In this thesis, the density profile of a hard sphere fluid around various hard and soft spheres has been studied using a well-established version of the density functional theory called “modified fundamental measure theory”. The results obtained from this study show that an increase in the size of the central molecule leads to both an enhanced density profile at the contact point and an amplified layering structure. Similar effects can be observed when attraction between original and bulk molecules increases. Another finding of the present study is that increasing the size of the central molecule has a quasi-attractive role of an entropic origin called ‘depletion potential’. Since the molecule is not very large, the depletion potential can be mapped in a hard core with an attractive Yukawa tail. In addition, the structure and configurational properties of hard spheres confined in a nanoslit with hard and structureless walls are studied using the density functional theory. For this purpose, the local chemical potentials of various hypothetical and real structures are obtained for a hard sphere fluid confined within a nanoslit. The inhomogeneity in some of these local chemical potentials originates from the tendency of the system to maximize its density in the vicinity of walls. In a later stage, comparisons are made among the configurational entropies of these different structures. Our results show that the prohibition of the walls leads to decreased entropy of the confined fluid, which is due to a decrease in the number of configurations for confined molecules. Therefore, to compensate for this entropy decrease, the confined fluid selects an inhomogeneous structure. The inhomogeneous layering structure of the confined fluid allows a greater average free volume for the molecules, which in turn may lead to an increase in the entropy. Along this line, the fundamental measure density functional results, represented in this work, indicate the happening of a layering phase transition in hard sphere fluid confined in a nano-slit with hard and structureless walls which has wholly entropic origin. In addition, in this thesis, the perturbative fundamental measure density functional theory is used to study energy effects on the structure and properties of a hard core two-Yukawa fluid confined in a nanoslit. Our results show how the structure and phase transition of the nano-confined fluids will change as a result of energy effects. Finally, the structure and phase diagram of a Lennard-Jones fluid confined in a single wall carbon nanotube (SWCNT) are studied using perurbative Tarazona-Rosenfeld DFT. The obtained results show how the structure and phase transition of the Lennard-Jones fluid confined in a SWNT are affected seriously by confinement effects.
