Pickering Emulsions Stabilized by Monodisperse Latex Particles:  Effects of Particle Size

The preparation, type, and stability of emulsions of oil and water stabilized solely by spherical, monodisperse polystyrene latex particles of different size is described. Two types of behavior occur depending on whether particles remain intact (in the case of cyclohexane) or dissolve to give free polymer chains (in the case of toluene). Emulsions formed with cyclohexane and either “hydrophilic” aldehyde/sulfate particles or “hydrophobic” sulfate particles are water-in-oil (w/o) over a wide range of salt concentrations and water volume fractions. Average emulsion drop diameters initially increase from 35 to 75 μm with increasing particle diameter and then remain constant. Although such emulsions sediment, there is no sign of coalescence for over 6 months. We show evidence of the transition from nonflocculated to flocculated emulsions upon increasing the water volume fraction, as predicted theoretically for charged drops in oil. By use of toluene and “hydrophilic” particles however, emulsions can be inverted from oil-in-water (o/w) to w/o with increasing salt concentration. The concentration of salt required to screen the repulsions between negatively charged adsorbed polymers increases with initial particle size as the average molecular weight also increases. Water-in-oil emulsions, of around 1 μm diameter, are stable to coalescence for long periods.