Effective Surface Diffusion Coefficient Research Articles

Biopolyelectrolyte Surface Diffusion Within a Planar Slit Geometry

The effective surface diffusion of biopolyelectrolytes within nanoscale confinement is integral to biosensing, biocatalysis, bioseparations (such as chromatography columns utilizing nanoporous media), as well as biophysical processes. Confinement influences the surface-mediated diffusion of biopolyelectrolyte chains due to the presence of electric double layers and the increased frequency of transient polymer/surface interactions. Here, the diffusion of poly-L-lysine (PLL) in a nanoslit was studied using Convex Lens-induced Confinement (CLiC) single-molecule tracking microscopy. Three surface chemistries were employed to understand and compare the effects of electrostatic and short-range interactions, respectively. In all cases, the effective surface diffusion coefficient increased rapidly with slit height until it saturated at its value for a semi-infinite interface for slit heights <30 nm. The evolution of the diffusion coefficient with slit height was faster for amine-functionalized surfaces with which PLL exhibited stronger short-range interactions. These hydrogen-bonding interactions influenced the intermittent random walk behavior by increasing the waiting times between flights and the re-adsorption probability (a.k.a. sticking coefficient) during flights. Intermittent random walks were simulated within a planar slit geometry, using the appropriate adsorption probabilities and waiting time distributions (obtained from diffusion at a single interface) to account for the characteristic short-range interactions between PLL and each type of surface chemistry. The simulated step-size distributions were in good agreement with experimental measurements, indicating that the fine details of diffusion in nanoscale confinement can be quantitatively described by a model that combines confined Brownian motion with transient surface adsorption and desorption, using parameters obtained only from the behavior at a single interface in a semi-infinite geometry. These results highlight the importance of short-range biopolyelectrolyte/surface interactions on diffusion within confined environments and moreover, demonstrate the complex relationship between electrostatics, confinement, and local polymer and surface chemistry on biopolyelectrolyte surface diffusion within a nanoslit.

Thermal and saline effect on mineral-water interactions in compacted clays: A NMR-based study

The behavior of the confined pore water in three compacted clays, which are saturated with different fluids at temperature ranging from 22 to 75 °C, is investigated via the NMR technique. It is shown that the pore water is mainly retained by surface forces and effective surface diffusion coefficient is estimated to be equal to the same in all the water-saturated clays, approximately two-order lower than that of bulk water. The optimal pore size in clays is roughly estimated to equal to or smaller than several micrometers based on the effective surface diffusion coefficient, which has been validated by pore size distributions obtained with mercury intrusion porosimetry. The measured effective activation energy are different for these three clays, and the water molecules in soil B with the largest specific surface and cation exchange capacity have the largest effective activation energy. Effective activation energy is also obtained for soil C (natural bentonite) saturated with NaCl, CaCl2 and KCl solutions with a concentration of 1 mol/L and distilled water. It is shown that salt solution increases the effective activation energy for soil C when saturated with KCl or NaCl (monovalent ion), whereas decreases the effective activation energy when saturated with CaCl2 (divalent ion). The influence of cationic type on effective activation energy is discussed based on the diffuse double layer theory and the cation hydration mechanism. It is also shown that permeability coefficient of the clays increases with temperature. A larger effective activation energy means that the permeability coefficient is more sensitive to temperature.

Concentration-dependent branched pore kinetic model for aqueous phase adsorption

A model that takes into account of a concentration-dependent intra-particle effective surface diffusion coefficient in the branched pore kinetic model is presented. This model was applied to adsorption data obtained from the adsorption of phenol solution by activated carbons prepared from oil-palm shells. Parametric sensitivity analyses were carried out to study the effects of each parameter on the kinetic process. The simulations showed good agreement with experimental adsorption results. The introduction of a concentration-dependent effective surface diffusion coefficient provides a scheme to distinguish the intra-particle mass transfer mechanism for specific adsorption systems using kinetic experimental data. The concentration-dependent branched pore kinetic model is able to correlate together the adsorbent surface characteristics, the adsorbate properties and the adsorption kinetics.

