Unlike interactions in simulated methane clathrate hydrates.

Ph. D. University of KwaZulu-Natal, Durban 2014.Clathrate hydrates are an ice-like substance consisting of networks of water molecules, held together by hydrogen bonds, enclosing trapped gas molecules. Natural gas clathrate hydrates (in which the trapped gas species is chiefly methane) are of interest in the field of offshore gas exploitation, where they frequently form blockages in natural gas pipelines. Knowledge of the phase equilibria of methane clathrate hydrate can thus reduce the overall monetary cost of natural gas extraction. Computer simulation of molecular systems is useful to understand fundamental mechanisms, and serves as a complementary method to laboratory experiments in the study of chemical systems. The Lennard-Jones potential is frequently used to describe intermolecular interactions in molecular simulations. Correction factors are often applied to the Lennard-Jones potential, although the effect of these correction factors on the behaviour of simulated molecular systems is not fully understood. This thesis examines the effect of Lennard-Jones correction factors on simulated methane clathrate hydrates using three different computational approaches: lattice distortion theory, grand canonical Monte Carlo simulations (which emulate gas adsorption into the clathrate lattice), and direct estimation of the heat of dissociation coupled with the Clausius-Clapeyron equation. In addition, the use of the results of grand canonical Monte Carlo simulations to infer phase equilibria was demonstrated in this thesis. The application of Lennard-Jones correction factors in lattice distortion calculations was found to not be viable, due to the extreme sensitivity of the perturbation potential (the quantity of interest in this theory) to changes in the values of the correction factors. Unlike interactions were found to weakly influence methane adsorption into the clathrate hydrate crystal, and so the application of correction factors in grand canonical Monte Carlo simulations is demonstrated to be ineffectual. The direct estimation of the heat of dissociation was shown to be viable when matching to calorimetric data, and the inference of phase equilibria by coupling the Clausius-Clapeyron equation with this approach was shown to yield agreeable results

Monte Carlo simulations of systems of light alcohols + water + n-dodecane and water solubility and structures in polytetrafluoroethylene.

Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2011.Polytetrafluoroethylene (Teflon®) is encountered in many environments – frying pans, clothing, osmotic distillation membranes, to name a few – yet the solubility and clustering behaviour of water with this material was not found in the open literature. This information may be useful in applications where an absence of water is desired, such as in clothing and textiles. Previous work on polyethylene + water has shown that small water clusters form in the amorphous portion of the polymer. This work investigated this phenomenon for the case of polytetrafluoroethylene + water. Initially, a test system of light alcohols + water + n-dodecane was investigated using Gibbs Ensemble Monte Carlo simulations and compared to previous laboratory experiments. This test system was investigated in order to gain expertise in the methodologies and theory behind Monte Carlo simulation, as well as to gain experience with using the necessary software. For this test system, it was found that the TraPPE parameters representing the interactions between the alcohols and the n-dodecane were not adequate and lead to increasing deviations with increasing carbon number in the alcohol. To replicate the conditions of the amorphous polymer matrix, liquid-liquid equilibrium between water and the polymer was investigated. Gibbs Ensemble Monte Carlo simulations have been performed for systems of perfluoroalkanes and water to determine the influence of temperature and carbon number on the solubility and clustering behaviour of water within the perfluoroalkanes. The temperature range in this study was from 450 K to 600 K, and the perfluoroalkane carbon number range was from 8 to 300 carbon atoms. With increasing carbon number, it was found that there was an asymptotic value of 98.0 mole percent water in the polymer phase. With increasing temperatures it was found that there were exponential increases in solubility of water into the polymer matrix. Previous work on clustering and supramolecular structure of perfluoroalkanes described the rigidity of the perfluoroalkane chains in comparison to alkane chains, thus explaining the large increases in free volume with increasing temperature in the polymer matrix observed in this work. A discontinuity with regard to both solubility and clustering behavior was observed for a polymer carbon number of 10 to 12 carbon atoms. Prior work on the energy contributions towards the helical structure of perfluoroalkanes showed a shift in the energy contribution regime for carbon numbers larger than ~10 carbon atoms, which may explain this discontinuity. It was found that linear water clusters accounted for up to ~90 percent of the water clusters, concurring with previous work on water clustering in polyethylene

