Physicochemical study of the ‘anatase/electrolyte solution’ interface in suspension and closely packed particles of the solid

The electric double layer (EDL) developed at the interface of anatase in contact with aqueous electrolyte solutions was investigated at 25 °C. Potentiometric titrations (PT), measurements of the electrophoretic mobility (EM) in suspensions, and streaming potential (SP) measurements were taken. The surface charge over a wide pH range (ca. 3–10) and the point of zero charge (pzc = 6.3 ± 0.1) of anatase was easily determined by means of the suspension titration curve and the blank one, obtained at a single ionic strength value. Streaming potential measurements were conducted in anatase particles appropriately packed to form plugs. Two different plugs were prepared differing in the degree of particles' packing and, consequently, in the respective porosities. It was found that surface conductivity is lower at higher packing (lower porosity), because of the reduction of the total surface area in contact with the electrolyte. Moreover, it was found that the surface conductivity of the anatase samples increased at pH values away from pzc, while the mobility of the counter ions behind the shear plane decreased. This trend was attributed to the increase of the absolute surface charge. This increase caused an increase in the amount of the counter ions and, therefore, in the conductivity due to these ions. On the other hand, stronger electrostatic interactions between the surface of the solid and the counter ions reduced their mobility. The packing density of the anatase particles in the respective plugs, affected the values of ζ‐potential calculated from SP measurements when the effect of surface conductivity was neglected. Copyright © 2010 John Wiley & Sons, Ltd.     

Model Studies on the Interaction of Amino Acids with Biominerals: The Effect of L-Serine at the Hydroxyapatite-Water Interface

The effect of L-serine in supersaturated solutions of calcium phosphate was investigated under plethostatic conditions. The rates of crystal growth measured in the presence of L-serine at relatively high concentrations and in the range between 2x10(-3) and 1x10(-2) mol dm(-3) were appreciably reduced. The inhibitory effect of L-serine was found to be due to blocking of a portion of the active growth sites by adsorption. Kinetics measurements in the presence of L-serine as well as adsorption isotherm analysis suggested Langmuir-type adsorption of L-serine on the surface of hydroxyapatite (HAP) with a relatively low affinity for the substrate. Adsorption experiments showed that at pH 7.4 considerable adsorption of L-serine onto HAP takes place, whereas at pH 10.0 the adsorption was negligible, suggesting that electrostatic interactions are dominant. Attraction between the positively charged protonated amino group of the L-serine molecule and the negatively charged HAP surface contributed largely to the adsorption. This was corroborated by the fact that, in the presence of L-serine in the solution, a significant shift of zeta-potential of the HAP particles to less negative values was found at pH values close to 7.4. At pH values higher than 10.0 essentially no shift of zeta-potential takes place. On the basis of the experimental results, a model was proposed according to which L-serine absorbs on the surface of HAP through electrostatic attractions exerted between one negative site of the HAP surface, i.e., phosphate or hydroxyl ion, and the positively charged protonated amino group of one L-serine molecule, forming a surface ion pair. Copyright 2001 Academic Press.     

Estimation and comparison of zeta-potentials of silica-based anion-exchange type porous particles for capillary electrochromatography from electrophoretic and electroosmotic mobility

Mobilities of different chromatographic particles obtained from two electrokinetic methods were determined and compared. The particles were all based on porous silica, between 3 and 15 microm diameter, and were either native, or derivatized. As intermediate of chemical modification 3-mercaptopropyl-modified silica particles (TP-silica) are obtained. These particles were finally transformed into weakly basic anion exchangers with O-9-(tert-butylcarbamoyl)quinine (tBuCQN) as chiral selector. The electrophoretic mobility of the particles was determined from their migration velocity in an electric field using microelectrophoresis. Electrokinetic chromatography with a capillary column packed with the same particles was used to measure the electroosmotic flow generated. All measurements were carried out in background electrolytes of equal ionic strength (10(-2) mol/L), at pH varying between 3.5 and 9.5. From these data a rough estimation of the zeta-potential was made, taking Helmholtz-Smoluchowski conditions into consideration. With both methods the zeta-potential of the native silica particles is negative throughout, and its value increases with pH. The weakly basic tBuCQN particles have positive zeta-potentials at pH lower than about 7.5, but exhibit a negative zeta-potential above this pH, indicating the dominating effect of residual silanol groups at the silica surface. The zeta-potential for these anion-exchange particles ranged between +30 and -40 mV. The zeta-potentials derived with electrophoresis and electroosmosis agree, showing the adequacy of the approach, although many limitations must be taken into account in the treatment of the electrokinetic phenomena in such porous systems. These restrictions in interpreting mobility and zeta-potential were discussed.     

