Biomedical subjects

Eli Ruckenstein

Publications and source records attributed to Eli Ruckenstein.

At least 55 records · Page 3Linked to original sources

PubMed2004

Specific ion and ph effects on supramolecular assembly of mesostructured V-Mg oxides.

Mesostructured V-Mg oxides were synthesized using the surfactant cetyltrimethylammonium bromide (CTAB) as template, V2O5, V(acac)3 (vanadium acetylacetonate), or NH4VO3 as vanadium source, and Mg(NO3)2, MgCl2, MgSO4, (MgCO3)4.Mg(OH)2, Mg(CH3CO2)2, or Mg(C2H5O)2 as magnesium source. The factors that influence the formation of mesostructured V-Mg oxides, such as the pH, the natures of magnesium and vanadium sources, and the ionic strength, were identified. The formation of mesophases could be related to the presence of anionic vanadium species, to the electrostatic interactions between the oppositely charged vanadates and micellar headgroups, and to the nature of the counterion of Mg2+ in the magnesium source. The main role was played by the pH and only when the pH allowed the formation of vanadates was a mesostructure generated. The counterions of Mg2+ also played a role, which could be explained via specific ion effects and the formation of complexes between them and the vanadium-containing species, which are attracted by the headgroups of the micellar templates.

Journal Article↗

PubMed2004

Simple model for grafted polymer brushes.

The first theories of grafted polymer brushes assumed a step profile for the monomer density. Later, the real density profile was obtained from Monte Carlo or molecular dynamics simulations and calculated numerically using a self-consistent field theory. The analytical approximations of the solutions of the self-consistent field equations provided a parabolic dependence of the self-consistent field, which in turn led to a parabolic distribution for the monomer density in neutral brushes. As shown by numerical simulations, this model is not accurate for dense polymer brushes, with highly stretched polymers. In addition, the scaling laws obtained from the analytical approximations of the self-consistent field theory are identical to those derived from the earlier step-profile-approximation and predict a vanishing thickness of the brush at low graft densities, and a thickness exceeding the length of the polymer chains at high graft densities. Here a simple model is suggested to calculate the monomer density and the interaction between surfaces with grafted polymer brushes, based on an approximate calculation of the partition function of the polymer chains. The present model can be employed for both good and poor solvents, is compatible with a parabolic-like profile at moderate graft densities, and leads to an almost steplike density for highly stretched brushes. While the thickness of the brush depends strongly on solvent quality, it is a continuous function in the vicinity of the temperature. In good and moderately poor solvents, the interactions between surfaces with grafted polymer brushes are always repulsive, whereas in poor solvents the interactions are repulsive at small separations and become attractive at intermediate separation distances, in agreement with experiment. At large separations, a very weak repulsion is predicted.

Journal Article↗

PubMed2004

Double layer interaction between two plates with polyelectrolyte brushes.

When polyelectrolyte chains are grafted to colloidal particles, the electric field between particles is affected by the charges of the chains. In some previous theoretical attempts, the charge density of the polyelectrolyte chains per unit length was considered constant, and its effect was accounted for by introducing an additional constant charge density into the unidimensional Poisson-Boltzmann equation, which was evaluated assuming that it is uniformly distributed in the polyelectrolyte volume of the brush. In this paper, a more detailed model is employed for the calculation of the electrical potential between two plates on which polyelectrolyte brushes are present. In this model, the polyelectrolyte chain is viewed as a rigid cylinder, on the surface of which charges are generated through the dissociation of ionizable sites and adsorption of the cations of the electrolyte. To each of the chains an atmosphere is attached which for simplicity is assumed cylindrical. In the brush region, the electrical potential is described by a two-dimensional Poisson-Boltzmann equation, while in the region free of polyelectrolyte chains by a unidimensional Poisson-Boltzmann equation. Such a model is physically suitable when the charges of the chains are sufficiently large for the repulsion they generate to ensure that the chains are fully extended. Such cases are quite frequent, because relatively low charges lead to an almost complete extension of the chains. In this paper, both the plate surface and the surface of the cylinders are considered charged. The effects of electrolyte concentration, pH, brush thickness and chain coverage density on the repulsion between plates are examined.

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PubMed2004

Long-range ordered structures in diblock copolymer melts induced by combined external fields.

The structure of diblock copolymer melts under a single external electric or shear field, as well as under combined orthogonal external fields was investigated using a cell dynamic system. The phase structure was determined by coupling the effects of the external fields with the original structure of the bulk free of external fields. The single electric or shear field generated long-range cylinders in asymmetric A4mB6m diblock copolymers and distorted lamellae in symmetric A5mB5m diblock copolymers. Successive orthogonal shear followed by an electric external field generated long-range lamellae in symmetrical A5mB5m systems. However, the simultaneous orthogonal electric and shear fields could more easily form long-range lamellae than the sequential orthogonal fields. The dynamical processes in diblock copolymer melts under orthogonal fields have been also examined.

