We propose a new hybrid reverse Monte Carlo (HRMC) procedure for atomistic modeling of the microstructure of activated carbons whereby the guessed configuration for the HRMC construction simulation is generated using the characterization results (pore size and pore wall thickness distributions) obtained by the interpretation of argon adsorption at 87 K using our improved version of the slit-pore model, termed the finite wall thickness (FWT) model (Nguyen, T. X.; Bhatia, S. K. Langmuir 2004, 20, 3532) . This procedure overcomes limitations arising from the use of short-range potentials in the conventional HRMC method, which make the latter unsuitable for carbons such as activated carbon fibers that are anisotropic with medium-range ordering induced by their complex pore structure. The newly proposed approach is applied specifically for the atomistic construction of an activated carbon fiber ACF15, provided by Kynol Corporation (Nguyen, T. X.; Bhatia, S. K. Carbon 2005, 43, 775) . It is found that the PSD of the ACF15's constructed microstructure is in good agreement with that determined using argon adsorption at 87 K. Furthermore, we have also found that the use of the Lennard-Jones (LJ) carbon-fluid interaction well depth obtained from scaling the flat graphite surface-fluid interaction well depth taken from Steele (Steele, W. A. Surf. Sci. 1973, 36, 317) provides an excellent prediction of experimental adsorption data including the differential heat of adsorption of simple gases (Ar, N(2), CH(4), CO(2)) over a wide range of temperatures and pressures. This finding is in agreement with the enhancement of the LJ carbon-fluid well depth due to the curvature of the carbon surface, found by the use of ab initio calculations (Klauda, J. B.; Jiang, J.; Sandler, S. I. J. Phys. Chem. B 2004, 108, 9842) .
We propose a new algorithm based on application of cluster analysis to group adsorbate molecules of a highly dense adsorbed phase in the atomistic structural model of a disordered material into connected and disconnected clusters, through which pore network connectivity of the material is identified. Our proposed algorithm is then validated using a synthetic pore structure, as well as the reconstructed structure of a saccharose char obtained in our recent work using hybrid reverse Monte Carlo simulation. The algorithm also identifies kinetically closed pores in the latter structural model that are not accessed by adsorbate molecules at low temperature, at which their kinetic energy cannot overcome potential barriers at the mouths of pores that can otherwise accommodate them. The results are validated by transition state theory calculations for N2 and Ar adsorption, showing that N2 can equilibrate in narrow micropores at practical time scales at 300 K, but not at 77 K. Large differences between time scales for micropore entry and exit are predicted at low temperature for N2, the latter being larger by over 3 orders of magnitude, suggesting hysteresis. Similar behavior is predicted for Ar in the same char at 87 K. The results explain several long standing issues such as the observed increase of adsorption of nitrogen with an increase in temperature in coals, , hysteresis phenomena in microporous carbons, , and underprediction of adsorption of supercritical gases using structural parameters extracted from subcritical adsorption of nitrogen. Finally, the determination of pore accessibility and connectivity in disordered porous carbons using our proposed model enables one to obtain correct adsorbed quantities as well as self-diffusivities and transport diffusivities using conventional grand canonical Monte Carlo and molecular dynamics simulations.
We present characterization results of silicon carbide-derived carbons (Si-CDCs) prepared from both nano- and micron-sized betaSiC particles by oxidation in pure chlorine atmosphere at various synthesis temperatures (600-1000 degrees C). Subsequently, the adsorption modeling study of simple gases (CH4 and CO2) in these Si-CDC samples for a wide range of pressures and temperatures using our Finite Wall Thickness model [Nguyen, T. X.; Bhatia, S. K. Langmuir 2004, 20, 3532] was also carried out. In general, characterization results showed that the core of Si-CDC particles contains predominantly amorphous material while minor graphitization was also observed on the surface of these particles for all the investigated synthesis temperatures (600-1000 degrees C). Furthermore, postsynthetic heat treatment at 1000 degrees C for 3 days, as well as particle size of precursor (betaSiC) were shown to have slight impact on the graphitization. In spite of the highly disordered nature of Si-CDC samples, the adsorption modeling results revealed that the Finite Wall Thickness model provides reasonably good prediction of experimental adsorption data of CO2 and CH4 in all the investigated Si-CDC samples at the temperatures of 273 K, 313 K, and 333 K for a wide range of pressure up to 200 bar. Furthermore, the impact of the difference in molecular size and geometry between analysis and probing gases on the prediction of the experimental adsorption isotherm in a disordered carbon using the slit-pore model is also found. Finally, the correlation between compressibility of the Si-CDC samples under high pressure adsorption and their synthesis temperature was deduced from the adsorption modeling.
Hitherto, adsorption has been traditionally used to study only the porous structure in disordered materials, while the structure of the solid phase skeleton has been probed by crystallographic methods such as X-ray diffraction. Here we show that for carbons density functional theory, suitably adapted to consider heterogeneity of the pore walls, can be reliably used to probe features of the solid structure hitherto accessibly only approximately even by crystallographic methods. We investigate a range of carbons and determine pore wall thickness distributions using argon adsorption, with results corroborated by X-ray diffraction.
Since desirable in vitro and in vivo characteristics were achieved, chitosan-coated nano-liposomes are promising release devices for the oral delivery of berberine hydrochloride increasing the bioavailability of the drug.
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