On the concept of ionicity in ionic liquids Douglas R. MacFarlane,* a Maria Forsyth, a Ekaterina I. Izgorodina, a Andrew P. Abbott, b Gary Annat a and Kevin Fraser a Received 7th January 2009, Accepted 20th February 2009 First published as an Advance Article on the web 31st March 2009 DOI: 10.1039/b900201d Ionic liquids are liquids comprised totally of ions. However, not all of the ions present appear to be available to participate in conduction processes, to a degree that is dependent on the nature of the ionic liquid and its structure. There is much interest in quantifying and understanding this ‘degree of ionicity’ phenomenon. In this paper we present transport data for a range of ionic liquids and evaluate the data firstly in terms of the Walden plot as an approximate and readily accessible approach to estimating ionicity. An adjusted Walden plot that makes explicit allowance for differences in ion sizes is shown to be an improvement to this approach for the series of ionic liquids described. In some cases, where diffusion measurements are possible, it is feasible to directly quantify ionicity via the Nernst–Einstein equation, confirming the validity of the adjusted Walden plot approach. Some of the ionic liquids studied exhibit ionicity values very close to ideal; this is discussed in terms of a model of a highly associated liquid in which the ion correlations have similar impact on both the diffusive and conductive motions. Ionicity, as defined, is thus a useful measure of adherence to the Nernst–Einstein equation, but is not necessarily a measure of ion availability in the chemical sense. Introduction Ionic liquids are currently under widespread investigation for a very broad range of solvent and electrolyte applications as a result of the attractive combination of properties that some members of the family possess. 1,2 For example, the bis(trifluoromethanesulfonyl) amide salts typically offer a very wide liquid range with very low vapour pressure (hence low flammability) and high ionic conductivity. Some level of ion conductivity is to be expected of all ionic liquids, 3 but interestingly some exhibit much lower molar conductivity than others, even after differences in viscosity are allowed for; 4 this is to be expected of concentrated ionic media in which ion pairs and other correlations can strongly influence conductivity. Ion pairs, if sufficiently long-lived, appear neutral in the electric field and thus cannot contribute to conductivity. Similarly, long-lived groups or aggregates of ions contribute less to conductivity than the independent ions would. Such conductivity effects are of extreme interest in most applications of ionic liquids as electrolytes in electrochemistry 5,6 and electrochemical devices, including lithium batteries, 7,8 double layer capacitors, 9–11 fuel cells 12 and photo-electrochemical cells. 13–15 Lack of conductivity, especially at low temperatures, is often one of the main disadvantages in the use of an ionic liquid and a number of research programs worldwide are dedicated to developing higher conductivity examples for these applications. An understanding of the factors that influence such ion correlation effects is also of importance as their impact extends beyond the electrolyte and electrochemistry to their solvent properties and vapour pressure. Thus the question of ‘‘how ionic is this ionic liquid?’’ becomes of quite general significance. Angell and co-workers 16–18 have described a qualitative approach to this question based on the Walden rule: LZ = k (1) where L is the molar conductivity and Z is the viscosity; k is a temperature dependent constant. The Walden rule was originally based on observations of the properties of dilute aqueous solutions, but has since been found to be applicable in non-aqueous electrolyte solutions 19 and molten salts. 20 On a plot of log L vs. log Z this rule predicts a straight line that passes through the origin; this has become known as a ‘‘Walden plot’’. Data for a 0.01M KCl solution provide a useful calibration point 18 that effectively allows estimation of the constant k and hence allows the placement of a reference line on the Walden plot as shown in Fig. 1. Data from a wide range of electrolytes can then be placed on the Walden plot, including any ionic liquid for which viscosity and conductivity measurements are available. Except for one or two notable exceptions, 21 most ionic liquids fall below the line, more or less so depending on their structure. Many protic ionic liquids fall well below the line, 18,22 suggesting that full ionisation (proton transfer) is not complete in those cases. Some aprotic ionic liquids also fall well below this reference line indicating that their conductivity is as much as an order of magnitude lower than would be expected on the basis of the Walden rule, presumably as a result of ion association in its various forms. 4 On the other hand it could be considered remarkable that so many ionic liquids actually lie as close to (within 20% of) the line as they do, given that one would expect strong ion a School of Chemistry and Department of Materials Engineering, Monash University, Wellington Rd., Clayton, Victoria, Australia 3800. E-mail: D.Macfarlane@sci.monash.edu.au b Chemistry Department, University of Leicester, Leicester, UK LE1 7RH 4962 | Phys. Chem. Chem. Phys., 2009, 11, 4962–4967 This journal is  c the Owner Societies 2009 PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics