The role of particles in stabilising foams and emulsions

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Abstract

The use of particles as foam and emulsion stabilising species, with or without surfactants, has received great interest in recent years. The majority of work has studied the effects of particles as stabilisers in emulsion systems, but recent successes has widened consideration into foams, where industries such as flotation and food processing have encountered the effects of particle stabilisation for many years. This review seeks to clarify studies into emulsions, highlighting new research in this area, and relate similarities and differences to foam systems. Past research has focused on defining the interaction mechanisms of stability, such as principles of attachment energies, particle–particle forces at the interface and changes to the interfilm, with a view to ascertain conditions giving optimum stability. Studied conditions include effects of particle contact angle, aggregation formations, concentration, size and interactions of other species (i.e. surfactant). Mechanisms can be complex, but overall the principle of particles creating a steric barrier to coalescence, is a straitforward basis of interaction. Much research in emulsions can be applied to foam systems, however evidence would suggest foam systems are under a number of additional constraints, and the stability ‘window’ for particles is smaller, in terms of size and contact angle ranges. Also, because of increased density differences and interfilm perturbations in foam systems, retardation of drainage is often as important to stability as inhibiting coalescence.

Introduction

Foams and emulsions are practically important to many chemical and engineering fields, and as such, investigations into the stability, interaction and structural relationships are of great significance. Liquid and solid foams are encountered extensively in the food and beverage industries (e.g. cake and beer), dermatology and personal care industries (e.g. shampoo), textile industries, fire retardants, general polymers/plastics, oil recovery and mineral flotation processing [1]. Emulsions are also heavily used in the food industry, cosmetics and paints, the pharmaceutical industry, agricultural products and the petroleum industry [2], [3]. Emulsions are typically categorized as oil-in-water (denoted O/W), such as milk dairy emulsions, or water-in-oil (W/O), such as oil based skin creams [4]. General pure liquid foams can be considered a highly concentrated “emulsion” of, say, air and water (denoted A/W). Hence there are many similarities between the mechanisms and underlying physical principles of stable emulsions and foams; so the inclusive study of both fields can yield results significant for a spectrum of industrial and practical applications. An example of a general similarity is the high surface area per volume of the interface.

There are also some differences that play a key role in observed behaviour. Firstly, most gas in liquid foams are formed with bubbles up to 103 times as large as emulsion droplets, mainly the result of the increased gas solubility in water. Also, gas bubbles are about 105 times as compressible as oil droplets, meaning they are easily deformed, which changes foam structure from an individual ‘bubble’ type to typically a polyhedral type structure [5], [6]. These are not observed for emulsions, except in some cases where the disperse phase is highly concentrated. Other significant differences, such as the higher disproportionation of gas bubbles, the higher Hamaker constants of gas–water systems (i.e. stronger van der Waals attractive force), the different surfactant systems required for stability and of course the differences in particle stabilised systems is given in Table 1 below. These features will be discussed throughout this review.

Foams, like pure liquid-in-liquid macroemulsions (emulsions where droplet size ranges from 1 to 100 μm) can be considered thermodynamically unstable, as their decay results in a decrease of the free energy [15]. However, kinetic mechanisms involved with breakdown can be so slow that the foams or emulsions can be thought of as at least metastable for their applications [4]. The primary processes for instability in foams and emulsions are creaming and sedimentation (for O/W and W/O emulsions respectively), analogous to drainage in foams, coalescence and flocculation [5], [16]. Creaming and sedimentation are gravity induced movements, caused by the difference in density of the oil and water phases. Creaming involves the rising of oil in an O/W emulsion, whereas sedimentation involves the settling of water in a W/O emulsion [5]. Because gravity induced movement is increased as differences in density become more pronounced, water drainage (and gas “creaming”) is most significant in foams. In fact, interfilm drainage is the basic driving force forming the ‘dry’ polyhedral type structures in foam systems, which in turn heightens conditions for bubble coalescence [6], [15]. Coalescence involves the irreversible binding of two or more foam bubbles (or emulsion droplets), where the interfacial film drains and is eventually ruptured forming a single larger bubble, and is generally the most severe form of instability [17]. Flocculation involves interaction between emulsion droplets (and can also be observed in ‘dilute’ foam systems, such as the initial bubble zone of a flotation column) where the free energy minimum is appreciably low for the droplets to interact. Unlike coalescence, the interfacial film wall does not drain to rupture, and the droplets remain held together as ‘clusters’[5].

