Bulk Oil Phase Research Articles

Formation and stability of emulsions produced by dilution of emulsifiable concentrates. Part III. The coalescence of oil droplets at a planar oil—water interface

The coalescence of oil droplets at a planar xylene/water interface has been studied by measuring their rest-times before coalescence with the bulk oil phase. The arithmetic mean, t mean, standard deviation, σ, drainage time, t D, and half-life for film rupture, T built:1 2 , were calculated from the results. Measurements were made as a function of surfactant concentration (a 1:1 mixture of Synperonic NPE 1800 and Ethoduomeen T20). At low surfactant concentrations (< 4 × 10 −3% w/v), droplet rest-times were found to decrease to a value below that obtained in the absence of surfactant. This may have been due to interfacial turbulence occurring as a result of non-equilibrated distribution of the surfactant at such low surfactant concentrations. However, above 4 × 10 −3% both t D and T 1 2 increased considerably with increase in surfactant concentration. This implies a considerable resistance to film thinning. This increase in film stability is not directly related to reduced interfacial tension, but coincides with a change in zeta potential from small negative values to high positive ones. However, the increase in droplet rest-times occurred at surfactant concentrations much lower than those required to produce stable emulsions in bulk.

The lippmann equation and the characterization of black lipid films

An optically black lipid film in aqueous media approximates closely to an ideally polarizable system and, in this respect, can be described by a form of the Lippmann equation. This equation is derived and integrated assuming the film capacitance to be independent of applied potential. Measurements of the contact angle and of the bulk interfacial tension are used to find accurate values of the film tension and it is shown that, for a range of lipid films, the integrated equation is obeyed for potential differences up to approximately 100 mV. Various ways are then described in which the Lippmann equation may be used to find film properties not readily accessible by other methods. Thus, for phospholipid films the accurate measurement of both film and bulk interface tensions presents difficulties, but these can be overcome if the specific capacitance is known. In this way the specific free energy of thinning of the film may be obtained. Conversely, for films in media of low conductivity or in systems in which film area measurements are difficult the determination of the specific capacitance is not readily achieved. A knowledge of the tension of the Plateau-Gibbs border together with the contact angle at different applied potentials, however, circumvents this problem. Finally it is pointed out that the use of the Lippmann equation provides an independent means of finding the interfacial tension between bulk oil and water phases and hence of checking the accuracy of the nonabsolute direct methods, such as the drop-volume technique, which have to be employed in such systems.

The flocculation of aqueous sols of a graphitized carbon black by a potential-determining electrolyte

Recent work on the film thinning behavior during the coalescence of oil drops in water is summarized. In the experimental work, color movies were taken of the light interference patterns produced by the thin film of water trapped between the rising drop and the bulk oil phase. The drops were relatively small (0.2–0.32 cm diam.); sufficient KCl was added to minimize double layer repulsion. Concentrations of 10 −6 –10 −2 g l −1 of sodium lauryl sulfate were used. The oils used were toluene, anisole, cyclohexanol and a mixture of anisole and cyclohexane. Five different patterns of film behavior were observed. The mechanisms to explain these patterns are based on concepts of dynamic immobility (when the distribution of surfactant along the interface gives an interfacial tension gradient that every-where balances the interfacial shear stress) and of steric immobility. The mechanisms include rapid approach (when both interfaces are mobile), dimple formation (when the interfacial tension gradient exceeds the interfacial shear stress), even drainage (when the interfaces are dynamically or sterically immobile) and uneven drainage (when there is local mobility). Reasons for uneven drainage are discussed. The different theoretical models developed to describe film thinning are reviewed. The emphasis is on the assumptions and the relationship between the equations describing flow in the film and those describing surface behavior. The predictions of the different models are compared but the emphasis is on the success of more recent models based on the above mentioned mechanisms to describe symmetrical film drainage.

The stability of emulsions

Recent work on the film thinning behavior during the coalescence of oil drops in water is summarized. In the experimental work, color movies were taken of the light interference patterns produced by the thin film of water trapped between the rising drop and the bulk oil phase. The drops were relatively small (0.2–0.32 cm diam.); sufficient KCl was added to minimize double layer repulsion. Concentrations of 10−6–10−2 g l−1 of sodium lauryl sulfate were used. The oils used were toluene, anisole, cyclohexanol and a mixture of anisole and cyclohexane.

Studies at phase interfaces: II. The stabilization of water-in-oil emulsions using oil-soluble emulsifiers

A range of oil-soluble emulsifying agents, representing various chemical classes, has been examined for the stabilization of water-in-oil (w/o) emulsions of high water/oil ratio. Some of the physical properties of oil solutions of these emulsifiers have been measured, including their interfacial tensions against water, and the relative rates of coalescence of oil drops in water with bulk oil phase and of water drops in oil with bulk water phase. These properties have provided some information on the type of interfacial film required for w/o emulsification. It is concluded that a strong interfacial film should be formed which prevents the coalescence of water droplets while permitting the coalescence of oil droplets. In almost all systems where stable w/o emulsions are formed, it is possible to postulate some form of molecular interaction, either electrostatic attraction or hydrogen bonding, in the interface. The emulsifier should be hydrophobic in character and have the correct hydrophilic-lipophilic balance (HLB) for the particular solvent concerned. However, the published methods for expressing the HLB value quantitatively do not appear to be suitable for selecting emulsifiers for w/o emulsions. A low interfacial tension between the two phases is desirable, but is not essential if a sufficiently strong interfacial film is formed. Interfacial tension may be important, however, in cases where very rapid emulsification is required, and in these cases the emulsifier should also be adsorbed rapidly at the interface.