Standard nitrogen adsorption data for α-alumina and their use for characterization of mesoporous alumina-based materials

Nitrogen adsorption isotherm measured at ?196 °C for a macroporous α-alumina (α-Al₂O₃) is reported. This isotherm is compared with the previously reported adsorption data measured on LiChrospher 1000 silica and with available reference isotherms measured at moderate and high relative pressures on macroporous aluminas. The isotherm reported in this work for α-Al₂O₃ and that recorded previously on LiChrospher 1000 silica were used as reference data for adsorption characterization of ordered and disordered mesoporous aluminas by α _s -plot analysis and pore size analysis. It is shown that both reference isotherms provide almost identical adsorption characteristics of the aforementioned mesoporous aluminas, indicating that the available reference data for the silica surface are also suitable for adsorption analysis of alumina-based materials.     

Determination of competitive adsorption isotherms applying the nonlinear frequency response method: Part II. Experimental demonstration

This paper demonstrates an experimental application of the nonlinear frequency response (FR) method extension to determine adsorption isotherms of binary mixtures. This method, based on the analysis of the response of a chromatographic column subjected to the sinusoidal inlet concentration changes, is shown to be an alternative for isotherm determination. The critical issue related to the successful application of the method is to reach experimentally the low frequency asymptotic behaviour of the corresponding frequency response functions (FRFs). Although, there are different possibilities to perform periodical inlet concentration changes, in this paper only simultaneous changes for both components were considered. The adsorption of phenol and 2-phenylethanol on octadecyl silica was analyzed experimentally using a mixture of methanol and water as a solvent. Parameters of competitive isotherms were also estimated for comparison using the classical perturbation method. Despite certain differences between competitive isotherms estimated with the two methods that were found, the obtained results show the potential of the nonlinear FR method for measuring competitive isotherms.     

Displacement processes on hydrophilic/hydrophobic surfaces in methanol-water mixtures

The adsorption from methanol/water and the enthalpy of displacement of methanol by water were studied on K-60 silica gel, on graphitized PRINTEX-80, and onn-alkylammonium vermiculites. The adsorption between the hydrophobized silicate layers was followed by x-ray measurements. The excess isotherms of alkylammonium vermiculites in methanol/water exhibit two maxima corresponding to the two steps of the individual isotherm. The equilibrium constant of the exchange of water by methanol is calculated. For calculating the adsorption capacities and molar adsorption potentials, a new equation is proposed which combines the adsorption excess quantities with free energy and enthalpy functions.     

Influence of free-space calibration using He on the measurement of adsorption isotherms

It is well known that helium (He) molecules that remain inside micropores after free-space calibration at a low temperature (77.4 K) affect the shape of an adsorption isotherm, especially in a very low relative pressure region. This negative effect of the remaining He leads to a misunderstanding of the porous characteristics, such as micropore size distribution and surface properties. However, it is still believed that such erroneous interpretations are limited to narrow microporous materials such as activated carbon and measurements at low temperatures, namely the measurement of the adsorption of N₂ and Ar at their boiling points. Here we report a systematic investigation of the influence of free-space calibration using He on microporous, mesoporous and non-porous materials. Zeolite Y, mesoporous silica, carbon black and aerosil 200 were used for the measurements. N₂, H₂O and CO₂ adsorption isotherms were measured at 77.4, 298 and 298 K, respectively. Free-space calibration was carried out before and after the isotherm measurement for each sample. Although the influence of the He that remained in the sample was small for the non-porous sample, the shape of the isotherms for the other samples in a low relative pressure region was rather affected by the timing of the free-space calibration even for the mesoporous sample, and at an ambient temperature.     

HPLC study of the adsorption from solutions on two silicagels

Summary In this paper, we report and discuss the adsorption isotherms of hydro-organic mixtures and of compounds used as mobile phase in reversed phase and normal phase high performance liquid chromatography (RP-HPLC and NP-HPLC, respectively). This work is the first attempt to study the solid-liquid interface between two types of chromatographic silica surfaces and pure organic adsorbates from water and from organic eluents by HPLC. Frequently, among the dynamic techniques the method of choice for the measurement of an adsorption isotherm is frontal analysis. We suggest here the combination of the technique based on peak asymmetry calibration and peak profiles, which allows calculations directly from integration data. The group of systems studied permits the analysis of the intermolecular interactions on the silica surface. Particular attention was given to the system methanol-water and the measurement of the adsorption ofn-octanol from methanol on RP-silica was also carried out.     

