Interaction energy emulsion

Fig. XrV-6. (a) The total interaction energy determined from DLVO theory for n-hexadecane drops for a constant ionic strength - 5.0 nm) at various emulsion pH (b) enlargement of the secondary minimum region of (a). (From Ref. 39.)...

But the droplets are fragile, and must be lucidly protected. Formulating an industrial emulsion implies numerous conditions stability, efficiency, easy delivery, price,. .. This is an art, and like all forms of art it requires experience and imagination. The present book provides both. It describes basic experiments on realistic model systems. I like this matter of fact approach. For instance, instead of beginning by formal discussions on interaction energies, the book starts with methods offabrication. And, all along the text, the theoretical aspects are restricted to basic needs. 

The interactions between two emulsions droplets can be described in terms of the interaction energy, or inter-droplet pair potential, w(h), which is the energy required to bring two emulsion droplets from an infinite distance apart to a surface-to-surface separation distance, h (McClements, 1999) ... 

The presence of surfactants, either natural or added, promotes emulsion stability by the reduction of interfacial tension and the formation of highly rigid films on the surface of the droplets. This reduction of interfacial tension can increase the maximum, M, in Figure 4 significantly through charge stabilization or steric stabilization (J5). Because the nature and shape of the interaction energy curve determine the stability of OAV (and other types) of emulsions, any process, parameter, or phenomenon that affects the shape of this curve will ultimately control emulsion stability. 

Short Range Interactions and Emulsion Stability. The stability of macroemulsions in terms of short range (e.g. inter-droplet) interactions will be discussed in this section. The dispersion (London) forces arise from charge fluctuations within a molecule associated with the electronic motion (21). Therefore, these forces can operate even between nonpolar molecules. London (21) reported an equation for mutual attractive energy between two molecules in vacuum in the form... 

From the interaction energy curves of Figures 4, 6, and 7 and the the corresponding data of Table IV, it is evident that the interparticle distance, at which energy is equal to zero, decreases with increasing concentrations of RNA and DNA, which indicates the flocculation of the emulsion system. 

Figure 6. Plots of interaction energy vs. interparticle distance for sulfapyridine-stabilized emulsion with DNA concentrations.

Fig. 5 Variations of the interaction energy between particles versus the center-to-center distance, (a) Hertzian potentials for particles with bulk elasticity the dashed line represents the usual Hertz potential (1), and the solid line the generalized Hertzian potential (2). (b) Potentials for emulsions with surface elasticity the dashed line denotes the approximate solution for small compression ratios (3), and the solid line the general solution (4). (c) Ultrasoft potentials for star polymers (where a 1.3Rg, with Rq being the radius of gyration of the star, following [123]) (5) dashed line f = 256 solid line f = 128 dashed and dotted line f = 64. (d) Hard-sphere potential...

Interaction Energies (Forces] between Emulsion Droplets and Their Combinations... 

Generally speaking, there are three main interaction energies (forces) between emulsion droplets and these are discussed below. 

Another geometric configuration, which corresponds to two colliding deformable emulsion droplets, is sketched in Figure 5.23. hi this case the interaction energy is given by the expression ... 

The phase behaviour at equilibrium turned out to be the main property reported in Win-sor s work in the late 1940s. Winsor interpreted the phase behaviour through the so-called R ratio of molecular interaction energies at interface. The R ratio was a handy theoretical concept to understand the variations of the phase behaviour of surfactant-oil-water systems and somehow of the emulsion properties. It is essentially qualitative, but for the first time the phase behaviour was linked with a condition that depended on all formulation variables, but could be expressed as a single generalised variable, i.e. the R ratio [1]. The original R ratio was... 

Particle shape. Calculation of the interaction energy works well for perfect spheres, i.e., for most O W emulsions. Although equations for a number of simple particle geometries are to be found in the literature, application to real nonspherical particles often involves several difficulties. For instance, the shape may be irregular and variable, and the particles can encounter each other in a number of orientations. 

In an attempt to evaluate the effect of electrostatic interaction on the stability of crude oil-caustic or orthosilicate emulsions, the total interaction energy (Vt) between two oil... 

The role of the interaction energy between surfactant molecules and liquids in the stabilization of emulsions is reflected in so-called Bancroft s rule [36,46]. This rule states that in the emulsification, the liquid in which the emulsifying agent is more soluble becomes the dispersion medium. Thus, water soluble surfactants stabilize direct oil-in-water emulsions, while oil soluble surfactants stabilize inverse water-in-oil emulsions. 

To determine the rate constants of coalescence, Danov et al. (147) examined the effects of the droplet interactions and the Brownian motion on the coalescence rate in dilute emulsions of micrometer- and submicrometer-sized droplets. The processes of film formation, thirming, and rupture were included as consecutive stages in the scheme of coalescence. Expressions for the interaction energy due to various DLVO and nonDLVO surface forces between two deformed droplets were obtained (143). 

The introduction by Winsor (15) of the theoretical concept that an emulsion formulation could be represented by a single parameter relating the ratio of the interaction energies between adsorbed surfactant molecules and the oil and water in the system was the next step in understanding emulsion formulation. It was shown that the state and properties of a system at equilibrium were directly related to a particular combination of the interactions between surfactant, water and oil. This combination of interactions was expressed as the ratio R. A ratio R < 1 means that the interactions between surfactant and oil are stronger than those between surfactant and water. In this case, the surfactant/oil/water systems have a tendency to form w/o emulsions. When / = 1, the surfactant-oil and surfactant-water interactions are balanced. Such a system forms a thermodynamically stable bicontinuous microemulsion. Finally, R > 1 means that the interactions between the surfactant and water molecules are stronger than the interactions between the surfactant and oil molecules, which thus leads to o/w emulsions. 

In the case of small Brownian deformable droplets the interaction energy, when the continuous phase is a micellar surfactant solution of sodium nonylphenol polyoxy-ethylene-25 (SNP-25S), is illustrated in Fig. 8 as a function of the thickness, A, and film radius, R (see Fig. 3 for the definition of the geometry). The parameters of the micro-emulsion system are R = 2 im, d = 9.8 nm, (j> = 0.38, Ah = 5x10" J, <7= 7.5 mN/m, % = -135 mV, r = 1.91 nm, the electrolyte concentration 25 mM. The points on the contour plot (Fig. 8) correspond to tree local minima of -406 AbT, -140 k T and -37 k T corresponding to film containing 0, 1 and 2 micellar layers, respectively (Ivanov et al. 1999). These three possible films are thermodynamically stable and they act like barriers against the closer approach and flocculation (or coalescence) of the droplets in emulsions. 

Under certain circumstances it is possible to utilize physical interactions to maintain surfaces at some minimum distances of separation as a result of an energy maximum in the interaction energy. The practical result of such long-range energy maxima is that, properly utilized, they can prevent or at least retard the natural tendency of surfaces to approach and join spontaneously, thereby reducing interfacial area. This effect is especially important in colloids, foams, emulsions, and similar systems. 

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