A Comparision of Laboratory and Field Based Determinations of Molecular Diffusion Coefficients in a Low Permeability Geologic Medium

2009 ◽  
Vol 43 (17) ◽  
pp. 6730-6736 ◽  
Author(s):  
M. Jim Hendry ◽  
S. Lee Barbour ◽  
Brigitte E. J. Boldt-Leppin ◽  
Laura J. Reifferscheid ◽  
Leonard I. Wassenaar
Keyword(s):  
Diffusion Coefficients ◽  
Molecular Diffusion ◽  
Low Permeability ◽  
Geologic Medium

Molecular Diffusion Processes in Crystalline Microporous Materials

1998 ◽  
Vol 527 ◽  
Author(s):  
G. Sastre ◽  
A. Corma ◽  
C. R. A. Catlow
Keyword(s):  
Molecular Dynamics ◽  
Chemical Composition ◽  
Diffusion Coefficients ◽  
Molecular Diffusion ◽  
Diffusion Processes ◽  
Diffusion Path ◽  
Microporous Structure ◽  
Microporous Material ◽  
Para Xylene ◽  
Channel Systems

ABSTRACTAtomistic Molecular Dynamics are used to simulate diffusion of hydrocarbons inside the microporous structure of siliceous zeolite CIT-I, with chemical composition SiO2. CIT-1 is a crystalline microporous material containing channels formed by rings containing 12 and 10 Si atoms (Figure 1). The dimensions of these two channel systems are sufficient to cause substantial differences in the diffusion of para-xylene and ortho-xylene. Diffusion coefficients as a function of loading of each isomer, and activation energies have been calculated from the simulations. The effect of the isomer size in the diffusion path is also analysed.

Diffusion in Biological Systems

Drug Delivery ◽  
2001 ◽  
Author(s):  
W. Mark Saltzman
Keyword(s):  
Diffusion Coefficient ◽  
Diffusion Coefficients ◽  
Molecular Diffusion ◽  
Biological Systems ◽  
Biological Tissues ◽  
Solute Concentration ◽  
Diffusion Resistance ◽  
Experimental Conditions ◽  
Composition Structure ◽  
Reliable Mechanism

Drug diffusion is an essential mechanism for drug dispersion throughout biological systems. Diffusion is fundamental to the migration of agents in the body and, as we will see in Chapter 9, diffusion can be used as a reliable mechanism for drug delivery. The rate of diffusion (i.e., the diffusion coefficient) depends on the architecture of the diffusing molecule. In the previous chapter a hypothetical solute with a diffusion coefficient of 10-7 cm2/s was used to describe the kinetics of diffusional spread throughout a region. Therapeutic agents have a multitude of sizes and shapes and, hence, diffusion coefficients vary in ways that are not easily predictable. Variability in the properties of agents is not the only difficulty in predicting rates of diffusion. Biological tissues present diverse resistances to molecular diffusion. Resistance to diffusion also depends on architecture: tissue composition, structure, and homogeneity are important variables. This chapter explores the variation in diffusion coefficient for molecules of different size and structure in physiological environments. The first section reviews some of the most important methods used to measure diffusion coefficients, while subsequent sections describe experimental measurements in media of increasing complexity: water, membranes, cells, and tissues. Diffusion coefficients are usually measured by observing changes in solute concentration with time and/or position. In most situations, concentration changes are monitored in laboratory systems of simple geometry; equally simple models (such as the ones developed in Chapter 3) can then be used to determine the diffusion coefficient. However, in biological systems, diffusion almost always occurs in concert with other phenomena that also influence solute concentration, such as bulk motion of fluid or chemical reaction. Therefore, experimental conditions that isolate diffusion—by eliminating or reducing fluid flows, chemical reactions, or metabolism—are often employed. Certain agents are eliminated from a tissue so slowly that the rate of elimination is negligible compared to the rate of dispersion. These molecules can be used as “tracers” to probe mechanisms of dispersion in the tissue, provided that elimination is negligible during the period of measurement. Frequently used tracers include sucrose [1, 2], iodoantipyrene [3], inulin [1], and size-fractionated dextran [3, 4].

scholarly journals Flory–Huggins Photonic Sensors for the Optical Assessment of Molecular Diffusion Coefficients in Polymers

2019 ◽  
Vol 11 (18) ◽  
pp. 16872-16880 ◽  
Author(s):  
Paola Lova ◽  
Giovanni Manfredi ◽  
Chiara Bastianini ◽  
Carlo Mennucci ◽  
Francesco Buatier de Mongeot ◽  
...  
Keyword(s):  
Diffusion Coefficients ◽  
Molecular Diffusion ◽  
Photonic Sensors

scholarly journals Specific features of eddy turbulence in the turbopause region

2014 ◽  
Vol 32 (4) ◽  
pp. 431-442 ◽  
Author(s):  
M. N. Vlasov ◽  
M. C. Kelley
Keyword(s):  
Diffusion Coefficient ◽  
Heat Transport ◽  
Diffusion Coefficients ◽  
Transport Coefficient ◽  
Molecular Diffusion ◽  
Eddy Diffusion ◽  
Cooling Rates ◽  
Eddy Diffusion Coefficient ◽  
Eddy Heat Transport ◽  
The Mean

Abstract. The turbopause region is characterized by transition from the mean molecular mass (constant with altitude) to the mean mass (dependent on altitude). The former is provided by eddy turbulence, and the latter is induced by molecular diffusion. Competition between these processes provides the transition from the homosphere to the heterosphere. The turbopause altitude can be defined by equalizing the eddy and molecular diffusion coefficients and can be located in the upper mesosphere or the lower thermosphere. The height distributions of chemical inert gases very clearly demonstrate the transition from turbulent mixing to the diffusive separation of these gases. Using the height distributions of the chemical inert constituents He, Ar, and N2 given by the MSIS-E-90 model and the continuity equations, the height distribution of the eddy diffusion coefficient in the turbopause region can be inferred. The eddy diffusion coefficient always strongly reduces in the turbopause region. According to our results, eddy turbulence above its peak always cools the atmosphere. However, the cooling rates calculated with the eddy heat transport coefficient equaled to the eddy diffusion coefficient were found to be much larger than the cooling rates corresponding to the neutral temperatures given by the MSIS-E-90 model. The same results were obtained for the eddy diffusion coefficients inferred from different experimental data. The main cause of this large cooling is the very steep negative gradient of the eddy heat transport coefficient, which is equal to the eddy diffusion coefficient if uniform turbulence takes place in the turbopause region. Analysis of wind shear shows that localized turbulence can develop in the turbopause region. In this case, eddy heat transport is not so effective and the strong discrepancy between cooling induced by eddy turbulence and cooling corresponding to the temperature given by the MSIS-E-90 model can be removed.

The determination of molecular diffusion coefficients by the barometric method

2008 ◽  
Vol 82 (7) ◽  
pp. 1225-1228 ◽  
Author(s):  
O. A. Kashirskaya ◽  
V. A. Lotkhov ◽  
V. V. Dil’man
Keyword(s):  
Diffusion Coefficients ◽  
Molecular Diffusion
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