Adsorption-desorption isotherm hysteresis of phenol on a C18-bonded surface

Single component adsorption and desorption isotherms of phenol were measured on a high-efficiency Kromasil-C18 column (N = 15000 theoretical plates) with pure water as the mobile phase. Adsorption isotherm data were acquired by frontal analysis (FA) for seven plateau concentrations distributed over the whole accessible range of phenol concentration in pure water (5, 10, 15, 20, 25, 40, and 60 g/l). Desorption isotherm data were derived from the corresponding rear boundaries, using frontal analysis by characteristic points (FACP). A strong adsorption hysteresis was observed. The adsorption of phenol is apparently modeled by a S-shaped isotherm of the first kind while the desorption isotherm is described by a convex upward isotherm. The adsorption breakthrough curves could not be modeled correctly using the adsorption isotherm because of a strong dependence of the accessible free column volume on the phenol concentration in the mobile phase. It seems that retention in water depends on the extent to which the surface is wetted by the mobile phase, extent which is a function of the phenol concentration, and of the local pressure rate, which varies along the column, and on the initial state of the column. By contrast, the desorption profiles agree well with those calculated with the desorption isotherms using the ideal model, due to the high column efficiency. The isotherm model accounting best for the desorption isotherm data and the desorption profiles is the bi-Langmuir model. Its coefficients were calculated using appropriate weights in the fitting procedure. The evolution of the bi-Langmuir isotherm parameters with the initial equilibrium plateau concentration of phenol is discussed. The FACP results reported here are fully consistent with the adsorption data of phenol previously reported and measured by FA with various aqueous solutions of methanol as the mobile phase. They provide a general, empirical adsorption model of phenol that is valid between 0 and 65% of methanol in water.     

Effect of pore expansion and amine functionalization of mesoporous silica on CO2 adsorption over a wide range of conditions

Adsorption of CO₂ was investigated over a wide range of conditions on a series of mesoporous silica adsorbents comprised of conventional MCM-41, pore-expanded MCM-41 silica (PE-MCM-41) and triamine surface-modified PE-MCM-41 (TRI-PE-MCM-41). The isosteric heat of adsorption, calculated from adsorption isotherms at different temperatures (298–328 K), showed a significant increase in CO₂–adsorbent interaction after amine functionnalization of PE-MCM-41, consistent with the high CO₂ uptake in the very low range of CO₂ concentration. The CO₂ adsorption isotherm and kinetics data showed the high potential of TRI-PE-MCM-41 material for CO₂ removal in gas purification and separation applications. With TRI-PE-MCM-41, the CO₂ selectivity over N₂ was drastically improved over a wide range of conditions compared to pure mesoporous silica. Moreover, the adsorption was reversible and fast, and the adsorbent was thermally stable and tolerant to moisture.     

Influence of Water Vapor on Silica Membrane: Adsorption Properties and Percolation Effect

The influence of water vapor on silica membrane with pore size of 4 ? has been investigated in terms of adsorption properties and percolation effect at 50 and 90 oC. Two methods are employed: spectroscopic ellipsometry for water vapor adsorption and gas permeation of binary mixture of helium and H₂O. The adsorption behaviors on the silica membrane comply with the first-order Langmuir isotherm. The investigation demonstrates that helium flux through the silica membrane decreases dramatically in presence of H₂O molecules. The transport of gas molecules through such small pores is believed not to be continuous any more, whereas it is reasonably assumed that the gas molecules hop from one occupied site to another unoccupied one under the potential gradient. When the coverage of H₂O molecules on the silica surface increases, the dramatic decrease of helium flux could be related to percolation effect, where the adsorbed H₂O molecules on the silica surface block the hopping of helium molecules.     

