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PubMed1986

A diffusion-diffusion model for percutaneous drug absorption.

Several theories describing percutaneous drug absorption have been proposed, incorporating the mathematical solutions of differential equations describing percutaneous drug absorption processes where the vehicle and skin are regarded as simple diffusion membranes. By a solution derived from Laplace transforms, the mean residence time MRT and the variance of the residence time VRT in the vehicle are expressed as simple elementary functions of the following five pharmacokinetic parameters characterizing the percutaneous drug absorption: kd, which is defined as the normalized diffusion coefficient of the skin, kc, which is defined as the normalized skin-capillary boundary clearance, the apparent length of diffusion of the skin 1d, the effective length of the vehicle lv, and the diffusion coefficient of the vehicle Dv. All five parameters can be obtained by the methods proposed here. Results of numerical computation indicate that: concentration-distance curves in the vehicle and skin approximate two curves which are simply expressed using trigonometric functions when sufficient time elapses after an ointment application; the most suitable condition for the assumption that the concentration of a drug in the uppermost epidermis can be considered unchanged is the case where the partition coefficient between vehicle and skin is small, and the constancy of drug concentration is even more valid when the effective length of the vehicle is large; and the amount of a drug in the vehicle or skin and the flow rate of the drug from vehicle into skin or from skin into blood becomes linear on a semilogarithmic scale, and the slopes of those lines are small when Dv is small, when the partition coefficient between vehicle and skin is small, when lv is large, or when kc is small. A simple simulation method is also proposed using a biexponential for the concentration-time curve for the skin near the skin-capillary boundary, that is, the flow rate-time curve for drug passing from skin into blood.

Animals↗

PubMed1998

A high diffusion coefficient for coenzyme Q10 might be related to a folded structure.

We measured the lateral diffusion of different coenzyme Q homologues and analogues in model lipid vesicles using the fluorescence collisional quenching technique with pyrene derivatives and found diffusion coefficients in the range of 10(-6) cm2/s. Theoretical diffusion coefficients for these highly hydrophobic components were calculated according to the free volume theory. An important parameter in the free volume theory is the relative dimension between diffusant and solvent: a molecular dynamics computer simulation of the coenzymes yielded their most probable geometries and volumes and revealed surprisingly similar sizes of the short and long homologues, due to a folded structure of the isoprenoid chain in the latter, with a length for coenzyme Q10 of 21 A. Using this information we were able to calculate diffusion coefficients in the range of 10(-6) cm2/s, in good agreement with those found experimentally.

Coenzymes↗

PubMed1988

Salt dependence of DNA structural stabilities in solution. Theoretical predictions versus experiments.

The predictions of currently available theories for treating DNA-diffuse ionic cloud free energy contributions to conformational stability have been tested against experimental data for salt induced B-Z and B-A transitions. The theories considered are (i) Manning's counterion condensation approach (CC), (ii) the idealized Poisson-Boltzmann approximation (PB), and (iii) the potentials of mean force (PMF) approach proposed by Soumpasis. As far as we can judge from comparison with the set of experimental data currently available, it is found that only the latter theory yields satisfactory quantitative results for the dependence of the B-Z and B-A relative stabilities on monovalent salt concentration. The correct application of the PB and CC theories does not yield very low salt Z-B transitions, in contradiction to earlier assertions. At low salt concentrations the PB theory is qualitatively correct in predicting that the B form is electrostatically more favorable than both the A and Z forms, whereas the CC theory is qualitatively wrong predicting that Z-DNA is more stable than both B and A DNA.

DNA↗

PubMed2001

Kinetics and thermodynamics of protein adsorption: a generalized molecular theoretical approach.

