Hydrodynamics studies of the behaviour of traditional and two-phase ionic liquid solvent systems in countercurrent chromatography (CCC)

The hydrodynamics and retention of 2D countercurrent chromatographic systems were investigated for the first time for ionic liquid based biphasic solvents using high-speed imaging. The results were compared against 3D chromatographic systems and different solvent systems for a wide range of beta values (0.27-0.83). The studies revealed details of the phase distribution, the mixing and settling areas within the column, as well as the interfacial characteristics that can enhance mass transfer (such as waves). There was no single-phase flooding in the beta range 0.27-0.5 contrary to previous limited visualisation studies, which suggested that for the biphasic solvents they utilized, single-phase flooding at low beta values (less than 0.5) occurs. Previously unseen, stable, six segment phase distributions, involving alternate high and low stationary phase retention were observed for the ionic liquid biphasic solvent system but not for the organic-aqueous and aqueous-aqueous biphasic solvent systems also studied. This unusual behaviour and the resulting considerable variations in interfacial height and thus local velocity of each phase along the coil, are discussed in the text. The ionic liquid biphasic solvent also demonstrated novel behaviour in other aspects. In the mixing zone around the proximal key node, interfacial waves were observed; however, it was found that their amplitude and frequency depended on the chosen elution mode as well as the mobile phase flow rate and beta value. While there was some agreement in the performance of both column geometries, it was found that for the serial 2D column there was always some retention of the stationary phase over the entire range of flow rates tested, unlike the case for the 3D column. (C) 2018 The Authors. Published by Elsevier Ltd.