Akshay Malik

Lithium ion batteries are important in a number of applications, and the demand for Lithium is expected to increase. In typical industrial Li⁺ extraction, brine is pumped to the surface of large reservoirs, followed by chemical treatment. This method requires large quantities of water and can pollute groundwater. There has been considerable interest in developing membranes that can be used for preferential Lithium recovery. It has been suggested that para-toluene sulfonate (PTS) can preferentially bind Li⁺ over other cations. In this work, we study the physical chemistry of Li⁺, Na⁺, and K⁺ binding to solutes in an aqueous solution via combined classical and ab initio molecular dynamics simulations. The solutes we consider are PTS, a trimer of PTS, and 4-(trifluoromethyl)benzenesulfonate (TBS). From the analysis of radial distribution functions and the potential of mean force (PMF), we find that the sulfonate group in the cations has the strongest spatial correlation with Li⁺ followed by Na⁺ and then K⁺. The magnitude of PMF, however, is only of the order of the thermal energy. The correlation between the sulfonate group and Li⁺ can be enhanced by polymerizing the PTS molecule, providing a possible means of extracting Li⁺ from the aqueous phase. For PTS, K⁺ is more strongly correlated with the carbon atoms in the benzene ring than the sulfonate group. This might provide a resolution to the puzzling difference between the behavior of sulfonated polystyrene polymers in the presence of K⁺ and Li⁺ counterions.

Rising atmospheric concentrations of anthropogenic hydrogen sulfide (H₂S) and carbon monoxide (CO) as a result of industrialization have encouraged researchers to explore innovative technologies for capturing these gases. Deep eutectic solvents (DESs) are an alternative media for mitigating H₂S and CO emissions. Herein, we have employed ab initio molecular dynamics simulations to investigate the structures of the nearest-neighbor solvation shells surrounding H₂S and CO when they are dissolved in reline and ethaline DESs. We aim to delineate the structural arrangement responsible for favorable H₂S and CO capture by analyzing the key interactions between H₂S and CO solutes with various components of the DESs. We observe that in the reline-H₂S system, chloride and carbonyl oxygen of urea are found to have the closest distance interaction with hydrogen atoms of the H₂S solute. The sulfur atom of H₂S is found to be predominantly solvated by hydrogen and oxygen atoms of urea molecules and the hydroxyl hydrogen of choline cations. The chloride ions and ethylene glycol molecules predominantly govern the solvation of H₂S in the ethaline-H₂S system. In both the DESs, H₂S is solvated by the hydroxyl group of the choline cations rather than by their ammonium group. In the reline-CO system, all the atoms of urea along with chloride dominate the immediate solvation shell around CO. In the ethaline-CO system, hydroxyl oxygen and hydrogen atoms of ethylene glycol are found in the nearest solvation structure around CO. Both the DESs exhibit a stronger solvent-solute charge-transfer tendency toward the H₂S solute compared to CO.

Structural and dynamic heterogeneity in thymol–coumarin-based hydrophobic deep eutectic solvents.

Because of the rising concentration of harmful greenhouse gases like methane in the atmosphere, researchers are striving for developing novel techniques for capturing these gases. Recently, neoteric liquids such as deep eutectic solvents (DESs) have emerged as an efficient means of sequestration of methane. Herein, we have performed ab initio molecular dynamics (AIMD) simulations to elucidate the solvation structure around a methane molecule dissolved in reline and ethaline DESs. We aim to elicit the structural organization of different constituents of the DESs in the vicinity of methane, particularly highlighting the key interactions that stabilize such gases in DESs. We observe quite different solvation structures of methane in the two DESs. In ethaline, chloride ions play an active role in solvating methane. Instead, in reline, chloride ions do not interact much with the methane molecule in the first solvation shell. In reline, choline cations approach the methane molecule from their hydroxyl group side, whereas urea molecules approach methane from their carbonyl oxygen as well as amide group sides. In ethaline, ethylene glycol and Cl- dominate the nearest neighbor solvation structure around the methane molecule. In both the DESs, we do not observe any significant methane-DES charge transfer interactions, apart from what is present between choline cation and Cl- anion.