The thermodynamics and kinetics of protein adsorption are studied using a molecular theoretical approach. The cases studied include competitive adsorption from mixtures and the effect of conformational changes upon adsorption. The kinetic theory is based on a generalized diffusion equation in which the driving force for motion is the gradient of chemical potentials of the proteins. The time-dependent chemical potentials, as well as the equilibrium behavior of the system, are obtained using a molecular mean-field theory. The theory provides, within the same theoretical formulation, the diffusion and the kinetic (activated) controlled regimes. By separation of ideal and nonideal contributions to the chemical potential, the equation of motion shows a purely diffusive part and the motion of the particles in the potential of mean force resulting from the intermolecular interactions. The theory enables the calculation of the time-dependent surface coverage of proteins, the dynamic surface tension, and the structure of the adsorbed layer in contact with the approaching proteins. For the case of competitive adsorption from a solution containing a mixture of large and small proteins, a variety of different adsorption patterns are observed depending upon the bulk composition, the strength of the interaction between the particles, and the surface and size of the proteins. It is found that the experimentally observed Vroman sequence is predicted in the case that the bulk solution is at a composition with an excess of the small protein, and that the interaction between the large protein and the surface is much larger than that of the smaller protein. The effect of surface conformational changes of the adsorbed proteins in the time-dependent adsorption is studied in detail. The theory predicts regimes of constant density and dynamic surface tension that are long lived but are only intermediates before the final approach to equilibrium. The implications of the findings to the interpretation of experimental observations is discussed.

Adsorption↗

PubMed2004

External noise imposed on the reaction-diffusion system CO+O2-->CO2 on Ir(111) surfaces: experiment and theory.

We study experimentally and theoretically the influence of noise on the fractions of CO and oxygen in the constant gas flow directed at an Ir(111) surface during CO oxidation. Depending on the noise strength and the fraction Y of CO we observe in the deterministically bistable region a large variety of different types of behavior. These include bistable behavior for small noise intensities, transitions from the upper to the lower branch of the bistable loop and vice versa, island nucleation and growth and noise-induced switching. Near the boundary of the bistable region and in the presence of noise the transition between the two branches takes place via very slow domain wall motion with time scales of the order of 10(4)-10(5) s. The experiments were carried out in an UHV system for which the mass flow could be controlled very precisely. The modeling was using the reaction-diffusion system underlying the reaction studied for which all the kinetic coefficients are known rather precisely. Our numerical analysis was performed for one and two spatial dimensions showing qualitatively similar behavior. The comparison between the experimental results and the modeling shows semiquantitative to quantitative agreement.

Journal Article↗

PubMed2005

Effects of lateral diffusion on morphology and dynamics of a microscopic lattice-gas model of pulsed electrodeposition.

The influence of nearest-neighbor diffusion on the decay of a metastable low-coverage phase (monolayer adsorption) in a square lattice-gas model of electrochemical metal deposition is investigated by kinetic Monte Carlo simulations. The phase-transformation dynamics are compared to the well-established Kolmogorov-Johnson-Mehl-Avrami theory. The phase transformation is accelerated by diffusion, but remains in accord with the theory for continuous nucleation up to moderate diffusion rates. At very high diffusion rates the phase-transformation kinetic shows a crossover to instantaneous nucleation. Then, the probability of medium-sized clusters is reduced in favor of large clusters. Upon reversal of the supersaturation, the adsorbate desorbs, but large clusters still tend to grow during the initial stages of desorption. Calculation of the free energy of subcritical clusters by enumeration of lattice animals yields a quasiequilibrium distribution which is in reasonable agreement with the simulation results. This is an improvement relative to classical droplet theory, which fails to describe the distributions, since the macroscopic surface tension is a bad approximation for small clusters.

Journal Article↗

PubMed1991

The use of gradient flow compensation to separate diffusion and microcirculatory flow in MRI.

This paper describes a new MR imaging technique termed Modified Stejskal Tanner versus Flow Compensation (MST/FC) for the separation of diffusion and microcirculatory flow. The theory behind the sequence is explained, along with a five-component model of microcirculation applicable to any "perfusion" imaging technique. Phantom data is presented showing that (1) diffusion effects can be matched between MST and FC (suggesting the possibility of flow-compensated diffusion imaging), and (2) the technique is a quantitative method of separating diffusion and slow (less than 0.25 mm/s) tortuous flow through a Sephadex column. Furthermore, animal images show the technique to be feasible and quantitative in measuring rat brain microcirculation under normal, vasodilated (hypercarbia), and no-flow (post mortem) conditions.

Acetone↗

PubMed1996

On measuring curvature and electrical diffusion coefficients in anisotropic myocardium: comments on "effects of bipolar point and line simulation in anisotropic rabbit epicardium: assessment of the critical radius of curvature for longitudinal block".

