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PubMed1986

A measurement of the process of adolescence: standardization and interpretation.

The Objective Measure of Ego Identity Status (Adams, Shea, & Fitch, 1979) was administered to 350 adolescents (12 to 18 years old) to test the Eriksonian theory of a developmental continuum from role diffusion to identity achievement. The results offer construct validity for the dynamic process proposed by Blos and Erikson, but do not support the categorization procedure as set forth by Adams, Shea, and Fitch. The OMEIS questionnaire lends itself to a qualitative and dynamic interpretation rather than to a quantitative and normative approach. It is suggested that the dynamic interpretation is more sensitive to the psychological reality of the developmental process.

Achievement↗
PubMedDate not supplied

High-frequency ventilation: a promising new approach to mechanical ventilation.

Adequate pulmonary ventilation can be achieved in experimental animals and in humans using tidal volumes on the order of the anatomic dead space volume applied at very high ventilatory frequencies (3-30 Hz). Classical mechanism of gas exchange cannot account for these paradoxical observations, but theories based on the concept of augmented diffusion may provide an adequate explanation for this phenomenon. Even though the exact mechanisms accounting for gas exchange are not well understood, a number of successful ventilators have been designed and tested based on the concept of small volume/high frequency ventilation. The differences among these various ventilators are compared and contrasted and possible clinical uses of the technique are discussed.

Animals↗
PubMed1998

Cation diffusion in calcite: determining closure temperatures and the thermal history for the Allan Hills 84001 meteorite.

The presence of zoned Fe, Mg, Ca, and Mn in the carbonate phases associated with the cracks and inclusions of the Allan Hills (ALH) 84001 meteorite provides evidence for constraining the thermal history of the meteorite. Using self- and tracer-diffusion coefficients obtained from laboratory experiments on natural calcite, cooling rates are calculated for various temperatures and diffusion distances to assist in the evaluation of the compositional zoning associated with the carbonate phases in ALH 84001. The closure temperature model provides the average temperature below which compositional zoning will be preserved for a given cooling rate, that is, the temperature at which diffusion will be ineffective in homogenizing the phase. The validity of various theories for the formation of the carbonate globules may be examined, therefore, in view of the diffusion-limited kinetic constraints. Experiments using a thin film-mineral diffusion couple and ion microprobe for depth profiling analysis were performed for the temperature range of 550-800 degrees C to determine self- and tracer-diffusion coefficients for Ca and Mg and in calcite. The resulting activation energies for Ca (Ea(Ca) = 271 +/- 80 kJ/mol) and for Mg (Ea(Mg) = 284 +/- 74 kJ/mol) were used then to calculate a series of cooling rate, grain size, and closure temperature curves. The data indicate, for example, that by the diffusion of Mg in calcite, a 10 micrometers compositional zone would be completely homogenized at a temperature of 300 degrees C for cooling rates <100 K/Ma. These data provide no constraint on formation models that propose a low-temperature fluid precipitation mechanism; however, they indicate that the carbonate globules were not exposed to a high-temperature environment for long time scales following formation.

Calcium↗
PubMed2004

Tractable molecular theory of transport of Lennard-Jones fluids in nanopores.

We present here a tractable theory of transport of simple fluids in cylindrical nanopores, which is applicable over a wide range of densities and pore sizes. In the Henry law low-density region the theory considers the trajectories of molecules oscillating between diffuse wall collisions, while at higher densities beyond this region the contribution from viscous flow becomes significant and is included through our recent approach utilizing a local average density model. The model is validated by means of equilibrium as well nonequilibrium molecular dynamics simulations of supercritical methane transport in cylindrical silica pores over a wide range of temperature, density, and pore size. The model for the Henry law region is exact and found to yield an excellent match with simulations at all conditions, including the single-file region of very small pore size where it is shown to provide the density-independent collective transport coefficient. It is also shown that in the absence of dispersive interactions the model reduces to the classical Knudsen result, but in the presence of such interactions the latter model drastically overpredicts the transport coefficient. For larger micropores beyond the single-file region the transport coefficient is reduced at high density because of intermolecular interactions and hindrance to particle crossings leading to a large decrease in surface slip that is not well represented by the model. However, for mesopores the transport coefficient increases monotonically with density, over the range studied, and is very well predicted by the theory, though at very high density the contribution from surface slip is slightly overpredicted. It is also seen that the concept of activated diffusion, commonly associated with diffusion in small pores, is fundamentally invalid for smooth pores, and the apparent activation energy is not simply related to the minimum pore potential or the adsorption energy as generally assumed.

Journal Article↗
PubMed1954

Filtration, diffusion, and molecular sieving through porous cellulose membranes.

