Electrolyte under Molybdenum Disulfide Surfaces: Molecular Insights on Structure and Dynamics of Water.

Molybdenum disulfide (MoS2) is a two-dimensional (2D) material that offers molecular transport and sieving properties and might be a potential candidate for membrane technologies for energy and environmental applications. To facilitate the separation application, understanding the structural and dynamic properties of water near the substrate-aqueous solution is essential. Employing the molecular dynamics simulation, we investigate the density, local water network at the solid-liquid interface, and water dynamics in aqueous electrolyte solutions with various chloride salts confined in MoS2 nanochannels with different pore sizes and electrolyte concentrations. Our simulation results confirm that the layering of interfacial water at the hydrophilic MoS2 surface and the water density variation depends on the nature of the ions. The simulation results imply a strong attraction of cations to the surface-liquid interfaces, whereas anions are expelled from the surface due to electrostatic interaction. An examination of the dynamical property of water reveals that the confinement effect is more pronounced on water mobility when the pore width is less than 3 nm, and the salt concentration is below 1 M, whereas the electrolyte concentration greater than 1 M, ions predominantly drive the water mobility as compared to confinement one. These simulation results enhance experimental observations and provide molecular insights into the local ordering mechanism that can be crucial in controlling water dynamics in nanofiltration applications.

Rational design for novel heterocyclic based Donepezil analogs for Alzheimer's disease: an in silico approach.

Alzheimer's disease (AD) is a progressive neurodegenerative disease and has devastating impacts on the elderly population. During the last two decades, there has been a significant focus on developing effective and safe treatments for AD. Acetylcholinesterase (AChE) has been identified as one of the primary therapeutic targets for developing drug candidates for AD. However, there is still a need for more efficient therapies. In this study, our aim is to design a new series of heterocyclic-based AChE inhibitors inspired by a standard drug. Here, we carried out molecular docking, drug-likeliness characteristics, and molecular dynamics (MD) to predict important pharmacophore features and understand the inhibitory mechanism of the designed inhibitors towards the AChE. We have designed 112 new derivatives by replacing the piperidine moiety of Donepezil with the different five and six-membered heterocyclic rings and selected 15 compounds that show higher or comparable docking scores as compared to standard Donepezil and pose no risk for carcinogenicity. Furthermore, MD results imply the structural stability of the selected docked complexes and seven exhibit a stronger binding affinity towards the AChE than Donepezil. Thus, heterocyclic-based derivatives based on oxazole, pyrazole, and tetrahydropyran may be potential therapeutic candidates for AD. Our structure-based drug design approach allows us to identify and gain insight into the structural stability of the inhibitor-protein complex and the inhibition mechanism of the newly designed inhibitors. The present finding might be an initial selection for developing a new inhibitor for AD and provide a direction for further experiments on its biological activities.Communicated by Ramaswamy H. Sarma.

Insight into the Structure and Dynamics of Ethanol-Water Binary Mixture Confined in Nanochannel by Mica and Graphene.

Investigation of the structural properties and dynamics of fluid mixture confined in nanochannels has become an essential topic in many fields due to potential applications in nanofluidic devices and biological systems. Here, we study the ethanol-water blend confined between the mica and single or multilayer graphene for different slit pore widths, ethanol content, and temperatures. Our molecular dynamics simulation indicates that water molecules are adsorbed at the mica surface, while ethanol molecules prefer to be adsorbed near the graphene surface. We find that distinct layers of ethanol molecules form as the channel width and ethanol content in the mixture are increased. The diffusion of confined ethanol and water molecules depends on the nanopore widths, concentrations, and temperatures. Interestingly, at nanopore widths of 1.0 and 1.3 nm, the mobility of confined ethanol molecules is greater than that of water molecules for all ethanol concentrations. In contrast, at pore width of 0.7 nm, the opposite behavior is observed at lower concentrations of ethanol (xEtOH = 0.1 and 0.3) in the mixture. Furthermore, the diffusivity of ethanol and water in the mixtures increases with increasing the temperatures. The hydrogen bond and cluster analysis imply the segregation of water molecules near the mica surface, while ethanol molecules are near the opposite pore wall (graphene).

Effects of interfaces on structure and dynamics of water droplets on a graphene surface: A molecular dynamics study.

The structure and dynamics of water droplets on a bilayer graphene surface are investigated using molecular dynamics simulations. The effects of solid/water and air/water interfaces on the local structure of water droplets are analyzed in terms of the hydrogen bond distribution and tetrahedral order parameter. It is found that the local structure in the core region of a water droplet is similar to that in liquid water. On the other hand, the local structure of water molecules at the solid/water and air/water interfaces, referred to as the interface and surface regions, respectively, consists mainly of three-coordinated molecules that are greatly distorted from a tetrahedral structure. This study reveals that the dynamics in different regions of the water droplets affects the intermolecular vibrational density of states: It is found that in the surface and interface regions, the intensity of vibrational density of states at ∼50 cm-1 is enhanced, whereas those at ∼200 and ∼500 cm-1 are weakened and redshifted. These changes are attributed to the increase in the number of molecules having fewer hydrogen bonds in the interface and surface regions. Both single-molecule and collective orientation relaxations are also examined. Single-molecule orientation relaxation is found to be marginally slower than that in liquid water. On the other hand, the collective orientation relaxation of water droplets is found to be significantly faster than that of liquid water because of the destructive correlation of dipole moments in the droplets. The negative correlation between distinct dipole moments also yields a blueshifted libration peak in the absorption spectrum. It is also found that the water-graphene interaction affects the structure and dynamics of the water droplets, such as the local water structure, collective orientation relaxation, and the correlation between dipole moments. This study reveals that the water/solid and water/air interfaces strongly affect the structure and intermolecular dynamics of water droplets and suggests that the intermolecular dynamics, such as energy relaxation dynamics, in other systems with interfaces are different from those in liquid water.

