SOLVATOCHROMIC STUDIES OF STATIONARY PHASES ON THIN-LAYER CHROMATOGRAPHIC PLATES

Three polarity-indicating dyes, Reichardt's betaine, N,N-diethyl-4-nitroaniline, and N-methyl-2-nitroaniline, have been used to characterize the mobile-phase-stationary-phase interface of thin-layer chromatographic plates. Diffuse reflectance spectroscopy was used to detect the shifts in the wavelength maxima of these dyes, which have been related to the pi* and E(T)(30) solvent polarity scales. For acetonitrile and methanol/water mobile-phase mixtures, the results indicate that the solvated octadecyl stationary phase is considerably more polar than the analogous bulk alkane solvent. However, the experimental evidence supports the idea that the octadecyl chains protect the dye molecules from exposure to bulk solvent. Control experiments on solvated silica surfaces indicate that some of the dyes are capable of specific interactions with the silanol groups on the surface.