Molecular Simulation Study Of Capillary Confined Fluids & Alkaline Polyelectrolytes

Modern civilization is established on the utilization of fossil fuels, especially the "black gold" petroleum; but now has to face severe energy and environmental crises due to the overexploitation. As a short-term resolution to the crises, the efficiency of oil recovery and utilization must be improved while effectively reducing environmental pollutions. As a long-term resolution, new clean energy sources are necessary, such as hydrogen, whose production, storage, distribution, and fuel-cell application are very hot research subjects nowadays.This thesis is about the molecular simulations of key processes and materials in oil recovery and fuel cells. The contents include two parts. The first one is related to oil recovery and groundwater remediation, which share common features of capillary-confined fluids. Many-body dissipative particle dynamics (MDPD) methods are employed in this work, resulting in the following main results:1. MDPD study of spontaneous capillary imbibition (SCI) and drainage (SCD).The relationship between static contact angles and solid-liquid interaction parameters is established, which enables the definition of fluid wettability. Startup conditions for SCI and SCD are studied and a series of such processes are simulated for fluids with distinct wettability. We have refined the modified Lucas-Washburn equation (with the static contact angle replaced by the dynamic contact angle, and the inertial resistance included) to incorporate the slip length b, such that SCI and SCD processes with partly-wetting fluids can also be well described.2. MDPD study of forced capillary displacement (FCDis).The relationship between the oil/water interfacial tensionγE and the oil-water interaction parameter AE is established, which enables the control of the interfacial tension between oil and water. A simulation model for FCDis is designed, in which external force f,water/capillary interfacial tension yw, oil/water interfacial tension yo, and oil/water interfacial tensionγE can be quantitatively regulated. With fixed water-capillary interaction parameter AW, oil/capillary interaction parameter AO and oil/water interaction parameter AE, it is found that FCDis can only take place when f exceeds a critical value and the flow rate is proportional to f.Accordingly, a parameter named starting force fs is derived and the influences of AW and AE are studied. Meanwhile, the residual oil content (ROC) could be extracted from the simulation, which is influenced by AW, AE, and f. Systematic investigations have produced a set of optimized conditions for FCDis, which may instruct realistic applications.3. MDPD study of spontaneous capillary displacement (SCDis).An appropriate oil/water/capillary 3-phase static contact angle model is designed, and a new definition for meniscus position is proposed. A simulation model for SCDis is designed, and a series of simulations under various conditions are conducted. A differential equation is derived to make use of the simulated dynamic contact anglesθd, which can describe the SCDis process very well. Further, after plugging in the relationship betweenθd and the meniscus velocity obtained from the MKT theory, the above differential equation is refined to be independent of the simulatedθd.The second part of the thesis is about molecular dynamics (MD) simulations on the static and dynamic properties of alkaline polymer electrolyte (APE) membrane, a novel material for fuel cells. The main results are summarized as follows:1. The static and dynamic properties of three types of APE membranes with different side chains are simulated using the force field for Nafion simulations reported in the literature.2. It is found that the density of hydrated APE membranes peaks at certain water content, according to which the concept of vacuum volume is proposed and demonstrated to be useful in explaining the filling behavior of water, and helpful in understanding the swelling nature of APE membranes.3. Radial distribution functions (RDFs) are calculated between typical atoms, including the water oxygen Ow, the hydroxide ion oxygen Oh, and the quaternary ammonium (QA) ion nitrogen Nq, which provide a variety of structural information of APE membranes. The QA functional groups are found to distribute uniformly in the APE membrane, while the hydroxide ions can be dissociated and assemble in the aqueous domains. There exist hydrophilic/hydrophobic phase separations in small range in APE membranes, differing from the wide-range phase separation observed in Nafion.4. Self-diffusion coefficients of water molecules and hydroxide ions are obtained by calculating their mean squared displacement (MSD) in the APE membranes. Although both coefficients are smaller than those in Nafion, the corresponding particle diffusion activation energies are larger in APE membranes, indicating greater conductivity can be obtained at elevated temperatures.