1. A study has been made of the diffusion and filtration of a graded series of molecules (including tritium-labelled water, urea, glucose, antipyrine, sucrose, raffinose, and hemoglobin) in aqueous solution through porous cellulose membranes of three degrees of porosity. 2. Experimental results were in close agreement with predictions based on the membrane pore theory of Pappenheimer et al. (1,2). Restriction to molecular diffusion is a function of pore radius and molecular radius described by equation (11) in the text. Molecular sieving during ultrafiltration is a function of total pore area per unit path length, pore radius, molecular radius, and filtration rate given by equations (16) and (19). 3. Estimates of average pore radius made by means of this theory were considerably larger than estimates made by the method of Elford and Ferry (3) (Table II). Sources of error in the latter method are discussed and a new method of membrane calibration is proposed in which the total cross-sectional area of the pores is measured by direct diffusion of isotope-labelled water. 4. Steady-state osmotic pressures of solutions of sucrose and raffinose measured during molecular sieving through cellulose membranes were found to be close to the "ideal" osmotic pressures calculated by van't Hoff's law. Thus the present experimental data support the methods used by Pappenheimer et al. in their studies on living capillary walls as well as their theory of membrane pore permeability.

Cellulose↗
PubMedDate not supplied

[Molecular diffusion/cerebrospinal fluid flow theory].

AIM: To review the fundamental aspects of the theory of the molecular flow/ cerebrospinal flux described recently and it can explained a group of events in the physiology of the cerebrospinal fluid and the physiopathology of neurological diseases. DEVELOPMENT: This theory was based on the postulate that a decrease of the flux rate of the cerebrospinal fluid was accompanied by an increment of the protein concentration in it and in the nervous system tissue. The increment of the protein transport from the blood to the cerebrospinal fluid not require structural changes or an increase of permeability. The reibergram or Reiber's quotient diagram, with the discriminatory hyperbolic function with its theoretical basis and its clinical relevance confirm the acceptance of the present theory. This theory was based on the first and second Fick's diffusion laws The increment of the molecular diffusion is the cause of the non-linear decrease of the cerebrospinal flux rate because of the blood-cerebrospinal fluid barrier dysfunction. CONCLUSIONS: This theory explain that an increase of the albumin quotient does not means a morphologic change on the barrier structures. The change in the cerebrospinal flux rate it has been considered the principal modulator of the protein concentration in cerebrospinal fluid in pathological conditions characterized by a blood-cerebrospinal fluid barrier dysfunction.

Albumins↗
PubMed1989

The concentration dependence of the diffusion coefficient for bovine pancreatic trypsin inhibitor: a dynamic light scattering study of a small protein.

This paper reports the use of dynamic light scattering to investigate the concentration dependence of the diffusion coefficient for bovine pancreatic trypsin inhibitor (BPTI). BPTI is a small molecular weight protein (6511 Da) that has been the subject of numerous experimental studies. In addition to addressing questions that remain in the literature concerning the aggregation behavior of BPTI, we show that dynamic light scattering can be practically applied to proteins as small as BPTI, and that it can provide a useful means of parameterizing the solution behavior for proteins. We obtained values for the apparent diffusion coefficient of BPTI as a function of concentration over a range of pH values from 2.59 to 9.92 at an ionic strength of 0.3M, and over a range of ionic strength values from 0.1 to 0.5M at a pH of 7.0. The concentration dependence is linear for nearly all the conditions examined, even up to concentrations as high as 65 mg/mL. The average diffusion coefficient obtained at infinite dilution is 14.4 +/- 0.2 x 10(-7) cm2/s. This value agrees with that expected for a BPTI monomer hydrated with less than a monolayer of water. We used the theories of Felderhof, of Batchelor, and of Phillies, along with the DLVO theory to interpret the concentration dependence of the apparent diffusion coefficient. The variations observed with pH and ionic strength can be primarily attributed to screened coulombic interactions. In addition, there is an attractive interaction that is slightly stronger than the repulsive coulombic one, and that is essentially independent of pH and ionic strength. The attractive interactions appear to arise from nonspecific van der Waals interactions and do not lead to the formation of stable aggregates of BPTI.

Animals↗
PubMed2000

Thermodynamic analysis of diffusion of non-electrolytes across plant cuticles in the presence and absence of the plasticiser tributyl phosphate.

