Biomedical subjects

Alexander V Neimark

Publications and source records attributed to Alexander V Neimark.

17 recordsLinked to original sources

PubMed2006

Density functional theory model of adsorption deformation.

Molecules adsorbed in pores cause elastic deformations of the solid matrix leading to either contraction or swelling of the material. Although experimental manifestation of adsorption-induced deformation in clays, coals, carbons, silicas, and other materials has been known for a long time, a rigorous theoretical description of this phenomenon is lacking. We report the nonlocal density functional theory (NLDFT) calculations that reproduce almost quantitatively the adsorption and strain isotherms of Kr and Xe on zeolite X. This system exhibits characteristic contraction at low vapor pressures and swelling at high vapor pressures. We show that the experimentally observed changes in the adsorbent volume are proportional to the solvation (disjoining) pressure caused by the adsorption stress exerted on the pore walls. The proposed NLDFT model can be used for the interpretation of adsorption measurements in micro- and mesoporous materials and for the characterization of their mechanical properties.

Journal Article↗

PubMed2006

Density functional theory model of adsorption on amorphous and microporous silica materials.

We present a novel quenched solid density functional theory (QSDFT) model of adsorption on heterogeneous surfaces and porous solids, which accounts for the effects of surface roughness and microporosity. Within QSDFT, solid atoms are considered as quenched component(s) of the solid-fluid system with given density distribution(s). Solid-fluid intermolecular interactions are split into hard-sphere repulsive and mean-field attractive parts. The former are treated with the multicomponent fundamental measure density functional. Capabilities of QSDFT are demonstrated by drawing on the example of adsorption on amorphous silica materials. We show that, using established intermolecular potentials and a realistic model for silica surfaces, QSDFT quantitatively describes adsorption/desorption isotherms of Ar and Kr on reference MCM-41, SBA-15, and LiChrosphere materials in a wide range of relative pressures. QSDFT offers a systematic approach to the practical problems of characterization of microporous, mesoporous, and amorphous silica materials, including an assessment of microporosity, surface roughness, and adsorption deformation. Predictions for the pore diameter and the extent of pore surface roughness in MCM-41 and SBA-15 materials are in very good agreement with recent X-ray diffraction studies.

Journal Article↗

PubMed2006

Ribbon-to-fiber transformation in the process of spinning of carbon-nanotube dispersion.

We describe a phenomenon of ribbon-to-fiber transformation observed in the process of spinning of single wall carbon nanotubes dispersed in polymer solutions. In the process of spinning, a gel-like ribbon comprised of nanotube bundles bound by polymer is withdrawn from a solvent bath. We show that upon crossing the liquid-air interface, the ribbon may either retain its flat shape or fold into a compact hairlike fiber. The ribbon-to-fiber transformation is caused by the capillary action of the liquid meniscus embracing the ribbon. Only sufficiently stiff ribbons can withhold the capillary compression. The critical conditions of folding, as well as the number of folds in the contractive ribbon, depend on the ribbon width, its flexural rigidity, and the solvent surface tension. We show that the ribbon rigidity can be efficiently modulated by varying the solvent composition, allowing us to control the pore structure of carbon-nanotube fibers.

Microscopy, Electron, Scanning↗

PubMed2006

Phase transitions and criticality in small systems: vapor-liquid transition in nanoscale spherical cavities.

Phase transformations in fluids confined to nanoscale pores, which demonstrate characteristic signatures of first-order phase transitions, have been extensively documented in experiments and molecular simulations. They are characterized by a pronounced hysteresis, which disappears above a certain temperature. A rigorous interpretation of these observations represents a fundamental problem from the point of view of statistical mechanics. Nanoscale systems are essentially small, finite volume systems, in which the concept of the thermodynamic limit is no longer valid, and the statistical ensembles are not equivalent. Here, we present a rigorous approach to the description and molecular simulations of phase transitions and criticality in small confined systems, as illustrated by the example of vapor-liquid transition (capillary condensation) in spherical cavities. The method is based on the analysis of the canonical ensemble isotherms, which can be generated by the gauge cell Monte Carlo simulation method. The method allows one to define the critical temperature of phase transition, conditions of phase equilibrium, limits of stability of metastable states, and nucleation barriers, which determine hysteretic phase transformations.

