions are defined as dispersions of two immiscible liquids
The catastrophic phase inversion of W/O emulsion to O/W type emulsion occurred at the aqueous phase concentration of 44.5% vol The W/O emulsions were nearly Newtonian up to an aqueous phase (0.25% wt starch nanoparticles (SNPs) dispersion) volume percent concentration of 29.5
At higher aqueous phase concentrations, the W/O emulsions showed some degree of shear-thickening, that is, the viscosity increased with the increase in shear rate
Summary
Emulsions are defined as dispersions of two immiscible liquids. They find applications in various fields such as petroleum, pharmaceuticals, foods, cosmetics, paints, etc. Some other emulsions (oil-in-oil and multiple) fall outside this classification but they are uncommon. In the absence of an effective interfacial stabilizer, the bare droplets of emulsions tend to coalesce readily. Emulsions separate out into oil and water phases when the mechanical agitation is stopped. In order to impart kinetic stability to emulsions various interfacial stabilizers such as surfactants and solid nanoparticles are utilized.
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