Abstract
Surfactants and/or salts at low concentrations are inherently relevant to gas hydrate formation in nature and industry. However, the combined effects of surfactants and salts at low concentrations on gas hydrate formation are poorly understood. Here, we aim to fill this gap of knowledge by examining the effects of sodium dodecyl sulfate (SDS) and sodium iodide (NaI) at millimolar concentrations on methane hydrate formation. We show that adding NaI to dilute SDS solutions reduces the induction time significantly, promoting gas hydrate nucleation. Meanwhile, adding NaI adversely reduces the growth of methane hydrate. Fundamental studies based on sum frequency generation spectroscopy indicate an electrostatically aligned water layer at the gas–SDS solution interface caused by the charged interface as a result of surface adsorption of dodecyl sulfate anions. This aligned interfacial water layer hinders the nucleation of gas hydrate at the gas–SDS solution interface. Added NaI diminishes the alignment of interfacial water, thereby favoring the nucleation of the gas hydrate. Even though, the adverse (inhibiting) effect of added NaI on the methane hydrate growth remains a puzzling observation that requires further investigations. This study sheds more light on gas hydrate formation in surfactant- and salt-containing systems that are important to many phenomena in nature and applications in the industry.
Highlights
Gas hydrates are crystalline solids composed of water and hydrophobic gas molecules
It was followed by a hydrate growth period, which was featured by a sudden rise of the temperature coupling with a steeper drop of pressure
These results demonstrate an extraordinary synergy between NaI at low concentrations and Sodium dodecyl sulfate (SDS) in promoting methane hydrate nucleation
Summary
Gas hydrates are crystalline solids composed of water and hydrophobic (nonpolar) gas molecules. Adding SDS at concentrations above the critical micellization concentration (CMC) increased the rate of ethane hydrate formation by a factor greater than 700.28 inorganic salts are well-known hydrate formation inhibitors. It is believed that surfactant micelles are responsible for increasing hydrate formation kinetics because they facilitate gas dissolution and act as mini gas reservoirs (i.e., storing gas molecules in the interior micellar core) in the solution.[28] In contrast, inorganic salts inhibit gas hydrate formation by reducing water reactivity and radically perturbing the tetrahedral network of water.[29,30] With their strong electrostatic interactions with water molecules, inorganic ions advantageously compete with the dissolved gas molecules for water, reducing the amount of water available for forming gas hydrates
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