Langmuir Behavior Research Articles

Adsorption of CO on Supported Gold Nanoparticle Catalysts: A Comparative Study

The adsorption of CO on three different gold nanoparticle catalysts supported on high surface area TiO2 was studied using infrared transmission spectroscopy at room temperature and CO pressures typically used in CO oxidation reactions. The three, real-world catalysts were Au catalysts synthesized in our laboratory from thiol monolayer protected clusters (MPCs) and two commercial catalysts from the World Gold Council (WGC and AuTEK). Within experimental reproducibility, the adsorption data for the three catalysts are indistinguishable. While showing approximately Langmuir behavior, the adsorption data also show coverage dependence, as others have observed for many catalyst systems. Two approaches were used to fit the data, a two-site model and a variable binding constant model. The two-site Langmuir model yielded strong (36%) and weak (64%) binding constants of 2740 and 146 atm−1, respectively. Alternatively, using a sliding-tangent Langmuir fit gave a variable binding constant of 2670−120 atm−1 at room temperature for coverage θ = 0−0.8. The heat of adsorption was then extracted from the binding constants using a literature value for −TΔS. These values were determined as ΔH = −64 and −56 kJ/mol for strong and weak binding according to the two-site model and ΔH = −63 to −56 kJ/mol for coverage θ = 0−0.8 for the variable binding constant model. These values agree well with literature values obtained (i) using supported catalysts under higher pressures and (ii) using model catalysts under higher pressures and ultrahigh vacuum conditions.

The behavior of the liquid-like Langmuir's adsorbed film on air bearing surfaces

The contamination of slider air bearing surfaces may lead to a critical failure of hard disk drive systems. The contamination may be classified into two types: the liquid-like contamination and the particle contamination. In this study, the liquid-like contamination is concerned. It is assumed that the liquid-like contamination is caused by the adsorption of contaminant molecules in air. A mathematic model has been developed to predict the accumulating process of the liquid-like contaminant on air bearing surfaces. Numerical analysis of the behavior of the liquid-like adsorbed film for two different types of air bearing surfaces was conducted by using the proposed model based on the Langmuir adsorption. The results show that the contaminant tends to accumulate on the recession area of the air bearing surface when the viscosity of the liquid-like adsorbed film is low. However it may also accumulate in the front area of the air bearing surface when the viscosity is as high as 20 Pa s.

Quantitative Affinity of Genetically Engineered Repeating Polypeptides to Inorganic Surfaces

Binding kinetics of platinum-, silica-, and gold-binding peptides were investigated using a modified surface plasmon resonance spectroscopy (SPR). Platinum binding septa-peptides, quartz-binding dodecapeptides, and gold-binding 14-aa peptides were originally selected using phage or cell surface display libraries using the mineral or pure forms of these materials. All of the peptides were synthesized singly to investigate their binding kinetics and to assess quantitatively the specific affinity of each to its material of selection. The peptides were also postselection engineered to contain multiple copies of the same original sequences to quantify the effects of repeating units. SPR spectroscopy, normally using gold surfaces, was modified to contain a thin film (a few nm thick) of the material of interest (silica or platinum) on gold to allow the quantitative study of the adsorption kinetics of specific solid-binding peptides. The SPR experiments, carried out at different concentrations, on all three materials substrates, resulted in Langmuir behavior that allowed the determination of the kinetic parameters, including adsorption, desorption, and equilibrium binding constants for each of the solids as well as free energy of adsorption. Furthermore, we also tested multiple repeats of the peptide sequences, specifically three repeats, to see if there is a general trend of increased binding with increased number of binding domains. There was no general trend in the binding strength of the peptides with the increase of the repeat units from one to three, possibly because of the conformational changes between the single and multiple repeat polypeptides. In all cases, however, the binding was strong enough to suggest that these inorganic binding peptides could potentially be used as specific molecular linkers to bind molecular entities to specific solid substrates due to their surface recognition characteristics.

