Competitive Adsorption Behavior Research Articles

Structure of and Competitive Adsorption in Alkyl Dicarbamate Two-Dimensional Crystals

The potential for relatively minor structural changes to dramatically impact materials properties is one of the primary obstacles to achieving the rational design of functional materials. For example, having an odd versus an even number of carbons between functional groups in polymers can cause large variation in melting point and mechanical properties. This odd-even effect is especially pronounced in hydrogen-bonded polymers and oligomers. To shed light on the structural basis of this phenomenon, physisorbed monolayers and single crystals of alkyl dicarbamates were investigated by scanning tunneling microscopy and X-ray diffraction, respectively. The related two- and three-dimensional crystal structures both demonstrated a clear odd-even effect in packing geometry. The differing accommodation of intermolecular interactions between odd and even packing motifs was directly related to the melting point trends and further dissected through computation. In addition, these oligomers displayed unusual competitive adsorption behavior; the relative preference for adsorption of a smaller species from a binary solution was increased compared to alkanes. These results were explained in the context of hydrogen bond density effects that arise due to competition for a limited substrate surface area. This study provides a model for understanding oligourethane surface coatings and demonstrates the importance of molecular structure and hydrogen bonding in determining adsorption behavior.

Competitive adsorption of bacteriophage T4 lysozyme stability variants at hydrophilic glass surfaces

The competitive adsorption behavior exhibited by the wild-type T4 lysozyme and two of its structural stability variants was studied by 125I radioisotope labeling. The mutant lysozymes were produced by substitution of the isoleucine residue at position 3 in the wild type with a tryptophan residue, resulting in a protein with lower structural stability, or with a cysteine residue, resulting in a protein with higher structural stability. Adsorption kinetics were recorded for binary protein mixtures in contact with a clean glass surface, in which one variant had been radiolabeled and the other had not. All pair permutations were tested. The kinetic data show that in instances in which exchange reactions between adsorbed protein and dissolved protein occur, they occur such that more stable variants are removed from the surface by less stable variants. The less stable proteins thus exhibited an advantage in competitive adsorption over the more stable proteins, in these tests.

Study of the competitive isotherm model and the mass transfer kinetics for a BET binary system

The competitive adsorption behavior of the binary mixture of phenetole (ethoxy-benzene) and propyl benzoate in a reversed-phase system was investigated. The adsorption equilibrium data of the single-component systems were acquired by frontal analysis. The same data for binary mixtures were acquired by the perturbation method. For both compounds, the single-component isotherm data fit best to the multilayer BET model. The experimental overloaded band profiles are in excellent agreement with the profiles calculated with either the general rate model or the modified transport-dispersive models. The competitive adsorption data were modeled using the ideal adsorbed solution (IAS) theory. The numerical values of the coefficients were derived by fitting the retention times of the perturbation pulses to those calculated using the IAS theory compiled with the coherence conditions. Finally, the elution profiles of binary mixtures were recorded. They compared very well with those calculated. As a characteristic feature of this case, an unusual retainment effect of the chromatographic band of the more retained component by the less retained one was observed. The combination of the General Rate Model and the adsorption isotherm model allowed an accurate prediction of the band profiles.

Competitive adsorption behavior of human serum albumin and fibrinogen on titanium oxide films coated on LTI-carbon by IBED