ESR study of aqueous micellar solutions of perfluoropolyether surfactants with the use of fluorinated spin probes

The acid-base properties of synthetically prepared and well-characterized hydroxyapatite (HAP) in contact with KNO3 solutions were investigated at 25 degrees C, through potentiometric titrations, zeta-potential measurements, and surface complex modeling. Aliquots of suspension were withdrawn every 0.5 pH unit during the titration procedure and analyzed for calcium and phosphate. It was found that, even for rapid titration experiments, a remarkable amount of H+ ions (H+dissol.) is consumed in the bulk solution in reacting with species coming from the dissolution of HAP. These H+ ions must be taken into account in the H+ mass balance, in order for true value for the point of zero charge (pzc=6.5+/-0.2) and consequently true value for the surface charge (sigma0) to be obtained. Besides the conventional potentiometric titration technique, it was found that pzc may be determined much more easily as the intersection point of the suspension titration curve and the blank one modified to include the amount of H+dissol. obtained at one ionic strength. Finally, a surface complexation model was proposed for the development of surface charge. Experimental data were satisfactorily fitted by using the value of 4.2 F m-2 for the capacitance.     

Removal of Co(II) from aqueous solution by using hydroxyapatite

Herein, hydroxyapatite (HAP) was prepared by aqueous precipitation technique and was characterized by using FT-IR and XRD to determine its chemical functional groups and micro-structure. The removal of cobalt from aqueous solution to HAP was studied by batch technique as a function of various environmental parameters such as contact time, pH, ionic strength, foreign ions, fulvic acid (FA), and temperature under ambient conditions. The results indicated that the sorption of Co(II) on HAP was strongly dependent on pH and ionic strength. The presence of FA enhanced the sorption of Co(II) on HAP at low pH, whereas reduced Co(II) sorption on HAP at high pH. The Langmuir, Freundlich and D-R models were used to simulate the sorption isotherms at three different temperatures of 303.15, 323.15 and 343.15 K. The thermodynamic parameters (ΔH°, ΔS° and ΔG°) calculated from the temperature dependent sorption isotherms indicated that the sorption process of Co(II) on HAP was spontaneous and endothermic. The sorption of Co(II) was dominated by outer-sphere surface complexation and ion exchange at low pH, whereas inner-sphere surface complexation or surface precipitation was the main sorption mechanism at high pH values. The results suggest that the HAP is a suitable material in the preconcentration and solidification of Co(II) from large volumes of aqueous solutions.     

Electrokinetic characterization of porous plugs from streaming potential coupled with electrical resistance measurements

The zeta potential of mixed nickel-iron oxide particles is evaluated by a new laboratory instrument. This latter allows the measurement of streaming potential together with the electrical resistance of porous plugs. The conductivity of electrolyte inside plug (pore conductivity) is deduced from electrical resistance measurements and is used together with streaming potential to evaluate the zeta potential by accounting for the surface conduction phenomenon. It is shown that neglecting the surface conduction phenomenon leads to a substantial underestimation of the zeta potential. The coupled measurements of streaming potential and plug electrical resistance yield zeta potential values that are in very good agreement with those obtained by electrophoresis. The densification of the porous plug with increasing pressure increments is put in evidence by the decrease in measured streaming potentials. Electrical resistance measurements make it possible to account for the increase in surface conductivity resulting from the more compacted structure of the plug. By doing so, the calculated zeta potential is found to be virtually independent of the pressure difference involved in streaming potential experiments, whereas the negligence of surface conduction phenomenon leads to a decrease in the apparent zeta potential with increasing pressure level.     

羟基磷灰石对牛血清白蛋白的吸附特性研究

研究了羟基磷灰石表面电位随溶液PO₄^3-浓度、Ca²⁺浓度、离子强度和pH的变化规律;测定了不同操作条件下牛血清白蛋白在羟基磷灰石上的吸附容量;吸附等温线的测定结果表明该吸附属于Langmuir型;通过对该吸附过程的动力学研究,计算得到的表观活化能和吸附热数值较低,表明该吸附是物理吸附.     

