Molecular Simulation and Modeling for the Thermodynamic Properties of Polyatomic Molecules and for Surfactant Adsorption.

In Chapter I, molecular dynamics simulations are performed for surfactant adsorption on mineral solid surfaces. Three arrangements of admicelles bilayer are studied: with the ad hoc assumption of even surfactant adsorption density at top layer and bottom layer, we test three different head group densities per layer: 2.29/nm^2 , 3/nm^2 and 4/nm ^2. The simulation results show that the structures of adsorbed surfactant aggregates change abruptly with changes in the head group surface densities. The projections of aggregates group density profiles along the normal to the surface, the tilt angle distribution, the gauche bond distribution and the structure order for each case are reported. Also, the two dimensional radial distribution functions of head groups and end groups are studied. In Chapter II, mixtures of chain molecules: monomers, dimers, trimers, and tetramers are studied using the soft interaction site model. The method of solution, based on the efficient algorithm of Labik and employing Newton -Raphson accelerations, is shown to be fast, accurate and stable; it also shows good convergence behavior even with inaccurate initial estimates. Utilizing the numerical method developed in Chapter II, an integral equation approximation--(RISM)--is solved for chain molecules interacting with the site-site Lennard -Jones potentials in order to test the underlying assumptions of the group contribution (GC) solution theory. By using "model molecules", we are able to satisfy precisely the prerequisites in GC, then test its hypotheses unequivocally. Our tests show that the group conformality does not apply to the microscopic structures (the site-site pair correlation functions) for all categories of solutions (failure of the strong hypothesis). However, the integrated properties (such as internal energy) can be made to obey group conformality (a success of the weak condition). For internal energy, the GC prescription is found to best describe the negative (~ attractive) energy contributions. In Chapter IV, a new numerical technique for solving the Ornstein-Zernike (OZ) equations for supercritical fluids mixtures based on Baxter's formula is described. The new method allows us to calculate the physical properties near the critical region with greter accuracy than is permitted by existing methods.