Surface Charges at the CaF₂ /Water Interface Allow Very Fast Intermolecular Vibrational-Energy Transfer

Area of Science:

• Physical Chemistry
• Surface Science
• Spectroscopy

Background:

• Fluorite (calcium fluoride) dissolution can create localized charges at the solid/liquid interface, particularly at low pH.
• Understanding interfacial water dynamics is crucial for various chemical and geological processes.

Purpose of the Study:

• To quantify the heterogeneity of interfacial water (D2O) molecules at the fluorite/water interface.
• To investigate the sub-picosecond vibrational energy relaxation dynamics at this buried interface.

Main Methods:

• Utilized 2D surface-specific vibrational spectroscopy to probe interfacial water.
• Employed atomistic simulations based on electronic-structure theory to understand molecular dynamics.

Main Results:

• Identified distinct dynamics for strongly and weakly hydrogen-bonded OD groups in interfacial water.
• Strongly H-bonded OD groups (<2500 cm⁻¹) showed rapid spectral diffusion and vibrational relaxation.
• Weakly H-bonded OD groups (>2500 cm⁻¹) exhibited significantly slower dynamics.

Conclusions:

• Localized charge defects at the fluorite interface influence water molecule orientation and dynamics.
• Strongly oriented water molecules, pinned by charge defects, facilitate rapid vibrational energy exchange via collective dipoles.
• This rapid energy transfer compensates for incomplete solvation shells at the interface.