Water Evaporation: A Transition Path Sampling Study

TL;DR

This study uses transition path sampling to analyze water evaporation at the molecular level, revealing the characteristics of the transition state and showing that evaporation can be modeled as ballistic escape without additional barriers.

Contribution

It provides a detailed characterization of the evaporation transition state and introduces a simple model that captures the evaporation process in water.

Findings

Transition state exhibits a liquid-vapor interface with negative mean curvature.

Evaporated water molecules have Maxwellian velocity distributions at the liquid temperature.

Evaporation can be modeled as ballistic escape from a potential well without extra barriers.

Abstract

We use transition path sampling to study evaporation in the SPC/E model of liquid water. Based on thousands of evaporation trajectories, we characterize the members of the transition state ensemble (TSE), which exhibit a liquid-vapor interface with predominantly negative mean curvature at the site of evaporation. We also find that after evaporation is complete, the distributions of translational and angular momenta of the evaporated water are Maxwellian with a temperature equal to that of the liquid. To characterize the evaporation trajectories in their entirety, we find that it suffices to project them onto just two coordinates: the distance of the evaporating molecule to the instantaneous liquid-vapor interface, and the velocity of the water along the average interface normal. In this projected space, we find that the TSE is well-captured by a simple model of ballistic escape from a deep potential well, with no additional barrier to evaporation beyond the cohesive strength of the liquid. Equivalently, they are consistent with a near-unity probability for a water molecule impinging upon a liquid droplet to condense. These results agree with previous simulations and with some, but not all, recent experiments.