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

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.