Notion of "curvature" and "propagation perpendicular to the activation front" inherited from electrophysiological theory of isotropic reaction-diffusion media do not apply directly to experimental data taken in uniformly anisotropic myocardium. They apply directly only after the concept of "curvature" is normalized and propagation is made to look perpendicular to activation fronts by rescaling distances to achieve isotropy. Without rescaling, the curvature-dependence of longitudinal propagation speed turns out counter-intuitively to measure transverse intercellular electrical coupling.

Action Potentials↗

PubMed1965

The effects of thymus and other lymphoid organs enclosed in millipore diffusion chambers on neonatally thymectomized mice.

When neonatally thymectomized CBA mice were implanted at 9 to 12 days of age with Millipore diffusion chambers (pore size, 0.1 micro) containing either syngeneic or allogeneic neonatal thymus, they were subsequently found to have the capacity to reject skin homografts and to form antibodies to sheep erythrocytes. In spite of displaying restored immune reactivity, thymectomized mice bearing thymus-filled diffusion chambers still had a lymphopenia and diminished numbers of small lymphocytes in their spleens, lymph nodes and Peyer's patches. Comparison of the lymphoid organs of these mice with those of the thymectomized control mice did not reveal any appreciable difference in the numbers of primary follicles or small lymphocytes. It is postulated that the thymus humoral factor induced immunological competence in lymphoid cells which had left the thymus prior to neonatal thymectomy. The paucity of circulating and tissue small lymphocytes in thymectomized animals, the immune reactivity of which was restored by thymus tissue in diffusion chambers, argues against the theory that the thymus humoral factor has a lymphocytosis-stimulating effect. There was no restoration of immune reactivity in those neonatally thymectomized mice which had been implanted with diffusion chambers containing neonatal or adult spleens, or adult lymph nodes. Thus, the competence-inducing factor is elaborated by the thymus but not by the spleen or lymph nodes. Allogeneic (C57Bl) neonatal thymus tissue, enclosed within diffusion chambers, had the capacity to restore the immune reactivity of totally thymectomized CBA mice, not only to skin homografts of a totally unrelated strain (Ak), but also to grafts isogeneic with the donor of the allogeneic thymus. Therefore, there is no strain barrier to the action of thymus humoral factor. To explain the apparent lack of full participation of thymus lymphocytes in immune reactions it is postulated that thymus lymphocytes are functionally immature in situ, and that they leave the thymus before attaining immunological competence. In the periphery, they undergo further maturation under the influence of the competence-inducing factor produced by the thymus.

Animals↗

PubMed2006

Mechanisms influencing levitation and the scaling laws in nanopores: oscillator model theory.

We provide here a detailed theoretical explanation of the floating molecule or levitation effect, for molecules diffusing through nanopores, using the oscillator model theory (Phys. Rev. Lett. 2003, 91, 126102) recently developed in this laboratory. It is shown that on reduction of pore size the effect occurs due to decrease in frequency of wall collision of diffusing particles at a critical pore size. This effect is, however, absent at high temperatures where the ratio of kinetic energy to the solid-fluid interaction strength is sufficiently large. It is shown that the transport diffusivities scale with this ratio. Scaling of transport diffusivities with respect to mass is also observed, even in the presence of interactions.

Journal Article↗

PubMed2000

Determination of Bulk and Surface Diffusion Coefficients from Experimental Data for Thin Liquid Film Drainage.

This note presents a method for the determination of the surface diffusion coefficient and surface diffusion flux. The theoretical considerations are based on the Onsager linear theory for the definition of the surface diffusion flux and on the Einstein theorem for the definition of the surface diffusion parameter. In this interpretation the surface diffusion coefficient differs from the one commonly defined in the literature. It does not depend on the surfactant concentration and it is a function only of the type of surfactant and the liquid/liquid interface. The theoretical calculations indicate that the effect of the surface diffusion on the film drainage is stronger than that predicted by previous theoretical studies. The experimental data for thin liquid film drainage in the case of low surfactant concentration in the continuous phase could be used for the calculation of the bulk and surface diffusion coefficients. In the present study we utilized the experimental data for the drainage of nitrobenzene films stabilized by different concentrations of dodecanol. Copyright 2000 Academic Press.

