Change In Adsorption Mechanism Research Articles

An updated study on CH4 isothermal adsorption and isosteric adsorption heat behaviors of variable rank coals

Understanding the gas sorption behavior of coals not only benefits for enhancing coalbed methane (CBM) recovery but also provides important insights for simultaneous coal and gas extraction. This study aims to quantitatively evaluate the CH4 adsorption behaviors by a series of experiments and the applicability of different adsorption dynamic models in different rank coals (0.48–2.59% of Ro,m). Moreover, the isosteric heat and ultimate heat of CH4 adsorption in different rank coals were also analyzed. The results indicate that the fitting accuracy of different adsorption models is presented as Dubinin-Astakhov (D-A) model > Langmuir model > Dubinin-Radushkevich (D-R) model > Freundlich model for different rank coal samples, and the maximum adsorption volume reaches the lowest value at the Ro,m of ~1.4%. This phenomenon indicates that the CH4 adsorption in coals is a micropore filling or multi-layer adsorption process (especially in anthracites at high pressure stage) rather than a simple Langmuir monolayer adsorption process. Moreover, the CH4 adsorption process in coal samples can be divided into three stages: the low relative pressure stage (<0.15 MPa), the medium relative pressure stage (0.15–0.30 MPa), and the high relative pressure (>0.30 MPa). As the relative pressure continuously increases, the adsorption mechanism changes from the collision stage between pore surface and CH4 molecules to the intermediate stage of monomolecular adsorption and the multilayer adsorption stage. Through calculating the isosteric heat and ultimate heat of CH4 adsorption, the adsorption heat generally increases with the adsorption capacity increasing, and shows a positive correlation with the micropore surface area. The effects of micropore structure and surface area on the adsorption heat should be crucial in coals. These results may be significant for understanding the process of CH4 adsorption.

A theoretical study on the application of different carbonaceous nanostructures in K-ion batteries

The possible usage of different carbon nanostructures, including C24 nanocage, carbon nanotube (CNT), and hexa-peri-hexabenzocoronene nanographene (HBC) is studied in the anode of K-ion batteries (KIBs) by DFT calculations. Based on the calculations, both of K and K+ species preferentially adsorb on the center of a hexagon of the studied nanostructures. The results show that the underlying mechanism of the K+ adsorption is the cation-π interaction, and by increasing the curvature of the adsorbents the interaction becomes weaker. When the curvature is reduced, the adsorption energy of the K atom becomes more positive and the adsorption mechanism changes from ionic to nonionic. This study suggests that the mechanism of the strength of K interaction and the structure curvature are the main factors in determining the cell voltage (Ucell) of the KIBs. The C24 cage generates a negative Ucell which is physically meaningless, and it cannot be used as an anode material. The CNT produces a very small Ucell of 0.09 V, while the HBC is suggested to be a proper nanostructure for use in the KIBs because of its large Ucell about 1.45 V.

Length and Charge of Water-Soluble Peptoids Impact Binding to Phospholipid Membranes.

In this study, we provide a quantitative description of the adsorption of water-soluble N-substituted glycine oligomers (peptoids) to supported lipid bilayers that mimic mammalian plasma membranes. We prepared a small array of systematically varied peptoid sequences ranging in length from 3 to 15 residues. Using the nonlinear optical method second harmonic generation (SHG), we directly monitored adsorption of aqueous solutions of 3- and 15-residue peptoids to phospholipid membranes of varying physical phase, cholesterol content, and head group charge in physiologically relevant pH buffer conditions without the use of extrinsic labels. Equilibrium binding constants and relative surface coverages of adsorbed peptoids were determined from fits to the Langmuir model. Three- and 15-residue peptoids did not interact with cholesterol-containing lipids or charged lipids in the same manner, suggesting that a peptoid's adsorption mechanism changes with sequence length. In a comparison of four three-residue peptoids, we observed a correlation between equilibrium binding constants and calculated logD7.4 values. Cationic charge modulated surface coverage. Principles governing how peptoid sequence and membrane composition alter peptoid-lipid interactions may be extended to predict physiological effects of peptoids used as therapeutics or as coatings in medical devices.

