Electric Dichroism. The Effect of Dialysis on the Color of Crystal Violet Adsorbed to Montmorillonite

Electric dichroism was measured in aqueous suspensions of Na-montmorillonite which had been treated with ZnCl₂ and stained with crystal violet (CV⁺). Salt had to be removed by dialysis in order to reduce ionic conductivity. On dialysis, new absorption bands of CV appeared simultaneously at 435 nm (weak) and 665 nm (strong), at the expense of metachromic bands β and γ. The new bands were attributed to CVH²⁺, protonated during dialysis by hydrolyzed water adjacent to Zn²⁺. CV⁺ has chiral D₃ symmetry and two overlapping absorption bands at 590 nm. The bands would separate by the action of a force of lower symmetry than that of the molecule. The exceptionally large separation of the new bands indicated that the extra proton was attached at an apex of the molecule, i.e., an amine group. Our MO calculations showed that protonation alone could not account for the observed band splitting and a tilt of a phenylamine ring was essential. The hydrolyzed samples were stable for months, unlike those acidified with HCl. Electric dichroism revealed that the absorption dipoles of the CV⁺ cations which yield bands α and β, as well as those of CVH²⁺, were parallel to the aluminosilicate layers of the montmorillonite. In contrast, the absorption dipoles of aggregated CV, which was trapped in the interparticle space of a floe of the clay and which yielded band y, were not parallel to the layers of the clay mineral.