Interpretation of fusion and vaporisation entropies for various classes of substances, with a focus on salts

• Bonding and configurational characteristics of various substances are related to the magnitude of their phase change entropics.
• Trouton’s law is obeyed by polyatomic substances that form non-ideal vapours.
• Simple salts appear to retain their ionicity over all 3 phases, while ionic liquids become of a molecular nature in the vapour.

Entropies of fusion and vaporisation of a variety of elements and compounds have been derived from literature data. Fusion entropies range from low values for metals and certain cyclic hydrocarbons (e.g., cyclopentane) through modest values for salts to high values for materials undergoing drastic rearrangement or disentanglement such as aluminium chloride and n-alkanes. Entropies of vaporisation for most substances are close to the Trouton’s Law value of ∼100 J · deg.−1 · mol−1, with low values for species which associate on boiling (e.g., acetic acid) and higher values signifying simple dissociation (e.g., nitrogen tetroxide) or total decomposition (e.g., some ionic liquids). The nature of inorganic and semi-organic salts in all 3 phases is discussed.