Cost Effective Natural Adsorbent for Fluoride Removal

In this study, the uptake of fluoride by raw diatomite and diatomite activated by heat treatment was investigated. Influential parameters such as contact time, pH and adsorption isotherm were also studied. The results showed that the diatomite heated to 500 oC performed better than other heat treatment products and the equilibration time needed for fluoride removal by the adsorbents was reached at 60 min. Results revealed that the capacity of the adsorbents for removing fluoride is independent of the pH when the pH is higher than 4. The adsorption isotherm of the 500 oC treated diatomite showed its Langmuir behavior. The maximum adsorption capacity of raw diatomite and 500 oC calcinated diatomite are 2.253 and 4.162 mg/g, respectively. Based on the results it can be concluded that the diatomite appears to be an economical and environmentally friendly material for defluoridation of water.

Equilibrium and kinetic analysis of methyl orange sorption on chitosan spheres

Chitosan can be used as adsorbent in the treatment of effluents from the textile industry, especially for negatively charged dyes, due to its cationic polyelectrolyte nature. In this work, the sorption of a model dye, methyl orange, on chitosan hydrobeads is analyzed in terms of equilibrium and kinetic approaches. Equilibrium studies showed that dye adsorption had a mixed Freundlich–Langmuir behavior that had its Langmuir character increased as the pH was increased. In terms of adsorption kinetics, it was found to be of nth-pseudo-order, with fractional n increasing from ∼2 to ∼2.5 as pH and initial dye concentration in the continuous phase were increased. The increase in the apparent pseudo-order was related to changes in mathematical approximation for the solution of the sorption rate equation, which were the result of the decrease in the ratio (number of active sites for adsorption)/(number of adsorbate molecules).

Ordered thiol-functionalized mesoporous silica with macrostructure by true liquid crystal templating route

Ordered thiol-functionalized mesoporous silica has successfully been synthesized via co-condensation of tetramethyl orthosilicate (TMOS) and 3-mercaptopropyltrimethoxysilane (MPTMS) in lyotropic liquid crystalline phase of Brij 56 surfactact (C 16EO 10). In contrast to the common synthesis in micellar solution, true liquid crystal templating synthesis can easily fabricate organic-functionalized mesoporous silica with desirable macro-morphology and the loading amount of organic moieties can also be controlled well. The resultant materials were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), nitrogen sorption, polarized optical microscopy, Fourier transform infrared (FT-IR) spectra, differential scanning calorimetry and thermogravimetric analysis (DSC–TGA), and elemental analyses (EA). Direct evidence of the presence of chemically attached thiol groups was provided by FT-IR, DSC–TGA and EA. Adsorption measurement indicates that the materials are effective for the removal of heavy metal ions from aqueous solutions. The Hg 2+ adsorption isotherm fits Langmuir behavior, but the Type-IV adsorption isotherm for Cd 2+ was unexpectedly observed, which is probably attributed to significantly different Cd 2+ affinity ability for the thiol groups on the external surface and those lining the framework pore channels of the nanoporous adsorbent.

Adsorption Isotherm of Formic Acid on the Surface of Ice, as Seen from Experiments and Grand Canonical Monte Carlo Simulation

The adsorption isotherm of formic acid on proton disordered hexagonal (Ih ) ice is determined by computer simulation (grand canonical Monte Carlo, GCMC) at 200 K and by experiments (flow tube measurements) at 187, 197, 209 and 221 K. The simulation results are in a good agreement with the experimental data; all of the deviations can be explained by a small (10−20 K) temperature shift of the properties of the model system simulated on the phase diagram. The adsorption behavior of formic acid is found to be very complex. Up to the relative pressure of about 0.04, the isotherms exhibit Langmuir behavior. In this pressure range, only the adsorption sites corresponding to the deepest adsorption energy are occupied, and the orientation of the adsorbed molecules is such that there is no considerable lateral interaction between them. Above this pressure, the adsorption isotherm exhibits another steeply rising part, corresponding to the filling of the first molecular layer by molecules occupying different adsorption sites in different orientations. This part of the isotherm can still be described by the Brunauer−Emmett−Teller (BET) equation. In this pressure range, the adsorbed molecules strongly interact with each other, often forming cyclic hydrogen bonding dimers. Finally, the first molecular layer becomes saturated at the relative pressure of about 0.5; above this pressure, multilayer adsorption occurs, as reflected in the sudden increase of the adsorption isotherm. Here, the neighboring formic acid molecules often form cyclic hydrogen-bonding dimers with each other both within the same layer and also between two consecutive layers, involving both oxygen−hydrogen···oxygen (O−H···O) and carbon−hydrogen···oxygen (C−H···O) type hydrogen bonds.

