Particles as surfactants—similarities and differences
Introduction
It is well known 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 (including microemulsions in their mixtures) and are commonly 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 surfactants but certain differences in behaviour are inevitable, e.g. individual particles do not assemble to give aggregates in the same way that surfactant molecules form micelles and hence solubilisation phenomena are absent in the particle case. Partly as a result of the current activity in nanoparticle technology for producing new materials [1], [2], there has been a resurgence of interest recently in the field of particles at interfaces (both planar and curved). This review aims to highlight the similarities and differences in the behaviour of surfactants and particles and includes their adsorption to interfaces, their partitioning between immiscible liquids and their ability to stabilise emulsions and foams. Since this area represents a new perspective, references to existing older literature are included which serve as relevant background and maintain continuity.
For surfactants present in oil+water mixtures, the system HLB is the most important variable in determining whether aggregated surfactant (micelles or microemulsion droplets) resides in either water, oil or a third phase. 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 [3]. This in turn is set by the geometry of the surfactant molecules, hydrated by water on one side of the monolayer and solvated by oil on the other. Thus, for hydrophilic surfactants (ionics in the absence of salt or non-ionics with a high degree of ethoxylation), the area per head group is larger than that of the chain and the monolayers curve around oil resulting in oil-in-water (o/w) micro- and macroemulsions. For more lipophilic surfactants (ionics in the presence of sufficient salt or nonionics with low degrees of ethoxylation), the area per chain exceeds that of the head group and water becomes the dispersed phase in water-in-oil (w/o) micro- and macroemulsions. For conditions in which the head group and chain areas are similar, monolayers have a net curvature of zero and new aggregates form including lamellar phases and bicontinuous microemulsions. The sequence of transitions for multiphase microemulsion systems (Winsor I–Winsor III–Winsor II) and the corresponding emulsion inversion can be achieved by changes in salt concentration, oil type or temperature and by addition of cosurfactant, and is reasonably well understood [4].
In the case of spherical particles which adsorb to interfaces (water–air or water–oil), the relevant parameter is thought to be the contact angle θ which the particle makes with the interface. For hydrophilic particles, e.g. metal oxides, θ measured into the aqueous phase is normally<90° and a larger fraction of the particle surface resides in water than in the non-polar phase. For hydrophobic particles, e.g. suitably treated silica, θ is generally greater than 90° and the particle resides more in air or oil than in water. By analogy with surfactant molecules, the monolayers will curve such that the larger area of the particle surface remains on the external side, giving rise to air or oil-in-water when θ<90° and water-in-air or oil when θ>90° (Fig. 1). The former include aqueous foams and o/w emulsions, respectively, whilst the latter include aerosols and w/o emulsions.
Section snippets
Adsorption to interfaces
There are now many methods to synthesise small, monodisperse particles of different shape and surface coating. If the coating, e.g. alkylsilane or fluorocarbon, is homogeneous over the particle surface following, say, reaction in the vapour phase, such particles are surface-active but, unlike surfactants, are not amphiphilic. If, however, the coating can be restricted to a particular area of the surface only, heterogeneous or ‘Janus’ particles result which are both surface-active and
Partitioning of particles between phases
The rules are reasonably well established now for describing the partitioning of both monomeric and aggregated surfactant molecules between oil and water phases. Thus, most ionic surfactants partition exclusively in favour of water as monomers whereas nonionic surfactant molecules containing ethylenoxy groups partition into both phases as monomer, with the distribution normally heavily in favour of the oil phase. Micelles or microemulsion droplets occur in only one of the phases at any overall
Emulsions stabilised by particles
The fact that finely divided solid particles can act as the stabiliser in emulsions has been known since the beginning of the last century. The credit is usually given to Pickering [43], hence the term ‘Pickering emulsions’, who noted that particles which were wetted more by water than by oil acted as emulsifiers for o/w emulsions by residing at the interface. However, in a paper 4 years earlier and cited by Pickering, Ramsden [44] described the formation of a membrane of solid particles
Solid-stabilised foams
Solid particles have been incorporated into surfactant-stabilised aqueous foams for many years, and their influence on the formation and stability of the foam is very dependent on the surfactant type, particle size and concentration [73], [74]. In some cases, particularly if the particles are fairly hydrophilic, foam stability is enhanced since particles present in the aqueous phase of the foam films collect in their Plateau borders slowing down film drainage. In other cases, hydrophobic
Conclusions
There are many similarities in the behaviour of small particles and surfactant molecules at fluid–fluid interfaces and in various kinds of dispersions. Important differences however also exist. The field of particles at interfaces, both flat and curved, is an exciting one and is beginning to form part of the research of many groups. Future challenges include designing a method for measuring the contact angles sub-micron sized spherical particles make with interfaces and synthesising and
Acknowledgements
It is a pleasure to thank Dr Herbert Barthel of Wacker-Chemie GmbH, Burghausen (Germany) for the continued supply of fumed silica powders of varying hydrophobicity, without which some of our work on solid-stabilised emulsions would not have been possible. I also thank Prof. P.D.I. Fletcher for useful discussions.
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