Separation of hydrocarbon mixture of neopentane and n-hexane using NaY zeolite: Large distinct diffusivity.

A recently proposed method based on Levitation and Blow torch effects, is employed here to see if it can separate a mixture of neopentane and n-hexane. The results show that the mixture can be separated with a hot zone temperature of just 40 K above the ambient temperature, 300 K. The two components are found to accumulate at the two extreme ends of the zeolite column.

Separating a linear C hydrocarbon from a branched C hydrocarbon: n-pentane from 2,2-dimethyl butane using levitation and blow torch effects.

The separation of linear from branched hydrocarbons is often required in many situations. There are several methods through which they can be separated but none provides a very high degree of purity or works without considerable expenditure of energy. Recently, a novel method was proposed to separate a mixture of neopentane and n-pentane.

High purity separation of n-pentane from neopentane using a nano-crystal of zeolite Y.

A method for the separation of a mixture of n-pentane and neopentane using a nano-crystallite of zeolite Y is reported. This method judiciously combines two well-known, counter-intuitive phenomena, the levitation and the blowtorch effects. The result is that the two components are separated by being driven to the opposite ends of the zeolite column.

Understanding fast diffusion of solutes in solid solutions: A molecular dynamics study of solutes in body centered cubic solid.

In some binary alloys, the solute exhibits high or fast diffusion with low activation energy. In order to understand this, diffusion of solute atoms through a lattice of body centered cubic solvent atoms has been investigated with molecular dynamics technique. Surprisingly, solutes exhibit two distinct diffusivity maxima.

Separating Hydrocarbon Mixtures by Driving the Components in Opposite Directions: High Degree of Separation Factor and Energy Efficiency.

A radically different approach for separation of molecular mixtures is proposed. A judicious combination of levitation effect observed in zeolites with a counter intuitive Landauer blow torch effect provides driving forces for the two components of the mixture to move in opposite directions. Using nonequilibrium Monte Carlo simulations, we illustrate the efficacy of the method for separating real mixtures of both linear n-pentane and its branched isomer, neopentane, and linear n-hexane and its branched isomer, 2,2-dimethylbutane.

Diffusion processes in a poly-crystalline zeolitic material: A molecular dynamics study.

Extensive molecular dynamics simulations of xenon in two classes of zeolite crystal systems, one consisting of purely intra-crystalline space and the other with both intra- and inter-crystalline space are reported. The latter mimics a typical poly-crystalline sample of zeolite. Comparison of results from these two systems provides insights into the structure and dynamics in the presence of inter-crystalline space.

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.

Understanding Translational-Rotational Coupling in Liquid Water through Changes in Mass Distribution.

A molecular dynamics study of liquid water and models of water has been carried out to understand the effect of changes in the mass distribution on molecular translation and rotation. Calculations on the motion of HO and HO, where m and n vary over a range of values by varying the mass at the hydrogen and oxygen positions, show that these form two distinct series. The two series exhibit different translational and rotational properties.

Rotation driven translational diffusion of polyatomic ions in water: A novel mechanism for breakdown of Stokes-Einstein relation.

While most of the existing theoretical and simulation studies have focused on simple, spherical, halide and alkali ions, many chemically, biologically, and industrially relevant electrolytes involve complex non-spherical polyatomic ions like nitrate, chlorate, and sulfate to name only a few. Interestingly, some polyatomic ions in spite of being larger in size show anomalously high diffusivity and therefore cause a breakdown of the venerable Stokes-Einstein (S-E) relation between the size and diffusivity. Here we report a detailed analysis of the dynamics of anions in aqueous potassium nitrate (KNO) and aqueous potassium acetate (CHCOOK) solutions.

Concentration Dependent Self-Assembly of TrK-NGF Receptor Derived Tripeptide: New Insights from Experiment and Computer Simulations.

Early research has shown that many neurodegenerative diseases are associated with the absence of a short and natural tripeptide sequence, Lys-Phe-Gly (KFG). Herein we report results of both experiments and extensive MD simulations of this tripeptide to understand the self-assembly and morphology as a function of its concentration. Morphologies of the aggregates formed by the tripeptide at low concentration (vesicles), and at high concentration (nanotubes) are studied by several independent 3 μs long Martini coarse-graining MD simulation runs.

