Intensity-Calibrated Variable-Angle Surface Spectroscopy Provides the Electrostatic Potential at the Interface...

Area of Science:

• Physical Chemistry
• Surface Science
• Spectroscopy

Background:

• Nonlinear optical spectroscopy can differentiate water molecules at charged surfaces from those in the diffuse electrical double layer.
• The electrical potential at the interface between these water regions is a key characteristic.
• Understanding interfacial water structure is crucial for various chemical and biological processes.

Purpose of the Study:

• To develop a method for calibrating the absolute second-order nonlinear optical susceptibility of the silica-aqueous interface.
• To measure the electrical potential at the interface between surface-bonded and diffuse water structures using intensity-based on-resonance sum-frequency generation.
• To investigate the relationship between interfacial water potential and surface properties at low ionic strength.

Main Methods:

• Utilizing second-order nonlinear optical spectroscopy, specifically intensity-based on-resonance sum-frequency generation.
• Implementing a novel scheme for calibrating the absolute second-order susceptibility of the silica-aqueous interface.
• Analyzing phase-matching conditions to separate contributions from different water molecule populations.

Main Results:

• The study provides the first quantitative measurement of the electrical potential at the bonded-diffuse water interface.
• The measured potential was found to be slightly greater than the zeta-potential.
• This suggests that the influence of surface silanol groups on water structure does not extend to the slipping plane.

Conclusions:

• The developed spectroscopic method allows for precise measurement of interfacial electrical potential.
• The findings offer new insights into the structure and electrical properties of water at charged interfaces.
• The results contribute to a better understanding of surface-induced water ordering and its implications.