The zeta-potential of the endogenous particles of a wine of Champagne in relation to the foaming behaviour

It was shown that the endogenous particles of the champagnes influence the lifetime, and not the maximum expansion of their evanescent foam (Food Hydrocolloids (1999) 12, 217-226). Actually, champagnes are electrolytic solutions with pH 3 and ionic strength equal to 0.02 mol/l in which bentonites, diatomites, and yeast cells are the more numerous colloids and particles present. In this context, we have investigated the electrophoretic properties of these particles to determine whether they can electrostatically interact with the foam bubbles. Results are that in model alcoholic solutions of proteins at same pH and ionic strength as the champagne, the zeta-potential was not vanishing whereas it dropped down to zero in wines. The zeta-potential of the particles does not vanish either when they are suspended in nanofiltered wines on molecular weight cut-off membrane (porosity=200-300 Da) or when the wines are basified upon addition of sodium hydroxide. This particular behaviour was tentatively assigned to the adsorption of some endogenous organic cationic ions on the particle surfaces, which screened out their electrostatic charge. The possible candidates are discussed.     

Effects of injector geometry and sample matrix on injection and sample loading in integrated capillary electrophoresis devices

The double-T injector design employed in many microchip capillary electrophoresis devices allows for the formation of very small (50-500 pL) sample plugs for subsequent analysis on-chip. In this study, we show that sample plugs formed at the channel junction can be geometrically defined. The channel width and injector symmetry prove to be of great importance to good performance. A unique pushback of solvent into the side channels can be induced when the side channels have a very low resistance to flow, and this helps to better define the injected sample plug. Samples and running buffers of differing ionic strength (e.g., 10 mM KCl buffer and 20 mM KCl sample) can yield widely variable results in terms of plug shape and amount injected (variations of 1.5 to 10x). Applying bias voltages to all the intersecting channels aids in controlling the plug shape. However, when the ionic strengths of buffer and sample are not matched, the actual amount injected (up to 10x variations) can be inconsistent with the appearance of the plug formed in the injector (up to only 30 % variations). Operating at constant pH and ionic strength produced the most consistent results. This report examines the effects of altering the injector geometry and solution ionic strengths, and presents the results of using bias voltages to control plug formation. The observed results should provide a benchmark for modeling of the fluid dynamics in channel intersections.     

Surface electrochemical properties of mixed oxide ceramic membranes: Zeta-potential and surface charge density

The surface electrochemical properties of alumina based ceramic microfiltration membranes were studied by measuring electroosmotic rates and surface charge densities obtained from potentiometric titrations. The rate of electroosmosis, which determines the zeta-potential, was measured on the membrane itself, whereas the surface charge was titrated on a suspension obtained by crushing of the membrane. The zeta-potential was measured in the presence of salts including NaCl, CaCl₂ and Na₂SO₄, for a wide range of pH values (4–9) at ionic strengths of 0.01 and 0.001 M. The pH value of the isoelectric point (iep) show a specific adsorption of SO₄²⁻ and Ca²⁺ ions onto the membrane surface. The iep in NaCl solutions occurs at pH 4.7 ± 0.1. The low iep is due to the large amount of silicium oxide in the membrane. The surface charge density is relatively high with respect to the low values of zeta-potentials. The point of zero charge pH(pzc) determined from surface charge and pH profiles occurs at pH 8.2 ± 0.1 in NaCl solution. The pH(pzc) value was also determined by two ‘addition’ methods. Similar pH(pzc) values were obtained. The difference between the pH(pzc) and pH(iep) may be correlated to a loss of acidity that is due to using crushed-membrane powder to perform potentiometric measurements.     

Temperature effect on the stability of bentonite colloids in water

The stability of natural bentonite suspensions has been investigated as a function of temperature at pH 9 and ionic strength 10(-3) M. The sedimentation rate of the particles is directly related to their stability. The sedimentation kinetics was determined by examining the variation of particle concentration in solution with time. The observed kinetics for sedimentation is discussed quantitatively in terms of the potential energy between particles. The zeta-potential of the particles was measured and the DLVO theory was used to calculate attractive and repulsive potentials. Experimental observations are consistent with DLVO model predictions and show that the stability of bentonite colloids increases with temperature. Differences with other colloidal systems can be attributed to the temperature dependence of the surface charge of bentonite particles.     

Balance of hydrophobic and electrostatic forces in the pH response of weak polyelectrolyte capsules

A detailed study of the role of solution pH and ionic strength on the swelling behavior of capsules composed of the weak polyelectrolytes poly(4-vinylpyridine) (P4VP) and poly(methacrylic acid) (PMA) with different numbers of layers was carried out. The polyelectrolyte layers were assembled onto silicon oxide particles and multilayer formation was followed by zeta-potential measurements. Hollow capsules were investigated by scanning electron microscopy and atomic force microscopy. The pH-dependent behavior of P4VP/PMA capsules was probed in aqueous media using confocal laser scanning microscopy. All systems exhibited a pronounced swelling at the edges of stability, at pHs of 2 and 8.1. The swelling degree increased when more polymer material was adsorbed. The swollen state can be attributed to uncompensated positive and negative charges within the multilayers, and it is stabilized by counteracting hydrophobic interactions. The swelling was related to the electrostatic interactions by infrared spectroscopy and zeta-potential measurements. The stability of the capsules as well as the swelling degree at a given pH could be tuned, when the ionic strength of the medium was altered.     

