Bonded Phases - Dispersive Interactions
Figure 6. Effect of Solvent Composition on the Orientation of a Brush Phase.
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 collapse of the chains is depicted in figure 6. The relationship between the retention of ethanol on the brush phase with water/methanol mixtures, as demonstrated in figure 5, can now be explained. In contact with 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. As a consequence, the retention volume of the solute will also be reduced, as its retention will be proportional to the available surface area. As the concentration of methanol is increased, the interactive character of the solvent becomes more dispersive. As a result the hydrocarbon chains can begin to interact with the mobile phase and they will begin to unfold. The increase in entropy of the chains from the surface (their freedom of movement in the mobile phase is increased) will result in an increase in the effective surface area of the stationary phase and the retention of the solute will also start to increase. This loosening of the hydrocarbon chains will continue until there is sufficient methanol in the solvent for the hydrocarbon chains to completely interact with the solvent and will no longer interact with themselves. At this concentration (about 3%w/v of methanol) the retention volume of the ethanol arrive at a maximum. Subsequently, the stationary phase will behave in the normal way and any further increase in methanol concentration will increase the interactions of the ethanol with the mobile phase and retention will steadily decreases in the same way as the bulk phase..
It is now interesting to consider the behavior of the bulk phase. There seems to be no interaction between the hydrocarbon chains themselves and no change in surface area at high water concentrations. Actually, the retention of ethanol falls steadily as the methanol content increases in the expected manner. The reason for this is the cross-linking that takes place when the bulk phase is synthesized (as a result of the use of the trichlor-silane), causes the polymeric chain system to become rigid and, consequently, does not allow the individual hydrocarbon chains to interact with each other and collapse on the surface. As a result, the surface area is not reduced and there is normal retention behavior. Another piece of information that is provided from the curves shown in figure 25 is that they confirm the cross-linked nature of the bulk phase. It might also appear that for certain solutes, the best reverse phase for operation with aqueous mixtures containing very little solvent (e.g. samples of biological origin) might be a bulk reverse phase. The retention mechanism on brush type phases under these conditions could well be highly anomalous.