Surface Charge Neutralization Research Articles

Effect of the preparation procedure on the structural properties of oligonucleotide/cationic liposome complexes (lipoplexes) studied by electron spin resonance and Zeta potential

Lipoplexes with different surface charge were prepared from a short oligonucleotide (20 mer, dsAT) inserted into liposomes of 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and 1,2-dioleoyl- sn-glycero-3-phospho-ethanolamine (DOPE). The starting liposomes were prepared by two different procedures, i.e. progressive dsAT addition starting from plain liposomes (titration) and direct mixing of dsAT with pure liposomes (point to point preparation). Lipoplexes were characterized from a molecular point of view by Electron Spin Resonance (ESR) of a cationic spin probe and by Nuclear Magnetic Resonance. Structural and surface features were analysed by Zeta potential (ζ) measurements and Cryo-TEM micrographs. The complete set of results allowed to demonstrate that: i) the interactions between dsAT and cationic lipids were strong and occurred at the liposome surface; ii) the overall shape and physicochemical properties of liposomes did not change when short nucleic acid fragments were added before surface charge neutralization; iii) the bilayer structure of the lipids in lipoplexes was substantially preserved at all charge ratios; iv) the physical status of lipoplexes with electrical charge far from neutrality did not depend on the preparation method.

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

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

Small-Angle Neutron Scattering of Mixed Ionic Perfluoropolyether Micellar Solutions

Aqueous mixed micellar solutions of perfluoropolyether carboxylic salts with ammonium counterions have been studied by small-angle neutron scattering. Two surfactants differing in the tail length were mixed in proportions n2/n3 = 60/40 w/w, where n2 and n3 are the surfactants with two and three perfluoroisopropoxy units in the tail, respectively. The tails are chlorine-terminated. The mixed micellar solutions, in the concentration range 0.1-0.2 M and thermal interval 20-40 degrees C, show structural characteristics of the interfacial shell that are very similar to ammonium n2 micellar solutions previously investigated; thus, the physics of the interfacial region is dominated by the polar head and counterion. The shape and dimensions of the micelles are influenced by the presence of the n3 surfactant, whose chain length in the micelle is 2 A longer than that of the n2 surfactant. The n3 surfactant favors the ellipsoidal shape in the concentration range 0.1-0.2 M with a 1/2 ionization degree of n2 micelles. The very low surface charge of the mixed micelles is attributed to the increase in hydrophobic interactions between the surfactant tails, due to the longer n3 surfactant molecules in micelles. The closer packing of the tails decreases the micellar curvature and the repulsions between the polar heads, by surface charge neutralization of counterions migrating from the Gouy-Chapman diffuse layer, leading to micellar growth in ellipsoids with greater axial ratios.

Role of the Charge–Charge Interactions in Defining Stability and Halophilicity of the CspB Proteins

Charge–charge interactions on the surface of native proteins are important for protein stability and can be computationally redesigned in a rational way to modulate protein stability. Such computational effort led to an engineered protein, CspB-TB that has the same core as the mesophilic cold shock protein CspB-Bs from Bacillus subtilis, but optimized distribution of charge–charge interactions on the surface. The CspB-TB protein shows an increase in the transition temperature by 20 °C relative to the unfolding temperature of CspB-Bs. The CspB-TB and CspB-Bs protein pair offers a unique opportunity to further explore the energetics of charge–charge interactions as the substitutions at the same sequence positions are done in largely similar structural but different electrostatic environments. In particular we addressed two questions. What is the contribution of charge–charge interactions in the unfolded state to the protein stability and how amino acid substitutions modulate the effect of increase in ionic strength on protein stability (i.e. protein halophilicity). To this end, we experimentally measured the stabilities of over 100 variants of CspB-TB and CspB-Bs proteins with substitutions at charged residues. We also performed computational modeling of these protein variants. Analysis of the experimental and computational data allowed us to conclude that the charge–charge interactions in the unfolded state of two model proteins CspB-Bs and CspB-TB are not very significant and computational models that are based only on the native state structure can adequately, i.e. qualitatively (stabilizing versus destabilizing) and semi-quantitatively (relative rank order), predict the effects of surface charge neutralization or reversal on protein stability. We also show that the effect of ionic strength on protein stability (protein halophilicity) appears to be mainly due to the screening of the long-range charge–charge interactions.

