Glucose and Fructose Hydrates in Aqueous Solution by IR Spectroscopy

In the mid-infrared attenuated total reflectance (MIR-ATR) spectra of aqueous d-glucose and d-fructose solutions, two hydrates were found by factor analysis (FA) for each sugar, d-glucose penta- and dihydrates and d-fructose penta- and monohydrates. We obtained the spectra and abundances for these hydrates as a function of carbohydrate concentrations. The biggest difference in these spectra lies in the CO stretch region. From the distribution of the species, the equilibrium between d-glucose pentahydrate and dihydrate is 3(H2O)2+2(C6H12O(6).2H2O) right arrow over left arrow 2(C6H12O(6).5H2O), with the equilibrium constant KG=(3.2+/-0.6)x10(-5) L3 mol-3. For d-fructose, the equilibrium is between pentahydrate and monohydrate, 2(H2O)2+C6H12O6.H2O right arrow over left arrow C6H12O(6).5H2O, with the equilibrium constant KF=(7.1+/-1.2)x10(-3) L2 mol-2. The four hydrates are present only in aqueous solutions and cannot be obtained in the solid state.