Review of non-crystalline and crystalline quaternary ammonium ions: Classification, structural and thermal insight into tetraalkylammonium ions

Abstract

In this review, all non-crystalline and crystalline quaternary ammonium substances, specifically tetraalkylammonium ions (NR4+) – namely ionic liquids (ILs), ionic liquid crystals (ILCs), and ionic plastic crystals (IPCs), have been briefly discussed according to their analogue structures and thermal properties. The proposed classification of organic salts is according to their chemical properties (chemical structure, and carbon substituents) and physical properties (phases of matter, orientational and positional order, which further reflect their crystalline and non-crystalline properties). This review also summarized the general information of these three subclasses – ILs, ILCs and IPCs. The recent definition of ILs has been stated as the substances solely having cations and anions without the presence of other molecules. The structure of ILCs, in contrast, comprises the main headgroups (similar to ILs), but commonly bear longer substituents, which result in phase segregation. The type of mesophases formed throughout melting process was influenced by functional group in the substituents, position, and length of chains attached to headgroup. However, the substances comprising both symmetrical cations and anions also exhibited similar patterns. These substances were labelled as IPCs with satisfied entropy of fusion’s value. This paper provides a comprehensive classification information about these ions.