Evaporation of Ethanol from Liquid Ethanol and Water-Ethanol Mixtures: Mechanism, Kinetics, and Free Energy Changes for Different Compositions of the Mixtures.

Understanding the evaporation mechanism of liquid ethanol and ethanol-water binary mixtures is important for numerous scientific and industrial processes. The amount of water in liquid water-ethanol mixtures can significantly affect how quickly ethanol molecules evaporate. Here, we study the mechanism and rate of evaporation of ethanol from pure liquid ethanol and ethanol/water binary mixtures through both unbiased molecular dynamics simulations and biased simulations using the umbrella sampling method. We first calculated the surface tension of pure ethanol and the mixtures, which are found to be consistent with those obtained in experiments. The rate of evaporation decreases as the surface tension increases with an increasing amount of water in ethanol. The free energy of evaporation is found to be positive, and it also increases with an increasing water concentration, which again can be linked to the slower evaporation rate for higher water concentration. The making and breaking of hydrogen bonds are found to play vital roles in the process of evaporation. The energy transfer to the evaporating molecule occurs through collision and hydrogen bonding interactions, which lead to the translation of the ethanol molecule from the surface to the vapor phase. Molecular identifications are made for the last interacting molecule, which provides the required kinetic energy to the evaporating ethanol molecule through collisional encounters before the breaking of the last hydrogen bond of the evaporating molecule.