Srinivasa Rao Varanasi

Relation between the diffusivity, viscosity, and ionic radius of LiCl in water, methanol, and ethylene glycol: a molecular dynamics simulation.

A molecular dynamics (MD) investigation of LiCl in water, methanol, and ethylene glycol (EG) at 298 K is reported. Several structural and dynamical properties of the ions as well as the solvent such as self-diffusivity, radial distribution functions, void and neck distributions, velocity autocorrelation functions, and mean residence times of solvent in the first solvation shell have been computed. The results show that the reciprocal relationship between the self-diffusivity of the ions and the viscosity is valid in almost all solvents with the exception of water. From an analysis of radial distribution functions and coordination numbers the nature of hydrogen bonding within the solvent and its influence on the void and neck distribution becomes evident. It is seen that the solvent−solvent interaction is important in EG while solute−solvent interactions dominate in water and methanol. From Voronoi tessellation, it is seen that the voids and necks within methanol are larger as compared to those within water or EG. On the basis of the void and neck distributions obtained from MD simulations and literature experimental data of limiting ion conductivity for various ions of different sizes, we show that there is a relation between the void and neck radius on the one hand and dependence of conductivity on the ionic radius on the other. It is shown that the presence of large diameter voids and necks in methanol is responsible for maximum in limiting ion conductivity (λ0) of TMA+, while in water and EG, the maximum is seen for Rb+. In the case of monovalent anions, maximum in λ0 as a function ionic radius is seen for Br− in water and EG but for the larger ClO4 − ion in methanol. The relation between the void and neck distribution and the variation in λ0 with ionic radius arises via the Levitation effect which is discussed. These studies show the importance of the solvent structure and the associated void structure.

Porphyrin–graphene oxide frameworks for long life sodium ion batteries

Herein, we demonstrate that a porphyrin interspersed graphene-oxide framework with a d-spacing of ∼7.67 A can significantly enhance the cycling stability of graphene-based anodes in sodium-ion batteries. These robust electrodes can deliver a reversible capacity of ∼200 mA h g−1 at a current density of 100 mA g−1 in the 20th cycle with negligible capacity fading over 700 cycles. In addition to the superior rate tolerance, the specific capacity was stable even after a resting time of one month. The excellent performance may be nested in the larger interlayer spacing, and rich nitrogen content along with the defect sites available for sodium interaction. Experimental studies and density functional theory calculations presented in this work give insights into the structure–property relationship of porphyrin–graphene oxide frameworks and their electrochemical performance.

Dependence of diffusivity on density and solute diameter in liquid phase: A molecular dynamics study of Lennard-Jones system

Investigations into the variation of self-diffusivity with solute radius, density, and degree of disorder of the host medium is explored. The system consists of a binary mixture of a relatively smaller sized solute, whose size is varied and a larger sized solvent interacting via Lennard-Jones potential. Calculations have been performed at three different reduced densities of 0.7, 0.8, and 0.933. These simulations show that diffusivity exhibits a maximum for some intermediate size of the solute when the solute diameter is varied. The maximum is found at the same size of the solute at all densities which is at variance with the prediction of the levitation effect. In order to understand this anomaly, additional simulations were carried out in which the degree of disorder has been varied while keeping the density constant. The results show that the diffusivity maximum gradually disappears with increase in disorder. Disorder has been characterized by means of the minimal spanning tree. Simulations have also been carried out in which the degree of disorder is constant and only the density is altered. The results from these simulations show that the maximum in diffusivity now shifts to larger distances with decrease in density. This is in agreement with the changes in void and neck distribution with density of the host medium. These results are in excellent agreement with the predictions of the levitation effect. They suggest that the effect of disorder is to shift the maximum in diffusivity towards smaller solute radius while that of the decrease in density is to shift it towards larger solute radius. Thus, in real systems where the degree of disorder is lower at higher density and vice versa, the effect due to density and disorder have opposing influences. These are confirmed by the changes seen in the velocity autocorrelation function, self part of the intermediate scattering function and activation energy.

