Examining the potential of particles to act as Pickering emulsifiers

particles is to consider the particle properties. The critical factors thought to determine the performance of particles as Pickering stabilisers are:

 Particle size

 Particle shape

 Particle concentration

 Particle wettability

 Interactions between particles (Binks, 2002)

The importance of each of these factors is outlined below. 2.10.5.1.Effect of particle contact angle

The particle contact angle gives an indication of wettability for spherical particles and has been suggested to be the major determinant of emulsion stability (Binks & Clint, 2002). The contact angle is similar to the hydrophile-lipophile balance (HLB) of surfactants, determining whether the surfactant resides in the water, oil or some other phase and the type of emulsion that is formed. Hydrophobic particles generally have a contact angle greater than 90° and a greater portion of the particle resides in the oil or air phase than in the water phase. The opposite is true for hydrophilic particles (Binks, 2002). The particle layer at the interface curves so that the larger area of the particle surface stays on the external side. Therefore, air or oil in water emulsions are formed for more hydrophilic particles and water in air or oil emulsions are formed for more hydrophobic particles (Figure 13).

The contact angle also affects the energy required to attach or remove a particle from the interface between the two fluids, which influences the stability of an emulsion. Particles that are strongly adsorbed can be considered to be irreversibly held at the interface and can provide long-term stability against changes in droplet size (i.e. stable to Ostwald ripening and coalescence). Equation 3 defines the relationship between the

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energy of attachment, E, of particles to an interface and the particle size, contact angle and the interfacial tension between the two fluids in an emulsion.

Figure 13. Top: Positioning of hydrophilic (left), intermediate hydrophobicity (centre) and hydrophobic (right) particles at an air or oil/water interface, showing contact angles < 90°, equal to 90° and > 90°, respectively. Bottom: The curvature of

the interface depending on the particle contact angle; aqueous foams or o/w emulsions formed when θ < 90° and aerosols or w/o emulsions formed when θ >

90° (Binks, 2002).

( ) Equation 3

Where r = particle radius (m), γαβ = interfacial tension between fluids α and β (Nm-1), θ = contact angle that the particle makes with the interface (degrees) and the sign inside the bracket is negative for particles that are removed into the water phase, and positive for particles that are removed into the oil phase (Binks, 2002).

Equation 3 shows that particles with contact angles close to 90° are held strongly at the interface, whereas above and below 90°, E decreases rapidly. For example, E for a 10nm particle with a contact angle of 90° is several orders of magnitude higher than that of a surfactant adsorbed at the interface. Surfactants ―are considered to be in a state of dynamic equilibrium constantly undergoing adsorption and desorption,‖ which is known to enhance coalescence and Ostwald ripening in droplets. Conversely, particles

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are considered to be irreversibly adsorbed at the interface and therefore confer extreme stability against changes in droplet size distribution (Gould et al., 2013). This has even been found to be true for millimetre-sized droplets, with the large droplets rolling over each other without deformation in the same manner as we would expect for true solid spheres (Binks, 2002). This extreme stability is one of the major advantages of Pickering stabilisation.

Another significant advantage of particles with intermediate hydrophobicity is that they can sometimes be used to form both o/w and w/o emulsions depending on the oil:water ratio. However, more knowledge still needs to be gained as to how one type of particle can stabilise both types of emulsions (Binks, 2002).

The wettability of particles can be altered by coating with various reagents, such as alkylsilanes and fluorocarbon (Binks, 2002; Binks & Lumsdon, 2001). For example, Frelichowska et al. (2010) investigated the ability of two types of silica that had been grafted with dichloromethylsilane to make their surfaces more hydrophobic to act as Pickering stabilisers.

However, although coatings make the particles surface active, the particles do not necessarily become amphiphilic. Amphilicity influences the strength of attachment of particles to the interface; Binks and Fletcher (2001) suggest that amphiphilic particles remain strongly surface active even for particles with contact angles approaching either 0 or 180°C. Particles can be made amphiphilic by applying the coating to only certain areas of the particle such that it has specific areas that are oil-liking and specific areas that are water-liking (Binks, 2002).

2.10.5.2.Effect of particle size

It has been suggested that smaller particles pack more efficiently at an interface than larger particles, thus producing a more homogeneous layer around the droplets that makes them more stable (Gould et al., 2013; Kurukji et al., 2013). Monodisperse particles may also be expected to pack more efficiently at the interface. It is also known that to effectively stabilise an emulsion, the size of the stabilising particle ―should be at least an order of magnitude smaller than the emulsion droplet‖ (Gould et al., 2013). For

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example, to stabilise emulsion droplets of 0.5-10 μm, the particles must be in the sub- micron/nanometre size range (Dickinson, 2012).

