Effect of polyelectrolytes and polyelectrolyte mixtures on the electrokinetic potential of dispersed particles. 2. Electrokinetic potential of silica particles in electrolyte, polyelectrolyte and polyelectrolyte mixtures solutions

The effect pH, ionic strength (KCl concentration), weakly and medium charged anionic and cationic polyelectrolytes (PEs) as well as their binary mixtures on the electrokinetic potential of silica particles as a function of the polyelectrolyte/mixture dose, its composition, charge density (CD) of the PE, and way of adding the polymers to the suspension has been studied. It has been shown that addition of increasing amount of anionic PEs increases the absolute value of the negative zeta-potential of particles at pH > pH isoelectric point (IEP = 2.5); this increase is stronger the charge density of the polyelectrolyte is higher. Adsorption of cationic polyelectrolytes at these pH values gives a significant decrease in the negative ζ-potential and overcharging the particles; changes in the ζ-potential are more pronounced for PE samples with higher CD. In mixtures of cationic and anionic PE at pH > pHIEP, the ζ-potential of particles is determined by the adsorbed amount of the anionic polymer independently of the CD of PEs, the mixture composition and the sequence of addition of the mixture components. Unexpectedly, the ζ-potential of silica at pH = 2.1, i.e. < pHIEP, turned out to be positive in the presence of both anionic PE and cationic + anionic PE mixtures. This is explained by formation (and adsorption onto positively charged silica surface) of pseudo-cationic PEs from anionic ones due to transfer of protons from the solution to the amino-group of the anionic polymer. Considerations about the role of coulombic and non-coulombic forces in the mechanism of PE adsorption are presented.     

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.     

Dissimilar pH-dependent adsorption features of bovine serum albumin and α-chymotrypsin on mica probed by AFM

We studied bovine serum albumin (BSA) and α-chymotrypsin adsorption onto mica surfaces over a large pH range by atomic force microscopy (AFM) measurements in liquid. Data analyses (height, roughness and roughness factor) brought new insights on the conformation of proteins in soil environments, with mica as a model of soil phyllosilicates and non-hydrophobic surfaces. Validation of AFM approach was performed on BSA, whose behavior was previously described by nuclear magnetic resonance and infra-red spectroscopic methods. Maximum adsorption was observed near the isoelectric point (IEP). A stronger interaction and a lower amount of adsorbed proteins were observed below the IEP, which contrasted with the progressive decrease of adsorption above the IEP. We then studied the adsorption of α-chymotrypsin, a proteolytic enzyme commonly found in soils. AFM pictures demonstrated a complete coverage of the mica surface at the IEP in contrast to the BSA case. Comparison of the AFM data with other indirect methods broadened the understanding of α-chymotrypsin adsorption process through the direct display of the protein adsorption patterns as a function of pH.     

Interfacial orientation and secondary structure change in tachyplesin I: molecular dynamics and sum frequency generation spectroscopy studies

Recent advances in the collection and interpretation of surface-sensitive vibrational spectroscopic measurements have made it possible to study the orientation of peptides and proteins in situ in a biologically relevant environment. However, interpretation of sum frequency generation (SFG) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) vibrational spectroscopy is hindered by the fact that orientation cannot be inferred without some prior knowledge of the protein structure. In this work, molecular dynamics simulations were used to study the interfacial orientation and structural deformation of the short β-sheet peptide tachyplesin I at the polystyrene/water interface. By combining these results with ATR-FTIR and SFG measurements, reasonable agreement was found with the simulation results, suggesting that tachyplesin I lies parallel to the surface, although the simulation results imply a broader distribution of peptide twist angles than could be characterized using available experimental measurements. The interfacial structure was found to be deformable even when disulfide bonds were preserved, and these local deviations from a purely extended β-sheet conformation may be of importance to future developments in the interpretation of SFG and ATR-FTIR spectra.     

