Effects of ionic strength and surface charge on protein adsorption at PEGylated surfaces

PEGylated Nb2O5 surfaces were obtained by the adsorption of poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) copolymers, allowing control of the PEG surface density, as well as the surface charge. PEG (MW 2 kDa) surface densities between 0 and 0.5 nm(-2) were obtained by changing the PEG to lysine-mer ratio in the PLL-g-PEG polymer, resulting in net positive, negative and neutral surfaces. Colloid probe atomic force microscopy (AFM) was used to characterize the interfacial forces associated with the different surfaces. The AFM force analysis revealed interplay between electrical double layer and steric interactions, thus providing information on the surface charge and on the PEG layer thickness as a function of copolymer architecture. Adsorption of the model proteins lysozyme, alpha-lactalbumin, and myoglobin onto the various PEGylated surfaces was performed to investigate the effect of protein charge. In addition, adsorption experiments were performed over a range of ionic strengths, to study the role of electrostatic forces between surface charges and proteins acting through the PEG layer. The adsorbed mass of protein, measured by optical waveguide lightmode spectroscopy (OWLS), was shown to depend on a combination of surface charge, protein charge, PEG thickness, and grafting density. At high grafting density and high ionic strength, the steric barrier properties of PEG determine the net interfacial force. At low ionic strength, however, the electrical double layer thickness exceeds the thickness of the PEG layer, and surface charges "shining through" the PEG layer contribute to protein interactions with PLL-g-PEG coated surfaces. The combination of AFM surface force measurements and protein adsorption experiments provides insights into the interfacial forces associated with various PEGylated surfaces and the mechanisms of protein resistance.     

Th(IV) adsorption on alumina: effects of contact time, pH, ionic strength and phosphate

Adsorption of Th(IV) (total concentration, 10(-5)-10(-4) mol/L) was studied by a batch technique. The effects of pH, ionic strength, contact time, and phosphate on the adsorption of Th(IV) onto alumina were investigated. Adsorption isotherms of Th(IV) on alumina at approximately constant pH and three ionic strengths (0.05, 0.1, 0.5 mol/L KNO3) were determined. It was found that the pH values of aqueous solutions of both the Th(IV)-alumina and phosphate-alumina adsorption systems increase with increasing contact time, respectively. Adsorption of Th(IV) on alumina steeply increases with increasing pH from 1 to 4.5 and the adsorption edge consists of three regions. The phosphate added clearly enhances Th(IV) adsorption in the pH range 1-4. From the adsorption isotherms at approximately constant pH and three different ionic strengths, a reduced ionic strength effect was observed and is contradictory to the insensitive effect obtained from the adsorption edges on three oxides of Fe, Al, and Si at different ionic strengths. Compared with the adsorption edges at different ionic strengths, the adsorption isotherms at approximately constant pH and different ionic strengths are more advantageous in the investigation of ionic strength effect. The significantly positive effect of phosphate on Th(IV) adsorption onto alumina was attributed to strong surface binding of phosphate on alumina and the subsequent formation of ternary surface complexes involving Th(IV) and phosphate.     

The adsorption of hydrophobically modified carboxymethylcellulose on a hydrophobic solid: effects of pH and ionic strength

Carboxymethylcellulose with 1.2% dodecyl groups (per glucose unit) was prepared by amidation with dodecylamine. This polymer behaves as a hydrophobically modified polyelectrolyte with the following thickening properties which are determined from viscosity data. It adsorbs from aqueous solution on spin-coated polystyrene films to various extents, depending on pH and ionic strength. The adsorbed amount has a surprising minimum at around pH 6 which coincides with a maximum in viscosity of a dilute solution, and with a very pronounced maximum in the hydrodynamic radius as determined from dynamic light scattering. To our knowledge, such behavior has not been reported before. It suggests that at low pH the polymer is present in the form of small aggregates which upon increasing the pH first swell and then break up into single molecules. Received: 19 June 1997 Accepted: 5 January 1998     

Am(III) adsorption on oxides of aluminium and silicon: effects of humic substances,pH, and ionic strength

The adsorption of Am(III) (total concentration 10(-9) mol/l) on alumina, silica, and hematite was studied by a batch technique. The effects of pH, ionic strength, and humic substances on the adsorption of Am(III) on alumina and silica were investigated, and the adsorption isotherms of Am(III) on alumina and silica at different pH values were determined. It was found that compared with the adsorption of Am(III) on alumina, the adsorbability of silica on the basis of mass is less, the relative adsorption rate on silica is slower, the sensitivity of adsorption on silica to ionic strength is less, the dependence of adsorption on silica on pH is gentler, and consequently that the adsorption characteristics of Am(III) on alumina and silica are distinctly different. The negative effect of fulvic acid on the adsorption on silica and the positive effect of humic acid on the adsorption on alumina were found. In contrast to the Am(III) adsorption on alumina and silica, a tremendously high adsorbability of Am(III) on hematite was found. The sequence of adsorbabilities of Am(III) on the basis of mass is Fe2O3 > Al2O3 > SiO2.     

