Bernard P. Binks

Particles as surfactants—similarities and differences

Abstract Colloidal particles act in many ways like surfactant molecules, particularly if adsorbed to a fluid–fluid interface. Just as the water or oil-liking tendency of a surfactant is quantified in terms of the hydrophile–lipophile balance (HLB) number, so can that of a spherical particle be described in terms of its wettability via contact angle. Important differences exist, however, between the two types of surface-active material, due in part to the fact that particles are strongly held at interfaces. This review attempts to correlate the behaviour observed in systems containing either particles or surfactant molecules in the areas of adsorption to interfaces, partitioning between phases and solid-stabilised emulsions and foams.

Emulsions stabilised solely by colloidal particles

Abstract The preparation and properties of emulsions, stabilised solely by the adsorption of solid particles at the oil–water interface, are reviewed especially in the light of our own work with particles of well-controlled surface properties. Where appropriate, comparison is made with the behaviour of surfactant-stabilised emulsions. Hydrophilic particles tend to form oil-in-water (o/w) emulsions whereas hydrophobic particles form water-in-oil (w/o) emulsions. Many of the properties can be attributed to the very large free energy of adsorption for particles of intermediate wettability (contact angle at the oil–water interface, say, between 50 and 130°). This effectively irreversible adsorption leads to extreme stability for certain emulsions and is in contrast to the behaviour of surfactant molecules which are usually in rapid dynamic equilibrium between the oil–water interface and the bulk phases. There is evidence that, in some systems, weak flocculation of the particles improves the emulsion stability. Phase inversion from w/o to o/w can be brought about by increasing the volume fraction of water. Emulsions close to this inversion point tend to be the most stable, again in contrast to surfactant systems. The volume fraction needed for inversion depends on the particle wettability and the nature of the oil and these effects have been rationalised in terms of surface energy components. Stable multiple emulsions (w/o/w and o/w/o) can be made using two types of particles with slightly different wettability. Similar multiple emulsions prepared with two types of surfactant tend to be much less stable. The possibility of preparing novel solid materials by evaporating solid-stabilised emulsions is also proposed. Finally we report on some extensions to the work of Levine et al. who obtained expressions for the free energy of formation of emulsion drops covered with close-packed monolayers of monodisperse spherical particles. In particular in the light of the observations that nanoparticles can act as excellent emulsion stabilisers, we have considered potential effects on the free energy of emulsion formation of the action of small (physically realistic) positive and negative line tensions in the 3-phase contact lines skirting particles adsorbed at the droplet interfaces. We also explore the possibility that curvature properties of close-packed particle monolayers can affect emulsion properties in much the same way that surfactant monolayer properties influence emulsion type and stability.

Colloidal particles at liquid interfaces.

1. Particles at liquid interfaces - an introduction B. P. Binks and T. S. Horozov 2. Structure and formation of particle monolayers at liquid interfaces L. Bergstrom 3. Theory for interactions between particles in monolayers J. C. Fernandez-Toledano, A. Moncho-Jorda, F. Martinez-Lopez and R. Hidalgo-Alvarez 4. Particle-assisted wetting W. A. Goedel 5. Particle-laden interfaces: rheology, coalescence, adhesion and buckling G. G. Fuller, E. J. Stancik and S. Melle 6. Solids-stabilized emulsions: a review R. J. G. Lopetinksy, J. H. Masliyah and Z. Xu 7. Novel materials derived from particles assembled on liquid surfaces K. P. Velikov and O. D. Velev 8. Interfacial particles in food emulsions and foams E. Dickinson 9. Collection and attachment of particles by air bubbles in froth flotation A. V. Nguyen, R. J. Pugh and G. J. Jameson 10. Antifoam effects of solid particles, oil drops and oil-solid compounds in aqueous foams N. D. Denkov and K. G. Marinova 11. Metal foams: towards high temperature colloid chemistry N. Babcsan and J. Banhart.

Modern aspects of emulsion science

Emulsions - Recent Advances in Understanding Emulsion Formation Emulsion Formation by Nucleation and Growth Mechanisms Emulsion Flocculation and Creaming Rheology of Emulsions - The Relationship to Structure and Stability Phase Inversion and Drop Formation in Agitated Liquid-Liquid Dispersions in the Presence of Nonionic Surfactants Coalescence in Emulsions Lifetime and Destruction of Concentrated Emulsions Undergoing Coalescence Molecular Diffusion in Emulsions and Emulsion Mixtures Interactions and Macroscopic Properties of Emulsions and Microemulsions Gel Emulsions - Relationship between Phase Behaviour and Formation Applications of Emulsions Subject Index.

