Solution Depletion Method Research Articles

Adsorption of asphaltenes on multiscale porous alumina

Alumina catalysts are frequently used in refineries for the hydrotreatment of heavy petroleum fraction that are enriched in asphaltenes. The transport of real and model asphaltenes molecules through powder and alumina’s extrudates treated or not at 150 °C to remove or not surface-adsorbed water was studied. The kinetics and isotherms of adsorption at 298 K were obtained by the solution depletion method. Calorimetric experiments were also investigated. The kinetic is faster on the powder than on the alumina extrudates where the equilibrium is reached after 24 h (against 1 h for the powder) due to mass transfer limitation. The capacity of adsorption of model asphaltenes on untreated powder and extrudates is comparable around 1.1 and 1.2 mg.m− 2 and increases with the heat treatment due to water removal. Both adsorption strength and capacity of real asphaltenes on alumina is lower compared to the model asphaltene molecule which could be explained by the strong interaction between the acidic function of the model molecule and the alumina surface. The calorimetric study in absence of alumina shows the dimerization of model asphaltene molecules. In presence of alumina, the enthalpy of adsorption of model and real asphaltenes on alumina is determined. The enthalpy of adsorption of model asphaltenes on treated powder is higher than on untreated powder meaning that more energetic sites are available (probably due to the release of water-occupied sites) and the curve obtained for treated powder suggests different adsorption sites. The enthalpy of adsorption of model asphaltene is higher for the treated extrudates but these results must be taken carefully because the kinetic of adsorption is very slow (24 h) for extrudates. The effect of flow rate was studied by saturating an extrudate column with model asphaltene molecules. The adsorption increases as the flow rate decreases which could be explained by higher friction in the macropores leading to the release of weakly retained asphaltenes as the flow rate increases or by less intermediate pore blocking by asphaltenes as the flow rate and thus the pressure increases. This study shows that the transport of asphaltenes through porous alumina supports is a complex process depending on many parameters.

Different relationships of fine root traits with root ammonium and nitrate uptake rates in conifer forests

ABSTRACT Nitrogen (N) uptake by fine roots of trees is important for understanding root physiological function in forest ecosystems. However, direct investigations of in situ ammonium and nitrate uptake rates are limited. Thus, we aimed to clarify inorganic N uptake rates among tree species and determine the factors controlling N uptake through relationships with fine root traits in cool temperate coniferous forests. Using a solution depletion method for measuring N uptake, we observed the relationship of N uptake rate in the form of NH4 + and NO3 – by an intact root system with root morphological traits, such as root diameter, specific root length (SRL), and root tissue density (RTD). The coniferous roots in this study preferred NH4 + to NO3 −. Across species, there were significant relationships between NH4 + uptake and diameter, SRL, and RTD; in contrast, only RTD had a significant impact on NO3 − uptake. Relationships between N uptake rates and root morphological traits differed between NH4 + and NO3 –. We found that the relationship of inorganic N uptake with morphological traits depended on the characteristics of the N form adsorbed through soil and on tree N assimilation efficiency. Our results make a breakthrough in the understanding of root physiological function and the prediction of fundamental N acquisition strategies.

Formation of Myoglobin Corona at Polymer Microparticles

Adsorption of myoglobin molecules at negatively charged polystyrene microparticles was studied using the dynamic light scattering (DLS), electrophoresis (LDV) and the solution depletion method involving atomic force microscopy (AFM). The measurements were carried out at pH 3.5 and NaCl concentration of 10−2 and 0.15 M. Initially, the stability of myoglobin solutions and the particle suspensions as a function of pH were determined. Afterward, the formation of myoglobin molecule corona was investigated via the direct electrophoretic mobility measurements, which were converted to the zeta potential. The experimental results were quantitatively interpreted in terms of the general electrokinetic model. This approach yielded the myoglobin corona coverage under in situ conditions. The maximum hard corona coverage was determined using the AFM concentration depletion method. It was equal to 0.9 mg m−2 for the NaCl concentration in the range 0.01 to 0.15 M and pH 3.5. The electrokinetic properties of the corona were investigated using the electrophoretic mobility measurements for a broad pH range. The obtained results confirmed that thorough physicochemical characteristics of myoglobin molecules can be acquired using nM amounts of the protein. It was also argued that this method can be used for performing electrokinetic characteristics of other proteins such as the SARS-Cov-2 spike protein exhibiting, analogously to myoglobin, a positive charge at acidic pHs.