Multiinstrument study of the electrophoretic mobility of quartz

The electrophoretic mobility of quartz in 0.01 mol dm(-3) NaCl and NaNO3 over the pH range 2-8 was studied using five different commercial instruments. The mobilities over the pH range 4-8 were relatively consistent, but the mobilities over the pH range 2-4 and the position of the isoelectric point IEP varied from one instrument to another. This result suggests that the discrepancies in the apparent IEP of quartz (and other silicas) reported in the literature are due to the instrument artifacts.     

Sum frequency generation from Langmuir-Blodgett multilayer films on metal and dielectric substrates

Sum frequency generation (SFG) vibrational spectra of cadmium arachidate multilayer films adsorbed on a substrate with high nonresonant susceptibility, i.e., gold, and on a low nonresonant susceptibility substrate, i.e., fused quartz, have been investigated in the C-H stretching region in air. The films were formed by Langmuir-Blodgett (LB) deposition and their spectra recorded using SFG spectrometers employing both 532-nm nanosecond and 800-nm femtosecond lasers, with counter-propagating and co-propagating beam geometries, respectively. Both kinds of substrate were rendered hydrophobic by coating them with per-deuterated octadecanethiol (gold) or per-deuterated cadmium arachidate (fused quartz) monolayers. Single per-protonated arachidate layers in otherwise per-deuterated 10-layer films were used to show that the SFG resonances arise only from the topmost and lowermost layers in a LB film comprised of an even number of per-protonated layers, although the SFG spectra from the two hydrophobic substrates are different from each other. The differences in the spectra from the same ten-layer per-protonated films deposited on the two types of hydrophobic substrate have been explained in terms of a simple model that accounts for resonant and nonresonant contributions.     

Piperidine adsorption on hydrated alpha-alumina (0001) surface studied by vibrational sum frequency generation spectroscopy

The adsorption of piperidine vapor on the hydrated alumina (alpha-Al2O3, corundum) (0001) surface was investigated using vibrational broad bandwidth and scanning sum frequency generation (SFG) spectroscopy. The interfacial vibrational signature in the C-H stretching region of piperidine at the alumina (0001) surface is shown to be a sensitive spectroscopic probe revealing the adsorption mechanism. The neat piperidine surface, aqueous piperidine surface, and aqueous piperidium chloride surface were also investigated in the C-H stretching region by SFG to establish vibrational reference frequencies. After piperidine adsorption, piperidine vapor was removed and piperidine was found to be chemisorbed onto the alumina (0001) surface through protonation by surface hydroxyl groups. The O-H stretching region of the alumina surface before and after piperidine adsorption was also investigated, and the results revealed the decrease of the surface number density of alumina surface hydroxyl groups.     

Measurements and theoretical interpretation of points of zero charge/potential of BSA protein

The points of zero charge/potential of proteins depend not only on pH but also on how they are measured. They depend also on background salt solution type and concentration. The protein isoelectric point (IEP) is determined by electrokinetical measurements, whereas the isoionic point (IIP) is determined by potentiometric titrations. Here we use potentiometric titration and zeta potential (ζ) measurements at different NaCl concentrations to study systematically the effect of ionic strength on the IEP and IIP of bovine serum albumin (BSA) aqueous solutions. It is found that high ionic strengths produce a shift of both points toward lower (IEP) and higher (IIP) pH values. This result was already reported more than 60 years ago. At that time, the only available theory was the purely electrostatic Debye-Hu?ckel theory. It was not able to predict the opposite trends of IIP and IEP with ionic strength increase. Here, we extend that theory to admit both electrostatic and nonelectrostatic (NES) dispersion interactions. The use of a modified Poisson-Boltzmann equation for a simple model system (a charge regulated spherical colloidal particle in NaCl salt solutions), that includes these ion specific interactions, allows us to explain the opposite trends observed for isoelectric point (zero zeta potential) and isoionic point (zero protein charge) of BSA. At higher concentrations, an excess of the anion (with stronger NES interactions than the cation) is adsorbed at the surface due to an attractive ionic NES potential. This makes the potential relatively more negative. Consequently, the IEP is pushed toward lower pH. But the charge regulation condition means that the surface charge becomes relatively more positive as the surface potential becomes more negative. Consequently, the IIP (measuring charge) shifts toward higher pH as concentration increases, in the opposite direction from the IEP (measuring potential).     

