Polyanions and their complexes. Part VII. Mechanism of methylene blue–polyanion interactions

Pulse radiolysis has been employed to study the ion-binding between methylene blue and the anionic sites of a number of polyanions. A comparison between the extent of ion-binding measured by pulse radiolysis, and the apparent amount of free methylene blue indicated by absorption spectra, showed that an equilibrium exists between bound aggregated dye, bound monomeric dye and free dye in solution. The results indicate that dye aggregation rather than ion-binding is mainly responsible for metachromasia. The effect of temperature on the ion-binding was examined. The chemical nature and density of the anionic sites exert a significant influence on the driving force of the interaction, but the reversible aggregation and disaggregation of the dye appears to be the dominant effect.