Titanium oxide films were deposited onto low temperature isotropic carbon (LTI-carbon), a prevailing material used for mechanical heart valve fabrication, by ion beam enhanced deposition (IBED). The composition and structure of the film were studied by glancing angle X-ray diffraction (GAXRD) and Auger electron analysis (AES). The results show that the film is dominated by polycrystalline Ti 2O 3, as well as TiO 2. The elements are well distributed along the depth profile. The surface morphologies of LTI-carbon and titanium oxide film are very similar when observed by atomic force microscopy (AFM), which indicated that the surface morphology has no significant influence on the protein adsorption properties of the samples. The protein adsorption behavior was investigated in a HSA/HFG binary system by means of 125I labeling. The results show that both proteins adsorbed preferentially on LTI-carbon rather than on titanium oxide film. In the HSA/HFG binary system, the adsorption behavior of one protein is inhibited as the concentration of the other protein increases. When the concentrations of both proteins were kept constant, the amount of adsorbed HSA strongly increased with time on titanium oxide, but less so on LTI-carbon. However, the adsorption of HFG showed the opposite behavior. It is concluded that titanium oxide film exhibits a HSA-preferred adsorption property, whereas LTI-carbon shows no preferential adsorption. This is mainly attributed to the different surface energy of LTI-carbon and titanium oxide film.

Competitive adsorption behavior of selected heavy metals in three soil types of India amended with fly ash and sewage sludge

Laboratory batch experiments were carried out to study the competitive sorption behavior of metals in three types of Indian soils, differing in their physicochemical properties: acid laterite (SL1), red alfisol (SL2) and black vertisol (SL3) treated with different proportions of fly ash and sewage sludge mixture. Representative samples were equilibrated with 10 to 200 µM L-1 concentrations of metals simultaneously containing Cd, Cu, Ni, Pb and Zn in 5 mM of Ca(NO3)2 solution. In most of the cases the affinity sequence of metals was Pb>Cu>Zn>Ni>Cd based on their amount of sorption, which varied little with either metal equilibrating concentrations or the soil/mixture type. The observed metal affinity sequences in different soils amended with mixtures were compared to the predicted affinity sequences based on metal properties and a good match was found with those predicted by metal hydrolysis constants. This indicated that formation and subsequent sorption of metal hydrolysis products on soil surface is the predominant mechanism for sorption. In all the cases, Pb and Cu showed higher affinity followed by Zn, Ni or Cd. The increase in the metal additions further enhanced the competition among metals for exchange sites. Adsorption isotherms showed that metal sorption was linearly related to its concentration in the equilibrium solution. The distribution coefficients (KD) computed from the slopes of linear regression for different metals were higher in SL3 than in both SL2 and SL1. All the mixture amended soils produced higher KD values than their respective controls. Selectivity between metals resulted in the following affinities based on their KD values—Pb>Cu>Zn>Ni or Cd—which was in line with the value of the hydrolysis constant of the metals under study.

The Competitive Adsorption of Heavy Metals under Various Incineration Conditions.

The main objective of this study is to investigate the competitive adsorption behavior of multiple components on sorbents (silica sand with limestone) at different operating conditions (temperature, additives of chloride, operating time). Moreover, the Toxicity Characteristic Leaching Process (TCLP) analyzed the leaching rates of metals from the spent sorbents.The results show that the adsorption capacities of Pb and Cd are better among those four metals on the sorbents. At 850°C, the adsorption of Pb and Cd can reach the adsorption saturation points. However, the adsorption amounts on the sorbents will increase as the operating temperature increases. X-ray powder diffract meter (XRD) analysis revealed the adsorption components on the sorbents had metal fusion formations. The TCLP tests show that the leaching rate of metals are significantly lower at 600°C, however, as the operating temperature increases (700°C and 850°C), the sorbents became more stable. At the same temperature, increasing the operating time on the incinerator shows a decreasing leaching rate on the sorbent. Adding PVC or NaCl will decrease the adsorption amounts of Pb and Cd, but increase the leaching rates of all metals on the sorbent. Competitive effects were analyzed by comparing single-component adsorption and multiple-component adsorption cases. The result shows that a multiple-component case has a better adsorption capacity than that of a single-component case.

Simulated moving bed chromatography (SMB) for application in bioseparation.

