Sohail Murad

A computer simulation study of the separation of aqueous solutions using thin zeolite membranes

A recently developed molecular simulation scheme for studying solutions undergoing osmosis and reverse osmosis was used to study the separation of aqueous solutions using thin zeolite membranes. This method allows for the preservation of the atomic roughness of the membranes, while the molecules that constitute the membranes are also allowed to vibrate. In the simulations, two thin membranes cut from a cubic cell of ZK-4 zeolite were used as the semi-permeable membranes to separate water from aqueous NaCl solutions. Both osmosis and reverse-osmosis phenomena were observed. The study showed that ZK-4 zeolite membranes show promise for use in membrane-based separation of aqueous electrolyte solutions, as well as other similar systems.

A molecular dynamics simulation of droplet evaporation

Abstract A molecular dynamics (MD) simulation method is developed to study the evaporation of submicron droplets in a gaseous surrounding. A new methodology is proposed to specify initial conditions for the droplet and the ambient fluid, and to identify droplet shape during the vaporization process. The vaporization of xenon droplets in nitrogen ambient under subcritical and supercritical conditions is examined. Both spherical and non-spherical droplets are considered. The MD simulations are shown to be independent of the droplet and system sizes considered, although the observed vaporization behavior exhibits some scatter, as expected. The MD results are used to examine the effects of ambient and droplet properties on the vaporization characteristics of submicron droplets. For subcritical conditions, it is shown that a spherical droplet maintains its sphericity, while an initially non-spherical droplet attains the spherical shape very early in its lifetime, i.e., within 10% of the lifetime. For both spherical and non-spherical droplets, the subcritical vaporization, which is characterized by the migration of xenon particles that constitute the droplet to the ambient, exhibits characteristics that are analogous to those reported for “continuum-size” droplets. The vaporization process consists of an initial liquid-heating stage during which the vaporization rate is relatively low, followed by nearly constant liquid-temperature evaporation at a “pseudo wet-bulb temperature”. The rate of vaporization increases as the ambient temperature and/or the initial droplet temperature are increased. For the supercritical case, the droplet does not return to the spherical configuration, i.e., its sphericity deteriorates sharply, and its temperature increases continuously during the “vaporization” process.

Influence of hydrophilic surface specificity on the structural properties of confined water.

The influence of chemical specificity of hydrophilic surfaces on the structure of confined water in the subnanometer regime is investigated using grand canonical Monte Carlo simulations. The structural variations for water confined between hydroxylated silica surfaces are contrasted with water confined between mica surfaces. Although both surfaces are hydrophilic, our study shows that hydration of potassium ions on the mica surface has a strong influence on the water structure and solvation force response of confined water. In contrast to the disrupted hydrogen bond network observed for water confined between mica surfaces, water between silica surfaces retains its hydrogen bond network displaying bulklike structural features down to surface separations as small as 0.45 nm. Hydrogen bonding of an invariant contact water layer with the surface silanol groups aids in maintaining a constant number of hydrogen bonds per water molecule for the silica surfaces. As a consequence, water depletion and rearrangement upon decreasing confinement is a strong function of the hydrophilic surface specificity, particularly at smaller separations. An oscillatory solvation force response is only observed for water confined between silica surfaces, and bulklike features are observed for both surfaces above a surface separation of about 1.2 nm. We evaluate and contrast the water density, dipole moment distributions, pair correlation functions, and solvation forces as a function of the surface separation.

Thermal transport across nanoscale solid-fluid interfaces

An explanation for the effective thermal resistance RK can be based on the impedance to the passage of thermal phonons across an interface. We conjecture that (1) increasing the fluid pressure, and (2) making an interface more hydrophilic should facilitate better acoustic matching and thus lower RK. Our molecular dynamics simulations confirm this. Overall, RK decreases with increasing temperature and is inversely proportional to the heat flux.

A computer simulation study of fluids in model slit, tubular, and cubic micropores

Computer simulation studies have been carried out, using a novel method, to examine the behavior of fluids in various confined geometries, including, slit pores, square and cylindrical tubular pores, cubic pores, and pores with rough walls. The method used to model these pores allows for the permeability of the pore wall to the confined fluid to be controlled precisely between the impermeable and totally permeable limits, while at the same time maintaining the atomic nature of the pore wall. These systems have been studied with several models for the pore wall for a wide range of state conditions. The results obtained for nonuniform density distributions, wall permeabilities, and diffusion coefficients are examined in detail.

