Inversion of particle-stabilized emulsions of partially miscible liquids by mild drying of modified silica particles

• Published in: Journal of colloid and interface science 2011-07, Vol.359 (1), p.126-135
• Main Authors: White, Kathryn A., Schofield, Andrew B., Wormald, Philip, Tavacoli, Joseph W., Binks, Bernard P., Clegg, Paul S.
• Format: Article
• Language: English
• Subjects: 2,6-Dimethylpyridine; Adsorption; Bijel; Binary fluid; Chemistry; Colloidal particles; Colloidal state and disperse state; confocal microscopy; Droplets; Drying; emulsifying; Emulsions; Emulsions - chemistry; Emulsions. Microemulsions. Foams; Exact sciences and technology; fluorescein; fluorescence; General and physical chemistry; Hydration; Inclusions; Inversions; Nuclear magnetic resonance; nuclear magnetic resonance spectroscopy; Particle Size; Physical and chemical studies. Granulometry. Electrokinetic phenomena; Pickering emulsion; Pyridines - chemistry; silica; Silicon dioxide; Silicon Dioxide - chemistry; Solid-liquid interface; Stöber silica; Surface physical chemistry; Surface Properties; thermogravimetry; Transitional inversion; Water - chemistry
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2011.03.074

Confocal microscopy images of emulsions/bijels formed via phase separation by particles dried for increasing lengths of time at 170°C (left to right 30, 130min. and 10hrs.). Scale bar=100 μm; green=silica particles, red=lutidine-rich phase labeled with rhodamine B; sample composition: 28 wt% lutidine in water, 2 vol% particles; T=40°C. [Display omitted] ► For Pickering emulsions of water and lutidine mixtures with silica particles, the hydration of the particle surface causes a change in emulsion structure. ► For water and lutidine at critical composition, decreasing silica particle hydration inverts the emulsion. ► For silica particles with some amino surface groups, a bijel forms at inversion. ► The character of the silica surfaces can change during phase separation. Using a system of modified silica particles and mixtures of water and 2,6-lutidine to form particle-stabilized emulsions, we show that subtle alterations to the hydration of the particle surface can cause major shifts in emulsion structure. We use fluorescence confocal microscopy, solid state nuclear magnetic resonance (NMR) and thermo-gravimetric analysis (TGA) to explore this sensitivity, along with other shifts caused by modifications to the silica surface chemistry. The silica particles are prepared by a variant of the Stöber procedure and are modified by the inclusion of 3-(aminopropyl)triethoxysilane and the dye fluorescein isothiocyanate. Treatment prior to emulsification consists of gently drying the particles under carefully controlled conditions. In mixtures of water and 2,6-lutidine of critical composition, the particles stabilize droplet emulsions and bijels. Decreasing particle hydration yields an inversion of the emulsions from lutidine-in-water (L/W) to water-in-lutidine (W/L), with bijels forming around inversion. So dependent is the emulsion behavior on particle hydration that microscopic differences in drying within a particle sample can cause differences in the wetting behavior of that sample, which helps to stabilize multiple emulsions. The formation of bijels at emulsion inversion is also crucially dependent on the surface modification of the silica.