The Electronic Structure of Ions in Solution and at Interfaces

Ionic liquids - liquids composed solely of mobile ions - show enticing potential for the development of many energy applications, e.g. supercapacitors, batteries.1 Electronic structure underpins all ionic liquid-derived applications, but there is currently a lack of understanding of the electronic structure of these complex ionic liquid-based systems. This is because there is a dearth of methods for studying the electronic structure of applied ionic liquid-based systems. I will present results to aid the understanding of the electronic structure of ions in solution and at interfaces, recorded using a mixture of laboratory- and synchrotron-based experimental methods.

To achieve reliable models for understanding and prediction of properties, experimental measures of the electron densities on different atoms are required.2 These electron densities can be used to provide reliable and realistic force fields for molecular dynamics simulations of ionic liquids. We have investigated the relative electron density on atoms in ionic liquids by probing the energies of core levels using both X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Time-dependent DFT (TD-DFT) calculations were conducted on individual ion pairs to allow for a comparison with the experimental data. Good agreement was found between the XPS, NEXAFS spectroscopy and TD-DFT results. For example, we find that the electron densities on sulfur atoms correlate with the valence number of the sulfur atom, irrespective of whether the sulfur atom was located on the cation or the anion.

The ionic liquid-gas surface underpins applications such as gas storage, gas separation, nanoparticle synthesis and supported ionic liquid phase catalysis.3 For all of these applications, one needs to know which elements of the ionic liquid-based system are located at the outer atomic surface. Low energy ion scattering (LEIS) is the most surface sensitive, element specific technique available. I will present LEIS results that identify the elements present at the ionic liquid-vacuum outer atomic surface for 23 neat ionic liquids, systematically varying both the cation and the anion charged headgroups, and also their alkyl chain lengths (where feasible).4 In addition, I will present LEIS results to show that ionic liquid-vacuum outer atomic surfaces can be created that are remarkably different from the bulk composition.5 I will demonstrate that for ionic liquid mixtures the outer atomic surface shows significantly more atoms from anions with weaker cation-anion interactions (and vice versa).

1. D. R. MacFarlane, N. Tachikawa, M. Forsyth, J. M. Pringle, P. C. Howlett, G. D. Elliott, J. H. Davis, M. Watanabe, P. Simon and C. A. Angell, Energy Environ. Sci., 2014, 7, 232-250.
2. J. Rigby and E. I. Izgorodina, Phys. Chem. Chem. Phys., 2013, 15, 1632-1646.
3. K. R. J. Lovelock, Phys. Chem. Chem. Phys., 2012, 14, 5071-5089.
4. I. J. Villar-Garcia, S. Fearn, G. F. De Gregorio, N. L. Ismail, F. J. V. Gschwend, A. J. S. McIntosh and K. R. J. Lovelock, Chem. Sci., 2014, 5, 4404-4418.
5. I. J. Villar-Garcia, S. Fearn, N. L. Ismail, A. J. S. McIntosh and K. R. J. Lovelock, Chem. Commun., 2014, DOI: 10.1039/C4CC06307D.