Effect of pressure on the ionic conductivity of Li+ and Cl- ions in water.

A molecular dynamics simulation study of aqueous solution of LiCl is reported as a function of pressure. Experimental measurements of conductivity of Li(+) ion as a function of pressure shows an increase in conductivity with pressure. Our simulations are able to reproduce the observed trend in conductivity. A number of relevant properties have been computed in order to understand the reasons for the increase in conductivity with pressure. These include radial distribution function, void and neck distributions, hydration or coordination numbers, diffusivity, velocity autocorrelation functions, angles between ion-oxygen and dipole of water as well as OH vector, mean residence time for water in the hydration shell, etc. These show that the increase in pressure acts as a structure breaker. The decay of the self part of the intermediate scattering function at small wave number k shows a bi-exponential decay at 1 bar which changes to single exponential decay at higher pressures. The k dependence of the ratio of the self part of the full width at half maximum of the dynamic structure factor to 2Dk(2) exhibits trends which suggest that the void structure of water is playing a role. These support the view that the changes in void and neck distributions in water can account for changes in conductivity or diffusivity of Li(+) with pressure. These results can be understood in terms of the levitation effect.

High Interfacial Barriers at Narrow Carbon Nanotube–Water Interfaces

Water displays anomalous fast diffusion in narrow carbon nanotubes (CNTs), a behavior that has been reproduced in both experimental and simulation studies. However, little is reported on the effect of bulk water-CNT interfaces, which is critical to exploiting the fast transport of water across narrow carbon nanotubes in actual applications. Using molecular dynamics simulations, we investigate here the effect of such interfaces on the transport of water across arm-chair CNTs of different diameters. Our results demonstrate that diffusion of water is significantly retarded in narrow CNTs due to bulk regions near the pore entrance. The slowdown of dynamics can be attributed to the presence of large energy barriers at bulk water-CNT interfaces. The presence of such intense barriers at the bulk-CNT interface arises due to the entropy contrast between the bulk and confined regions, with water molecules undergoing high translational and rotational entropy gain on entering from the bulk to the CNT interior. The intensity of such energy barriers decreases with increase in CNT diameter. These results are very important for emerging technological applications of CNTs and other nanoscale materials, such as in nanofluidics, water purification, nanofiltration, and desalination, as well as for biological transport processes.

Variation of diffusivity with the cation radii in molten salts of superionic conductors containing iodine anion: A molecular dynamics study

AbstractA molecular dynamics study of the dependence of diffusivity of the cation on ionic radii in molten AgI is reported. We have employed modified Parinello-Rahman-Vashistha interionic pair potential proposed by Shimojo and Kobayashi.1 Our results suggest that the diffusivity of the cation exhibits an increase followed by a decrease as the ionic radius is increased. Several structural and dynamical properties are reported. Graphical AbstractDiffusivity exhibits a maximum when cation radius is varied in a molten salt containing iodide anion. Computed properties include radial distribution function, diffusivity of cation and anion, velocity autocorrelation function, activation energy and intermediate scattering function. The results suggest that the anomalous maximum arises from the Levitation Effect.

Structure and dynamics of cumene and 1,2,4-trimethylbenzene mixture in NaY zeolite: a molecular dynamics simulation study

A molecular dynamics study of 1:1 mixture of the C9 isomers, isopropyl benzene (cumene) and 1,2,4-trimethylbenzene (124TMB) (pseudocumene), in zeolite NaY is reported. Structural and dynamical properties have been computed to understand possible difficulties in the separation of these isomers. Cumene exhibits a slightly higher self-diffusivity. 124TMB encounters a larger barrier as compared with cumene at the 12-ring window during migration from one supercage to another. 124TMB has a significantly larger backscattering during rotation which may be attributed to its shape and large cross-sectional diameter as compared with cumene. Cumene has a higher rotational diffusivity. Results suggest that there is larger difference between the rotational diffusivities of the two isomers and little difference in their translational diffusivity. It may be possible to exploit this difference in separating the two isomers.