Adsorption of human serum albumin (HSA) onto colloidal TiO2 particles, Part I

The adsorption of human serum albumin (HSA) onto colloidal TiO2 (P25 Degussa) particles was studied in NaCl electrolyte at different solution pH and ionic strength. The HSA-TiO2 interactions were studied using adsorption isotherms and the electrokinetic properties of HSA-covered TiO2 particles were monitored by electrophoretic mobility measurements. The adsorption behavior shows a remarkable dependence of the maximum coverage degree on pH and was almost independent of the ionic strength. Other characteristic features such as maximum adsorption values at the protein isoelectric point (IEP approximately 4.7) and low-affinity isotherms that showed surface saturation even under unfavorable electrostatic conditions (at pH values far away from the HSA IEP and TiO2 PZC) were observed. Structural and electrostatic effects can explain the diminution of HSA adsorption under these conditions, assuming that protein molecules behave as soft particles. Adsorption reactions are discussed, taking into account acid-base functional groups of the protein and the surface oxide in different pH ranges, considering various types of interactions.     

Deposition of Oil Drops on a Glass Substrate in Relation to the Process of Washing

The attachment of emulsion drops to glass substrates is investigated in relation to the redeposition of oil drops in the process of washing. It turns out that the drops of a surfactant-stabilized oil-in-water emulsion cannot be attached to an immersed glass plate simply by the buoyancy force. However, the same drops can be deposited on the plate when the latter is pulled out of the emulsion, i.e., when the drops are pressed against the substrate by a receding meniscus. We measured the amount of the oily deposit as a function of the pH, ionic strength, and composition of an amphoteric-anionic surfactant mixture. The enhanced oil deposition at low pH correlates with the domain in which the emulsion drops and the solid substrate bear opposite electric charges. This was established by zeta-potential measurements with oil drops and glass particles. The anionic surfactant brings negative surface charge to the oil droplets and suppresses the oil deposition on the negatively charged glass. With the increase of the fraction of the amphoteric surfactant in the mixture, the zeta-potential is converted from negative to positive, and the oil deposition grows almost linearly with the potential. In general, the deposition of oil drops by a receding meniscus is governed by an interplay of electrostatic and hydrodynamic factors. Copyright 2000 Academic Press.     

Adsorption of cationic lipid bilayer onto flat silicon wafers: effect of ion nature and concentration

The effect of monovalent salt nature and concentration over a range of low ionic strengths (0-10 mM LiCl, NaCl, KCl, or CsCl) and at two different pH values (6.3 and 10.0) on adsorption of dioctadecyldimethylammonium bromide (DODAB) bilayer fragments (BF) onto flat SiO(2) surfaces was systematically evaluated by means of in situ ellipsometry. High-affinity adsorption isotherms fitted by the Langmuir model indicated that adsorption maxima were consistent with bilayer deposition only around 10 mM monovalent salt at both pH values. In pure water, the mean thickness of the DODAB adsorbed layer was close to zero with bilayer deposition taking place only around 10 mM ionic strength. In the presence of 10 mM CsCl or LiCl, the highest and the lowest affinity constants for DODAB adsorption onto SiO(2) were, respectively, obtained consistently with the expected facility of cation exchange at the surface required for DODAB adsorption. The cation more tightly bound to the solid surface should be Li(+), which would present the largest resistance to displacement by the DODAB cation, whereas the less tightly bound cation should be Cs(+) due to its largest ionic radius and lowest charge density. In other words, DODAB adsorption proceeds in accordance with charge density on the solid surface, which depends on the nature and concentration of bound counterions as well as DODAB cation ability to displace them. AFM images show a very smooth DODAB film adsorbed onto the surface in situ with a large frequency of BF auto-association from their edges. The present results for flat surfaces entirely agree with previous data from our group for DODAB adsorption onto silica particles.     

