Crystal violet adsorbed on Degussa P-25 titanium dioxide, under certain conditions of dye loading and after evaporation of liquid solvent, exhibits a reversible color change from pink-violet to blue-violet when exposed to water vapor. The spectral changes, measured by reflectance spectroscopy, correspond to metachromasy of crystal violet that occurs in solution at different dye concentrations, due to aggregation of dye molecules. We propose a mechanism, reversible metachromasy, in which the extent of aggregation of the dye molecules in the surface water monolayers changes as water molecules adsorb or desorb. Specifically, as water adsorbs, the aggregates separate into monomers, and then as water desorbs, the molecules reaggregate.

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The refractive indices no and ne, the linear electrooptic coefficient r33, and the relative (low frequency) permittivities ε3 and ε2 = ε2 of nominally pure KTN crystals with tetragonal symmetry (4 mm) are measured at different temperatures. In the case of the optical properties (no, ne, and r33) the light wavelength is varied, too.

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