Water Separation
4.2 Particulate Emulsion Stabilizer
4.2.2 Special Features About Particulate Emulsion Stabilizers
Colloidal-sized particles can also be adsorbed to the interface and stabilize emulsions.18 the surface polarity of the particle usually lies between the polarities of the liquids. it means that these particles can be wetted simul- taneously by both the dispersed phase and the continuous phase. this gives the particle interfacial activity. therefore, they are energetically favourable to be adsorbed onto the interface and reduce the contact area between the two immiscible phases.
generally speaking, the two immiscible phases in an emulsion consist of an oil phase with low polarity and an aqueous phase with high polarity. then, part of the particle stays in the oil phase and the other part of the particle stays in the aqueous phase. this equilibrium of wetting areas is determined by the hydrophilicity or the hydrophobicity of the surface of the particle. the resulting special physical quantity is called the contact angle, which is the equilibrated angle of the particle at the interface after the interfacial tensions between the particle, oil phase and aqueous phase are all balanced.19 First, homogeneous particles are concerned. in Figure 4.2, the three possible situ- ations for a homogeneous spherical particle at the interface are shown. if the surface of the particle is hydrophilic, the interaction with the aqueous phase will be more favourable. it means that the water–particle interfacial tension γwp is lower than the oil–particle interfacial tension γop. Figure 4.3 illustrates the Young’s equation for a homogeneous spherical particle at an oil–water interface.19
γop − γwp = γow cos θ where θ is the contact angle (4.2)
Because (γop − γwp) is a positive number for hydrophilic particles, the contact angle will lie between 0° and 90°. as a result, the majority of the particles stay in the aqueous phase when the minority part of it is wetted by the oil phase.
on the contrary, hydrophobic particles favour interaction with the oil phase rather than interaction with the aqueous phase, and the majority of the parti- cles remain in contact with the oil phase. the contact angle for a hydrophobic particle at the oil–water interface is then between 90° and 180°. Moreover, if the hydrophilicity of the particle is intermediate, the contact angle of the par- ticle will be very close to 90°. the concept of contact angle can be treated as an analogy of hydrophile–lipophile balance (hLB), which is extensively used for surfactant molecules. it is because the hLB is related to the tendency of cur- vature of the interface with surfactant molecules.11 Similar to that of hLB, the contact angle is also related to the tendency of curvature of a particle adsorbed at the interface. if the contact angle of the particle is smaller than 90°, the inter- face will bend to the oil phase.14 then, an oil-in-water (o/w) emulsion forms.
however, if the contact angle of the particle is larger than 90°, the aqueous Figure 4.2 homogeneous spherical particles at oil-water interface. they can be hydrophilic (left), hydrophobic (right) and can have intermediate hydrophilicity (middle). their contact angles depend on their surface hydrophilicity.
Figure 4.3 Schematic drawing of a homogeneous particle at an oil–water interface.
the three interfacial tensions balanced each other according to the Young’s equation.
phase will form a convex interface. in this case, a water-in-oil (w/o) emulsion forms.20 in addition, if the contact angle is around 90°, then both w/o and o/w emulsions may form. it depends on some other factors, such as the type of oil, the ratio between the two phases, etc.21
although colloid particles and surfactant molecules are both capable of attaching onto the oil–water interface, the energy involved to remove them from the interface can be very different. generally, surfactant molecules or amphiphilic polymers are considered to establish an equilibrium between the interface and the bulk solution. the low molecular weight molecules leave the interface easily and molecules in the bulk are adsorbed back simul- taneously.11 however, when the diameters of the colloid particles are larger than around 50 nm, the energy involved in the desorption of individual par- ticles would be very large compared to the thermal energy kBT.
when a homogeneous spherical particle is adsorbed from the aqueous phase to the interface, it replaces some of the oil–water interface with a par- ticle–oil interface. at the same time, the area of the particle–water interface is reduced. therefore, the energy involved to reverse this process, the desorption energy, can be calculated by the following equation (gravity is neglected),22
Ed = γowπr2(1 − |cos θ|)2 (4.3) For example, when a hydrophilic particle with diameter and contact angle of 100 nm and 45° is desorbed from a typical oil–water interface with γow equal to 50 mn m−1, the energy required will be 3.3 × 104 kBT (T = 298 K).
