Organic salts with low melting points (below about 100 °C) are commonly termed ionic liquids (IL). These liquids have unique physico-chemical characteristics: no significant vapour pressure, low flammability, good thermal stability and wide useable temperature range. There is a large (and rapidly growing interest) in their use as “green” solvents in organic synthesis, catalysis, extraction, etc. Ionic liquids are highly concentrated electrolytes and have potentially important electrochemical applications (e.g. batteries and fuel cells).

In this study we have looked at the interfacial properties of several imidazolium- and pyrrolidinium-based ionic liquids. The surface tension at room temperature was measured with the pendant drop and Wilhelmy plate techniques. The interfacial tension of ILs against a series of alkanes was determined by the pendant drop method. The contact angles on smooth Teflon surfaces were measured with the sessile drop method. Commercial grade ionic liquids (Merck) were used without further purification and some of these contained significant amounts of impurities (mainly halides and water). Nevertheless the results were reproducible and amenable to a simple physical interpretation.

The surface tension of all ILs studied strongly correlates with their molecular volume. The dependence is adequately described by the scaled particle theory. The interfacial tension measurements against non-polar liquids allow the splitting of the surface tension into an apolar (Lifshitz–van der Waals, LW) component and an acid-base (AB) component. The Hamaker constants estimated from the LW components are in good correlation with the values calculated through the Tabor-Winterton approximation. The interpretation of contact angles measured on smooth surfaces further confirms these findings.