Trigonal Tricationic Ionic Liquids: A Generation of Gas Chromatographic Stationary Phases

Trigonal tricationic ionic liquids (ILs) are a new class of ILs that appear to be unique when used as gas chromatographic stationary phases. They consist of four core structures; (1) A = mesitylene core, (2) B = benzene core, (3) C = triethylamine core, and (4) D = tri(2-hexanamido)ethylamine core; to which three identical imidazolium or phosphonium cationic moieties were attached. These were coated on fused silica capillaries, and their gas chromatographic properties were evaluated. They were characterized using a linear solvation parameter model and a number of test mixtures. On the basis of the literature, it is known that both monocationic and dicationic ILs possess almost identical polarities, solvation characteristics, and chromatographic selectivities. However, some of the trigonal tricationic ILs were quite different. The different solvation parameters and higher apparent polarities appear to generate from the more rigid trigonal geometry of these ILs, as well as their ability to retain the positive charges in relatively close proximity to one another in some cases. Their unique selectivities, retention behaviors, and separation efficiencies were demonstrated using the Grob mixture, a flavor and fragrance test mixture, alcohols/alkanes test, and FAME isomer separations. Two ILs C1 (methylimidazolium substitution) and C4 (2-hydroxyethylimidazolium substitution) had higher apparent polarities than any know IL (mono, di, and tricationic ILs) or commercial stationary phases. The tri(2-hexanamido)ethylamine core IL series proved to be very interesting in that it not only showed the highest separation efficiency for all test mixtures, but it also is the first IL stationary phase (containing NTf₂⁻ anions) that eliminates peak tailing for alcohols and other H-bonding analytes. The thermal stabilities were investigated using three methods: thermogravimetric analysis (TGA) method, temperature programmed gas chromatographic method (TPGC), and isothermal gas chromatographic method. The D core series had a high working temperature range, exceptional selectivities, and higher separation efficiencies than comparable polarity commercial columns. It appears that this specific type of multifunctional ILs may have the most promising future as a new generation of gas chromatographic stationary phases.