Classification of countercurrent chromatography solvent systems on the basis of the capillary wavelength

Abstract

Countercurrent chromatography (CCC) is a separation method based on the partition of solutes between two liquid phases, as they interact in a thin tube under centrifugal forces. Many different versions of CCC instruments have been developed over the last twenty years.’ Some of them have been commercialized. In the last few years, two major CCC instrument types have received most attention due to their relatively large efficiency and speed capabilities. In these systems, the use of an intense centrifugal force field leads to a significant retention of one phase (called the stationary phase) even at large flow rates of the other phase (the mobile phase). Basically, in one of these two systems (centrifugal droplet CCC, CDCCC), the column is made of numerous small channels connected in series, in which the flow of the mobile phase is parallel to the direction of the centrifugal force. In this system, the direction of the flow in the channels depends on the density of the mobile phase relative to that of the stationary phase (outward flow when the mobile phase has a lower density). In the other system (high-speed CCC, HSCCC), the flow and field vectors are not parallel. The column is made of a Teflon tube helically coiled in one or several layers wound on a core sylinder (with radius r) rotating with an angular speed il around its own axis. This assembly (column and cylinder) is itself rotating with the same angular velocity in the same direction around the main axis of the system. R is the distance between the two parallel axes. The ends of the column can be labeled “head” or “tail”, depending on both the type of helix (right-handed or left-handed) and the direction of rotation. It is found experimentally that in order to more or less retain the stationary phase in the column, the lighter phase has to be pumped either from the tail to the head or from the head to the tail, depending on the nature of the two-phase solvent system and on the /3 ratio, where 8 = r/Re2 A classification of solvent systems, established by Ito according to their hydrodynamic behaviors in the HSCCC device,’ is reported in Table 1. The first group encompasses solvent systems for which the lighter phase has to flow toward-the head of the column to ensure some retention of the heavier phase. Because the polarities of the two phases (one of which being water) are significantly different for all the investigated systems in this group, Ito labeled these solvent systems as “hydrophobic” systems. A second group of so-called ‘hydrophilic” solvent systems, for which the nonaqueous phase is quite polar, exhibits an opposite behavior (flow of the lighter phase toward the tail). A third group of “intermediate” systems exhibits a more complex behavior as they behave like hydrophobic systems for > 0.3 and like hydrophilic systems for /3 < 0.3. The detailed hydrodynamic behavior of two-phase solvent systems in CCC devices is extremely complex and is at the present stagevery poorly understood. Yet it would bedesirable to have good models of this flow behavior based on intrinsic physicochemical properties of solvent systems in order to optimize the separation and to select the operating conditions of the CCC apparatus, especially for the HSCCC device. This paper is a preliminary attempt toward this goal. It is well-known that the physics of biphasic liquid systems in a gravitational field is controlled by thecapillary wavelength. One therefore expects that this parameter plays a key role in