Thermochimica Acta 421 (2004) 87–93 Lattice and phase transition thermodynamics of ionic liquids Leslie Glasser ∗ Department of Applied Chemistry, Nanochemistry Research Institute, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia Received 23 February 2004; received in revised form 24 March 2004; accepted 27 March 2004 Available online 20 May 2004 Abstract The thermodynamic systematics of the formation and phase changes of a range of materials which form ionic liquids is examined, based upon experimental values of densities and calorimetric quantities. Certain results are shown to be consistent across groups of these materials, namely: the lattice potential energies decrease with increasing alkyl chain length; the molecular volumes and ‘total phase change’ entropies (from crystal through liquid crystalline phases to melt) increase linearly with alkyl group chain length for pyridinium cations. However, the total phase change transition entropies for the two 1-alkyl-3-methylimidazolium cation systems examined are anomalous, having dome-shaped graphs with a dip in value around C 14 . © 2004 Elsevier B.V. All rights reserved. Keywords: Phase transitions; Lattice energies; Ionic liquids; Thermodynamics 1. Introduction Thermodynamic considerations are important in un- derstanding the stability and behaviour of solid, liquid crystalline and molten materials. An important quantity in assessing the stability of an ionic material is its lattice en- ergy (that is, the energy required to remove the ions from their positions in the crystal structure to infinite separation). The lattice energy, in a Born–Haber–Fajans cycle, often determines whether the material can be synthesised or not; indeed, Bartlett [1] used just such a quantity in predicting the stability of the first known noble gas complex, XePtF 6 . Furthermore, as temperature rises and energy is gained, crystalline solids lose their long-range order correlation [2] which becomes reduced to disorder, whether through a ‘continuous’ (lambda) transition from one solid phase to another, or at a single fusion temperature (where the solid becomes a liquid melt), or through a sequence of transitions in the solid and/or liquid, to a final ‘clearing’ temperature. In the last case, the liquid may pass through one or more liquid crystalline states of considerable technological con- cern. These increases in disorder are observed as changes in the structures of the material concerned, accompanied by ∗ Tel.: +61-8-6293-1202; fax: +61-8-9266-4699; mobile: +61-422-713-475. E-mail address: leslieglasser@yahoo.co.uk (L. Glasser). enthalpy increases resulting from the altered interactions, and are described by increases in entropy. Although the enthalpy and entropy increases for any transition (tr) are closely connected, by the equilibrium relation:  tr S =  tr H T tr (1) the entropy increases generally over less than an or- der of magnitude for most substances and is largely temperature-independent, whereas enthalpy increases may vary over several orders of magnitude [3] and are somewhat temperature-dependent, so that the systematics of entropy change are more readily dealt with. ‘Ionic liquids’ are materials of increasing importance, providing new, low vapour pressure, non-flammable solvents for replacement of volatile organic compounds in chemical and electrochemical processing [4]. 1 However, little has yet been done in establishing correlations of their thermody- namic properties. It is the purpose of this paper to contribute to such analyses by (a) implementing a recently published method for estimating ‘lattice’ potential energies of ionic liquids, in order to provide information on their relative stabilities; (b) establishing some thermodynamic regular- ities; and (c) comparing entropies and entropy changes 1 This is the lead paper in issue 57(2) of this journal, resulting from an Ionic Liquid Workshop, Melbourne, May 2003. 0040-6031/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2004.03.015