Liquid Chromatography - Interactions Between 'Brush' and 'Bulk'' Reverse Phases and Aqueous Solvents 3
The relationship between the retention of ethanol on the brush phase with water/methanol mixtures, as shown in figure 35, can now be explained. In pure water the hydrocarbon chains of the brush phase are collapsed on the surface and thus, the effective surface area of the stationary phase is much reduced. Consequently, the retention volume of the solute, being proportional to the available surface area, is also reduced. As methanol is added to the solvent mixture, the solvent becomes more dispersive and the hydrocarbon chains can begin to interact with it and, as a consequence, begin to unfold. The liberation of the chains from the surface results in an increase in the effective surface area of the stationary phase and the retention of the solute also starts to increase. This process continues until there is sufficient methanol in the solvent for the hydrocarbon chains to totally interact with the solvent and be completely released from the surface. At this concentration (about 3%w/v of methanol) the retention volume of the ethanol reaches a maximum. Subsequent increase in methanol concentration merely increases the interactions of the ethanol with the mobile phase and, by adsorption of the solvent onto the surface of the reverse phase, reduces the interactive forces with the reverse phase. Consequently, the retention volume steadily decreases in the expected manner.
Having explained the behavior of the brush phase it is now interesting to consider the behavior of the bulk phase. There appears to be no interaction between the hydrocarbon chains themselves and no change in surface area at high water concentrations. In fact, the retention of ethanol falls steadily as the methanol content increases in the expected manner. The explanation given for this is that the cross-linking that takes place when the bulk phase is synthesized, keeps the polymeric chain system rigid and does not allow the individual hydrocarbon chains to collapse on the surface. As a consequence, the surface area is not reduced and the retention behavior is normal. Another aspect of the behavior of the bulk phase at low solvent concentrations is that it does, in fact, confirm the cross-linked nature of the bulk phase. It would also appear that for certain solutes, the best reverse phase for operation with aqueous mixtures containing very little solvent might be a bulk reverse phase. The retention mechanism on brush type phases under these conditions might be anomalous.