Recent development of novel water‐immiscible green solvents known as hydrophobic deep eutectic solvents (HDESs) has opened the gates for applications requiring media where the presence of water is undesirable. Ever since they were prepared, researchers have used HDESs in diverse fields such as extraction processes, CO₂ sequestration, membrane formation, and catalysis. The structure and dynamics associated with the species comprising HDESs guide their suitability for specific applications. For example, varying the alkyl tail length of the HDES components significantly affects the dynamics of the components and thus helps in tuning the efficiency of extraction processes. However, the development of HDESs is still in infancy, and very few theoretical studies are available in the literature that help in understanding the structure and dynamics of HDESs. This review highlights the recent studies focused on the microscopic structure and dynamics of HDESs and their potential applications, particularly in extraction processes. We have also provided a glimpse of how the integration of experiments and computational techniques can help delineate the mechanism of extraction processes.

Hydrophobic deep eutectic solvents (HDESs) have gained immense popularity because of their promising applications in extraction processes. Herein, we employ atomistic molecular dynamics simulations to unveil the dynamics of DL-menthol (DLM) based HDESs with hexanoic (C6), octanoic (C8), and decanoic (C10) acids as hydrogen bond donors. The particular focus is on understanding the nature of dynamics with changing acid tail length. For all three HDESs, two modes of hydrogen bond relaxations are observed. We observe longer hydrogen bond lifetimes of the inter-molecular hydrogen bonding interactions between the carbonyl oxygen of the acid and hydroxyl oxygen of menthol with hydroxyl hydrogen of both acids and menthol. We infer strong hydrogen bonding between them compared to that between hydroxyl oxygen of acids and hydroxyl hydrogens of menthol and acids, marked by a faster decay rate and shorter hydrogen bond lifetime. The translational dynamics of the species in the HDES becomes slower with increasing tail length of the organic acid. Slightly enhanced caging is also observed for the HDES with a longer tail length of the acids. The evidence of dynamic heterogeneity in the displacements of the component molecules is observed in all the HDESs. From the values of the α-relaxation time scale, we observe that the molecular displacements become random in a shorter time scale for DLM-C6. The analysis of the self-van Hove function reveals that the overall distance covered by DLM and acid molecules in the respective HDES is more than what is expected from ideal diffusion. As marked by the shorter time scale associated with hole filling, the diffusion of the oxygen atom of menthol and the carbonyl oxygen of acid from one site to the other is fastest for hexanoic acid containing HDES.

Deep eutectic solvents (DESs) are emerging as an alternative media for the sequestration of greenhouse gases such as CO₂ and SO₂. Herein, we performed ab initio molecular dynamics (AIMD) simulations to elucidate the solvation structure around CO₂ and SO₂ in choline chloride-based DESs, namely, reline and ethaline. We show that in all four systems the structures of the nearest neighbor shells around these molecules are distinct. We observe that because of the electrophilic character, the carbon atom of CO₂ and the sulfur atom of SO₂ are preferentially solvated by the chloride anions. The strength of the correlation between the chloride anions and the sulfur atom is much stronger because of charge transfer, which is more profound in ethaline DES. In both DESs, the choline cations are found to be closer to the oxygen atoms of CO₂ and SO₂. We observe that upon changing the solute from CO₂ to SO₂, the nearest neighbor solvation structure changes drastically; while the chloride anions prefer to stay in a circular shell around the carbon atom of CO₂, they are found to be much more localized near the sulfur atom of SO₂. The solvation shells formed by the urea molecules in reline and EG molecules in ethaline also overlap with that of the chloride anion around CO₂. In ethaline, the hydroxyl group of the choline cation is found to be closer to the solute molecules as compared to its ammonium headgroup.

