Density Functional Study on the Condensation and Evaporation Heats of the Nanoconfined Fluids

Within the framework of classical density functional theory, the isosteric heat and condensation/evaporation heat of the confined methane are studied. First, the theoretical expression for condensation/evaporation heat is derived on the basis of the first law of thermodynamics. The method for computation is also proposed. With the proposed method, the dependences of the condensation/evaporation heat on temperature, pore width, and surface strength are studied and analyzed in detail. First of all, it is found that the condensation/evaporation heat of the confined fluid decreases monotonically with the temperature. Moreover, it is also found that at the same temperature the condensation heat is usually larger than the corresponding evaporation heat, which confirms the irreversibility of the condensation and evaporation from an exothermic point of view. Furthermore, our results reveal that the condensation/evaporation heat of confined fluid in pores decreases with the width of the pore. With the further increase in the pore width, the condensation/evaporation heat tends gradually to that of the corresponding bulk fluids. In addition, an interesting result is found that the condensation/evaporation heat is insensitive to variation in surface strength.