Characterization of oxide layers on Ti6Al4V and titanium by streaming potential and streaming current measurements

Titanium and titanium alloys (e.g. Ti6Al4V) are increasingly used as medical implant materials in a wide variety of applications. So far, many surface properties of the passive layer considered to explain interactions with biological tissues are deduced from those of the crystalline phases of titanium dioxide (anatase, rutile, brookite), but do not necessarily correspond to those of naturally formed amorphous passive layers. We report on streaming potential and streaming current measurements on oxide layers on Ti6Al4V and Ti, carried out using a microslit electrokinetic set-up (MES) and a commercial electrokinetic analyzer (EKA, PAAR). Passive and anodic oxide layers on Ti6Al4V, as well as passive layers on titanium sputtered on glass, were investigated in this study. Isoelectric points (IEP) of ≈4.4 were found for all oxide layers. The IEP of the air-formed passive layer on Ti6Al4V did not depend on the KCl concentration. Hence, it was concluded that IEP is here identical to the point of zero charge (pzc). Controversially, the charge formation process seems to depend on the chloride ion concentration in the neutral and basic pH region.     

Adsorption of oleic acid at sillimanite/water interface

The interaction of oleic acid at sillimanite-water interface was studied by adsorption, FT-IR, and zeta potential measurements. The isoelectric point (IEP) of sillimanite obtained at pH 8.0 was found to shift in the presence of oleic acid. This shift in IEP was attributed to chemisorption of oleic acid on sillimanite. Adsorption experiments were conducted at pH 8.0, where the sillimanite surface is neutral. The adsorption isotherm exhibited a plateau around 5 micromol/m2 that correspond to a monolayer formation. Adsorption of oleic acid on sillimanite, alumina, and aluminum hydroxide was studied by FT-IR. Chemisorption of oleic acid on the above substrates was confirmed by FT-IR studies. Hydroxylation of mineral surface was found to be essential for the adsorption of oleic acid molecules. These surface hydroxyl sites were observed to facilitate deprotonation of oleic acid and its subsequent adsorption. Thus protons from oleic acid react with surface hydroxyl groups and form water molecules. Based on the experimental results, the mechanism of oleic acid adsorption on mineral substrate was proposed. Free energy of adsorption was estimated using the Stern-Graham equation for a sillimanite-oleate system.     

Study of interaction of proteins with fumed silica in aqueous suspensions by adsorption and photon correlation spectroscopy methods

The interaction of fumed silica A-300 (S(BET) = 297 m2 g(-1)) with bovine serum albumin (prepared by different methods), ovalbumin, human hemoglobin, and gelatin as a function of pH, salinity, and concentrations of components in aqueous medium was studied by adsorption and photon correlation spectroscopy (PCS) methods. Comparison of equilibrium (incubation time t(i) approximately 1 h) adsorption of proteins on A-300, minute (t(i) approximately 1 min) flocculation rate, and the particle size distributions measured by the PCS method shows different rearrangement of particle swarms depending on pH, salinity, and concentration of proteins, especially at pH close to IEP of silica or proteins. The electrokinetic mobility of protein/silica swarms is greater than that of individual components at pH far from the IEP of proteins. Changes in the Gibbs free energy (DeltaG) on protein adsorption depend on pH (-DeltaG is minimal at pH 2, close to the IEP of silica, and maximal at pH between the IEP of protein and silica), concentration (-DeltaG is maximal at C(p) between 1 and 6 mg/ml), type of proteins, and their preparation technique.     

