Molecular Dynamics Simulation to Evaluate the Stability of Tetra-n-butyl Ammonium/Phosphonium Bromide Semiclathrate Hydrates in the Presence and Absence of Methane, Carbon Dioxide, Methanol, and Ethanol Molecules

Abstract

Stability of tetra-n-butyl ammonium bromide (TBAB) and tetra-n-butyl phosphonium bromide (TBPB) semiclathrate hydrates with a hydration number of 38 is studied by the molecular dynamics (MD) simulation in the absence and presence of methane, carbon dioxide, methanol, and ethanol molecules. All of the simulation runs are performed under NVT (constant number of atoms, volume, and temperature) and NPT (constant number of atoms, pressure, and temperature) conditions using optimized potentials for liquid simulations-all-atom (OPLS-AA) force field with nonpolarizable water models (including SPC, TIP3P, TIP4P, and TIP4P/Ice), where the operating conditions include a temperature range of 250−350 K and pressures of 0.1 and 50 MPa. The potential energy and structural analysis, mean square displacement (MSD), diffusion coefficient, radial distribution function (RDF), lattice parameter, and the average number of hydrogen bonds between water−water molecules are evaluated by employing MD strategy. According to the potential energy results, the order for the water models in both TBAB and TBPB hydrate stabilities is as follows: TIP4P/Ice > TIP4P ≈ SPC > TIP3P. This order agrees well with the previous studies of gas hydrates and ices. In the absence and presence of methane and carbon dioxide guest molecules, the height of the peaks in the RDF of oxygen−oxygen atoms decreases with lowering pressure and increasing temperature; greater MSD, diffusion coefficient, and lattice parameter values are also achieved. Thus, in the absence and/or presence of guest gas molecules, the stability of TBAB and TBPB hydrate cages decreases with increasing temperature and reducing pressure. A good match is noticed between the results of MD simulation and previous experimental and theoretical studies, confirming the accuracy and validation of the simulation method. Reduction of the hydrogen bond balance between water molecules and formation of new hydrogen bonds between the water and the hydroxyl groups of methanol and/or ethanol molecules in the cages indicate that methanol and ethanol molecules have an inhibitory effect on TBAB and TBPB hydrates. Due to different hydrophilic versus hydrophobic behaviors of alcohols, the extent of disruption of hydrogen bonds between water−water molecules of TBAB and TBPB hydrates in the presence of methanol is larger than that in the presence of ethanol, which is consistent with the MD simulation results of clathrate hydrate. This study can help to further understand the semiclathrate hydrate stability or dissociation conditions at a molecular scale for better design and operation of corresponding processes.

Adsorption Efficiency of Carbon Materials for the Removal of Organic Pollutants: DDT from Aqueous Solution

“…To better understand the adsorption behavior of DDT molecules, their dynamics are examined both in terms of translational and orientational mobility. The dynamic translation is assessed through the diffusion coefficient ( D ), which is derived from the mean squared displacement (MSD) (eq ) using the Einstein relation (eq ) MSD = ( | r i − r 0 | 2 ) = 1 N ∑ i = 1 N ( | R i false( t false) − R i false( t 0 false) | 2 ) where R i ( t ) represents for the instantaneous position of atom i at time step t , R i ( t 0 ) denotes the initial position of atom i , and N represents the total number of particles. D i = lim normalΔ t → ∞ nobreak0em.25em⁡ d normald t 0.25em 1 6 | r i − r i 0 | 2 → 6 italicDt = MSD where r i ( t ) represents the position of the molecule, while D and t denote the diffusion coefficient and simulation time, respectively.…”

Further Molecular Insights into Formation of Tetra-n-Butyl Bromide Semi-Clathrate Hydrates

“…MD simulation runs are carried out using the GROMACS 5.1.4 program package to study the melting temperature, structural properties, and formation process of the TBPB and TBAB semiclathrate hydrates . In our pervious published work, the well-known SPC, TIP3P, TIP4P, and TIP4P/ice models for water and the optimized potentials for liquid simulations-all-atom (OPLS-AA) force field model set are utilized to assess the interactions between water–water, guest–guest, and water–guest particles. According to our previous findings and previous studies on gas hydrates and ices, , the TIP4P/ice water model shows the best performance in the thermodynamic theory of TBAB and TBPB hydrates.…”