Bancroft s rule

One may rationalize emulsion type in terms of interfacial tensions. Bancroft [20] and later Clowes [21] proposed that the interfacial film of emulsion-stabilizing surfactant be regarded as duplex in nature, so that an inner and an outer interfacial tension could be discussed. On this basis, the type of emulsion formed (W/O vs. O/W) should be such that the inner surface is the one of higher surface tension. Thus sodium and other alkali metal soaps tend to stabilize O/W emulsions, and the explanation would be that, being more water- than oil-soluble, the film-water interfacial tension should be lower than the film-oil one. Conversely, with the relatively more oil-soluble metal soaps, the reverse should be true, and they should stabilize W/O emulsions, as in fact they do. An alternative statement, known as Bancroft s rule, is that the external phase will be that in which the emulsifying agent is the more soluble [20]. A related approach is discussed in Section XIV-5. 

The well-known empirical Bancroft s rule [84] states that the phase in which the surfactant is preferentially soluble tends to become the continuous phase. An analogous empirical correlation has been reported by Shinoda and Saito [85]. Eor a nonionic surfactant of the polyethoxylated type [R-(CH2-CH2-0) -0H, where R is an alkyl chain], as temperature increases, the surfactant head group becomes less hydrated and hence the surfactant becomes less soluble in water and more soluble in oil. Its phase diagram evolves as schematically shown in Fig. 1.4. At low... 

But these byzantine discussions on mechanism are beyond the point. This book is based on experiments. It does not claim to solve all problems (e.g what is thereat origin of Bancroft s rule) but it presents them with common sense and precision. I am convinced that it will be of great help. 

Whether the system formed on mixing oil, water, and surfactant will be an oil-in-water or a water-in-oil emulsion is a central problem in emulsion technology. It was realized very early that the volume fractions of oil and water are not that important and that the type of emulsion is primarily determined by the nature of the surfactant. Simply speaking surfactants with Ns < 1 tend to form oil-in-water emulsions, while surfactants with Ns > 1 are more likely to form water-in-oil emulsions. Two more detailed guiding principles which are used for practical emulsion formulation are Bancroft s rule of thumb and the more quantitative concept of the HLB scale ... 

Surfactants are enriched at the interface but they are also dissolved in the aqueous and the oil phase. Some surfactants are more soluble in water, others are better soluble in oil. In essence, Bancroft s rule states that the continuous phase of an emulsion will be the phase in which the emulsifier is preferentially soluble [542,543],... 

Whether an oil-in-water or a water-in-oil emulsion is formed is largely determined by the surfactant. For macroemulsions Bancroft s rule and, more quantitatively, the concept of the HLB, are useful. They reflect the solubility of the surfactant in water and oil. 

The generalisation that the phase in which the emulsifying agent is the more soluble tends to be the dispersion medium is known as Bancroft s rule. 

An empirical generalization used to predict which phase in an emulsion will be continuous and which dispersed. It is based on a physical picture in which emulsifiers are considered to have a wedge shape and will favour adsorbing at an interface, such that most efficient packing is obtained that is, with the narrow ends pointed toward the centres of the droplets. A useful starting point, but there are many exceptions. See also Bancroft s Rule, Hydrophile-Lipophile Balance. 

There is a common rule, called Bancroft s rule, that is well known to people doing practical work with emulsions if they want to prepare an O/W emulsion they have to choose a hydrophilic emulsifier which is preferably soluble in water. If a W/O emulsion is to be produced, a more hydrophobic emulsifier predominantly soluble in oil has to be selected. This means that the emulsifier has to be soluble to a higher extent in the continuous phase. This rule often holds but there are restrictions and limitations since the solubilities in the ternary system may differ from the binary system surfactant/oil or surfactant/water. Further determining variables on the emulsion type are the ratios of the two phases, the electrolyte concentration or the temperature. 

Figure 4.39. Explanation of Bancroft s rule. Two droplets just after their formation.

