Computational investigations of fluid phase behavior: supercooled water, confined fluids and carbon dioxide-brine mixtures

In this dissertation, a combination of state-of-the-art molecular simulation techniques are used to investigate the interesting phase behavior of complex fluids, including supercooled water, the CO2-H2O and CO2-H2O-NaCl systems, and confined fluids.;In the first part of the dissertation, sufficient and robust computational evidence are obtained in support of the existence of a liquid-liquid transition in the ST2 model of water, which is distinct from the crystal-liquid transition. Using Ewald summation treatment of long-range electrostatic interactions, we locate the critical point of the liquid-liquid transition at T c = 237 ± 4 K, ρc = 0.99 ± 0.02 g/cc, Pc =167 ± 24 MPa. We perform umbrella sampling NPT Monte Carlo simulations and compute free energy surface as a function of density and a bond-orientational order parameter, Q 6, that is able to distinguish crystalline from disordered phases. We find two liquid basins at T = 228.6K and P = 2.2 kbar, which provides further evidence for the existence of a low- density liquid phase in this model.;In the second part of the dissertation, we perform a comprehensive test of several existing water (SPC, TIP4P, TIP4P2005, exponential-6), carbon dioxide (EPM2, TraPPE, exponential-6), and NaCl (SD and DRVH) models in predicting the phase behavior of CO2-H2O and CO2-H 2O-NaCl mixtures over a broad temperature and pressure range (50°C ≤ T ≤ 350°C, 0 ≤ P ≤ 1000 bar), and NaCl concentrations (1 mol/kg H2O to 4 mol/kg H2O), using conventional Lorentz-Berthelot combining rules for the unlike-pair parameters. Under conditions of moderate NaCl molality (~1 mol/kg H2O), the predictions of the CO2 solubility in the water-rich and CO 2-rich phase resemble those in the CO2-H2O system. Our work points to the challenge and importance of improving current atomistic models as well as combining rules so as to accurately predict the phase behavior of mixtures.;Finally, the vapor-liquid critical and coexistence properties of the Lennard Jones fluid confined between two parallel hard walls in the range σ ≤ H ≤ 6σ and 10σ ≤ Lx,Ly ≤ 28σ are investigated. Using the mixed-field finite size scaling approach, we establish a "phase diagram" in the (H, L ) plane, showing the boundary between four types of behavior: 3D, quasi-3D, quasi-2D and 2D. We show that the infinite-system-size critical points obtained by extrapolation from the apparent 3D and 2D critical points have only minor differences with each other.