Liquid Chromatography - Interactions Between 'Brush' and 'Bulk'' Reverse Phases and Aqueous Solvents 2
At methanol concentrations above 3%w/v the retention values begin to fall and eventually follow a curve parallel to that for the bulk phase but somewhat higher. Lochmüller and Wilder (25) carried out some similar experiments and obtained the same results while Gilpin and Squires (26) carried out some thermodynamic measurements on the two systems and showed an anomalous behavior by the brush type phase at very low concentrations of solvent. It was suggested that the behavior of the brush phase could be explained as a result of the free chains interacting with themselves in preference to interacting with the aqueous solvent. In effect, this was the same phenomena of immiscibility that occurs between water and a liquid hydrocarbon. The dispersive forces between the hydrocarbon chains themselves are greater than the forces between the hydrocarbon chains and the aqueous solvent. As a result the chains interact with themselves and collapse on the surface of the silica. When the methanol content of the mixture is increased, the solvent becomes more dispersive in nature and the hydrocarbon chains can then interact with the solvent and no longer exist in a collapsed state. The situation is depicted in figure 36.
Figure 36. Effect of Solvent Composition on the Orientation of the Brush Phase.