Site Dissociation Model Research Articles

Large-area nanostructured surfaces with tunable zeta potentials

Through a refined fabrication protocol based on block-copolymer self-assembly we synthesize nanostructured surfaces with conical nanopillars of different height (60, 120, and 200 nm) in hexagonal arrays with uniform spacing (∼50 nm) over large areas (> cm2). While the nanostructured surfaces fabricated on silicon substrates display superhydrophilic behavior, superhydrophobic properties are attained by coating with octadecyltrichlorosilane (OTS). Negative zeta potentials for all the studied surfaces are reported by electrokinetic flow measurements with aqueous KCl solutions at different concentrations (1 and 10 mM) and pH values between 4 and 8. While the surface nanostructure reduces the zeta potential magnitude, the hydrophobic OTS coating enhances it. Experimental results can be accounted for by a site-dissociation model for the surface charge density. The reported wetting properties and zeta potential tunability makes the studied surfaces particularly relevant for applications such as energy conversion and storage, membrane-based water treatment and molecular separation.

Combined electroosmotic and pressure driven flow in a microchannel at high zeta potential and overlapping electrical double layer

A mathematical model for combined electroosmotic and pressure driven flow in a rectangular microchannel at high zeta potential and overlapping electrical double-layer is presented. The potential distribution is solved numerically without Debye–Hückel approximation using site dissociation model. The corresponding hydrodynamic analysis along with the associated heat transfer problem was performed. For wall potential of 120 mv, the centerline potential could be 95% of wall potential at non-dimensional channel height κa = 0.3 and it was 90% for κa = 1.0, where κ−1 is Debye length and a is half height of the channel. Centerline potential was 84% of wall potential at pH 5.5 and beyond pH 6.5, centerline potential was close to zero, even at lower value of κa (=0.3). Centerline velocity decreases by 16% as κa decreases from 1 to 0.5, when both pressure driven flow and electroosmotic flow are equally dominant, for non-dimensional zeta potential α = 3 (77 mV). For ratio of Joule heating to surface cooling beyond 5, the non-dimensional temperature in the channel is always positive. In absence of Joule heating (G = 0), Nusselt number decreases by 28% as κa decreases from 1 to 0.5. Nusselt number also decreases by 34% with increase in non-dimensional wall zeta potential from 3 to 5.

Numerical Simulation of Proton Distribution with Electric Double Layer in Extended Nanospaces

Understanding the properties of liquid confined in extended nanospaces (10-1000 nm) is crucial for nanofluidics. Because of the confinement and surface effects, water may have specific structures and reveals unique physicochemical properties. Recently, our group has developed a super resolution laser-induced fluorescence (LIF) technique to visualize proton distribution with the electrical double layer (EDL) in a fused-silica extended nanochannel (Kazoe, Y.; Mawatari, K.; Sugii, Y.; Kitamori, T. Anal. Chem.2011, 83, 8152). In this study, based on the coupling of the Poisson-Boltzmann theory and site-dissociation model, the effect of specific water properties in an extended nanochannel on formation of EDL was investigated by comparison of numerical results with our previous experimental results. The numerical results of the proton distribution with a lower dielectric constant of approximately 17 were shown to be in good agreement with our experimental results, which confirms our previous observation showing a lower water permittivity in an extended nanochannel. In addition, the higher silanol deprotonation rate in extended nanochannels was also demonstrated, which is supported by our previous results of NMR and streaming current measurements. The present results will be beneficial for a further understanding of interfacial chemistry, fluid physics, and electrokinetics in extended nanochannels.

The formation of pepsin monomolecular layer by the Langmuir–Blodgett film deposition technique

We report herein the formation of pepsin monomolecular layer by the Langmuir–Blodgett film deposition technique. An effort was made to find an optimal subphase by adjusting the concentration of salt (KCl) and pH by monitoring the growth kinetics of pepsin for the formation of Langmuir monolayer by using as little as possible pepsin molecules to build up ultra thin film and to measure the extent of denaturation. Significant changes of area/molecule, compressibility, rigidity and unfolding of pepsin are observed at optimized subphase than pure water subphase. Observations at optimal subphase are explained in context of the modified DLVO theory and the site dissociation model. FTIR analysis of amide band together with the observed surface morphology of pepsin film in FE-SEM images indicate that at optimal subphase the pepsin molecules modify their structures by incrementing the β-structure, resulting into larger unfolding and inter-molecular aggregates.

