Molecular interactions at octadecylated chromatographic surfaces

Interactions between fluorescent solutes and an octadecylated silica surface are investigated using fluorescence emission spectra and quenching techniques under conditions similar to those found in high-performance liquid chromatography. Pyrene, benzo[a]pyrene, fluorene, biphenyl, propyldansylamide, and decyldansylamide are used as the fluorescent probes, and potassium iodide and N,N-dimethylaniline are used as quenchers. N,N-Dimethylaniline is a moderately retained quencher thought to probe deeper into the octadecylated surface than the ionic iodide salt. Solvent-dependent fluorescence emission maxima, solvent-dependent fluorescence vibronic band intensities, and quencher access to the probes are investigated using aqueous methanol mobile phase compositions ranging from 60 to 100% methanol. The resulting information is used to interpret differences in interfacial probe environments and to determine the location of the probes within the bonded phase layer. The data indicate that biphenyl and pyrene may remain in very nonpolar interfacial probe environments deep in the bonded phase layer over the mobile phase composition range tested. The fluorophore portion of both propyldansylamide and decyldansylamide may reside in an interfacial environment, which becomes more polar as the water content in the mobile phase is increased. Benzo[a]pyrene apparently becomes exposed to the mobile phase as the water content in the mobile phase increases. This is thought to be due to the relatively large size of the solute molecule and the collapse of the octadecyl chains with increased solvent polarity. Fluorene appears to interact strongly with silanol groups. The results are interpreted in light of the surface convolution and chain cluster octadecylated chromatography surface models and are found to be more constant with the chain cluster model. Implications of the results to reversed phase high-performance liquid chromatography separations are also discussed.