Emulsions stabilised solely by colloidal particles

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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.

Introduction

It is now well established that low molar mass surfactants and surface-active polymers are used to aid dispersion of powdered materials in a liquid, can form a variety of aggregated structures in aqueous or non-aqueous media and are frequently employed as emulsifiers in the preparation of emulsions and as stabilisers in the production of foams. Much less well appreciated is that solid particles (nano- or micro-) can function in similar ways to surfactant molecules but certain differences in behaviour are inevitable, e.g. particles do not assemble into aggregates in the same way that surfactant molecules form micelles and hence solubilisation phenomena are absent in the particle case. There has been a resurgence of interest recently in the field of particles at interfaces (both planar and curved), partly as a result of the current activity in nanoparticle technology for producing new materials [1]. A review by one of us [2] has described the recent advances in the areas of particle monolayers at fluid–fluid interfaces, determination of contact angles of small particles, partitioning of particles between phases and solid–stabilised foams, highlighting the similarities and differences between the two types of stabiliser. The present review deals with emulsions, i.e. mixtures of oil and water, stabilised solely by colloidal particles. The ideas originating from investigating simple emulsions are extended to the preparation of multiple emulsions and a brief account is given of the use of such emulsions as templates for the formation of porous solid phases.

We report also 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 we have considered potential effects on the free energy of emulsion formation of the action of line tension in the 3-phase contact lines around 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.

Section snippets

Simple emulsions

For surfactant molecules present in oil+water mixtures, the system hydrophile–lipophile balance (HLB) is the important parameter in determining whether aggregated surfactant (micelles or microemulsion droplets) resides in either water, oil or a third phase [3]. It has been shown that the packing parameter of the surfactant in situ at the oil–water interface determines the tendency of the surfactant monolayer to curve towards water or oil or remain effectively planar. This in turn is set by the

Multiple emulsions

In addition to acting as emulsifiers in simple emulsions, the possibility exists that particles may be effective at stabilising multiple or double emulsions. The latter have been termed ‘emulsions of emulsions’, i.e. the globules of the dispersed phase contain even smaller dispersed droplets themselves. In the surfactant case, two different surfactants are normally required in order to prepare such emulsions, one of which adsorbs primarily at the interface of inner drops whilst the other

New materials from solid-stabilised emulsion templates

In other work, we have been studying the kinetics and mechanism of evaporation of water and oil from creamed o/w emulsions stabilised by various surfactants [50]. Evaporation rates are of interest in many contexts including assessment of hazards from the spillage of volatile chemicals, drying processes and the release of volatile active species such as perfumes and flavours from cosmetic and food products. It is found that the continuous aqueous phase of the emulsions evaporates at a rate equal

Some aspects of the origins of stability of solid-stabilised emulsions

There are a number of factors that appear to govern the stability of particle-stabilised emulsions. For example it is known that for high stability, contact angles, θow of particles with the oil/water interfaces should not be too remote from 90o, and that particle roughness (and associated contact angle hysteresis) is beneficial for stability [53]. Usually, for oil and water volume fractions of approximately 0.5, particles with θow<90o favour the formation of o/w emulsions whilst w/o emulsions

Summary and conclusions

Solid particles adsorb strongly at the oil–water interface and are able to stabilise emulsions of oil and water. The key parameter is the contact angle (θow) made by the oil–water interface on the surface of the particle. For θow not too far from 90° the energy required to remove even nanoparticles from this interface can be several thousand times kT and hence the adsorption can be considered irreversible. Particles for which θow<90° tend to stabilise oil-in-water (o/w) emulsions whereas those

Acknowledgements

Dr Binks would like to thank Dr H. Barthel of Wacker-Chemie GmbH (Munich) for supplying us with large samples of silica powders of different hydrophobicity, without which some of our work would not have been possible.

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