Sorption of natural gas in cement hydrate by Monte Carlo simulation

Concrete, a combination of cement, water, sand, and aggregates, is a ubiquitous engineering and construction material. This composite material is exposed to a wide variety of environmental conditions that can cause degradation, such as extremes of temperature, and exposure to corrosive substances. This study is concerned with the sorption of natural gas constituents and their mixtures in cement hydrate using atomistic Monte Carlo simulation in the grand canonical ensemble. Pure species sorption isotherms were generated at 273, 298, and 323 K for gas fugacities up to 103 kPa. Comparison of gas uptake and the isosteric heat of adsorption in cement was undertaken for all of the species in the study, and the influences of both temperature and gas fugacity on sorption characteristics were considered. The selectivity of adsorption of hydrogen sulphide in a natural gas blend was also considered, as it is typically responsible for the degradation of concrete infrastructure

Influence of fluorination on barrier properties of polymers: Insights from Monte Carlo simulations of eicosanes + methane

Fluorination is widely used to improve the resistance and physical properties of polymers that are cheap to manufacture. This process improves the resistance properties of unfluorinated materials. This study examines the effects of varying the degree of fluorination on the clustering and absorption behaviour of methane n-eicosane. Monte Carlo simulations were performed for several different pressure values, at ambient temperature, to determine the uptake of methane into the eicosanes. The density of the pure eicosanes, simulated at ambient conditions, compared favourably with experimental data for the relevant polymers. The spatial configurations resulting from the absorption simulations were analysed to determine the clustering behaviour of absorbed methane. Both the prevalence of cluster formation in general, and the occurrence of specific cluster topologies of various sizes were considered. Cyclic clusters had a tendency to become more prevalent in unfluorinated eicosanes as the gas pressure was increased, while the presence of fluorine atoms on the eicosane backbone appeared to inhibit the formation of such clusters

Influence of gravitational potential on the thermodynamic stability of pure and mixed clathrate hydrates

Clathrate hydrates are an ice-like material consisting of gas molecules confined within cavities in a crystalline water lattice. Phase equilibria of clathrate hydrates systems was described using the statistical mechanical theory of van der Waals and Platteeuw. This theory makes use of the fractional occupancy of cavities within the clathrate hydrate lattice in the determination of chemical equilibria. Classical density functional theory with intermolecular interactions restricted to the first hydration shell was employed to determine the fractional occupancy. In addition to the external field describing the gas-water interactions, the effect of a gravitational field was introduced. The results of the calculations show that although the gravitational potential term may be orders of magnitude smaller than the thermal kinetic energy of the gas species or the hydrogen-bond energy holding the clathrate lattice together, it can nevertheless influence the phase equilibrium of the clathrate hydrate system to some degree. The effect of the magnitudes of both the gravitational potential and the local gravitational field are considered too

Influence of unlike dispersive interactions on methane adsorption in graphite: a grand canonical Monte Carlo simulation and classical density functional theory study

Activated carbons are popular adsorbents due to their large micro- and mesoporous volumes and high specific surface areas. Modeling adsorption behaviour using molecular computations is frequently undertaken, but the influence of the unlike intermolecular interactions on adsorption behaviour is often not well understood. This study employed grand canonical Monte Carlo simulations, and classical density functional theory coupled with a simple lattice gas model to study the influence of unlike intermolecular interactions on adsorption behaviour, with a focus on the dispersive interactions. Both approaches yielded qualitative agreement with experimental data from the literature, although only a fitted classical density functional theory approach agreed quantitatively. Changing the potential energy well depth of the methane-carbon interaction did not change the Langmuir-type adsorption behaviour observed, however, there was some dependence of the adsorption behaviour on the unlike interactions, depending on the thermodynamic conditions

Sorption of Perfluorinated and Pharmaceutical Compounds in Plastics: A Molecular Simulation Study

The aim of the current study is to investigate the effect of temperature and degree of polymerisation on the thermodynamic interaction of perfluorinated compounds (PFCs) into plastics. The occurrence of contaminants of emerging concern such as pharmaceutical drugs, PFCs, microplastics (MPs), etc., in sources of drinking water have posed significant health risks to aquatic life and humans in recent years. These organic pollutants can interact with MPs and pose much higher health risks; consequently, MPs become a transport vector and thus alter their migration as well as occurrence in the environment. The purpose of this paper is to examine the adsorption mechanism of perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), and sulfamethazine (SMT)—relative to water—on polyethylene (PE) and polypropylene (PP) using an extended Flory–Huggins approach. The results suggest that in an aqueous environment, both PFOA and PFOS may be taken up preferentially by PP and PE, although less strongly by PE. The degree of polymerisation of PE and PP did not significantly influence the observed behaviour. In terms of sorption affinity, the observed affinity was PFOA>PFOS>SMT which was consistence for both PE and PP