Simulated Moving Bed (SMB) technology is of rising interest in the field of bioseparation. This is particularly due to its advantages such as reduction of solvent consumption, high productivity and final purities as well as low investment costs in comparison to eluent chromatography. SMB units can operate under high productivity overloaded conditions. This leads to non-linear competitive adsorption behavior, which has to be accounted for when designing and optimizing new SMB separations. The so called "Triangle Theory", which is briefly reviewed in this chapter, provides explicit criteria for the choice of the operating conditions of SMB units to achieve the prescribed separation of a mixture characterized by Langmuir, modified Langmuir and bi-Langmuir isotherms. The application of the SMB-technique to the downstream processing of biotechnological products requires some specific changes to meet the special demands of bioproduct isolation. Some exemplary applications are given including separations of sugars, proteins, monoclonal antibodies, ionic molecules and optical isomers and for desalting.

Lateral phase separation in adsorbed binary protein films at the air-water interface.

Lateral phase separation in two-dimensional mixed films of soy 11S/beta-casein, acidic subunits of soy 11 (AS11S)/beta-casein, and alpha-lactalbumin/beta-casein adsorbed at the air-water interface has been studied using an epifluorescence microscopy method. No distinct lateral phase separation was observed in the mixed protein films when they were examined after 24 h of adsorption from the bulk phase. However, when the soy 11S/beta-casein and AS11S/beta-casein films were aged at the air-water interface for 96 h, phase-separated regions of the constituent proteins were evident, indicating that the phase separation process was kinetically limited by a viscosity barrier against lateral diffusion. In these films, beta-casein always formed the continuous phase and the other globular protein the dispersed phase. The morphology of the dispersed patches was affected by the protein composition in the film. In contrast with soy 11S/beta-casein and AS11S/beta-casein films, no lateral phase separation was observed in the alpha-lactalbumin/beta-casein film at both low and high concentration ratios in the film. The results of these studies proved that proteins in adsorbed binary films exhibit limited miscibility, and the deviation of competitive adsorption behavior of proteins at the air-water interface from that predicted by the ideal Langmuir model (Razumovsky, L.; Damodaran, S. J. Agric. Food Chem. 2001, 49, 3080-3086) is in fact due to thermodynamic incompatibility of mixing of the proteins in the binary film. It is hypothesized that phase separation in adsorbed mixed protein films at the air-water and possibly oil-water interfaces of foams and emulsions might be a source of instability in these dispersed systems.

Modeling chromatographic chiral separations under nonlinear competitive conditions

AbstractThe chromatographic separation of Tröger's base enantiomers with ethanol as eluent on microcrystalline triacetate cellulose (CTA) was studied. Binary competitive adsorption equilibrium data measured were interpreted using ideal adsorbed solution and real adsorbed solution (RAS) theory. Both models were used to simulate a set of column breakthrough experiments. The competitive behavior of the two enantiomers is quantitatively discussed and the reliability of the two models is assessed. The results show that the RAS model can describe with good accuracy the complex competitive adsorption behavior exhibited by the CTA/Tröger's base model system. This conclusion is expected to apply to the adsorption of the other enantiomeric mixtures on chiral stationary phases.

Studies on Polymer−Metal Interfaces. 2. Competitive Adsorption between Oxygen- and Nitrogen-Containing Functionality in Model Copolymers onto Metal Surfaces

The competitive adsorption behavior between oxygen- and nitrogen-containing functionality in poly(methyl methacrylate-co-2-vinylpyridine) (MMA2VP) and poly(methyl methacrylate-co-4-vinylpyridine) (MMA4VP) random copolymers onto alumina substrate was investigated. The adhesive strength of both the MMA2VP/alumina and MMA4VP/alumina systems increased with increasing vinylpyridine (VP) content in the copolymers, and the effect was more prominent for the MMA4VP/alumina system. Analysis of the reflection−absorption infrared spectroscopy shows that the carbonyl functional groups in MMA2VP and MMA4VP interact specifically with alumina while the pyridine rings in VP units are oriented more vertically to alumina surface than carbonyl groups. Results from X-ray photoelectron spectroscopy (XPS) reveal that both the nitrogen-containing functionalities in 2VP or 4VP units and the oxygen-containing functionality in MMA unit interact specifically with alumina surface. The quantitative analysis of XPS spectra indicates th...