Journal Article↗

PubMed2006

Adsorption hysteresis of nitrogen and argon in pore networks and characterization of novel micro- and mesoporous silicas.

We report results of nitrogen and argon adsorption experiments performed at 77.4 and 87.3 K on novel micro/mesoporous silica materials with morphologically different networks of mesopores embedded into microporous matrixes: SE3030 silica with worm-like cylindrical channels of mode diameter of approximately 95 angstroms, KLE silica with cage-like spheroidal pores of ca. 140 angstroms, KLE/IL silica with spheroidal pores of approximately 140 angstroms connected by cylindrical channels of approximately 26 angstroms, and, also for a comparison, on Vycor glass with a disordered network of pores of mode diameter of approximately 70 angstroms. We show that the type of hysteresis loop formed by adsorption/desorption isotherms is determined by different mechanisms of condensation and evaporation and depends upon the shape and size of pores. We demonstrate that adsorption experiments performed with different adsorptives allow for detecting and separating the effects of pore blocking/percolation and cavitation in the course of evaporation. The results confirm that cavitation-controlled evaporation occurs in ink-bottle pores with the neck size smaller than a certain critical value. In this case, the pressure of evaporation does not depend upon the neck size. In pores with larger necks, percolation-controlled evaporation occurs, as observed for nitrogen (at 77.4 K) and argon (at 87.3 K) on porous Vycor glass. We elaborate a novel hybrid nonlocal density functional theory (NLDFT) method for calculations of pore size distributions from adsorption isotherms in the entire range of micro- and mesopores. The NLDFT method, applied to the adsorption branch of the isotherm, takes into account the effect of delayed capillary condensation in pores of different geometries. The pore size data obtained by the NLDFT method for SE3030, KLE, and KLE/IL silicas agree with the data of SANS/SAXS techniques.

Journal Article↗

PubMed2006

Characterization of micro-mesoporous materials from nitrogen and toluene adsorption: experiment and modeling.

Universal mechanisms of adsorption and capillary condensation of toluene and nitrogen on ordered MCM-41 and PHTS materials are studied by means of high-resolution experiments and Monte Carlo molecular simulations. A molecular simulation model of toluene adsorption in silica nanopores, which accounts for surface heterogeneity, and a hybrid molecular-macsroscopic method for pore size distribution (PSD) calculations have been developed. For a range of reference materials, the PSD results obtained from toluene isotherms are consistent with the results of nitrogen adsorption using the nonlocal density functional theory method.

Journal Article↗

PubMed2005

Nitrogen and carbon dioxide adsorption by soils.

High-resolution nitrogen (77 K) and carbon dioxide (273 K) adsorption at subatmospheric pressures has been studied for a range of model soils of various origins with different organic matter (OM) contents. It is demonstrated that N2 and CO2 molecules probe different regions of soil particles. Nitrogen is adsorbed primarily on the outer surface of soil particles, while CO2 has a higher affinity to OM domains. Low-pressure nitrogen adsorption reveals that soil particle surfaces consist of clay/mineral domains with discrete patches of OM. A linear correlation has been found between the CO2 uptake and the amount of organic carbon reduced per unit of the external surface area. A new method for discriminating the microporosity of soil particles and accessibility of OM has been proposed.

Adsorption↗

PubMed2005

A simulation method for the calculation of chemical potentials in small, inhomogeneous, and dense systems.

