A thermodynamic study of some aqueous monosaccharide solutions

Abstract

The heats of fusion, solubility, heats of solution at saturation and activity coefficients in binary aqueous solutions of fructose, xylose and arabinose were measured up to their saturation points at 25 °C. The activity coefficients were likewise measured for ribose up to 20 molal at 25 °C. The partial molal heat contents of the sugars and water were calculated from the temperature variation of the water activity over a 20-40 °C range at 5 °C intervals. The activity data for fructose, xylose, and arabinose quantitatively fit a hydration model using a one parameter equation (viz. the stepwise equilibrium constant for hydration of the sugar). The model fit the ribose more qualitatively. The activity fo water was also measured for aqueous ternary solutions of fructose, xylose, and ribose in NaCl and MgCl2. A generalized method was derived for calculating the activity coefficients of two components of a ternary mixture when the activity of the third component is known over the composition range of the other two. This procedure was used to calculate the activity coefficients of the above sugars and salts in ternary solutions. From this data, salting-out parameters of NaCl and MgCl2 were calculated in each sugar. These were compared to the theories of Debye-McAulay, McDevit-Long and Kirkwood. While the salting-out parameter in fructose and xylose solutions qualitatively fir the Debye-McAulay theory, all these theories qualitatively failed to account for the salting parameters in ribose solution. A strong association between ribose and NaCl and MgCl2 is suspected.