Modeling the effect of pH on biosorption of heavy metals by citrus peels

Biosorption by materials such as citrus peels could be a cost effective technique for removing toxic heavy metals from wastewater. Orange peels, lemon peels and lemon-based protonated pectin peels (PPP) had Langmuir sorption capacities of 0.7–1.2 mequiv./g (39–67 mg/g) of Cd per biosorbent dry weight. A potentiometric titration was interpreted using a continuous p K a spectrum approach. It revealed four acidic sites with p K a values of 3.8, 6.4, 8.4 and 10.7, and a total site quantity of 1.14 mequiv./g. Sorption isotherms of untreated citrus peels showed an unusual shape with two plateau values. Protonated pectin peels on the other hand showed a typical Langmuir behavior with a higher sorption capacity than untreated peels. At lower pH, metal binding was reduced due to increased competition by protons. This was modeled using pH-sensitive isotherm equations. It was not necessary to assume four binding sites; using one site with p K a 3.8 and a quantity of 1.14 mequiv./g was sufficient. It was possible to accurately predict metal uptake at one pH using the metal binding constant determined at a different pH. A 1:1 stoichiometry model fit the sorption isotherms shape better than a 1:2 stoichiometry. For constant pH, the 1:1 stoichiometry reduces to the Langmuir model.

Removal of Cu from aqueous solutions by synthetic hydroxyapatite: EXAFS investigation

The sorption of aqueous Cu on synthetic hydroxyapatite has been investigated by means of the results of a combined structural simulation and extended X-ray absorption fine structure (EXAFS) analysis. The removal of Cu was studied in batch experiments at 25 ± 2 °C. The sorption of Cu follows Langmuir behaviour and was attributed to a two-step mechanism involving surface complexation and ion exchange with Ca resulting in the formation of a copper-containing hydroxyapatite. EXAFS results suggest that the heavy metal is present in the Cu 2+ form. The structural experimental and theoretical analysis shows that Cu is bond to about four O atoms at a distance of about 1.95 Å. In all the studied cases the immobilization site of Cu is the same. The fixation of Cu occurs in the surface sites of hydroxyapatite whereas the sorption in the Ca sites in the inner part of the structure is unlikely.

Thermodynamics of tetracycline adsorption on silica

The adsorption of antibiotic tetracycline onto silica particles was investigated. The effects of pH, electrolyte concentration and temperature on the adsorption equilibrium were examined and the data correlated with a dual site adsorption model. Adsorption of tetracycline onto approximately 9% of the adsorption sites was found to be irreversible, whereas the rest exhibited Langmuir behaviour. Increasing pH of the solution had a strong negative effect on the adsorbed amounts. Adsorption decreased also with increasing ionic strength and temperature. The enthalpy and entropy of adsorption were found to be approximately −16 and −25 J/mol, respectively.

Impact of an error in the column hold-up time for correct adsorption isotherm determination in chromatography: I. Even a small error can lead to a misunderstanding of the retention mechanism

The impact of a realistic error in the column hold-up time on the determination of the adsorption isotherm model was systematically investigated. Frontal analysis and the inverse method were used for the accurate determination of the adsorption isotherm. The true retention times of the breakthrough curves were used with a known hold-up time as reference. The adsorption isotherms were calculated using the same procedure that is used for real experimental adsorption isotherms, where the true hold-up time is unknown. The raw data were analyzed with calculations of adsorption energy distributions (AEDs), Scatchard plots, fitting to different rival adsorption models and finally their ability to predict true profiles. The results show that for a true Langmuir and bi-Langmuir model with an underestimated hold-up time the error may lead to a more heterogeneous model and for overestimated cases false adsorption processes like multi-layer adsorption or solute–solute interaction are assumed. The Scatchard plots for data obtained using a Langmuir adsorption isotherm are nonlinear and the AEDs show clear deviations from Langmuir behavior already at small deviations from the true hold-up time at a moderate surface coverage. The inverse method confirms the result that was obtained from the frontal analysis procedure.

