Evaporation of short-chain alcohols of methanol, ethanol, and propanol: Mechanism, kinetics, and potential of mean force calculations at room temperature

Abstract

We have performed a comparative study of the process of evaporation of three short-chain alcohols, namely, methanol, ethanol, and propanol, through unbiased and also biased molecular dynamics simulations, with an emphasis on the structural, kinetic, mechanistic, and thermodynamic factors that influence molecular escape into the vapor phase. The evaporation flux calculated directly from unbiased simulations is found to decrease systematically with increasing alkyl chain length. The potential of mean force profiles obtained from umbrella sampling simulations reveal that methanol has the lowest barrier for evaporation, facilitating the highest evaporation rate, whereas ethanol and propanol exhibit progressively higher barriers, implying lower evaporation rates, consistent with the fluxes calculated from unbiased simulations. Kinetic energy analysis shows that methanol evaporates with the lowest kinetic energy requirement, while higher kinetic energies are required for ethanol and propanol. These additional kinetic energies are acquired through strong intermolecular interactions at close distances, which play a key role in enabling molecules to overcome the evaporation barrier. Soon after the final interaction with an interfacial molecule, the last hydrogen bond of the evaporating molecule vanishes rapidly, promoting its escape to the vapor phase. The dynamics of hydrogen bonding, quantified through continuous and intermittent correlation functions, further reveal a progressively slower dynamics from methanol to ethanol and propanol. Together, these results provide a molecular-level picture of evaporation of short-chain alcohols, underscoring the roles of hydrogen bond dynamics, kinetic energy transfer, and loss of intermolecular interactions in governing the evaporation kinetics from alcohol surfaces.