Molecules characterization, diffusion gases

Adsorbents, and activated carbon in particular, are typically characterized by a highly porous structure. Adsorbents with the highest adsorption capacity for gasoline or fuel vapors have a large pore volume associated with pore diameters on the order of 50 Angstroms or less. When adsorption occurs in these pores, the process is comparable to condensation in which the pores become filled with hquid adsorbate. Fig. 5 depicts the adsorption process, including transfer of adsorbate molecules through the bulk gas phase to the surface of the solid, and diffusion onto internal surfaces of the adsorbent and into the pores. 

Interstellar molecules are grouped by their location in three types of clouds diffuse, dark, and black clouds [1]. The diffuse clouds are characterized by low gas density, predominately... 

Volatilization (also referred to as vaporization or evaporation) is the conversion of a chemical from the sohd or hquid phase to a gas or vapor phase. The partitioning of a volatile compound in the subsurface environment comprises two distinct patterns volatilization of contaminant molecules (from the liquid, sohd, or adsorbed phase) and dispersion of the resulting vapors in the subsurface gas phase or the overlying atmosphere by diffusive and turbulent mixing. Even though the two processes are fundamentally different and controlled by different chemical and environmental factors, they are not wholly independent under natural conditions only by integrating their effects can volatilization be characterized. 

In addition to colliding with other molecules, the particles in a DSMC simulation may interact with solid surfaces. An extensive review of the state of gas-surface interaction modeling is provided by Hurlbut. In almost all DSMC computations, a combination of two ideal limits is employed that may be characterized by surface accommodation coefficients, a. These limits are (1) specular reflection (a = 0) in which reversal of the velocity component normal to the surface is the only change made and (2) diffuse reflection with full thermal accommodation (a = 1), in which new velocity components are sampled from a Maxwellian distribution function at the surface temperature. In reality, the dynamics of particle interaction with real surfaces lies somewhere between these limits and it is common to try and capture such behavior phenomenologically through the use of... 

Knudsen flow is characterized by the mean free path (A) of the molecules, which is larger than the pore size, and hence collisions between the molecules and the pore walls are more frequent than intermolecular collisions. A lower limit for the significance of the Knudsen mechanism has usually been set at dp> 20 A [28]. The classical Knudsen equation for diffusion of gas is... 

Mesoporous structures are commonly characterized with diffraction, electron microscopy methods [14], and gas sorption techniques. The ensemble diffusion behavior of small molecules has been examined with pulsed-field gradient NMR spectroscopy [15] and neutron scattering [16]. Here, we are interested in techniques which give a more direct access to the real structure of the mesoporous host and to the dynamics on a single-molecule basis, and thus reveal structural and dynamic features which are not obscured by ensemble or statistical averaging as in conventional techniques. 

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