Influence of Particle Wettability on the Type and Stability of Surfactant-Free Emulsions

Abstract

A systematic investigation into the influence of the wettability of spherical, nanometer-sized silica particles on the type and stability of water-toluene emulsions is described. The particles range from hydrophilic to hydrophobic depending on the extent of chemisorbed silane on their surfaces. We show that predictions based on considerations of the energy of attachment of a single particle to the oil-water interface relate directly to the stability of emulsions. A combination of powder immersion, shelf stability, light diffraction, and microscopy measurements is used to characterize the systems. Emulsions stabilized by either very hydrophilic or very hydrophobic particles are large (>100 µm) and unstable to coalescence. Those with particles of intermediate hydrophobicity are submicrometer and stable to coalescence indefinitely. For these, catastrophic inversion of emulsions occurs upon increasing the volume fraction of water, φw, and their stability to sedimentation or creaming increases approaching inversion. The stability of emulsions to gravity-induced separation passes through a sharp maximum upon increasing the particle hydrophobicity, alongside a minimum in the average drop diameter. This is universal and independent of φw and the type of emulsion formed. In one system, an otherwise very stable water-in-oil emulsion is destabilized in an ultracentrifugal field leading to enhanced sedimentation and eventual coalescence.

Covalently Cross-Linked Colloidosomes

“…Figure 1.16 demonstrates this relationship for a 20 nm particle adsorbed at the toluene/water interface. 65 The calculated energy of detachment is greatest for θ = 90° and falls rapidly either side of this value. Therefore the most stable emulsions are formed when θ is close to 90° as the particles are held more strongly at the interface with a detachment energy of around 2750 kT.…”

“…Therefore the most stable emulsions are formed when θ is close to 90° as the particles are held more strongly at the interface with a detachment energy of around 2750 kT. 65 However, larger/less stable emulsions are formed at higher and lower values of θ. In fact, when the contact angle is less than 20° or greater than 160° the detachment energy is significantly smaller (< 10 kT).…”

“…Therefore it is easier (requires less energy) for particles to detach into the water phase if θ < 90° or detach into the oil phase if θ > 90°. 64 Chapter One -Introduction [64][65] Equation (18) shows that the energy of detachment of a particle at the oil/water interface is heavily dependent on the three-phase contact angle θ. Figure 1.16 demonstrates this relationship for a 20 nm particle adsorbed at the toluene/water interface.…”

Co-Assembled Supported Catalysts: Synthesis of Nano-Structured Supported Catalysts with Hierarchic Pores through Combined Flow and Radiation Induced Co-Assembled Nano-Reactors

“…Furthermore, due to the high storage modulus of the fluid, it is likely that such a fluid is capable of forming thin films despite the fact that it has a very large loading of nano-sized silica particles which can also promote the stability of emulsions/foams [32,33] as well as act as an anti-foaming agent/destabilizer [34] depending on the hydrophobichydrophilic balance on the silica surface [32,33].…”

Evolution of bubble size distribution in particle stabilised bubble dispersions: Competition between particle adsorption and dissolution kinetics

“…In this approach it is the adsorption of small nanoparticles, as oppose to molecular species, onto the surface of droplets or bubbles that is responsible for their colloidal stabilisation. Although known for quite some time, the first systematic studies of Pickering emulsions were carried out by Binks and Lumsdon in a series of studies [23][24][25][26] , and later extended to bubbles by the same and other researchers [27][28][29]. As with any other type of emulsion or foam, strictly speaking, droplets and bubbles stabilised by particles remain thermodynamically unstable.…”