Methods Applicable to Interfaces Between Two Liquids

In theory, every surface tension measurement method can be used to determine the interfacial tension between two liquids. However, the accuracy of these methods is reduced when applied to liquid-liquid interfaces, or when one or both of the liquids is viscous. In practice, the maximum bubble pressure and pendant drop methods are the most suitable, giving consistent and reliable values for interfacial tensions, although there is sometimes the 

It is important to allow liquid-liquid interfaces to achieve equilibrium before making an interfacial tension measurement. Thus, it is always difficult to measure the interfacial tension of viscous liquids because they are slow in reaching equilibrium. In addition, viscous liquids prevent injection of a liquid sample of the required volume into the instrument. The Wilhelmy plate (at a constant depth, without detachment) and sessile drop methods are generally preferred for viscous liquids, after equilibrating the samples for several hours before measurement. 

When the value of the interfacial tension is significantly less than 1 mN m 1, then we consider the measurement of ultra-low interfacial tension, which is common in liquid-liquid emulsification processes when effective surfactant solutions are used. The dynamic spinning drop tensiometer method is especially suitable for this purpose. Ultra-low interfacial tension measurement is important in the chemical industry because the cleaning of solid surfaces of dirt, grease, and oil the formulation of stable emulsions the recovery of petroleum, and other applications often rely on lowering the interfacial tension between immiscible liquids to ultra-low values by the use of surfactants. 

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