Interactions between cations and sugars. II. Enthalpies, heat capacities, and volumes of aqueous solutions of Ca2+–D-ribose and Ca2+–D-arabinose at 25 °C

Abstract

The thermodynamic parameters characterizing the interaction between Ca2+ and the suitably positioned sequences of hydroxyls of some sugar isomers have been determined. This was done by comparing the properties of D-ribose which bears such sequences of hydroxyls with the properties of D-arabinose chosen as an inactive reference. The enthalpies of solution and of dilution, the apparent molal heat capacities, and the apparent molal volumes of the two pentoses have been first measured in water at 25 °C. The measurement of these properties for the transfer of the sugars from water to CaCl2 solutions (and, conversely, for the transfer of CaCl2 from water to the sugar solutions) directly gives access to the Ca2+–hydroxyls pair interaction parameters. The thermodynamic properties of this reaction of association may then be estimated: [Formula: see text][Formula: see text] The analysis of these data shows that the weak association constant results from a large compensation between the favourable enthalpy and the unfavourable entropy of reaction.