In foam systems, another form of instability that is important is disproportionation. Disproportionation is the process whereby air molecules diffuse through the disperse phase, between bubbles. Because of the increased Laplace pressure of smaller bubbles, the diffusion flux generally results in the shrinking of smaller, and the growth of larger gas bubbles [5]. Coarsening of the bubble size distribution from disproportionation, can also lead to increased drainage and coalescence [8]. This diffusion process is also present in emulsion systems; termed ‘Ostwald ripening’. As it is controlled by molecular solubility, it is negligible for immiscible fluids, such as most oil and water emulsions, and is usually only significant for droplets under 0.1 μm [9], [17].

To help stabilise foams or emulsions of pure components, it is normally necessary to add an additional third component, a foaming/emulsifying agent. In, say, a traditional air–water system, foaming agents are required to first produce conditions that will create a foam, as well as stabilise it for required time periods, as simply, “clean” air bubbles passed through water will burst immediately on drainage [15]. Foamers/emulsifiers are classically molecular surfactants (e.g. fatty acids or alcohols [18]), polymers or larger protein type agents (e.g. egg albumen) [19]. Molecular surfactants generally contain a polar (hydrophilic) head group and a non-polar (hydrophobic) chain tail. Surfactants therefore preferentially adsorb to the air/oil–water interface. This reduces the free energy involved with producing a high surface area interface, and as a result, reduces the interfacial surface tension [4]. Polymers and proteins cause stability largely through electric and steric repulsion, controlled by the extent of unfolding (or ‘denaturing’ as unfolded proteins are known) and conformational layer structure on droplets [20], [21]. ‘Semi-dilute’ polymers and larger ‘globular’ proteins, which do not denature to the same extent, can further cause stability through changes to the rheological properties of the dispersion medium (namely through increase in viscosity) [5]. This may also be achieved through addition of agents such as glycerine [18]. Of course, many naturally occurring emulsions and foams will contain a variety of types of agents, leading to potentially very complex interactions giving overall characteristics.

Particles are naturally present in many types of emulsions, yet despite this, the pointed use of particles either solely, or in addition to other traditional surfactants as emulsion stabilisers, has only been recently (in the last 20 years) given extensive attention. In addition to the many recent developments, published study of particles in emulsions has been documented for 100 years. The person credited with first investigating particle stabilisation in emulsions is Pickering, who in 1907 (using work documented four years earlier by Ramsden [22]) noted that particles more wetted by water than oil, stabilised O/W emulsions by residing at the interface [23]. From this report, the term ‘Pickering emulsions’ has been given to emulsions stabilised by solid particles.

The role of particle wettability and emulsion stability was discussed further by Finkle et al [24], who considered that particles at an interface in an emulsion would most likely preferentially reside in one of the liquids, and this would become the disperse phase. This was considered in line with resulting emulsions from surfactant adsorption. It was not until the work of Schulman and Leja [25], that particle wettability and interface contact angle was comprehensively investigated. Using barium sulphate crystals and surfactant, they found that conditions giving particle contact angles (θ measured through the aqueous phase) slightly below 90° resulted in O/W emulsions, and for conditions giving θ slightly above 90°, W/O emulsions were formed. Interestingly, conditions giving extreme contact angles (close to 0° or 180°) no stable emulsions were formed.

The wetting behaviour of solid particles is generally considered a key descriptive rule for particle emulsion behaviour. Consider a particle at an oil–water interface. For a θ measured through the aqueous phase of below 90°, the particle is preferentially wetted by the water (i.e. it is preferentially hydrophilic in nature). For a particle θ of above 90°, it is preferentially wetted by the oil (i.e. hydrophobic). For a contact angle of 90°, it is equally wetted by both phases, and is considered at the point of inversion [14]. Similar to surfactant behaviour, the preferentially wetting phase becomes the disperse phase, as the particles minimise energy by curving round the inhibited phase, forming stable emulsion droplets [26], [27]. For particles with contact angles near 90°, whether O/W or W/O emulsions result depends on the particle and solution properties such as concentration [28], [29]. Fig. 1 shows how the bending behaviour of droplets coated in particles may dictate emulsion formation.

