The effects of hofmeister cations at negatively charged hydrophilic surfaces

This work explores the interactions of cations with highly negatively charged solid/aqueous interfaces. Vibrational sum frequency spectroscopy (VSFS) was exploited to observe interfacial water structure on the surfaces of fused quartz and titanium dioxide (TiO ₂) in the presence of nine chloride salts with different alkali, alkaline earth, and transition metal cations. The results showed prominent specific cation effects at low concentration. The cations followed a direct Hofmeister series. On quartz surfaces the series was: Li ⁺ > Cs ⁺ > Rb ⁺ > NH ₄ ⁺ > K ⁺ > Na ⁺ > Ca ²⁺ > Mg ²⁺ > Zn ²⁺. As such, Zn ²⁺ attenuated water structure to the greatest degree and therefore gave rise to the smallest peaks in the OH stretch region of the VSFS spectrum. The opposite was the case for Li ⁺. Such results indicate that Li ⁺ partitioned least to the surface, while Zn ²⁺ partioned there to the greatest extent. We also observed prominent specific cation effects on TiO ₂ surfaces. There were, however, some key differences between these surfaces. On TiO ₂, Li ⁺, which is the best hydrated monovalent cation, behaved more similarly to K ⁺ and Na ⁺. In addition, the ordering of Mg ²⁺ and Ca ²⁺ was reversed on TiO ₂ compared with quartz. Such reordering of the Hofmeister series should result from differences in the charge density, polarizability, and basicity of the two oxide surfaces. Finally, NH ₄ ⁺ was found to interact more strongly with TiO ₂ than any other monovalent cation. This result is in line with the greater role that hydrogen bonding should play on TiO ₂ compared with quartz under the conditions of the experiment.