Is a Stern and diffuse layer model appropriate to ionic liquids at surfaces?

Abstract

According to a recent article in PNAS by Gebbie et al., ionic liquids (ILs) should be considered as dilute electrolyte solutions, consisting of ion pairs of which only a small fraction are dissociated (1). This claim is based upon measurements of weak attractive forces between a mica surface and a gold surface across ILs, attributed to double-layer attraction. From the decay length of the attractive force (10–13 nm), an effective free-ion concentration is deduced by fitting to a modified Derjaguin–Landau–Verwey–Overbeek (DLVO) theoretical expression. However, the overwhelming evidence from the thermodynamic and transport properties of similar ILs is that they are not composed of neutral ion pairs with simple dissociation equilibria, and, as such, the DLVO framework is inappropriate. Rather, the liquid should be considered as strongly dissociated with ions interacting to varying extents with several nearby counterions (2, 3). The authors support their assertion of weak dissociation by calculating the binding energy of an ion pair, relative to isolated ions in a dielectric medium. However, they do not contrast this with the energetics of a dense, locally charge-ordered ionic melt that is the likely state of the fluid. In the same limited framework, NaCl would be expected to be an ion-pair fluid.