Water orientation and hydrogen-bond structure at the fluorite/water interface

Area of Science:

• Geochemistry
• Surface Science
• Physical Chemistry

Background:

• Water-mineral interactions are crucial for many natural and industrial processes.
• A molecular-level understanding of the calcium fluoride (CaF2)/water interface is lacking.
• This interface is ubiquitous and plays a significant role in various applications.

Purpose of the Study:

• To provide a quantitative, molecular-level understanding of the CaF2/water interface.
• To investigate the surface charge, ion distribution, and water ordering at the interface.
• To elucidate the origin of surface charge and the behavior of hydroxyl species at different pH levels.

Main Methods:

• Combined theoretical and experimental approach.
• Advanced phase-resolved Vibrational Sum Frequency Generation (VSFG) spectroscopy.
• State-of-the-art ab initio molecular dynamics (AIMD) simulations including electronic structure effects.

Main Results:

• At low pH, the CaF2 surface is positively charged due to fluoride ion dissolution, leading to ordered water.
• At high pH, Ca-OH species form and interact weakly with water, creating a distinct VSFG signature.
• Detailed quantification of surface termination, ion distribution, and water arrangement was achieved.

Conclusions:

• The study provides unprecedented molecular-level insights into the CaF2/water interface.
• Understanding surface charge mechanisms and water ordering is critical for predicting interfacial behavior.
• The findings advance the fundamental knowledge of mineral-water interactions.