Dehydration of Oxide:Water Interfaces by Brines Quantified with Nonlinear Spectroscopy

Area of Science:

• Physical Chemistry
• Surface Science
• Spectroscopy

Background:

• Determining ion and water molecule densities at aqueous:solid interfaces is challenging, particularly at high ionic strengths.
• Understanding interfacial behavior is crucial for various chemical and biological processes.

Purpose of the Study:

• To quantify cation and anion coverages at fused silica-water interfaces.
• To determine the number density and orientation of net-aligned water molecules.
• To investigate the impact of high ionic strengths (up to 8 M) on interfacial composition and water ordering.

Main Methods:

• Utilized phase- and amplitude-resolved second harmonic generation spectroscopy.
• Employed an all-optical, noncontact approach.
• Studied fused silica surfaces in contact with aqueous solutions (pH 5.8) with various salts (NaCl, NaClO4, Na2SO4, Na2HPO4) at ionic strengths up to 8 M.

Main Results:

• At 8 M NaClO4, the interface consists of approximately half a monolayer of ions and half a monolayer of water molecules.
• Water molecules at high ionic strength flip their orientation from 'protons to the surface' to 'oxygens to the surface'.
• The interfacial water to total ion ratio is significantly lower (tenfold) than in bulk solutions at high ionic strengths.
• Helmholtz free energy of water disordering increases abruptly from ~0 kJ mol-1 below 2 M to 10 kJ mol-1 at 8 M ionic strength.

Conclusions:

• The study provides quantitative insights into interfacial structure under extreme ionic conditions.
• Observed water reorientation suggests a transition towards a dehydrated interface at high ionic strengths.
• Results indicate a potential Kirkwood-like transition governing interfacial water behavior.