Interactions between cations and sugars. Part 8.—Gibbs energies, enthalpies and entropies of association of divalent and trivalent metal cations with xylitol and glucitol in water at 298.15 K

Abstract

Microcalorimetry has been used to determine the standard Gibbs energies, enthalpies and entropies of association of several divalent and trivalent metal cations, including alkaline-earth-metal and lanthanide cations, with xylitol and D-glucitol (sorbitol) in water at 298.15 K. Following a procedure previously used for small sugars, the contributions solely characterizing the specific interaction between the cation and the complexing triol of the ligand were isolated. It appears that xylitol and D-glucitol form weak complexes of comparable strength, and that all the stability constants are smaller than 10. As observed previously with D-ribose, the strongest complexes are formed with the trivalent lanthanide cations. However, there is no clear relationship between the thermodynamic properties of complexation and the size or charge of the cation. Xylitol and D-glucitol appear to be extremely sensitive to the inner-sphere hydration number change occurring within the lanthanide series. Furthermore, both ligands show a selectivity towards the lanthanide cations that is comparable to that observed with much stronger ligands such as acetate, glycolate or murexide.