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PubMed2006

DFT-based prediction of high-pressure H2 adsorption on porous carbons at ambient temperatures from low-pressure adsorption data measured at 77 K.

Hydrogen adsorption isotherms were measured both at cryogenic temperatures below 1 atm and at ambient temperature at high pressures, up to 90 atm, on selected porous carbons with various pore structures. The nonlocal density functional theory (NLDFT) model was used to calculate the pore size distributions (PSDs) of the carbons, from H2 adsorption isotherms measured at 77 K, and then to predict H2 adsorption on these carbons at 87 and 298 K. An excellent agreement between the predicted and measured data was obtained. Prior to analyzing the porous carbons, the solid-fluid interaction parameters used in the NLDFT model were derived from H2 adsorption data measured at 77 K on nonporous carbon black. The results show that the NLDFT model with appropriate parameters may be a useful tool for optimizing carbon pore structures and designing adsorption systems for hydrogen storage applications.

Letter↗
PubMed2005

Observation of a one-dimensional adsorption site on carbon nanotubes: adsorption of alkanes of different molecular lengths.

Three well-defined adsorption sites have been found on opened single-wall carbon nanotubes by temperature-programmed desorption measurements for several alkanes. A series of linear chain alkanes from pentane to nonane, as well as a branched alkane molecule, 2,2,4-trimethylpentane, were used to elucidate the effect of molecular length on the capacity of the adsorption sites. The two highest-energy adsorption sites were assigned as the nanotube interior sites and groove sites on the outside of the nanotube bundles. Hybrid Monte Carlo simulations were performed to probe the molecular-level details of adsorption. Both in experiments and in the simulation, the groove sites were seen to behave as one-dimensional adsorption space, demonstrating an inverse dependence of capacity on the length of the adsorbed molecule. In contrast, the capacity of the internal sites was found to depend inversely on the volume occupied by the molecule.

Journal Article↗
PubMed2004

Determining the adsorptive and catalytic properties of strained metal surfaces using adsorption-induced stress.

We demonstrate a model for determining the adsorptive and catalytic properties of strained metal surfaces based on linear elastic theory, using first-principles calculations of CO adsorption on Au and K surfaces and CO dissociation on Ru surface. The model involves a single calculation of the adsorption-induced surface stress on the unstrained metal surface, which determines quantitatively how adsorption energy changes with external strain. The model is generally applicable to both transition- and non-transition-metal surfaces, as well as to different adsorption sites on the same surface. Extending the model to both the reactant and transition state of surface reactions should allow determination of the effect of strain on surface reactivity.

Journal Article↗
PubMed2004

Adsorption from alkane+perfluoroalkane mixtures at fluorophobic and fluorophilic surfaces. II. Crossover from critical adsorption to complete wetting.

Using neutron reflectometry, adsorption from an equimolar mixture of hexane + perfluorohexane to a fluorophobic, octadecyl-coated, silicon substrate has been investigated as a function of temperature in the one-phase region upon approach to liquid-liquid coexistence. The composition of the investigated mixture, x(F) = 0.50, is well removed from the critical composition of x(F) = 0.36, where x(F) is the perfluorohexane mole fraction. To aid the modeling, mixtures with three different neutron refractive index contrasts have been used: namely, mixtures of C(6)H(14) + C(6)F(14) (H-F), C(6)D(14) + C(6)F(14) (D-F), and a mixture of C(6)H(14) + C(6)D(14) + C(6)F(14) which has been adjusted to have the same refractive index as silicon (CMSi). For all three contrasts, the principal features of the composition profile normal to the interface follow similar trends as the temperature T is reduced towards T(0), the coexistence temperature. These features consist of: (i) a hexane-rich primary adsorption layer appended to the octadecyl coupled layer. This primary layer is 22 +/- 5 A thick and becomes increasingly enriched in hexane as T(0) is approached. (ii) A tail that decays exponentially towards the bulk composition with a characteristic decay length zeta. As T(0) is approached, zeta increases. The scattering length density profiles have been converted to volume fraction profiles and the surface excess of hexane Gamma has been determined as a function of temperature for all three contrasts. As T(0) is approached Gamma increases, and its behavior can be represented using the scaling law Gamma approximately |T - T(0)|(-m). The resulting values of m are 0.71 +/- 0.09, 0.68 +/- 0.04, and 0.68 +/- 0.06 for the D-F, H-F, and CMSi contrasts, respectively. The behavior of Gamma with temperature does not adhere to the Gamma approximately |T - T(0)|(-1/3) law expected for complete wetting in systems with van der Waals interactions nor does it correspond to Gamma approximately |T - T(c)|(-0.305) expected for critical adsorption. The magnitude of the exponent m indicates that the adsorption resides in the crossover region between critical adsorption and complete wetting.

