Phase Inversion of Colored Pickering Emulsions Stabilized by Organic Pigment Particle Mixtures

Abstract

Pickering emulsions stabilized by a mixture of colored organic pigment particles of different hydrophobicity have been investigated at equal oil/water ratio and a fixed overall particle concentration by several complementary methods. Transitional phase inversion of emulsions from water-in-oil to oil-in-water can be effected by increasing the fraction of hydrophilic pigment orange in mixtures with either hydrophobic pigment yellow, red, indigo, or blue. In two mixtures, we find that a distinct change in the color of emulsions occurs at phase inversion. Although the fraction of pigment orange required depends on the particular hydrophobic pigment selected, phase inversion occurs at a similar surface energy of the particle mixture, which lies between that of the hydrophilic pigment and those of the hydrophobic pigments. We show that both pigment types are present at the oil-water interface simultaneously, giving rise to emulsions that are extremely stable to coalescence. The average drop size for both emulsion types increases toward phase inversion, in line with an increase in the extent of sedimentation/creaming at long times.

Amphiphilic Cellulose Nanocrystals for Enhanced Pickering Emulsion Stabilization

“…Emulsions stabilized by solid particles, known as Pickering emulsions, have attracted increasing attention as alternatives to surfactant-stabilized emulsions. − They require less emulsifier, benefit from increased stability, and possess other advantages when compared to emulsions stabilized by surfactants. − Pickering emulsions perform better over classical emulsions because the solid particles are irreversibly adsorbed onto the oil–water interface, and subsequently form an effective electro-steric protective shield for the emulsified droplets. − While in surfactant-based emulsions, thermodynamic equilibrium is usually observed, which means that the stabilizer molecules are exchanging between oil–water interface and bulk continuous phase …”

Illuminating the Impact of Submicron Particle Size and Surface Chemistry on Interfacial Position and Pickering Emulsion Type

“…Inverting the type of a Pickering emulsion conventionally requires significant manipulation of the system components. It has been acknowledged that inversion of Pickering emulsion type can be achieved through multiple handles such as altering the oil–water ratio, particle concentration, grafting functionalization, particle roughness, or the addition of a surfactant to modify particle wettability. − However, by utilizing less invasive handles we can feasibly create a series of stable Pickering emulsions from the same components but with different microstructures.…”

Phase Inversions Observed in Thermoresponsive Pickering Emulsions Stabilized by Surface Functionalized Colloidal Silica

“…36 This has been done in Pickering systems by using mixtures of particles as well as of particles and surfactants, where adsorption of surfactants onto the particle surface alters θ ow . 5,37,38 However, added surfactants can also adsorb at the oil/water interface and compete with the particles, causing antagonistic effects. 39 Controlled phase separations, achieved through the utilization of switchable surfactants, generating switchable Pickering emulsions, have also been reported.…”

“…Transitional (reversible) phase inversion is achieved by changing the chemical conditions, for instance by altering the surface affinity of the emulsifier but at a fixed oil/water ratio . This has been done in Pickering systems by using mixtures of particles as well as of particles and surfactants, where adsorption of surfactants onto the particle surface alters θ ow . ,, However, added surfactants can also adsorb at the oil/water interface and compete with the particles, causing antagonistic effects . Controlled phase separations, achieved through the utilization of switchable surfactants, generating switchable Pickering emulsions, have also been reported. − Temperature can be an important tool to control emulsion type and stability. , Pioneering work by Nagai et al addresses the utilization of temperature-sensitive microgels to achieve controlled phase separation, and by Binks et al .…”