Metachromasy in Clay Minerals. Part I. Sorption of Methylene‐Blue by Montmorillonite

UV, visible and IR spectra of methylene‐blue sorbed on Wyoming bentonite saturated with different cations were studied and characterized. The colloidal properties of a dilute suspension of Cu montmorillonite treated with methylene‐blue was studied by heterometric titration. The organic dye can be sorbed either at the edges of the clay platelets or on oxygen sheets of the silicate layer. In the latter case sorption leads to metachromasy of the dye molecule and a shift of the absorption bands to lower values in the visible range. X‐ray patterns show that metachromasy of methylene‐blue in montmorillonite occurs also when there is only one organic layer lying parallel to the silicate sheets. Thus, metachromasy cannot be attributed to polymerization of the organic dye as was previously suggested.