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PubMed2004

Interaction force between two charged plates immersed in a solution of charged particles. Coupling between double layer and depletion forces.

When two parallel plates are immersed in a solution of small charged particles, the center of the particles is excluded from a region of thickness D/2 near the plate, where D is their diameter. The approach which Langmuir developed for the double layer repulsion in the presence of an electrolyte with ions of negligible size is extended to the case in which one of the "ions" is a charged particle of finite, relatively small size. A general expression for the force generated between the two charged plates immersed in an electrolyte solution containing relatively small charged particles is derived. In this expression, only the electrical potential at the middle distance between the plates is required to calculate the force. A Poisson-Boltzmann equation which accounts for the volume exclusion of the charged particles in the vicinity of the surface is solved to obtain the electrical potential at the middle between the two plates. Starting from this expression, some results obtained previously for the depletion force acting between two plates or two spheres are rederived. For charged plates immersed in a solution of an electrolyte and charged small particles, the effects of the particle charge, particle charge sign, particle size, and volume fraction of the particles on the force acting between the two plates are examined.

Journal Article↗

PubMed2004

Kinetics of phase transformation on a Bethe lattice in the presence of spin exchange.

Kinetics of phase transformation on a Bethe lattice governed by single-spin-flip Glauber and spin-exchange Kawasaki dynamics is examined. For a general Glauber dynamics for which all processes (splitting and coagulation, growth and decay of clusters, as well as creation and annihilation of single-spin clusters) take place, the addition of the Kawasaki dynamics accelerates the transformation process without changing the qualitative behavior. In the growth-decay regime of the Glauber dynamics, regime in which the splitting and coagulation, and creation and annihilation processes due to single-spin flips are negligible, the Kawasaki dynamics strongly increases the fraction of transformed phase because of the splitting and coagulation of clusters induced by the spin-exchange processes. Acting alone, the Kawasaki dynamics leads to the growth of the clusters of each of the phases after the quenching of the temperature to a lower value. When the final temperature T(f) is smaller than a certain temperature T(f0), the average cluster radius grows linearly with time during both the initial and intermediate stages of the kinetic process, and diverges as log(2)(t(d)-t)(-1) when the time t approaches the value t(d) at which infinite clusters arise. It is shown that, among the various spin-exchange processes involved in Kawasaki dynamics, the main contribution is provided by those which decrease or increase the number of clusters by unity.

Journal Article↗

PubMed2004

Fluorescence study of aggregation in water of PEO-grafted polydiphenylamine.

Fluorescence spectra of water-soluble conducting poly(ethylene oxide)-grafted polydiphenylamines (PDPA-g-PEOs with the M(n)s of PEO 350, 750, and 2000) in water were determined and interpreted. The emissions of the PDPA-g-PEOs occurred in the range from 360 to 700 nm and were dependent on their concentrations, PEO chain length, extent of oxidation, pH, and temperature. An optimum concentration, above which the fluorescence intensity decreased dramatically because of quenching, was observed. This quenching was a result of the aggregation of PDPA-g-PEO macromolecules in water. The doped PDPA-g-PEO molecules provided a lower optimum concentration than the corresponding reduced-state macromolecules. A rod-shaped microstructure of nanoscale size was generated through the aggregation of the PDPA-g-PEO macromolecules. This microstructure was also confirmed by dynamic light scattering, atomic force microscopy, and surface elemental analysis.

Journal Article↗

PubMed2004

Double-layer interaction between two plates with hairy surfaces.

In most theoretical treatments of colloidal particles with hairy surfaces, only the steric effect is taken into account. The steric force is a short-range interaction and acts only when the chains on different particles begin to interpenetrate each other. However, since the hairy chains are extended into the continuous phase, they constrain the orientation of the water molecules near the surface and, as a result, the dielectric constant in that region can become very different from that in the bulk. The low dielectric constant affects the distributions of ion concentrations and the gradient of the electric field. Therefore, the double-layer interactions between two plates with hairy surfaces cannot be calculated on the basis of the classical Gouy-Chapman theory, which involves a uniform dielectric constant in the Poisson-Boltzmann equation. A model which accounts for the difference in dielectric constants in the hairy region and outside that region is therefore proposed. The ion specificity is also taken into account by using Born's expression for the free energy of hydration of ions. The repulsive forces calculated via the Gouy-Chapman theory and via the new model are compared. The hairy region can have a long range effect on the repulsive double-layer interactions.