EC-SERS study of phenolic acids sorption behavior on Au, Ag and Cu substrates – Effect of applied potential and metal used

p-coumaric and ferulic acids are natural phenolic phytochemicals present in seeds and leaves usually in very low concentrations. For this reason, their detection and identification using traditional techniques of vibrational spectroscopy is problematic and requires the use of advanced methods to reduce the detection limit. Both p-coumaric and ferulic acids are significant for their antioxidant properties and other biological activities such as antiallergic, hepatoprotective, anticarcinogenic or modulation of enzyme activity. Surface-enhanced Raman scattering (SERS) spectroscopy appears a good tool for their study and detection. In SERS spectroscopy, the mechanism of analytes adsorption on the surface of a plasmonic metal substrate plays a key role, since the formation of a surface complex and its geometry fundamentally influences the enhancement of spectral response. This work was aimed at in-situ SERS spectroelectrochemical study of p-coumaric and ferulic acid showing the dependence on applied potential and changes in adsorption mechanism. The SERS spectra were collected at 785 nm excitation wavelength on gold, silver and copper rough electrodes. The applied potential significantly influences the orientation of the molecules towards the active sites of the metal substrate and their overall spatial arrangement on the surface in terms of the individual adsorption mechanisms.

Adsorption equilibrium and dynamic surface tension of alkyl polyglucosides and their mixed surfactant systems with CTAB and SDS in the surface of aqueous solutions

The surface adsorption equilibrium and dynamic surface tension (DST) of alkyl polyglucosides (APGs) with single or mixed carbon chains (C8G1.46, C10G1.54, C12G1.46, C8/10G1.31 and C12/14G1.43), and of the mixed surfactant systems C8G1.46-CTAB and C8G1.46-SDS in aqueous solutions were studied. The magnitude in descending order for cmc and γcmc values of various APGs were C8G1 > C10G1 > C12G1 and C8/10G1.31 > C12/14G1.43. Both the APGs with single carbon chain and the APGs with mixed carbon chains, have approximately the same Γm and Amin values.When the concentration of APG is relatively small, the adsorption is controlled by diffusion. When APG concentrations increase, the adsorption mechanism changes from diffusion controlled into barrier controlled.The apparent diffusion coefficient (Da) decreases with an increase in APG concentrations. And the Da values of various APGs at the same concentration in descending order of magnitude is C8G1.46 > C10G1.54 > C12G1.46 and C8/10G1.31 > C12/14G1.43. The magnitude in descending order for the adsorption barrier values (Ea) of different APG products was C8G1.46 > C10G1.54 > C8/10G1.31 > C12/14G1.43 > C12G1.46. The longer the carbon chain of the APG molecules, the smaller the repulsive force of the APG molecules on the surface, and the faster the DST drops.In the C8G1.46-CTAB mixed system, when CTAB has relatively small concentration, adding C8G1.46 to CTAB solution will result in significantly lowered DST values. But when CTAB concentration is relatively large, adding C8G1.46 to CTAB solution will not result in significant change to the DST curves. The DST values of the C8G1.46-SDS mixed system is significantly lower than the single C8G1.46 and single SDS systems.

Acid-base treated vermiculite as high performance adsorbent: Insights into the mechanism of cationic dyes adsorption, regeneration, recyclability and stability studies

Additional treatment with NaOH of acid activated vermiculite results in even higher increase in the adsorption capacity in comparison to samples modified only in acidic solution (first step of activation) with respect to raw material. Optimization of treatment conditions and adsorption capacity for two cationic dyes (methylene blue (MB) and astrazon red (AR)), also as binary mixture, was evaluated. The capacity, based on column studies, increased from 48 ± 2 to 203 ± 4 mg g−1 in the case of methylene blue and from 51 ± 1 to 127 ± 2 mg g−1 in the case of astrazon red on starting and acid-base treated material, respectively. It was shown that adsorption mechanism changes for both cationic dyes after NaOH treatment and it results in decrease of adsorption rate. In binary mixtures methylene blue is bound stronger by adsorbent and astrazon red may be removed in initial stage of adsorption. Extensive studies on desorption/regeneration process proved high efficiency in recyclable use of all materials. Although cation exchange capacity decreases due to acid treatment, after base treatment exchange properties are used more efficiently. On the other hand, increased specific surface area has less significant contribution into the adsorption potential of studied materials. Obtained adsorbents worked efficiently in 7 adsorption-regeneration cycles and loss of adsorption capacity was observed only in two first cycles.

