Investigating the Vapor-Phase Adsorption of Aroma Molecules on the Water-Vapor Interface using Molecular Dynamics Simulations.

Abstract

Surfactants are amphiphilic additives primarily used to reduce the surface tension of water and manipulate its wettability on various surfaces. Recent reports suggest that volatile surfactants, such as aroma molecules, diffuse more quickly to the interface from the vapor-phase than conventional surfactants typically used in the aqueous phase. The ability to adsorb from the vapor phase, in addition to their use as cosurfactants, expands the potential applications of volatile surfactants, particularly in situations where adding surfactants from the liquid phase is difficult. Here, we present a molecular level understanding of the adsorption kinetics of linalool, a common aroma molecule, on the water interface using molecular dynamics simulations. We note that the value of surface tension while adsorption from vapor and liquid phases is dependent only on the surface coverage. A minimum surface tension of 32 ± 1.8 mN/m is obtained in both cases at a maximum surface coverage of 4.88 μmol/m2 at 300 K. We observe the extent of decrease of the H-bonds between linalool-water and linalool-linalool molecules at various surface coverages to explain the mechanism of surface tension reduction. We solve Gibb's adsorption equation to establish a correlation between the surface coverage of linalool and the corresponding bulk concentration in experiments. We investigate the free energy profile of linalool's adsorption behavior at different surface coverages and temperatures. Our report suggests that linalool adsorption onto the water interface is an enthalpy-driven process primarily dependent on the strength of the interaction between the hydroxyl group of linalool and water molecules. These insights are crucial for selecting a suitable aroma molecule for various applications that target the vapor-phase adsorption mechanism.