Bonded Phases - The Oligomeric Phases

The Oligomeric Phases

A theoretical model attempting to represent the physical nature of an oligomeric phase is rather difficult to portray diagrammatically and in figure 9 an effort is made to represent the bonded surface in a manner that is as simple as possible. The diagram is meant to represent an oligomeric phase, each bonded group having ten chains regularly attached to a theoretically ideal flat surface. Actually, the surface will be uneven and the bonding positions highly irregular. In addition, each bonded oligomer will probably be separated by at least one, un-reacted, silanol group.

Figure 9. The Diagrammatic Representation of an Oligomeric Reverse Phase

The small gray circles are meant to represent the silicon atoms that join the oligomers together and the larger black circles are meant to represent the surface silicon atoms to which the oligomer is attached. In order to reduce the complexity of an already complex diagram, the oxygen atoms joining the silicon atoms of the oligomer are not shown, as neither are the methyl groups at the base of each octyl chain. The left hand side of the diagram indicates the position of each oligomer relative to its neighbors and at the rear, four individual oligomers are supposed to indicate their relative proximity on the surface.

The oligomers are depicted as oriented perpendicular to the surface which, although may be partly true due to steric hindrance from neighboring groups, is a great over simplification. Theoretically, each oligomer should be free to move in all directions around their silicon oxygen bond with the surface and, in addition each hydrocarbon chain should be free to rotate. Despite these and other limitations, the diagram does clearly indicate the complexity of the surface of, what would be, a true oligomeric phase. It appears probable, that, in reality, the long oligomeric chains must, to some extent, lie over each other and it is obvious, that as the size of the oligomer increases, the effective interacting surface of the reverse phase retreats progressively farther away from the silica surface. In addition, this would also means that the sterically screened and un-reacted silanol groups will also be further away from the solvent and, consequently. less available as sites for surface erosion. It would seem, that the oligomeric phase could be significantly more resistant to erosion than either the brush or bulk phases.