Phosphate sorption and desorption on pyrite in primitive aqueous scenarios: relevance of acidic --> alkaline transitions.

Phosphate (P(i)) sorption assays onto pyrite in media simulating primeval aquatic scenarios affected by hydrothermal emissions, reveal that acidic conditions favour P(i) sorption whereas mild alkaline media--as well as those simulating sulfur oxidation to SO(2-) (4)--revert this capture process. Several mechanisms relevant to P(i) availability in prebiotic eras are implicated in the modulation of these processes. Those favouring sorption are: (a) hydrophobic coating of molecules, such as acetate that could be formed in the vicinity of hydrothermal vents; (b) water and Mg(2+) bridging in the interface mineral-aqueous media; (c) surface charge neutralization by monovalent cations (Na+ and K+). The increase of both the medium pH and the SO(2-) (4) trapping by the mineral interface would provoke the release of sorbed P(i) due to charge polarization. Moreover it is shown that P(i) self-modulates its sorption, a mechanism that depends on the abundance of SO(2-) (4) in the interface. The relevance of the proposed mechanisms of P(i) capture, release and trapping arises from the need of abundant presence of this molecule for primitive phosphorylations, since--similarly to contemporary aqueous media--inorganic phosphate concentrations in primitive seas should have been low. It is proposed that the presence of sulphide minerals with high affinity to P(i) could have trapped this molecule in an efficient manner, allowing its concentration in specific niches. In these niches, the conditions studied in the present work would have been relevant for its availability in soluble form, specially in primitive insulated systems with pH gradients across the wall.

Role of surface functionality in the adsorption of anionic dyes on modified polymeric sorbents

While synthetic polymeric sorbents effectively treat dye wastewaters by adsorption, the underlying mechanisms remain to be understood. This work determined the adsorption of an anionic dye by three polymers differing significantly in surface functionality. Surface functional groups of polymers were indicated in FTIR spectra and quantified by the Boehm titration. In reference to the commercial sorbent XAD-4 with a low degree of functionality, the laboratory synthesized NG-8 had primarily acidic functional groups, whereas its aminated product MN-8 had mainly basic amino groups. Electrophoresis determined the points of zero charge of 4.18, 3.23, and 4.51 for XAD-4, NG-8, and MN-8, respectively. The adsorption of Reactive Black 5 dye (RB5) by all the sorbents on a unit surface area basis increased with decreasing pH. At the same low pH (⩽4.40), the adsorption by NG-8 was similar to that by XAD-4, indicating little influence of protonated (neutral) surface functional groups. In contrast, the adsorption by NG-8 at pH 6.05 was 75% lower than that by XAD-4, resulting apparently from the strong electrostatic repulsion between RB5 and deprotonated (negative) groups. Amination substantially enhanced RB5 adsorption by eliminating acidic groups and creating a positive charge on the surface of MN-8. The adsorptive enhancement was also achieved in the presence of CaCl 2, due presumably to the neutralization of negative surface charge by Ca 2+ and the RB5-Ca 2+ pairing. These results manifest the important role of surface functionality in the adsorption of dyes by synthetic polymers.

