Separating Hofmeister Trends in the Stern and Diffuse Layer Structures at a Charged Aqueous Interface

Understanding the role of pH and ions on the electrical double layer (EDL) structure of charged mineral oxide/aqueous interfaces remains crucial in modeling various environmental and industrial processes. Yet the simultaneous contribution of pH and specific ion effects (SIEs) on the different layers of the EDL remains unknown. Here, we utilize zeta potential (ζ) measurements, vibrational sum frequency generation (vSFG), and the maximum entropy method (MEM) to ascertain the detailed structure of water in the Stern and diffuse regions of the EDL at the silica/water interface under varying pH conditions for different alkali chlorides. Both at pH 2, when the surface is nearly neutral, and at pH 12, when the surface is highly charged, we observe that the Li+ and Na+ disrupt existing water structures within the Stern layer while Cs+ enhances them. Moreover, the SIE trends for the diffuse and Stern layers are opposite with respect to one another at pH 2 (in the amount of ordered water) and at pH 12 (in amount of net oriented water). Finally, we observe pH-dependent SIE in ζ, which results in Cs+ exhibiting the least ordered diffuse layer at low pH and the most at high pH. These results indicate that SIEs play critical yet separable roles in governing both electrostatic and water structuring capabilities in the EDL at charged surfaces.