Solute mobility in cuticular membranes (CMs) of 14 plant species (Citrus aurantiumn L., Citrus grandis L., Hedera helix L., Ilex aquifolium L., Ilex paraguariensis St.-Hil., Malus domestica Borkh. cv. Golden Delicious, Populus alba L., Prunus laurocerasus L., Pyrus communis L. cv. Bartlett, Conference and Gellerts Butterbirne, Pyrus pyrifolia (Burm. f.) Nakai, Schefflera actinophylla (Endl.) Harms and Strophanthus gratus Baill.) was measured over the temperature range 25-55 degrees C. The five organic model compounds differed in size (130-349 cm3 mol(-1)) and cuticle/water partition coefficient (18-10(8)). For all individual CMs (n = 297), the data were plotted according to the thermodynamic relationship between the preexponential factor (which is proportional to entropy) of the Arrhenius equation and the activation energy (enthalpy) of diffusion (ED). A strict linear correlation was obtained, providing evidence that the five compounds diffused along the same lipophilic diffusion path in all plant species tested. Extracting cuticular waxes from CMs of four plant species (Hedera, Pyrus, Schefflera and Strophanthus) had no effect on the slope of the plot but a parallel displacement towards higher entropy was observed with these polymer matrix (MX) membranes. This displacement is interpreted as a temperature-independent tortuosity factor directly related to entropy. The influence of the plasticiser tributyl phosphate on solute mobility at various temperatures was measured for CM and MX membranes. The plasticiser increased solute mobility and ED was reduced drastically for both membrane types. This plasticiser effect was almost completely reversible, when tributyl phosphate was desorbed from the membranes. For both, plasticised CM and MX, the thermodynamic correlation exists whereby all data points lie on the same line. The data are used to characterise the lipophilic pathway across plant cuticles in terms of the free-volume theory.

Diffusion↗
PubMed1997

Kinetics of Dissolution of beta-Tricalcium Phosphate

The rate of dissolution of beta-tricalcium phosphate (beta-Ca3 (PO4 )2 ; beta-TCP) has been measured in the solution system Ca(OH)2 -H3 PO4 -NaOH-HNO3 -H2 O. The effects of different parameters such as pH, temperature, time, and saturation have been investigated. At zero saturation, the logarithm of the dissolution rate is a linear function of the pH (log(j 0 ) = 2.02 - 0.82 pH; r 2 = 0.993; in mmol/m2 s), indicating diffusion-limited dissolution. A simple calculation of the theoretical rate of a diffusion-controlled process showed that our data are consistent with theory. Moreover, the activation energy for this process is low (E act = 3.9 kcal/mol) also suggesting that the beta-TCP dissolution is controlled by diffusion processes. At increased saturation, the initial beta-TCP dissolution rate decreases much faster than that predicted assuming a diffusion-controlled model. However, this latter model gives a good prediction of the results if it is assumed that beta-TCP dissolution is controlled by the dissolution of an interfacial layer of hydroxyapatite (Ca5 (PO4 )3 OH; HAp): log(j ) = -1.47 + 1.34 log(1 - S HAp ); r 2 = 0.959. The beta-TCP dissolution rate decreases very sharply with time. This effect increases at higher pH or saturation. Several explanations are proposed and discussed, even though none is conclusive.

Journal Article↗
PubMed2006

Time dependent diffusion in a disordered medium with partially absorbing walls: a perturbative approach.

We present an analytical study of the time dependent diffusion coefficient in a dilute suspension of spheres with partially absorbing boundary condition. Following Kirkpatrick [J. Chem. Phys. 76, 4255 (1982)] we obtain a perturbative expansion for the time dependent particle density using volume fraction f of spheres as an expansion parameter. The exact single particle t operator for partially absorbing boundary condition is used to obtain a closed form time dependent diffusion coefficient D(t) accurate to first order in the volume fraction f. Short and long time limits of D(t) are checked against the known short time results for partially or fully absorbing boundary conditions and long time results for reflecting boundary conditions. For fully absorbing boundary condition the long time diffusion coefficient is found to be D(t)=5a(2)/(12fD(0)t)+O((D(0)t/a(2))(-2)) to the first order of perturbation theory. Here f is small but nonzero, D(0) the diffusion coefficient in the absence of spheres, and a the radius of the spheres. The validity of this perturbative result is discussed.

Absorption↗
PubMed1995

Theory of 1/T1 and 1/T2 NMRD profiles of solutions of magnetic nanoparticles.

