Doubly pH Responsive Emulsions by Exploiting Aggregation of Oppositely Charged Nanoparticles and Polyelectrolytes

Abstract

We present a simple and modular approach to realize highly stable pH responsive Pickering emulsion from mixtures of commercially available oppositely charged nanoparticle and polyelectrolyte. While highly charged nanoparticles and polyelectrolytes when used solely do not stabilize emulsions, we show that the electrostatic attraction between oppositely charged nanoparticles and polyelectrolytes can be exploited to formulate emulsions with long-term stability of up to 8 months. The Ludox CL nanoparticles and poly(4-styrenesulfonate) sodium salt (PSS) when dispersed in aqueous solution at pH 2-11 form particle polyelectrolyte complexes (PPCs) due to heteroaggregation. These complexes are effective in stabilizing oil-in-water Pickering emulsions. We demonstrate that this is due to the formation of weakly charged complexes that are surface active and hence readily adsorbed to the oil-water interface created during emulsification. We show that the composition of nanoparticles and polyelectrolytes in the mixture as well as the pH can be tuned to control the average diameter of the emulsions droplets. Immediate destabilization and doubled responsiveness of the emulsions stabilized by particle polyelectrolyte complexes are illustrated by changing the pH of the stable emulsions formed at intermediate pH to either 1 or 13. The aggregation behavior of nanoparticle-polyelectrolyte mixtures and the effect of various parameters such as mixing fraction, pH, and energy input on the formation of Pickering emulsions is discussed. Furthermore, we show that the formation of near charge neutral aggregates that exhibit optimal wetting conditions is a requirement to accomplish emulsion formation. The visualization of particle polyelectrolyte complexes around the emulsion droplets, their morphology prior to emulsification, and their wetting properties are also investigated to elucidate the mechanism of emulsification.

Redox-Responsive Pickering Emulsions Based on Silica Nanoparticles and Electrochemical Active Fluorescent Molecules

“…For example, in fuel production, oil transfer, and emulsion polymerization, the emulsion requires only temporary stabilization and subsequent demulsification. Therefore, the Pickering emulsions responding to various stimuli have received considerable attention in recent years. − Various triggers such as light irradiation, , pH, − CO 2 , − temperature, magnetic field, , and specific ion concentration have been reported. Among these triggers, CO 2 is widely studied because of its low cost, environment friendly, and easily removed from the system .…”

Porous Ceramics via Processable Pickering Emulsion Stabilized by Oppositely Charged Colloids

“…The emulsions formed in this composition range exhibit extraordinary stability of more than 12 months. The weakly charged or charge neutral heteroaggregates in the binary mixtures formed due to attractive electrostatic interaction readily adsorbed around the drop surface created during homogenization, thus forming stable Pickering emulsions. , The concentration of the surface-active heteroaggregates determines the final diameter of the emulsion drops formed because of arrested coalescence . In the intermediate composition (from Φ a = 0.2 to Φ a = 0.8), the diameters of the emulsion drops are in a narrow size ranging from 50 to 70 μm.…”

“…In such emulsion systems, the stability and size distribution of the emulsion droplets strongly depend on the microstructure and the dimensions of the heteroaggregates, which is a function of parameters such as the charge ratio, number ratio, size ratio, concentration, and the contact angles of the particles. 37,38 An understanding and optimization of the conditions to achieve controlled heteroaggregation are essential for the preparation of processable Pickering emulsion for the fabrication of porous ceramic structures.…”

“…When the particles available for emulsification are highly charged, surface charge neutralization can be achieved by the addition of particles of opposite polarity, which in turn can be exploited to facilitate the formation of Pickering emulsions. , Oppositely charged particles when dispersed in aqueous solution form aggregates because of electrostatic heteroaggregation, , which are effective in stabilizing the Pickering emulsions. In such emulsion systems, the stability and size distribution of the emulsion droplets strongly depend on the microstructure and the dimensions of the heteroaggregates, which is a function of parameters such as the charge ratio, number ratio, size ratio, concentration, and the contact angles of the particles. , An understanding and optimization of the conditions to achieve controlled heteroaggregation are essential for the preparation of processable Pickering emulsion for the fabrication of porous ceramic structures.…”

Temperature and CO2 Dual-Responsive Pickering Emulsions Using Jeffamine M2005-Modified Cellulose Nanocrystals

“…The stability of Pickering emulsions stabilized by Pickering emulsifiers based on electrostatic interactions is expected to be easily controlled by outside triggers, such as pH or CO 2 . For example, Shahid et al 56 reported a Pickering emulsion stabilized by the product of electrostatic binding between positively charged nanoparticles (SiO 2 @Al 2 O 3 ) and negatively charged polyelectrolytes (poly(4-styrenesulfonate) sodium salt), and the prepared Pickering emulsion was pH-responsive. Therefore, we speculate that the stability of the Pickering emulsion prepared from the electrostatic binding product of CNCs-M2005 is also pH-responsive.…”