pH-Dependent Inversion of Hofmeister Trends in the Water Structure of the Electrical Double Layer

Specific ion effects (SIEs) are known to influence the acid/base behavior of silica and the interfacial structure of water, yet evidence of the effect of pH on SIEs is lacking. Here broadband vibrational sum frequency generation (SFG) spectroscopy was used to study SIEs on the water structure at the electrical double layer (EDL) of silica as a function of pH and monovalent cation identity from pH 2-12 at 0.5 M salt concentration. SFG results indicate a direct Hofmeister series of cation adsorption at pH 8 (Li⁺ < Na⁺ < K⁺ < Cs⁺), with an inversion in this series occurring at pH > 10. In addition, an inversion in SFG intensity trends also occurred at pH < 6, which was attributed to contributions from asymmetric cation hydration and EDL overcharging. The highly pH-dependent SIEs for silica/water have implications for EDL models that often assume pH-independent parameters.