Chemisorption of carbon dioxide on sodium oxide promoted alumina

AbstractNew equilibrium and column dynamic data for chemisorption of carbon dioxide from inert nitrogen at 250, 350, and 450°C were measured on a sample of sodium oxide promoted alumina, which was found to be a reversible chemisorbent for CO2. The equilibrium chemisorption isotherms were Langmuirian in the low pressure region (p <2.0 kPa) with a large gas–solid interaction parameter. The isotherms deviated from the Langmuirian behavior in the higher pressure region. A new analytical model which simultaneously accounted for Langmuirian chemisorption of CO2 on the adsorbent surface and additional reaction between the gaseous and sorbed CO2 molecules was used to describe the measured equilibrium data. The heats of CO2 chemisorption and the additional surface reaction were, respectively, 64.9 and 37.5 kJ/mol. The column breakthrough curves for CO2 sorption from inert N2 on the chemisorbent as well as the desorption of CO2 from the chemisorbent by N2 purge at 350°C could be described by the linear driving force (LDF) model in conjunction with the new sorption isotherm. The same LDF mass transfer coefficients can be used to describe both sorption and desorption processes. The CO2 mass transfer coefficients were (i) independent of feed gas CO2 concentration in the range of the data at a given temperature, and (ii) a weak function of temperature. The ratio of the mass transfer zone length to the column length was very small due to highly favorable CO2 sorption equilibrium. Several sequential cyclic CO2 sorption–desorption column dynamic tests were conducted to demonstrate the apparent stability of the material. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Adsorption of guar gum and CMC on pyrite

The depression of pyrite in sulfide flotation by two types of polysaccharides, CMC and guar, was investigated using micro-flotation tests, zeta potential measurements and adsorption tests. Environmentally and from an occupational safety standpoint – this would be a better processing option than cyanide. The range of variables tested included molecular weight of guar, degree of substitution of CMC, pH and the concentration of calcium ions in the flotation process water. The guar gums depressed pyrite considerably even at low dosages. An optimum dosage of 10 ppm was recommended. The CMC’s were not as effective as the guar gums, and required much higher dosages to achieve pyrite depression. Within the range of CMC’s tested, lower degree of substitution was more effective than high degree of substitution. This was presumably due to lower electrostatic repulsion between the negatively charged CMC and pyrite. When calcium ions were introduced with CMC’s, the adsorption of the CMC was considerably enhanced. No such effect was observed for guar gums. Both CMC and guar gums demonstrated Langmuirian behavior in their adsorption isotherms. Two different adsorption mechanisms were displayed. The guar gum adsorption was attributed to hydrogen bonding and Bronsted acid–base interaction. The CMC was considered to occur through electrostatic interaction in the presence of calcium ions, and Bronsted acid–base interaction depending on pH.

Chemisorption of carbon dioxide on potassium-carbonate-promoted hydrotalcite

New equilibrium and column dynamic data for chemisorption of carbon dioxide from inert nitrogen at 400 and 520 °C were measured on a sample of potassium-carbonate-promoted hydrotalcite, which was a reversible chemisorbent for CO 2. The equilibrium chemisorption isotherms were Langmuirian in the low-pressure region ( p CO 2 < 0.2 atm ) with a large gas–solid interaction parameter. The isotherms deviated from Langmuirian behavior in the higher pressure region. A new analytical model that simultaneously accounted for Langmuirian chemisorption of CO 2 on the adsorbent surface and additional reaction between the gaseous and sorbed CO 2 molecules was proposed to describe the measured equilibrium data. The model was also capable of describing the unique loading dependence of the isosteric heat of chemisorption of CO 2 reported in the literature. The column breakthrough curves for CO 2 sorption from inert N 2 on the chemisorbent could be described by the linear driving force (LDF) model in conjunction with the new sorption isotherm. The CO 2 mass-transfer coefficients were (i) independent of feed gas CO 2 concentration in the range of the data at a given temperature and (ii) a weak function of temperature. The ratio of the mass-transfer zone length to the column length was very low due to highly favorable CO 2 sorption equilibrium.

