Emulsion stabilization, theories

The preceding treatment relates primarily to flocculation rates, while the irreversible aging of emulsions involves the coalescence of droplets, the prelude to which is the thinning of the liquid film separating the droplets. Similar theories were developed by Spielman [54] and by Honig and co-workers [55], which added hydrodynamic considerations to basic DLVO theory. A successful experimental test of these equations was made by Bernstein and co-workers [56] (see also Ref. 57). Coalescence leads eventually to separation of bulk oil phase, and a practical measure of emulsion stability is the rate of increase of the volume of this phase, V, as a function of time. A useful equation is... 

Graham, D.E. and Philips, M.C.,"The Confcamation of Proteins at Interfaces and their Role in Stabilizing Emulsions", in Theory and Practice of Emulsion Technology, cd. Smith, Ai., Academic Press, New York (1976).. 

Figure I. DLVO Theory for explaining emulsion stability...

Detailed description of theory, measurements, and applications in emulsion science covering all aspects of emulsion stability. 

Caims, R.J.R. Grist, D.M. Neustadter, E.L. The effects of Crude Oil-Water Interfacial Properties on Water-Crude Oil Emulsion Stability in Theory and Practice of Emulsion Technology, Smith, A.L. (Ed.), Academic Press New York, 1976 pp. 135-151. 

Schulman and Cockbain (see Appendix) have recently put forward a new theory of emulsion stability and inversion. 

Liquid emulsions are inherently unstable to a varying degree. It is important to understand, therefore, the mechanisms that contribute to emulsion stability. Before the solidification step, instability of an emulsion can arise due to either phase separation or phase inversion (Mulder and Walstra, 1974). It is evident that the likelihood of phase inversion will increase as the fraction of dispersed phase increases. The vast majority of literature references are concerned with the stability to phase separation as coalescence or creaming in oil-in-water emulsions (Hailing, 1981 Jaynes, 1983). In addition, a method for determining the stability of water-in-oil emulsions to inversion has not been reported. It is usually assumed that certain aspects of oil-in-water emulsion theory apply in reverse to water-in-oil emulsions. 

To quantify the increase of a due to pressure, a mean bubble diameter has been estimated using Taylor s stability theory [7] on bubble deformation and break-up in sheared emulsions. According to this theory, bubble size in a sheared emulsion results from a balance between viscosity and surface tension forces. The dimensionless number that describes the ratio of these forces is called the capillary number Q. For large bubble deformations, the maximum stable bubble diameter in a shear flow is expressed as [8] ... 

Very often, the microstructure and the macroscopic states of dispersions are determined by kinetic and thermodynamic considerations. While thermodynamics dictates what the equilibrium state will be, kinetics determine how fast that equilibrium state will be determined. While in thermodynamics the initial and final states must be determined, in kinetics the path and any energy barriers are important. The electrostatic and the electrical double-layer (the two charged portions of an inter cial region) play important roles in food emulsion stability. The Derjaguin-Landau-Verwey-Oveibeek (DLVO) theory of colloidal stability has been used to examine the factors affecting colloidal stability. 

Kabalnov, A.S. Shchukin, E.D. Ostwald ripening theory applications to fluorocarbon emulsion stability. Adv. Colloid Interf. Sci. 1992, 38, 69-97. 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

The DLVO theory, which was developed independently by Derjaguin and Landau and by Verwey and Overbeek to analyze quantitatively the influence of electrostatic forces on the stability of lyophobic colloidal particles, has been adapted to describe the influence of similar forces on the flocculation and stability of simple model emulsions stabilized by ionic emulsifiers. The charge on the surface of emulsion droplets arises from ionization of the hydrophilic part of the adsorbed surfactant and gives rise to electrical double layers. Theoretical equations, which were originally developed to deal with monodispersed inorganic solids of diameters less than 1 pm, have to be extensively modified when applied to even the simplest of emulsions, because the adsorbed emulsifier is of finite thickness and droplets, unlike solids, can deform and coalesce. Washington has pointed out that in lipid emulsions, an additional repulsive force not considered by the theory due to the solvent at close distances is also important. 

The DLVO theory does not explain either the stability of water-in-oil emulsions or the stability of oil-in-water emulsions stabilized by adsorbed non-ionic surfactants and polymers where the electrical contributions are often of secondary importance. In these, steric and hydrational forces, which arise from the loss of entropy when adsorbed polymer layers or hydrated chains of non-ionic polyether surfactant intermingle on close approach of two similar droplets, are more important (Fig. 4B). In emulsions stabilized by polyether surfactants, these interactions assume importance at very close distances of approach and are influenced markedly by temperature and degree of hydration of the polyoxyethylene chains. With block copolymers of the ethylene oxide-propylene oxide... 

Emulsions have been widely used as vehicles for oral, topical, and parenteral delivery of medications. Although the product attributes of an emulsion dosage form are dependent on the route of administration, a common concern is the physical stability of the system, in particular the coalescence of its dispersed phase and the consequent alteration in its particle-size distribution and phase separation. The stabilization mechanism(s) for an emulsion is mainly dependent on the chemical composition of the surfactant used. Electrostatic stabilization as described by DLVO theory plays an important role in emulsions (0/W) containing ionic surfactants. For 0/W emulsions with low electrolyte content in the aqueous phase, a zeta potential of 30 mV is found to be sufficient to establish an energy maximum (energy barrier) to ensure emulsion stability. For emulsions containing... 

Some progress toward an understanding of these systems is also possible by considering the influence of the presence of water within the oil drops on the interaction between the oil drops and by consideration of the influence of the size of the internal water droplets on their internal stability and on the possibility of coalescence with the external aqueous phase. It is premature to consider all this in detail as the application of colloid stability theory to simpler emulsions has not been particularly successful (37). For type A w/o/w emulsions, the approach of Void (38) may perhaps be used if the oil layer is thought of as the homogeneous adsorbed layer. 

It can be seen that there are a lot of parameters influencing emulsion stability. Some of them can be predicted by theory, while others are able to be explained by experimental results. An emulsion breaking device has been developed which enables the investigation of various parameters and different emulsion systems. Thus, on the one hand, emulsion stability can be compared with theory, while on the other hand, conclusions about coalescence processes are obtained. 

From the survey of the above literature, it is concluded that only a limited work is done on such type of problems. In the present study, the stability of emulsion has been discussed in the light of Derjaguin, Landau, Vervey and Overbeek theory (8) using Deoxyribonucleic acid and ribonucleic acid as flocculants for the emulsion stabilized by the drug sulphapyridine. 

Verrips, C.T. and Zaalberg, J. 1980. The intrinsic microbial stability of water-in-oil emulsions I. Theory. European Journal of Applied Microbiology and Biotechnology 10 187-196. 

Florence AT, Rogers JA. Emulsion stabilization by non-ionic surfactants experiment and theory. J Pharm Pharmacol 1971 23 153-169, 233-251. 

Interrelationship of Emulsion Stability and Interfacial Viscosity in Improved Oil Recovery," paper presented at the Engineering Foundation Conference on Theory, Practice... 

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