Neutron diffraction study of concentrated aqueous lithium benzoate solutions

Time-of-flight neutron diffraction and infrared (IR) double difference spectral measurements have been carried out on aqueous 4 mol% lithium benzoate solutions. The neutron diffraction with isotopic substitution technique has been applied to both lithium and benzyl-hydrogen atoms within the benzoate ion in order to obtain information on the hydration structure around both the lithium ion and the benzyl-hydrogen atom within the benzoate ion. The observed first-order difference functions, ΔLi(Q) and ΔHinter(Q), were analyzed by the least squares fitting procedure. The number of water molecules within the first hydration shell of the Li+ was determined to be 5.3(1) with the Li+···O and Li+···D distances of 1.95(1) and 2.58(1) Å, respectively. A well-defined second hydration shell involving 6.0(1) water molecules, has been found and characterized by the Li+···O and Li+···D distances of 4.12(1) and 4.73(1) Å, respectively. The first hydration shell of the benzyl-hydrogen atoms (Hb) within the benzoate ion was found to consist of 1.0(1) water molecule with the average Hb···D2O intermolecular distance of 2.78(1) Å. Results of the IR double difference spectra exhibited that the number of water molecules within the first hydration shell of Li+ is 5.7(1), in reasonable agreement with that obtained from the neutron diffraction data. The hydration number of the carboxyl group within the benzoate ion was determined to be 4.9(1) from the IR difference spectra.