Pore-surface characterization of amphoteric charged membranes by means of zeta potential measurements

Abstract Weak amphoteric charged membranes were prepared by the radiation-induced graft copolymerization of the poly(ethylene glycol) (PEG) derivatives with pendant ionizable groups onto polyethylene (PE) porous membranes. In this study, two types of amphoteric charged membranes, which are the amphoteric ion-pair side chain (ASC) type and the mixed grafted chain (MGC) type, were prepared and then characterized by means of zeta (ζ)-potential measurements. The ζ-potential measurements were performed using streaming potential method over the pH range of 2–12. The ζ-potential/pH profiles for the amphoteric charged membranes showed their characteristic behavior. The pH dependence of the apparent surface charge density derived from the ζ-potential was compared with the calculated result from the theoretical model, site dissociation model (SDM), which was based on the dissociation of ionizable groups on the pore surface. Our analysis indicated that the interfacial charge of the ASC-type membrane was ascribed to the dipolar ion structure of cysteine residues. On the other hand, not only the charge groups but also the conformation of grafted polyelectrolyte chain affected the electrokinetic pore-surface charge of the MGC-type membrane.

Surface chemical and electrokinetic characterizations of membranes containing different carriers by x‐ray photoelectron spectroscopy and streaming potential measurements: study of the effect of pH

AbstractTwo activated membranes containing 200 mM of di‐(2‐ethylhexyl) phosphoric acid (DEHPA‐2) and di‐(2‐ethylhexyl)dithiophosphoric acid (DTPA‐2) as carriers have been studied. The electrokinetic characterization of the membrane/solution interface was carried out by means of tangential streaming potential measurements, with the membranes in contact with an NaCl solution (5 × 10−4 M) at different pH (3.5 ≤ pH ≤ 8.5). These measurements allow the determination of the zeta potential, the electrokinetic surface charge density and the isoelectric point of the membranes. Other membrane characteristic parameters such as the acidic dissociation constant or the number of acidic sites per membrane area (density of acidic sites) can be determined if a site dissociation model is used. On the other hand, chemical changes in the membrane surfaces due to contact with solutions at different pH were determined by comparing XPS spectra of the used samples with the original ones. In order to see the influence of each type of carrier on electrokinetic parameters, a comparison with the polymeric base membrane (B0) has also been made. Copyright © 2002 John Wiley & Sons, Ltd.

Preparation and Characterization of Novel Weak Amphoteric Charged Membrane Containing Cysteine Residues

Novel weak amphoteric charged membranes having cysteine residues were prepared. Cysteine (Cys) is one of the amino acids and has an amphoteric ion pair, which consists of an amino group and a carboxyl group. The poly(ethylene glycol) derivatives, with cysteine residues added to their side chains (PEG–Cys), were graft-polymerized onto a porous cellulose acetate (CA) membrane. The zeta potential was measured using a streaming potential method at various pH values ranging from 2.5 to 10 to investigate the behavior of the charge groups on the pore surface of these membranes. The zeta potential of this PEG–Cys-grafted membrane was pH-dependent, and the zeta potential/pH profile showed a plateau which was characteristic of an amphoteric ion pair in the range pH 4/9. The apparent surface charge density on these membranes was obtained from the zeta potential. The experimental results could be well explained by a calculated value using a site dissociation model including the contribution of not only the amphoteric ion pairs on the grafted polymer chains but also the negative charge groups originating from the base CA membrane.