Enhanced flocculation of oil-in-water emulsions by hydrophobically modified chitosan derivatives

Flocculation efficiency of hydrophobically modified (HM) chitosan derivatives in comparison with commercial cationic polyacrylamide flocculant and unmodified (UM) chitosans varying in molecular weights and acetylation degrees has been evaluated in sodium dodecyl sulfate (SDS)-stabilized and surfactant-free oil-in-water emulsions at pH 4–9. It was found that due to the synergetic effect of cationic and hydrophobic functionalities HM-chitosans provide superior phase separation in SDS-stabilized systems at twice- and four-fold lower doses than UM-chitosan and cationic polyacrylamide, respectively. For this type of emulsions, pH had little influence on the efficiency of phase separation. In surfactant-free emulsions with low surface charge density of oil droplets, hydrophobic substitution had weaker effect on chitosan flocculation performance. Complete phase separation was reached only at pH > 7.5, and at UM and HM chitosan doses considerably higher than it was required for surface charge neutralization. This suggests that in surfactant-free emulsions hydrophobic interactions could not compensate a dramatic decrease of electrostatic attraction between polymers and droplet surface, thus, flocculation occurred mainly through the “sweep floc” process in contrast to the “charge patch” mechanism in SDS-stabilized systems.

Impact of Surface Immobilization and Solution Ionic Strength on the Formal Potential of Immobilized Cytochrome c

Four different self-assembled monolayer (SAM) electrode systems were examined electrochemically in order to better understand surface charge effects on the redox thermodynamics of immobilized horse heart cytochrome c (cyt c). Neutralization of protein surface charge upon adsorption on anionic COOH-terminated SAMs was found to cause substantial changes in the formal potential, as determined by cyclic voltammetry. For cyt c immobilized on negatively charged surfaces, the formal potential shifted to more negative values as the ionic strength was decreased, which is opposite to the trend displayed by solution cyt c. In contrast, immobilization to uncharged interfaces resulted in an ionic strength dependence for cyt c that is similar to its solution behavior. The results provide insight into the importance of surface charge on the formal potential of cyt c.

Starch derivatives of high degree of functionalization: 10. Flocculation of kaolin dispersions

Flocculation properties of cationic potato starch derivatives (2-hydroxy-3-trimethylammonium-propyl starch chloride) with degree of substitution (DS) up to 1.54 were investigated in kaolin dispersions at various solid/liquid ratios and ionic strengths by means of turbidity measurements and colloid titration. It was found that flocculation has occurred long before the kaolin surface charge neutralization was reached. Flocculant amount required for a complete phase separation decreased with the increase of DS, and dispersion restabilization was observed at polymer overdose for all the studied starch derivatives and solid/liquid ratios. The mechanism of the kaolin dispersion flocculation with cationic starch derivatives was suggested to be a combination of bridging and “charge patch” processes. Addition of electrolyte proved to enhance the dispersion flocculation through lowering the required polymer dose and broadening the flocculation window, although no substantial influence of ionic strength on adsorption of the starch derivatives was found.

Charge Neutralization in the ESEM for Quantitative X-ray Microanalysis

Quantitative chemical analysis by energy-dispersive X-ray spectrometry (EDS) in the environmental scanning electron microscope (ESEM) is difficult. This analysis is complicated by the spread of the electron beam by chamber gas molecules and the necessity for surface charge neutralization. Without charge neutralization, errors in quantitative analysis can range up to 15-20% relative. It is possible to achieve the error expected of traditional EDS, +/- 5% relative error, using a newly developed surface charge neutralization scheme for the ESEM. Estimates of accuracy and precision are based on studies of the National Bureau of Standards (now National Institutes for Science and Technology) Standard Reference Material 482, a series of certified copper-gold alloys. The scheme for charge neutralization requires an independent path to ground at or near the surface of the specimen. The current through the ground path must be maintained at zero by adjusting the voltage on the Gaseous Secondary Electron Detector when the sample chamber is at a gas pressure of 1-2 torr. This procedure forms the exact number of chamber gas positive ions to neutralize negative electrical charge on the specimen surface from electron bombardment.