Development of Silica-Immobilized Vaccines for Improving Thermo-Tolerance and Shelf-Life

It is estimated that 50% of vaccines produced annually are wasted because effectivity is dependent on protein structure and heat exposure disrupts the intermolecular interactions that maintain this structure. Since 90% of vaccines require a temperature-controlled supply chain, it is necessary to create a cold chain system to minimize vaccine waste. We have developed a more sustainable technology via the adsorption of Invasion Plasmid Antigen D (IpaD) onto mesoporous silica gels, improving the thermal stability of protein-based therapeutics. The solution depletion method using UV-Vis was utilized to study the adsorption of IpaD onto silica gels. The silica-IpaD complex is heated above the denaturing temperature of the protein and then the IpaD is removed using N,N-Dimethyldodecylamine N-oxide (LDAO) and their secondary structure is tested using circular dichroism (CD). Pore diameter, pore volume and surface area were characterized for seven different silica gels. Silica gels designated as 6389, 6378, and 6375 had an adsorption percentage above 95% at pore volumes of 2.2, 2.8 and 3.8 cm3 mg-1, respectively. CD analyses confirmed that the adsorbed IpaD after the heat treatment displayed a similar "W" shape CD signal as the native IpaD, indicating the conservation of α-helices. In contrast, the unprotected IpaD after being exposed to high temperature shows a flat CD signal, demonstrating the loss of secondary structure. We have successfully increased the thermo-tolerance for IpaD using mesoporous silica and continue to further optimize mesoporous silica's physiochemical properties to improve adsorption and desorption yields.

Influence of the Templating Amine on the Nanostructure and Charge of Layered Vanadates for Radioactive Wastewater Treatment

Theoretical and experimental characterization methods have been combined to shed more light on the potential impact of the various templating amines that they may have on the formation of layered vanadates with a scroll morphology and their cation-exchange capacity. Eight branched- and straight-chain alkylamines were used as the structure-directing agents to make the V2O5 sheets roll up into a scroll structure during microwave-assisted hydrothermal synthesis. The as-synthesized materials were characterized by X-ray diffraction (XRD), transmission electronic microscopy (TEM), and elemental analysis in order to obtain the necessary information on their nanostructure and morphology as a function of the template. On the basis of these experimental results, the density functional theory (DFT) calculations completed with Monte Carlo simulation methods were applied to model the template conformation and the way it had been intercalated into the vanadate layers. It was then postulated that similar structuring mechanisms had been involved in the formation of different hybrid nanotubes with amine molecules forming tilted bilayers of overlapping hydrocarbon tails. In line with what one would expect from the hypothesis that the driving force of vanadate scrolling is due to a partial reduction of V(V) to V(IV) induced by interactions of amine template with the 2D V2O5 layers, the variations in the surface charge density were monitored by studying the cation-exchange capacity of the samples prepared previously by exchanging the amine units for ammonium cations. Then the isotherms for ion exchange of cesium onto such samples from aqueous solutions were measured by the solution depletion method. The use of cesium to probe the surface density of negative charge can also be regarded as viable in view of potential application of vanadate nanotubes in radioactive wastewater treatment.

The effect of chelating anions on the retention of Co(II) by γ-alumina from aqueous solutions under the unadjusted pH condition of supported catalyst preparation

