Interaction between stabilized droplets of different phases in the same continuous phase of an aqueous three-phase system.

Water-in-water (W/W) emulsions, also called aqueous two-phase systems, are formed by mixing two incompatible polymers in water that phase separate into two distinct phases. They can be stabilized by addition of colloidal particles. Droplets of the dispersed phase can be used to compartmentalize ingredients and induce localized reactions. By mixing more types of incompatible polymers, emulsions containing droplets of different phases can be formed that can potentially capture different ingredients. Here the interaction between dispersed droplets of different types was studied by gently mixing a W/W emulsion containing droplets rich in dextran (DEX) dispersed in a continuous phase rich in polyethylene oxide with an emulsion containing droplets rich in fish gelatin (GEL) dispersed in the same continuous medium. Bis-hydrophilic microgels (MG) consisting of DEX grafted with poly(N-isopropylacrylamide) were added and their effect on the stability of each binary emulsion was investigated. Interestingly, when two very stable emulsions were gently mixed, droplets of different types were observed with confocal scanning laser microscopy to coalesce immediately upon contact. In this manner, Janus-type droplets were formed containing a DEX and a GEL compartment with no MG at the GEL/DEX interface that further associated into strings of alternating droplets. Contact angles between the different phases in emulsions with and without MG were compared and used to determine the effect of the microgels on the interfacial tension between the phases.

Effect of charge on the stabilization of water-in-water emulsions by thermosensitive bis-hydrophilic microgels.

HYPOTHESIS: Molecular surfactants are not able to stabilize water-in-water (W/W) emulsions, unlike nano or micro-particles, which can achieve this in some cases. However, the effect of electrostatic interactions between particles on the emulsion stability has rarely been investigated. We hypothesize that introducing charges modifies the stabilization capacity of particles and renders it both pH- and ionic strength-dependent. EXPERIMENTS: Charge was introduced into bis-hydrophilic and thermoresponsive dextran/polyN-isopropylacrylamide microgels by replacing a small fraction of polyN-isopropylacrylamide with acrylic acid groups. The size of the microgels was obtained by dynamic light scattering. The stability and microstructure of dextran/poly(ethyleneoxide)-based W/W emulsions, was studied as a function of pH, NaCl concentration and temperature using confocal microscopy and by analytical centrifugation. FINDINGS: The swelling degree of charged microgels depends on the pH, ionic strength and the temperature. In the absence of salt, charged microgels do not adsorb at the interface and have little stabilizing effect even after neutralization. However, the interfacial coverage and the stability increase with rising concentration of NaCl. Saltinduced stabilization of these emulsions was also observed at 50 °C. Increasing the temperature strongly influences the emulsion stability at low pH.