Effects of Water Concentration on the Free Volume of Amino Acid Ionic Liquids Investigated by Molecular Dynamics Simulations

Amino acid ionic liquids (AAILs) are gaining attention because of their potential in CO₂ capture technology. Molecular dynamics simulations of AAILs tetra­methyl­ammonium glycinate ([N₁₁₁₁]­[Gly]), tetra­butyl­ammonium glycinate ([N₄₄₄₄]­[Gly]), and 1,1,1-tri­methyl­hydrazinium glycinate ([aN₁₁₁]­[Gly]) and their corresponding mixtures with water were performed to investigate the effect of water concentration on the cation–anion interactions. The water content significantly influenced the free volume (FV) and fractional free volume (FFV) of the AAILs that varied with the hydrophobic and hydrophilic nature of the ion pairs. Under dry conditions, the FFV increased with increasing cation molecular sizes, indicative of proportional adsorption of any inert gases, such as N₂, as consistent with experimental observations. Furthermore, the polarity of the cation played an important role in FFV and hence the diffusion of the AAILs. Density functional theory calculations suggested that hydrophilic [aN₁₁₁]­[Gly] featured stronger interactions in the presence of water, whereas the hydrophobic IL showed weaker interactions. The carboxylate group of glycinate displayed stronger interactions with water than the cation. The computational study provided qualitative insight into the role of FV of the AAILs on CO₂ and N₂ absorption and suggests that [aN₁₁₁]­[Gly] has CO₂ adsorption capacity in the presence of water superior to that of other studied AAILs

Quantum Chemical Molecular Dynamics Study of the Water–Gas Shift Reaction on a Pd/MgO(100) Catalyst Surface

The water–gas shift (WGS) reaction on a Pd/MgO(100) catalyst surface was studied using the tight binding-quantum chemical molecular dynamics (TB-QCMD) method. Molecular adsorption of CO was observed. In contrast, we observed that H₂O adsorption occurs first molecularly but the molecule then dissociates on the surface. The resultant hydroxyl group reacts with preadsorbed CO to form an OCOH intermediate and a single H atom. This process is relevant as the initial hydroxylation step, and it is part of the catalyzed hydrolysis mechanism. During the molecular dynamics simulation the OCOH intermediate inverted into an H–CO₂ like molecule and finally HCO₂ decomposed to CO₂ and H. Later on, the resultant H interacts with the previously dissociated single H atom (H released from the H–OH dissociation) and forms the WGS product H–H molecule. It was observed that the CO₂ desorbed from the supported Pd cluster while the H₂ molecule remains attached to the Pd cluster during the simulation. The geometries and dissociation energies of water molecules were obtained and the type of adsorption assessed. Chemical changes, changes in electronic and adsorption states, and structural changes were also investigated through TB-QCMD calculations, which indicate that the metal-oxide interface plays an essential role in the catalysis, helping in the dissociation of water and the formation of the OCOH intermediate. The present study indicates that the MgO(100) support has a strong interaction with the Pd catalyst, which may cause an increase in Pd activity as well as enhancement of the metal catalyst dispersion, hence, increasing the rate of the WGS reaction. Furthermore, from the molecular dynamics and electronic structure calculations, we have identified a number of consequences for the interpretation and modeling of the WGS reaction

Adsorption of Bovine Serum Albumin on Poly(vinylidene fluoride) Surfaces in the Presence of Ions: A Molecular Dynamics Simulation

Adsorption of bovine serum albumin (BSA) on poly­(vinylidene fluoride) (PVDF) surfaces in an aqueous environment was investigated in the presence and absence of excess ions using molecular dynamics simulations. The adsorption process involved diffusion of protein to the surface and dehydration of surface–protein interactions, followed by adsorption and denaturation. Although adsorption of BSA on PVDF surface was observed in the absence of excess ions, denaturation of BSA was not observed during the simulation (1 μs). Basic and acidic amino acids of BSA were found to be directly interacting with PVDF surface. Simulation in a 0.1 M NaCl solution showed delayed adsorption of BSA on PVDF surfaces in the presence of excess ions, with BSA not observed in close proximity to PVDF surface within 700 ns. Adsorption of Cl^– on PVDF surface increased its negative charge, which repelled negatively charged BSA, thereby delaying the adsorption process. These results will be helpful for understanding membrane fouling phenomena in polymeric membranes, and fundamental advancements in these areas will lead to a new generation of membrane materials with improved antifouling properties and reduced energy demands