therefore, the adsorption of colloid particles is usually considered to be irre- versible, unless the contact angle is very close to 0° or 180°.14 the result- ing pickering emulsion usually possesses very high stability. it might not undergo complete phase separation even in the time scale of years, which is normally quite difficult to be achieved by traditional small molecule surfac- tants. this is a very important feature of a particle-stabilized pickering emul- sion. the strongly adsorbed particles are capable of forming a close-packed layer which surrounds the emulsion droplets. Figure 4.4 shows a SeM image of a colloidosome made by self-assembly in an o/w pickering emulsion.23 it is not always a must for the particles to align themselves to a hexagonal-liked packing at the interface. nevertheless, such packing is often observed, for both o/w and w/o emulsions. this physical barrier effectively protects the droplets from coalescence. it is because a large amount of energy is required to disrupt and deform the particle layer for coalescence to happen. also, there are interesting interactions between these adsorbed particles, which will be discussed later. the behaviour of this monolayer of particles under compres- sion was studied by aveyard et al. using a Langmuir–Blodgett trough.24 they found out that the particle monolayer distorted to result in a rhombohedral array. they suggested that particle repulsion and buckling might be general features of such particle monolayers.
Colloidal particle dispersions usually possess very high colloidal stability.
the stability of these particles usually originates from the steric stabilization
and electrostatic stabilization. obviously, the electrostatic stabilization refers to the repulsion of the charges on the particle surface. particles can be synthesized or modified so that the surface of the particles possesses ioniz- able functional groups, such as carboxylic groups, amine groups, sulfonate groups, etc.25 they are ionized in aqueous solution and the surface of the par- ticle will be charged. these charged particles can also stabilize a pickering emulsion. this electrostatic repulsion of the particles is going to provide the resulting droplets’ electrostatic stabilization as well. in Figure 4.5, two drop- lets covered by charged particles are illustrated. when they approach each other, the repulsion between the charges on the particles repels the droplets.
For steric stabilization, it is given by the dangling chains on the particle surface (Figure 4.5). these dangling chains are solvated by the continuous phase. when the emulsion droplets are very close to each other, the dangling chains layer may be overlapped. as it happens, the osmotic pressure of the overlapping region increases. then, the continuous phase will be drawn to the overlapping region and prevent the droplets from getting closer. this steric stabilization is quite common for polymeric colloidal particles, such as Figure 4.4 SeM image of a colloidosome made by self-assembly of polystyrene par-
ticles in o/w pickering emulsion.23 reprinted (adapted) with permis- sion from M. F. hsu, M. g. nikolaides, a. d. dinsmore, a. r. Bausch, V. d. gordon, X. Chen, J. w. hutchinson and d. a. weitz, Langmuir, 2005, 21, 2963–2970. Copyright 2005 american Chemical Society.
poly(N-isopropylacrylamide) microgel particles. Similar to the case of electro- static stabilization of the particles, steric repulsion of the particles also con- tributes in enhancing the stability of the emulsion droplets in a pickering emulsion. they stabilize the emulsion by slowing down the flocculation and coalescence process as they avoid the droplets from getting to close proximity.
particle–particle interactions are not limited to the particles on different droplets. they also exist between the particles on the same droplet. Kral- chevsky and nagayama discussed and reviewed different capillary interac- tions between particles at interfaces.26,27 Figure 4.6 shows some of these interactions. normally flotation interactions, which are driven by the grav- ity, are negligible for colloid particles. however, the immersion interactions, which are driven by the wetting of particles, are usually large compared to the thermal energy kBT.26 this happens even if the diameter of the particle is just a few nanometers. therefore, Brownian force might not be able to counteract the capillary interactions between the particles. then, they are attracted by each other and aggregate at the interface. at the same time, the interactions originated on the particles themselves, which are mentioned above, balance this capillary attraction. these complicated particle–particle interactions affect the packing of the adsorbed particles at the interface. thus the emul- sion stability of the resulting pickering emulsion is also dependent on them.
