Transport in nanoporous zeolites: relationships between sorbate size, entropy, and diffusivity

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Previous studies indicated unusually high self-diffusivity for certain sorbates in specific adsorbents.
• Experimental findings suggested a minimum loss of entropy for these sorbates.
• Kemball proposed a link between minimum entropy loss and high self-diffusivity.

Purpose of the Study:

• To investigate the relationship between sorbate diameter, entropy, and self-diffusivity in zeolite NaY using molecular dynamics simulations.
• To evaluate the entropy of the adsorbed phase using two distinct methods.
• To validate the correlation between entropy loss and self-diffusivity in confined systems.

Main Methods:

• Molecular dynamics simulations of monatomic sorbates confined within zeolite NaY.
• Evaluation of adsorbed phase entropy via two-phase and multiparticle expansion methods from simulation trajectories.
• Analysis of self-diffusivity as a function of sorbate diameter and excess entropy.

Main Results:

• An anomalous maximum in entropy was observed as a function of sorbate diameter.
• The study confirmed Kemball's observation: minimum entropy loss correlates with maximum self-diffusivity.
• Dimensionless self-diffusivity exhibited an exponential dependence on excess entropy, similar to bulk fluids.

Conclusions:

• The findings support the hypothesis that minimal entropy loss leads to enhanced sorbate mobility in confined environments.
• The study provides microscopic insights into entropy changes during confinement.
• The observed excess entropy scaling offers a generalized understanding of transport phenomena in confined systems.