We present a modification of the gauge cell Monte Carlo simulation method [A. V. Neimark and A. Vishnyakov, Phys. Rev. E 62, 4611 (2000)] designed for chemical potential calculations in small confined inhomogeneous systems. To measure the chemical potential, the system under study is set in chemical equilibrium with the gauge cell, which represents a finite volume reservoir of ideal particles. The system and the gauge cell are immersed into the thermal bath of a given temperature. The size of the gauge cell controls the level of density fluctuations in the system. The chemical potential is rigorously calculated from the equilibrium distribution of particles between the system cell and the gauge cell and does not depend on the gauge cell size. This scheme, which we call a mesoscopic canonical ensemble, bridges the gap between the canonical and the grand canonical ensembles, which are known to be inconsistent for small systems. The ideal gas gauge cell method is illustrated with Monte Carlo simulations of Lennard-Jones fluid confined to spherical pores of different sizes. Special attention is paid to the case of extreme confinement of several molecular diameters in cross section where the inconsistency between the canonical ensemble and the grand canonical ensemble is most pronounced. For sufficiently large systems, the chemical potential can be reliably determined from the mean density in the gauge cell as it was implied in the original gauge cell method. The method is applied to study the transition from supercritical adsorption to subcritical capillary condensation, which is observed in nanoporous materials as the pore size increases.

Journal Article↗

PubMed2005

Monte Carlo simulation study of droplet nucleation.

A new rigorous Monte Carlo simulation approach is employed to study nucleation barriers for droplets in Lennard-Jones fluid. Using the gauge cell method we generate the excess isotherm of critical clusters in the size range from two to six molecular diameters. The ghost field method is employed to compute the cluster free energy and the nucleation barrier with desired precision of (1-2)kT. Based on quantitative results obtained by Monte Carlo simulations, we access the limits of applicability of the capillarity approximation of the classical nucleation theory and the Tolman equation. We show that the capillarity approximation corrected for vapor nonideality and liquid compressibility provides a reasonable assessment for the size of critical clusters in Lennard-Jones fluid; however, its accuracy is not sufficient to predict the nucleation barriers for making practical estimates of the rate of nucleation. The established dependence of the droplet surface tension on the droplet size cannot be approximated by the Tolman equation for small droplets of radius less than four molecular diameters. We confirm the conclusion of ten Wolde and Frenkel [J. Chem. Phys. 109, 9901 (1998)] that integration of the normal component of the Irving-Kirkwood pressure tensor severely underestimates the nucleation barriers for small clusters.

Journal Article↗

PubMed2005

Vapor-to-droplet transition in a Lennard-Jones fluid: simulation study of nucleation barriers using the ghost field method.

We report a comprehensive Monte Carlo (MC) simulation study of the vapor-to-droplet transition in Lennard-Jones fluid confined to a spherical container with repulsive walls, which is a case study system to investigate homogeneous nucleation. The focus is made on the application of a modified version of the ghost field method (Vishnyakov, A.; Neimark, A. V. J. Chem. Phys. 2003, 119, 9755) to calculate the nucleation barrier. This method allows one to build up a continuous trajectory of equilibrium states stabilized by the ghost field potential, which connects a reference droplet with a reference vapor state. Two computation schemes are employed for free energy calculations, direct thermodynamic integration along the constructed trajectory and umbrella sampling. The nucleation barriers and the size dependence of the surface tension are reported for droplets containing from 260 to 2000 molecules. The MC simulation study is complemented by a review of the simulation methods applied to computing the nucleation barriers and a detailed analysis of the vapor-to-droplet transition by means of the classical nucleation theory.

Journal Article↗

PubMed2005

The birth of a bubble: a molecular simulation study.