another important feature of a pickering emulsion is called maximum capillary pressure, Pmax.28 First, two colliding emulsion droplets are consid- ered in an o/w pickering emulsion. each of the oil droplets is protected by a Figure 4.5 illustration of electrostatic and steric stabilization given by microgel
particles.
single layer of particles. when they are pushed together, each particle shell is deformed and rearranged. a bridged interface with single layer of particles between the two oil droplets will then be formed (see Figure 4.7).29 then, there will be a thin aqueous film located right between the oil droplets. if these two touching droplets are further pushed together, it causes the aque- ous interfilm to drain. Finally, the interfilm is completely drained, and the droplets will touch and combine. however, as the particles here are hydro- philic, there will be a capillary pressure which counteracts the interfilm drainage. the capillary pressure increases as the interfilm drainage goes on, until the pressure pushing the two droplets together exceeds the maximum capillary pressure of the system. at this point, the aqueous interfilm between them is ruptured and coalescence of the droplets occurs.
denkov and co-workers28 have introduced this idea as a stabilization mech- anism of particle-stabilized emulsions. Later, this study was also studied by other groups.30 the maximum capillary pressure of a particle layer between two oil droplets can be calculated by the follow equation:
ow max
2P cos
P r
= γ θ
(4.4) Figure 4.6 different capillary forces acting on solid particles at an interface.26 reprinted from p. a. Kralchevsky and K. nagayama, Capillary interac- tions between particles bound to interfaces, liquid films and biomem- branes, Adv. Colloid Interface Sci., 2000, 85, 145–192, Copyright 2000, with permission from elsevier.
where P in the equation is called theoretical packing parameter and r is the radius of the particles. in another study, which was done by tcholakova et al.,31 the maximum capillary pressure was studied with β-lactoglobulin protein by the film trapping technique. Besides that, they also measured the maximum osmotic pressure of the emulsion before coalescence by putting the emulsion into a centrifuge. they found out that this maxi- mum osmotic pressure was a complete analogy of the maximum capillary pressure. it was because centrifugation applied a pressure to the aqueous film and drained it by the density difference of the two phases. never- theless, measuring the maximum osmotic pressure by centrifugation was very convenient. it was also applied by other researchers for comparing emulsion stability.32 Maximum capillary pressure or maximum osmotic pressure of the emulsion gives us a general and convenient way to com- pare and study the emulsion stability of a pickering emulsion. note that there is an interesting relationship between the desorption energy and the maximum capillary pressure. in eqn (4.3), desorption energy of the particle increases as the contact angle approaches 90°. on the other hand, eqn (4.4) indicates that the maximum capillary pressure of the interfilm with the particles increases as the contact angle approaches 0° (or 180° for w/o pickering emulsion). For a contact angle approaching 90°, the parti- cle is likely to be adsorbed to the interface more strongly for both o/w and w/o emulsion. however, the particle would not be preferentially wetted by Figure 4.7 Schematic drawing of two colliding oil droplets. eventually, an aqueous film will be formed in between the droplets. Further pushing causes the film to drain and a capillary pressure is built up.
the continuous phase significantly. the capillary pressure which draws in the continuous phase and maintains the interfilm would be much lower.
therefore, Kapkay et al. evaluated the combined effect of them.30 it was predicted that the best contact angle for stabilizing a pickering emulsion would be 70° (or 110° for w/o pickering emulsion), which is shown in Figure 4.8.
therefore, to control the stability of a particle-stabilized pickering emul- sion, the above factors are to be altered by designing specific particles and changing the external conditions. this means that the emulsion can be prepared with excellent stability, yet it can be destabilized in a controlled manner when needed. these responsivenesses will be discussed in the next section.
Figure 4.8 Combined effect of desorption energy and maximum capillary pres- sure, which is considered energetically.30 reprinted from g. Kaptay, on the equation of the maximum capillary pressure induced by solid par- ticles to stabilize emulsions and foams and on the emulsion stability diagrams, Colloids Surf., A, 2006, 282, 387–401, Copyright 2006, with permission from elsevier.