Molecular and theoretical identification of adsorption phase transition behaviors via thermo-kinetics analysis

Abstract

In order to identify the adsorption phase transition behaviors, a series of thermo-kinetics investigation is performed through theoretical analysis integrated with molecular dynamics simulations. Vary with the vapor pressures and cooling temperatures, three regions are distinguished during the phase transition process, they are stable adsorption region, metastable adsorption and condensation regions. For the stable state, the adsorption equilibrium is reached and the thickness of adsorption layer remains constant. In the metastable state, the thickness of the adsorption layer grows slightly. In the final state, condensation is initiated, a liquid film is formed and it thickens unlimitedly. From the theoretical adsorption model and statistical rate theory, the thermos-kinetics behaviors are presented and the temperature jump across the interface is discussed. Also, the boundaries for dividing the transition process are determined. Results show that the critical pressure ratio for the transition from stable state to the metastable adsorption is decreased with the increase of solid cooling temperature. Once the temperate is beyond 129 K, the critical pressure ratio keeps constant. For the transition from metastable adsorption to condensation, the critical condition is determined from the kinetic behaviors of adsorption clusters. With the increase of pressure ratio, the clusters are formed and aggregated. Once the pressure ratio approaches a certain value, the first-order phase transition occurs, then a homogenous liquid film is formed. The adsorption phase transition process presented by molecular dynamics simulations coincides well with that predicted from theoretical models.