Journal Article↗

PubMed2003

Studies on self and electrolyte diffusion in cesium halides.

The present paper deals with the determination of the self-diffusion coefficients of 134 Cs(+) ions in CsI, CsBr and CsCl over the concentration range of 5 x 10(-6)-0.01 M and of electrolyte diffusion coefficients of CsI, CsBr and CsCl (5 x 10(-5)-0.1M) labelled with 134 Cs(+) in 1% agar gel at 25 degrees C. These diffusion coefficients are compared with those expected from the Onsager-Fouss and Onsager's theory for electrolyte and self-diffusion, respectively. The disparity between the theoretical and experimental diffusion coefficients over the wide range of concentrations is accounted on the basis of various types of interactions occurring in the ion-gel-water system.

Agar↗

PubMed2004

From innovation to social norm: bounded normative influence.

CORRECTION: Every innovation begins as a deviation from existing social norms. Given the strong effect of social norms and pressure, how can any innovation ever diffuse to the point where it becomes a new social norm? The seeming paradox of how a minority can influence the majority has not been explained well by prevailing social science theory. Computer simulations of the diffusion of a new behavior within the social network of a Bangladesh village led to the discovery of a new principle of social change that resolves this paradox. The results revealed the important but overlooked role played by boundaries that emerge within a social network and how such local boundaries affect the creation of a new social norm. A minority position can become the social norm by means of the process of bounded normative influence. As long as a minority maintains its majority status within its own, locally bounded portion of the network, then it can survive, recruit converts in the near surround, and establish its behavior as the norm for the network as a whole. The process is accelerated when the minority subgroup is centrally located in the network and communicates more frequently and persuasively than the majority.

Bangladesh↗

PubMed1976

Diffusion frequency factors in some simple examples of transition-state rate theory.

In the Eyring rate theory, the rate constant is expressed as a product of a frequency factor (kT/h) and a quotient of partition functions. Continuing an earlier paper, it is shown here by means of simple examples that the Eyring formalism may be extended to include diffusion-controlled processes if a new frequency factor, D/Rlambda, is substituted for kT/h, where D = diffusion coefficient, lambda = thermal de Broglie wavelength, and R = a characteristic distance that depends on the particular case. The Eyring formalism is also applicable in hybrid cases, intermediate between diffusion (D/Rlambda) and dynamics (kT/h). Because these modified frequency factors are not "universal" (as kT/h is), their main use (other than conceptual) would appear to be in cases in which one considers a simple model (with calculable frequency factor) together with a related more complicated model. In the latter case, as an approximation, one would combine the "simple" frequency factor with the "complicated" quotient of partition functions in order to obtain the desired rate constant. Examples are given.

Diffusion↗

PubMed1985

Physicochemical studies on turnip-yellow-mosaic virus. Homogeneity, relative molecular masses, hydrodynamic radii and concentration-dependence of parameters in non-dissociating solvents.

Turnip-yellow-mosaic virus, with its stable, highly spherical and monodisperse character, was chosen as a suitable model substance with which to test hydrodynamic theories of transport. Sedimentation coefficients, diffusion coefficients (obtained through photon correlation spectroscopy) and viscosities were measured accurately as a function of concentration in well-defined and nearly neutral buffer systems. Ancillary information was also obtained from very-low-speed sedimentation-equilibrium experiments. The coefficients expressing the variation in sedimentation and diffusion coefficients with weight concentration were obtained, and by combination with other data it was possible to avoid assumptions concerning solvation and transform such regression coefficients into the form appropriate to volume fractions. Some measure of support for Batchelor's [(1972) J. Fluid Mech. 52, 245-268] calculations was thus obtained, but over most of the pH range the coefficients were significantly smaller than those calculated from his theory. It seems likely that electrostatic interactions are responsible for the discrepancies. Hydrodynamic radii (from diffusion coefficients) were in very fair agreement with those calculated from the thermodynamic excluded-volume term, but were higher than indicated by electron microscopy and X-ray diffraction, a discrepancy ascribable to solvation.