Molecular origin of decreased diffusivity with loading for benzene/HY

On the basis of our recently proposed “ideal” and “insertion” adsorption mechanisms of aromatics in HY zeolites, changes in energetic and dynamic properties with loading were investigated through Monte Carlo and molecular dynamic (MD) calculations. Loading‐dependent isosteric heat could be divided into three linear ranges. The two inflection points were attributed to adsorbate interactions and inherent adsorption mechanism changes. With regard to the loading dependence of diffusivity, diffusivity decreased faster at high loadings than at low and medium loadings, separated by an inflection point. This result confirmed a two‐stage diffusion mechanism based on the distribution of adsorbate from MD simulations which was able to qualitatively predict the further restriction of the mobility. This study provided insights into the modeling of mobility at high loadings on the basis of site‐hopping mechanism. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2778–2785, 2016

Influence of sodium chloride on hydrophobic adsorption of PEGylated lysozyme

Interaction of macromolecules in aqueous salt‐containing solution with a hydrophobic adsorbent is studied by adsorption equilibrium measurements and by independent isothermal titration calorimetry. The macromolecules are native as well as mono‐, di‐, and tri‐PEGylated lysozyme and four pure PEGs. The hydrophobic adsorbent is Toyopearl PPG‐600M. The salt is sodium chloride. The sodium chloride concentration in the aqueous 25 mM sodium phosphate buffer is varied from 2000 to 4500 mM at pH 7.0 and 25°C. PEGylation of the lysozyme is carried using 5 and 10 kDa PEG chains. The molar enthalpy of adsorption is calculated from the adsorption equilibrium and the calorimetric data. The results show that the adsorption of the PEGylated lysozyme is caused by both the interaction of the lysozyme and the interaction of the PEG chains with the adsorbent, respectively, but the interaction of the lysozyme is stronger than that of PEG. The comparison of the results of the present study on the influence of sodium chloride with a corresponding study on the influence of ammonium sulfate shows that the adsorption mechanism changes upon the variation of the salt. The knowledge of the adsorption mechanisms supports the systematic development of chromatographic purification steps.

Thermodynamic Study of Effects of Mixed Aqueous Electrolytes on Adsorption of Native and PEGylated Lysozyme on a Mildly Hydrophobic Resin

PEGylation of pharmaceutical proteins, i.e., the covalent binding of poly(ethylene glycol) (PEG) to the protein, can lead to improved residence time of the drug in the body. Usually, the protein is already purified before PEGylation. Therefore, the purification task after PEGylation is reduced to a separation of the PEGylated isoforms or a fraction with a certain PEGylation degree from the unPEGylated protein. As the PEGylation changes the hydrophobicity of the proteins, the hydrophobic interaction chromatography (HIC) is a promising technique for the separation of PEGylated protein forms. The state-ofthe-art is using pure electrolytes for reducing the protein solubility. Applying mixed electrolytes for that task has great potential due to different interactions of the different anions and cations with the different parts of the proteins. This can lead to synergetic effects in protein adsorption. In the present work, the adsorption of different forms of PEGylated lysozyme as well as native lysozyme and pure PEGs on a mildly hydrophobic resin (Toyopearl PPG-600M) was studied by measuring both the adsorption equilibrium isotherms and the specific enthalpies of adsorption along the isotherms. This gives a complete thermodynamic picture of the adsorption. The measurements of the adsorption equilibrium isotherms were carried out using a fully automated robotic liquid handling station. For the calorimetric measurements, a high-precision isothermal titration microcalorimeter was used. The studied salts are sodium chloride, ammonium sulfate, sodium sulfate, ammonium chloride, and their mixtures. A sodium phosphate buffer was used to adjust the pH value. It was found that the adsorption mechanism changes completely upon variation of the salt. A molecular picture is discussed which describes the complex adsorption mechanisms based on the interpretation of the interactions of the different ions with the proteins. It is demonstrated that the specific interactions of the individual ions with particular regions of the proteins are responsible for the varying adsorption behavior. A quantitative structure property relationship (QSPR) is discussed, which describes the influence of the different salt ions and their cross interactions on the adsorption.

Quantum chemical study of inhibition of the corrosion of mild steel in 1 M hydrochloric acid solution by newly synthesized benzamide derivatives

The inhibitory effect of some new synthesized benzamide compounds on corrosion of mild steel in 1 M HCl solution has been studied by use of weight loss measurements and the electrochemical techniques potentiodynamic polarization and electrochemical impedance spectroscopy. The inhibiting action is more pronounced with increasing concentration. Inhibition efficiency is maximum (approximately 99 %) at 10−3 M. Polarization measurements also show that the compounds act as mixed inhibitors. The cathodic curves indicate that reduction of protons at the mild steel surface occurs as a result of a pure activating mechanism. EIS measurements reveal increased transfer resistance with increasing inhibitor concentration. The presence of heteroatoms increases inhibition efficiency without causing a drastic change in adsorption mechanism, which follows the Langmuir isotherm model. Significant correlations were obtained between inhibition efficiency with the chemical indexes calculated, by use of the standard software Gaussian03, on the basis of density functional theory (DFT) at the B3LYP/6-31G** level of theory, indicating that variation of inhibition with inhibitor structure may be explained in terms of electronic properties. The effect of temperature on the corrosion behaviour of steel in 1 M HCl without and with inhibitors at 10−3 M was studied in the temperature range from 308 to 333 K, and the associated activation energy was determined.