Sorption behavior of hydroxyapatite for 109Cd(II) as a function of environmental conditions

In this work, hydroxyapatite (HAP) was prepared by aqueous precipitation technique and was characterized by using FT-IR to determine its chemical functional groups. A series of batch experiments were carried out to investigate the effect of various environmental factors such as contact time, pH, ionic strength, foreign ions, fulvic acid (FA) and temperature on the sorption behavior of HAP towards radionuclide ¹⁰⁹Cd(II). The results indicated that the sorption of ¹⁰⁹Cd(II) on HAP was strongly dependent on pH and ionic strength. A positive effect of FA on ¹⁰⁹Cd(II) sorption was found at pH <7.0, whereas a negative effect was observed at pH >7.0. The Langmuir, Freundlich and D-R models were used to simulate the sorption isotherms at three different temperatures of 298.15, 323.15 and 343.15 K. The thermodynamic parameters (ΔH⁰, ΔS⁰ and ΔG⁰) calculated from the temperature dependent sorption isotherms indicated that the sorption process of ¹⁰⁹Cd(II) on HAP was spontaneous and endothermic. At low pH, the sorption of ¹⁰⁹Cd(II) was dominated by outer-sphere surface complexation and ion exchange on HAP surfaces, whereas inner-sphere surface complexation was the main sorption mechanism at high pH. From the experimental results, it is possible to conclude that HAP has good potentialities for cost-effective treatments of ¹⁰⁹Cd(II)-contaminated wastewaters.     

Multiscale dynamics of the cell envelope of Shewanella putrefaciens as a response to pH change

The bacterial surface properties of gram-negative Shewanella putrefaciens were characterized by microbial adhesion to hydrocarbons (MATH), adhesion to polystyrene dishes, and electrophoresis at different values of pH and ionic strength. The bacterial adhesion to these two apolar substrates shows significant variations according to pH and ionic strength. Such behavior could be partly explained by electrostatic repulsions between bacteria and the solid or liquid interface. However, a similar trend was also observed at rather high ionic strength where electrostatic interactions are supposed to be screened. The nanomechanical properties at pH 4 and 10 and at high ionic strength were investigated by using atomic force microscopy (AFM). The indentation curves revealed the presence of a polymeric external layer that swells and softens up with increasing pH. This suggests a concomitant increase of the water permeability and so did of the hydrophilicity of the bacterial surface. Such evolution of the bacterial envelope in response to changes in pH brings new insight to the pH dependence in the bacterial adhesion tests. It especially demonstrates the necessity to consider the hydrophobic/hydrophilic surface properties of bacteria as not univocal for the various experimental conditions investigated.     

Multiscale dynamics of the cell envelope of Shewanella putrefaciens as a response to pH change

The bacterial surface properties of gram-negative Shewanella putrefaciens were characterized by microbial adhesion to hydrocarbons (MATH), adhesion to polystyrene dishes, and electrophoresis at different values of pH and ionic strength. The bacterial adhesion to these two apolar substrates shows significant variations according to pH and ionic strength. Such behavior could be partly explained by electrostatic repulsions between bacteria and the solid or liquid interface. However, a similar trend was also observed at rather high ionic strength where electrostatic interactions are supposed to be screened. The nanomechanical properties at pH 4 and 10 and at high ionic strength were investigated by using atomic force microscopy (AFM). The indentation curves revealed the presence of a polymeric external layer that swells and softens up with increasing pH. This suggests a concomitant increase of the water permeability and so did of the hydrophilicity of the bacterial surface. Such evolution of the bacterial envelope in response to changes in pH brings new insight to the pH dependence in the bacterial adhesion tests. It especially demonstrates the necessity to consider the hydrophobic/hydrophilic surface properties of bacteria as not univocal for the various experimental conditions investigated.     

The ion sensitivity of surface conductive single crystalline diamond

Charge build-up at the solid/aqueous interface is a ubiquitous phenomenon that determines the properties of interfacial electrical double layers. Due to its unique properties, the surface of diamond offers an attractive platform to investigate charging mechanisms in aqueous solutions. We investigate the surface charge by studying the ion sensitivity of H-terminated single crystalline diamond surface conductive layers. The effect of monovalent and divalent salts has been probed at different pH values. For a pH above 3.5, increasing the ionic strength results in a decrease of the surface conductivity, in contrast to the results obtained for pH below 3.5. Electrokinetic experiments are in good agreement with the surface conductivity measurements, showing an isoelectric point at pH 3.5 for the H-terminated diamond surface. We discuss the results in terms of the Coulombic screening by electrolyte ions of the surface potential, which is induced by a pH-dependent surface charge. The origin of this surface charge is discussed in terms of charge regulation by amphoteric hydroxyl surface groups and unsymmetrical adsorption of hydroxide and hydronium ions induced by the hydrophobic nature of the H-terminated diamond surface. This surface charge can have important consequences for processes governed by the diamond/aqueous interface, such as electron transfer to charged redox molecules, adsorption of charged molecules and proteins, and ion sensitivity.