Gerhard Rychlicki

Estimation of the pore-size distribution function from the nitrogen adsorption isotherm. Comparison of density functional theory and the method of Do and co-workers

A comparative analysis of the results for the estimation of the pore-size distribution based on the methods of Do and co-workers (ND) and density functional theory (DFT) is given. A new algorithm (ASA, Adsorption Stochastic Algorithm) is adopted and it is shown that this algorithm can be successfully applied for the determination of the PSD curve from the ND method. The obtained results show that, generally, the ND and DFT methods lead to almost the same PSD curves and this similarity is observed for carbons of different origin and possessing different pore structures. However, if the contribution of micropores to the PSD increases, the differences in the fit of ND and DFT to the experimental data are more pronounced.

The influence of activated carbon surface chemical composition on the adsorption of acetaminophen (paracetamol) in vitro: The temperature dependence of adsorption at the neutral pH

Abstract The in vitro adsorption and desorption of acetaminophen from water solution on four activated carbons at three temperatures (300, 310 and 320 K) and at the neutral pH (7) were investigated. The carbons were characterized using the low temperature nitrogen adsorption, the mercury porosimetry, Bachmann’s method, Boehm’s method as well as the water immersion calorimetry. As an initial adsorbent, the de-ashed-commercial, ‘non-modified’ carbon D43/1 (Carbo-Tech, Essen, Germany) was applied. To change the chemical composition of its surface, concentrated nitric and sulfuric acids as well as gaseous ammonia were applied as chemical modificators. The acetaminophen adsorption and desorption isotherms on the ‘non-modified’ as well as on the chemically modified carbons were measured, together with the enthalpy of immersion in paracetamol solution. It is shown that, generally, for all the investigated carbons, acetaminophen adsorption increases with temperature. A slightly marked hysteresis on adsorption–desorption isotherms was observed at higher adsorption values. Among the applied procedures of the changing of carbon surface chemical composition, the modification with fuming sulphuric acid leads to the increase in paracetamol adsorption, whilst the opposite effect is observed for the carbon modified with concentrated nitric acid. The modification in the stream of ammonia practically does not change the adsorption properties towards paracetamol. The changes in the adsorption properties of carbons after modification are analysed using isotherms, adsorbability, relative enthalpy of displacement as well as the values of the integral enthalpy of adsorption. To calculate this enthalpy the solubility of acetaminophen in the investigated range of temperatures was determined, and the enthalpy of solution at infinite dilution was calculated using Abraham’s method.

Developing the solution analogue of the Toth adsorption isotherm equation

The well-known Toth adsorption isotherm equation developed formerly for adsorption of vapors is converted into its solution analogue. It is shown that this equation can be successfully applied to the description of adsorption data of organics on activated carbons.

Fractal dimension of microporous carbon on the basis of Polanyi–Dubinin theory of adsorption. Part IV. The comparative analysis of two alternative solutions of the overall adsorption isotherm equation for microporous fractal solids

Abstract The comparative analysis of two alternative adsorption isotherm equations, i.e. approximate (proposed by Avnir and Jaroniec) and analytical solutions (developed by us and called FRDA equation) of the global adsorption isotherm on microporous fractal solids is presented. The general conclusion is that if adsorption proceeds by a micropore filling mechanism and the pore size distribution is assumed to be Pfeifer and Avnir function, not only the adsorption value but also the process energetics cannot be correctly described by simpler—however generating some erroneous results—approximated isotherm equations and the corresponding thermodynamical formulas, proposed by Avnir, Jaroniec et al.

The new correlation between microporosity of strictly microporous activated carbons and fractal dimension on the basis of the Polanyi–Dubinin theory of adsorption

Abstract Adsorption on microporous fractal carbons is investigated in terms of their microporous structure. The structure is characterised by the micropore-size distribution (MSD) proposed by Pfeifer and Avnir, and the considerations are limited only to the range of micropores, following the IUPAC classification. Beginning with numerical studies performed for model solids with different microporosity, a new relationship between the parameters of the Dubinin–Astakhov equation (adsorption energy, E0, and parameter, nDA) and the fractal dimension (D) is derived. The obtained results are compared with those published by Jaroniec et al. and Ehrburger-Dolle. The general conclusion is that if adsorption proceeds by a micropore filling mechanism and the pore size distribution function is assumed to be the Pfeifer and Avnir one, the relationship between D and parameters of the Dubinin–Astakhov equation is more complicated than has been presumed up to the present.