This study analyzes the effect of the addition of acetate, citrate, and nitrilotriacetate anions on the retention of Co(II) cations by the γ-alumina surface in view of the preparation of alumina supported cobalt catalysts. The emphasis was placed on the way the Co(II) species attach to the solid surface when adsorbed from aqueous solutions under the unadjusted pH condition. The individual adsorption isotherms onto γ-Al2O3 support for cobalt and a given ligand were determined by following the solution depletion method in single-solute and bi-solute systems. These adsorption data were supplemented by the results of potentiometric titrations. In the case of bi-solute systems, the adsorption procedures allowed either co-impregnation of γ-alumina with equimolar solutions of cobalt and ligand salts or pre-impregnation of γ-alumina with the ligand anions and the subsequent adsorption of cobalt. Changes in the pH of the equilibrium solid-liquid suspension were also monitored along the adsorption isotherms. The adsorption of Co(II) onto γ-Al2O3 in the presence of acetate and nitrilotriacetate led to the formation of the type A (i.e., solid-metal-ligand) ternary complexes. The use of citrate anions together with Co(II) cations was shown to improve the impregnation process through the formation of ternary complexes of type B (i.e., solid-ligand-metal). The comparison with a system containing tricarballylate anions allowed concluding that the presence of the hydroxyl group in the citrate anion enhanced its affinity for the alumina surface by contributing to the inner-sphere character of its surface-bound complexes.

Direct electron transfer of bilirubin oxidase at a carbon flow-through electrode

The kinetics and isotherms of adsorption of Myrothecium verrucaria bilirubin oxidase (Mv BOD) on nanoporous carbons (CNP) and carbon nanofibers (CNF) were studied by the solution depletion method. The kinetics of adsorption of Mv BOD on both carbons are very fast, reaching equilibrium within 10 min and 30 min for CNP and CNF, respectively. The adsorption isotherms reveal a strong affinity between Mv BOD and both carbons. An original flow-through device based on electrodes filled with CNP and CNF connected to an UV–vis spectrophotometer was used to correlate the dioxygen catalytic current reduction to the amount of Mv BOD adsorbed on the carbons. It was shown that although the amount of BOD adsorbed on CNP is much lower than the amount adsorbed on CNF, the currents are comparable, suggesting that the Mv BOD orientation is more favourable for direct electron transfer in the case of CNP. Chronoamperometry experiments showed that the catalytic current is stable for a few days (70 h). It was emphasized by stop and flow experiments that the current is limited by mass transport inside the column.

Selective capture of glycoproteins using lectin-modified nanoporous gold monolith

The surface of nanoporous gold (np-Au) monoliths was modified via a flow method with the lectin Concanavalin A (Con A) to develop a substrate for separation and extraction of glycoproteins. Self-assembled monolayers (SAMs) of α-lipoic acid (LA) on the np-Au monoliths were prepared followed by activation of the terminal carboxyl groups to create amine reactive esters that were utilized in the immobilization of Con A. Thermogravimetric analysis (TGA) was used to determine the surface coverages of LA and Con A on np-Au monoliths which were found to be 1.31×1018 and 1.85×1015moleculesm−2, respectively. An in situ solution depletion method was developed that enabled surface coverage characterization without damaging the substrate and suggesting the possibility of regeneration. Using this method, the surface coverages of LA and Con A were found to be 0.989×1018 and 1.32×1015moleculesm−2, respectively. The selectivity of the Con A-modified np-Au monolith for the high mannose-containing glycoprotein ovalbumin (OVA) versus negative control non-glycosylated bovine serum albumin (BSA) was demonstrated by the difference in the ratio of the captured molecules to the immobilized Con A molecules, with OVA:Con A=2.3 and BSA:Con A=0.33. Extraction of OVA from a 1:3 mole ratio mixture with BSA was demonstrated by the greater amount of depletion of OVA concentration during the circulation with the developed substrate. A significant amount of captured OVA was eluted using α-methyl mannopyranoside as a competitive ligand. This work is motivated by the need to develop new materials for chromatographic separation and extraction substrates for use in preparative and analytical procedures in glycomics.

Adsorption of Polyvinylpyrrolidone and its Impact on Maintenance of Aqueous Supersaturation of Indomethacin via Crystal Growth Inhibition

This study explored the adsorption and crystal growth inhibitory effects of polyvinylpyrrolidone (PVP) on indomethacin crystals in an aqueous medium. A solution depletion method was used to construct adsorption isotherms of PVPs with different molecular weights and N-vinylpyrrolidone onto indomethacin crystals. The affinity for and extent of maximum adsorption of PVP on indomethacin crystals were significantly higher than that of N-vinylpyrrolidone, which was attributed to cooperative interactions between PVP and the surface of indomethacin. The extent of PVP adsorption onto indomethacin crystals in terms of mg/m(2) was greater for higher molecular weight PVP but less on a molar basis indicating an increased percentage of loops and tails for the higher molecular weight PVP. PVP significantly inhibited the crystal growth of indomethacin at a high degree of supersaturation as compared with N-vinylpyrrolidone, which was attributed to a change in indomethacin crystal growth mechanism leading to a change in the rate limiting step from bulk diffusion to surface integration. Higher molecular weight PVPs are better inhibitors of the crystal growth of indomethacin than lower molecular weight PVPs, which was attributed in part to a greater barrier for surface diffusion of indomethacin provided by a thicker adsorption layer of PVP.