Equation 3 indicates that large particles would be held most strongly at the interface. However, the equation does not hold true for particles with diameters greater than a few microns (Binks, 2002). Furthermore, it has been found that as long as the contact angle is close to 90°, even particles as small as 5-10 nm can adsorb essentially irreversibly to the interface, supporting the importance of the contact angle (Dickinson, 2012).

2.10.5.3.Effect of particle shape

Spherical particles are often investigated as Pickering stabilisers. However, other particle shapes such as rods and discs can also be used. Other shapes offer various advantages, for example, rods can out-perform equivalent spherical particles and create ―super-stable‖ foams and emulsions. This is because of the high surface area-to-volume ratio and the ability of rods to intertwine at interfaces, resulting in enhanced steric stabilisation (Campbell et al., 2008; Campbell, Stoyanov, & Paunov, 2009; Wege et al., 2008). Using this logic, Li et al. (2013) hypothesised that wheat and potato starch granules, which are oval-shaped, would provide more effective emulsification than polygonal-shaped rice and waxy corn starch granules. However, the rice and waxy corn starch granules were found to be the best emulsifiers, indicating that particle shape is not the critical factor controlling the effectiveness of native starch granules to act as emulsifiers. The rice and waxy corn starch granules were smaller than the wheat and potato starch granules, suggesting that in this case the difference in size has had more effect than the difference in shape. Perhaps it is the combination of particle properties that is important, rather than individual properties.

2.10.5.4.Effect of particle concentration

Particle concentration determines how much of an interfacial area can be stabilised and therefore the size of resulting emulsion droplets. Equation 4 shows how the mean emulsion droplet diameter (D) is related to the volume fraction of the dispersed phase (φv) and the interfacial area per unit volume of emulsion (A/V).

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The particle-to-oil ratio is an important parameter, since both the particle concentration and the amount of oil influence the interfacial area per unit volume of emulsion and thus the droplet diameter (Frelichowska et al., 2010).

2.10.5.5.Effect of particle-particle interactions

Many studies have shown that particles must be weakly flocculated or aggregated to enable stable emulsions to form (Briggs, 1921; Lucassen-Reynders & Van Den Tempel, 1963; Wei, Yang, Yang, & Wang, 2012). Briggs (1921) suggests that if solids are in suspension in one of the liquid emulsion components, these solids may not be sufficiently surface active enough to act as emulsifiers. Weak flocculating agents can be added to force the particles to go to the interface, thus improving emulsifying ability. Ashby and Binks (2000) also suggest that due to their larger size, flocs adsorbed to an interface have a greater energy of attachment compared to single droplets and are therefore less likely to desorb from the surface and destabilise the emulsion. However, flocculating agents that are too powerful may cause the solid to agglomerate into large masses or flocs that are unable to form a stabilising film around emulsion droplets (Briggs, 1921; Lucassen-Reynders & Van Den Tempel, 1963). Therefore, weak flocculating agents that cause moderately prevalent attraction between the particles are required (Lucassen-Reynders & Van Den Tempel, 1963). It is suggested that flocculation or aggregation can be induced by the addition of salt, change in pH or addition of surfactant, depending on the type of emulsion (Binks, 2002; Frelichowska et al., 2010; Gould et al., 2013; Liu & Tang, 2013).

Gould et al. (2013) found that larger emulsion droplets were produced when flocculated cocoa particles were used to stabilise the emulsions than when non-flocculated particles were used. It was mentioned that this may have been due to the effect of the acid conditions that were used to induce the flocculation on the hydrophobicity of the particles. However, this result may suggest that it is the overall suite of particle

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properties that are important in particle performance, rather than the domination of one certain factor.

Interactions between aggregates or single particles are also thought to be of importance. In addition to the stability against coalescence provided by the film of particles around dispersed droplets, the formation of a 3D-network of particles in the continuous phase surrounding the droplets may provide additional stabilisation. The network increases the viscosity of the continuous phase and prevents movement of the dispersed phase droplets (Binks, 2002).

Finally, the density of packing of particles at the oil-water interface is suggested to be a major determinant of the strength of the steric protection (Dickinson, 2012). Binks (2002) has reviewed the formation of particle monolayers and occurrence of hexagonal packing in monolayers. Interactions between particles may influence the packing that occurs.