Relative permittivity in the electrical double layer from nonlinear optics

Area of Science:

• Surface science
• Nonlinear optics
• Physical chemistry

Background:

• Second harmonic generation (SHG) spectroscopy is a powerful nonlinear optical technique for studying interfacial phenomena.
• Understanding the electrical double layer at solid-liquid interfaces is crucial in various scientific and industrial applications.
• Previous studies have explored interfacial properties, but a comprehensive understanding of salt concentration effects on SHG response at different interfaces remains incomplete.

Purpose of the Study:

• To investigate the effect of varying salt concentrations on the fused silica/water and sapphire/water interfaces using SHG spectroscopy.
• To probe the behavior of the electrical double layer under different ionic strengths and salt compositions.
• To evaluate the validity of existing theoretical models in explaining the observed experimental phenomena.

Main Methods:

• Utilized second harmonic generation (SHG) spectroscopy to analyze the fused silica/water and sapphire/water interfaces.
• Exposed interfaces to aqueous solutions of various salts (NaCl, NaBr, NaI, KCl, RbCl, CsCl) at controlled pH levels.
• Systematically varied salt concentrations from low micromolar to higher millimolar ranges.

Main Results:

• Observed an initial reversible increase in SHG response with increasing salt concentration up to approximately 0.1 mM for all salts studied.
• Further increases in salt concentration led to a monotonic decrease in the SHG signal.
• The observed SHG signal behavior (increase followed by decrease) aligns with predictions related to phase interference and phase matching in nonlinear optics.

Conclusions:

• Standard mean-field theories for electrical double layers require modifications to accurately recapitulate the experimental observations.
• Effective models necessitate considering a relative permittivity of the diffuse layer equal to bulk water, variable surface charge density with salt concentration, and/or variations in Stern layer charge or thickness.
• Experimental data suggest a sensitivity to the direction of salt concentration change (increasing vs. decreasing) not fully captured by current models, indicating areas for future theoretical and experimental refinement.