Adsorption of Anionic Dye and Surfactant from Water onto Organomontmorillonite

Single- and bisolute competitive adsorptions were carried out to investigate the adsorption behavior of an anionic dye, Orange II, and an anionic surfactant, dodecylbenzenesulfonate (DBS), from water at 25°C onto montmorillonite treated with hexadecyltrimethylammonium (HDTMA) cation. The monovalent HDTMA cation was exchanged for the metal cations on the montmorillonite to prepare HDTMA-montmorillonite, changing its surface property from hydrophilic to organophilic. Adsorption affinity of DBS on HDTMA-montmorillonite was found to be slightly higher than that of Orange II, probably due to hydrophobic or nonpolar interactions between the long hydrocarbon chains of the HDTMA with an anion. The single-solute adsorption data were fitted to the Langmuir and the Redlich-Peterson (RP) adsorption models. The competitive Langmuir model and the ideal adsorbed solution theory (IAST) coupled with the single-solute adsorption models were used to predict the bisolute competitive adsorption equilibria. Among the models, the IAST coupled with the Langmuir and the RP models for DBS and Orange II, respectively, yielded the most favorable representation of the bisolute competitive adsorption behavior.

Required Properties for Functional Monomers To Produce a Metal Template Effect by a Surface Molecular Imprinting Technique

We investigated the properties required to design functional monomers which produce an imprint effect in prepared metal-imprinted polymers. Novel organophosphorus functional monomers were synthesized, and zinc-imprinted polymers were prepared with the functional monomers by a surface molecular imprinting technique. The competitive adsorption behavior of zinc and copper ions on the surface-imprinted polymers was examined, and the template effect was characterized. A slight structural change in the functional monomers, which interact with target metal ions (zinc ions in this study), resulted in a favorable change in metal recognition by the zinc-imprinted polymers. It was found that the presence of aromatic rings and a suitable straight alkyl chain in the functional monomer in addition to a high binding affinity to the target metals renders molecular recognition on the surface of the imprinted polymers effective.

Influence of competitive adsorption on flocculation and rheology of high-pressure-treated milk protein-stabilized emulsions.

The effect of high-pressure treatment (HPT) on the droplet-size distribution and small-deformation rheology of oil-in-water emulsions containing beta-lactoglobulin and a nonionic surfactant or sodium caseinate has been investigated at neutral pH. Addition of Tween 20 (polyoxyethylenesorbitan monolaurate) to a beta-lactoglobulin-stabilized emulsion results in competitive displacement of the adsorbed globular protein film and, following HPT, the formation of a less flocculated emulsion. The age of the beta-lactoglobulin-stabilized emulsion prior to addition of sodium caseinate influences the competitive adsorption behavior. The strengthening of the beta-lactoglobulin layer with time makes it more resistant to disruption by sodium caseinate. The level of pressure-induced flocculation of beta-lactoglobulin-coated oil droplets depends on the intensity of processing conditions and on the degree of interfacial displacement. In contrast, beta-lactoglobulin added after emulsification appears to show little evidence of competitive adsorption behavior at the caseinate oil-water interface. Changes in the rheological properties of these latter systems following HPT can be attributed to pressure-induced denaturation and gelation of beta-lactoglobulin in the continuous phase of the emulsion.