Influence of the structure of boundary layers and the nature of counterions on the position of the isoelectric point of silica surfaces

Dependences of electrokinetic potentials of different silica materials (nano-and ultraporous glasses, a quartz glass plane-parallel capillary, and monodisperse spherical particles of silicon oxide) on the pH of solutions containing single-, double-, and triple-charged cations have been compared. It has been shown that the degree of hydration of a single-charged cation and the structure of an interface substantially affect the position of the isoelectric point (IEP). The most hydrated Na⁺ ions have virtually no effect on the position of the IEP up to their concentration of 0.1 M irrespective of the thickness of an ion-permeable layer at the surface of a solid phase. A reduction in the radius of a hydrated cation (K⁺, Cs⁺) enables its penetration into an ion-permeable layer and, as a consequence, causes the IEP to shift toward larger pH values depending on the parameters of this layer. Two IEPs are observed in LaCl₃ solutions: one at a pH value close to pH_IEP in NaCl solutions and another at a higher pH value corresponding to the charge reversal of the Stern layer.     

Doubly pH-responsive pickering emulsion

A pH-responsive Pickering emulsion has been designed on the basis of commercially available alumina-coated silica nanoparticles (Ludox CL silica particles) and potassium hydrogen phthalate (KHP). KHP was found to bind to cationic particle surfaces at pH values between 3.5 and 5.5, enabling the resulting surface-active particles to stabilize emulsions of xylenes in water. Above and below this pH range, the system demulsifies, resulting in a reversible Pickering emulsifier having two pH-controlled, reversible transitions.     

Effect of pH on monolayer properties of colloidal silica particles at the air/water interface

Monodisperse colloidal silica particles were prepared by the St?ber method and hydrophobized by grafting a silane coupling agent, octadecyltrimethoxysilane. Two different types of silica particles, i.e., hydrophilic and hydrophobic silica particles were spread at the air/water interface to form the Langmuir monolayers. Monolayer properties of those particles were investigated by measuring surface pressure–area (π–A) isotherms at different subphase pH. At pH above the isoelectric point (IEP) of silica, as pH increased the π–A isotherms for the hydrophobic particles slightly shifted to larger surface area whereas those for the hydrophilic particles showed a reverse trend. At pH below the IEP, the π–A isotherms for both types of particles shifted to much larger surface area with different shapes. In order to analyze the π–A isotherm results further, the time dependence of π was examined. When pH is above the IEP, the π for the hydrophilic particles significantly decreased with increasing time and it did more at higher pH. On the other hand, the decrease in π for the hydrophobic particles was insignificant regardless of pH. For both types of silica particles, the decrease in π was minimal at pH below the IEP. These results were discussed in terms of particle desorption into the water subphase and interparticle electrostatic repulsion which is directly influenced by zeta potential.     

Synthesis and characterization of goethite and goethite-hematite composite: experimental study and literature survey

Aging of synthetic goethite at 140 degrees C overnight leads to a composite material in which hematite is detectable by M?ssbauer spectroscopy, but X-ray diffraction does not reveal any hematite peaks. The pristine point of zero charge (PZC) of synthetic goethite was found at pH 9.4 as the common intersection point of potentiometric titration curves at different ionic strengths and the isoelectric point (IEP). For the goethite-hematite composite, the common intersection point (pH 9.4), and the IEP (pH 8.8) do not match. The electrokinetic potential of goethite at ionic strengths up to 1 mol dm(-3) was determined. Unlike metal oxides, for which the electrokinetic potential is reversed to positive over the entire pH range at sufficiently high ionic strength, the IEP of goethite is rather insensitive to the ionic strength. A literature survey of published PZC/IEP values of iron oxides and hydroxides indicated that the average PZC/IEP does not depend on the degree of hydration (oxide or hydroxide). Our material showed a higher PZC and IEP than most published results. The present results confirm the allegation that electroacoustic measurements produce a higher IEP than the average IEP obtained by means of classical electrokinetic methods.     

