Alkylammonium Formate Ionic Liquids as Organic Mobile Phase Replacements for Reversed‐Phase Liquid Chromatography

Abstract

Alkylammonium formate (AAF) ionic liquids, RNH3 + [HCO2 −], R=C2H5, C3H7, or C4H9, are prepared in high yield and purity by slowly reacting an alkylamine and formic acid in an ice bath under N2. UV spectra of undiluted ethylammonium formate (EAF), n‐propylammonium formate (PAF), n‐butylammonium formate (BAF) show no appreciable background absorbance down to about 250 nm, 270 nm, and 295 nm, respectively. Physical properties such as viscosity, melting point, decomposition temperature, density, and miscibility with common organic solvents are also determined. The viscosity of EAF of 11.5 cP at 24°C is considered very low, as compared to previously reported imidazolium or alkylammonium ionic liquids. Because of the higher viscosities, van Deemter plots using the AAF ionic liquids are shifted to lower flow rate optima, as compared to that taken with methanol. The polarity index P′ values for EAF, PAF, and BAF, chromatographically determined to be (6.4±0.1); (5.8±0.1), and (5.2±0.1) (n=5), are quite close to methanol (5.1) and acetonitrile (5.8). Kamlet‐Taft solvatochromic parameters were also measured and the AAFs were found to have acid, base character similar to that for methanol but polarizability like acetonitrile. The solvent strength S values of 1.9, 2.2, and 2.5 for EAF, PAF, and BAF, respectively, are smaller but comparable to those for methanol (about 2.6). Application of these AAF ionic liquids in reversed‐phase liquid chromatography to separate a test mixture of caffeine, p‐nitroaniline, and phenol or water soluble vitamins is demonstrated using the polystyrene‐divinylbenzene PRP‐1 column.