A system map for the ionic liquid stationary phase 1,12-di(tripropylphosphonium)dodecane bis(trifluoromethylsulfonyl)imide trifluoromethanesulfonate for gas chromatography

Abstract

The solvation parameter model is used to prepare a system map for the retention of volatile organic compounds on the ionic liquid stationary phase 1,12-di(tripropylphosphonium)dodecane bis(trifluoromethylsulfonyl)imide trifluoromethanesulfonate (SLB-IL61) over the temperature range 80–260°C. Retention is governed by dispersion, dipole-type and hydrogen-bonding interactions each with its own temperature dependence. The exchange of a bis(trifluoromethylsulfonyl)imide anion in SLB-IL60 for a trifluromethanesulfonate anion (SLB-IL61) results in a change in selectivity indicated by an increase in the hydrogen-bond basicity and a decrease in hydrogen-bond acidity of the stationary phase without change in either the cohesion or dipolarity/polarizability of the stationary phases. At high temperatures there are small differences in electron lone pair interactions but these are relatively unimportant in terms of selectivity differences. Since the disclosed chemical structures for SLB-IL60 and SLB-IL61 does not contain obvious hydrogen-bond acid functional groups the modest hydrogen-bond acidity of these stationary phases was unexpected but does not appear to be obviously connected to adsorption sites at the column wall. The polarity number is shown to be a poor indicator of column retention properties for SLB-IL61. Principal component analysis with the system constants as variables indicates that the retention properties of SLB-IL61 are not duplicated by any of the common poly(siloxane) and poly(ethylene glycol) stationary phase chemistries in current use for column preparation. The SLB-IL61 column is closest in separation properties to poly(cyanopropylphenyldimethylsiloxane) and poly(cyanopropylmethyldimethysiloxane) stationary phases with a high percentage of cyanopropyl-containing monomer but the two stationary phase types are not selectivity equivalent.