Effects of pH and ionic strength on the adsorption of phosphate and arsenate at the goethite-water interface

The surface properties of a well-crystallized synthetic goethite have been studied by acid-base potentiometric titrations, electrophoresis, and phosphate and arsenate adsorption isotherms at different pH and electrolyte concentrations. The PZC and IEP of the studied goethite were 9.3+/-0.1 and 9.3+/-0.2, respectively. Phosphate and arsenate adsorption decrease as the pH increases in either 0.1 or 0.01 M KNO(3) solutions. Phosphate adsorption is more sensitive to changes in pH and ionic strength than that of arsenate. The combined effects of pH and ionic strength result in higher phosphate adsorption in acidic media at most ionic strengths, but result in lower phosphate adsorption in basic media and low ionic strengths. The CD-MUSIC model yields rather good fit of the experimental data. For phosphate it was necessary to postulate the presence of three inner-sphere surface complexes (monodentate nonprotonated, bidentate nonprotonated, and bidentate protonated). In contrast, arsenate could be well described by postulating only the presence of the two bidenate species. A small improvement of the arsenate adsorption data could be achieved by assuming the presence of a monodentate protonated species. Model predictions are in agreement with spectroscopic evidence, which suggest, especially for the case of arsenate, that mainly bidentate inner-sphere complexes are formed at the goethite-water interface.     

Benzalkonium chloride and sulfamethoxazole adsorption onto natural clinoptilolite: effect of time, ionic strength, pH and temperature

The influence of different physical factors on the adsorption of the cationic surfactant benzalkonium chloride (BC) and the model drug sulfamethoxazole by a purified natural clinoptilolite (NZ) has been studied in order to employ zeolite-surfactant-drug composites as drug deliverer. It has been demonstrated that the adsorption of BC and sulfamethoxazole onto NZ depends of the time, the temperature, the ionic strength and the pH of the aqueous medium. The optimal conditions for the preparation of the zeolite-surfactant and zeolite-surfactant-drug composite materials are established. The results of the composite characterization support the presence of BC and sulfamethoxazole, as well as the structural stability of NZ during the treatments performed. The release experiments in acid medium demonstrate that the adsorption of sulfamethoxazole is reversible. It is also confirmed that the drug release profile corresponds to a diffusion or zero-order mechanism as a function of the compression pressure.     

Influence of pH,ionic strength,foreign ions and FA on adsorption of radiocobalt on goethite

This work contributed to the adsorption of radiocobalt on goethite as a function of contact time, pH, ionic strength and foreign ions in the absence and presence of fulvic acid (FA) under ambient conditions. The results indicated that adsorption of Co(II) was dependent on ionic strength and foreign ions at low pH values (pH < 7.8), and independent of ionic strength and foreign ions at high pH values (pH > 7.8). Outer-sphere surface complexation and/or ion exchange were the main mechanisms of Co(II) adsorption on goethite at low pH values, whereas inner-sphere surface complexation was the main adsorption mechanism at high pH values. The presence of FA enhanced Co(II) adsorption at low pH values, but reduced Co(II) adsorption at high pH values. The thermodynamic data (ΔH ⁰, ΔS ⁰, ΔG ⁰) were calculated from the temperature dependent adsorption isotherms, and the results suggested that adsorption process of Co(II) on goethite was spontaneous and endothermic. The results are crucial to understand the physicochemical behavior of Co(II) in the nature environment.     