Surface chemistry aspects of bastnaesite flotation with octyl hydroxamate

Surface chemistry features of bastnaesite were investigated with respect to octyl hydroxamate adsorption and the wetting characteristics at different levels of hydroxamate adsorption. The wetting characteristics of bastnaesite in the absence of collector adsorption were examined using both contact angle measurements and MD simulations. As expected, it was found that bastnaesite is naturally hydrophilic. Surface chemistry studies included octyl hydroxamate adsorption by the solution depletion method. The surface state was described by sum-frequency vibrational spectroscopy (SFVS) and molecular dynamic simulations (MDS). The adsorption isotherm at low levels of hydroxamate adsorption was established and the hydrophobic surface state was described from contact angle measurements. The relationship between hydrophobicity and adsorption density was examined for the first time by MD simulations. The SFVS spectra indicate that a well-ordered monolayer was formed at a hydroxamate concentration of about 1×10−4M.

On the real performance of cation exchange resins in wastewater treatment under conditions of cation competition: the case of heavy metal pollution

Sorption performance of cation-exchange resins Amberlite® IRN77 and Amberlite™ IRN9652 toward Cs(I) and Sr(II) has been tested in single-component aqueous solutions and simulated waste effluents containing other monovalent (Effluent 1) or divalent (Effluent 2) metal cations, as well as nitrate, borate, or carbonate anions. The individual sorption isotherms of each main component were measured by the solution depletion method. The differential molar enthalpy changes accompanying the ion-exchange between Cs+ or Sr2+ ions and protons at the resin surface from single-component nitrate solutions were measured by isothermal titration calorimetry and they showed a higher specificity of the two resins toward cesium. Compared to the retention limits of both resins under such idealized conditions, an important depression in the maximum adsorption capacity toward each main component was observed in multication systems. The overall effect of ion exchange process appeared to be an unpredictable outcome of the individual sorption capacities of the two resins toward various cations as a function of the cation charge, size, and concentration. The cesium retention capacity of the resins was diminished to about 25% of the "ideal" value in Effluent 1 and 50% in Effluent 2; a further decrease to about 15% was observed upon concomitant strontium addition. The uptake of strontium by the resins was found to be less sensitive to the addition of other metal components: the greatest decrease in the amount adsorbed was 60% of the ideal value in the two effluents for Amberlite® IRN77 and 75% for Amberlite™ IRN9652. It was therefore demonstrated that any performance tests carried out under idealized conditions should be exploited with much caution to predict the real performance of cation exchange resins under conditions of cation competition.

Formation of N719 Dye Multilayers on Dye Sensitized Solar Cell Photoelectrode Surfaces Investigated by Direct Determination of Element Concentration Depth Profiles

The structure of the dye layer adsorbed on the titania substrate in a dye-sensitized solar cell is of fundamental importance for the function of the cell, since it strongly influences the injection of photoelectrons from the excited dye molecules into the titania substrate. The adsorption isotherms of the N719 ruthenium-based dye were determined both with a direct method using the depth profiling technique neutral impact collision ion scattering spectroscopy (NICISS) and with the standard indirect solution depletion method. It is found that the dye layer adsorbed on the titania surface is laterally inhomogeneous in thickness and there is a growth mechanism already from low coverage levels involving a combination of monolayers and multilayers. It is also found that the amount of N719 adsorbed on the substrate depends on the titania structure. The present results show that dye molecules in dye-sensitized solar cells are not necessarily, as presumed, adsorbed as a self-assembled monolayer on the substrate.