Journal Article↗
PubMed2002

A study of the alumina-silica gel adsorbent for the removal of silicic acid from geothermal water: increase in adsorption capacity of the adsorbent due to formation of amorphous aluminosilicate by adsorption of silicic acid.

Two kinds of adsorbents (Si adsorbent and Al adsorbent) for the removal of silicic acid from geothermal water to retard the formation of silica scales were prepared using silicic acid contained in geothermal water. The Si adsorbent was prepared by evaporating geothermal water, and the Al adsorbent was prepared by evaporating geothermal water after the addition of aluminum chloride. The specific surface area of the Si adsorbent was small and it's adsorption capacity of silicic acid was low. Although the specific surface area of the Al adsorbent was also small, it was significantly increased by the adsorption of silicic acid and it's adsorption capacity was high. Based on the change in the local structure of aluminum ion by the adsorption of silicic acid, the Al adsorbent was considered to be silica particles covered with crystalline aluminum hydroxide. Moreover, it was concluded that the increase in the specific surface area of the Al adsorbent and the decrease in the zeta potential were due to the formation of an amorphous aluminosilicate with a large surface area and a negative charge (one 4-coordinated Al) by the reaction between aluminum ions and silicic acids.

Adsorption↗
PubMed1993

Pre-adsorption of a cellulose ether onto polymer surfaces: adsorption of adhesins and platelet activation.

The effect of pre-adsorbed cellulose ethers upon the adsorption of plasma proteins at polymer surfaces was measured by an enzyme-linked immunosorbent assay. The resulting adsorption of plasma proteins was correlated with the sticking and activation of platelets at the blood-material interface, measured by scanning electron microscopy and release of beta-thromboglobulin. Retained Ca2+ activity was made possible by hirudin, a specific thrombin inhibitor, from medicinal leeches as an anticoagulant drug, thereby keeping the initial coagulation cascade and calcium-dependent cell-protein interactions intact during the experiments. The results show that pre-adsorption of a hydrophobic cellulose ether with a flocculation temperature < 37 degrees C, on polyurethane and polytetrafluoroethylene, decreased the adsorption of fibrinogen, fibronectin and vitronectin. A corresponding decrease was found in the number of cells sticking to the surface and in the release of beta-thromboglobulin from platelets.

Adsorption↗
PubMed2003

Experimental determination of adsorption energies, adsorption isotherms, probability density functions, and lateral molecular interactions on CXHY/CaO systems.

Reversed-flow gas chromatography (RF-GC) is extended to the measurements of the probability density function for the adsorption energies as well as the differential energies of adsorption due to lateral interactions of molecules adsorbed on different heterogeneous solid surfaces. All these calculations are based on a non-linear adsorption isotherm model as it is well accepted that the linear one is inadequate for substances such as these used in this work. Thus, some new important physicochemical parameters have been obtained for the characterization of the heterogeneous systems studied. The adsorbent used in this study was calcium oxide. The adsorption of many significant hydrocarbons was investigated. With these systematic experiments under conditions which are similar to the atmospheric ones, an extrapolation of the results obtained to "real" atmospheres with a high degree of confidence is possible.

Adsorption↗
PubMed2005

Use of lipophilic ion adsorption isotherms to determine the surface area and the monolayer capacity of a chromatographic packing, as well as the thermodynamic equilibrium constant for its adsorption.

