INTERACTIONS BETWEEN CATIONS AND SUGARS .8. GIBBS ENERGIES, ENTHALPIES AND ENTROPIES OF ASSOCIATION OF DIVALENT AND TRIVALENT METAL-CATIONSWITH XYLITOL AND GLUCITOL IN WATER AT 298.15 K

Microcalorimetry has been used to determine the standard Gibbs energie s, enthalpies and entropies of association of several divalent and tri valent metal cations, including alkaline-earth-metal and lanthanide ca tions, with xylitol and D-glucitol (sorbitol) in water at 298.15 K. Fo llowing a procedure previously used for small sugars, the contribution s solely characterizing the specific interaction between the cation an d the complexing triol of the ligand were isolated. It appears that xy litol and D-glucitol form weak complexes of comparable strength, and t hat all the stability constants are smaller than 10. As observed previ ously with D-ribose, the strongest complexes are formed with the triva lent lanthanide cations. However, there is no clear relationship betwe en the thermodynamic properties of complexation and the size or charge of the cation. Xylitol and D-glucitol appear to be extremely sensitiv e to the inner-sphere hydration number change occurring within the lan thanide series. Furthermore, both ligands show a selectivity towards t he lanthanide cations that is comparable to that observed with much st ronger ligands such as acetate, glycolate or murexide.