PNNL

Abstract

The transport of molecules across the liquid/vapor interface of water is a fundamental process that is important in a number of areas. Uptake of trace molecules by aqueous droplets is important in the atmosphere as it enables heterogeneous new chemical pathways that are not available in the gas phase. The uptake of molecules by water and partitioning to the air/water surface is important in aquatic environmental systems. The summary of molecular simulations presented in this work supports a picture of solute transport across the water’s vapor/liquid interface in which a hydrophilic solute molecule impinging on the surface is rapidly equilibrated, sticks to the surface of the interface with nearly unit probability, then diffuses into the bulk liquid on an free energy surface. Analysis of a large number of experimental observations of uptake, using a variety of techniques, supports a view of mass transport that is diametrically opposed – most solute molecules that collide with the surface return to the vapor rather than being absorbed into the liquid. This discrepancy stresses the need for greater understanding of the important process of uptake. We provide a perspective on molecular-scale simulations and evaluate the type of accuracy we should expect for interfacial properties, particularly those associated with the update of molecular at the surface. We briefly review computational methods commonly used in studies of aqueous interfaces, including functional forms of the molecular interaction potentials, descriptions of the simulation methodologies, and descriptions of the molecular models of the interface. We present the results of simulations of select properties of the air/water interface and properties of molecular interactions at interfaces and present an overview of modeling approaches to the macroscopic process of uptake. In particular, we present detailed descriptions of the uptake process in flow tube experiments with the help of fluid dynamics calculations toward critical comparison between the uptake experiments and the molecular simulations.