Solvation of alkali metal ions in liquid hydrogen fluoride and water: A combined ab initio and molecular dynamics study

Abstract

Despite its high toxicity and corrosivity, hydrogen fluoride (HF) is widely used in industrial processes such as fluorine compound synthesis, aluminum production, and gasoline refining. As the only weak hydrohalic acid, HF may exist in its molecular, undissociated form in some aqueous and biological environments. HF has similar polarity to H₂O, and both liquids exhibit hydrogen bond-driven molecular associations. However, their electrostatic potential surfaces differ: the oxygen atom in H₂O carries a more negative potential than the fluorine atom in HF, while the hydrogen atom in HF carries a more positive potential than in H₂O. Although many salts dissolve in HF, their solvation properties remain poorly understood. Here, we present the first computational investigation of alkali metal ion (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) solvation in liquid HF. High-level ab initio calculations on (HF)ₙM⁺ clusters (n = 1–6) show gas-phase binding affinities 30–35 % lower than for water. Our results reveal that ion–dipole and hydrogen bonding interactions can act cooperatively or anti-cooperatively depending on cluster geometry, with cooperative effects generally dominant. Molecular dynamics simulations using a non-polarizable model yield average HF coordination numbers of 5.5, 8.1, 8.9, and 9.7 for Li⁺ to Cs⁺, compared to 4.2, 5.9, 7.1, 8.1, and 9.6 in water. The computed liquid–vacuum interfacial potential is +250 mV for HF and −500 mV for H₂O. The calculated solvation free energies are −91.5 to −52.6 kcal/mol in HF versus −122.4 to −67.6 kcal/mol in water. These results show that solvation is more favorable in water, consistent with higher ion–H₂O affinities and more negative interfacial potential of water.