Bonded Phases - The Wetting Characteristics of Brush and Bulk Reverse Phases
The Wetting Characteristics of Brush and Bulk Reverse Phases
Scott and Kucera also investigated the wetting characteristics of the same five reverse phases by measuring the maximum water concentration in the solvents methanol, acetonitrile and isopropanol that would allow the phase to remain wetted and not float to the surface of the solvent. The results they obtained are shown in table 3. It is clear from table 3 that reverse phase ODS, that was the incompletely silanized, is wetted by pure water, confirming that there are a large amount of unreacted silanol groups on the surface that permit polar interactions to occur with the water molecules.
Table 3. Wetting Characteristics of the Five Reverse Phases
It should also be noted that the bulk phase ODS2 is the most dispersive and, as a consequence, can only tolerate the smallest amount of water in the solvent before the phase is un-wettable . This is to be expected considering the high retentive characteristics of this bulk phase. All the brush phases are shown to tolerate higher concentrations of water in the solvent than the bulk phases before they become un-wettable. It is also seen that, the more dispersive and less polar the solvent, the more water the solvent can contain before any of the phases become un-wettable. Table 3 also shows that excluding the incompletely reacted phase ODS, the brush phase with only the 2 carbon chains was the least dispersive.
These results have significant implications when considering the exchange of solutes between the mobile and stationary phases in a chromatographic column. If the solvent is not adequately dispersive (i.e. the water content is above that which wets the stationary phase) the two phases will be immiscible in a similar way that water and octane are immiscible. Actually, except that the exchange process is far more efficient, the column will act in a similar manner to a Craig machine that contains two immiscible liquids. However, if the solvent can wet the reverse phase, the distribution is now between miscible phases but as one is attached to the solid silica matrix, and, thus, can remain stationary, a chromatographic separation can still be achieved. This emphasizes the importance of the silica support.
In chromatography, the support permits a distribution system to be employed, where the two phases are intrinsically miscible but still allow one phase to be kept stationary and at the same time permit the other phase to be mobile.