On the calculation of the dielectric properties of liquid ionic systems

The calculation of dielectric properties of fluids, which is straightforward for homogeneous dipolar liquids, presents several intricacies when applied to ionic liquids. We investigate, for a model sodium chloride aqueous solution, three different methods which could provide an estimate of the static permittivity, namely (a) the Einstein-Helfand approach, (b) the first moment of the current-current correlation functions, and (c) the analysis of the low frequency limit of the dielectric spectrum. The contribution to the static permittivity of the sodium chloride solution coming from the ion-ion correlations, often neglected in simulation studies but implicitly taken into account in experimental measurements, are shown to be important, although particularly hard to measure. In the case of our model sodium chloride solutions, we find these dynamic contributions to be the main cause of the saturation of the dielectric constant with increasing salt concentration.