The rôle of water in the metachromatic reaction

The effect of water on the metachromatic complexes of methylene blue and cellulosic polyanions has been studied by diffuse reflectance spectroscopy. Complete dehydration causes the complexes to become orthochromatic and rehumidification reverses the change. These effects are explained in terms of site‐binding. It is postulated that the dye cations are bound more tightly in the anhydrous state and this modifies the dye‐dye interactions that are the cause of the metachromatic colour change. Anionic celluloses produced hypsochromic shifts in the absorption spectrum of the dye in the order: sulfate > phosphate > carboxyl > carboxymethyl. This is the reverse of the order of electrostatic attractions between the sites and the dye cations. Such attractions are shown to modify dye‐dye interactions, with the result that for a given spacing of negative sites, the polycarboxylates initiate least metachromatic colour change.