MICROSCOPIC ORDER AS A FUNCTION OF SURFACE COVERAGE IN ALKYL-MODIFIEDSILICAS - SPIN PROBE STUDIES

The chemically modified surface plays an important role in retention i n reversed-phase liquid chromatography. One surface factor that might affect retention is solute expulsion due to partial ordering of the gr afted chains at sufficiently high grafting densities. This study has s hown by the use of the electron paramagnetic resonance technique of sp in probing that spectroscopic evidence exists for an increase in molec ular order of the bonded octadecyl phase as a function of grafting den sity. The spin probe, DOXYL-cholestane, shows significant increase in the order parameter, S, as a function of increased surface coverage. I n addition, DOXYL-cholestane undergoes a decrease in rotational freedo m and an increase rotational anisotropy as the grafting density increa ses.