Is Ice Nucleation by Organic Crystals Nonclassical? An Assessment of the Monolayer Hypothesis of Ice Nucleation.

Potent ice nucleating organic crystals display an increase in nucleation efficiency with pressure and memory effect after pressurization that set them apart from inorganic nucleants. These characteristics were proposed to arise from an ordered water monolayer at the organic-water interface. It was interpreted that ordering of the monolayer is the limiting step for ice nucleation on organic crystals, rendering their mechanism of nucleation nonclassical. Despite the importance of organics in atmospheric ice nucleation, that explanation has never been investigated. Here we elucidate the structure of interfacial water and its role in ice nucleation at ambient pressure on phloroglucinol dihydrate, the paradigmatic example of outstanding ice nucleating organic crystal, using molecular simulations. The simulations confirm the existence of an interfacial monolayer that orders on cooling and becomes fully ordered upon ice formation. The monolayer does not resemble any ice face but seamlessly connects the distinct hydrogen-bonding orders of ice and the organic surface. Although large ordered patches develop in the monolayer before ice nucleates, we find that the critical step is the formation of the ice crystallite, indicating that the mechanism is classical. We predict that the fully ordered, crystalline monolayer nucleates ice above -2 °C and could be responsible for the exceptional ice nucleation by the organic crystal at high pressures. The lifetime of the fully ordered monolayer around 0 °C, however, is too short to account for the memory effect reported in the experiments. The latter could arise from an increase in the melting temperature of ice confined by strongly ice-binding surfaces.

Computationally efficient approach for the identification of ice-binding surfaces and how they bind ice.

Recognition and binding of ice by proteins, crystals, and other surfaces is key for their control of the nucleation and growth of ice. Docking is the state-of-the-art computational method to identify ice-binding surfaces (IBS). However, docking methods require a priori knowledge of the ice plane to which the molecules bind and either neglect the competition of ice and water for the IBS or are computationally expensive. Here we present and validate a robust methodology for the identification of the IBS of molecules and crystals that is easy to implement and a hundred times computationally more efficient than the most advanced ice-docking approaches. The methodology is based on biased sampling with an order parameter that drives the formation of ice. We validate the method using all-atom and coarse-grained models of organic crystals and proteins. To our knowledge, this approach is the first to simultaneously identify the ice-binding surface as well as the plane of ice to which it binds, without the use of structure search algorithms. We show that biased simulations even identify surfaces that are too small or too weak to heterogeneously nucleate ice. The biasing simulations can be used to identify of IBS of antifreeze and ice nucleating proteins and to equilibrate ice seeds bound to an IBS for the calculation of heterogeneous ice nucleation rates using classical nucleation theory.

Ice nucleation on nanotextured surfaces: the influence of surface fraction, pillar height and wetting states.

In this work, we address the nucleation behavior of a supercooled monatomic cylindrical water droplet on nanoscale textured surfaces using molecular dynamics simulations. The ice nucleation rate at 203 K on graphite based textured surfaces with nanoscale roughness is evaluated using the mean fast-passage time method. The simulation results show that the nucleation rate depends on the surface fraction as well as the wetting states. The nucleation rate enhances with increasing surface fraction for water in the Cassie-Baxter state, while contrary behavior is observed for the case of Wenzel state. Based on the spatial histogram distribution of ice formation, we observed two pathways for ice nucleation. Heterogeneous nucleation is observed at a high surface fraction. However, the probability of homogeneous ice nucleation events increases with decreasing surface fraction. We further investigate the role of the nanopillar height in ice nucleation. The nucleation rate is enhanced with increasing nanopillar height. This is attributed to the enhanced contact area with increasing nanopillar height and the shift in nucleation events towards the three-phase contact line associated with the nanotextured surface. The ice-surface work of adhesion for the Wenzel state is found to be 1-2 times higher than that in the Cassie-Baxter state. Furthermore, the work of adhesion of ice in the Wenzel state is found to be linearly dependent on the contour length of the droplet, which is in line with that reported for liquid droplets.

Surface effect on the electromelting behavior of nanoconfined water.

Electric field induced phase transitions of confined water have an important role in cryopreservation and electrocrystallization. In this study, the structural and dynamical properties of nano-confined water in nano-slit pores under the influence of an electric field varying from 0 to 10 V nm(-1) are investigated under ambient conditions using molecular dynamics simulations. In order to replicate the nature of different materials, a systematic approach is adopted, including pore-size and lattice constant variations in different lattice arrangements viz., triangular, square and hexagonal, with hydrophilic and hydrophobic surface-fluid interactions. The structural behavior of water is investigated using radial distribution functions, bond order parameters and hydrogen bond calculations; the dynamical properties are analyzed using lateral and rotational diffusivity calculations. The lateral diffusivity with increasing electric field E increases by order(s) of magnitude during electromelting. The pore-size, lattice constant, lattice arrangement and hydrophobic/hydrophilic nature of the pore surface strongly influence the electromelting behavior for E≤∼7 V nm(-1). Higher values of lattice constants and/or hydrophobic pores enhance the electromelting behavior of nanoconfined water.