Journal Article↗

PubMed2004

Quantum chemical density-functional theory calculations of the structures of defect C60 with four vacancies.

Quantum chemical density-functional theory (DFT) calculations have been carried out for the six isomers obtained by removing four adjacent atoms from C60. The most stable isomer consists of twelve 5-member and eighteen 6-member rings, indicating that the removal of some atoms from C60, which contains twelve 5-member rings and twenty 6-member rings, does not always generate larger holes. Each of the other five isomers contains at least one 4-member ring and one larger ring (7-, 8-, 9-, or 10-member ring) besides the 5- and 6-member rings. All isomers have similar structures for singlet and triplet spin multiplicities but with different stabilities. The ground states for two of the isomers are triplets, whereas the ground states for the other isomers are singlets. Furthermore, a comparison between the various isomers allowed one to examine the effect of the structure on the stability of fullerene cages.

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PubMed2004

Attractive interactions in dispersions of identical charged colloidal particles: a Monte Carlo simulation.

Using the Debye-Hückel pair potential, the collective interactions between identical charged particles were examined via Monte Carlo simulations. The results have shown that when the number of charges per particle and the particle volume fraction were sufficiently large, the pair long-range electrostatic repulsion generated an effective attractive interaction between identical charged particles because of many-body effects. Disordered liquid-like structures, ordered crystal-like structures, ordered structures dispersed in disordered ones, and disordered structures dispersed in ordered ones have been found. The structures are dependent on the volume fraction and charge of the particles, as well as on the electrolyte concentration.

Journal Article↗

PubMed2004

Phase transformation in a lattice system in the presence of spin-exchange dynamics.

A joint action of the Glauber single-spin-flip and the Kawasaki spin-exchange mechanisms upon the processes of phase transformation is examined in the framework of the one-dimensional kinetic Ising model. It is shown that the addition of the Kawasaki dynamics to that of Glauber accelerates the process of phase transformation in the initial stage, but slows it down in later stages. For the truncated form of Glauber dynamics, which excludes the processes of splitting and coagulation of clusters, the addition of the Kawasaki dynamics always accelerates the phase transformation process. Acting alone, the Kawasaki mechanism provides a cluster growth proportional to t(1/2) (where t is the time) in the initial stage and proportional to t(1/3) (Lifshitz-Slyozov-Wagner law) in the intermediate stage. In the final stage, a cluster size approaches exponentially its equilibrium value.

Journal Article↗

PubMed2004

Kinetics of phase transformation on a Bethe lattice.

A kinetic Ising model is applied to the description of phase transformations on a Bethe lattice. A closed set of kinetic equations for a model with the coordination number q=3 is obtained using a procedure developed in a previous paper. For T close to Tc(T>Tc), where Tc is the phase transition temperature, and zero external field (absence of supersaturation), the rate of phase transformation (RPT) for small deviations from equilibrium is independent of time and tends to zero as (T-Tc). At T=Tc, the RPT depends on time and for large times behaves as t(-1). For T 0. The role of different mechanisms responsible for growth (decay), splitting (coagulation), and creation (annihilation) of clusters are examined separately. In all cases there is a critical value Bc of the external field, such that the phase transformation takes place only for Bf>Bc. This result is also obtained from a more simple consideration involving spherical-like clusters on a Bethe lattice. The characteristic time tR at which the polarization becomes larger than zero diverges as (Bf-Bc)(-b) for Bf-->Bc with b=0.47. The RPT has a rapid growth near tR and remains constant for t>tR. The average cluster size (number of spins in a cluster) exhibits a rapid unrestricted growth at a time td approximately tR which indicates the creation of infinite clusters. The only exception to the latter behavior occurs when the kinetics is dominated by cluster growth and decay processes. In this case, the average cluster size remains finite during the transformation process. In contrast to the classical theory, the present approach does not separate the processes of creation of clusters of critical size (nucleation) and of their growth, both being accounted for by the kinetic equations employed.

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PubMed2003

Crosslinked glass fiber affinity membrane chromatography and its application to fibronectin separation.

Macroporous glass membranes were prepared from glass fiber filters via chemical crosslinking and modification, and used for the membrane affinity chromatography of fibronectin from human blood plasma. The filters were first treated with a piranha solution (a concentrated solution of H2SO4 + H2O2 in water), and then crosslinked with bifunctional organosilanes and modified to introduce amino or aniline moieties. Ligand immobilizations via diazotization and glutaraldehyde pathways were carried out and compared. Characterization of the membranes was performed using bovine serum albumin and trypsin as test ligands. By using a cartridge containing gelatin immobilized affinity membranes followed by another cartridge containing heparin immobilized membranes, fibronectin from human blood plasma could be separated.