There have been a number of different particle types used as stabilisers in both O/W and W/O emulsions, including silica, latex particles, metal oxides and sulphates, clays and carbon. The effectiveness of a specific particle type in stabilising an emulsion, depends on the emulsion medium, the particle shape and size, particle wettability and inter-particle interactions [14]. Particles impart stability on emulsions primarily through a steric barrier, created at the interface. With a certain concentration of particles, a close packed network is generally formed, which further increases stability. It is also thought that a certain level of particle flocculation is advantageous for stability [30]. In addition, at certain concentrations, particles may affect the rheological properties of the disperse medium (much like free-proteins), and particles such as clays, can also act to increase stability by formation of inter-droplet networks [31], [32], [33]. Unlike surfactants, particles do not affect emulsion stability by significantly reducing the oil–water surface tension [34].

There is a large size range of particles that can be used to stabilise emulsions. For successful stabilisation, it is necessary that the particles be approximately orders of size smaller than the droplets, for the particles to be properly located around the droplets [2]. The actual size of particles that can be used ranges from small nanometre [35] to micrometer [36] to successfully stabilise emulsions. The size of the particles does correlate to the size of the droplets formed in stable emulsions, with even droplets of up to millimetre having been found stable to coalescence, something not easily possible with surfactants [14]. Generally the overall stability is inversely proportional to particle size, with smaller particles giving a higher packing efficiency, and so producing a more homogenous layer [16], [35], [37].

Because of the extensive attention particle stabilised emulsions have gained in recent years, behaviour has previously been well studied and documented, as evidenced from reviews by Binks, Aveyard and co-workers, from the University of Hull [14], [26] and a recent book covering this topic edited by Binks & Horozov [38] . Because of this, an effort will be made to focus on presenting recent work on the subject of particles in emulsions, while maintaining required continuity. The second and major focus of this report is to link well detailed emulsions behaviour, to the study of particles in foams, a subject that has been less extensively investigated, but currently is an area of increasing interest.

As with particles in emulsions, the phenomena of particle stabilisation of foams, has only recently been given pointed attention, and interest is largely the result of successes in emulsions. Yet, it is well known that finely dispersed particles play an important role in the stabilisation of many different types of foams. For example, particles in the absence of surfactant are known to cause foaming in rivers, treatment of radioactive wastes, dispersed sludges, distillation towers, oil-well drilling, pulping in the paper industry, fabrication of cellular metal foams and the preparation of foods, etc. In the oil industry, obnoxious foams are produced in boilers and various stages of distillation and are thought to be stabilised by asphaltene particles [39], [40]. In waste water, foams are stabilised by colloidal particles such as bacteria, soil and viruses. In other cases, heterogeneous nucleation of insoluble precipitates (hydrolysed cations such as iron hydroxides) at the air/water surface can also cause foaming in waste water or rivers. Many other everyday processed products frequently involve particle stabilised foam systems, which ensure long-term stability. In food and drink production (bread and beer have foam like structure) and in the dairy industry when whipping cream, partially crystalline ‘solid’ oil droplets accumulate at the interface and stabilise the foam [5].

It is also important to note that the adsorption of particles at bubble interfaces is the basic driving mechanism in many dynamic foaming processes, such as dissolved air flotation to treat waste water, effluent treatment and froth flotation [41]. In the froth flotation process, which is widely used as a particle separation method for minerals, the attachment of particles to bubbles occurs when the particles are dispersed in the liquid pulp [42]. The bubble/particle aggregates then ascend to the interface where the behaviour of the particles has a critical influence on the stability of the thin films in the froth. It is of interest that much early work on particles as foam stabilisers has occurred, from a perspective of mineral processing and flotation. Throughout industry, there are many examples of particle stabilised foaming processes in both aqueous and non aqueous environments, with and without surfactant addition. For example, during the boiling of radioactive sludge suspensions, foaming has been reported to frequently occur [43].

