In the footsteps of August Michaelis: Syntheses and Thermodynamics of Extremely Low‐Volatile Ionic Liquids

A series of nine different known ionic liquids or low melting salts was synthesised and purified. They are composed of the [NTf(2)](–) (bis(trifluoromethane)sulfonimide), [OTf](–) (trifluoro‐methane‐sulfonate), or [B(CN)(4)](–) (tetracyanidoborate) anion and [Ph(4)P](+) (tetraphenylphosphonium), [Ph(3)BzP](+) (triphenylbenzyl phosphonium), [(n)Bu(4)P](+) (tetra‐(n)butylphosphonium), [(n)BuPh(3)P](+) (tri‐phenyl‐(n)butylphosphonium), [(n)Bu(4)N](+) (tetra‐(n)butylammonium), or the [PPN](+) (bis(triphenylphosphine)iminium) cation. Precise vapour pressure data and enthalpies of vaporisation were measured using the Quartz Crystal Microbalance (QCM) method and evaluated. Structure‐property relations are established using the obtained data as well as literature known data of ILs with alkyl‐substituted imidazolium cations. It turns out that ILs with the tetracyanidoborate anion have even higher values of the enthalpy of vaporisation than those with the common [NTf(2)](−) or [OTf](−) anion and therefore are even less volatile.