A method that champions the approaches of two independent research groups, to quantitate the chromatographic stationary phase surface available for lipophilic ion adsorption, is presented. For the first time the non-approximated expression of the electrostatically modified Langmuir adsorption isotherm was used. The non approximated Gouy-Chapman (G-C) theory equation was used to give the rigorous surface potential. The method helps model makers, interested in ionic interactions, determine whether the potential modified Langmuir isotherm can be linearized, and, accordingly, whether simplified retention equations can be properly used. The theory cultivated here allows the estimates not only of the chromatographically accessible surface area, but also of the thermodynamic equilibrium constant for the adsorption of the amphiphile, the standard free energy of its adsorption, and the monolayer capacity of the packing. In addition, it establishes the limit between a theoretical and an empirical use of the Freundlich isotherm to determine the surface area. Estimates of the parameters characterising the chromatographic system are reliable from the physical point of view, and this greatly validates the present comprehensive approach.

Adsorption↗
PubMed2004

Adsorption of Co(II), Ni(II), Cu(II), and Zn(II) on hexagonal templated zirconia obtained thorough a sol-gel process: the effects of nanostructure on adsorption features.

Using zirconium tetrabutoxide, diaminedecane, and diamineoctane as precursors, a templated hexagonal zirconia matrix is synthesized and characterized by X-ray diffractometry and scanning electron microscopy. The adsorption capacity of such a matrix toward Co(II), Ni(II), Cu(II), and Zn(II) from aqueous solutions is studied. The adsorption affinity of the synthesized hexagonal templated zirconia toward the cations is Cu(II)>Zn(II) >>Ni(II)>Co(II). It is also verified that the adsorption of the cations follows a Langmuir and not a Freundlich isotherm. All obtained isotherms are of type I, according to the IUPAC classification. The observed adsorption affinity sequence can be explained by taking into account the velocity constant for the substitution of water molecules into the cation coordination spheres, as well as the Irving-Williams series.

Adsorption↗
PubMed2005

Interactions of anionic dyes with silica-aminopropyl 1. A quantitative multivariate analysis of equilibrium adsorption and adsorption Gibbs free energies.

In this work silica-aminopropyl (Sil-NH2) was synthesized and employed to evaluate the quantitative roles of temperature, pH, dye concentration, and Hg(II) or anionic surfactant SDB interferents in the adsorptions of blue and red remazol dyes in aqueous medium using four distinct 2(4) factorial designs. The results were analyzed statistically using multiple regressions, Student's t-test, analysis of variance, and F-test. Polynomial modelings were used to define the most important factors affecting dye adsorption. The results indicate that the principal effects of dye concentration and pH, as well as most of the interactions of all factors, are statistically very important in relation to the equilibrium adsorption quantities. However, the adsorption Gibbs free energies are influenced, in general, only by pH, dye concentration, and some binary interactions. Temperature changes do not affect the deltaG values significantly.

Adsorption↗
PubMed2006

Adsorption of mixtures of nonionic sugar-based surfactants with other surfactants at solid/liquid interfaces II. Adsorption of n-dodecyl-beta-D-maltoside with a cationic surfactant and a nonionic ethoxylated surfactant on solids.

Synergy and antagonism between sugar-based surfactants, a group of environmentally benign surfactants, and cationic surfactants and nonionic ethoxylated surfactants have been investigated in this study with solids which adsorbs only one or other when presented alone. Sugar-based n-dodecyl-beta-D-maltoside (DM) does not adsorb on silica by itself. However, in mixtures with cationic dodecyltrimethylammonium bromide (DTAB) and nonionic nonylphenol ethoxylated decyl ether (NP-10), DM adsorbs on silica through hydrophobic interactions. In contrast, although DM does adsorb on alumina, the presence of NP-10 reduces the adsorption of DM as well as that of the total surfactant adsorption. Such synergistic/antagonistic effects of sugar-based n-dodecyl-beta-D-maltoside (DM) in mixtures with other surfactants at solid/liquid interfaces were systematically investigated and some general rules on synergy/antagonism in mixed surfactant systems are identified. These results have implications for designing surfactant combinations for controlled adsorption or prevention of adsorption.

Adsorption↗
PubMed2001

Retention modelling of electrostatic and adsorption effects of aliphatic and aromatic carboxylic acids in ion-exclusion chromatography. II. Calculations of adsorption coefficients in unbuffered eluents.

