Conditions for Equilibrium Solid-Stabilized Emulsions

Abstract

Particular types of solid-stabilized emulsions can be thermodynamically stable as evidenced by their spontaneous formation and monodisperse droplet size, which only depends on system parameters. Here, we investigate the generality of these equilibrium solid-stabilized emulsions with respect to the basic constituents: aqueous phase with ions, oil, and stabilizing particles. From systematic variations of these constituents, we identify general conditions for the spontaneous formation of monodisperse solid-stabilized emulsions droplets. We conclude that emulsion stability is achieved by a combination of solid particles as well as amphiphilic ions adsorbed at the droplet surface, and low interfacial tensions of the bare oil-water interface of order 10 mN/m or below. Furthermore, preferential wetting of the colloidal particles by the oil phase is necessary for thermodynamic stability. We demonstrate the sufficiency of these basic requirements by extending the observed thermodynamic stability to emulsions of different compositions. Our findings point to a new class of colloid-stabilized meso-emulsions with a potentially high impact on industrial emulsification processes due to the associated large energy savings.

Evolution of Equilibrium Pickering Emulsions—A Matter of Time Scales

“…11 Other stabilizing colloidal particles, such as Bindzil silica, cobalt ferrite, and gold also induce emulsification within a day. 3,11 The time scale for the formation of these solid-stabilized emulsions is linked to the time scale of partial hydrolysis and subsequent dissociation of the TPM molecules in the oil phase ( Figure 1). The dissociated hydrolyzed TPM molecules are amphiphilic and hence adsorb at the oil-water interface, lowering the interfacial tension.…”

“…The dissociated hydrolyzed TPM molecules are amphiphilic and hence adsorb at the oil-water interface, lowering the interfacial tension. 3,12 In the presence of 6 mM aqueous tetramethyl ammonium hydroxide (TMAH), the interfacial tension continuously decreases from 8 to 3 mN/m over the course of 24 h. 12 Therefore, using an oil with the much less readily hydrolyzing ethoxy 13 instead of methoxy groups, (3-methacryloxypropyl) triethoxysilane (MPTES), does not yield emulsification within a few days. The interfacial tension between MPTES and 6 mM aqueous TMAH is 17 mN/m immediately after mixing (Table S1, Supporting Information), too high for spontaneous emulsification in the presence of colloidal particles.…”

“…10 The samples were prepared by first filling glass tubes with the denser phase, after which droplets of the lighter phase were injected. Mixtures of MPTES and TPM at 70/30, 50/50, and 30/70 with densities close to 1 g/cm 3 were measured against an aqueous phase containing 75% w/w D 2 O (density of the mixture 1.08 g/cm 3 ). For mixtures of 10/90 TPM/MPTES, 50% D 2 O was used in the aqueous phase to obtain a density of 1.05 g/cm 3 .…”

“…Mixtures of MPTES and TPM at 70/30, 50/50, and 30/70 with densities close to 1 g/cm 3 were measured against an aqueous phase containing 75% w/w D 2 O (density of the mixture 1.08 g/cm 3 ). For mixtures of 10/90 TPM/MPTES, 50% D 2 O was used in the aqueous phase to obtain a density of 1.05 g/cm 3 . The angular velocity of the spinning tube was measured using an optical sensor.…”

“…2,3 Their equilibrium state is determined by the volume fractions of the basic components, oil and aqueous phase, the type and amount of stabilizer, the temperature, and the pH. For stabilization surfactants, cosurfactants, solid particles, salt, and combinations thereof may be employed.…”