Neural Network-Assisted Analysis of Free O-H Orientational Distribution at the Air-Water Interface: Gaussian or...

Area of Science:

• Physical Chemistry
• Surface Science
• Computational Chemistry

Background:

• The orientational distribution of free O-H groups at the air-water interface is a subject of ongoing scientific debate.
• Existing theoretical frameworks include stepwise angular distribution, Gaussian patterns, and exponential decay.
• Understanding O-H group orientation is challenged by insufficient consideration of conformational states.

Purpose of the Study:

• To investigate the influence of conformational states on the orientational distribution of free O-H groups.
• To propose a new model that accurately describes free O-H group orientation at the air-water interface.
• To reconcile conflicting theoretical frameworks regarding O-H group orientation.

Main Methods:

• Neural network (NN)-based molecular dynamics (MD) simulations using DeePMD.
• Categorization of free O-H groups into "doubly free" (f-HOH-f) and "mixed" (f-HOH-h) states.
• Development and application of a dual-Gaussian superposition model.

Main Results:

• The "mixed" (f-HOH-h) state is significantly more prevalent than the "doubly free" (f-HOH-f) state.
• The f-HOH-h state exhibits a narrow, Gaussian-like distribution (ordered water) with an average tilt angle of ~40°.
• The f-HOH-f state shows a broad Gaussian distribution (disordered water) around 90°.
• The dual-Gaussian model accurately reproduces experimental sum frequency generation (SFG) intensity ratios and orientational parameter D.
• The ordered f-HOH-h state critically influences SFG signals at 3700 cm⁻¹, while the disordered f-HOH-f state has minimal impact.

Conclusions:

• The conformational state of free O-H groups (ordered vs. disordered) is crucial for understanding their orientation at the air-water interface.
• The proposed dual-Gaussian superposition model provides a comprehensive explanation for free O-H orientation, resolving previous debates.
• The ordered f-HOH-h state plays a dominant role in determining SFG signals, highlighting its importance in interfacial water studies.