What makes ionic fluids characteristically ionic? A corresponding-states analysis of the surface tension of an ionic model fluid with variable dispersion interactions.

Inorganic molten salts, such as NaCl, are known to show characteristically lower values of Guggenheim's corresponding-states surface tension γ(red) at a given reduced temperature T∕T(c) than simple or aprotic polar fluids. Recently, the corresponding values of γ(red) for (some) room temperature ionic liquids (RTILs) were found in the same region as those for weakly polar fluids, that is, markedly above the values typical of inorganic molten salts despite the ionic character of RTILs. Here, we present the results of simulations of an ionic model fluid in which the strength of attractive dispersion interactions among the ions is varied relative to the Coulomb interactions. For weak dispersive interactions, the behavior known for real inorganic molten salts is found. If the attractive dispersion energy of two unlike ions at contact exceeds 20% of the Coulombic attraction in such an isolated ion pair, γ(red) increases markedly and approaches the region of values for simple and polar fluids. Rough theoretical estimates of the relative strengths of dispersive and Coulombic attractions in molten inorganic salts and in RTILs support our conclusion that the dispersion interactions in RTILs are strong enough for their corresponding-states surface tension to behave regularly and, thus, to deviate from the values one would expect for strongly ionic systems.