Tetracycline adsorption on chitosan: A mechanistic description based on mass uptake and zeta potential measurements

Adsorption phenomena occurring at the solid/liquid interface of chitosan particles are of extreme importance in the kinetics of drug release/upload as well as in effluent treatment by adsorption. In this work, equilibrium and kinetic aspects of protonated tetracycline adsorption on chitosan are explored using classic solution depletion method and zeta potential measurements. Equilibrium experiments showed that for solutions with tetracycline initial concentration of ca. 1.2gL−1, corresponding to a pH around 3, chitosan structure disrupted, as indicated by an increase in magnitude of tetracycline sorption. Adsorption and zeta potential isotherms before disruption suggested that the process of adsorption had a Langmuir character up to a point at which subsurface was exposed to adsorption; at this point, a second mode of sorption began: zeta potential tended to an equilibrium value, following Sips isotherm and tetracycline sorption had a linear dependence on its continuous phase concentration. The kinetics of tetracycline sorption suggested that sorption of tetracycline was divided between the sorption of protonated and non-protonated tetracycline; tetracycline in its non-protonated form seemed to rule the sorption of tetracycline.

Effect of Nanoscale Pore Space Confinement on Cadmium Adsorption from Aqueous Solution onto Ordered Mesoporous Silica: A Combined Adsorption and Flow Calorimetry Study

The competitive adsorption of Cd(II) ions has been studied from aqueous solutions of cadmium nitrate in the presence of 0.1 M sodium nitrate at 298 K onto nonporous Spherosil XO75LS and ordered porous silica of the SBA-15 type with the purpose of tracking the effect of confinement in pores. Four SBA-15 mesoporous samples, prepared using pluronic P103 and P123 as templates at two temperatures of 303 and 373 K and differing in both the pore size (from 3.8 to 6.4 nm) and the areal density of acid sites reactive toward gaseous ammonia, were used. The equilibrium adsorption isotherms and the enthalpy curves for the related displacement process were determined by means of solution depletion method and liquid flow microcalorimetry, respectively. The experimental adsorption curves were transformed into normalized plots showing the ratio between the amount of cadmium retained at the solid–liquid interface and the number of surface silanol groups. Estimates of the standard molar Gibbs free energy and entropy of adsorption were made in the pseudoplateau adsorption region. It was postulated that the adsorbing Cd2+ cations ion exchanged with the initially adsorbed Na+ ions, the overall displacement process being endothermic and entropy-driven in the whole concentration range studied, mostly due to small modifications of the hydration shells of adsorbing and desorbing species. The areal density of surface silanols, influenced by the surface curvature within pore space, appeared to be the major factor that induced significant changes in the molar enthalpy of displacement.

Measurements of water sorption enthalpy on polymer surfaces and its effect on protein adsorption

The molar enthalpy of sorption ( ΔH m s` ) of water vapor onto three polymer surfaces and its effect on nonspecific protein adsorption were investigated by inverse gas chromatography (IGC). The values of ΔH m s measured by IGC were found to be −16.9 ± 1.2, −18.6 ± 1.3, and −29.9 ± 2.4 kJ/mole for polystyrene (PS), polymethylmethacrylate (PMMA), and poly(2-hydroxyethyl methacrylate) (PHEMA), respectively, over a temperature range of 333–423 K. Protein adsorption to three polymer-coated substrates was conducted as a function of the bulk protein concentration using lysozyme, fibrinogen, and bovine serum albumin (BSA), and the amount of adsorbed protein was measured by the solution depletion method. For a given bulk protein concentration, a larger amount of protein is adsorbed on PS and PMMA surfaces which have greater ΔH m s than that of PHEMA surfaces. Although ΔH m s for PS and PMMA are close to each other, PS surfaces were found to exhibit a higher adsorption affinity than PMMA surfaces over the proteins and concentrations investigated. Our results indicate that the strength of water–polymer interactions and the functional groups on the polymer surface are important factors for controlling the amount of nonspecifically adsorbed protein.

