Effects of additives on the formation of methane and carbon dioxide gas hydrates

Abstract

Gas hydrates (GHs) are ice-like crystalline solids comprising water and suitable gases in which gas molecules are physically encaged in a cage-like hydrogen-bonded structure formed by water molecules. Under the presence of appropriate additives, the formation of GHs can be controlled in a desired manner thereby opening novel ways of using GHs for gas storage and transportation, carbon dioxide sequestration, gas separation, desalination, etc. Although significant works have been undertaken to investigate the effects of additives on gas hydrate formation, there still remains a substantial gap in the understanding of the fundamentals behind the experimental observations. This thesis aims to provide new molecular insights into the effects of surfactants, hydrophobic solid surfaces and sodium halides on the formation of GHs. These effects are studied at the molecular level using synergic combinations of experimental and computational techniques. Kinetics experiments using a high-pressure reactor are carried out to quantify the effect of additives on the kinetics of gas enclathration. A see-through reactor is used for in situ visual observations of the effects of the hydrophobicity of solid surfaces on the formability of GHs. Interface-susceptible sum frequency generation (SFG) vibrational spectroscopy is employed to analyse water structure at gas-solution interfaces. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) is employed to analyse water structure in the bulk solution. Molecular dynamics (MD) simulation is used to calculate surface adsorption of surfactants, concentration profiles, interfacial water alignment, water ordering and local concentration of gases. The results from kinetics measurements are correlated with the spectroscopic and simulation data to unravel the molecular mechanisms underlying the experimental observations. The key findings of this thesis are as follows. First, surfactants display an extraordinary inhibition of gas hydrate formation at ultralow concentrations (e.g., 0.3 mM for sodium dodecyl sulfate and 3.6 mM for tetra-butyl ammonium bromide) beside the well-known promotion effect in high concentration regime. The analysis of water structure based on SFG and MD results reveals a strong alignment of water underneath surface adsorption of surfactants which gives rise to the extraordinary inhibition effect. The perturbation of water structure in the bulk by bulk-residing surfactants gives rise to the promoted gas hydrate formation in concentrated surfactants solutions. Secondly, when one hydrophobic solid surface and one hydrophilic solid surface are placed simultaneously in a see-through reactor, in situ visual observations show a preferential formation of CO2 hydrate at a hydrophobic solid surface. The analysis of MD results reveals an interfacial gas enrichment and a structured water network both happening at the hydrophobic solid surface, in contrast with an interfacial gas depletion and a disrupted water network happening at the hydrophilic solid surface. This finding uncovers the origin of the promoted hydrate formation in the presence of hydrophobic solid surfaces (including dry water) to be the interfacial gas enrichment and the structured water network at the hydrophobic solid-water interface. Finally, it is very interesting that sodium halides can act as hydrate promoters when they are used at low concentrations (50 ─ 70 mM). In high-concentration regime, the salts become effective hydrate inhibitors as expected. The analysis of this result using the concept of ion-specific effect points out that such anomalous promotion effect originates from the hydrophobic nature of large-size and low-charge-density halide ions (e.g., I⁻ and Br⁻). All of these findings ultimately infer that the formation of GHs is actually governed by the additives-induced change in local water structure, especially water structure at the interfaces. The additives-induced change in local water structure, in turn, is governed by the hydrophobic effect. From this new perspective, this thesis develops a hypothesis of hydrophobic effect on gas hydrate formation in the presence of additives, which serves as a universal explanation for the effects of additives on gas hydrate formation. In summary, this thesis sheds new molecular insights into the effects of interfacial water alignment, hydrophobic hydration, hydrophobic interaction and ions-specificity on the formation of GHs. It provides steps towards mastering hydrate-based processes in many industrial and environmental applications.