Ultraviolet–visible absorption spectra of Thymol Blue during the sol–gel–xerogel transition of tetraethyl orthosilicate: in situ probe of microchemical environment

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Abstract
The absorption spectra of Thymol Blue (TB) in TEOS dispersions during a sol–gel–xerogel transition catalysed by acid or base at constant temperature have been observed as a function of time. We have observed for the first time the spectra of TB throughout the transitions for one of the systems studied. On mixing the solutions, TB exists mainly in the anionic (yellow) form for both mild acid and base conditions. The system is assumed to have an effective pH of ca. 7 at this stage. No change in colour was observed just before and after gelation. At the next stage of the sol–gel reaction, the zwitterion (red) of TB became the major species in the doped systems under mild conditions. These colour changes indicate that the doped systems increase in acidity throughout the transitions, regardless of the catalysts used. In the xerogel, the weak proton donating tendency of the surface functional group is insufficient to convert TB to the cation but is strong enough to convert it to the zwitterion.

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