L. D. Asnin

Features of the adsorption of naproxen enantiomers on weak chiral anion-exchangers in nonlinear chromatography

The retention mechanism of the enantiomers of naproxen on a Pirkle-type chiral stationary phase (CSP) was studied. This CSP is made of a porous silica grafted with quinidine carbamate. It can interact with the weak organic electrolyte naproxen either by adsorbing it or by ion-exchange. Using frontal chromatography, we explored the adsorption equilibrium under such experimental conditions that naproxen dissociates or cannot dissociate. Under conditions preventing ionic dissociation, the adsorption isotherms were measured, the adsorption energy distributions determined, and the chromatographic profiles calculated. Three different types of the adsorption sites were found for both enantiomers. The density and the binding energy of these sites depend on the nature of the organic modifier. Different solute species, anions, neutral molecules, solvent-ion associates, and solute dimers can coexist in solution, giving rise to different forms of adsorption. This study showed the unexpected occurrence of secondary steps in the breakthrough profiles of S-naproxen in the adsorption mode at high concentrations. Being enantioselective, this phenomenon was assumed to result from the association of solute molecules involving a chiral selector moiety. A multisite Langmuir adsorption model was used to calculate band profiles. Although this model accounts excellently for the experimental adsorption isotherms, it does not explain all the features of the breakthrough profiles. A comparison between the calculated and experimental profiles allowed useful conclusions concerning the effects of the adsorbate-adsorbate and adsorbate-solvent interactions on the adsorption mechanism.

Retention of Naproxen enantiomers on the chiral stationary phase Whelk-O1 under reversed-phase conditions. A reconsideration of the adsorption mechanism in the light of new experimental data

A model of adsorption of the Naproxen enantiomers on the (S,S)-Whelk-O1 chiral stationary phase that we recently proposed was found to be inconsistent with new experimental data. A new model taking into account the variation of the dissociation coefficient of the analyte during elution of the band is discussed.

Thermodynamic parameters of adsorption described by the logarithmic Temkin isotherm

Thermodynamic analysis of the adsorption Temkin isotherm was performed. The equations that describe the dependence of the thermodynamic functions on the amount adsorbed were obtained. A relationship between the excess differential and mean molar thermodynamic functions was found. The thermodynamic approach does not contradict the molecular statistical theory and appears to be more general.

Calculation of the sticking coefficient in the case of the linear adsorption isotherm

The expression of the sticking coefficient for the linear adsorption isotherm was derived. The sticking coefficient is an increasing temperature function and directly related to the entropy factor. The limits of applicability of the derived equation are discussed.

Adsorption of chlorobenzene on γ-Al2O3 obtained by calcination of boehmite at various temperatures

The influence exerted by the temperature of boehmite calcination in air within the range 450-600°C on the adsorption properties of the resulting γ-Al2O3 with respect to chlorobenzene was studied.

Adsorption of chlorobenzene and benzene on γ-Al2O3/atl>

Adsorption of chlorobenzene and benzene on γ-Al2O3 was investigated in the 413--572 K temperature region at an adsorbate partial pressure ranging from 2 to 1000 Pa. The adsorption isotherms were measured and the isosteric heats and the entropy characteristics of adsorption were determined. The experimentally found and theoretically calculated entropy changes upon adsorption were compared. The mobility of the molecules of both adsorbates in the adsorption layer was limited with respect to that predicted by the ideal two-dimensional gas model. The mechanism of adsorption of benzene and chlorobenzene is discussed.

Thermodynamic quantities of adsorption described by Freundlich isotherm

Thermodynamic analysis of the Freundlich adsorption isotherm was performed. Equations describing the thermodynamic characteristics of adsorption as a function of the adsorbed quantity are presented, and relationships between the excess differential and average molar thermodynamic adsorption quantities were obtained. It was shown that the thermodynamic approach does not contradict the molecular statistical theory and provides more general equations. A model of adsorption described by the Freundlich isotherm is discussed.

Adsorption of benzene on the V2O5/γ-Al2O3 catalyst

Adsorption of benzene on the V2O5/γ-Al2O3 catalysts was studied in the temperature interval from 443 to 493 K and at partial pressures of the adsorbate ranging from 1 to 400 Pa. The adsorption isotherms were plotted. The isosteric heats and various entropy characteristics of adsorption were determined. Mobility of benzene in the adsorption layer is restricted compared to the model of ideal dimeric gas. The adsorbed amounts of benzene and chlorobenzene are compared.

Adsorption of Chlorobenzene on V2O5/γ-Al2O3 Catalyst

Isotherms of chlorobenzene adsorption on V2O5/γ-Al2O3 catalyst within the 0.07-18 Pa range of adsorbate partial pressure were measured, and certain thermodynamic characteristics of adsorption were found.

Description of the dynamics of vapor adsorption in a fixed bed of an adsorbent using various approximations of the mixed-diffusion model

The use of the mixed-diffusion model for calculating the dynamics of adsorption of the vapors of volatile organic compounds in a fixed bed of a porous adsorbent is considered. The V2O5/Al2O3 system is used as an example. It is shown that a simplified version of the mixed-diffusion model in which the influence of axial dispersion is neglected and the establishment of equilibrium on the external boundary of a grain is assumed gives distorted knowledge of the real dynamics of the process. The complete mixed-diffusion model adequately reflects the influence of the contributions of axial dispersion and external and internal mass transfer in the smearing of the sorption front, but diversions from the assumptions of the model that manifest themselves in the concentration dependence of an apparent internal-diffusion coefficient are also observed in this case. The considerable advantage of the model is that using a single-fitting parameter allows experimental data to be approximated with a satisfactory accuracy.