Surface Potential at Electrolyte/Air Interfaces: A Quantitative Analysis via Sum-Frequency Vibrational Spectroscopy.

Area of Science:

• Physical Chemistry
• Surface Science
• Spectroscopy

Background:

• The air/water interface plays a crucial role in atmospheric processes.
• Understanding ion interactions at this interface is vital for atmospheric chemistry.
• Sum-frequency vibrational spectroscopy (SFVS) is a powerful tool for probing interfacial structures.

Purpose of the Study:

• To quantitatively investigate the effect of atmospherically relevant ions on the air/water interface.
• To determine the ion specificity and interfacial structural changes induced by electrolytes.
• To measure surface potentials at the air/water interface with varying ionic compositions.

Main Methods:

• Quantitative phase-sensitive sum-frequency vibrational spectroscopy (SFVS).
• Investigation of the OH-stretching resonance at the air/water interface.
• Analysis of spectral changes induced by submolar concentrations of various electrolytes.

Main Results:

• No ion specificity was observed in spectral changes of the OH-stretching resonance below 0.1 M.
• Ion-induced spectral changes resemble the third-order nonlinear optical susceptibility of bulk water.
• Invariant free OH resonance indicates mean-field alignment of water molecules in a subsurface network.

Conclusions:

• The primary effect of ions on interfacial structure is mean-field alignment of water molecules.
• Surface potentials were quantitatively determined for six electrolyte solutions.
• Results align with Levin's continuum theory, suggesting minimal electrostatic correlations for divalent ions.