Evolution of adsorption isotherm and isosteric heat from sub-triple to super-critical points

Abstract: We carried out a comprehensive molecular simulation to investigate the evolution of the excess isotherms and the isosteric heat with respect to temperature for argon adsorption on strong and weak substrates. The temperature ranges from well below the bulk triple point temperature to above the bulk critical temperature to show the first- (second-) order transitions and the state of the system at the bulk coexistence pressure P₀, whether it is non-wetting, partial wetting or complete wetting (preceded by pre-wetting). It is found that the key parameter that affects the dependence of the transitions on temperature and the state of the system at P₀ is the relative difference between the isosteric heat and the heat of sublimation (or condensation). For strong substrates, the state of the system changes from partial wetting to complete wetting when the temperature crosses the bulk triple point temperature, and for temperatures well below the bulk triple point the 2D-condensation occurs in the first and second (and possible higher) layers. For weak substrates, the state of the system changes from non-wetting to complete wetting when the temperature crosses the wetting temperature T_W, which is specific to the substrate. For temperatures greater than T_W, complete wetting in weak substrates only occurs at pressures close to the bulk coexistence pressure P₀ via the initial stage of clustering (unfavourable adsorption), followed by a pre-wetting (known as thin-to-thick transition), and this is reflected in the increase of the isosteric heat from a value less than the heat of condensation λ and approaching λ as the pressure tends to P₀.