Variable-Angle Surface Spectroscopy Reveals the Water Structure in the Stern Layer at Charged Aqueous Interfaces

Area of Science:

• Physical Chemistry
• Surface Science
• Nonlinear Optics

Background:

• Charged aqueous interfaces feature an electrical double layer comprising Stern and diffuse layers.
• Water molecules in these layers exhibit ordering influenced by surface fields and specific interactions.
• Second-order nonlinear optical (NLO) response probes interfacial water structure.

Purpose of the Study:

• To disentangle the NLO contributions from water in the Stern and diffuse layers.
• To investigate the orientation of water molecules at the silica-water interface.
• To understand the effect of ionic strength on interfacial water structure.

Main Methods:

• Utilized a second-order nonlinear optical technique.
• Varied the angle of incidence to alter the coherence length of laser-interfacial water interaction.
• Applied the method to the silica-water interface under varying ionic strengths and neutral pH.

Main Results:

• Demonstrated a method to vary coherence length by changing the angle of incidence.
• Observed that water molecules in both Stern and diffuse layers orient hydrogen atoms towards the silica surface at low ionic strength and neutral pH.
• Found that increasing ionic strength decreases the NLO signal due to hydrated cation adsorption competing for surface sites.

Conclusions:

• The orientation of water molecules at the silica-water interface is dependent on ionic strength.
• Hydrated cations play a significant role in altering interfacial water structure by competing for surface binding sites.
• The developed method allows for the differentiation of NLO signals from different interfacial water environments.