Stability of foams and emulsions

We have used film interferometry to reveal a new mechanism for the stabilization of foams and emulsions due to layering inside the thinning films, as will be discussed below. 

A closer relationship between foam stability and HLB has been reported for two- or three-phase systems surfactant solution-oil or oil-surfactant phase-water [60,109-111]. The effect of various parameters changing HLB on the stability of foams and emulsions has been studied in [111]. These were the concentration of amyl alcohol and sodium chloride, the number of the ethylene oxide groups in the molecule of the oxyethylated octylphenol. As a general parameter of HLB the authors used the surfactant affinity difference concept (SAD) which is an empirical generalised formulation. It measures the deviation from the optimum formulation for three phase behaviour. For anionic surfactants... 

Liquid crystal structures are important not only in the viscosity modification of surfactant solutions, but also in the stabilization of foams and emulsions, in detergency, in lubrication (Boschkova, 2002), and in other applications. 

Most of the traditional adsorption studies of surfactants correspond to dilute systems without aggregation in the bulk phase. At the same time micellar solutions are much more important from a practical point of view. To estimate the equilibrium adsorption, neglecting the effect of micelles can usually lead to reasonable results. However, the situation changes for nonequilibrium systems when the adsorption rate can increase by orders of magnitude when the of surfactant concentration is increased beyond the CMC. Current interest in the adsorption from micellar solutions is mainly caused by recent observations that the stability of foams and emulsions depends strongly on the concentration in the micellar region [1]. This effect can be explained by the influence of the micellisation rate on the adsorption kinetics. 

The behavior of proteins at interfaces influences the formation of foams and emulsions (32). Stabilization of foams and emulsions depends, to a great extent, on the formation, rheological, and mechanical properties of the interfacial film ( ). Factors which ensure optimum film properties in simple systems may retard film formation or cause destabilization in foams or emulsions (3 ) for example, many rheological properties of films are maximum in the isoelectric pH range of specific proteins, yet most proteins have minimum solubility in this pH range (34). Thus, environmental and processing factors which... 

The stability of foams and emulsions depends critically on whether formation of a stable Newton black film or a hole leading to coalescence is favored. Kabalnov and Wen-nerstrom (4) addressed this question by developing a temperature-induced hole nuclea-tion model applicable to emulsions. They point on that the coalescene energy barrier is strongly affected by flic spontaneous monolayer curvature. The aufliors consider a flat emulsion film, covered by a saturated surfactant monolayer, in thermodynamic equi-... 

It is generally found that good stabilization of foams and emulsions requires compounds that give condensed monolayers. Compounds that give expanded or gaseous monolayers do not, as a rule, act as stabilizers in fact, they are more likely to be destabilizers. The compounds that enhance loaf volume or have little effect are those that give condensed... 

Oscillatory structural forces appear in thin films of pure solvent between two smooth solid surfaces and in thin liquid films containing colloidal particles including macromolecules and surfactant micelles (Israelachvili 1992). In the first case, the oscillatory forces are called the solvation forces and they are important for the short-range interactions between solid particles and dispersions. In the second case, the structural forces affect the stability of foam and emulsion films as well as the flocculation processes in various colloids. At lower particle concentrations, the structural forces degenerate into the so-called depletion attraction, which is found to destabilize various dispersions. 

Kruglyakov, P.M., Equilibrium properties of free films and stability of foams and emulsions, in Thin Liquid Films, Ivanov, LB. (Ed.), Marcel Dekker, New York, 1988, p. 767. 

Not only the equilibrium sruface tension but also the kinetic properties of a surfactant adsorption monolayer play an important role in various phenomena related to the stability of foams and emulsions [5,30], rising of bubbles and flotation [31]. Indeed, many processes are accompanied by disturbances (expansion, compression) of the adsorption monolayer or by formation of new surface of the solution. The surfactant solution has the property to damp the disturbances by diffusion of the surfactant from the bulk to the interface, or vice versa. The main subject of this section is the theory of adsorption and surface tension under such dynamic conditions. 

In addition to the thermodynamic characteristics of the adsorption equilibrium the dynamic dilational visco-elasticity of the surfactant interfacial layers is a very important quantity [5]. This Ireqnency dependent property of a liquid interface is a significant quantity in the stabilization of foams and emulsions. Of course, in many practical situations mixtures of surfactants with particles, polymers or proteins are used, however, these rather complex systems are not the subject of this work. 

We have now stated clearly the main features characterizing the stability of foams and emulsions in terms of their correlation with interfacial phenomena. The following section illustrates such correlation. 

The ultimate aim of this research work is to understand the stability of colloidal dispersions on the basis of the fundamental surface properties displayed by the same systan. To this end, we show several situations that illustrate the existing relationship between the different phenomena. In particular, we show the differences between the foam stability of two food proteins the complex behavior of foams formed with mixed systems piotein/surfactant, and finally, the stability of foams and emulsions of a model protein. In all the cases, the systems are evalnated over different length scales ranging from structural to functional properties. 

The behavior of particles at interfaces undoubtedly has important practical consequences, not the least in determining the stability of foams and emulsions containing particles. Whereas aqueous foam stability and breakdown in the presence of hydrophobic particles have been reasonably well studied in recent years, there are still a number of problems to be resolved, including how particle irregularities, particle size, and dynamic wetting effects can be adequately treated. When it comes to emulsion stability in the presence of particles, far less systematic experimental work has been done. There are intriguing similarities between particle wettability (in particle-stabilized emulsions) and smfactant HLB (in surfactant-stabilized emulsions). The study of such similarities, both experimentally and theoretically, will no doubt prove rewarding. 

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