Previous models for the retention behaviour of carboxylic acids in ion-exclusion chromatography are applicable only when the degree of ionisation of the analyte is constant over the entire chromatographic peak. When solutions of sulfuric acid are used as eluents, this condition applies only when the eluent concentration is considerably higher than that of the analyte. Since it is common for dilute solutions of sulfuric acid to be used as eluents, a retention model which accounts for unbuffered eluents has been developed. This model also considers the effects on retention of hydrophobic adsorption of the undissociated and dissociated forms of the analyte onto the stationary phase substrate, as well as the effects of organic solvents added to the eluent. The derivation of this model is presented and it has been evaluated using a comprehensive set of retention data obtained using three different sulfonated stationary phases over a range of eluent conditions. The adsorption coefficients calculated from the model are in accordance with expected trends and showed that both the undissociated and dissociated forms of the analyte acids were retained by hydrophobic adsorption effects, although this adsorption was much stronger for the undissociated analytes.

Adsorption↗
PubMed2003

Effect of natural organic matter on powdered activated carbon adsorption of trace contaminants: characteristics and mechanism of competitive adsorption.

Batch adsorption experiments using powdered activated carbon (PAC) to remove trace synthetic organic chemicals (SOCs) from water containing natural organic matter (NOM) were conducted. The percentage of SOC removed at any contact time and at any PAC dose was observed to be independent of the initial SOC concentration. Equations derived from the ideal adsorbed solution theory and the pore surface diffusion model validated this observation. For the strongly adsorbing SOCs (simazine and simetryn), the percentage of SOC removed was independent only at low initial SOC concentrations. The NOM fraction competing with the weakly adsorbing SOC (asulam) constituted a larger percentage of the total NOM than that competing with the strongly adsorbing SOCs. Although the adsorptive capacities of the SOCs were greatly reduced in water containing NOM compared with those in pure water, the change in the pore diffusion coefficient was insignificant. Therefore, NOM competed with the SOCs for adsorption sites, reducing the adsorptive capacity, but the amount of NOM loading was not so severe that it blocked or filled the pores, hindering the internal diffusion of the SOCs.

Adsorption↗
PubMed1988

Effects of adsorbed water vapor on the adsorption rate constant and the kinetic adsorption capacity of the Wheeler kinetic model.

A recent trend in occupational safety and health has focused on the use of respiratory protective equipment to supplant engineering controls as the primary means of protecting workers from toxic substances. Respirator adsorbent cartridges have been demonstrated to have a finite capacity to adsorb toxic vapors. The knowledge of when this limit is approached or has been exceeded is crucial to the user. The Wheeler kinetic breakthrough model has been shown to describe accurately organic vapor penetration through beds of activated carbon. The model, however, does not account for competitive adsorption of water vapor or other organic vapors. The investigations reported here demonstrate the effect of adsorbed water vapor on the kinetic adsorption parameters (kinetic rate constant and kinetic saturation capacity) of the Wheeler equation. Adsorbent beds were equilibrated at varying concentrations of water vapor and then challenged with carbon tetrachloride vapor-laden air. Dry carbon had an initial rate constant of 62.5 s-1 and a kinetic adsorption capacity of 0.36 g of adsorbed CCl4/gram (g/g) of adsorbent. These parameters decreased in proportion to the amount of water vapor adsorbed, with the minimum predicted values occurring at 100% relative humidity. The minimum experimental value for the kinetic rate constant was 17.6 s-1, a decrease of 73% from the dry carbon values. The minimum predicted value for the kinetic adsorption capacity was 0.16 g/g, a decrease of 45%.

Adsorption↗
PubMed2004

Copper adsorption with Pb and Cd in sand-bentonite liners under various pHs. Part I. Effect on total adsorption.