Effect of adsorption,ionic strength and pH on the potential of the diffuse electric layer

Summary The potential, ₀, of the diffuse electric layer was determined by the equilibrium liquid film method (1). The method is based on calculating the ₀-potential by means of the DLVO theory of the electrostatic disjoining pressure at known concentration,C _el of the solution forming the microscopic liquid film and at known thickness,h _r of the film. The thickness of the film was measured by the microinterferometric method (7, 8). The calculation of the potential was carrried out by means of the more general eqs. [2] and [3], valid for a wide range ofC _el and ₀. The dependence of the potential of the diffuse electric layer on the concentration of surface activ agent (surfactant) stabilizing the liquid film was found. It was established that ionogenic SAS show a higher potential than the non-ionogenic ones. The potential of SAS-free solutions was determined — ca. 30 mV.The dependence of the potential of the diffuse electric layer on concentration and kind of electrolyte was also found.A strong effect of p_H on ₀-potential was observed and the isoelectric points were found. An analysis is made of the ₀-potential values obtained by the method of equilibrium liquid film with respect to their similarity to those on the boundary of the bulk liquid.

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Zusammenfassung Das Potential ₀ der diffusen elektrischen Doppelschicht wurde nach der Methode des flüssigen Gleichgewichtfilms in Schäumen (1) bestimmt. Die Dicke der Filme wurde interferometrisch (7, 8) gemessen.Es wurde die Abhängigkeit des ₀-Potentials von der KonzentrationC _S des grenzflächenaktiven Stoffes (Schaumfilmstabilisator) untersucht. Die ₀ (C _S)-Kurve hat am Anfang ein kleines Plateau, danach folgt ein Anstieg bis zu einem Sättigungswert. Dieser Sättigungswert ₀ ist für die verschiedenen Tenside unterschiedlich. Nichtionogene Tenside besitzen ein geringes ₀, die ionogenen Tenside geben höhere Werte.Es wurde weiterhin der Einfluß der Elektrolytkonzentration auf das ₀-Potential untersucht. Eine spezifische Wirkung der Elektrolyte wurde nicht gefunden. Dagegen wurde ein starker Einfluß des p_H-Wertes auf das ₀-Potential festgestellt. Bei einem bestimmten p_H-Wert fällt ₀ bis zu Null ab, d. h. ein isoelektrischer Punkt tritt auf. Die an Schaumfilmen gemessenen ₀-Werte wurden mit den ₀-Werten an der Oberfläche derselben Lösung verglichen.

     

Th(IV) adsorption on mesoporous molecular sieves: effects of contact time,solid content,pH, ionic strength,foreign ions and temperature

The mesoporous molecular sieves (Al-MCM-41) are synthesized with montmorillonite as silica–alumina source by hydrothermal method. The application of Al-MCM-41 for the adsorption of Th(IV) from aqueous solution is studied by batch technique. The effects of contact time, solid content, pH, ionic strength, foreign ions, and temperature are determined, and the results indicate that the adsorption of Th(IV) to Al-MCM-41 is strongly dependent on pH values but independent of ionic strength. The adsorption isotherms are simulated by D–R and Freundlich models well. The thermodynamic parameters (ΔH ⁰, ΔS ⁰, ΔG ⁰) are calculated from the temperature dependent adsorption isotherms at 293, 313 and 333 K, respectively, and the results suggest that the adsorption of Th(IV) on Al-MCM-41 is a spontaneous and endothermic process. Al-MCM-41 is a suitable material for the preconcentration of Th(IV) from large volumes of aqueous solutions.     

Adsorption of humic acids onto goethite: effects of molar mass, pH and ionic strength

In this paper, the LCD (ligand charge distribution) model is applied to describe the adsorption of (Tongbersven) humic acid (HA) to goethite. The model considers both electrostatic interactions and chemical binding between HA and goethite. The large size of HA particles limits their close access to the surface. Part of the adsorbed HA particles is located in the compact part at the goethite surface (Stern layers) and the rest in the less structured diffuse double layer (DDL). The model can describe the effects of pH, ionic strength, and loading on the adsorption. Compared to fulvic acid (FA), adsorption of HA is stronger and more pH- and ionic-strength-dependent. The larger number of reactive groups on each HA particle than on a FA particle results in the stronger HA adsorption observed. The stronger pH dependency in HA adsorption is related to the larger number of protons that are coadsorbed with HA due to the higher charge carried by a HA particle than by a FA particle. The positive ionic-strength dependency of HA adsorption can be explained by the conformational change of HA particles with ionic strength. At a higher ionic strength, the decrease of the particle size favors closer contact between the particles and the surface, leading to stronger competition with electrolyte ions for surface charge neutralization and therefore leading to more HA adsorption.     