Estimation of single solute adsorption isotherms applying the nonlinear frequency response method using non-optimal frequencies

In order to estimate single solute adsorption isotherms, the nonlinear frequency response (FR) of a chromatographic column is analyzed experimentally and evaluated using the concept of higher order frequency response functions (FRFs) based on the Volterra series and generalized Fourier transform. In this case study, it has been investigated the adsorption of ethyl benzoate on octadecyl silica from a mixture of methanol and water (60:40) as a solvent. Experiments are performed using a standard gradient HPLC unit. For estimation of adsorption isotherms by the nonlinear FR method the column inlet concentration is changed in a nearly sine waveform around several steady-state concentrations. Using this method the first three local derivatives of a single solute adsorption isotherm are estimated from the low frequency asymptotes of the corresponding functions, i.e. the phase and first order derivative of the FRFs. For an accurate estimation of isotherm coefficients periodical experiments should be preformed for frequencies below a certain critical frequency. This is the frequency needed for approaching the low frequency asymptotic behaviour of the corresponding functions close enough, so that errors due to the non-feasibility of experiments with zero frequency can be neglected. Unfortunately, depending on the properties of the system, it can happen (as for the system investigated here) that experiments for the critical frequency would be too long and cannot be realized. In order to study the loss of accuracy of the nonlinear FR method, when it is applied for non-optimal frequencies, experiments are performed for frequencies approximately one order of magnitude higher than the critical frequency required to evaluate the FRF phases. The obtained isotherm model coefficients are compared with the ones estimated using conventional frontal analysis as a reference method. The isotherms determined by two methods are similar, however a closer look reveals that peaks predicted under overloading conditions differ.     

Determination of standard isotherms of the sorption of some vapors with cellulose

Standard isotherms of the sorption of water, methanol, and benzene vapors on cellulose using a cellulose standard are determined. The standard, namely, mesoporous cellulose with specific surface of up to 350 m2/g, is obtained by the method of exchanging water in swollen cellulose with organic solvents. A comparison of the experimental sorption isotherm with the standard isotherm makes it possible to determine the specific surface of celluloses accessible a the given sorbate and, in combination with the Brunauer-Emmett-Teller adsorption equation, to characterize their surface properties. The identity of the sorption properties of the initial and dewatered (porous) celluloses relative to active vapors is shown, which evidences the assumed mechanism of swelling as the sorbent's division into morphological structures with the formation of new surface. A comparative analysis of the sorption properties of cellulose and silica, whose nature of active sorption centers is similar (weak acid hydroxyl groups), has been made. The affinity of the standard isotherms and close values of the cross-sectional area of different sorbates on both sorbents testify the similarity in their sorption behavior. Thus, the processes of sorption with rigid and swelling sorbents can be regarded in a unified context. Therefore, the adsorption models developed for rigid sorbents can be applied to cellulose sorbents to analyze their sorption properties.     

Evaluation of pore size distribution in boundary region of micropore and mesopore using gas adsorption method

This paper discusses an accurate method of pore size distribution evaluation in boundary regions of micropores and mesopores using the gas adsorption process on the basis of the capillary condensation theory, which is liable to be underestimated with the existing BJH and DH methods. A typical nitrogen adsorption isotherm for highly ordered mesoporous silica, which has cylindrical pores with diameter smaller than 4 nm, is considered to be type IV and it is well known for the steep increase of the amount adsorbed through capillary condensation in the region of the relative pressure P/P0 smaller than 0.4. In calculating the distribution of the pore size from the change of the amount adsorbed due to capillary condensation, it is important to accurately predict both the multilayer thickness t of the adsorbed nitrogen molecules and the critical radius rc where capillary condensation occurs. It is necessary to consider the curvature of the adsorption layer-gas phase interface when predicting the multilayer thickness t of nitrogen adsorbed within the pore of highly ordered mesoporous silica. Revision of the Kelvin equation is also required when rc is to be predicted. While the predicted value of t based on the Broekhoff and de Boer theory is matched well with the value of t which is actually measured using highly ordered mesoporous silica, and the predicted value of rc based on the GTKB-Kelvin-cylindrical equation that has been revised considering the effect of the interfacial curvature on the interfacial tension of the adsorption layer-gas phase interface is matched with the value of rc which is actually measured using highly ordered mesoporous silica. A combination method of the Broekhoff and de Boer equation and the GTKB-Kelvin-cylindrical equation is proposed as a means of accurately evaluating, from the nitrogen adsorption isotherm, the pore size distribution in the highly ordered mesoporous silica in boundary region of micropore and mesopore. The proposed new method of pore size evaluation features high accuracy and offers the convenience of obtaining the pore size distribution without repeated calculations by employing the same algorithm as DH method. The pore size predicted by the Halsey equation and the Kelvin equation of the conventional DH method is about 20% smaller than the pore size predicted by the newly proposed evaluation method.     

