COMPUTATIONAL INVESTIGATIONS OF FLUID PHASE BEHAVIOR: SUPERCOOLED WATER, CONFINED FLUIDS AND CO2-BRINE MIXTURES

Abstract

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 Tc = 237 ± 4 K, c = 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, Q6, that is able to distinguish crystalline from disordered phases [Steinhardt et al., Phys. Rev. B, 28, 784(1983)]. 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-H2O-NaCl mixtures over a broad temperature and pressure range (50oC ≤ T ≤ 350oC, 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 CO2-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.