Investigation on effects of aggregate structure in water and wastewater treatment

The fractal structure and particle size of flocs are generally recognized as the two most crucial physical properties having impact on the efficiency of operation of several unit processes in water and wastewater treatment. In this study, an experimental investigation is undertaken on the effect of aggregate structure in water and wastewater treatment in Hong Kong. The fractal dimension of the resulting aggregate is employed as a measure of the aggregate structure. Small angle light scattering technique is used here. Different amounts of polymers are mixed to bacterial suspensions and the resulting structures are examined. The addition of polymer may foster aggregate formation by neutralization of the bacterial surface charge and enhance inter-particle bridging. The aggregation behavior may affect the efficiency of certain water and wastewater treatment processes such as dewatering and coagulation. The impacts of aggregate structure on two representative processes, namely, ultra-filtration membrane fouling and pressure filter dewatering efficiency, are studied. It is found that the looser flocs yield a more porous cake and less tendency to foul whilst more porous filter cakes yield more ready biosolids dewatering.

Adsorption and Structural Arrangement of Cetyltrimethylammonium Cations at the Silica Nanoparticle−Water Interface

Although the sorption of cetyltrimethylammonium ion (CTA+) on silica (SiO2) surfaces has been studied extensively, little is known about the interactions between large surfactant molecules and nanosized colloidal particles with a high specific surface area. The aim of the study was to understand the effects of structural arrangements of sorbed CTA+ ions on the stability and surface properties of SiO2 nanoparticles. The extent of the effect of CTA+ sorption on the aggregation behavior of the SiO2 nanoparticle suspension was investigated with the dynamic light scattering (DLS) technique. Both the Fourier transform infrared (FTIR) and Raman spectroscopic techniques were used to probe the sorbed layers of CTA+ on silica surfaces. Results indicate that, at a low surface coverage (less than a monolayer), CTA+ molecules were strongly bound to the SiO2 surface via their trimethylammonium headgroups. A bilayer sorption of CTA+ was observed at a high surface coverage, and the sorption is attributed to the hydrophobic interactions between aliphatic tails of CTA+. Sorption of CTA+ at a low surface coverage also caused the destabilization of the SiO2 nanoparticle dispersion as a result of surface charge neutralization, but redispersion and surface charge reversal of SiO2 colloids occurred at a high surface coverage. The present study thus confirms the adsorption mechanism of the reverse orientation model and contributes to a better understanding of the sorption and structural arrangements of sorbed surfactants at the SiO2 nanoparticle−water interface.

Accuracy and precision of quantitative energy-dispersive x-ray spectrometry in the environmental scanning electron microscope.

The accuracy and precision of quantitative energy-dispersive x-ray spectrometry in the environmental scanning electron microscope have been estimated using a series of copper / gold alloys of known composition. The mean values (five to six replicate experiments) had relative errors within +/- 5%, and most were within +/- 3.5%. All relative standard deviations were < 5% and most were < 3%. Since the standard specimens were large (approximately 500 microm) in diameter, electron scattering in the 2 torr of water vapor above the specimen did not affect the results. This level of accuracy and precision was possible only by using a novel specimen surface charge neutralization scheme.

Defect-induced nonpolar-to-polar transition at the surface of CuInSe 2

In contrast to zinc-blende semiconductors, where the nonpolar (110) surface has the lowest energy, our first-principles calculations on the chalcopyrite semiconductor CuInSe 2 reveal that facets terminated by the (112)-cation and (1̄1̄2̄)-Se polar surfaces are lower in energy than the unfaceted (110) plane, despite the resulting increased surface area. This explains the hitherto puzzling existence of polar microfacets on nominally nonpolar (110) chalcopyrite surfaces. The extraordinary stability of these polar facets originates from the effective neutralization of surface charge by low-energy ordered Cu In antisite or Cu vacancy surface defects, while the relaxed but defect-free (112) surface is metallic and much higher in energy. We explain the low carrier density of the observed faceted surface in terms of autocompensation between opposite-polarity facets.