Recently, the introduction of novel deep eutectic solvents having a hydrophobic character with greener starting materials has motivated researchers to explore these water-immiscible solvents, owing to their new potential applications. In this regard, herein atomistic molecular dynamics simulations have been performed to understand the bulk phase morphology present in DL-menthol based hydrophobic deep eutectic solvents (HDESs) with organic acids of different chain lengths used as hydrogen bond donors. From the appearance of a prepeak in the simulated total X-ray scattering structure function (S(q)), we found an evidence of intermediate-range structural organization in these HDESs. We show that the prepeak originates from the self-segregation of DL-menthol and the polar hydroxyl groups of the acids, as witnessed from partitioning of the total S(q). Surprisingly, even for a very long tail containing HDES, the apolar–apolar component shows only nominal contribution to the prepeak. We show that the structure of these HDESs is dominated by a set of very strong intermolecular hydrogen bonding between menthol–menthol, acid–acid, and menthol–acid molecules. Our results show that there is competition between the hydroxyl hydrogen of the acids and menthol to form an intermolecular hydrogen bond with the carbonyl oxygen of the acids and hydroxyl oxygen of menthol.

Deep eutectic solvents (DESs) have emerged as a promising class of solvents for application in nanotechnology, particularly for designing new functional nanomaterials based on carbon. Here, we have employed molecular dynamics simulations to understand the structuring of choline chloride and urea-based DES, reline, nanodroplets on carbon sheets with varying strength of the DES–sheet interaction potentials. The wetting–dewetting nature of reline has been investigated by analyzing simulated contact angles formed by its nanodroplets on the carbon sheets. Through this investigation, we find that at the lowest DES–sheet interaction strength, the contact angle formed by the reline nanodroplet on the carbon surface exceeds 150°, indicating that the surface is supersolvophobic. On the other hand, at the higher interaction potentials, reline DES wets the surface of the sheets, forming an adlayer primarily consisting of urea molecules. The choline cation and urea molecules are observed to exhibit stronger interactions with the carbon surface as compared to that of chloride anions. At the supersolvophobic carbon surface, the urea molecules have relatively higher density in the bulk of the nanodroplet, whereas the choline cation and chloride have major contributions to the outer layers of the droplets. Moreover, at the solvophilic surfaces, urea molecules are present in the adlayer, as well as in the bulk of the droplets, whereas the reline–vapor interface majorly consists of choline and chloride ions.

Atomistic molecular dynamics simulations have been performed to investigate the microscopic structure of ethaline deep eutectic solvent (DES), a mixture of choline chloride ([Ch][Cl]) and ethylene glycol (EG) in molar ratio of 1:2, respectively. As much as the structure of a DES is derived by the composition of the species present in it, the arrangement of hydrogen bond donor species also plays a crucial role in laying down the microscopic structure of DESs. By virtue of its inherent chemical structure, EG renders both intra- and inter-molecular hydrogen bonds. Therefore, the molecular level structural landscape of DESs containing EG as hydrogen bond donor is reckoned to be a bit complex. In the present study, we aim to understand the structural morphology of ethaline using optimum force-field parameters for EG recently proposed by our group. After initial assessment of the refined force-field parameters for ethaline DES, we have presented an in-depth analysis of the arrangement and ordering of its components at molecular level. Simulated X-ray scattering structure function and its partial components reveal the presence of short as well as long range interactions in ethaline. The role of hydrogen bonding interactions between all the three species [Ch]^{+}, [Cl]^{-} and EG were predominantly observed through radial and radial-angular distribution functions and substantiated by spatial distribution functions. The observation of competitive nature of [Ch]^{+} and EG to form hydrogen bond with the anion is one of the major outcomes of the present study. Also, weaker intra- and inter-molecular hydrogen bonding interactions between EG molecules were seen along with their simultaneous involvement with the ammonium group of choline cation.