We study the nucleation of a bubble in a metastable Lennard-Jones (LJ) fluid, confined to a spherical pore with wetting walls, by a combination of grand canonical, canonical ensemble, and gauge cell Monte Carlo simulation methods complemented by the Voronoi-Delaunay tessellation analysis of statistical geometry of intermolecular cavities. We construct the isotherm of confined fluid in the form of a continuous van der Waals' loop, in which the unstable backward trajectory between the spinodals corresponds to bubble states. We show that as the degree of metastability increases and the fluid becomes progressively stretched, the decrease of fluid density is associated with the evolution of a population of interstitial intermolecular cavities. At the spinodal, the fluid becomes mechanically unstable: Interstitial cavities partly coalesce into a larger cavity located due to the system symmetry around the pore center. This cavity represents a bubble embryo, which grows at the expense of interstitial cavities. The nucleation barrier is calculated by direct thermodynamic integration along the isotherm. We compare our simulation results to the predictions of the classical nucleation theory and experiments on capillary condensation-evaporation of nitrogen in pores of hybrid organic-inorganic mesoporous molecular sieve HMM-3.

Journal Article↗

PubMed2003

"Humic coverage index" as a determining factor governing strain-specific hydrocarbon availability to contaminant-degrading bacteria in soils.

We report development of a novel parameter for quantifying the amount of humic and fulvic acids per unit surface area in a particular soil. This quantity, the "humic coverage index" (HCI), provides a measurement of the relative spatial extents and/or thicknesses of the humic/fulvic overlayers in different soils, and, therefore, can be used in modeling various soils' behavior in sequestration processes in which humic materials are involved. HCI is herein applied to modeling biodegradation of aromatic and aliphatic hydrocarbons (phenanthrene, pyrene, and hexadecane) by several bacterial strains. Results indicate that, for the cases studies here, contaminant biodegradation is highest at a particular HCI and decreases if the coverage density of humic material is lower or higher than this optimum value. The HCI value at which maximal degradation was observed varied across different strains (indicating strain-specific differences in ability to degrade contaminants sorbed to humic materials) and, to a lesser extent, across different contaminants. The HCI concept is also demonstrated to be useful in explaining soil-, strain-, and contaminant-specific variations in the ability of fulvic acid supplementation to enhance contaminant biodegradation. Finally, we show that, in general, strains which are comparatively better at degrading contaminants in high-HCI soils also show enhanced contaminant mineralization in vitro in the presence of humic acids, such as when hydrocarbons are adsorbed onto these materials.

Alkanes↗

PubMed2003

Spontaneous adsorption of viscous and viscoelastic fluids by capillaries and porous substrates.

We have developed a new technique to monitor spontaneous adsorption of fluids by porous substrates. The method is based on an optical electronic measuring system providing millisecond resolution. The method capabilities are demonstrated with the example of the absorption of millimeter-size droplets of water and aqueous solutions of polyethylene oxide and polyacrylamide by capillaries. It is shown that polymer additives even in a small amount reduce significantly the rate of adsorption. We have introduced a generalized Lucas-Washburn equation to account for the fluid elasticity. This equation is shown to explain the observed kinetics quantitatively without invoking adjustable parameters. We have derived a modified Bosanquet equation for the initial velocity of penetration, which accounts for the fluid elasticity. This simple formula gives a reasonable estimate of the rate of absorption of small droplets. We report visualization experiments on absorption of water and polymer solutions by sugar cubes as an example of porous substrates. Although the kinetics of droplet adsorption by porous substrates is similar to the kinetics of droplet adsorption by capillaries, the interpretation of experimental data is more complex and requires a plausible hydrodynamic model for lateral spreading in pores.

Absorption↗

PubMed2003

Modeling of spontaneous penetration of viscoelastic fluids and biofluids into capillaries.

A theoretical model was developed to describe the dynamics of spontaneous penetration of viscoelastic fluids into capillaries. The model agrees quantitatively with recent experiments on absorption of droplets of polymer solutions by glass capillaries [A.V. Bazilevsky, K.G. Kornev, A.N. Rozhkov, A.V. Neimark, J. Colloid Interface Sci. (2003)]. The rate of penetration progressively reduces with the increase in fluid elasticity. Analysis revealed two main contributions to the viscoelastic drag of the liquid column: (i) viscous resistance, which is independent of fluid elasticity, and (ii) viscoelastic resistance, known as the Weissenberg effect. We analytically derived an augmented Bosanquet equation for the maximal velocity of penetration by balancing capillary, inertia, and viscoelastic forces. For slow creep of a liquid column, the Lucas-Washburn equation was modified by accounting for the Weissenberg effect. A series of numerical calculations were performed to demonstrate characteristic features of absorption of fluids at different conditions. This article also discusses some problems specific to absorption of biofluids. We show that deformations of cell membranes in the external converging flow may cause their rupture at the pore entrance.

