B(OH)4 hydration and association in sodium metaborate solutions

In relation to the excellent performance of borohydride in storing hydrogen gas, aqueous sodium metaborate solutions have attracted attention in matters regarding their application in direct borohydride fuel cell technologies. Previous studies have already unveiled the wood preservation abilities, fire retardant attributes and the impact on the fuel performance of pressurized water nuclear reactors that borate ion hydration promulgates due to the borate mineral sedimentation and crystallization process. Despite having several techniques applied, the polyborate distribution and solid–liquid equilibrium prediction in aqueous borate solutions are still unsatisfactory. Several studies reported borate ion association in aqueous sodium metaborate solutions over a wide range of salt concentrations. However, no clear atomic/molecular level information about the borate ion association is available.

Dr. Yongquan Zhou from Qinghai Institute of Salt Lakes, Chinese Academy of Science and his colleagues at Fukuoka University in Japan studied borate ion hydration and association in sodium metaborate solutions over a wide salt concentration by X-ray diffraction and empirical potential structure refinement. Their work is currently published in the research journal, Phys. Chem. Chem. Phys.

The research team recrystallized sodium metaborate from distilled water. The x-ray scattering intensities of the empty capillary and the sample solutions were then measured under ambient conditions with an X-ray diffractometer and a curved imaging plate detector. The researchers then integrated the two-dimensional diffraction pattern of the samples recorded on an imaging plate, to one-dimensional intensities with a 2DP program. Empirical potential structure refinement modelling that utilized the Monte Carlo style approach to diminish the difference between experimental diffraction data and of those generated from the simulation of the structure was conducted. Eventually, density functional theory calculations were undertaken as a circumstantial evidence.

The authors observed that for the case of the hydrated sodium cations, the cations were seen to be surrounded by around six water molecules in an octahedral geometry in the diluted solution. Besides, octahedral hydrated geometry was observed to remain stable, while some of the water molecules were being replaced by the oxygen from borate ion to form contact ion pairs with increasing concentration.

The study successfully presented an in-depth analysis of solution structures involving borate ion hydration and sodium borate association. In addition, the effects of the bulk water in aqueous sodium metaborate solutions have been studied by X-ray diffraction and empirical potential structure refinement modelling. It has been seen that, from both X-ray diffraction and Density Functional Theory calculations of sodium metaborate clusters, there are two forms of contact ion pairs between sodium cation and borate ion in aqueous sodium metaborate solutions.