Streaming potential along the surface of polysulfone membranes: a comparative study between two different experimental systems and determination of electrokinetic and adsorption parameters

Because of the diversity of streaming potential and zeta potential values reported in the literature, even for similar membranes and external conditions (hydrodynamics and solution chemistry), the influence of measuring cells and procedures in electrokinetic parameters has been studied. For this purpose, two different experimental systems (named EKA and NIS) with similar hydrodynamic parameters, but different electrodes and electrical devices, are used for the electrokinetic characterization of a commercial polysulfone membrane. Detailed descriptions of measurement procedures and the influence of external conditions (flow direction, electrode polarization, etc.) are made since they could contribute to the variability of experimental results. On the other hand, an electrokinetic characterization of the polysulfone membrane in contact with NaCl solutions at different concentrations and pH was carried out by using the EKA system. An heterogeneous specific adsorption of anions on the membrane surface (Freundlich isotherm) was assumed in order to determine parameters such as the membrane surface charge density, the Gibb’s free energy and the number of adsorption sites per unit of membrane area. However, at low salt concentration (∼10 −3 M) the membrane proper charge is mainly due to dissociation of some kind of acid groups on the membrane surface (isoelectric point: 3.1±0.1), and the site dissociation model has been used to obtain the membrane acid equilibrium constant and the surface density of acid sites.

Pore-surface characterization of poly(acrylonitrile) membrane having amphoteric charge groups by means of zeta potential measurement

Pore-surface characteristics of poly(acrylonitrile)-based porous membranes having amphoteric charge groups were investigated by zeta potential obtained from streaming potential measurements. The amphoteric-charged pore surface was theoretically examined by a single-acid, single-base site dissociation model, whose model is based on the assumption that the surface charge arises only from the protonation and deprotonation of the charged surface groups. From the theoretical approach, some characteristic parameters of the amphoteric pore surface such as the isoelectric point, dissociation constant, apparent site density of each acidic and basic group, and acid-to-base ratio were successfully determined. These parameters were changed in response to the variation of the feed monomer ratio. This approach can be expected to provide useful information concerning the pore-surface charge of an amphoteric-charged membrane.

Characterization of an Amphoteric-Charged Layer Grafted to the Pore Surface of a Porous Membrane

Porous membranes having amphoteric charge groups were prepared by heterogeneous graft polymerization from aqueous solution containing acrylic acid and (N,N-dimethylamino)propyl acrylamide. The charging properties of the amphoteric charge groups grafted to the pore surface were investigated by ζ potentials obtained from streaming potential measurements. Theoretical development in terms of the amphoteric pore surface was attempted by introducing a site dissociation model of pH-dependent ζ potentials. The theoretical model is based on the assumption that the surface charge arises only from the protonation and deprotonation of the charged surface groups exposed to electrolyte solution. Good fits between experimental results and theoretical equations were attained, and thereby the isoelectric point, dissociation constant, apparent surface site density, and acid-to-base ratio of the amphoteric pore surface were determined. Depending on the variation in the feed monomer ratio for graft polymerization, these para...

Surface Characterization of Poly(acrylonitrile) Membranes Graft-Polymerized with Ionic Monomers As Revealed by ζ Potential Measurement

Anionic and cationic monomers were graft-polymerized onto poly(acrylonitrile) (PAN) membrane surfaces. The graft polymerization was initiated by an oxidation−reduction system at 25 °C. Grafted PAN membrane surfaces yielded FT-IR/ATR spectra clearly different from that of an unmodified PAN membrane, because of the presence of the graft chains. ζ potentials of the pore surfaces of the grafted PAN membranes were obtained from streaming potential measurements. The surface properties are discussed with respect to a single site dissociation model, which is based on the protonation and deprotonation of charged surface groups. In this study, we obtained some characteristic parameters of the pore surfaces from the theoretical fits using the site dissociation model. This approach gives direct insight into the pore surface properties of the membrane. The results could be well explained by an ion-pairing effect in terms of different charge characteristics between membranes having hydrophobic or hydrophilic graft chai...

Poly(ethylene glycol) Grafted to Quartz: Analysis In Terms Of A Site-Dissociation Model of Electroosmotic Fluid Flow

Poly(ethylene glycol) coatings are known to control a variety of surface phenomena including nonspecific protein adsorption, wetting, and electrokinetic effects. These and other properties make such coatings useful for biocompatible materials and in improving the efficiency of analytical and preparative separations where analyte-surface interactions play a role. In the present study terminally activated poly(ethylene glycol) derivatives were grafted onto quartz functionally activated with (3-aminopropyl)triethoxysilane, (3-mercaptopropyl)trimethoxysilane, or poly(ethylenimine). Grafts and activated surfaces were electro-kinetically characterized by measuring the pH dependence of induced electroosmotic fluid flow. A site-dissociation model of electroosmotic fluid flow based on the Smoluchowski equation, the Gouy-Chapman theory of the electrical double layer, and exact solutions to the Poisson-Boltzmann equation is presented in order to relate electroosmosis to the chemistry of the surface. Results include indications of relative coating stability and estimates of grafting density as well as effective coating thickness.

