Molecular modeling the microstructure and thermodynamic properties of complex fluids

Abstract

The accurate prediction of a complex fluid's equilibrium microstructure and corresponding thermodynamic properties relies on the capability to describe both the molecular level architecture and specific governing physics. This thesis makes key contributions to furthering the application and understanding of molecular models for complex bulk and inhomogeneous fluids with a specific interest in mixtures involving trace components. Such developments have potential for wide-ranging application to fields from consumer goods and medicine to energy and targeted specialized material design. In the bulk, the perturbed-chain statistical associating fluid theory (PC-SAFT), an equation of state based on Wertheim's first order thermodynamic perturbation theory (TPT1) is used to demonstrate the robustness and performance of intrinsic molecular parameters determined for a complex fluid (water) with a new fitting strategy. Experimental solubility data at ambient conditions was used to find the PC-SAFT parameters for water which where capable of reproducing water content for binary mixtures with liquid and vapor n -alkanes under a myriad of physical conditions. The model gave excellent qualitative and very good quantitative agreement without the need of a binary interaction parameter. For inhomogeneous fluids, the application of a density functional theory (DFT) also based on TPT1, is extended to model the self-assembly of amphiphilic molecules at a liquid-liquid interface. This DFT, interfacial SAFT ( i SAFT), is validated against molecular simulation results for the microstructure and interfacial tension of a simple diatomic surfactant based on the continuum oil-water-surfactant model of Telo da Gama and Gubbins. A comprehensive systematic study is conducted for characterizing the affects of part of the vast parameter space governing the fluid microstructure and observed interfacial tension. The role of surfactant structure, oil structure, surfactant concentration, nonionic cosurfactant mixtures, and temperature play in altering molecular level phenomena such as surfactant aggregation, solvent depletion, and surfactant chain conformation as a result of the balance between enthalpic and entropic driving forces are described.