Emulsion stabilization by non-ionic surfactants: experiment and theory

Abstract

Two recent texts (Becher, 1965, Sherman, 1968 have dealt in some detail with emulsions, but little attention has been given to mechanisms of emulsion stabilization by non-ionic materials, in spite of the widespread industrial use of non-ionic surfactants. Emulsions stabilized by these non-ionic compounds present interesting problems not encountered in systems which contain ionic surfactants.At first sight it would appear that stability cannot be explained by the electrostatic repulsive forces which operate in many dispersions, but it is known that pure hydrocarbon droplets dispersed in pure water possess a net negative charge. The problems which arise in the theory of non-ionic dispersions are these. To what extent do electrostatic forces contribute to stability? How is any charge on the dispersed particles affected by the presence of non-ionic detergents or polymer molecules at the interface? What is the nature of other stabilizing forces? Lyklema (1968) recently discussed the principles of stability of colloidal dispersions in non-aqueous media. Much of what he has written is relevant here, although we will restrict our discussion mainly to oil-in-water emulsions. The outline of the theories of stability will thus be kept to a minimum.It is the purpose of this review to consider relevant aspects of colloid stability theory and the relation of some recent experiments to this theory, and to discuss this with regard to the use of these emulsion systems. We will deal mainly with stabilizers which are derivatives of polyoxyethylene glycols. ASPECTS OF EMULSION STABILITYEmulsions have been defined as heterogeneous systems of one liquid dispersed in another in the form of droplets usually exceeding 0.1 pm (Becher, 1965) in diameter. The two liquids are immiscible, chemically unreactive, and form systems characterized by a minimal thermodynamic stability.Unless the free energy of the oil-water interface is zero, an emulsion cannot be a thermodynamically stable system, since reduction of the area of oil in contact with the water will always result from coalescence of the droplets. "Stability" is therefore a relative term; but the degree of stability can be assessed by observing the rate of change of a parameter such as interfacial area or droplet diameter. Unstabilized emulsions, referred to as oil hydrosols by King (1941), coalesce rapidly, while stabilized emulsions can retain a highly dispersed internal phase for months or years. As the free energy of the interface is the driving force for coalescence, emulsions can be stabilized by the inclusion of a surface-active substance in the * The first of two articles, the second of which will appear on p. 233 et seq. and includes all the references. t Present address :

Mixed Composition Films of Spans and Tween 80 at the Air−Water Interface

“…Surfactants stabilize emulsions by reduction of interfacial tension and formation of electrical or mechanical barriers. 8,9 When a rigid interfacial complex forms at the primary W-O interface, emulsion stability is enhanced. 6,7 Improved stability can also be achieved by maximizing the interfacial strength.…”

Polymerization of aniline hydrochloride in reverse of microemulsion by batch and semicontinuous process using ionic and nonionic surfactants

“…Thus, for the aniline hydrochloride-water/AOT/cyclohexane system, the interactive attraction between the droplets decreases by making the layer more rigid due to the packing of the polar groups so that the degree of interpretation of the droplet is reduced and the reduction of the effect of curvature [ 19 ]. However, for the system where was used a nonionic surfactant such as PEG-SORHEX, the solubility of aniline hydrochloride-water is favored by the effects of the composition chemical of surfactant nonionic [ 20 ], which changed the radius of curvature.

Figure 1.

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Colloidal and thermal characteristics of concentrated dispersions of polymethacrylate-based latices for aqueous enteric coating

“…16,17 Some simple experiments were performed in order to gain some insight into the interparticle repulsion/ stabilization mechanism. Addition of small amounts of salt (e.g.…”