Estimation of surface polarity of silicas by absorption spectroscopy of glycerin suspensions of adsorbed 6-nitrobenzoindolinopyran.

The title compound (6-NO2-BIPS) is adsorptiochromic, becoming colored upon adsorption to a polar surface. Powders of 6-NO2-BIPS adsorbed to silica gel or silicic acid are suspended in glycerin, and the absorption spectrum of the adsorbate is recorded by conventional absorption spectroscopy. The wave number of maximum absorption is related to the effective surface polarity by v*/cm-1 = 90.85 Z + 11,571, where Z is the Kosower polarity measure. Silica surface polarity corresponds to Z = 86-89.