Understanding the Effects of Ionicity in Salts, Solvates, Co-Crystals, Ionic Co-Crystals, and Ionic Liquids, Rather than Nomenclature, Is Critical to Understanding Their Behavior

The incorporation
of active pharmaceutical ingredients (APIs) into
multicomponent solid forms (such as salts and co-crystals) or liquid
forms (such as ionic liquids (ILs) or deep eutectic mixtures) is important
in optimizing the efficacy and delivery of APIs. However, there is
a current debate regarding the classification of these multicomponent
systems based on their ionicity which could interfere with their consideration
in important applications. Multicomponent systems of intermediate
ionicity can show a combination of properties, leading to behavior
that is neither strictly typical of either purely ionic or purely
neutral compounds, nor easily described as intermediate between the
two. In this perspective, we attempt to illustrate the problems in
classifying multicomponent APIs based on one of two categories by
discussing selected literature regarding solid and liquid multicomponent
APIs and presenting the crystal structures of some relevant systems
as case studies. It is clear that a focus on restrictive nomenclature
carries with it the risk that a thorough examination of the physicochemical
properties of the compounds will be overlooked.