Stabilizing an emulsion

Two Liquids Plus a Solid. SoHd particles may be used to stabilize an emulsion, avoiding the problem of simultaneous stabilization of both the oil drops of the emulsion and the soHd particles of the suspension. The key factor for the use of particles as stabilizers is their location. If they are located at the iaterface between the two Hquids, they will stabilize the emulsion, serving as a mechanical barrier to prevent the coalescence of the droplets (Fig. 17). 

Emulsion Stability is the property of the emulsifier (the interphase molecules) to stabilize an emulsion following its formation and, sometimes, following certain stress conditions. In this test, only three steps are observed emusification, stress, and measurement. The stress may be only an applied force (gravity or centrifugal forces) or heat or a combination of both. 

The theory has certain practical limitations. It is useful for o/w (oil-in-water) emulsions but for w/o (water-in-oil) systems DLVO theory must be applied with extreme caution (16). The essential use of the DLVO theory for emulsion technology lies in its ability to relate the stability of an o/w emulsion to the salt content of the continuous phase. In brief, the theory says that electric double-layer repulsion will stabilize an emulsion, when the electrolyte concentration in the continuous phase is less than a certain value. 

The most common agents to stabilize an emulsion are surfactants. Different effects contribute to the stabilization of emulsions. Steric repulsion between those parts of the surfactant, which are in the continuous phase, is an important effect. For a water-in-oil emulsion the hydrocarbon chains are hindered in their thermal movements if two water drops approach each other too closely. For an oil-in-water emulsion there is an additional effect the hydrophilic head groups have to be dehydrated to come into close contact. The resulting hydration repulsion stabilizes the emulsion. 

Of course, interfacial tension lowering alone may not be sufficient to stabilize an emulsion, in which case other interfacial properties must be adjusted as well. These simple calculations do, however, show how important the interfacial properties can become when colloidal-sized species are involved, as in the case of emulsions. 

In food science, ice crystals play a role in stabilizing ice cream, while fat crystals play a role in stabilizing whipping cream. It should be emphasized that in practice the contact angles should neither be too high nor too low, or else the partides will remain in the water or oil phases, respectively, and not stabilize an emulsion [297]. Although there are many exceptions to such rules they remain useful for making initial predictions. The particles used for emulsion stabilization have to be much smaller than the size of the emulsion droplets they are intended to stabilize so the particles are usually quite small, typically less than 1 pm in diameter. 

Any agent that acts to stabilize an emulsion. The emulsifier can make it easier to form an emulsion and to provide stability against aggregation and possibly coalescence. Emulsifiers are frequently but not necessarily surfactants. 

Proteins are dynamic molecules with respect to structure. The preferred folded structure for a given set of environmental conditions is that which has the minimum free energy. The driving force to assume a given folded structure is a thermodynamic force. In aqueous systems, the hydrophobic side-chains will endeavour to orient away from the surrounding water and towards the core of the molecule. However, for high surface activity, it is essential that the protein molecule should unfold and orient its hydrophobic side-chains towards the oil phase. A lack of hydrophilic residues usually does not restrict protein functionality at interfaces. Thus, flexible proteins can create a highly hydrated, mobile layer to stabilize an emulsion particle. 

The energy plotted in Figure 7 was obtained by multiplying the total area by the interfacial tension. Now if a small quantity of a surfactant was added to the water, possibly a few tenths of a percent, that lowered the interfacial tension to 0.35 mN/m, it would lower the amount of mechanical energy needed in the example by a factor of 100. From the area per molecule that the adsorbed emulsifying agent occupies, the minimum amount of emulsifier needed for the emulsion can also be estimated. In practice, lowering interfacial tension alone may not be sufficient to stabilize an emulsion, in which case other interfacial properties must be adjusted as well. These simple calculations do, however, show how important the interfacial properties can become when colloidal-sized species are involved, as in emulsions. 

Figure 11. Schematic representation of the electrophoretic mobility (A) measurement showing the major components. In an applied electric field, emulsion droplets move according to their surface charge. These charges can electrostatically stabilize an emulsion system by preventing the droplets from coming into contact and coalescing. The motion of the droplets is visually observed, and the electrophoretic mobilities of a number of particles are measured to determine zeta potential. The sedimentation potential (B) is also illustrated.

