Increased fraction of low-density structures in aqueous solutions of fluoride.

X-ray absorption spectroscopy (XAS) and small angle x-ray scattering (SAXS) were utilized to study the effect of fluoride (F(-)) anion in aqueous solutions. XAS spectra show that F(-) increases the number of strong H-bonds, likely between F(-) and water in the first hydration shell. SAXS data show a low-Q scattering intensity increase similar to the effect of a temperature decrease, suggesting an enhanced anomalous scattering behavior in F(-) solutions. Quantitative analysis revealed that fluoride solutions have larger correlation lengths than chloride solutions with the same cations but shorter compared to pure water. This is interpreted as an increased fraction of tetrahedral low-density structures in the solutions due to the presence of the F(-) ions, which act as nucleation centers replacing water in the H-bonding network and forming stronger H-bonds, but the presence of the cations restricts the extension of strong H-bonds.