Continuous chromatographic separation through simulated moving beds under linear and nonlinear conditions

Simulated Moving Bed (SMB) technology is receiving more and more attention as a convenient technique for the production scale continuous chromatographic separation of fine chemicals. Thanks to the efficient simulated countercurrent contact between the stationary and the fluid phase, SMB units can operate under high productivity overload conditions. These lead to nonlinear competitive adsorption behavior, which has to be accounted for when designing and optimising new SMB separations. The so called `Triangle Theory', which is briefly revised here, provides explicit criteria for the choice of the operating conditions of SMB units to achieve the prescribed separation of a mixture characterized by Langmuir, modified Langmuir and bi-Langmuir isotherms. In all these cases, the effect of increasing nonlinearity of the separation, due to increased feed concentration can be predicted. In this paper, the use of this approach for the design of linear and nonlinear SMBs is considered, with reference to examples reported previously. Alternative strategies have been proposed in the literature, which are based on the use of safety factors for linear systems and on an adaptation of this for nonlinear ones. In the cases considered, which involve experimental data for a linear system and numerical experiments for a nonlinear Langmuir system, it is shown that `Triangle Theory' allows attainment of a better understanding of and a deeper insight into the behavior of SMB units.

Adsorption of Phenol and Nitrophenol Isomers onto Montmorillonite Modified with Hexadecyltrimethylammonium Cation

ABSTRACT Single- and two-component competitive adsorptions were carried out in a batch adsorber to investigate the adsorption behavior of phenol and 2-, 3-, and 4-nitrophen-ols in aqueous solution at 25°C onto hexadecyltrimethylammonium (HDTMA)-treated montmorillonite. HDTMA cation was exchanged for metal cations on the montmorillonite to prepare HDTMA—montmorillonite, changing its surface property from hydrophilic to organophilic. Effective solid diffusivity of HDTMA cation in the montmorillonite particle was estimated to be about 3 × 10−12 cm2/s by fitting the film-solid diffusion model to a set of HDTMA adsorption kinetic data onto mont-morillonite. Adsorption affinity on HDTMA—montmorillonite was found to be in the order 3-nitrophenol = 4-nitrophenol > 2-nitrophenol > phenol. The Langmuir and the Redlich—Peterson (RP) adsorption models were used to analyze the single component adsorption equilibria. The ideal adsorbed solution theory (IAST) and the Langmuir competitive model (LCM) were used to predict the multicomponent competitive adsorption equilibria. These models yielded favorable representations of both individ-ual and competitive adsorption behaviors.

Tafel behaviour and competitive adsorption of amino acids and peptides with surface oxide formation at the Pt electrode

The electrochemical oxidation reactions and competitive adsorption behaviour of glycine, the peptides glycyl-glycine and glycyl-glycyl-glycine, α- and β-alanine, and α-, β-, and γ-aminobutyric acid have been investigated by cyclic voltammetry at a platinum electrode in a pH 7.0 phosphate buffer solution at ambient temperature. Strong adsorption of all species at anodic potentials in neutral solution was accompanied by an efficient ability to block both the initial stages of OH monolayer deposition as well as the second stage involving an irreversible process of continued deposition of OH or O species. Glycine and the peptides blocked the initially and more reversibly deposited OH species in preference to those formed at higher potentials. Surface concentrations determined with the lower anodic end potentials (0.50 V) gave information on the orientation of the adsorbed molecules. Steady-state polarization measurements gave Tafel slopes in the range of 253–498 mV, considerably higher than that normally seen for oxygen evolution from a phosphate buffer solution. These results suggest that, in the pH 7.0 buffer, the negatively charged carboxylate group preferentially adsorbs under these anodic potentials in a mechanism similar to the Kolbe reaction, with the decarboxylation step being rate determining. Keywords: electrochemical oxidation, glycine, alanine, aminobutyric acid, peptides, platinum oxide.

