Bancroft rule

Abstract: Here, we investigate water-in-oil (W/O) emulsions that are stabilized by polystyrene latex particles with sulfate surface groups. The particles, which play the role of emulsifier, are initially contained in the disperse (water) phase. The existence of such emulsions formally contradicts the empirical Bancroft rule. Theoretical considerations predict that the drop diameter has to be inversely proportional to the particle concentration, but should be independent of the volume fraction of water. In addition, there should be a second emulsification regime, in which the drop diameter is determined by the input mechanical energy during the homogenization. The existence of these two regimes has been experimentally confirmed, and the obtained data agree well with the theoretical model. Stable W/O emulsions have been produced with hexadecane and tetradecane, while, in the case of more viscous and polar oils (soybean and silicone oil), the particles enter into the oily phase, and Pickering emulsions cannot be obtained. The formation of stable emulsions demands the presence of a relatively high concentration of electrolyte that lowers the electrostatic barrier to particle adsorption at the oil -water interface. Because the attachment of particles at the drop surfaces represents a kind of coagulation, it turns out that the Schulze -Hardy rule for the critical concentration of coagulation is applicable also to emulsification, which has been confirmed with suspensions containing Na + ,M g 2+ , and Al 3+ counterions. The increase of the particle and electrolyte concentrations and the decrease of the volume fraction of water are other factors that facilitate emulsification in the investigated system. To quantify the combined action of these factors, an experimental stability -instability diagram has been obtained.

Abstract: In this paper, we investigate the two mechanisms that are responsible for the release of a chemical substance from water-in-oil-in-water double emulsions. (i) One is due to the coalescence of the thin liquid film separating the internal droplets and the oil globule surface. (ii) The other mechanism occurs without film rupturing, that is, by diffusion and/or permeation of the chemical substance across the oil phase. The thin liquid film that forms between the internal droplets and the globule surface is composed of two mixed monolayers covered by hydrophilic and hydrophobic surfactant molecules. Following the well-known Bancroft rule, such inverted films possess a long-range stability with respect to coalescence when essentially covered by hydrophobic surfactant. In that limit, the release is governed by diffusion and/or permeation. However, the film becomes very unstable when a strong proportion of hydrophilic surfactant is adsorbed and the release is then controlled by coalescence. We show that the trans...

Abstract: In most cases, in both micro- and macroemulsions, the phase in which the surfactant is most soluble constitutes the continuous phase. There are, however, violations to this Bancroft rule, and the goal of the paper is to delineate the conditions of validity of the rule. For microemulsions two ratios are significant from this point of view. One of them, R1, compares the interactions in the bulk between surfactant and oil to those between surfactant and water; the other, R2, compares the interactions between the hydrocarbon chains of the surfactant in the interfacial layer and oil to those between the head groups of the surfactant molecules in the interfacial layer and water. Ratio R1 indicates the preferred phase, while ratio R2 indicates the preferred curvature. When both ratios are smaller or larger than unity, the Bancroft rule is obeyed. When one of them is smaller and the other larger than unity, the Bancroft rule is violated. The violations observed by Binks regarding the phase behavior of the ternary...