Determination of the Hydration of Methylene Blue Aggregates and Their Dissociation Constants Using Visible Spectroscopy

Abstract

The visible spectra of methylene blue (MB) in aqueous-methanol solutions were used to determine the extent of hydration of MB aggregates. The investigation was carried out at MB concentrations of 1.000 × 10−4 M and 1.000 × 10−3 M. Abstract factor analysis (AFA) was used to determine the number of chemical species responsible for the spectral data. The species were identified as aqueous MB monomer, dimer, and trimer, and a methanoic MB monomer. The absorptivity spectra of the aqueous MB species obtained from the previous studies were used to extract their concentration profiles in methanol–water mixtures. With the addition of methanol to the aqueous solvent, the concentration of water could be varied from 0% to 100%. This variation was used to determine the extent of hydration and solvation of each of the four MB species, as well as their mutual equilibria. The species were identified as (1) MB+CH3OH, MB monomer monomethanol; (2) MB+H2O, MB monomer monohydrate; (3) (MB)2++ 6H2O, MB dimer hexahydrate; and (4) (MB)3Cl++ (H2O)13, MB trimer monochloro tridecahydrate. The dissociation constants for the hydrated monomer, dimer, and trimer were determined to be 0.48, 1.29 × 103 M5 and 7.2 × 106 M13, respectively.

Optical humidity sensor using methylene blue immobilized on a hydrophilic polymer

“…1) consist of a band at 664, and two shoulders at 610 and 560 nm. The 664 nm band is assigned to the monomer of MB, the 610 nm shoulder belongs to the dimer of MB [21] and the 560 nm shoulder is similar to that of the trimer [36]. Fig.…”

Polymeric Adsorption of Methylene Blue in TiO2 Colloids—Highly Sensitive Thermochromism and Selective Photocatalysis

“…The absorption at 570 nm becomes more and more prominent with the increase of MB concentration; this absorption represents the trimer state of MB according to previous studies . [31,32] For comparison, Figure 3 shows the visible light-absorption spectra of a pure MB water solution of different concentrations. There is no obvious trimerization for pure MB in water, so we believe that the TiO 2 nanoparticles in www.chemeurj.org the hydrosol provide a good surface for the polymerization of MB from monomer, to dimer, and to trimer.…”

Novel temperature-activated humidity-sensitive optical sensor

“…This high degree of aggregation is not readily observed in aqueous solutions of MB, even at high concentrations (>10 mM) [23,30], and thus the mechanism responsible for formation of this species for MB, and all the other thiazine dyes (except TH), must be due to RH-related, significant changes in the encapsulated dye's microenvironment [25].…”