Measurement of the Interfacial Tension in an Ionic Liquid–Dielectric Liquid System Using an Electrically Deformed Droplet

The unique chemical and physical properties of ionic liquids (ILs) are attracting an increasing amount of interest in energy and biological fields. However, some physicochemical properties of ILs are not well-known. Here, we present an alternative method for the measurement of the interfacial tension in an IL–dielectric liquid system by means of the small deformation of a charged IL droplet in a dielectric medium. Using the pendant drop method, the validity of the proposed measurement method is confirmed. The interfacial tension between ILs and silicone oil has been found to be lower than that between water and silicone oil, and the tension is negatively correlated to the size of the IL ion. Further analyses of Girifalco-Good’s interaction parameters of the two liquid–liquid systems demonstrate that the lower interfacial tension of IL can be attributed to the stronger intermolecular interaction between IL and silicone oil molecules. Due to a universal feature of the interaction parameter for a given class of liquid–liquid systems, it can provide an approximate estimate of the interfacial tension without a direct measurement.