Effect of pH, ionic strength, and temperature on the phosphate adsorption onto lanthanum-doped activated carbon fiber

Phosphate removal from polluted water is crucial to preventing eutrophication. Herein, we present the investigation on phosphate adsorption in aqueous solutions by using lanthanum-doped activated carbon fiber (ACF-La). Various batch sorption conditions, e.g., pH, ionic strength, and temperature were tested, and the adsorption mechanisms were discussed. The sorption capacity of ACF-La was higher in acidic solutions than that in basic ones, suggesting that the Lewis acid-base interaction gradually dominated the adsorption process with the increase in pH values. The degree of phosphate removal decreased with the enhancement of the ionic strength of the solution, meaning that the adsorption of phosphate on ACF-La was strongly dependent on ionic strength. Employing the pseudo first- and second-order, and intra-particle diffusion models to evaluate the adsorption kinetics of phosphate onto ACF-La indicated that the second-order model best fits the experimental data. The presence of chloride ion in solutions increased the effect of intra-particle diffusion on the adsorption of phosphate onto ACF-La but reduced the overall rate of the adsorption. The thermodynamic parameters were determined which revealed the feasibility, spontaneity, and endothermic nature of adsorption.     

Residence time, loading force, pH, and ionic strength affect adhesion forces between colloids and biopolymer-coated surfaces

Exopolymers are thought to influence bacterial adhesion to surfaces, but the time-dependent nature of molecular-scale interactions of biopolymers with a surface are poorly understood. In this study, the adhesion forces between two proteins and a polysaccharide [Bovine serum albumin (BSA), lysozyme, or dextran] and colloids (uncoated or BSA-coated carboxylated latex microspheres) were analyzed using colloid probe atomic force microscopy (AFM). Increasing the residence time of an uncoated or BSA-coated microsphere on a surface consistently increased the adhesion force measured during retraction of the colloid from the surface, demonstrating the important contribution of polymer rearrangement to increased adhesion force. Increasing the force applied on the colloid (loading force) also increased the adhesion force. For example, at a lower loading force of approximately 0.6 nN there was little adhesion (less than -0.47 nN) measured between a microsphere and the BSA surface for an exposure time up to 10 s. Increasing the loading force to 5.4 nN increased the adhesion force to -4.1 nN for an uncoated microsphere to a BSA surface and to as much as -7.5 nN for a BSA-coated microsphere to a BSA-coated glass surface for a residence time of 10 s. Adhesion forces between colloids and biopolymer surfaces decreased inversely with pH over a pH range of 4.5-10.6, suggesting that hydrogen bonding and a reduction of electrostatic repulsion were dominant mechanisms of adhesion in lower pH solutions. Larger adhesion forces were observed at low (1 mM) versus high ionic strength (100 mM), consistent with previous AFM findings. These results show the importance of polymers for colloid adhesion to surfaces by demonstrating that adhesion forces increase with applied force and detention time, and that changes in the adhesion forces reflect changes in solution chemistry.     

pH and ionic strength effect on single fibrinogen molecule adsorption on mica studied with AFM

Although several investigations have been reported on the effect of pH or ionic strength on protein adsorption, most of them have been carried out with protein monolayers and not with single molecules. We have used atomic force microscopy to image, in phosphate buffer, single fibrinogen molecules adsorbed on mica and compare the surface coverage at variable pH (7.4, 5.8, 3.5) or ionic strength (15, 150, 500 mM) conditions. The images obtained and the statistical analysis of the surface coverage indicate adsorption enhancement at the IEP of fibrinogen (pH 5.8) and minimum adsorption at pH 3.5. On the other hand, more protein was adsorbed when the salt concentration of the buffer at pH 7.4 was increased from 15 to 150 mM. However, further increase of salt concentration up to 500 mM resulted in decreased adsorption. To confirm the aforementioned results an approaching bare Si(3)N(4) tip was used as an electrostatic analogue to a protein molecule and interaction force curves between it and the substrate were recorded. The results were in consistence with the double layer theory which justifies the screening of electrostatic repulsion as the salt concentration increases.     

Sorption of Th(IV) on goethite: effects of pH,ionic strength,FA and phosphate

Sorption of thorium (IV) on goethite was investigated as a function of contact time, pH, ionic strength, anions, solid-to-liquid ratio (m/V) and Th(IV) concentration using batch technique. The results showed that the sorption of Th(IV) was strong pH-dependence, and increased from ~10 to ~100% over the pH range of 2.0–4.0, and then kept a constant level in the higher pH range. The sorption of Th(IV) increased with increasing m/V and independent of ionic strength. It was clear that phosphate and FA significantly enhanced Th(IV) sorption on goethite. The sorption and desorption isotherms were investigated at pH 2.90 ± 0.05 and analyzed with Freundlich and Langmuir models, respectively. Compared to Langmuir model, Freundlich model could fit the experimental data better, according to the high relative coefficients.     

