Liquid Chromatography - Aqueous Solvent Mixtures 1

Aqueous Solvent Mixtures

When the relationship between the distribution coefficient of a solute and solvent composition, or the corrected retention volume and the solvent composition, was tested with aqueous solvent mixtures it was found that the relationship identified by Purnell and Laub and Katz et al failed. It was suspected that the failure was due to the solvent strongly associating with the water and, in fact, an aqueous solution of methanol, for example, contained methanol, water and methanol associated with water. The solvent mixture was thus, a ternary system and thus the linear relation ship between the volume fraction (before mixing) and retention would not be expected to hold.

The association of methanol and water was examined by Katz, Lochmüller and Scott (36) using volume change on mixing and refractive index data and established that the methanol/water solvent system was indeed a complex ternary system. They calculated both the association equilibrium constant and the distribution of the different components of a methanol water mixture form zero to 100% methanol. The curves they obtained are shown in figure 47.

Figure 47. Diagram of the Ternary Solvent System for Methanol/Water Mixtures