Volumetric interpretation of protein adsorption: Interfacial packing of protein adsorbed to hydrophobic surfaces from surface-saturating solution concentrations

The maximum capacity of a hydrophobic adsorbent is interpreted in terms of square or hexagonal (cubic and face-centered-cubic, FCC) interfacial packing models of adsorbed blood proteins in a way that accommodates experimental measurements by the solution-depletion method and quartz-crystal-microbalance (QCM) for the human proteins serum albumin (HSA, 66 kDa), immunoglobulin G (IgG, 160 kDa), fibrinogen (Fib, 341 kDa), and immunoglobulin M (IgM, 1000 kDa). A simple analysis shows that adsorbent capacity is capped by a fixed mass/volume ( e.g. mg/mL) surface-region (interphase) concentration and not molar concentration. Nearly analytical agreement between the packing models and experiment suggests that, at surface saturation, above-mentioned proteins assemble within the interphase in a manner that approximates a well-ordered array. HSA saturates a hydrophobic adsorbent with the equivalent of a single square or hexagonally-packed layer of hydrated molecules whereas the larger proteins occupy two-or-more layers, depending on the specific protein under consideration and analytical method used to measure adsorbate mass (solution depletion or QCM). Square or hexagonal (cubic and FCC) packing models cannot be clearly distinguished by comparison to experimental data. QCM measurement of adsorbent capacity is shown to be significantly different than that measured by solution depletion for similar hydrophobic adsorbents. The underlying reason is traced to the fact that QCM measures contribution of both core protein, water of hydration, and interphase water whereas solution depletion measures only the contribution of core protein. It is further shown that thickness of the interphase directly measured by QCM systematically exceeds that inferred from solution-depletion measurements, presumably because the static model used to interpret solution depletion does not accurately capture the complexities of the viscoelastic interfacial environment probed by QCM.

Volumetric interpretation of protein adsorption: Kinetics of protein-adsorption competition from binary solution

The standard solution-depletion method is implemented with SDS-gel electrophoresis as a multiplexing, separation-and-quantification tool to measure competition between two proteins ( i and j) for adsorption to the same hydrophobic adsorbent particles (either octyl sepharose or silanized glass) immersed in binary-protein solutions. Adsorption kinetics reveals an unanticipated slow protein-size-dependent competition that controls steady-state adsorption selectivity. Two sequential pseudo-steady-state adsorption regimes (State 1 and State 2) are frequently observed depending on i, j solution concentrations. State 1 and State 2 are connected by a smooth transition, giving rise to sigmoidally-shaped adsorption-kinetic profiles with a downward inflection near 60 min of solution/adsorbent contact. Mass ratio of adsorbed i, j proteins ( m i / m j ) remains nearly constant between States 1 and 2, even though both m i and m j decrease in the transition between states. State 2 is shown to be stable for 24 h of continuous-adsorbent contact with stagnant solution whereas State 2 is eliminated by continuous mixing of adsorbent with solution. In sharp contrast to binary-competition results, adsorption to hydrophobic adsorbent particles from single-protein solutions (pure i or j) exhibits no detectable kinetics within the timeframe of experiment from either stagnant or continuously mixed solution, quickly achieving a single steady-state value in proportion to solution concentration. Comparison of binary competition between dissimilarly-sized protein pairs chosen to span a broad molecular-weight (MW) range demonstrates that selectivity between i and j scales with MW ratio that is proportional to protein-volume ratio (ubiquitin, Ub, MW = 10.7 kDa; human serum albumin, HSA, MW = 66.3 kDa; prothrombin, FII, 72 kDa; immunoglobulin G, IgG, MW = 160 kDa; fibrinogen, Fib, MW = 341 kDa). Results are interpreted in terms of a kinetic model of adsorption that has protein molecules rapidly diffusing into an inflating interphase that is spontaneously formed by bringing a protein solution into contact with a physical surface (State 1). State 2 follows by rearrangement of proteins within this interphase to achieve the maximum interphase concentration (dictated by energetics of interphase dehydration) within the thinnest (lowest volume) interphase possible by ejection of interphase water and initially-adsorbed proteins. Implications for understanding biocompatibility are discussed using a computational example relevant to the problem of blood–plasma coagulation.

