The energy effects on phase transition of nano-confined fluids

System walls may have three effects on the thermodynamic properties of a fluid. The first is due to the wall-molecule interactions[1]. Introduction of these interactions, which in macroscopic systems are ignorable compared to intermolecular interactions, can give rise to new properties like wetting or new kinds of phase transitions. In addition to this direct wall effect, when the distance between nanopore walls tends to a few molecular diameters,the range of intermolecular interactions will be greater than the confined dimensions of the pore. Therefore, part of the intermolecular interactions, which are usually long-range attractive forces, will remain unaccounted for in the calculation of thermodynamic properties. The two effects just mentioned have effects on the energy of the system; hence, it can be designed as energy effects [2]. However, in the absence of any long-range attractions in a confined fluid system, its thermodynamic properties show a significant difference with those of the macroscopic fluid. An example would be the differences observed between a hard sphere fluid confined between two parallel hard walls and a macroscopic hard sphere fluid. In the absence of any long-range forces and for specific numbers of molecules, the tendency of the system to maximize its entropy leads to these changes in the behavior of the system, which can be designated as entropy effects [2].