Structure of Concentrated Aqueous Urea Solutions Involving Alkali Metal Salts Studied by Neutron Diffraction with 14N/15N, 6Li/7Li, and 35Cl/37Cl Isotopic Substitution Methods

Neutron diffraction with N-14/N-15, Cl-35/Cl-37, and Li-6/Li-7 isotopic substitution methods have been applied to aqueous 10 mol % urea solutions involving 5 mol % NaCl and 10 mol % LiCl, in order to obtain information on the hydration structure around the amino group of urea which is affected by coexisting ions, Cl- and Li+. Observed first-order difference functions, Delta(inter)(N)(Q), Delta(Cl)(Q), and Delta(Li)(Q), were respectively analyzed using least-squares fitting. The nearest neighbor intermolecular N center dot center dot center dot O-w (O-w: oxygen atom in D2O) distance was determined to be 3.09(1) and 3.19(1) angstrom, from the analyses for sample solutions involving NaCl and LiCl, respectively. These intermolecular N center dot center dot center dot O-w distances are slightly longer than those reported for aqueous urea solutions of similar concentrations. These results suggest that hydrogen bonds among amino groups of urea and neighboring water molecules are weakened by coexisting alkali metal salts. The structural parameters concerning the first hydration shell of Cl- and Lit, have been determined to be r(Cl-center dot center dot center dot D-w) = 2.27(1) angstrom (D-w: deuterium atom in D2O) and r(Li+center dot center dot center dot O-w) = 1.95(1)angstrom, respectively. The results indicate that the structure of the first hydration shell around Cl- and Li+ is not affected by the presence of urea molecules in the aqueous solution.