The chemical stabilization of methylene blue in colloidal dispersions of smectites

Abstract

The model reaction of methylene blue (MB) with ascorbic acid (AA) to give the reduced leuco form of MB and dehydroascorbic acid was chosen to evaluate the chemical reactivity of the dye in aqueous solution and smectite colloidal dispersions. Saponite Sumecton (Sap) and montmorillonite Kunipia (Mt) from Kunimine Ind. were used. To obtain a large series of spectral data, UV–Vis absorption spectrophotometer equipped with a diode-array detector was used. The spectral components of the reaction were analyzed by chemometric methods. MB decomposition was evident only in aqueous solutions without smectites. The reaction kinetics was dependent on the AA concentration and the acidity of the solution but was not significantly affected by the concentration of MB. The half-life of the reaction without addition of HCl was in the range 69 to 23 s and further reduced in the presence of HCl. In the systems based on smectite colloidal dispersions, no similar reaction was observed. MB formed molecular aggregates depending on the surface properties of the smectite particles. Relatively complex spectral forms indicated the presence of monomers (672, 690 nm), J-aggregates (761 nm) and H-aggregates absorbing at a wavelength lower than 500 nm. Nothing indicated the bleaching of MB in smectite colloids as a consequence of the reaction with AA. The importance of this phenomenon to stabilize reactive dyes from the point of view of application and to interpret some of the earlier results of MB antimicrobial efficacy is discussed.