'Covalent' effects in 'ionic' liquids

Many binary systems (and their mixtures) which might be expected to be 'ionic', from electronegativity considerations, are found to exhibit pronounced 'covalent effects' in their condensed-phase structure and dynamical properties. Recent work, involving both electronic structure calculations and computer simulation, has suggested that the interactions arise and are describable within the ionic model-provided that many-body effects, whose origin is the change in an ion's properties caused by interaction with its environment, are included. In systems where they are substantial, the many-body effects promote remarkably rich changes in the intermediate-range structure of an ionic liquid. AlCl3 becomes molecular, BeCl2 a 'living polymer' of extended chains, and the distinctive intermediate-range order (IRO) of the three-dimensional-network, glass-forming systems ZnCl2, BeF2 and SiO2 is reproduced. The structural changes have considerable dynamical consequences: for ZnCl2 the slow structural relaxation, leading to the glass transition, may be traced back to the relaxation of the IRO; On shorter timescales (higher frequency) these liquids exhibit spectroscopic bands usually assigned to quasi-molecular units. The formation and dissociation of these units is crucial in ionic conduction, and other transport properties.