Classical density functional theory of orientational order at interfaces: application to water

Area of Science:

• Physical Chemistry
• Computational Fluid Dynamics
• Statistical Mechanics

Background:

• Understanding the behavior of fluids at interfaces is crucial in various scientific and engineering applications.
• Classical density functional theory provides a framework for studying inhomogeneous fluids.

Purpose of the Study:

• To develop and apply a classical density functional formalism for predicting the position-orientation number density of structured fluids.
• To investigate the liquid-vapor interface of pure water, including molecular orientation and surface properties.
• To explore the behavior of water molecules near droplet surfaces and their effect on vapor pressure.

Main Methods:

• Developed a classical density functional formalism incorporating classical, gradient correction, and anisotropic terms.
• Applied the formalism to the liquid-vapor interface of pure water.
• Calibrated the model to predict molecular orientation and surface properties.

Main Results:

• The model predicts water molecules orient with dipole moments parallel to a planar interface.
• Molecular planes are parallel on the liquid side and perpendicular on the vapor side.
• Calculated surface tension is twice the experimental value, with qualitative agreement for surface potential.

Conclusions:

• The developed density functional formalism effectively predicts the position-orientation number density of structured fluids at interfaces.
• The model provides insights into water molecule orientation at planar and curved interfaces.
• Further refinement is needed to match experimental surface tension values.