Surfactants are amphiphilic molecules consisting of both hydrophobic and hydrophilic groups. The unique structure of surfactants and their ability to self-assemble into interesting microstructures lead to extensive application in industrial and commercial fields. The study of their interfacial phenomena and self-assembling behavior has also been of interest to academic researchers for years. One key that led to the wide application of surfactant micelles is their ability to enhance the solubility of hydrophobic compounds in water. However, there is a lack of thorough theoretical understanding of the micellar structure and solubilization process in solution due to its inhomogeneous nature. Also, wide industrial application leads to continued interest in the development of predictive theoretical models to understand the system comprehensively and guide the design of optimal surfactant solutions.

This thesis aims to develop a thermodynamic model for surfactant self-assembly in solution. A molecular density functional theory (DFT) that accounts for molecular structure, van der Waals attraction, and hydrogen bonding, i.e., interfacial statistical associating fluid theory (iSAFT), is applied to study the fluid structure and phase behavior of nonionic surfactant micellar systems, with particular interest in understanding the partitioning of solute into the micelle. The model predicts the macroscopic physical properties and provides an explanation of the microscopic density profile. This work aims to provide a reliable tool that can be used in surfactant design and material screening. Key contributions of this thesis include:

1. A quantitative iSAFT approach to model polyethylene oxide alkyl ether surfactant micelle formation in water and to model the effect of surfactant architecture on micelle properties and solute partitioning. The effects of size and branching of surfactant head and tail groups on micelle size distribution are also studied.

2. The iSAFT approach is applied further to predict the partition equilibrium behavior between micelles and various solutes. The model is able to predict properties such as partition coefficient and, at the same time, explain the exact solute partitioning behavior at the microscopic level. The mechanism behind the competitive solubilization of benzene and hexane is also studied.

3. The role of alcohol as an additive in the surfactant micellar system is investigated. The dualistic effect of alcohol as both a cosolvent and a cosurfactant is captured, and the model predictions are in agreement with experimental results. The change in behavior with changing alcohol carbon numbers is also studied and discussed.