A Spectroscopic Study of Methylene Blue Monomer, Dimer, and Complexes With Montmorillonite

Abstract

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.

Preparation and properties of potassium-vermiculite films

“…The volume was then made up to 100 cm 3 with an aqueous solution of MB of known concentration (about 5 ? 10 24 M or 0.1 g͞l) and placed in a water bath at 27 ± C and shaken for 16 h. The amount of MB that remained in the filtered supernatant was then measured after dilution to assure the MB was in its monomer form, 16,18 using a spectrophotometer (Perkin-Elmer Lamba 6 UV/VIS, at 664 nm for monomer absorption) and by difference the amount of MB adsorbed by the vermiculite calculated.…”

“…18,21 The XRD results on Bu, Li, and K exchanged vermiculites after MB adsorption show reflections between 11.8 and 12.2Å [e.g., Fig. 1(e)].…”

“…Without a delamination step the amount of MB adsorbed onto Li or Bu exchanged vermiculites was lower by a factor of 5. 18 In order to estimate the surface area of these delaminated vermiculites a knowledge of the internal and external surface is therefore necessary. The use of nitrogen adsorption to estimate the external surface could be a possibility but Shelden et al 6 have shown that this is viable only for low surface area micas and is not really applicable to our delaminated vermiculites.…”

Adsorção do corante azul de metileno em partículas de argilominerais: análise dos tamanhos das partículas

“…Os espectros de absorção de luz visível são diferentes para as moléculas que se encontram nas formas agregadas daqueles vistos para os monômeros do corante (i.e., moléculas que se encontram separadas umas das outras), uma vez que a agregação altera significativamente os níveis de energia para as transições eletrônicas das mesmas. 19,22,23 Como exemplo, em soluções de azul de metileno, AM, com concentrações da ordem de 10 -6 mol L -1 , nota-se uma banda de absorção máxima em 670 nm correspondente aos monômeros de AM. Em concentrações mais altas, a banda dos monômeros diminui e outra banda com máximo em 605 nm, atribuída aos dímeros de AM, passa a predominar.…”

“…Em concentrações ainda maiores é possível observar outra banda por volta de 575 nm, referente aos trímeros e agregados maiores. 22,23 Em meio aquoso altamente ácido o AM exibe também uma banda com máximo em 765 nm, que é atribuída aos monômeros protonados do corante.…”