Effect of inorganic positive ions on the adsorption of surfactant Triton X-100 at quartz/solution interface

The electrode-separated piezoelectric sensor (ESPS), an improved setup of quartz crystal microbalance (QCM), has been employed to investigate the adsorption behavior of nonionic surfactant Triton X-100 at the hydrophilic quartz-solution interface in mineralized water medium in situ, which contained CaCl2 0.01 mol·L−1, MgCl2 0.01 mol·L−1, NaCl 0.35 mol·L−1. In a large scale of surfactant concentration, t effects of Ca2+, Mg2+ and Na+ on the adsorption isotherm and kinetics are obviously different. In aqueous solution containing NaCl only, adsorption of Triton X-100 on quartz-solution interface is promoted, both adsorption rate and adsorption amount increase. While in mineralized water medium, multivalent positive ions Ca2+ and Mg2+ are firmly adsorbed on quartz-solution interface, result in the increasing of adsorption rate and adsorption amount at low concentration of surfactant and the peculiar desorption of surfactant at high concentration of Triton X-100. The results got by solution depletion method are in good agreement with which obtained by ESPS. The “bridge” and “separate” effect of inorganic positive ions on the adsorption and desorption mechanism of Triton X-100 at the quartz-solution interface is discussed with molecular dynamics simulations (MD), flame atomic absorption spectrometry (FAAS) and atomic force microscopy (AFM) methods.

Volumetric interpretation of protein adsorption: Ion-exchange adsorbent capacity, protein pI, and interaction energetics

Adsorption of lysozyme (Lys), human serum albumin (HSA), and immunoglobulin G (IgG) to anion- and cation-exchange resins is dominated by electrostatic interactions between protein and adsorbent. The solution-depletion method of measuring adsorption shows, however, that these proteins do not irreversibly adsorb to ion-exchange surfaces, even when the charge disparity between adsorbent and protein inferred from protein pI is large. Net-positively-charged Lys (pI = 11) and net-negatively-charged HSA (pI = 5.5) adsorb so strongly to sulfopropyl sepharose (SP; a negatively-charged, strong cation-exchange resin, −0.22 mmol/mL exchange capacity) that both resist displacement by net-neutral IgG (pI = 7.0) in simultaneous adsorption competition experiments. By contrast, IgG readily displaces both Lys and HSA adsorbed either to quaternary ammonium sepharose (Q; a positively-charged, strong anion exchanger, +0.22 mmol/mL exchange capacity) or to octadecyl sepharose (ODS; a neutral hydrophobic resin, 0 mmol/mL exchange capacity). Thus it is concluded that adsorption results do not sensibly correlate with protein pI and that pI is actually a rather poor predictor of affinity for ion-exchange surfaces. Adsorption of Lys, HSA, and IgG to ion-exchange resins from stagnant solution leads to adsorbed multi-layers, into or onto which IgG adsorbs in adsorption competition experiments. Comparison of adsorption to ion-exchange resins and neutral ODS leads to the conclusion that the apparent standard free-energy of adsorption Δ G ads ° of Lys, HSA, and IgG is not large in comparison to thermal energy due to energy-compensating interactions between water, protein, and ion-exchange surfaces that leaves a small net Δ G ads ° . Thus water is found to control protein adsorption to a full range of substratum types spanning hydrophobic (poorly water wettable) surfaces, hydrophilic surfaces bearing relatively-weak Lewis acid/base functionalities that wet with (hydrogen bond to) water but do not exhibit ion-exchange properties, and surfaces with strong Lewis acid/base functional groups that exhibit ion-exchange properties in the conventional chemistry sense of ion-exchange.

Adsorption of itaconic acid from aqueous solutions onto alumina

Itaconic acid, IA (C5H6O4), was investigated as a potential flocculant for the aqueous processing of alumina powders. The adsorption of IA, as a function of its concentration and pH value of the solution, onto the alumina surface was studied by the solution depletion method. The stability of the suspensions in the presence of itaconic acid was evaluated in light of the surface charge of the alumina powder used, the degree of dissociation of IA, as well as the sedimentation behavior and rheology of the suspensions. It was found that the adsorption process is extremely pH dependent; the maximum adsorption of IA onto alumina surface occurring at a pH close to the value of the first IA dissociation constant, pKa1. Also, IA does not influence the value of the point of zero charge of alumina. It was shown that IA represents an efficient flocculant for concentrated acidic alumina suspensions.