Na⁺, F^-, Br^-, and Cl^- Adsorptions and Penetrations on an Ice Surface

With the help of our quantum mechanical/effective fragment potential (QM/EFP) scheme, the adsorptions of Na⁺, F^-, Br^-, and Cl^- ions on a hexagonal ice (0001) surface were theoretically studied. Drastically different adsorption behaviors depending upon ion signs and surface heterogeneity were observed. The positive Na⁺ ion forms 4-5 Na⁺-O interfacial bondings, regardless of the number of hydrogen dangling bonds (HDBs), yielding consistent adsorptions with large stabilization energies from -49.2 to -65.6 kcal/mol. On the other hand, the binding strengths of negative ions are sensitive to the number of HDBs. In the particular binding sites where there is no HDB, both Cl^- and Br^- cannot form a stable surface adsorption product. At the same binding site, more reactive F^- can undergo insertion reaction into surface hydrogen bonding. A molecular HF and a hydroxide are formed on a site with one HDB, showing that the surface acid-base chemistry may depend upon the surface heterogeneity. In general, the versatile bonding ability of Na⁺ through s and empty p orbitals provides strong interactions with the ice surface by disrupting the surface hydrogen-bonding network, which, in turn, reduces its initial penetration barriers into the bulk. However, the ice surface structures are intact in the case of negative ion adsorptions, making their penetrations into the bulk difficult.