[Paper Review] Investigation of Six Imidazolium-Based Ionic Liquids as Thermo-Kinetic Inhibitors for Methane Hydrate by Molecular Dynamics Simulation

Mohammad Ebrahim Haji Nasrollahebrahim, Bagher Abareshi+3 more|arXiv (Cornell University)|Jul 19, 2017

Methane Hydrates and Related PhenomenaEnvironmental Science25 references18 citations

TL;DR

This study investigates six imidazolium-based ionic liquids (ILs) as thermo-kinetic inhibitors for methane hydrate formation using classical molecular dynamics simulations. [C2OHmim][f2N] and [C3(OH)2mim][f2N] exhibit superior inhibition due to strong hydrogen bonding with water, high diffusion to hydrate surfaces, and effective hindrance of water and guest molecule adsorption, outperforming conventional inhibitors like methanol and NaCl.

ABSTRACT

The thermo-kinetic inhibition mechanism of six imidazolium-based ionic liquids (ILs) on methane clathrate hydrate formation and growth is studied in this work using classical molecular dynamics (MD) simulation. The ionic liquids investigated include 1-(2,3-dihydroxypropyl)-3-methylimidazoliumbis(fluorosulfonyl)imide ([C3(OH)2mim][f2N]), 1-(2-hydroxyethyl)-3-methylimidazolium bis(fluorosulfonyl)imide ([C2OHmim][f2N]), 1-ethyl-3-methylimidazolium tetrafluoroborate ([C2mim][BF4]), 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]), 1-butyl-3-methylimidazolium acetate ([C4mim][OAc]) and 1-ethyl-3-methylimidazolium ethylsulfate ([C2mim][EtSO4]). Simulations showed that [C2OHmim][f2N] and [C3(OH)2mim][f2N] are strongly hydrated compared to other ILs because of hydrogen bonding between OH groups of the cation and water molecules. They also exhibit high diffusion rates towards crystal surface and bond to it through strong intermolecular interactions. As a result, these two ILs are stronger thermo-kinetic inhibitors for formation and growth of methane hydrates compared to other ILs studied in this work as well as conventional inhibitors like methanol and NaCl. The simulations also revealed that cations of [C3(OH)2mim][f2N] and [C2OHmim][f2N] show that the presence of ions near the hydrate crystal causes hindrance for water and guest molecules adsorbing on the hydrate surface, which inhibits the growth of hydrate crystals. In addition, it is shown that [C3(OH)2mim][f2N] and [C2OHmim][f2N] are more likely to inhibit hydrate formation.

Motivation & Objective

• To evaluate the thermo-kinetic inhibition performance of six imidazolium-based ionic liquids on methane clathrate hydrate formation and growth.
• To understand the molecular-level mechanisms by which these ILs inhibit hydrate nucleation and crystal growth.
• To compare the inhibitory efficiency of the ILs with conventional inhibitors such as methanol and NaCl.
• To identify structural features in ILs that enhance their hydrate inhibition performance.

Proposed method

• Classical molecular dynamics (MD) simulations were employed to model the interaction between the six ionic liquids and water molecules in the presence of methane.
• The simulations tracked the diffusion of IL ions toward the hydrate surface and analyzed their adsorption behavior.
• Hydrogen bonding interactions between hydroxyl groups on IL cations and water molecules were quantified to assess hydration strength.
• The extent of water and methane guest molecule adsorption on the hydrate surface was monitored to evaluate growth inhibition.
• Ion distribution and radial distribution functions were used to analyze the spatial organization of ions near the hydrate interface.
• The results were benchmarked against conventional inhibitors like methanol and NaCl to assess relative performance.

Experimental results

Research questions

• RQ1 How do the six imidazolium-based ionic liquids affect the nucleation and growth kinetics of methane hydrates at the molecular level?
• RQ2 What role do hydroxyl groups on the cationic side chains play in enhancing hydration and inhibition efficiency?
• RQ3 How does the diffusion rate of IL ions toward the hydrate surface correlate with their inhibition performance?
• RQ4 To what extent do the ILs hinder the adsorption of water and methane molecules on the hydrate surface?
• RQ5 How do the inhibitory effects of these ILs compare quantitatively to those of methanol and NaCl?

Key findings

• [C2OHmim][f2N] and [C3(OH)2mim][f2N] exhibit significantly stronger hydration due to hydrogen bonding between their hydroxyl groups and water molecules.
• These two ILs display the highest diffusion rates toward the hydrate crystal surface among all studied ionic liquids.
• The cations of [C2OHmim][f2N] and [C3(OH)2mim][f2N] form strong intermolecular interactions with the hydrate surface, effectively blocking water and guest molecule adsorption.
• The presence of these ILs near the hydrate surface causes steric and energetic hindrance, reducing the rate of hydrate crystal growth.
• These two ILs are more effective at inhibiting hydrate formation than conventional inhibitors such as methanol and NaCl.
• The simulation results indicate that hydroxyl-functionalized imidazolium cations are key structural features for superior thermo-kinetic inhibition performance.