The remarkable similarity between the acid-base properties of ISFETs and proteins and the consequences for the design of ISFET biosensors

Studying the acid-base properties of protein molecules led us to reconsider the operational mechanism of ISFETs. Based on the site-dissociation model, applied to the amphoteric metal oxide gate materials used in ISFETs, the sensitivity of ISFETs is described in terms of the intrinsic buffer capacity of the oxide surface, β s, and the electrical surface capacitance, C s. The ISFET sensitivity towards changes in the bulk pH is fully described by the ratio β s/C s. Practical measurements support this theoretical approach. The new approach to the description of the acid-base properties of ISFETs is analogous to the classical description of the acid-base properties of protein molecules. The acid-base titration of proteins is also determined by the ration between the intrinsic buffer capacity and the electrical double layer capacitance. In addition to the amazing conclusion that ISFET surfaces and protein molecules behave in a similar way with respect to their acid-base properties, further conclusions are drawn with respect to the possibility of protein characterization by means of dynamic measurements with protein covered ISFETs. Design rules are given for this type of biosensors, based on the theoretical understanding of the acid-base behaviour of both sensor parts.

Influence of Solvent on the Electrophoretic Mobility of Polystyrene Latex

Latices exhibit anomalous electrokinetic behavior. Instead of the expected decrease in mobility with increasing electrolyte concentration, a maximum occurs. Hairy layer models (HLM) and co-ion enrichment models (CEM) offer incomplete explanations for this. In order to understand the underlying mechanisms, electrophoresis and Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR) experiments were conducted using latex suspended in various solvents. Anomalous electrokinetic behavior, previously reported only for aqueous systems, was also observed in methanol. HLM describe electrokinetic behavior for electrolyte concentrations greater than 10-4M. Both solvation and electrical interactions influence hairy layer thickness. FTIR-ATR results suggest that methanol adsorbs onto latex, consistent with HLM. CEM describe electrokinetic behavior over the entire concentration range. FTIR-ATR results suggest that conditions suitable for hydrophobic adsorption exist. Adsorption maxima vary with the solvent in a manner consistent with the site dissociation model. Adsorption constants vary with the solvent, but the chemical components are similar, contrary to postulated hydrophobic adsorption mechanisms. Instead, the electrical component controls solvent effects. Although both models explain the results, the underlying mechanisms need clarification. HLM must consider solvation of the surface. Proposed hydrophobic adsorption mechanisms are incomplete. Further work is needed to understand the physical processes at latex-solvent interfaces.

Studies of the cell‐wall properties of Saccharomyces cerevisiae during fermentation

The cell-wall properties of three strains of the yeast Sacharomyces cerevisiae have been experimentally studied at various times during fermentation. The cell walls have been characterized by electrophoretic mobility measurements, from which zeta potentials may be calculated. They have also been characterized by computerized pH titration, which gives direct information on the number and nature of groups in the yeast cell wall. The data have been quantitatively analyzed in three ways. First, a simplified analysis of the electrokinetic data of a type used by previous workers has been applied. Second, such a simplified analysis of the electrokinetic data has been developed more rigorously by means of a two-dimensional site-dissociation model of the outer cell wall-solution interface. Third, a description of the yeast cell-wall electrochemical properties in terms of a three-dimensional gel model incorporating site dissociation has been developed. The advantages and disadvantages of the three analyses are discussed. Only the three-dimensional gel model can account simultaneously for both the electrokinetic and pH surface titration data. It provides new insights into the changes that occur to the yeast cell wall during fermentation.

Surface chemistry and suspension stability of oxide-nitride powder mixtures

The effect of bimetallic oxide sintering aids on the colloidal stability of homogeneous (surface-coated) and heterogeneous (mechanically mixed) silicon nitride powder mixtures in aqueous solution was studied by acoustophoretic analysis. While the surface charge generation and colloidal stability of single-phase and oxide-coated silicon nitride powder mixtures may be described according to the site dissociation model of amine and hydroxyl surface groups, the surface charge observed in heterogeneous multiphase powder mixtures is associated with the adsorption of soluble metal ion hydroxocomplexes on the nitride particles. Segregation of the mixtures by heteroflocculation, which causes the formation of agglomerates and microstructural defects upon sintering, may be avoided by generation of a threshold surface potential of equal sign on all powder constituents at a pH where the metal hydroxide dissolution is subcritical.