Coupled jump rotational dynamics in aqueous nitrate solutions.

A nitrate ion (NO) with its trigonal planar geometry and charges distributed among nitrogen and oxygen atoms can couple to the extensive hydrogen bond network of water to give rise to unique dynamical characteristics. We carry out detailed atomistic simulations and theoretical analyses to investigate these aspects and report certain interesting findings. We find that the nitrate ions in aqueous potassium nitrate solution exhibit large amplitude rotational jump motions that are coupled to the hydrogen bond rearrangement dynamics of the surrounding water molecules.

Mode coupling theory analysis of electrolyte solutions: Time dependent diffusion, intermediate scattering function, and ion solvation dynamics.

A self-consistent mode coupling theory (MCT) with microscopic inputs of equilibrium pair correlation functions is developed to analyze electrolyte dynamics. We apply the theory to calculate concentration dependence of (i) time dependent ion diffusion, (ii) intermediate scattering function of the constituent ions, and (iii) ion solvation dynamics in electrolyte solution. Brownian dynamics with implicit water molecules and molecular dynamics method with explicit water are used to check the theoretical predictions.

A molecular dynamics study of ambient and high pressure phases of silica: structure and enthalpy variation with molar volume.

Extensive molecular dynamics studies of 13 different silica polymorphs are reported in the isothermal-isobaric ensemble with the Parrinello-Rahman variable shape simulation cell. The van Beest-Kramer-van Santen (BKS) potential is shown to predict lattice parameters for most phases within 2%-3% accuracy, as well as the relative stabilities of different polymorphs in agreement with experiment. Enthalpies of high-density polymorphs - CaCl2-type, α-PbO2-type, and pyrite-type - for which no experimental data are available as yet, are predicted here.

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.

Neutron scattering and molecular dynamics evidence for levitation effect in nanopores.

Neutron scattering measurements and molecular dynamics simulations have been carried out on the three isomers of pentane (neopentane (neo), isopentane (iso), and n-pentane (n-)) adsorbed in zeolite NaY. The results show that the self-diffusivity of these isomers follow the order Ds(neo)>Ds(iso)>Ds(n-), suggesting that the larger the cross section perpendicular to the molecular axis of the isomer, the higher the self-diffusivity. This counterintuitive result provides the first direct experimental evidence in support of the mutual cancellation of forces on the diffusant leading to a diffusivity maximum and is often referred to as the levitation effect.

Adsorption isotherm and other properties of methane in zeolite A from an intermolecular potential derived from ab initio calculations.

A classical Lennard-Jones potential is derived from a fit to the ab initio energies obtained from an all-electron mixed-basis calculation for methane in zeolite LTA. The potential predicts the heat of adsorption, adsorption isotherm, and self-diffusivity of methane in excellent agreement with experiment. This study suggests, for the first time, that ab initio energies-in addition to experimental data-can form a good basis for derivation of accurate classical potentials between organic and inorganic elements.

Diffusion anomaly as a function of molecular length of linear molecules: levitation effect.

Previous work on monatomic spherical sorbates has shown the existence of an anomalous peak in self-diffusivity (D) when plotted as a function of size of the diffusant. Molecular dynamics studies on linear molecules of different lengths l in zeolite NaY at 140 and 200 K are reported. It is seen that there is a peak in D as a function of l, suggesting that the levitation effect exists for linear molecules, the simplest member of polyatomics.

A full interionic potential for Na(1+x)Zr(2)Si(x)P(3-x)O(12) superionic conductors.

Most inorganic solids are made up of octahedral and tetrahedral units interconnected to give an infinite framework. Use of computer simulation to study these materials has not been as prevalent as in the organic or biomolecules. Na(1+x)Zr(2)Si(x)P(3-x)O(12) is a typical inorganic solid with ZrO(6) octahedra and (Si/P)O(4) tetrahedra which are shown along with a few Na(+) sites marked M1, M2, and M3.