Studies on adsorptiochromism. II: Diffuse reflectance spectroscopy of adsorptiochromic spiropyrans adsorbed to some pharmaceutically useful solids

The colored powders produced by the adsorption of four adsorptiochromic spiropyrans to many solids (silica gel, silicic acid, fumed silica, alumina, microcrystalline cellulose, talc, titanium dioxide) were examined by diffuse reflectance spectroscopy. The reflectance spectra were dominated by two bands, one at 550 nm and the other in the range 400-500 nm, often at 472 nm. Plots of the Kubelka-Munk function [F(R' infinity)] against g, the coverage expressed in nmol/m2, were linear at very low g and approached a limiting value independent of g at high coverage. The color formation upon adsorption terminates at coverages much lower than the maximum binding capacity of the solid. The slope of the plot of F(R' infinity) against g, at low g (denoted f0), appears to be sensitive to the scattering properties of the solid. For a single solid (silica gel), comparison of f0 for adsorptiochromic adsorbates with f0 for permanent dyes allowed estimates to be made of the fraction of adsorbed spiropyran in the colored form on the surface.