Monoethanolamine oleate is a typical protic ionic liquid. A protic ionic liquid is an

ionic liquid An ionic liquid (IL) is a salt (chemistry), salt in the liquid state at ambient conditions. In some contexts, the term has been restricted to salts whose melting point is below a specific temperature, such as . While ordinary liquids such as wate ...

that is formed via

proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...

transfer from a Brønsted acid to a Brønsted base. Unlike many other types of ionic liquids, which are formed through a series of synthesis steps, protic ionic liquids are easier to create because the acid and base must simply be mixed together.

Properties

Because the proton transfer reaction is reversible, the equilibrium between reactants and ionic products can shift depending on the conditions. This equilibration significantly impact the properties of protic ionic liquids since some neutral acid and base species are generally present in the solution. Many protic ionic liquids have non-negligible

vapor pressure Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indicat ...

, even though traditional ionic liquids are often touted for their low vapor pressures. Some protic ionic liquids are distillable. Additionally, during distillation protic ionic liquids exhibit what appears to be a reactive

azeotrope An azeotrope () or a constant heating point mixture is a mixture of two or more liquids whose proportions cannot be changed by simple distillation.Moore, Walter J. ''Physical Chemistry'', 3rd e Prentice-Hall 1962, pp. 140–142 This happens beca ...

. In other words, initially only the acid or base is vaporized, but at a specific composition both the acid and base start to vaporize and the composition of the vapor and liquid is the same. This composition depends on the anion and cation and contains more acid than base. However, when mixed with water the vaporization behavior is different and an azeotrope no longer occurs at that same composition. The density of a protic ionic liquid mixture also appears to be higher for the composition at which the azeotrope forms. That is, if some excess of the acid is present in the mixture the density increases.

References

{{Reflist Acid–base chemistry Ionic liquids