Reversible metachromasy of crystal violet on titanium dioxide: A new surface photophysical phenomenon

Crystal violet adsorbed on Degussa P-25 titanium dioxide, under certain con ditions of dye loading and alter evaporation of liquid solvent, exhibits a reversible color change from pink-violet to blue-violet when exposed to wat er vapor, The spectral changes, measured by reflectance spectroscopy, corre spond to metachromasy of crystal violet that occurs in solution at differen t dye concentrations, due to aggregation of dye molecules. We propose a mec hanism, reversible metachromasy, in which the extent of aggregation of the dye molecules in the surface water monolayers changes as water molecules ad sorb or desorb. Specifically, as water adsorbs, the aggregates separate int o monomers, and then as water desorbs, the molecules reaggregate.