Inverse condensation of adsorbed molecules with two conformations

TL;DR

This paper explores the counterintuitive phenomenon of molecules adsorbed on surfaces condensing upon heating, using a lattice model and simulations to understand the underlying mechanisms and potential applications.

Contribution

It introduces a simple lattice model and analytical methods to study inverse condensation phenomena in molecules with multiple conformations, expanding understanding beyond traditional phase transitions.

Findings

Inverse condensation occurs upon heating in the model.

Monte Carlo and mean-field methods confirm the phenomenon.

Results are applicable to molecules with different conformations and entropies.

Abstract

Conventional gas-liquid phase transitions feature a coexistence line that has a monotonic and positive slope in line with our intuition that cooling always leads to condensation. Here we study the inverse phenomenon, condensation of adsorbed organic molecules into dense domains upon heating. Our considerations are motivated by recent experiments [Aeschlimann et al., Angew. Chem. (2021)], which demonstrate the partial dissolution of an ordered molecular monolayer and the mobilization of molecules upon cooling. We introduce a simple lattice model in which each site can have three states corresponding to unoccupied and two discernible molecular conformations. We investigate this model through Monte Carlo simulations, mean-field theory, and exact results based on the analytical solution of the Ising model in two dimensions. Our results should be broadly applicable to molecules with distinct conformations that have sufficiently different entropies or heat capacities.