Fluorination effect in the volatility of imidazolium-based ionic liquids

Abstract

The volatility and cohesive energy of fluorinated ionic liquids, FILs, was evaluated and used to explored and understand the effect of fluorination of the ILs cations and anions on the cohesive interaction and structuration. For this, the vapour pressures at different temperatures using a Knudsen effusion apparatus coupled with a quartz crystal microbalance were measured for three fluorinated imidazolium-based ionic liquids. The enthalpies and entropies of vaporization were derived from the temperature dependence of the vapour pressures. The degree of fluorination of the ions leads to an increase of the IL volatility following the order: [C8mim][NTf2] < [C8mim][BETI] < [C8H4F13mim][NTf2] < [C8H4F13mim][BETI]. The Gibbs energy of vaporization follows the opposite trend proving that the fluorination effect on the volatility of the studied ionic liquids is ruled by the increase of the entropy of vaporization that overlaps the increase of the cohesive energy of the liquids. The binding energies of gas-phase FILs ions were measured using electrospray tandem mass spectrometry (ESI-MS-MS). The energy required for extracting the fluorinated cation from the [(cation)2(anion)]+ aggregates in these fluorinated ionic liquids is significantly higher than in their non-fluorinated counterparts. The measured binding energies trends are in agreement with those found for the enthalpies of vaporization.