Organically coated iron oxide crystallites with diameters of 5-50 nm ("nanoparticles") are potential magnetic resonance imaging contrast agents. 1/T1 and 1/T2 of solvent water protons are increased dramatically by magnetic interactions in the "outer sphere" environment of the nanoparticles; subsequent diffusive mixing distributes this relaxation throughout the solvent. Published theory, valid for the solute magnetic energy small compared with thermal energy, is applicable to small magnetic solutes (e.g., gadolinium and manganese diethylenetriaminopentaacetic acid, and nitroxide free radicals) at generally accessible fields (< or = 50 T). It fails for nanoparticles at fields above approximately 0.05 T, i.e., at most imaging fields. The authors have reformulated outer sphere relaxation theory to incorporate progressive magnetic saturation of solute nanoparticles and, in addition, indicate how to use empirical magnetization data for realistic particles when their magnetic properties are not ideal. It is important to handle the effects of rapid thermally induced reorientation of the magnetization of the nanoparticles (their "superparamagnetism") effectively, including their sensitivity to particle size. The theoretical results are presented as the magnetic field dependence (NMRD profiles) of 1/T1 and 1/T2, normalized to Fe content, for three sizes of particles, and then compared with the limited data extant for well-characterized material.

Algorithms↗
PubMed2006

Rotating wave solutions of the FitzHugh-Nagumo equations.

This paper will treat the bifurcation and numerical simulation of rotating wave (RW) solutions of the FitzHugh-Nagumo (FHN) equations. These equations are often used as a simple mathematical model of excitable media. The dependence of the solutions on a uniformly applied current, and also on the diffusion coefficients or domain size will be studied. Ranges of applied current and/or diffusion coefficients in which RW solutions are observed will be described using bifurcation theory and continuation methods. The bifurcation of time-periodic solutions of these FHN equations without diffusion is described first. Similar methods are then used to find RW solutions on a circular ring and numerical simulations are described. These results are then extended to investigate RW solutions on annular rings of finite cross-section. Scaling arguments are used to show how the existence of solutions depends on either the diffusion coefficient or on the size of the region.

Action Potentials↗
PubMed2006

Thermochemistry of the chromium hydroxides Cr(OH)n, n = 2-6, and the oxyhydroxide CrO(OH)4: ab initio predictions.

We here present a high-level ab initio study of the thermochemistry of the chromium hydroxides Cr(OH)n, n = 2-6, and of the oxyhydroxide CrO(OH)4. Optimum geometries and harmonic vibrational frequencies were determined at the B3LYP level of theory using basis sets of triple-zeta quality including polarization and diffuse functions. Heats of formation were obtained from isogyric reaction energies computed at the CCSD(T) level of theory using large basis sets and including corrections for core-valence correlation, scalar relativistic effects, and basis set incompleteness. Additionally, polynomial fits were performed for the heat capacity and the standard enthalpy and entropy over the 100-3000 K temperature range. While our computed heats of formation agree well with previously obtained experimental data for some of these species, our results suggest that revision of the experimental data for others may be appropriate.

Journal Article↗
PubMed1995

Continuing the dialogue: a response to Draper's critique of Fawcett's 'Conceptual models and nursing practice: the reciprocal relationship'.

In their response to Draper's (1993) critique of Fawcett's (1992) paper 'Conceptual models and nursing practice: the reciprocal relationship', the authors argue that Draper's critique is flawed. They point out that Draper fails to acknowledge the diversity within the positivist movement, makes logical leaps, and fails to acknowledge Fawcett's long-standing distinction between middle-range theories and conceptual models and her discussions of the differences in the criteria to be used when evaluating conceptual models and theories.

Clinical Nursing Research↗
PubMed2026

Diffusive Noise Controls Early Stages of Genetic Demixing.

Theoretical descriptions of the stepping-stone model, a cornerstone of spatial population genetics, have long overlooked diffusive noise arising from migration dynamics. We derive a fluctuating hydrodynamic description of this model from microscopic rules, which we then use to demonstrate that diffusive noise significantly alters early-time genetic demixing, which we characterize through heterozygosity, a key measure of diversity. Combining macroscopic fluctuation theory and microscopic simulations, we demonstrate that the scaling of allele-number fluctuations in a spatial domain displays an early-time behavior dominated by diffusive noise. Our results underscore the need for additional terms in existing continuum theories and highlight the necessity of including diffusive noise in models of spatially structured populations.

Models, Genetic↗
PubMed1985

Interaction of spatial diffusion and delays in models of genetic control by repression.

A class of models based on the Jacob and Monod theory of genetic repression for control of biosynthetic pathways in cells is considered. Both spatial diffusion and time delays are taken into account. A method is developed for representing the effects of spatial diffusion as distributed delay terms. This method is applied to two specific models and the interaction between the diffusion and the delays is treated in detail. The destabilization of the steady-state and the bifurcation of oscillatory solutions are studied as functions of the diffusivities and the delays. The limits of very small and very large diffusivities are analyzed and comparisons with well-mixed compartment models are made.

Biometry↗