Study of the Effect of Small Ions as a Key Parameter in the Adsorption of Polyvinylimidazole on Silica and Gold

The adsorption of polyvinylimidazole (PVI) from aqueous solutions on gold and silica substrates has been studied using reflectometry and compared to that measured on colloidal silica by the solution depletion method. The adsorption trends were studied as a function of pH, monovalent electrolyte (KCl, KNO 3), and divalent electrolyte (ZnSO 4) concentrations. Uptake on gold compares very well with that on silica. The main objective of this paper is to show experimentally the importance of small electrolyte ions in the adsorption of PVI by measuring their electrical contribution in the charge balance of the adsorption layer. We shall use the adsorption data on colloidal silica, which compare also very well with those on the silica- and gold-modified wafers, and combine them with electrophoretic measurements to calculate and discuss small ion contribution. The results demonstrate that up to 80% of the charge in the interface is neutralized by small ions and, consequently, that they play a significant role in the free energy of polymer adsorption. PVI adsorption does not obey the principle of surface charge neutralization. The results show that, for the significantly charged polyelectrolyte (in acidic media at pH 7 and below), the main part of the adsorbed polymer charge is screened by its counterions. Conversely, at pH 8 and above, the weakly charged polymer cancels only a very small fraction of the high surface charge but the counterion of the surface (K +) is now largely in excess. With use of the gold and silica substrates, it is shown that when the ploymer is weakly charged (pH 8 and above), the increase of the negative surface charge leads to the displacement of the polyelectrolyte by small K + ions. Our results are in agreement with current SFC modelization of the adsorption of polyelectrolytes but they illustrate more quantitatively the predominant contribution of small ions; they also explain several features of polyelectrolyte uptake with regard to the specific effect of the type and the concentration of the dissolved electrolyte, in particular, the occurrence of irregular isotherms and of endothermic adsorption reactions.

Determination of Intrinsic Bacterial Surface Acidity Constants using a Donnan Shell Model and a Continuous p Ka Distribution Method

Intrinsic acidity constants (p K a int) for Bacillus subtilis (Gram+) and Escherichia coli (Gram−) cells were calculated from potentiometric titration data at different salt concentrations. Master curves were generated by replotting charge excess data as a function of pH S (pH at the location of surface reactive sites) where pH S was determined as a function of Donnan potential, Ψ DON. This potential decreased in magnitude with increasing ionic strength, from −48.5±0.2 to −3.5±0.0 mV for B. subtilis and −47.9±0.3 to −3.5±0.0 mV for E. coli at 0.01 and 0.5 M K +, respectively, indicating an efficient surface charge neutralization by counterions. A fully optimized continuo us (FOCUS) p K a distribution method revealed four binding sites on B. subtilis and E. coli surfaces from the master curves with p K a int values of 3.59±0.38, 4.33±0.57, 5.94±0.66, and 8.64±0.57 for B. subtilis and 3.73±0.44, 4.85±0.71, 6.56±0.64, and 8.79±0.62 for E. coli. These were assigned to functional groups according to reported p K a ranges of 2.0–6.0 (carboxylic acid), 3.2–3.5 (phosphodiesters), 5.6–7.2 (phosphoric acid), and 9.0–11.0 (amine groups). Average points of zero salt effect (pH pzse) for B. subtilis experiments were 6.63±0.21 and 6.42±0.08 as a function of pH bulk and pH S, respectively. Under the same criteria, E. coli calculations yielded 5.73±0.23 and 5.45±0.05. An understanding of metal and proton reactivity on bacterial cell surfaces can be addressed quantitatively through the use of electrostatic and chemical equilibrium modeling techniques proposed in this study. The results are consistent with those of electrical force microscopy studies used to document the intrinsic electrochemical heterogeneity of bacterial cell surfaces.

Surface Modification of Colloidal Gold by Chemisorption of Alkanethiols in the Presence of a Nonionic Surfactant

Surface modification of colloidal gold with 11-mercaptoundecanoic acid or 16-mercaptohexadecanoic acid was performed in the absence or in the presence of the nonionic surfactant polyoxyethylene (20) sorbitan monolaurate (Tween 20). The stability of the colloidal systems was assessed with optical absorption spectroscopy. The surface-modified nanoparticles were stable only within a narrow range of intermediate pH values when chemisorption of alkanethiols was performed in the absence of Tween 20. This was explained in terms of partial ionization of the surface carboxylic groups and charge neutralization at high pH values by counterions present in the buffer solutions. Formation of a physisorbed monolayer of Tween 20 onto the nanoparticles prior to chemisorption of alkanethiols resulted in surface-modified colloidal gold that was stable over a broader range of pH values. Parallel experiments demonstrated that self-assembled monolayers could form on flat substrates in the presence of Tween 20. Therefore, possi...

