Structuring of fluid adlayers upon ongoing unimolecular adsorption

TL;DR

This study investigates how the rate of molecular adsorption affects fluid structuring on surfaces, revealing that slower adsorption rates lead to larger phase-separated regions, challenging existing phenomenological models.

Contribution

The paper demonstrates that the size of phase-separated regions depends strongly on adsorption rate, highlighting limitations of current phenomenological descriptions in non-instantaneous destabilization scenarios.

Findings

Region size depends on adsorption rate more than predicted

Phenomenological models are inadequate for gradual destabilization

Surface phase separation dynamics are influenced by adsorption kinetics

Abstract

Fluids with spatial density variations of single or mixed molecules play a key role in biophysics, soft matter and materials science. The fluid structures usually form via spinodal decomposition or nucleation following an instantaneous destabilisation of the initially disordered fluid. However, in practice an instantaneous quench is often not viable, and the rate of destabilisation may be gradual rather than instantaneous. In this work we show that the commonly used phenomenological descriptions of fluid structuring are inadequate under these conditions. We come to that conclusion in the context of surface catalysis, where we employ kinetic Monte Carlo simulations to describe the unimolecular adsorption of gaseous molecules onto a metal surface. The adsorbates diffuse at the surface and, as a consequence of lateral interactions and due to an ongoing increase of the surface coverage, phase separate into coexisting low- and high-density regions. The typical size of these regions turns out to depend much more strongly on the rate of adsorption than predicted from recently reported phenomenological models. We discuss how this finding contributes to the fundamental understanding of the crossover from liquid-liquid to liquid-solid demixing of solution-cast polymer blends.