Glass Transition Temperature of Saccharide Aqueous Solutions Estimated with the Free Volume/Percolation Model

Abstract

The glass transition temperature of trehalose, sucrose, glucose, and fructose aqueous solutions has been predicted as a function of the water content by using the free volume/percolation model (FVPM). This model only requires the molar volume of water in the liquid and supercooled regimes, the molar volumes of the hypothetical pure liquid sugars at temperatures below their pure glass transition temperatures, and the molar volumes of the mixtures at the glass transition temperature. The model is simplified by assuming that the excess thermal expansion coefficient is negligible for saccharide-water mixtures, and this ideal FVPM becomes identical to the Gordon-Taylor model. It was found that the behavior of the water molar volume in trehalose-water mixtures at low temperatures can be obtained by assuming that the FVPM holds for this mixture. The temperature dependence of the water molar volume in the supercooled region of interest seems to be compatible with the recent hypothesis on the existence of two structure of liquid water, being the high density liquid water the state of water in the sugar solutions. The idealized FVPM describes the measured glass transition temperature of sucrose, glucose, and fructose aqueous solutions, with much better accuracy than both the Gordon-Taylor model based on an empirical kGT constant dependent on the saccharide glass transition temperature and the Couchman-Karasz model using experimental heat capacity changes of the components at the glass transition temperature. Thus, FVPM seems to be an excellent tool to predict the glass transition temperature of other aqueous saccharides and polyols solutions by resorting to volumetric information easily available.

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“…An issue to consider is the possibility that vitrified samples could crystallize during the process of heating to -90ºC, which is made to sublimate free water in the solid state lakes during cryo-SEM procedure. Although estimative methods of glass transition on simple aqueous solution containing carbohydrates could support this circumstance [39], the extender used in the present study also contained Tris, citric acid, and even proteins from the egg yolk. In addition glassy state has a near null sublimation rate, so vitrified solutions do not present enhanced contrast after the etching process [40,41].…”

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“…(C) Gordon–Taylor curves for binary aqueous sugar mixtures showing T g versus sugar weight fraction. For trehalose, sucrose, and glucose, the values of k are 4.76, 4.94, and, 4.52, respectively . (D) Natural logarithm of trehalose diffusion coefficient versus inverse temperature for different trehalose weight fractions as seen in molecular dynamics simulations .…”