Heats of adsorption and adsorption heterogeneity for methane, ethane, and carbon dioxide in MCM-41

We report the adsorption isotherms and the isosteric heats of adsorption of pure methane, ethane, and CO2 and a mixture of methane and CO2 in the periodic mesoporous silica MCM-41 using a multicomponent adsorption calorimeter of the Tian-Calvet type, looking in particular at the degree of heterogeneity in the adsorption of these species. The adsorption of methane and ethane in MCM-41 was found to be essentially homogeneous, while the adsorption of pure CO2 and of CO2 from a CO2/methane mixture was found to be significantly heterogeneous, reflecting the electrostatic interactions between CO2 and the adsorbent.     

Molecular organization of hydrophobic molecules and co-adsorbed water in SBA-15 ordered mesoporous silica material

The purpose of this study was to improve our understanding of the molecular organization of hydrophobic guest molecules in the presence of co-adsorbed water inside SBA-15 ordered mesoporous silica material. Understanding this adsorption competition is essential in the development of applications of controlled adsorption and desorption. The poorly water soluble drug compound itraconazole and the fluorescent probe Nile red were selected for the study. The interaction between itraconazole and SBA-15 was investigated using FT-IR, (1)H MAS NMR and (29)Si MAS NMR spectroscopy, by determination of adsorption isotherms and release kinetics in simulated gastric fluid. The distribution and migration of the hydrophobic fluorescent probe Nile red was visualized in situ using confocal fluorescence microscopy. For both molecules, there was a pronounced influence of the co-adsorbed water on adsorption, hydrophobic aggregation and migration in SBA-15 pores. These insights contribute to the development of practical methods for loading ordered mesoporous silica materials with hydrophobic molecules.     

Determination of the adsorption isotherm of methanol on the surface of ice. An experimental and grand canonical Monte Carlo simulation study

The adsorption isotherm of methanol on ice at 200 K has been determined both experimentally and by using the Grand Canonical Monte Carlo computer simulation method. The experimental and simulated isotherms agree well with each other; their deviations can be explained by a small (about 5 K) temperature shift in the simulation data and, possibly, by the non-ideality of the ice surface in the experimental situation. The analysis of the results has revealed that the saturated adsorption layer is monomolecular. At low surface coverage, the adsorption is driven by the methanol-ice interaction; however, at full coverage, methanol-methanol interactions become equally important. Under these conditions, about half of the adsorbed methanol molecules have one hydrogen-bonded water neighbor, and the other half have two hydrogen-bonded water neighbors. The vast majority of the methanols have a hydrogen-bonded methanol neighbor, as well.     

Adsorption hysteresis of nitrogen and argon in pore networks and characterization of novel micro- and mesoporous silicas

We report results of nitrogen and argon adsorption experiments performed at 77.4 and 87.3 K on novel micro/mesoporous silica materials with morphologically different networks of mesopores embedded into microporous matrixes: SE3030 silica with worm-like cylindrical channels of mode diameter of approximately 95 angstroms, KLE silica with cage-like spheroidal pores of ca. 140 angstroms, KLE/IL silica with spheroidal pores of approximately 140 angstroms connected by cylindrical channels of approximately 26 angstroms, and, also for a comparison, on Vycor glass with a disordered network of pores of mode diameter of approximately 70 angstroms. We show that the type of hysteresis loop formed by adsorption/desorption isotherms is determined by different mechanisms of condensation and evaporation and depends upon the shape and size of pores. We demonstrate that adsorption experiments performed with different adsorptives allow for detecting and separating the effects of pore blocking/percolation and cavitation in the course of evaporation. The results confirm that cavitation-controlled evaporation occurs in ink-bottle pores with the neck size smaller than a certain critical value. In this case, the pressure of evaporation does not depend upon the neck size. In pores with larger necks, percolation-controlled evaporation occurs, as observed for nitrogen (at 77.4 K) and argon (at 87.3 K) on porous Vycor glass. We elaborate a novel hybrid nonlocal density functional theory (NLDFT) method for calculations of pore size distributions from adsorption isotherms in the entire range of micro- and mesopores. The NLDFT method, applied to the adsorption branch of the isotherm, takes into account the effect of delayed capillary condensation in pores of different geometries. The pore size data obtained by the NLDFT method for SE3030, KLE, and KLE/IL silicas agree with the data of SANS/SAXS techniques.     

