Spectrophotometry was employed to study binding of methylene blue (MB) dye to sodium montmorillonite (NaM) in dilute aqueous suspension. The adsorption isotherm may be expressed u = h + fee1/", the first term (k, = 80 meq. MB/100 g. NaM) arising from ion exchange, the second from physical adsorption. Large spectral changes (metachromasy) were found to accompany changes in the coverage, u, at values well below k¡. These are similar to the spectral shift accompanying dimerization of MB in aqueous solution and are attributed to dye-dye interactions. The spectral properties showed large aggregates were formed at moderate coverage. The spectra of free MB monomer and dimer were determined quantitatively. The peak molar absorbancy index of the monomer was found to be 9.5 X 104 at 6640 A. The spectral variations up to concentrations of 2 X 10 "4 M were interpreted quantitatively in terms of a monomer-dimer equilibrium, with a dimer dissociation constant of 1.7 X 10-4 at 25°. The dimer spectrum was found to contain a long wave length peak which can be explained by a sandwich structure having the monomer transition moments at a mean angle of about 13°to each other. Trimethylthionme, formed by base-catalyzed demethylation, was found chromatographically to be a common impurity in methylene blue; a spectral criterion for this impurity was introduced and an extraction procedure was developed for removing it.

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