Chemical Phenomena↗

PubMed2005

Field observation and modeling of dissolved fraction sediment-water exchange coefficients for PCBs in the Hudson River.

Chemical fate and transport models that simulate sediment-water exchange of contaminants typically employ empirically determined sediment-water exchange coefficients for the dissolved fraction to describe the net effect of poorly understood mechanisms. This paper presents field-derived observations of the coefficient for 12 PCB congeners and two PCB mixtures in the Thompson Island Pool, Hudson River, and also presents an evaluation of a theoretical sediment-water exchange model. An extensive PCB data set was used to compute apparent coefficients for PCBs in the pool. Average exchange coefficients for the 12 congeners ranged from 2.6 to 18.8 cm/ day, and results showed a strong seasonal dependence. Peak coefficient values occurred in mid-May to early July, preceding peak water temperatures by 1 month and lagging the spring high-flow period. The coefficients increase with increasing partition coefficients, suggesting a dependence on congener properties. The large magnitude of the coefficients and the variation among the congeners is inconsistent with the pore-water molecular-diffusion transport process. A theory-based, mechanistic two-layer model reproduces the nonlinear relationship between the sediment-water exchange coefficients and partition coefficients. This model includes transfer through the mixed sediment layer by bioturbation and diffusion transfer through a water-side boundary layer governed by flow velocity. Results suggest that this algorithm can provide increased accuracyto future system-level fate and transport models for hydrophobic chemicals. The seasonal variation in the transfer coefficient appears to be a poorly understood interaction of physical and biological processes and merits further study.

Diffusion↗

PubMed2000

Molecular dynamics simulations of supercooled and amorphous Co(100-x)Zr(x): atomic mobilities and structural properties

Molecular dynamics simulations are reported on Co(100-x)Zrx in the complete range of compositions. The simulations are based on Hausleitner-Hafner potentials. For the glassy states of the alloys, a comparison of simulated pair-correlation functions with experimental data is presented. Diffusivities were evaluated for the liquid and supercooled melt from isothermal simulation runs. Therefrom a strong dependency on alloy composition is found for the critical temperature T(c)(x) of the mode-coupling theory. Lines of constant averaged diffusivity in the supercooled melts scale closely with this temperature. The ratio of the component diffusivities shows a more involved variation with temperature and composition. These variations reflect differences in the amorphous structure of the alloys, which are quantified in terms of the chemical short-range-order at the level of tetrahedral clusters.

Journal Article↗

PubMed2001

Influence of neutral cyclodextrin concentration on plate numbers in capillary electrophoresis.

A quantitative theory of plate number N in capillary electrophoresis was developed for buffers containing neutral cyclodextrins (CDs) capable of forming inclusion complexes. In the theory, N was modeled by longitudinal diffusion, injection extent, width of the detection window, and the detector time constant. The apparent mobility was modeled as a weighted sum of the mobilities of the free-solution analyte and the inclusion complex. The apparent diffusion coefficient was modeled as a similarly weighted sum. Both the apparent mobility and diffusion coefficient were corrected by functions that compensated for increases of buffer temperature caused by Joule heat. The experimental N's and apparent mobilities of neutral thiourea and of the anions, dansyl D- and L-leucine, dansyl D- and L-aspartic acid, benzoate, and 4-nitrophenolate, were determined in buffers containing from 0 to 15 mM beta-CD. The binding constants, and mobilities and diffusion coefficients of the free-solution analyte and inclusion complex, were calculated as regression coefficients by fitting theory to these determinations. The regression coefficients were shown to have physicochemical meaning, as assessed by literature values, independent measurements, and theoretical predictions. The assessment showed the Nernst-Einstein equation does not relate mobilities and diffusion coefficients at the electrolyte concentration used. The interdependence of mobilities, diffusion coefficients, binding constants, and other dispersion sources was interpreted to evaluate the factors affecting the variation of N with CD concentration. From the interpretation, an approximate equation for N in low-concentration CD buffers was derived. The equation depends on free-solution and inclusion-complex mobilities and diffusion coefficients, the binding constant, the potential difference over the effective capillary length, and the number of plates in a CD-free buffer.

Buffers↗