Decoding the molecular water structure at complex interfaces through surface-specific spectroscopy of the water...

Area of Science:

• Physical Chemistry
• Surface Science
• Spectroscopy

Background:

• Interfacial water structure impacts atmospheric chemistry, material wetting, and protein folding.
• Surface-specific sum-frequency generation (SFG) vibrational spectroscopy, using the O-H stretch mode, is common but struggles with complex interfaces due to overlapping signals from non-water species.
• Distinguishing water signals from surface hydroxyl or amine groups is a significant challenge.

Purpose of the Study:

• To develop and demonstrate a protocol for selectively measuring the hydrogen bond strength of interfacial water.
• To overcome limitations of existing methods for analyzing complex aqueous interfaces.
• To provide a new spectroscopic approach for understanding interfacial water structure.

Main Methods:

• Utilizing the water bending mode in SFG vibrational spectroscopy to isolate water signals.
• Analyzing the vibrational frequency distribution of the bending mode to probe hydrogen bonding.
• Applying the method to water-CaF2 and water-protein interfaces.

Main Results:

• The water bending mode effectively distinguishes water O-H groups from surface hydroxyls.
• Increasing pH strengthens the hydrogen-bonded network of interfacial water at the water-CaF2 interface.
• Water at the human serum albumin protein interface exhibits enhanced hydrogen bonding compared to bulk water.

Conclusions:

• The water bending mode offers a novel and selective pathway to investigate interfacial water hydrogen bonding at complex interfaces.
• This method provides deeper insights into the structure and dynamics of water at mineral and biological interfaces.
• The findings highlight the importance of interfacial water structure in biological and material systems.