An ion-beam technique for measuring surface diffusion coefficients

The effective surface diffusion coefficient of Ga along the [110] direction on vicinal GaAs(001)2×4 surfaces during molecular-beam epitaxy was measured using specular ion current measurements. In this technique, 3 keV Ar ions were impinged upon the surface at a glancing angle (typically 3°), and the specularly scattered ion current was measured. Since specular reflections require a locally flat surface, adatoms cause a decrease in the measured current, allowing an average adatom density measurement. The time dependence of the Ga adatom population was measured during and after Ga deposition. Diffusion coefficients, obtained from the adatom lifetimes using a simple model of diffusion to the step edges, were fit well by the expression D=2×10−9 exp(−0.73 eV/kT)cm2/s from 400 to 600 °C.

Auger electron spectroscopy of the kinetics of evaporation of palladium beaded films from sapphire substrate

) surface of sapphire were investigated by Auger electron spectroscopy in the temperature range of 1053–1083 K. It was found that the decrease of the effective thickness during heat treatments under 10-8 mbar was caused by evaporation from the surface of the substrate. Using the model developed by Kaganovskii and Beke we have established that the process is controlled by surface diffusion. It was shown that the evaporation rate of palladium adatoms from an alumina surface is given by Io=1.45×1036exp{-(393±46 kJ/mol)/RT}ms, and, on the basis of an estimation of the surface diffusion length, the effective surface diffusion coefficient Ds was evaluated:

Kinetics of Adsorption from Single and Binary Solutions on Activated Carbons with Chemically Different Surfaces

The effective surface diffusion coefficient DS for model adsorption systems was determined experimentally in a mixer tank with a constant solution volume. As an adsorbent, modified activated carbons with chemically different surfaces were used. The adsorbates were single and binary aqueous solutions of 10 phenolic compounds. The effect of the physicochemical parameters characteristic for the adsorption system (pure energy of adsorption, dipole moment, molar reaction, parachor, solubility, molecular diffusion coefficient in water) on changes in the DS values was analyzed. It was observed that the extent to which DS decreased for adsorption from binary solutions relative to single solutions depended on the adsorbent–adsorbate interactions and the content of oxygen on the activated carbon surfaces.

Surface diffusion of hydrogen sulfide and sulfur dioxide in alumina membranes in the continuum regime

Surface diffusion of H 2S and SO 2 in alumina membranes with an average pore diameter of ca. 350 mm, impregnated with γ-Al 2O 3, is studied as a function of pressure at temperatures between 446 and 557 K. Diffusion through the gas phase in the pores of the membrane almost entirely takes place in the continuum regime. The pressure dependence of transport by diffusion through the gas phase and by surface diffusion is different, a fact which is used for the interpretation of the steady-state diffusion measurements. It is observed that the contribution of surface diffusion to the total diffusion through the membrane can be as high as about 40%. The surface diffusion data obtained are interpreted by a theoretical surface diffusion model using two parameters to be fitted, the product of the effective surface diffusion coefficient, the concentration of the adsorption sites and the adsorption parameter as well as the adsorption parameter itself. The parameters fitted for SO 2 show a remarkable change between 498 and 525 K. This probably maybe attributed to a change in the adsorbed species also reported in literature. The parameters fitted for H 2S at different temperatures show a maximum; however, the product of the effective surface diffusion coefficient and the concentration of the adsorption sites increases with temperature.

A variational calculation of the effective surface diffusion coefficient and tortuosity

A variational upper bound on the effective diffusion coefficient for a porous slab of arbitrary pore geometry is derived for liquid or gas phase Fickia bulk diffusion in the void and Fickian surface diffusion on the pore walls. Calculations of the effective diffusivity are performed for a model pore st generated by randomly placed, freely overlapping solid spheres all of the same radius. An analytical for the effective diffusion coefficient is compare usual engineering model of parallel surface and void diffusion. An equation for the surface tortuosity for liquid and gas filled pores in terms of the is obtained.