Fractal dimension of microporous carbon on the basis of Polanyi-Dubinin theory of adsorption. Dubinin-Radushkevich adsorption isotherm equation

A new adsorption isotherm equation has been proposed which allows calculation of the structural parameters of activated carbons, including the fractal dimension. The equation is based on the extension of the potential theory of adsorption which was formulated by Dubinin and Stoeckli, and the pore size distribution proposed by Pfeifer and Avnir. In this paper the properties of the new equation and the energetics of adsorption in fractal microporous solids are discussed. The adsorption results, obtained on the basis of some model calculations, are not in agreement with those published by Avnir and Jaroniec which were based on the approximate solution of the problem presented here.

Study of solvent adsorption on chemically bonded stationary phases by microcalorimetry and liquid chromatography.

A detailed, molecular-level description of the sorption mechanism in reversed-phase liquid chromatography is of great interest to analytical chemists. For this purpose, solvent adsorption in the octadecyl stationary bonded phase was investigated. Preferential adsorption of solvents from an acetonitrile-water and methanol-water mobile phase was measured on a series of non-end-capped octadecyl bonded phases with different coverage densities of bonded ligands using the minor disturbance method. For a comparison, a microcalorimetric study of organic solvent adsorption on the stationary phase was executed. The results from the excess isotherm measurement agree well with the experimental measurement of the heat of immersion of the bonded stationary phases by the test solvents. The microcalorimetric measurement is another method for determination of solvation processes of the stationary phases. Changes of the heat of immersion provide information about the surface accessibility for interaction with solvent molecules. The increase of the stationary phase coverage density reduces the free space between bonded chains and penetration of solvent between organic chains.

Fractal dimension of microporous carbon on the basis of the Polanyi-Dubinin theory of adsorption. Part 3: Adsorption and adsorption thermodynamics in the micropores of fractal carbons

Abstract Fundamental thermodynamic relations are formulated based on the equation of physical adsorption on microporous fractal solids, proposed previously and derived from the Polanyi-Dubinin theory of volume filling of micropores. A new adsorption isotherm and corresponding adsorption heat equations are verified using the experimental data published by Dubinin and Polstyanov of benzene and cyclohexane adsorption and adsorption heat on three microporous carbons. The obtained average correlation coefficients are compared with those from the original Dubinin-Astakhov (DA) equation. The correlation between the theoretical and experimental data is satisfactory, especially in the range of relative adsorptive pressures for which, following Stoeckli, the potential theory is accepted as appropriate. It is shown that, for nearly all the cases, micropore volumes are similar to those obtained from the original DA equation. Fractal dimensions calculated from adsorption data of both sorbates on the same carbon are practically equal and can be treated as constants that characterize the micropores of a solid. Average pore diameters, calculated from the obtained fractal dimensions, and also minimal and maximal pore widths are similar to those determined by the methods proposed by other authors, especially those obtained using the equation of McEnaney, developed from the analysis of SAXS data. The explanation of why the approximate adsorption isotherm equation proposed by Avnir and Jaroniec cannot be applied for the correct determination of a micropore fractal dimension is given.

New phosphorus-containing spherical carbon adsorbents as promising materials in drug adsorption and release.

A simple method of preparation of new high surface area spherical carbon adsorbents is presented. The phosphoric acid activation upon hydrothermally formed spherules was employed to produce carbons having controlled high specific surface area (over 2100m(2)/g), large volumes of pores (1.2cm(3)/g), and high acidity. Prepared from sucrose materials show high adsorption capacities (i.e. 220mg/g(C)) toward paracetamol. It is proved that for these materials the contents of surface phosphorus are responsible for the reversibility of drug adsorption/release process.

Determination of accessible silanols groups on silica gel surfaces using microcalorimetric measurements

The calorimetric measurements of methanol and hexane heats of immersion were carried out on different silica gels. Based on the difference in immersion heats, a methodology for the determination of the number of silanols on the surface is presented. The calculated concentration of residual silanols on the silica gel surface agreed with data found in the literature. The proposed methodology, based on a calculation of possible hydrogen bond formation, was also tested on the series of bonded stationary phases with different coverage densities. A very good correlation between the calculated number of accessible residual silanols and the coverage density of bonded ligands was observed.