Henryk Grajek

Comparison of the differential isosteric adsorption enthalpies and entropies calculated from chromatographic data

The differential isosteric enthalpies, -deltaH(ads), and entropies, -deltaS(ads), of adsorption were calculated taking the retention times of the peak maxima and the centres of gravity of peaks into account and compared with the results obtained from the adsorption second virial coefficients. A mathematical link between the -deltaH(ads) and -deltaS(ads) magnitudes and experimental data was derived through the Antoine-type equation which enables the -deltaH(ads) and -deltaS(ads) magnitudes to be found from adsorption second virial coefficients, B2S, calculated on the basis of chromatographically determined adsorption isotherm data. The virial coefficients were calculated employing the values of the Tóth and Unilan equation parameters. There are no significant differences to be found between the isosteric enthalpies obtained, whereas the values of the adsorption entropies were the highest for the centre of peak gravity data.

Regeneration of adsorbents by the use of liquid, subcritical and supercritical carbon dioxide

The literature concerning the adsorption and desorption of environmental impurities from adsorbents by means of liquid, subcritical and supercritical carbon dioxide and the authors work on the subject have been reviewed. The influence of the adsorption and desorption temperature, the pressure and the density of the extraction solvent, the solubility of the adsorbate in the extraction solvent, the activation energy for adsorbate desorption and the particle size of the adsorbent on the adsorbate desorption efficiency by this method were discussed.

Application of Kováts retention indices for investigation of adsorption properties of activated carbons

Abstract Inverse gas chromatography has been used to measure the enthalpy and entropy of adsorption of benzene, cyclohexane, cyclohexene, 1,3-cyclohexadiene, n -hexane, n -pentane on three active carbons. The high values of adsorption enthalpy at zero coverage indicated high-energy sites on the surface of the tested carbons. The magnitudes of the enthalpy and entropy of adsorption have been calculated in two ways: from the specific retention volumes referred to column temperature and zero coverage of adsorbate, and from Kovats retention indices.

Changes in the Surface Chemistry and Adsorptive Properties of Active Carbon Previously Oxidised and Heat-Treated at Various Temperatures. I. Physicochemical Properties of the Modified Carbon Surface

Several active carbon samples with different quantities of surface oxides were obtained by annealing previously oxidised material under vacuum at various temperatures (ranging from 180°C to 900°C). Concentrated HNO3 was used as the oxidiser. The oxygen content, the pH of the aqueous slurry and the concentration of surface functional groups were determined for all the carbons. Spectral studies (FT-IR, XPS) were performed for selected samples. The heterogeneous decomposition of hydrogen peroxide was also studied. The catalytic activity for H2O2 decomposition correlated well with the acid consumption on the carbon materials. A linear relationship was derived between the oxygen reduction potentials and the logarithm of the peroxide decomposition rate constants.

Changes in the Surface Chemistry and Adsorptive Properties of Active Carbon Previously Oxidized and Heat-Treated at Various Temperatures. III. Studies of the Adsorption of Organic Solutes from Aqueous Solutions

The dependence between the surface chemistry characteristics of thermally modified active carbon samples with previously oxidized surfaces and their ability to adsorb organic solutes from aqueous solutions was investigated. The characteristics of the surface functional groups and the porous structure of the samples obtained were estimated using various experimental methods. The adsorption isotherms of nitrobenzene and 4-nitrophenol from dilute aqueous solutions onto all the carbon samples obtained were measured. The experimental systems were analyzed in terms of the theory of adsorption onto energetically heterogeneous solids. The relationships between the optimized isotherm parameters, i.e. equilibrium constants and heterogeneity parameters, and the characteristics of the active carbons were discussed.

Determination of dependencies between the specific retention volumes and the parameters characterising adsorbent properties

Propane adsorption isotherms have been chromatographically determined on active carbon for different amounts of the injected adsorbate on column. The dependencies between the specific retention volume corrected to the standard temperature (273.15 K), Vg(273), and the molar differential work of adsorption, A, have been calculated on the basis of the propane isotherms and using the retention times of the peak maxima. The obtained equations: ln Vg(273) = f1(A) and (dW/dA)T.F(C) = f2(ln Vg(273)) have been used to explain the dependency between the chromatographic peak profile and the distribution function of pore volumes filled with propane with respect to the molar differential work of adsorption at different column temperatures (303-318 K).

Liquid crystalline stationary phases with a cyclohexyl ring in the molecule

Abstract The selectivities and polarities of five liquid crystalline stationary phases, derived from cyclohexane and differing as regards the terminal substituents, were determined. The efficiency of the columns in which these stationary phases were deposited was established. The selectivities of the stationary phases are related to the type of mesophase and are comparable to those of other liquid crystalline stationary phases; likewise the effciencies of the columns with these phases are comparble but the polarity is low.

Elucidating the Changes in Activated Carbon Pore Structure as a Result of tert-Butylbenzene Adsorption and its Desorption by Means of Supercritical Carbon Dioxide

Inverse gas chromatography was applied to characterize the changes in pore structure of activated carbon samples (commercial, de-ashed, and de-ashed and oxidized) before and after the adsorption of tert-butylbenzene, and after its desorption by means of supercritical carbon dioxide. The slit-like micropore model was used for the description of the pore structure. It was shown that tert-butylbenzene was desorbed with greater difficulty from samples containing basic impurities. Changes in the amounts of surface groups (acidic and basic) during the desorption process were not significant.

A New Method of Calculating Kováts Retention Indices for Substances Chromatographed on Carbon Adsorbents

The net retention volumes relative to zero adsorbate coverage of adsorbent at 273.2 K, i.e. V0S(273), have been determined on two active carbons with well-developed micropore structures. Kováts retention indices have been calculated on the basis of these net retention volumes using two methods. The first involved the substitution of the net retention volumes in the Kováts equation. The second method, which is a new method, involved a description of the net retention volumes by the function log V0S(273) = f(Tc), where Tc is the column temperature, followed by the calculation of the average values for the net retention volumes using the best-fit curve. Kováts retention indices were then calculated from the average values of the net retention volumes. A comparison of the results obtained indicates that the new method appears to yield better values. The Kováts retention indices obtained via both methods were also calculated using a quadratic equation.