On the origin of the electrostatic potential difference at a liquid-vacuum interface

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Surface Science

Background:

• Interface potential is crucial for understanding molecular behavior at surfaces.
• Computer simulations provide insights into microscopic origins of interfacial phenomena.
• Previous models lacked a clear link between molecular charge distribution and interface potential.

Purpose of the Study:

• To elucidate the microscopic origin of interface potential using a simplified molecular model.
• To correlate molecular charge distribution with the sign and magnitude of interface potential.
• To validate the model against molecular dynamics simulations of different interface systems.

Main Methods:

• Development of a simple model for molecules near an interface with isotropic orientation and Gaussian charge distribution.
• Calculation of interface potential using the developed model.
• Comparison of model predictions with interface potentials computed via molecular dynamics simulations for vacuum-methane and vacuum-water systems.

Main Results:

• The model demonstrates that interior-negative charge distributions yield negative interface potentials, while interior-positive distributions yield positive potentials.
• The model accurately predicted the vacuum-methane interface potential (-220 mV).
• The model showed good agreement for the vacuum-water interface potential (-400 mV vs. -510 mV from simulation).

Conclusions:

• Molecular charge distribution is a key determinant of interface potential.
• The simplified model provides a quantitative understanding of interface potential origins.
• The findings have implications for understanding phenomena like ion solvation at interfaces.