Structural analysis of potassium borate solutions.

In this work, H/D isotopic substitution neutron diffraction was combined with empirical potential structure refinement (EPSR) and DFT-based quantum calculations to study the interactions between B(OH) boric acid molecules, B(OH) metaborate ions, water molecules, and potassium cations in borate solutions. The results show that the solute ions and molecules have a marked effect on the second coordination shell of the water molecules, causing a greater deviation from a tetrahedral structure than is observed for pure water. Potassium ions and -B(OH) tend to form a monodentate contact ion pair (MCIP) with a K-B distance ∼3.8 Å, which remains constant upon changing the solution concentration. Potassium ions and -B(OH) form both a MCIP at K-B ∼3.8 Å and a bidentate contact ion pair (BCIP) at K-B ∼3.4 Å. As the solution concentration increases, there is a BCIP to MCIP transformation. Boric acid molecules can undergo hydration in one of three ways: direct hydration, interstitial hydration, and axial hydration. The energetic hydration preference is direct hydration → interstitial hydration → axial hydration. Nine water molecules are required when all water molecules directly interact with the -OH groups of B(OH), and a tenth water molecule is located at an interstitial position. The hydrogen bonding between boric acid molecule/metaborate ion and water molecules is stronger than that between water molecules in the hydration layer.