Core-level binding energies describe electrostatic potentials at nuclei for ionic liquids

Electrostatic interactions in ionic liquids (ILs) dictate many of their physical properties and hence underpin a plethora of potential applications. It is vital to develop both experimental and theoretical electronic descriptors for ILs, to drive deeper understanding of the interactions that may be tuned for applications. A possible descriptor for ILs is the readily measurable core-level binding energy from experimental core-level X-ray photoelectron spectroscopy (XPS), E_B(core). To establish that differences in E_B(core) capture the differences in electrostatic potential at nuclei, V_n, we use a computational approach based on ab initio molecular dynamics (AIMD). We demonstrate clear quantitative (linear) correlations between experimental E_B(core) and calculated V_n for carbon, nitrogen, sulfur, oxygen and fluorine for both cations and anions. Our work shows that E_B(core) are chemically interpretable descriptors of electrostatic interactions in ILs. This has implications for the ability to predict, out of the vast number of ILs that can form from the array of available cations and anions, the best IL compositions for target applications. We also discuss how this work could open up new areas of enquiry, and the potential usefulness of V_n to characterise interactions of ILs with surfaces and interfaces.