There are limitations on the size. As has been estimated by theoretical calculations, for spherical particles, 4 nm will be the approximate minimum size that should stabilize an emulsion or foam. This represents the smallest particle that will not be enveloped by the amplitude of surface corrugations. ... 

Apart from the protein concentration at an interface, the structure of protein at the interface also plays an important role in stabilizing an emulsion (Friberg et al.,... 

Emulsion Stability. An emulsion is defined as a macroscopic dispersion of two liquids, one of which forms the continuous phase of the system and the other forms the discrete phase. An emulsion of two liquids without a stabilizer will quickly break into two liquid layers. Emulsions destabilize by three distinct processes breaking, creaming, and flocculation (Figure 14). When emulsions break, the initial small droplets of the emulsion spontaneously join to form larger droplets. This process is termed coalescence , and it ultimately leads to two separate liquid layers. 

McMahon studied the effect of waxes on emulsion stability as monitored by the separation of water over time (46). The size of the wax crystals showed an effect in some emulsions but not in others. Interfacial viscosity indicated that the wax crystals form a barrier at the water/oil interface which retards the coalescence of colliding water droplets. Studies with octacosane, a model crude oil wax, show that a limited wax/asphaltene/resin interaction occius. A wax layer, even with absorbed asphaltenes and resins, does not by itself stabilize an emulsion. McMahon concludes that the effect of wax on emulsion stability does not appear to be through action at the interface. Instead, the wax may act in the bulk oil phase by inhibiting film thinning between... 

Emulsifying agent A compound that has a water-soluble part as well as an oil-soluble part and that stabilizes an emulsion... 

FIGURE 1L3. To effectively stabilize an emulsion, colloidal particles must have a proper balance of surface properties. If the particles are preferentially wetted by the continuous phase, they will be poorly adsorbed and easily desorbed from the interface and provide little stability (a). If they are preferentially wetted by the dispersed phase they will not be adsorbed at the interface and will again provide Uttle stabUity (b). For optimum effectiveness the particles should be partially wetted by both phases to insure their location at the interface (c). 

Silicones bearing both hydrophilic and lipophilic substituents are amphiphilic compounds. They are well known as W/0 emulsifiers that are used in a broad range of cosmetic applications [8]. When cholesterol is used as a lipophilic substituent, an amphiphilic molecule is formed that has the ability to form both lyotropic and thermotropic LC structures at the water/oil interface. These structures are in principle able to stabilize an emulsion excellently . ... 

The above thermodynamic argument implies that to stabilize an emulsion against flocculation and coalescence, one needs to create an energy barrier between the droplets to prevent their close approach (whereby the van der Waals attraction is strong). Several... 

Micro-encapsulation involves stabilizing an emulsion by forming strong films around each droplet. This can be done with a combination of colloidal silica and gelatin as described by Brockett (685). Further study of the interaction of colloidal silica with cationic polymers or with cationic surfactants in oil or polymer emulsions and dispersions in water appears worthwhile, particularly from the standpoint of highly stabilized interfaces. 

From the above discussion, a surfactant/co-surfactant combination for a microemulsion is clearly of little use to stabilize an emulsion. This is a disadvantage when a multiple emulsion of the 5 O/W type is to be formulated, whereby the W/ O system is a microemulsion. This problem has been resolved by Larsson et al. [13], who used a surfactant combination to stabilize the microemulsion and a polymer to stabilize the emulsion. 

From the beginning, the formulator has to decide which type of formulation will be developed emulsion (O/W, W/O, W/O/W), microemulsion, or suspension. This section addresses primarily the emulsions and their most frequent form O/W. The orientation O/W or W/O calls for hydrophilic surfactants, sequence of addition, a relatively high proportion of water or hydrophobic surfactants, and a higher amount of oil. The ingredients have to be selected on the basis of their electrical compatibility. Stabilizing an emulsion wherein materials of opposite charges are present is always a challenge. 

---