The association interaction plays a significant role in self-assembly and determining the properties of associating fluids. The patch-patch attraction in patchy colloids, and hydrogen bonding are two examples of association. Due to the strength, range, and directionality of association, an accurate theory including information at the level of the structure of self-assembling species is required for a precise prediction of the behavior of these fluids. Wertheim\textquoteright s thermodynamic perturbation theory, which uses density expansion method, has presented a promising performance in capturing the thermo-physical properties of both hydrogen bonding and patchy colloidal fluids through prediction of all possible states of the bonding of associating species. While most of the previous studies were focused on utilizing the first order limit of Wertheim\textquoteright s theory, recent simulation and theoretical studies have shown that the simplifying assumptions included in the first order make it not capable of modeling complex self-assembling species.

In this thesis, we develop Wertheim\textquoteright s theory beyond its first order to include accurate information about the structure of associating species like the size of association sites and their relative positions (in case of fluids with multiple sites), and possible self-assembled clusters of associated species. The theory developments are applied for both hydrogen bonding in molecular fluids and patchy colloids and verified with Monte Carlo simulations and previous experimental measurements results, where the agreements were excellent.

Beyond the introduction and conclusion chapters, the scope of this thesis can be summarized into the followings:

Chapter 2: the prediction of the self-assembly of a binary mixture of patchy colloids with two similar patches and small bond angle.

Chapter 3: modeling the effect of hydrogen bond cooperativity and bond angle dependent ring formation for associating hard spheres and Lennard Jones spheres. Applying the final equations to predict the thermodynamic properties of hydrogen fluoride.

Chapter 4: developing an asymmetric model for water including the effect of hydrogen bond cooperativity and multiple bonding at an association site.

Chapter 5: the extension of Wertheim\textquoteright s theory for fluids of patchy colloids with two divalent patches confined between two planar hard walls in a classical density functional theory formalism.

Chapter 6: investigating the effect steric hindrance for association between an associating fluid and a planar hard wall with discrete divalent active sites.