Dendritic nanopattern for Langmuir layer of Gemini/TiO 2 nanoparticles complex at air–water interface

Nanocomposite Langmuir film of Gemini surfactant/TiO 2 nanoparticles was prepared on clear hydrosols subphase of TiO 2 nanoparticles with average diameters of 8 nm. In comparison with ultrapure water subphase, the surface pressure–area isotherm of Gemini monolayer was significantly expanded in the presence of the titanium oxide nanoparticles. The collapse pressure of Gemini monolayer on pure water is 34 mN/m. While Gemini/TiO 2 nanoparticles composite monolayer is still stable even increasing the surface pressure to 40 mN/m on TiO 2 hydrosols. The dissimilar Langmuir behavior of Gemini/TiO 2 nanoparticles nanocomposite monolayer at air–water interface suggests that interaction between Gemini monolayer and titanium oxide nanoparticles at subphase obviously exists. Morphology of the Langmuir layer investigated via transmission electron microscope indicates that TiO 2 nanoparticles transferred from subphase into Gemini monolayer to form complex. The complex becomes a regular dendritic nanocomposite system gradually as the surface pressure increasing. A mechanism involves the unique molecular structure of Gemini, and the electrostatic interaction between charged TiO 2 nanoparticles and the ionic head groups of Gemini was proposed to explain this phenomenon. The method maybe allows us to introduce some small molecules, instead of conventional dendritic large molecules, to design nanocomposite Langmuir films with desirable nanopattern.

Molecular Hydrogen Storage in Binary THF−H2 Clathrate Hydrates

The hydrogen storage capacity of binary THF-H(2) clathrate hydrate has been determined as a function of formation pressure, THF composition, and time. The amount of hydrogen stored in the stoichiometric hydrate increases with pressure and exhibits asymptotic (Langmuir) behavior to approximately 1.0 wt % H(2). This hydrogen concentration corresponds to one hydrogen molecule occupying each of the small 5(12) cavities and one THF molecule in each large 5(12)6(4) cavity in the hydrate framework. Contrary to previous reports, hydrogen storage was not increased upon decreasing the THF concentration below the stoichiometric 5.6 mol % solution to 0.5 mol %, at constant pressure, even after one week. This provides strong evidence that THF preferentially occupies the large 5(12)6(4) cavity over hydrogen, for the range of experimental conditions tested. The maximum amount of hydrogen stored in this binary hydrate was about 1.0 wt % at moderate pressure (<60 MPa) and is independent of the initial THF concentration over the range of conditions tested.

Design of Simulated Moving Bed Separations: Generalized Langmuir Isotherm

In this paper, exact criteria for the complete separation of binary mixtures in simulated moving bed (SMB) units are developed. They apply to systems subject to the generalized Langmuir isotherm, whose definition yields four different types of isotherms corresponding to different combinations of Langmuir and anti-Langmuir behavior of the components to be separated. These criteria are derived in a similar way in the different cases using the equilibrium theory of chromatography applied to the equivalent true moving bed (TMB) unit. Separation performance is controlled by the dimensionless flow rate ratios. The complete separation region in the operating parameters space has boundaries that are constituted of straight lines and curves, defined through simple and easy-to-use equations. These in turn depend on the adsorption isotherm parameters and on the composition of the feed mixture. The analogies and differences observed among the four isotherms in terms of complete separation region are analyzed and discussed.