Adsorption of Organic Phenojs onto Dual Organic Cation Montmorillonite from Water

Single- and multicomponent competitive adsorptions were carried out in a batch reactor to investigate the removal of three toxic organic phenols, 2-chlorophenol, 3-cyanophenol, and 4-nitrophenot, using organically modified montmorillonite. To study the removal capacity of phenolic contaminants dissolved in aqueous solution, two organic cations, tetramethylammonium (TMA) and hexadecyltrimethylammonium (HDTMA), were exchanged for the metal cations on the montmorillonite to the extent of about 40 and 45% of the cation-exchange capacity (CEC) of the montmorillonite, respectively, resulting in a surface property change from hydrophilic to organophilic. From the experimental results it was observed that adsorption affinity on TMA/HDTMA–montmorillonite is in the order 2-chlorophenol > 4-nitrophenol > 3-cyanophenol. The Langmuir and the Redlich–Peterson models were used to analyze the single component adsorption equilibria. On the other hand, the IAST and LCM models were used to predict the multicomponent competitive adsorption equilibria. These models yielded favorable representations of both individual and competitive adsorption behaviors.

Optimal operation of simulated moving bed units for nonlinear chromatographic separations

The simulated moving bed (SMB) technology is receiving more and more attention as a convenient technique for the production scale continuous chromatographic separation of fine chemicals. Characteristic features of SMBs are improved performances with respect to preparative chromatography and nonlinear competitive adsorption behavior. The selection of the operating conditions to achieve high separation performances under nonlinear conditions is acknowledged to be the major problem in running a SMB unit for a new application. This problem is solved in this paper, where a general theory is developed which provides explicit criteria for the choice of the operating conditions of SMB units to achieve the prescribed separation of a mixture characterized by both constant selectivity Langmuir isotherms and variable selectivity modified Langmuir isotherms. The space of the operating parameters, i.e. the fluid to solid flow-rate ratios, is divided in regions with different separation regimes. The effect of increasing nonlinearity of the system on the operating conditions and the separation performances, namely desorbent requirement, enrichment, productivity and robustness of the separation, is thoroughly analyzed. The obtained results are shown to provide a very convenient tool to find both optimal and robust operating conditions of SMB units. Finally, a comparison between model predictions and experimental data dealing with the resolution of different racemic mixtures assesses the reliability and accuracy of the obtained theoretical findings.

Competitive Adsorption Behaviour of Wheat Flour Components and Emulsifiers at an Air–Water Interface

This study aimed to characterise the competitive adsorption behaviour of the different components present in wheat flour and emulsifiers used in breadmaking at an air–water interface using surface balance measurements. It was found that the proteins could be arranged in increasing order of surface activity: albumin < globulin < glutenin < gliadin. Furthermore, the ability of the proteins to interact with and/or to insert into lipid monolayers was found to correlate with their surface activities. The ability of gliadin to insert into emulsifier monolayers was comparable to its ability to insert into lipid monolayers. Competition experiments with pure protein monolayers, and the subsequent addition of different lipid or emulsifier suspensions, revealed that non-bilayer forming lipids were more effective at adsorbing to and competing at the air–water interface than bilayer forming lipids. It was found that proteins could not insert into lipid or emulsifier monolayers above a surface pressure of 37 mN/m, whereas a wheat lipid mixture could reach a surface pressure of approximately 50 mN/m. If sufficient amounts of lipids are available to reach the gas cell interface in dough, it is likely that lipids will expel the proteins from the interface and that lipids and/or emulsifiers will be the only constituents left in the gas cell film. This study indicates that future studies addressing the issue of gas-cell stability in a wheat flour dough should focus on the behaviour of lipid and emulsifier monolayers.

Flocculation and competitive adsorption in a mixed polymer system Relevance to casein-stabilized emulsions

In a colloidal system containing two chemically similar adsorbable polymers, one a good steric stabilizer and the other not so good, the state of flocculation of the polymer-coated particles is determined by the competitive adsorption behaviour of the polymer. The key factors controlling the limits of stability can be expressed in terms of a simple model based on a system of sticky hard spheres. Evidence is presented to support the hypothesis that this approach may be useful in understanding the stability of oil-in-water emulsions containing a mixture of the milk proteins α s1 -casein and β-casein under conditions of moderate ionic strength.