Data Interpretation, Statistical↗

PubMed2003

Sorption hysteresis of benzene in charcoal particles.

Charcoal is found in water, soil, and sediment where it may act as a sorbent of organic pollutants. The sorption of organic compounds to natural solids often shows hysteresis. The purpose of this study was to determine the source of pronounced hysteresis that we found in the sorption of a hydrophobic compound (benzene) in water to a maple-wood charcoal prepared by oxygen-limited pyrolysis at 673 K. Gas adsorption (N2, Ar, CO2), 13C NMR, and FTIR show the charcoal to be a microporous solid composed primarily of elemental (aromatic) C and secondarily of carboxyl and phenolic C. Nonlocal density functional theory (N2, Ar) and Monte Carlo (CO2) calculations reveal a porosity of 0.15 cm3/g, specific surface area of 400 m2/g, and appreciable porosity in ultramicropores < 10 A. Benzene sorption-desorption conditions were chosen to eliminate artificial causes of hysteresis (rate-limiting diffusion, degradation, colloids effect). Charcoal sorbed up to its own weight of benzene at approximately 69% of benzene water solubility. Sorption was highly irreversible over most of the range tested (10(-4)-10(3) microg/mL). A dimensionless irreversibility index (/i) (0 < or = /i < or = 1) based on local slopes of adsorption and desorption branches was evaluated at numerous places along the isotherm. /i decreases as C increases, from 0.9-1 at low concentration to approximately 0 (approximately fully reversible) at the highest concentrations. Using sedimentation and volumetric displacement measurements, benzene is observed to cause pronounced swelling (up to > 2-fold) of the charcoal particles. It is proposed that hysteresis is due to pore deformation by the solute, which results in the pathway of sorption being different than the pathway of desorption and which leads to entrapment of some adsorbate as the polyaromatic scaffold collapses during desorption. It is suggested that intra-charcoal mass transport may be influenced by structural rearrangement of the solid, in addition to molecular diffusion.

Adsorption↗

PubMed2002

Inside the hysteresis loop: multiplicity of internal states in confined fluids.

We study the equilibrium and stability of metastable states and capillary condensation hysteresis of a Lennard-Jones fluid in cylindrical pores by means of the canonical ensemble density functional theory and gauge cell Monte Carlo simulations. We demonstrate a possibility for the existence of multiple laterally uniform internal states of equal density inside the hysteresis loop. The region of multiple states is bounded by the states of zero compressibility. The internal states can be stabilized in Monte Carlo simulations constraining the density fluctuations.

Journal Article↗

PubMed2002

Capillary condensation as a morphological transition.

The process of capillary condensation/evaporation in cylindrical pores is considered within the idea of symmetry breaking. Capillary condensation/evaporation is treated as a morphological transition between the wetting film configurations of different symmetry. We considered two models: (i) the classical Laplace theory of capillarity and (ii) the Derjaguin model which takes into account the surface forces expressed in terms of the disjoining pressure. Following the idea of Everett and Haynes, the problem of condensation/evaporation is considered as a transition from bumps/undulations to lenses. Using the method of phase portraits, we discuss the mathematical mechanisms of this transition hidden in the Laplace and Derjaguin equations. Analyzing the energetic barriers of the bump and lens formation, it is shown that the bump formation is a prerogative of capillary condensation: for the vapor-liquid transition in a pore, the bump plays the same role as the spherical nucleus in a bulk fluid. We show also that the Derjaguin model admits a variety of interfacial configurations responsible for film patterning at specific conditions.

Journal Article↗