Protic ionic liquids are formed via a proton transfer from an acid to a base. In contrast to other ionic liquids, which generally are formed through a sequence of synthesis steps, protic ionic liquids can be created more easily by simply mixing the acid and base.
Phosphonium cations (RP) are less common but offer some advantageous properties. Some examples of phosphonium cations are trihexyl(tetradecyl)phosphonium (P) and tributyl(tetradecyl)phosphonium (P).