Interactions between cations and sugars. III. Free energies, enthalpies, and entropies of association of Ca²⁺, Sr²⁺, Ba²⁺, La³⁺, Gd³⁺ with D-ribose in water at 25 °C

The standard enthalpies of transfer of ribose and arabinose from water to aqueous solutions of electrolytes (CaCl₂, SrCl₂, BaCl₂, LaCl₃, and GdCl₃) have been measured at 25 °C. A method is described to calculate from these data the equilibrium constant and the enthalpy for the association between the cations and the complexing isomers of ribose. Mean values relative to these isomers are given: the constants vary from 2.0 to 4.3 and the enthalpies from −5.9 to −17.9 kJ mol⁻¹ for the different cations studied. The thermodynamic properties of association are not related to the size nor to the charge of the complexed cation in a simple way. On the other hand, the enthalpies of reaction are linearly correlated to the entropies of reaction.