二组分气体在固体上吸附的研究(IV)

测定了丙酮－正己烷、甲苯－正己烷、苯－正己烷、正戊烷－正己烷四个二组分气体在部分石墨化的灯黑上的吸附等温线,并测定了单组分气体吸附等温线．对比了上述四个二组分气体,分别在硅胶、硅烷化硅胶和灯黑上吸附的十二个吸附体系的吸附规律．结果表明,二组分气体在吸附剂上的竞争吸附的强弱,基本上可以通过它们的纯组分气体在吸附剂上第一层吸附热Q1数值的大小加以预测．     

Adsorption at liquid interfaces: The generalized Frumkin isotherm and interfacial structure

The thermodynamics of adsorption of amphiphilic surface-active compounds at the interface between two immiscible liquids is considered. At the interface, these molecules are supposed to replace a few of the adsorbed molecules of both solvents. Classical isotherms of adsorption (Frumkin, Frumkin-Damaskin, Langmuir, Henry) were based on the model of non-penetrable interface, where an adsorbate can substitute only molecules of one solvent. At the interface between two immiscible electrolytes, nonpolar oil/water interfaces, and liquid membranes amphiphilic molecules can substitute molecules of both solvent and classic isotherms cannot be used. The generalization of Frumkin isotherm for permeable and non-permeable interfaces, known as the Markin-Volkov isotherm, gives the possibility to analyze adsorption in a general case. The adsorption isotherms of pentafluorobenzoic acid at the octane/water interface at different pHs were measured by the drop-weight method. The thermodynamic parameters of pentafluorobenzoic acid (PFBA) adsorption at octane/water interface were determined. From the measurements of PFBA adsorption, the structure of the octane/water interface was determined. Substitution of one adsorbed octane molecule requires approximately three adsorbed PFBA molecules. This result shows that the orientation of solvent molecules at the interface is different from the bulk. Adsorbed octane molecules have a lateral orientation with respect to the interface. Gibbs free energy of adsorption equilibrium and thermodynamic parameters of PFBA adsorption show that the adsorption of PFBA at the octane/water interface is accompanied by a reduction in the attraction between adsorbed PFBA molecules as the pH decreases to the acidic region. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 10, pp. 1194–1200. The text was submitted by the authors in English.     

二组分气体在固体上吸附的研究(Ⅳ)

测定了丙酮－正己烷、甲苯－正己烷、苯－正己烷、正戊烷－正己烷四个二组分气体在部分石墨化的灯黑上的吸附等温线，并测定了单组分气体吸附等温线．对比了上述四个二组分气体，分别在硅胶、硅烷化硅胶和灯黑上吸附的十二个吸附体系的吸附规律．结果表明，二组分气体在吸附剂上的竞争吸附的强弱，基本上可以通过它们的纯组分气体在吸附剂上第一层吸附热Q1数值的大小加以预测．     

Modified Langmuir Isotherm for the Description of Cluster Adsorption on Surface Lyophilic Centers

An adsorption isotherm that generalizes the Langmuir equation for the cluster adsorption of several interacting molecules at one center is proposed. In the case of the adsorption of water molecules at surface hydroxyl groups on modified silica it was shown that the energy and structure characteristics of such a system, determined by the ab initio method, agree with the experimental data and lead to satisfactory agreement with the parameters of the proposed isotherm. __________ Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 41, No. 5, pp. 283–289, September–October, 2005.