Equilibrium and kinetics analysis of carbon dioxide capture using immobilized amine on a mesoporous silica

Thermogravimetric analysis is used to study the adsorption kinetics, equilibrium, and thermodynamics of CO2 on immobilized polyethylenimine sorbent impregnated on a mesoporous silica over the range of 300–390 K and 5–100% CO2 concentration. Adsorption isotherm models were fitted to the experimental data indicating that a change in adsorption mechanism occurred near 70°C. Below this temperature, the adsorption data followed the heterogeneous isotherms, while data taken at higher‐temperatures followed isotherms for homogeneous surfaces. Heat of sorption was estimated to be 130 kJ/mole for the low‐temperature regime, but this decreased to 48 kJ/mole above 70°C. The rate of CO2 fractional uptake decreased as temperature increased. A phenomenological kinetic model was derived from the Weibull distribution function using a nucleation growth theory to describe the two‐step process. The kinetic model was used to predict the uptake at different operating conditions and resulted in good agreement with experimental data. Published 2012 American Institute of Chemical Engineers AIChE J, 2013

Comparative Study of Methane Adsorption on Graphite

To study methane adsorption on graphite in a wide range of coverages and temperatures, we compare experimental results with Monte Carlo simulations (MCSs) of the grand canonical ensemble (GCE) and mean-field approximation (MFA) of the lattice gas model (LGM). MCSs were performed by employing two models for the substrate description; we utilized Steele's 10-4-3 analytical potential, and as a second approach, we represented the graphite surface as composed of several graphene layers (at the atomic level). We obtained adsorption isotherms and density profiles that confirm a layer-by-layer mechanism at low temperatures; the later results in the analytical model having a denser condensed phase than the atomistic one. LGM calculations show a close-packed lattice configuration and also allow us to describe the adsorption mechanism changes with temperature. The isosteric heat of adsorption that was found was approximately 13 kJ/mol. We can also conclude that, in spite of the greater computational cost, the atomistic model could be employed for surfaces that are not necessarily homogeneous and beyond the low-pressure range that are not covered by the simple, fast description given by the analytical model.

Protein adsorption onto polystyrene surfaces studied by XPS and AFM

Albumin has been adsorbed onto polystyrene substrates which have been prepared by controlled oxidation using an ultraviolet/ozone system. The composition and chemistry of PS and albumin adsorbed PS surfaces have been characterised using X-ray photoelectron spectroscopy and topography has been studied using atomic force microscopy. PS surface oxygen levels from 0 to 22 at.% were produced with minimal topographical change to the surface (AFM mean roughness within the range 0.2–0.5 nm across the whole oxidation range) which allowed isolation of the surface chemical effects on the albumin adsorption. Untreated (non-polar) PS surfaces with albumin adsorbed from aqueous solutions containing 0.1, 0.05 and 0.01 mg/cm 3 albumin all gave effectively the same proteinaceous nitrogen level (4.5–5.0 at.%) indicating that the adsorbed levels are independent of the initial solution concentration. For PS surfaces containing <14 at.% oxygen the measured nitrogen levels are inversely related to oxygen level, these surfaces contain mainly C–OR groups. At substrate oxygen levels of 15–22 at.% surface nitrogen levels appear independent of substrate chemistry. Nitrogen levels of 0.8, 2.2 and 3.8 at.% were recorded for the three respective solution concentrations and AFM roughness increased with surface albumin concentrations. Quartz balance measurements indicate that more albumin is adsorbed on PS than on oxidised PS. Comparison of AFM images shows that the adsorbed albumin layer on PS is thicker than that on the oxidised material. AFM images of protein adsorbed onto non-polar surfaces from all three starting concentrations are characterised by ‘peak and ridge’ topography. This changes to a ‘grainy’ type structure for albumin adsorbed on oxidised substrates. Overall the observations are consistent with a substrate driven change in adsorption mechanism from non-polar to polar substrate which is accompanied by a change in albumin conformation.