A new model for permeable micropores

A model is proposed for the computer simulation of a fluid in pores of various geometries having walls of controllable permeability. The walls are produced by tethering atoms to the cyclically replicated faces of the basic cube. A particular example with variable anisotropic diffusion is discussed in detail.

A molecular dynamics simulation of fluid hydrogen chloride

We report a molecular dynamics computer simulation study of hydrogen chloride in the liquid and dense gas states. The intermolecular pair potential model consists of four cut off, shifted-force atom-atom Lennard-Jones (LJ) interactions, to which are added molecular electric point dipolar and quadrupolar interaction terms. The LJ parameters are fixed using the experimental density and vapour pressure of the coexisting liquid. The simulation is used to predict the internal energy and the self-diffusion coefficient for the liquid, and the internal energy and pressure for the supercritical fluid. Good agreement is obtained with experiment for these properties, and the model gives somewhat better results than those found in a previous study [2] in which the electrostatic forces were omitted. The mean squared torque for the liquid is now somewhat larger rather than smaller than the experimental value. We also calculate the site-site correlation functions and partial structure factors, and find in particular tha...

Ion permeation dynamics in carbon nanotubes.

Molecular dynamics simulations are carried out to investigate the permeation of ions and water in a membrane consisting of single wall carbon nanotubes possessing no surface charges connecting two reservoirs. Our simulations reveal that there are changes in the first hydration shell of the ions upon confinement in tubes of 0.82 or 0.90 nm effective internal diameter. Although the first minimum in the g(r) is barely changed in the nanotube compared to in the bulk solution, the hydration number of Na(+) ion is reduced by 1.0 (from 4.5 in bulk to 3.5 in the 0.90 nm tube) and the hydration number is reduced further in the 0.82 nm tube. The changes in the hydration shell of Cl(-) ion are negligible, within statistical errors. The water molecules of the first hydration shell of both ions exchange less frequently inside the tube than in the bulk solution. We compare ion trajectories for ions in the same tube under identical reservoir conditions but with different numbers of ions in the tubes. This permits investigation of changes in structure and dynamics which arise from multiple ion occupancy in a carbon nanotube possessing no surface charges. We also investigated the effects of tube flexibility. Ions enter the tubes so as to form a train of ion pairs. We find that the radial distribution profiles of Na(+) ions broaden significantly systematically with increasing number of ion pairs in the tube. The radial distribution profiles of Cl(-) ions change only slightly with increasing number of ions in the tube. Trajectories reveal that Na(+) ions do not pass each other in 0.90 nm tubes, while Cl(-) ions pass each other, as do ions of opposite charge. An ion entering the tube causes the like-charged ions preceding it in the tube to be displaced along the tube axis and positive or negative ions will exit the tube only when one or two other ions of the same charge are present in the tube. Thus, the permeation mechanism involves multiple ions and Coulomb repulsion among the ions plays an essential role.

Thermal conductivity in molecular fluids

The theory of heat flow in fluids composed of what may be regarded as rigid-body molecules, is not a straightforward generalization of the corresponding situation for classical atomic fluids. In this short note we derive an expression for the heat flux vector valid for fluids composed of rigid bodies possessing rotational degrees of freedom. We then generalize the Evans heat flow algorithm

Molecular simulation of osmosis, reverse osmosis, and electro-osmosis in aqueous and methanolic electrolyte solutions

Computer simulations of aqueous and methanolic electrolyte (NaCl, LiCl, NaBr, LiBr) solutions undergoing osmosis, reverse osmosis, and electro-osmosis have been carried out using semi-permeable membranes. These studies used a novel technique developed by the present authors in which the atomic roughness of the membrane is preserved. In addition, the molecules that constitute the membrane are, allowed to vibrate. The effect of the important driving forces in these separations, viz., pressure, concentration, temperature and electric field strength, has been investigated. These results show that the water and methanol molecules cluster strongly around the ions in these simulations, and this plays a significant role in membrane based separations in both aqueous and methanolic solutions-an effect of which the importance was not previously recognized. In addition, studies have been made of the self-diffusion coefficients and density profiles in these systems. It has been found that external electric fields usua...