Chromatography, Affinity↗

PubMed2003

Specific ion effects via ion hydration: I. Surface tension.

A simple modality is suggested to include, in the framework of a modified Poisson-Boltzmann approach, specific ion effects via the change in the ion hydration between the bulk and the vicinity of the surface. This approach can account for both the depletion of the interfacial region of structure-making ions as well as for the accumulation of structure-breaking ions near the interface. Expressions for the change in interfacial tension as a function of electrolyte concentrations are derived. On the basis of this theory, one explains the dependence of the surface potential on pH and electrolyte concentration, the existence of a minimum in the surface tension at low electrolyte concentrations and the linear dependence, with a positive or sometimes negative slope, of the surface tension on the electrolyte concentration at sufficiently high ionic strengths.

Chemical Phenomena↗

PubMed2003

Specific ion effects via ion hydration: II. Double layer interaction.

A simple modified Poisson-Boltzmann formalism, which accounts also for those interactions between electrolyte ions and colloidal particles not included in the mean potential, is used to calculate the force between two parallel plates. It is shown that the short-range interactions between ions and plates, such as those due to the change in the hydration free energy of a structure-making/breaking ion that approaches the interface, affect the double layer interaction at large separations through the modification of the surface potential and surface charge density. While at short separations (below the range of the short-range ion-hydration forces) the interaction can be attractive, at larger separations the interaction is always repulsive, as in the traditional theory. When the long-range van der Waals interactions between the ions and the system (ion-dispersion interactions) are accounted for in the modified Poisson-Boltzmann approach, an attractive force between plates can be generated. At sufficiently large separations, this attraction can become even stronger than the traditional van der Waals attraction between plates of finite thickness, thus generating a dominant long-range 'double layer attraction'. At small plate separations, the attraction generated by the ion-dispersion forces combined with the electrostatic repulsion due to the double layers overlap can lead to a variety of interactions, from a weak attraction (which is typically by at least one order of magnitude smaller than the traditional van der Waals attraction between plates) to a strong double layer repulsion (for sufficiently large surface charges). Both types of ion interactions (long-range van der Waals or short-range ionic hydration) strongly affect the magnitude of the double layer interaction, and can account for the specific ion effects observed experimentally. However, they do not affect qualitatively the traditional theory of the colloid stability, which predicts that the colloid is stable when there is a sufficiently large charge on the surface, and coagulates when the van der Waals interactions between two colloidal particles dominate. The only qualitative difference found when the ion-dispersion interactions were incorporated into the traditional double layer theory was the emergence of a 'double layer attraction' at very large separations, which, however, does not affect much the stability of colloids.

Colloids↗

PubMed2003

The effect of surface dipoles and of the field generated by a polarization gradient on the repulsive force.

Double-layer and hydration interactions have been coupled into a single set of equations because both are dependent on the polarization of the water molecules. The coupled equations involve the electric fields generated by the surface charge and surface dipoles, as well as the field due to the neighboring dipoles in water. The dipoles on the surface are generated through the counterions' binding to sites of opposite charge. The equations obtained were employed to explain the restabilization observed experimentally at large ionic strengths for colloidal particles on which protein molecules were adsorbed. Polar molecules adsorbed on a charged surface of colloidal particle can generate a field either in the same direction as that generated by the charge or in the opposite direction. The effect of the sign of the dipole of the adsorbed polar molecules on the interaction between surfaces was also examined.

Journal Article↗

PubMed2003

Multiple transient response methods to identify mechanisms of heterogeneous catalytic reactions.

Identification of mechanisms of heterogeneous catalytic reactions is challenging because there are few effective in-situ techniques with which to determine reactive intermediates on surfaces. An effective multiple transient technique consisting of a combination of a broadened pulse with step changes and a sharp isotopic pulse was developed in our group. This technique provides much richer dynamic information than is obtainable from experiments with individual pulses or step changes in reactant concentration. In this Account, we illustrate the method by its application to high-temperature heterogeneous catalytic reactions.

Journal Article↗

PubMed2002

Scaling analysis of coating of a plate or a fiber.

The problem of coating a plate or a fiber through their withdrawal from a large reservoir was treated in the literature in various limiting cases. In the present paper, interpolation expressions are derived by scaling for Newtonian, Bingham, and viscoelastic fluids. The scaling analysis was also employed to explain the sharp increase in film thickness produced by inertia and the occurrence of a maximum thickness for an optimum velocity. It is also shown that, for a viscoelastic fluid, the film thickness tends to a finite value at very large withdrawal rates. An explanation is also provided for the existence of a maximum film thickness for an optimum surfactant concentration.

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