There has been published material, in which the role of particles as stabilising species in foams has been noted, dating back to 1913, where Hoffmann suggested finely divided particles had a role to play in stabilisation of foams or froth, specifically in mineral frothers and ore flotation ([44] as cited in [18]). In 1925, Bartsch found partially hydrophobic particles stabilised froths, whereas completely wetted particles had no effect on the stability [45]. Similarly Hausen found stable three phase froths formed with various metal oxide and clay particles [46], and Lekki and Laskowski found that particles could enhance the stabilisation power of frothers such as diacetone and ethyl acetal [47]. Ottewill et al., in a non-aqueous system, reported increased foam stability in systems of alkali metal carbonates, that was attributed to increase in bulk viscosity of the dispersion with solid particles [48]. More recently, Tang [49] showed that fine silica particles enhanced stability of an SDS foam, and that stability was proportional to particle concentration, and inversely proportional to particle size (much like emulsion systems).

This initial evidence would signify that many similarities lay in the stabilisation mechanism of particles in foams and emulsions. The reliance on particles to create a steric barrier to coalescence is again a major contribution to foam stabilisation, and as with emulsions, smaller particles in high concentrations that form a more complete layer gives the most effective barrier. Because of the increased effects of drainage in foam systems, the affect of particles on the interfilm (for example, causing retardation of drainage and increasing maximum capillary pressure) is also significant in some systems, and generally is a more important mechanism than in emulsions. Bridging of particles between bubbles is also a very important process, because of the high forced contact area in foam phases, as they become dryer [6]. Fig. 2 shows how particles may stabilise polyhedral type foam.

Hydrophobic particles (of contact angle greater than 90°) should not work as effective stabilisers in foams, obviously as foams will break (collapse), upon opposing bending energies, as there is no ability for foams to ‘invert’ (although, recent evidence suggests so-called free-flowing liquid marbles and powders containing numerous water drops, termed ‘dry water’, may be formed [50]). In fact, hydrophobic particles can pointedly be used as foam breakers, further destabilising the foam from bridging drainage, as comprehensively investigated by Dippenaar [51], [52], [53]. Also, because of the deformability and the ‘delicate’ nature of foams, they can be subject to foam destruction from particle piercing, especially in the case of larger, non uniform, particles (an environment often encountered flotation). Further, because of the weak affect on surface tension without the use of other frothers, it is difficult to first establish frothing conditions in clean particle–water systems. So collectively, the increased bubble–bubble contact area from drainage drying, deformability, and inhibitive effects of hydrophobic particles means particle stabilised foams are under a number of additional constraints when compared to emulsions.

Further research into the finite stabilisation mechanism of particles in foams may possibly yield results of great significance, and is in some ways, a more complex proposal than in emulsions. Overcoming the constraints of stable foam behaviour may lead to many potentially profitable investigations, using particles in addition to, or as a way of completely removing surfactants from a foaming process. Particles can prove both economically and environmentally attractive to industry. This is because particles may cause stability over and above generally used surfactants and also because of the benefits of reducing dependence on potentially harmful organics. Knowledge of the stabilisation process is especially important to the flotation industry, where accurate prediction of mineral yields is a difficult prospect, and the great range of particles often encountered leads to many possibilities in overall foam structure and so flotation potential.

Section snippets

Particle detachment energy

A useful factor that theoretically links how well particles of different contact angles stabilise emulsions and foams is the particle detachment energy. This detachment energy is related to the free energies involved in removing an adsorbed particle from an interface (either oil–water or air–water). As particles create a steric barrier to coalescence, it is obvious that strong particle detachment energy will result in more force being required to disrupt the particle layers and allow

Emulsion systems

Previous research has focused on particle emulsion stabilisation with a view to ascertaining the inherent mechanism of stabilisation, including determination of the structure of interparticle networks at the interface, conditions that give optimum emulsion stabilisation and also ways to augment emulsions for specific purposes. Work has been undertaken to investigate aspects such as determination of particle to droplet size relationships, comparing particle types and performance, mixtures of

Conclusions

The recent interest in particle-stabilised foams and emulsions has identified the major factors affecting the stability of respective systems. It should now be possible equate rules governing the effect of particle size, concentration, aggregated nature and contact angle to help improve the performance of a given system. However, great complexities arise from the fact that these structures attain equilibrium as a result of colloidal forces acting on the particles and bubbles in the bulk (such

Acknowledgments

Mr. Hunter would like to thank the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Australia for additional project funding.

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