Bonded Phases - Dispersive Interactions

Interactions Between 'Brush' and 'Bulk' Reverse Phases and Aqueous Solvents

A striking example of the effect of different types reverse stationary phases on chromatographic performance is given by the different characteristics of the brush and bulk phases. In truth, their different behavior confirms the basic nature of the two phases. The interactions between aqueous solvent mixtures and brush reverse phases differ very significantly from those with a bulk reverse phase particularly at very low concentrations of solvent. The physical chemical behavior of the brush and bulk phases, when in contact with pure water and aqueous mixtures containing very low concentrations of solvent, have been rigorously investigated by Lochm ller and Wilder (24), Gilpin and Squires (25) and Scott and Simpson (26). They determined the retention of a series of solutes on bulk and brush reverse phases using solvent mixtures ranging from 0 to 10%w/v methanol in water. The results reported by Scott and Simpson for the retention of ethanol are shown in figure 5.

Figure 5. Graph of the Corrected Retention Volume of Ethanol against the Concentration of Methanol in the Mobile Phase for a Bulk and Brush Reverse Phase

The phases evaluated were the RP-18 brush, reverse phase manufactured by E. M. Laboratories, (coated with a C18 (dimethyloctadecyl) chain) and ODS-3 a bulk reverse phase (coated with a cross-linked C18 (octadecyl) chain) manufactured by Whatman Inc. The curves relating retention volume with solvent composition exhibit, two, very different retention characteristics for the two respective reverse phases. The relationship between the retention of ethanol and the methanol concentration in the mobile phase for the ODS-3 bulk reverse phase is conventional and what might be expected. As the concentration of methanol is increased, the retention volume of the ethanol decreases smoothly and continuously across the range from zero to 10%w/v of methanol. The relationship for the brush phase, is however, very unexpected. The retention volume of ethanol at first increases as the methanol content is increased, to a maximum concentration of about 3%w/v. At methanol concentrations above 3%w/v the retention values then begin to decrease and eventually follow a curve parallel to that for the bulk phase but at somewhat higher retention levels. Lochm ller and Wilder (24) performed some similar experiments and obtained analogous results while Gilpin and Squires (25) made some thermodynamic measurements on the two systems and demonstrated a clearly anomalous behavior by the brush type phase at very low methanol concentrations. It was considered that the performance of the brush phase could be explained as due to the free hydrocarbon chains interacting with themselves in preference to interacting with the aqueous solvent. It might be considered that, this effect was due to 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 .