Structural behavior and dynamics of an anomalous fluid between solvophilic and solvophobic walls: templating, molding and superdiffusion

TL;DR

This study uses molecular dynamics to explore how structured solvophilic and unstructured solvophobic walls influence the phases, structure, and dynamics of an anomalous fluid, revealing long-range templating effects and heterogeneous superdiffusive behavior.

Contribution

It demonstrates the long-range templating and molding effects of structured walls on an anomalous fluid's layers, extending understanding of confinement effects beyond the first layer.

Findings

Structured wall induces high-density layer at low temperatures.

Layer templating propagates structural effects through multiple layers.

Heterogeneous superdiffusive dynamics occur near the solvophilic wall.

Abstract

Confinement can modify the dynamics, the thermodynamics and the structural properties of liquid water, the prototypical anomalous liquid. By considering a general anomalous liquid, suitable for globular proteins, colloids or liquid metals, we study by molecular dynamics simulations the effect of a solvophilic structured and a solvophobic unstructured wall on the phases, the crystal nucleation and the dynamics of the fluid. We find that at low temperatures the large density of the solvophilic wall induces a high-density, high-energy structure in the first layer ("tempting" effect). In turn, the first layer induces a "molding" effect on the second layer determining a structure with reduced energy and density, closer to the average density of the system. This low-density, low-energy structure propagates further through the layers by templating effect and can involve all the existing layers at the lowest temperatures investigated. Therefore, although the high-density, high-energy structure does not self-reproduce further than the first layer, the structured wall can have a long-range effect thanks to a sequence of templating, molding and templating effects through the layers. We find dynamical slowing down of the solvent near the solvophilic wall but with largely heterogeneous dynamics near the wall due to superdiffusive liquid veins within a frozen matrix of solvent. Hence, the partial freezing of the first hydration layer does not correspond necessarily to an effective reduction of the channel section in terms of transport properties.