Acid–Base and Monomer–Dimer Equilibria of Methylene Blue in Dip-Coated Thin Films

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Abstract
Thin films including Methylene Blue (MB) have been prepared. They were dip-coated using the sol–gel reaction of tetraethyl orthosilicate (TEOS). The absorption spectra of the individual thin films have been observed as a function of time after preparation of the films. The relative contributions of the monomer (MB+), dimer ((MB+)2), and protonated species (MBH2+) existing in the individual thin films to the total absorption spectra were obtained. The acidity in each film was estimated from the relative abundance of the MBH2+. In the case of the system including a large amount of HCl as a catalyst, MB formed MBH2+ in addition to MB+ and (MB+)2 just after preparation of the films. However, MBH2+ in every film disappeared 5—10 min after the preparation. The formation of MBH2+ indicates the very high acidity around MB molecules. In each film just after the preparation, the rate of the hydrolysis became slower and the concentration of the released proton remarkably increased because the polycondensation rapidly proceeded with the contraction in volume. Although growing –SiOSi– networks prevent the evaporation of the acid together with water and ethanol, the decrease of MBH2+ indicates the lowering of acidity in the films due to the evaporation.
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