Forces between Glass Surfaces in Aqueous Polyethylenimine Solutions

Interaction forces between flame-polished glass surfaces were measured in a range of aqueous solutions of a branched cationic polyelectrolyte, polyethylenimine (PEI) MW ≈ 70 000 g/mol. Short incubation in 1 ppm PEI solution leads to neutralization of the glass negative surface charge. At this point the interaction forces are dominated by a bridging attraction detectable at separations below 10 nm. Prolonged incubation in the same solution results in charge reversal. Upon increasing the bulk polymer concentration, an additional adsorption takes place, and the magnitude of the observed charge reversal increases. In 50 ppm PEI an additional electrosteric force is present at short distances, and the force measured on approach remains repulsive at all separations. The pull-off forces measured on separation were shown to be dependent on the time the two surfaces spent in contact and increased with increasing contact time. In addition, the concentration dependence of the pull-off force in PEI solutions was compared to two linear, highly charged polyelectrolytes polyvinylamine (PVAm) and poly([2-(propionyloxy)ethyl]trimethylamonium chloride) (PCMA). It was found that the two linear polyelectrolytes induce lower adhesion between glass surfaces than PEI. Further, for the linear polyelectrolytes the adhesion force was not strongly influenced by the polyelectrolyte concentration for concentrations higher than 2 ppm. In contrast, PEI generates higher adhesive forces at low bulk concentrations, which then sharply decrease upon increasing the PEI concentration. These differences are discussed in terms of both the PEI highly branched structure and the fact that the PEI charge is concentration dependent due to changes in solution pH.

Neutralization of static surface charges by an ac ionizer in a nitrogen and dry air environment

To elucidate the dynamics of static charge elimination, we measured charge decay and residual potential (balance or offset voltage) in gases with various ion mobilities. It was observed that surface charge decay, especially for positive charges, occurs much faster in nitrogen than in air. The residual potential on the probe is negative in pure nitrogen and increases toward positive values with the injection of small quantities of air in front of the ionizer. The fluctuations in the residual potential are generally less than 3 V peak-to-peak. For ionizer operations in nitrogen environments, the charge decay rate increases with superficial gas-flow rate. The results are consistent with a theory of the interaction between bipolar ions and a charged object.

Weakly Charged Polyelectrolyte Adsorption to Glass and Cellulose Studied by Surface Force Technique

Interaction forces were measured between glass surfaces and between one glass and one cellulose surface in the presence of a 10% charged, random copolymer of acrylamide and cationic [3-(2-methylpropionamido)-propyl]trimethylammonium chloride (AM-MAPTAC-10), using a noninterferometric surface force technique (MASIF). Adsorption to glass from a 1 ppm solution leads to neutralization of the substrate negative surface charge. When the bulk polymer concentration is increased to 50 ppm, a weak charge reversal occurs as indicated by the reappearance of the double-layer force. Throughout the concentration range studied (1-50 ppm), the forces measured on approach between two glass surfaces could be adequately described by the DLVO theory. The interactions between one glass and one model cellulose surface under the same conditions follow a different pattern. AM-MAPTAC-10 adopts an extended conformation upon adsorption to cellulose and steric repulsion is generated when the thick adsorbed layer is compressed. The different structure of the adsorbed layers on cellulose is explained in terms of its lower surface charge density compared to glass. This leads to fewer attachment spots on the solid surface and an increased amount of polymer loops and tails proturding into solution; i.e., thicker adsorbed layers are formed and the resulting system is sterically stabilized.