Acid-base and monomer-dimer equilibria of methylene blue in dip-coated thin films

Thin films including Methylene Blue (MB) have been prepared. They were dip- coated using the sol-gel reaction of tetraethyl orthosilicate (TEOS). The a bsorption spectra of the individual thin films have been observed as a func tion of time after preparation of the films. The relative contributions of the monomer (MB+), dimer ((MB+)(2)), and protonated species (MBH2+) existin g in the individual thin films to the total absorption spectra were obtaine d. The acidity in each film was estimated from the relative abundance of th e MBH2+. In the case of the system including a large amount of HCl as a cat alyst, MB formed MBH2+ in addition to MB+ and (MB+)(2) just after preparati on of the films. However, MBH2+ in every film disappeared 5-10 min after th e preparation. The formation of MBH2+ indicates the very high acidity aroun d MB molecules. In each film just after the preparation, the rate of the hy drolysis became slower and the concentration of the released proton remarka bly increased because the polycondensation rapidly proceeded with the contr action in volume. Although growing -SiOSi- networks prevent the evaporation of the acid together with water and ethanol, the decrease of MBH2+ indicat es the lowering of acidity in the films due to the evaporation.