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On the progress of mutarotation above and below the Tg. Studying the relationship between constant rates and structural relaxation times

Abstract

Comprehensive FTIR studies on the progress of mutarotation in D-fructose mixed with maltitol have been carried out over a wide range of temperatures, both above and below the glass transition temperature (Tg). Except for the analysis of single bands enabling monitoring selective pathways of mutarotation (α→β), we developed completely new approach taking into account full spectral range to follow overall progress of the reaction. We found that at the calorimetric Tg, there is a clear change in temperature dependence of constant rates. The activation barrier for the mutarotation changes from 107 kJ/mol (in supercooled state) to around 250 kJ/mol (in glassy state). Such a dramatic variation in activation barrier is closely related to the change in mechanism of this specific chemical conversion, as considered theoretically by Wlodarczyk et al. [Phys. Chem. Chem. Phys. 2014, 16, 4694-4698]. Additionally, we tested a relationship between constant rates and structural relaxation times and found that above the Tg there is a strong decoupling between both quantities. This experimental observation agrees well with the data published for many glass forming systems, showing universal character of this phenomenon. However, it is the first report about interrelationship between structural relaxation time (τα) and chemical reaction rate constant (k) in the supercooled regime. Interestingly, below the glass transition temperature the degree of decoupling decreases. It is related to the fact that both constant rates as well as τα are sensitive to the glass transition temperature. Our results also indicate that a single activation barrier describing mutarotation might be not sufficient.

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Publication details

The article was received on 30 Mar 2017, accepted on 10 Jul 2017 and first published on 10 Jul 2017


Article type: Paper
DOI: 10.1039/C7CP02046E
Citation: Phys. Chem. Chem. Phys., 2017, Accepted Manuscript
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    On the progress of mutarotation above and below the Tg. Studying the relationship between constant rates and structural relaxation times

    K. Wolnica, M. Dulski, E. Kaminska, A. Cecotka, M. Tarnacka, R. Wrzalik, K. Kaminski and M. Paluch, Phys. Chem. Chem. Phys., 2017, Accepted Manuscript , DOI: 10.1039/C7CP02046E

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