Cation-Dependent Water Dynamics in Concentrated Aqueous Tetrafluoroborate Solutions: Insights from Ultrafast IR Spectroscopy and Molecular Dynamics Simulations

Abstract The freezing behavior of aqueous tetrafluoroborate solutions differs markedly between Li+ and Zn2+, yet the microscopic origin remains unclear. By combining ultrafast infrared spectroscopy and molecular dynamics (MD) simulations, we compare the dynamics of bulk-like water and water molecules hydrogen-bonded to BF4– in LiBF4 and Zn(BF4)2 solutions. Our results reveal that the vibrational lifetime of both water species is only slightly affected by the cation identity. The rotational dynamics, however, show a clear cation-dependent slowing: in Zn(BF4)2, the rotational time constant of bulk-like water increases from ∼2.7 to ∼4.6 ps, while that of anion-bound water increases from ∼5.1 to ∼8.7 ps. These observations indicate that Zn2+ imposes orientational constraints on water molecules, particularly on anion-bound water, suggesting a cation-dependent influence on water–anion interactions. The different freezing behavior of the two electrolytes is therefore proposed to be associated with collective changes in the intermolecular interactions and water structure, rather than being determined solely by individual water rotational dynamics. These changes include hydrogen-bond network disruption and suppressed diffusion, as supported by our MD simulations. Our findings provide a molecular-level understanding of how cation identity modulates water dynamics. These insights help explain the distinct freezing behavior of aqueous electrolytes and may guide the design of low-temperature aqueous electrolytes.