Redox,ionic strength,and pH sensitive supramolecular polymer assemblies

Formation of micelle‐type assembly from supramolecular complexation of a surfactant and an oppositely charged homopolymer is demonstrated. The lower CAC observed for these assemblies suggest that the electrostatic interaction provides an amphiphilic homopolymer‐like structure. The stimulus‐induced disassembly of these supramolecular structures has been accomplished with variations in redox characteristics, ionic strength, and pH of the medium. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1052–1060, 2009     

Adsorption of paraquat on mesoporous silica modified with titania: effects of pH, ionic strength and temperature

The adsorption of the herbicide paraquat (PQ(2+)) on the binary system titania-silica has been studied in batch experiments by performing adsorption isotherms under different conditions of pH, supporting electrolyte concentration, and temperature. Adsorption kinetic on the studied material has also been carried out and discussed. PQ(2+) adsorption is very low on the bare silica surface but important on the composed TiO(2)-SiO(2) adsorbent. In this last case, the adsorption increases by increasing pH and decreasing electrolyte concentration. There are no significant effects of temperature on the adsorption. The increase of the adsorption in TiO(2)-SiO(2) seems to be related to an increase in acid sites of the supported titania and to the homogenously dispersion of the TiO(2) nanoparticles over the silica support. The adsorption takes place by direct binding of PQ(2+) to TiO(2) leading to the formation of surface species of the type SiO(2)-TiO(2)-PQ(2+). Electrostatic interactions and charge-transfer and outer-sphere complexes formations seem to play a key role in the adsorption mechanism. The analysis of thermodynamic parameters suggests that the adsorption on TiO(2)-SiO(2) is endothermic and spontaneous in nature.     

Effect of pH,ionic strength and humic substances on the adsorption of Uranium (VI) onto Na-rectorite

In this study, the adsorption of U(VI) from aqueous solution on Na-rectorite was studied as a function of various environmental conditions such as contact time, pH, ionic strength, soil humic acid (HA)/fulvic acid (FA), solid contents, and temperature under ambient conditions by using batch technique. The kinetic adsorption is fitted by the pseudo-second-order model very well. The adsorption of U(VI) on Na-rectorite was strongly dependent on pH and ionic strength. A positive effect of HA/FA on U(VI) adsorption was found at low pH, whereas a negative effect was observed at high pH. The presence of HA/FA enhanced the U(VI) adsorption at low pH values, but reduced U(VI) adsorption at high pH. The thermodynamic parameters (ΔH ⁰, ΔS ⁰, and ΔG ⁰) were also calculated from the temperature dependent adsorption isotherms, and the results suggested that the adsorption of U(VI) on Na-rectorite was a spontaneous and endothermic process.     

The impact of ionic strength and background electrolyte on pH measurements in metal ion adsorption experiments

Our understanding of metal ion adsorption to clay minerals has progressed significantly over the past several decades, and theories have been promulgated to describe and predict the impacts of pH, ionic strength, and background solution composition on the extent of adsorption. Studies evaluating the effects of ionic strength on adsorption typically employ a broad range of background electrolyte concentrations. Measurement of pH in these systems can be inaccurate when pH values are measured with liquid junction pH probes calibrated with standard buffers due to changes in the liquid junction potential between standard, low ionic strength (0.05 M) buffers and high ionic strength solutions (>0.1 M). The objective of this research is to determine the extent of the error in pH values measured at high ionic strength, and to develop an approach for accurately measuring pH over a range of ionic strengths using a combined pH electrode. To achieve this objective, the adsorption of cobalt (10(-5) M) onto gibbsite (10 g/L) from various electrolyte solutions (0.01-1 M) was studied. The pH measurements were determined from calibrations with standard buffers and ionic strength corrected buffer calibrations. The results show a significant effect of the aqueous solution background electrolyte anion and ionic strength on pH measurement. The 0.5 and 1 M ionic strength metal ion adsorption edges shifted to lower pH with increasing ionic strength when pH was calibrated with standard buffers whereas no shift in the adsorption edges was observed when calibrated with ionic strength corrected buffers. Therefore, to obtain an accurate pH measurement, pH calibration should contain the same electrolyte and ionic strength as the samples.