How to Measure SF
Figure 1 indicates the principle of use of any CCC device for the equilibrium of two liquid, nonmiscible phases. In this case, the stationary phase which is chosen is the lighter phase of the solvent system (dark gray in Fig. 1), whereas the mobile phase is indicated in white. For simplification, the coil is considered as an empty cylinder and the phenomena which occur inside the column are highly schematized as a stack of disks of mobile and stationary phases. This allows us to visualize the progression of the mobile phase inside the column. After the solvent system has reached equilibrium (complete settling of the two phases), the phase chosen as the stationary phase is pumped into the apparatus. The latter is considered as filled as soon as droplets of this stationary phase are expelled out of the column; this is Step 1 of Fig. 1.
The apparatus is then started, and when the desired rotational speed is reached, the mobile phase is pumped into the apparatus. A graduated cylinder is then put at the outlet of the apparatus. The two phases undergo a hy-drodynamic or hydrostatic equilibrium inside the column while the mobile phase progresses toward the outlet of the column; this is Step 2. After a certain time, the mobile phase has reached the end of the column and then the first droplet of the mobile phase falls into the graduated cylinder; this Step 3. The experimenter then reads the volume, V1, of the stationary phase which has been expelled from the column. The experiment is continued until the desired total volume is reached in the graduated cylinder. The experimenter can read the respective volumes of the stationary phase, named V2, and the mobile one; this is Step 4. Finally, the apparatus can be emptied (for instance, by pushing with nitrogen gas) and the liquids collected in another graduated cylinder; the volume of the stationary phase is V3. For simplification purposes, the extracolumn volumes are neglected. Three measurements of the stationary phase retention are available:
• One just after the equilibrium inside the column: SFi = (Vcolumn - V)/^column
( Vcolumn V2 2 / Vcolumn
• One by emptying the apparatus: SF3 =
V3/Vcolumn
If the equilibrium of the two phases was stable and not disturbed by any external event (change in rotational speed, flow rate, etc.), the three values of SF should be similar by a few percent of precision.
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