Gaussian and Non-Gaussian Solvent Density Fluctuations within Solute Cavities in a Water-like Solvent

Area of Science:

• Computational chemistry and physics
• Statistical thermodynamics
• Soft matter physics

Background:

• Water density fluctuations in nanoscale cavities are crucial for understanding various physical and chemical processes.
• Previous studies indicated Gaussian fluctuations for small cavities, but deviations in larger ones remained unexplained.

Purpose of the Study:

• To investigate length-scale-dependent density fluctuations in water cavities using simulations.
• To develop a theoretical framework explaining the transition from Gaussian to non-Gaussian fluctuations.

Main Methods:

• Monte Carlo simulations employing test particle insertion and umbrella sampling techniques.
• Analysis of water occupation states in spherical cavities (up to 6.3 Å radius).
• Development of a statistical thermodynamic approach based on radial distribution functions.

Main Results:

• Water density fluctuations exhibit Gaussian behavior in atomic-scale cavities but show non-Gaussian 'fat-tail' distributions in larger cavities at low occupancy.
• Non-Gaussian fluctuations are linked to bubble formation within the cavity and water adsorption on its surface.
• A modified theory incorporating surface tension accurately describes fluctuations across different cavity sizes.

Conclusions:

• The study successfully explains the emergence of non-Gaussian density fluctuations in water cavities.
• The developed theoretical framework accurately predicts the transition point from Gaussian to non-Gaussian behavior.
• Findings provide a deeper understanding of water behavior at interfaces and in confined environments.