The temperature dependence of the surface potential at the Al 2O 3/electrolyte interface

Electrolyte/insulator/silicon structures can be used to measure the variations of surface potential at the insulator/electrolyte interface. We have used this method to measure the temperature dependence of the surface potential Ψ 0 at the γ-Al 2O 3/electrolyte interface. By applying the surface site dissociation model, this measurement can be interpreted in terms of the enthalpy changes of the ionization reactions of OH surface sites. As this theory predicts, it is found that ∂Ψ 0 ϖT varies linearly with pH. From the slope and intercept of this experimental line, the following enthalpies can be deduced: ΔH a1 = +34.8 ± 2.0 kJ/mole, for the dissociation of a proton from a SOH 2 + site, and ΔH 2 a = +54.0 ± 2.0 kJ/mole, for the dissociation of a proton from a SOH site. These results imply that proton dissociation on Al 2O 3 surfaces is endothermic, and are in reasonable agreement with those obtained by Griffiths and Furstenau with a colloidal suspension.

Adsorption of dodecylammonium ion on quartz in relation to its flotation

The flotation of −10μm diameter quartz particles using dodecylammonium acetate (DAA) as a collector was carried out at pH 5 and pH 9.8 by varying DAA concentration. The adsorption density of DAA and the zeta potential of quartz were also measured in order to analyze the flotation results. At pH 5, complete flotation of quartz was achieved at a DAA adsorption density of only about 1% surface monolayer coverage, while at pH 9.8 the complete flotation of quartz required the adsorption density of DAA to be more than equivalent monolayer adsorption. The isoelectric point of quartz in the presence of DAA at pH 5 was best described by adopting Δp K (=p K −−p K +) = 6, where K − and K + are the dissociation constants of anionic and cationic sites on quartz surfaces, respectively. Analysis of the results of zeta potential measurements at pH 9.8 on the basis of the site dissociation model suggests that the adsorbed species on quartz surfaces are dominated by undissociated free amine molecules at this pH. The mechanism of the free amine adsorption on fine quartz in alkaline media is discussed.

Sensitivity control of ISFETs by chemical surface modification

The response of ISFETs (ion-sensitive field-effect transistors) to concentrations of ions, especially H + ions, is determined by the type of gate surface. Both the number of active surface sites and (proton) association and dissociation constants influence the sensitivity. In the case of a chemically-modified gate surface, a new surface is formed, which generally has a different sensitivity. It is shown that the original pH response of the gate oxide can be either lowered or increased, depending on the reactivity of the added groups. In general, coverage with apolar groups and reduction of the number of sites result in a lower pH response, while addition of basic or acidic groups as well as an increase of active sites give a higher pH response. Using the extended site-dissociation model, which describes the behaviour of a surface composed of two types of sites, theoretical curves for surface potential versus pH are calculated. Measurements with chemically-treated siO 2 and Ta 2O 5 ISFETs confirm the theoretical expectations. The conclusion has been drawn that by a proper choice of chemical treatment, both the point of zero charge (pzc) and the pH-insensitive rage can be changed.

Operation of chemically sensitive field-effect sensors as a function of the insulator-electrolyte interface

All chemical sensors based upon the field-effect principle share a common quality. Their measurable properties can be described in terms of a flat-band voltage. The various terms in the expression of the flat-band voltage are described and discussed, and in particular the voltage drop at the insulator-electrolyte interface. It is shown that this voltage drop depends on the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pH</tex> of the electrolyte and is determined by two parameters, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pH</tex> at the point of zero charge, and a sensitivity parameter which is introduced in this paper. These parameters are obtained from the site-dissociation model of the insulator-electrolyte interface, combined with the Gouy-Chapmann-Stern theory of the electrical double layer at this interface. The theoretical description is used to interpret experimental results obtained from insulators with widely different properties, namely SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> and Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> .