A protic ionic liquid is an

ionic liquid An ionic liquid (IL) is a salt in the liquid state. 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 water and gasoline are predominantly made of ...

that is formed via

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

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 products can shift depending on the conditions. This has a significant impact on the properties of protic ionic liquids since some neutral acid and base species are generally present in the solution. One notable affect is that many protic ionic liquids have non-negligible

vapor pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases ...

. Ionic liquids have often been touted for their low vapor pressures, but for protic ionic liquids vaporization may need to be taken into account. As a result of this higher vapor pressure, some protic ionic liquids are distillable, which means they boil at a temperature lower than their decomposition point. 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 altered or changed by simple distillation.Moore, Walter J. ''Physical Chemistry'', 3rd e Prentice-Hall 1962, pp. 140–142 This ...

. 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