In situ adsorption studies of a 14-amino acid leucine-lysine peptide onto hydrophobic polystyrene and hydrophilic silica surfaces using quartz crystal microbalance, atomic force microscopy, and sum frequency generation vibrational spectroscopy

The adsorption of a 14-amino acid amphiphilic peptide, LK14, which is composed of leucine (L, nonpolar) and lysine (K, charged), on hydrophobic polystyrene (PS) and hydrophilic silica (SiO2) was investigated in situ by quartz crystal microbalance (QCM), atomic force microscopy (AFM), and sum frequency generation (SFG) vibrational spectroscopy. The LK14 peptide, adsorbed from a pH 7.4 phosphate-buffered saline (PBS) solution, displayed very different coverage, surface roughness and friction, topography, and surface-induced orientation when adsorbed onto PS versus SiO2 surfaces. Real-time QCM adsorption data revealed that the peptide adsorbed onto hydrophobic PS through a fast (t < 2 min) process, while a much slower (t > 30 min) multistep adsorption and rearrangement occurred on the hydrophilic SiO2. AFM measurements showed different surface morphologies and friction coefficients for LK14 adsorbed on the two surfaces. Surface-specific SFG spectra indicate very different ordering of the adsorbed peptide on hydrophobic PS as compared to hydrophilic SiO2. At the LK14 solution/PS interface, CH resonances corresponding to the hydrophobic leucine side chains are evident. Conversely, only NH modes are observed at the peptide solution/SiO2 interface, indicating a different average molecular orientation on this hydrophilic surface. The surface-dependent difference in the molecular-scale peptide interaction at the solution/hydrophobic solid versus solution/hydrophilic solid interfaces (measured by SFG) is manifested as significantly different macromolecular-level adsorption properties on the two surfaces (determined via AFM and QCM experiments).     

Characterization of glass,quartz, and fused silica capillary column surfaces from contact-angle measurements

Summary This paper describes the systematic characterization of glass and the newly introduced fused silica and quartz capillary columns from surface wettability measurements. Common gas chromatographic stationary phases were used in capillary-rise measurements at temperatures up to 300°C. By construction of Zisman plots and using the Cassie equation, the relative surface concentrations of wettable and non-wettable groups were determined. By application of the Fowkes equation, the dispersion force component of the surface energy was investigated. The influence of various surface treatments such as leaching, silylation, and polymeric film formation are discussed. Wettability measurements were also used to evaluate the thermostability of various treated surfaces and to compare the surface properties of glass, quartz, and fused silica. The wettability of the surfaces with selected stationary phases as a function of temperature is also discussed.     

Hydronium and hydroxide at the interface between water and hydrophobic media

The behavior of hydronium and hydroxide ions at the water/alkane, water/vapor, and water/rigid wall interfaces was investigated by means of molecular dynamics simulations. All these interfaces exhibit a strong affinity for hydronium, which is in agreement with spectroscopic and low pH zeta-potential measurements. Except for the water/rigid wall interface, which strongly structures water and weakly attracts OH(-), none of the other investigated interfaces shows an appreciable accumulation of hydroxide. This computational result is at odds with the interpretation of higher pH zeta-potential and titration experiments, however, it is supported by surface selective spectroscopies of the surface of water and hydroxide solutions.     

An SFG study of interfacial amino acids at the hydrophilic SiO2 and hydrophobic deuterated polystyrene surfaces

Sum frequency generation (SFG) vibrational spectroscopy was employed to characterize the interfacial structure of eight individual amino acids--L-phenylalanine, L-leucine, glycine, L-lysine, L-arginine, L-cysteine, L-alanine, and L-proline--in aqueous solution adsorbed at model hydrophilic and hydrophobic surfaces. Specifically, SFG vibrational spectra were obtained for the amino acids at the solid-liquid interface between both hydrophobic d(8)-polystyrene (d(8)-PS) and SiO(2) model surfaces and phosphate buffered saline (PBS) at pH 7.4. At the hydrophobic d(8)-PS surface, seven of the amino acids solutions investigated showed clear and identifiable C-H vibrational modes, with the exception being l-alanine. In the SFG spectra obtained at the hydrophilic SiO(2) surface, no C-H vibrational modes were observed from any of the amino acids studied. However, it was confirmed by quartz crystal microbalance that amino acids do adsorb to the SiO(2) interface, and the amino acid solutions were found to have a detectable and widely varying influence on the magnitude of SFG signal from water at the SiO(2)/PBS interface. This study provides the first known SFG spectra of several individual amino acids in aqueous solution at the solid-liquid interface and under physiological conditions.