Municipal solid wastes could be segregated based on their specific heavy metal content and disposing of them in separate landfill cells. Therefore, the objective of the project was to investigate the interaction between copper (Cu) and either lead (Pb) or cadmium (Cd) using equilibrium batch adsorption experiments. A first test consisted of soaking three types of sand-bentonite liner samples (0, 5, and 10% bentonite) with a respective cation exchange capacities (CEC) of 2, 6.4, and 10.8 cmol(+)/kg in one of nine solutions consisting of a combination of three pH levels (3.7, 5.5, and 7.5) and three heavy metal solutions (Cu alone, Cu with Cd, Cu with Pb) each offering a respective heavy metal equivalence of 1, 2, and 2cmol(+)/kg of liner. A second test set consisted in soaking 5% bentonite liner samples in three solutions at a pH of 3.7, with either Cu alone or with Pb or Cd, at 4.8 cmol(+)/kg of liner. For up to 14 days, duplicate samples were sacrificed to determine the supernatant Cu level and pH. The results indicated that under acidic conditions (pH< 6.5), the liner bentonite content, the solution pH and the presence of Pb or Cd significantly influenced Cu adsorption. Lead, and to a lesser extent Cd, competed with Cu for adsorption sites. Under alkaline conditions (pH> 6.5), carbonate and hydroxyl precipitation governed and masked the Pb and Cd competition. Thus, at low pH, limiting the presence of Pb in landfill leachate can improve Cu adsorption.

Adsorption↗
PubMed1990

Protein adsorption on polymer surfaces: calculation of adsorption energies.

In an attempt to understand the mechanisms of protein adsorption at the solid-liquid interface, we have calculated the interaction potential energy between the protein and the polymer surface by a computer simulation approach. The adsorption of four proteins--lysozyme, trypsin, immunoglobulin Fab, and hemoglobin--on five polymer surfaces was examined. The model polymers used for the calculation were polystyrene, polyethylene, polypropylene, poly(hydroxyethyl methacrylate), and poly(vinyl alcohol). All possible orientations of the protein on the polymer surfaces were simulated and the corresponding interaction energies for the initial contact stage of protein adsorption were calculated. In the calculation of interaction energies, the hydrophobic interaction was not treated explicitly owing to the difficulty in the theoretical treatment. The results showed that the interaction energy was dependent on the orientation of the protein on the polymer surfaces. The energy varied from -850 to +600 kJ/mol with an average of about -155 kJ/mol. The interaction energy was also dependent on the type of polymer. The average interaction energies of the four proteins with poly(vinyl alcohol) were always lower than those with the other polymers. The interaction energy was not dependent on the protein size. It was found that the dispersion attraction played the major role in protein adsorption on neutral polymer surfaces.

Adsorption↗
PubMed2001

Adsorption of chloroacetanilide herbicides on soil (I). Structural influence of chloroacetanilide herbicide for their adsorption on soils and its components.

Adsorption of chloroacetanilide herbicide acetochlor, alachlor, metolachlor and propachlor on soils and soil components was determined, and the structural differences of these herbicides were used to explain the order of sorptivity. Adsorption isotherms for all herbicide-soil combinations conformed to the Freundlich equation, and Ki increased with increasing soil organic carbon content. Kd on soil humic acid was greater than that on clay, but association of humic acid with clay reduced the overall adsorption. On all soils and soil humic acids, herbicide adsorption decreased in the order: metolachlor > acetochlor > propachlor > alachlor. On Ca(2+)-montmorrilonite, the order changed to metolachlor > acetochlor > alachlor > propachlor. FT-IR spectra of herbicide-clay or herbicide-humic acid-clay mixtures showed that H-bonding and charge transfer were the primary interaction pathways between these compounds and the surface of clay or humic acids. The different moieties attached to 2-chloro-acetanilide and their unique arrangement may have influenced the binding mechanisms and thus the sorptivity of these herbicides. This study indicates that the structural difference of pesticides in the same classes may be used as a molecular probe to obtain a better understanding of sorption mechanisms of pesticides on soil.

Acetamides↗
PubMed2004

[Determination of soil organic carbon adsorption coefficients of pollutants using soil column liquid chromatography with stepwise adsorption].

A novel method for the determination of soil organic carbon adsorption coefficients (K(oc)) of pollutants was established using soil column liquid chromatography with stepwise adsorption. With this method the adsorption amount of samples at various concentrations on soil can be obtained in a single experiment. The values of K(oc) can be calculated through adsorption isotherms. The soil column can be used without change or desorption throughout the whole experiment. This method leads to a shorter experimental duration, simplified experimental procedure and experimental error reduction. The values of log K(oc) for tebuconazole, azinphos-methyl, diuron, atrazine, prometryn, phenol and naphthalene on the European reference soil of 3# were measured to be 2.70, 2.87, 2.47, 2.26, 3.09, 1.51 and 2.77, respectively, which are consistent with the values reported in the literature.

Adsorption↗