Microstructure and phase behavior of inhomogeneous complex fluids
Chapman, Walter G.
Doctor of Philosophy
Broadly defined as fluids possessing multiple length scales, complex fluids, typified by polymers, hydrocarbons, surfactants, emulsions etc., exhibit microstructures even when macroscopically homogeneous. This dissertation introduces a classical density functional theory (DFT) that provides structural and thermodynamic information at the molecular level in these fluids near interfaces and in confinement. The microstructure in such systems is a function of both fluid and substrate characteristics, and varies on the order of molecular length scale (sometimes even smaller). The developments presented here can be broken down into two components that separately focus on the fluid and the interface aspects of the system. On the fluid side, the theory provides a very simple method for modeling polymeric mixtures, by treating the polyatomic system as a strongly associating atomic fluid mixture. Derived in terms of segment density, it offers accuracy comparable to the computationally intensive multi-point-density-based theories at a modest expense comparable to those of atomic DFTs. Comparisons with molecular simulations demonstrate its capability to accurately capture the entropic and enthalpic effects dictating the microstructure in inhomogeneous solutions and blends of linear and branched chains. On the interface side, the DFT provides the capability to describe adsorption of associating fluids on functionalized surfaces. These surfaces are activated with polar sites to which fluid molecules can bond, such as water adsorbing on activated carbon, silica, clay minerals, etc. The theory, in excellent agreement with simulations, shows that surface association significantly changes the fluid structure and adsorption behavior. An impressive feature of the theory enables one to estimate the distribution of fluid along the interface, i.e., the three-dimensional (3D) structure, while retaining the one-dimensional (1D) form. This translates into orders of magnitude of savings in computation time. The DFT is based on thermodynamic perturbation theory of the first order (TPT1), which is also the basis of the most widely used theory for bulk polymer solutions and melts---Statistical Associating Fluid Theory (SAFT). This consistency facilitates a seamless integration of the two into a common platform to model combined bulk-interfacial phase behavior and microstructure. This is of critical importance to several applications where interfacial properties need to be predicted based on bulk conditions.
Chemical engineering; Molecular physics