Effect of temperature on the adsorption behavior of tryptophan in reversed-phase liquid chromatography

Single-component adsorption isotherm data of l-tryptophan on a C 18-bonded silica column were acquired by frontal analysis (FA), with aqueous mobile phases containing 5% of acetonitrile at five different temperatures between 23 and 62 °C. The non-linear fitting of these data provided the bi-Moreau model for all temperatures as the best isotherm model. The inverse method (IM) was used to derive the parameters at these temperatures from the parameters of the 25 °C isotherm. The adsorption constants and the saturation capacities of the low and high-energy sites decreases by increasing the temperature, while the adsorbate–adsorbate parameters of both sites increase. An excellent agreement was found between the experimental and calculated overloaded band profiles at all the temperatures used. The breakthrough curves obtained and the overloaded band profiles obtained were found to have different shapes according to the range of concentration studied and the temperatures. At low concentration 0.05–0.5 g/L the breakthrough curves and the overloaded band profiles have a front shock and diffuse rear, which indicates langmuirian behavior, but at intermediate 1–2 g/L and high concentration 8 g/L they start to have diffuse fronts and shocks at the rear or more than one shock at the rear which indicates non-langmuirian behavior. At 23 °C the isotherm has another langmuirian part, which appears at high concentration. The behavior of the breakthrough curves is explained by the shape of the isotherm in which all of the isotherms have a langmuirian part (the isotherm is concave upward) and an antilangmuirian part (the isotherm is concave downward). The temperature affected the breakthrough curves by decreasing the time of the appearance of the fronts for all concentration ranges studied, and by decreasing the time difference between the highest concentration and lowest concentration of the fronts, especially the low concentration range at 0.5 g/L. The fronts of the breakthrough curves at high concentration seems to be the most affected by temperature.

Surface Modification of Magnetic Nanoparticles Using Gum Arabic

Magnetite nanoparticles were synthesized and functionalized by coating the particle surfaces with gum arabic (GA) to improve particle stability in aqueous suspensions (i.e. biological media). Particle characterization was performed using transmission electron microscopy (TEM) and dynamic light scattering (DLS) to analyze the morphology and quantify the size distribution of the nanoparticles, respectively. The results from DLS indicated that the GA-treated nanoparticles formed smaller agglomerates as compared to the untreated samples over a 30-h time frame. Thermogravimetric analyses indicated an average weight loss of 23%, showing that GA has a strong affinity toward the iron oxide surface. GA most likely contributes to␣colloid stability via steric stabilization. It was determined that the adsorption of GA onto magnetite exhibits Langmuir behavior.

Effect of the mobile phase composition on the adsorption behavior of tryptophan in reversed-phase liquid chromatography

Single-component adsorption isotherm data of l-tryptophan on a C 18-bonded silica column were acquired by frontal analysis (FA), with aqueous mobile phases containing 2.5, 5, and 7.5% of acetonitrile (ACN) or 7, 10, 15, and 20% of methanol (MeOH). Most of these isotherms have two inflection points and three different parts. The low and the high concentration parts exhibit langmuirian behavior. The intermediate part exhibits anti-langmuirian behavior. The inflection points shift toward higher concentrations with increasing mobile phase concentration in ACN or MeOH, which causes the differences in the isotherm profiles. The nature of the organic modifier and its concentration affect only the isotherm profile and the numerical values of its parameters, not the nature of the best model, which is the bi-Moreau model in all cases. The isotherm profiles depend on the experimental conditions because they affect the intensity of the adsorbate–adsorbate interactions. Overloaded band profiles of tryptophan were recorded with the seven mobile phase compositions. They were used to determine the best values of the isotherm coefficients by the inverse method (IM) of chromatography. There is an excellent agreement between the values of these parameters obtained by FA and by IM. Increasing the concentration of either ACN or MeOH in the mobile phase causes a slight decrease in the saturation capacities of the low and the high energy sites, and in the adsorption constant of the low energy sites. The adsorption constant of the high energy sites increases with increasing concentration of either solvent or is little affected. The adsorbate–adsorbate interaction constants of both low and high energy sites increase for both solvents. Saturation capacities of the high energy sites are higher for ACN than for MeOH.