Melting and Boiling from Ionic to Molecular Phases

Ionic liquids, on boiling, are expected to be in equilibrium with molecular vapours. It is to be anticipated that the temperature range of the ionic liquid phase is extensive and comparable with the stability range of the solid. This is confirmed by available data. Boiling at low pressure is found from the data to occur when the Debye Hückel screening length in the ionic melt is a fixed fraction, for the whole family of alkali halides, of the molecular equilibrium spacing. Starting from this regularity, a corresponding states argument is used to make predictions about the critical point. Melting of a solid ionic phase into a molecular phase is a more system specific phenomenon. Characteristics favourable for such phase transitions are: (i) Large valence difference between cation and anion. (ii) Highly polarizable valence shells on the ions, such as arise with rather weakly bound d-electrons on the cation. To exemplify these points, the cases of the aluminium halides and the mercury halides are considered in some detail.