Copper and silver contamination of thick film gold surfaces

The surface composition of thick film gold fired onto alumina substrates was studied using Auger electron spectroscopy. Two configurations were evaluated: (1) isolated gold films and (2) gold films contacted by partially overlapping Pd Ag films. The necessary Auger calibration factors were developed so that compositions could be given in atomic per cent. Two principal results were obtained. (1) The “gold” surface contained copper oxide immediately after the gold firing step. The surface copper was believed to come from copper oxide which is added to the gold paste to promote adhesion of the gold to the Al 2O 3 substrate. Auger surface analysis showed that the gold surface contained about 7 at.% Cu and depth profiling showed that the copper concentration decreased to less than 1 at.% after a depth of about 300A˚into the gold. (2) The gold surface also contained about 9 at.% Ag after firing the Pd Ag. The silver was thought to surface diffuse over the gold from the contacting Pd Ag. An effective surface diffusion coefficient at 850°C was estimated to be at least 2 × 10 −6 cm 2 s −1.

Concentration dependence of effective surface diffusion coefficient in aqueous phase adsorption on activated carbon

Concentration dependence of effective surface diffusion coefficient in aqueous phase adsorption on activated carbon

Adsorption of poly(oxyethylene) of various molecular weights from aqueous solutions on activated carbon.

Adsorption of poly(oxyethylene) of nine different molecular weights ranging from 194 to 2.49×106 from aqueous solution of 100 mg/l on activated carbons was tried in a batch finite bath. Activated carbon of coconut shell base, which has micropores of 8 A radius, and of lignite base activated carbon showed a reasonable capacity of adsorption of even poly(oxyethylene) of molecular weight bigger than 104 which has Flory''s dimension of 84 A radius. This suggests that shrinkage of a polymer molecule occurs when it is adsorbed on the carbon surface. As for the rate of adsorption, surface diffusion possibly caused by migration of monomer units is considered dominant, and the effective surface diffusion coefficient was determined for each system by applying the concentration diagrams previously presented by the authors. The effective surface diffusion coefficient on each carbon decreased in proportion to the square root of molecular weight of the polymer.

EFFECTIVE SURFACE DIFFUSION COEFFICIENTS OF VOLATILE ORGANICS ON ACTIVATED CARBON DURING ADSORPTION FROM AQUEOUS SOLUTION

Adsorption of fifteen kinds of volatile organics on activated carbon from aqueous solution was carried out in a batch system. It was assumed that the rate of adsorption was controlled by intraparticle surface diffusion, and the effective surface diffusion coefficient for each organic was determined by applying the concentration-time diagrams appropriated for Freundlich isotherms. The effective surface diffusion coefficients, De, thus obtained, are successfully correlated to the ratio of the boiling point of adsorbate to adsorption temperature as De=1.1×10-4exp(-5.32Tb/T) [cm2/sec]

Surface diffusion of oxides (A review)

The existing data for surface diffusion on oxides have been gathered by two types of measurements: mass transport (grain boundary grooving and scratch smoothing) and tracers. Mass transport methods yield an effective surface diffusion coefficient, without being able to determine directly whether metal ions or oxide ions control the rate of mass transport, while tracer studies measure the diffusion of a single ion. The analysis of the experimental data for both methods must take into account volume diffusion and evaporation-condensation in addition to surface diffusion.Mass transfer measurements on aluminum oxide, magnesium oxide, and uranium dioxide indicate that surface diffusion is dominant over volume diffusion at high temperatures for these oxides. The only tracer measurements of surface diffusion on an oxide to date are measurements on uranium dioxide. Reanalysis of the tracer results indicates the volume diffusion contribution to the tracer motion may be considerable.Small amounts of impurities can greatly affect the apparent surface diffusion coefficient. Departures from stoichiometry can affect the mechanism of material transport by altering the absolute value of the surface diffusion coefficient and its value relative to the volume diffusion coefficient. The validity of models of surface diffusion on oxides is examined in relation to the existing experimental data.