Some general features of limited coalescence in solid-stabilized emulsions

Abstract.We produce direct and inverse emulsions stabilized by solid mineral particles. If the total amount of particles is initially insufficient to fully cover the oil-water interfaces, the emulsion droplets coalesce such that the total interfacial area between oil and water is progressively reduced. Since it is likely that the particles are irreversibly adsorbed, the degree of surface coverage by them increases until coalescence is halted. We follow the rate of droplet coalescence from the initial fragmented state to the saturated situation. Unlike surfactant-stabilized emulsions, the coalescence frequency depends on time and particle concentration. Both the transient and final droplet size distributions are relatively narrow and we obtain a linear relation between the inverse average droplet diameter and the total amount of solid particles, with a slope that depends on the mixing intensity. The phenomenology is independent of the mixing type and of the droplet volume fraction allowing the fabrication of both direct and inverse emulsion with average droplet sizes ranging from micron to millimetre.

Stability of oil-in-water emulsions stabilised by silica particles

We describe the preparation and properties of oil-in-water emulsions stabilised by colloidal silica particles alone. The charge on the particles and their extent of flocculation, assessed via turbidity measurements, can be modified by pH control and addition of simple electrolytes. The stability of emulsions to both creaming and coalescence is low in the absence of electrolyte, and the effects of adding salt are dependent on the type of salt. In systems containing NaCl, emulsions are less stable once the particles are flocculated. In the presence of either LaCl3 or tetraethylammonium bromide (TEAB), emulsion stability increases dramatically for conditions where the silica particles are weakly flocculated; extensive flocculation of the particles however leads to destabilisation of the emulsions. For TEAB, relatively large emulsions of diameter around 40 µm remain very stable for up to 3 months at salt concentrations corresponding to the onset of coagulation of the colloid. Such emulsions are themselves strongly flocculated.

Pickering emulsions stabilised by Laponite clay particles

The type and stability of Pickering emulsions stabilised by disc-like Laponite RD clay particles are described. By establishing the phase diagram of aqueous dispersions as a function of clay and salt (NaCl) concentration, we deduce that toluene-in-water (o/w) emulsions, stable to creaming and coalescence for at least 6 months, are only formed under conditions where the colloidal particles are flocculated. The initial average drop diameter is independent of clay concentration but depends markedly on oil volume fraction, ranging from 10 to 28 μm. Changes in the drop size distributions with time are shown to be due to Ostwald ripening, which, due to the irreversible nature of particle adsorption at oil/water interfaces, is rapid at first and ceases completely at long times. It is suggested that ripening is arrested when the Laplace pressure inside drops of different sizes becomes equal. For optimum conditions, emulsions prepared using a variety of oils including non-polar alkanes and polar alcohols are always o/w even at high oil phase volume fraction, reflecting the hydrophilic nature of this synthetic clay in oil–water systems.

Magnetic Pickering emulsions stabilized by Fe3O4 nanoparticles.

Superparamagnetic Fe(3)O(4) nanoparticles prepared by a classical coprecipitation method were used as the stabilizer to prepare magnetic Pickering emulsions, and the effects of particle concentration, oil/water volume ratio, and oil polarity on the type, stability, composition, and morphology of these functional emulsions were investigated. The three-phase contact angle (θ(ow)) of the Fe(3)O(4) nanoparticles at the oil-water interface was evaluated using the Washburn method, and the results showed that for nonpolar and weakly polar oils of dodecane and silicone, θ(ow) is close to 90°, whereas for strongly polar oils of butyl butyrate and 1-decanol, θ(ow) is far below 90°. Inherently hydrophilic Fe(3)O(4) nanoparticles can be used to prepare stable dodecane-water and silicone-water emulsions, but they cannot stabilize butyl butyrate-water and decanol-water mixtures with macroscopic phase separation occurring, which is in good agreement with the contact angle data. Emulsions are of the oil-in-water type for both dodecane and silicone oil, and the average droplet size increases with an increase in the oil volume fraction. For stable emulsions, not all of the particles are adsorbed to drop interfaces; the fraction adsorbed decreases with an increase in the initial oil volume fraction. Changes in the particle concentration have no obvious influence on the stability of these emulsions, even though the droplet size decreases with concentration.

On the origin of the remarkable stability of aqueous foams stabilised by nanoparticles: link with microscopic surface properties

We have performed a quantitative study of the coarsening of foams stabilised by partially hydrophobic silica nanoparticles. We have used a variety of techniques: optical and electron microscopy, microfluidics, and multiple light scattering. Using earlier studies of planar particle monolayers, we have been able to correlate the interfacial properties and the macroscopic temporal evolution of the foam. This has shed light on the origin of the absence of coarsening of particle-stabilised foams. Such particle-stabilised foams appear to be the only known foam system where coarsening is inhibited by surface elasticity.

Particle-stabilised foams: an interfacial study

In an attempt to elucidate the remarkable stability of foams generated from dispersions of partially hydrophobic nanoparticles (fumed silica), we present investigations into the static and dilational properties of the gas–liquid interfaces of such dispersions. By relating the dynamic surface tension γ(t) and the dilational elasticity E measured using an oscillating bubble device, we confirm that the Gibbs stability criterion E > γ/2 against foam coarsening is fulfilled. We complement these studies using ellipsometry and Brewster angle microscopy, which provide evidence for a pronounced adsorption barrier for the particles and a network-like structure in the interface at sufficiently high concentrations. We observe this structure also in freely suspended films drawn from the same particle dispersions.