Structure of Aqueous H3BO3 Solutions by DFT and Neutron Scattering

The micro-structure of aqueous boric acid (H3BO3) solutions is of broad interest to earth sciences, geochemistry, material science, as well as chemical engineering, etc. In the present work, the structure of aqueous H3BO3 solutions is studied by neutron scattering with 2H and 11B isotope labelling combined with empirical potential structure refinement (EPSR) modelling. In aqueous H3BO3 solutions, B(OH)3 is the dominant borate species. Density function theory (DFT) calculations show the boron hydroxyl with a lower electrostatic potential (ESP), which makes B(OH)3 a relatively weak hydration, compared with the bulk water. In the 0.95 mol L-1 H3BO3 solution at 298 K (saturated), ~18 water molecules enter the hydration sphere of B(OH)3 with the hydration distance (B-O(W)) of 3.75 A, while only 4.23 of them hydrate with H3BO3 in hydrogen bond (H-bond) acceptor or H-bond donor. Both neutron scattering and DFT calculations for 2B(OH)3·6H2O clusters at ωB97XD/6-311++g(3df,3pd) basis level show that B(OH)3 forms molecular-clusters in bidentate contact molecular pair (BCMP), mono-dentate molecular pair (MCMP), solvent-shared molecular pairs (SMP) and paralleled-solvent-shared molecular pairs (PSMP) in aqueous solutions. Their relative contents are both concentration and temperature sensitive. BCMP with the B-B distance of ~4.1 A is the dominant molecular pairs in aqueous solutions. The relative less content and van der Waals interaction stabilized PSMP, with a B-B distance of ~3.6 A between the two paralleled layers, is the crucial species for the crystallization of H3BO3 from aqueous solution