Emulsion Bancroft rule

The previous experimental observations reported in the preceding text are, at least to a certain extent, in agreement with the well-known Bancroft rule. Indeed, a double W/O/W emulsion turns into a simple direct one when a sufficient quantity of the water-soluble surfactant is added. Similarly, by shaking a 1 1 mixture of water and oil, each phase containing one of the two types of surfactants, a direct emulsion is obtained if the aqueous phase contains a large amount of water-soluble... 

The interfacial tension is a key property for describing the formation of emulsions and microemulsions (Aveyard et al., 1990), including those in supercritical fluids (da Rocha et al., 1999), as shown in Figure 8.3, where the v-axis represents a variety of formulation variables. A minimum in y is observed at the phase inversion point where the system is balanced with respect to the partitioning of the surfactant between the phases. Here, a middle-phase emulsion is present in equilibrium with excess C02-rich (top) and aqueous-rich (bottom) phases. Upon changing any of the formulation variables away from this point—for example, the hydrophilie/C02-philic balance (HCB) in the surfactant structure—the surfactant will migrate toward one of the phases. This phase usually becomes the external phase, according to the Bancroft rule. For example, a surfactant with a low HCB, such as PFPE COO NH4+ (2500 g/mol), favors the upper C02 phase and forms w/c microemulsions with an excess water phase. Likewise, a shift in formulation variable to the left would drive the surfactant toward water to form a c/w emulsion. Studies of y versus HCB for block copolymers of propylene oxide, and ethylene oxide, and polydimethylsiloxane (PDMS) and ethylene oxide, have been used to understand microemulsion and emulsion formation, curvature, and stability (da Rocha et al., 1999). 

While microemulsions are thermodynamically stable, and the stability of emulsions has a kinetic origin, in both cases the adsorption of the dispersant upon the interface of the globules is responsible for stability. For this reason it appears natural to attempt to explain the above equality between the two inversion temperatures on the basis of surfactant adsorption. In addition, both the micro and macro-emulsions obey in many cases the Bancroft rule [8,9], which indicates that the phase in which a larger amount of dispersant is present becomes the continuous phase there are, however, some violations of this rule which will be discussed later in the paper. 

The scope of the present review is to emphasize that thermodynamics can explain the above experimental observations. The next section (Section 2), which is based on ref. [10], will be concerned with the effects of HLB (denoted in what follows h) on the interfacial tension and on the stability of macroemulsions, the goal being to explain the observations of Boyd et al. [5] and of Berger et al. [4]. Section 3, which is based on ref. [11], will examine the effect of temperature on the interfacial tension at the oil-water interface by assuming that no microemulsion or emulsion is formed, as well as its effect on the stability of emulsions. Shinoda and Saito s observations regarding the equality of the two inversion temperatures will be thus explained. Finally, the Bancroft rule [8,9], and some of the violations of this rule, will be examined in the spirit of ref. [12],... 

Phase inversion in dead-end membrane emuisification When one prepares an oil-in-water emulsion and presses this through a hydrophilic membrane, die resulting fine emulsion will be an oil-in-water emulsion as well. However, if you would use a hydrophobic membrane, the resulting emulsion will be a water-in-oil emulsion (assuming that the surfactant system would support the formation of a water-in-oil emulsion—see the Bancroft Rule). In this case, the emulsion is inverted from an 0/W towards a W/0 emulsion (see Figure 15.21). The same can be done with a water-in-oil emulsion pressed through a hydrophilic membrane, which leads to an oil-in-water emulsion. 

Emulsions are commonly prepared by mixing the oil (o) and water (w) in the presence of one or more emulsifiers, under vigorous agitation. Emulsifiers are substances that adsorb strongly at the oil-water Interface. We shall assume that there is only one, and call it the surfactant. The type of emulsion that is formed depends primarily on the nature of the surfactant. According to the empirical Bancroft rule this type tends to be such that the phase into which the surfactant is more soluble becomes the continuous one. So, hydrophilic surfactants promote the formation of oil-water emulsions, for hydrophobic surfactants it is the other way around. A host of commercial emulsifiers are available, tailor-made for certain purposes, but the above rule remains generally valid. 

There have been numerous attempts to formulate simple rules connecting the emulsion stability with the surfactant properties. Historically, the first was the Bancroft rule, which states that to... 

Ivanov et al.4 >6 5oo.5oi,562 jj yg proposed a semiquantitative theoretical approach that provides a straightforward explanation of the Bancroft rule for emulsions. This approach is based on the idea of Davies and RideaP that both types of emulsions are formed during the homogenization process, but only the one with lower coalescence rate survives. If the initial drop concentration for both emulsions is the same, the coalescence rates for the two emulsions — (Rate)i for emulsion 1 and (Rate)2 for emulsion 2 (Figure 5.44) — will be proportional to the respective coalescence rate constants, and, 2 (ss Section 5.6, below), and inversely proportional to the film lifetimes, Xj and X2 ... 

Danov, K.D. et al., Bancroft rule and hydrodynamic stability of thin films and emulsions, in Proc. First World Congress on Emulsion, 19-22 Oct., Paris, 1993, p. 125. 

As seen in the typieal variation indicated in Fig. 7 the eleetrolytie eonduetivity ehanges drastically inside the three-phase region, indicating that emulsion inversion takes place (142, 153, 155). According to the Bancroft rule, the wedge theory, and more modem curvature conceptualization (156), SAD < 0 is associated with 0> 0 with W/0 emulsions. 

