Anisotropic Water Structure at Charged Interfaces Studied by Depth-Resolved Vibrational SFG/DFG Spectroscopy

Area of Science:

• Physical Chemistry
• Surface Science
• Spectroscopy

Background:

• Interfacial electric fields significantly influence molecular water structure at charged aqueous interfaces.
• This electric field-induced anisotropy extends over nanometer scales but remains poorly understood due to experimental limitations.

Purpose of the Study:

• To investigate the depth-dependent anisotropic water structure at interfaces with charged surfactants.
• To develop and apply a novel technique for probing molecular orientation with depth resolution.

Main Methods:

• Utilized a newly developed depth-resolved nonlinear vibrational spectroscopy technique.
• Correlated nonlinear vibrational spectra directly with nanometer-scale depth information.
• Reconstructed nonlinear vibrational responses as a function of depth.

Main Results:

• Identified two distinct regions of interfacial anisotropy with varying degrees of molecular orientation.
• Spectral analysis confirmed that the local hydrogen-bond structure of bulk water is largely unperturbed.
• Observed that water structure remains unperturbed even in close proximity to surface charges.

Conclusions:

• The study refines understanding of anisotropic water structure at hydrophilic charged interfaces.
• The developed depth-resolved technique offers significant potential for future interfacial studies.