Structure of Concentrated Aqueous Sodium Formate Solutions

Abstract

Abstract Time-of-flight neutron and X-ray diffraction as well as Raman spectroscopic measurements were carried out on an aqueous 15 mol% HCOONa solution in order to investigate the hydration structure of both the formate and sodium ions in the concentrated aqueous solution. The intramolecular bond distances of DCOO− (rCD = 1.07 ± 0.01 Å, rCO = 1.272 ± 0.007 Å and rOO = 2.18 ± 0.02 Å) were determined through a least-squares fit of the observed neutron interference term in the high-Q region. The average hydrogen-bond distances between the D2O molecules in the solution (r(O···D) = 1.92 ± 0.02 Å and r(D···D) = 2.42 ± 0.02 Å) were also obtained. The hydration parameters on the formate ion were estimated to be rOformate···H2O = 2.82 ± 0.01 Å and nOformate···H2O = 4.4 ± 0.2 from a least-squares fit of the X-ray intermolecular interference term. A preferred orientation between the formate ion and hydrated water molecules was suggested from the present X-ray data. The hydration number of Na+ in the solution was given to be 4.6 ± 0.2, besides the intermolecular distance (rNa+···H2O = 2.37 ± 0.01 Å). The symmetrical stretching vibrational band of the hydrated sodium ion, Na+(H2O)n, in a 15 mol% HCOONa solution was observed at 225 cm−1 in the present isotropic Raman spectrum.