The other two branches of the standard inversion line are essentially vertical, and are located typically at 30% water on the negative SAD side of optimum formulation, and at 70% water on the positive side. When the water content is low, the emulsion is always W/0, regardless of the formulation. Similarly, when the oil content is low, an 0/W can be expected, whatever the formulation. In these extreme WOR regions, the phase which is present in larger volume becomes the external phase of the emulsion. It may be said that the composition dominates. However, a closer look at the conductivity value indicates the presence of multiple emulsions in the B" and zones, i.e., where the composition effects dominate over the normal formulation trend. These B" and regions have been called abnormal in opposition to the other ones which are labeled normal because they follow the Bancroft rule and the wedge theory (172). 

Generally, oilfield emulsions are most often W/O with the surface-active emulsifiers residing in the crude-oil continuous phase. According to the Bancroft rule (109) the phase for which the emulsifiers are most soluble is the continuous phase. The emulsifiers possess some degree of polarity which attracts them to the water phase. Solid emulsifiers would be very fine particles in a state of incipient flocculation (110). The emulsifiers may be one or more of the following solids whieh are partially hydrophobic with contact angle (9>90°), polar asphaltenes and resins with some partial insolubility indueed by solvents which dilute the crude oils, or metalloporphyrins integrated within the asphaltenes (24, 25). 

The theoretical description of the mutual approach and coalescence of two emulsion drops is the subject of Sec. IV the Bancroft rule on emulsification is interpreted and generalized in Sec. V and the kinetics of flocculation is considered in Sec. Vt, where the size of the aggregates needed for the creaming to start is estimated. 

Hence, the rate of film thinning in System II is much greater than that in System I. Therefore, the location of the siufactant has a dramatic effect on the thinning rate and, thereby, on the drop lifetime. Note also that the interfacial tension in both systems is the same. Henee, the mere phase inversion of an emulsion, from Liquid 1-in-Liquid 2 to Liquid 2-in-Liquid 1 (Fig. 15), could change the emulsion lifetime by orders of magnitude. As diseussed in Sec. V, the situation with interaction in the Taylor regime (between spherical, nondeformed drops) is similar. These facts are closely related to the explanation of the Bancroft rule for the stability of emulsions (see Sec. V) and the process of chemical demulsifrcation (1). 

The Bancroft rule states that in order to have a stable emulsion the smfactant must be soluble in the eontinuous phase. Most of the emulsion systems obey this rule, but some exclusions have also been found (162). The results on drop-drop interactions, presented in Sec. IV, allow one to give a semiquantitative interpretation of the rule and the exclusions (1, 2, 163). 

In thick films the disjoining pressures, IIj and IIjj, are zero, and then the ratio in Eq. (89) will be very small. Consequently, emulsion I (surfactant soluble in the continuous phase) will coalesce much more slowly than emulsion II hence, emulsion I will survive. Thus, we obtain an explanation of the empirical Bancroft rule. The emulsion behav-... 

The finding that the hydrodynamie velocity of mutual approach of two emulsion drops is mueh higher when the surfactant is dissolved in the drop phase (rather than in the continuous phase) provides a natural explanation of the Bancroft rule in emulsification (Sec. V). A generalized version of the Bancroft rule is proposed, Eqs (89) and (91), which takes into account the role of various thermodynamic and hydrodynamic factors. For example, the existence of a considerable repulsive (positive) disjoining pressure may lead to exclusions from the conventional Bancroft rule, which are accounted for in its generalized version. 

Chapters 26—29 all discuss hydrodynamic aspects of emulsified systems. The contribution by Danov, Kralchevsky, and Ivanov presents a very fundamental and thorough survey of different phenomena in emulsions related to dynamic and hydrodynamic motions, such as the dynamics of surfactant adsorption mono-layers, which include the Gibbs surface elasticity, and characteristic time of adsorption, mechanisms of droplet-droplet coalescence, hydrodynamic interactions and drop coalescence, interpretation of the Bancroft rule with regard to droplet symmetry, and, finally, kinetics of... 

By the solubilities, ranging from oil-soluble to water-soluble. This was first expressed by the Bancroft rule, which states that water-soluble emulsifiers favour emulsions with an aqueous continuity, while oil-soluble emulsifiers favour emulsions with an oil continuity. 

For typical emulsion systems one has R her, and equation (49) yields Rate I/Rate II 1. Therefore, System I (with surfactant in the continuous phase. Fig. 20) will survive. This prediction for spherical drops is analogous to the conclusion for deformable drops. Both these predictions essentially coincide with the Bancroft rule and are valid for cases, in which the hydrodynamic stability factors prevail over the thermodynamic ones. The latter become... 

The Bancroft rule should be interpreted as a general rule. In practice, however, there are many exceptions. In addition, the type of emulsion that is formed will also depend on the volume ratio of the two phases, the method of preparation, the electrolyte concentration, etc. 

Often an emulsion is formed, or made more stable, by addition of a third fluid. Called the emulsifier, in semi-aqueous cleaning it is the surfactant added to make the solvent (SA) and water (RA) form a single fluid temporarily in the immersion chamber. In general, the Bancroft rule applies — the continuous phase (the water) is the one in which the emulsifier (the surfactant) is most soluble. 

---