Counterion Effects on Hexadecyltrimethylammonium Surfactant Adsorption and Self-Assembly on Silica

Combining optical reflectometry and atomic force microscopy (AFM), we have studied the effects of the surfactant counterion on the adsorption isotherms, kinetics, and layer structure for cationic hexadecyltrimethylammonium (C16TA+) surfactants on negatively charged silica surfaces. The adsorption kinetics suggest that the adsorption mechanism changes at the critical micelle concentration (cmc). A change in mechanism is also suggested by differences observed in the state of interfacial self-assembly on either side of the cmc. Above the cmc, increasing the binding affinity of the counterion (from chloride to bromide) increased the surface excess concentration by approximately 60% and changed the structure of the adsorbed surfactant layer from aggregates with circular projections to wormlike micelles. The addition of 10 mM KCl or KBr increased the surfactant surface excess concentration for both counterions. Below the cmc, the counterion has only a small effect on the structure of the adsorbed layer, and the...

Peculiarities Associated with the Adsorption of Inorganic and Organic Nitrogen-Containing Compounds on Amorphous Metal Surfaces

The surface of an intermetallic amorphous alloy has a low density of localized energy centres which does not facilitate the adsorption of substances on the alloy surface. However, the active diffusion of amorphizators (usually B, Si) in these materials leads to their concentration on the surface. These amorphizators have a high affinity for organic compounds and can form non-covalent bonds with some of these compounds. Such behaviour has been observed for the adsorption of organic nitrogen-containing inhibitors on an iron-based amorphous alloy surface, although when the quantity of iron atoms is sufficient chemisorption with the formation of surface compounds of planar orientation is possible. The chemical bond is formed via the free electron pair associated with nitrogen, which results in a high activation energy (Ea) for the adsorption process. The adsorption of inorganic ions (Cl−, NO−2, NO−3) increases as the iron content in the alloys increases (55–78.5%). This effect is most significant for the adsorption of chloride ions. The injection of piperidine, morpholine or diethylamine in a 3% sodium chloride solution results in significant changes in adsorption. In this case, it is believed that the adsorption mechanism changes leading to the formation of a two-layer adsorption film on the amorphous alloy surface.

Controls on polyacrylamide adsorption to quartz, kaolinite, and feldspar

Potentiometric titrations of quartz, kaolinite, feldspar, and partially hydrolysed polyacrylamide (HPAM), and sorption measurements of HPAM on the minerals, allows identification of the general mechanisms of polyacrylamide adsorption to aluminosilicates and quartz. Adsorption was monitored at the mineral solution interface by way of X-ray photoelectron spectroscopy (XPS). XPS spectra of the unreacted minerals show bands in the Ols, Si2p, Al2p, and Cls regions. Additional peaks are observed in the C1s and N1s regions after treatment with polyacrylamide and the latter is used in this study to monitor polymer adsorption. N1s peak intensities increase with polymer concentration to a maximum corresponding to surface site saturation. At a fixed polymer concentration, adsorption varies with pH-dependent surface charge. The adsorption mechanism changes with pH, reflecting variation in the pH-dependent concentrations of ionizable groups on polyacrylamide and at aluminosilicate surfaces, and the extent of hydrogen-bonding between uncharged mineral surface sites and polymer amide groups.

Moisture Adsorption Isotherms of Tarhana

In this study, moisture adsorption isotherms of tarhana were determined at 25 and 35°C and as the temperature increased, the decrease in the adsorbed moisture content was observed at water activity between 0.2 and 0.9. This case was explained by possible changes in adsorption mechanism that occured in water-water and water-adsorbent interactions to water adsorption. Isoteric heats that accompanied water adsorption, and free energy values were calculated by using modified Clausius-Clapeyron equation, and Chung and Pfost equation respectively.

Experimental measurements of fast adsorption kinetics of H2 on vicinal Si(100) and (111) surfaces

The Si(100) and (111) surfaces have generally been classified as relatively inert toward molecular hydrogen adsorption. In the present work, we show using surface second harmonic generation that the sticking probability increases dramatically for slightly misoriented substrates, which are of interest in many integrated circuit applications. At temperatures lower than 30–50 K below where desorption becomes significant, defect sites serve as adsorption conduits from which hydrogen atoms diffuse onto the rest of the surface. The adsorption rate therefore has the activation energy of surface diffusion, found to be 0.78 eV on Si(100) and 1.50 eV on Si(111). The small energy barrier and pre-exponential factor (∼10−11 cm2/s) for diffusion on (100) seem to result from the need to break Si–Si dimer-pair bonds prior to hopping of an H atom. At higher temperatures, the adsorption mechanism changes, and H2 sticks to both surfaces with a fairly large probability near 0.1.