If the disjoining pressures, flj and Il2, are zero, the ratio in Equation 5.292 will be very small. Hence, emulsion 1 (surfactant soluble in the continuous phase) will coalesce much more slowly and it will survive. This underlines the crucial importance of the surfactant location (which is connected with its solubility), thus providing a theoretical foundation for Bancroft s rule. The emulsion behavior in this case will be controlled almost entirely by the hydrodynamic factors (kinetic stability). 

The formulation has been related with the type and properties of emulsions since Bancroft s rule of thumb (1913) and Langmuirs wedge theory (1917). The hydrophilic-lipophilic balance (HLB) was introduced by Griffin 60 years ago, probably as a selling argument for the (by the time) new non-ionic surfactants. It accounts for the relative importance of the hydrophilic and lipophilic parts of an amphiphilic molecule on a weight basis [19]. For decades there was no other numerical yardstick. The simplicity of the HLB concept was its main advantage in spite of very serious limitations, such as an inaccuracy sometimes over two units, and the fact that it does not take into account several variables which are known to alter the phase behaviour, independently of the surfactant. 

An important characteristic of an emulsion is whether oil or water makes up the continuous phase. This is governed by Bancroft s rule, which... 

Bancroft s rule also explains why proteins cannot be used as surfactants to make a W-0 emulsion they are insoluble in oil. 

Proteins are not very suitable for making fine emulsions in other words, it takes more energy to obtain small droplets than with a small-molecule surfactant. This is primarily due to their large molar mass. It causes the effective y value that they can produce at the O-W interface to be fairly large. Moreover, their molar concentration is small at a given mass concentration, causing the Gibbs elasticity to be relatively small. This means that prevention of recoalescence is less efficient. Proteins are not suitable to make W O emulsions, as follows from Bancroft s rule they are insoluble in oil. The adsorption layer of proteins on the droplets obtained by emulsification is not an equilibrium layer, whereas it is for small-molecule surfactants. 

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. 

According to Bancroft s rule the best type of emulsion raises the solubility of the emulsifier in its homogeneous phase. Hence, nonionic ethylene oxide adducts with a low degree of ethoxylation, for example the tetra glycol dodecyl ether, stabilise a water/oil type and higher ethoxylated products, such as EO 10, stabilise oil/water emulsions. Given a sufficient solubility... 

The emulsifying effect of a copolymer can be characterized by determining the type of emulsion (DMF in hexane or hexane in DMF), its stability, its viscosity, and the particle size of the dispersed phase. These characteristics of oil-in-oil emulsions obtained with PS-PI block copolymers were studied as functions of solvent volume ratio, molecular weight, composition, and structure of the copolymer (5). Although Bancroft s rule was established for conventional oil-water emulsions, it appears to apply also to oil-in-oil emulsions—the continuous phase of the emulsion is preferentially formed by the solvent having the best solubility for the emulsifier (6, 7). Thus, block or graft copolymers can be prepared giving hexane/DMF, DMF/hexane, or both types of emulsions. 

Figure 21.1 points out the key parameters which determine the nature of the dispersion [3]. These are the emulsifier concentration (horizontal axis) and the hydrophile-lipophile balance (HLB) of the emulsifier (vertical axis). According to Bancroft s rule, HLB values higher than 7 favour the formation of oil-in-watCT... 

One limitation of the HLB concept is its failure to account for variations in system conditions from that at which the HLB is measured (e.g., temperature, electrolyte concentration). For example, increasing temperature decreases the water solubility of a nonionic surfactant, ultimately causing phase separation above the cloud point, an effect not captured in a temperature-independent HLB value. When both water and oil are present, the temperature at which a surfactant transitions from being water soluble to oil soluble is known as the phase inversion temperature (PIT). Below the PIT, nonionic surfactants are water soluble, while above the PIT. they are oil soluble. Thus, from Bancroft s rule, a nonionic surfactant will form an 0/W emulsion below its PIT and a W/0 emulsion above its PIT. Likewise, increasing salt concentrations reduces the water solubility of ionic surfactant systems. At elevated salt concentrations, ionic surfactants will eventually partition into the oil phase. This is illustrated in Fig. 13. which shows aqueous micelles at lower salt concentrations and oil-phase inverse micelles at higher salt concentrations. Increasing the system temperature will likewise cause this same transition for nonionic surfactant systems. 

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