Divalent Ion Specific Outcomes on Stern Layer Structure and Total Surface Potential at the Silica:Water Interface

Area of Science:

• Physical Chemistry
• Surface Science
• Electrochemistry

Background:

• The Stern layer and interfacial potential are crucial for understanding electrochemical interfaces.
• Nonlinear optical techniques like Second Harmonic Generation (SHG) probe interfacial properties.

Purpose of the Study:

• To investigate the impact of divalent cations on the silica:water interface.
• To quantify the second-order nonlinear susceptibility (χ(2)) and total interfacial potential drop (Φ(0)tot).

Main Methods:

• Utilized SHG amplitude and phase measurements.
• Studied divalent cations (Mg2+, Ca2+, Sr2+, Ba2+) at the silica:water interface.
• Varied pH, ionic strength, and employed anion substitution (SO42- for Cl-).

Main Results:

• Interfacial structure and potential depend strongly on ion valency.
• Significant differences in χ(2) were found between alkali earth cations and NaCl.
• Hydration structure in the Stern layer influences results, especially at intermediate concentrations.
• Hysteresis in ion adsorption/desorption was observed, notably for CaCl2 and MgSO4.

Conclusions:

• Ion valency and hydration significantly dictate interfacial properties at the oxide:water interface.
• SHG is a sensitive probe for detecting subtle changes in interfacial structure and potential.
• Understanding hysteresis is key for accurate interfacial characterization.