Electrokinetic Measurements Research Articles

A comparative study of some kaolinites surface properties

The surface properties of five kaolinites of various origins were analyzed using potentiometric titration, electrokinetic measurements and low-pressure gas adsorption associated to derivative isotherm summation (DIS) modeling. The data show that sample structure is important in determining the surface properties. The combined analytical results clearly shows the existence of a permanent negative layer charge for all the samples. The general features that could be observed for the five kaolinites are: the titration curves are shifted to lower pH with increasing ionic strength due to the layer charge; the amount of consumed proton is influenced by the permanent layer charge; although low, the permanent charge determines the electrokinetic behavior. The shape anisotropy and the charge distribution on the basal and edge surfaces are crucial parameters to understand the behavior of kaolinite particles in aqueous media. The Cameroonian sample of the set exhibits a significant permanent charge together with large specific surface area associated to fine particle size and considerable shape anisotropy.

Superior stability and high biosorbent efficiency of carboxymethylchitosan covalently linked to silica-coated core-shell magnetic nanoparticles for application in copper removal

Given the daily increase of polluted biomass (e.g. sludge) with heavy metals (such as copper), the removal of the latter is essential in order to recovery biomass resources into more beneficial products. By virtue of its higher efficiency and lower costs, environmental nanotechnology in combination with naturally-derived biopolymers must result into novel nanobiosorbents with high efficiency for metal ions cleaning process technology. This contribution presents the synthesis of novel environment friendly magnetic nanocomposites, based on 13 nm-sized magnetic maghemite core, coated with approximately 7 nm thick silica layer. For the first time, abundant silanol groups on silica coating were directly chemically coupled with amino-biopolymer carboxy-methyl chitosan (CMC) using carbodiimide chemistry, which resulted in strong and stable ester bond formation. Nanobiosorbent crystal structure was determined using X-ray powder diffraction (XRD), whereas morphology, size and silica-layer thickness (core-shell structure) were analysed with transmission electron microscopy (TEM). CMC chemical coupling of silica-coated magnetic nanoparticles was carefully executed experimentally and was examined via infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), electrokinetic measurement supported with dynamic light scattering (DLS) and termogravimetric analysis (TGA). Magnetic properties of novel nanoparticles were measured with vibration sample magnetometer (VSM). The stability of bounded CMC, as important however often neglected parameter for practical application, was tested at alkaline pH value using XPS analysis and biopolymer desorption was negligible. The resulted nanoadsorbent revealed superior tendencies for copper adsorption (∼350 mg g−1 maximum adsorption capacity) among other state-of-the-art reported magnetic-chitosan-based adsorbent, proving the promising potential of studied nanocomposite for heavy metal removal applications.

Effects of extracellular polymeric substances of Pseudomonas fluorescens, citrate, and oxalate on Pb sorption by an acidic Ultisol

The continuous production of low molecular weight (LMW) organic acids by plants and microorganisms coupled with the continuous presence of extracellular polymeric substances (EPS) in soils is a guarantee that the mobility of heavy metals in soils will be controlled. The effects of citrate, oxalate, and EPS on the adsorption of Pb by an acidic Ultisol were studied both as a function of pH and ionic strength. Electrokinetic potential measurements were also employed to observe to what extent each ligand affected the surface charge property of the Ultisol. All the ligands shifted the zeta potential of the Ultisol to the negative direction, implying that the surface charge of the soil became more negative. The effect on the zeta potential of the soil was observed in the order of oxalate ˃ citrate ˃ EPS. The quantity of Pb adsorbed at each pH (3.0–7.0) reflected the corresponding change in the zeta potential as induced by each ligand. The presence of the ligands shifted the isoelectric point of the Ultisol from 4.8 to 3.2 for the EPS system and below 3.0 for the citrate and oxalate systems. More Pb was adsorbed in the presence of oxalate than in the presence of citrate and EPS. The two most outstanding mechanisms that governed the adsorption of Pb by the Ultisol were (1) electrostatic attraction which was supported by the increase in negative zeta potential of the Ultisol and, (2) complexation which was supported by the lesser proportion of Pb adsorbed in the citrate system at higher pH and also by the spectroscopic data for EPS. The combination EPS + citrate + oxalate was more effective in enhancing the adsorption of Pb than the combination EPS + oxalate and EPS + citrate.

Effects of surface treatments on trapping with DC insulator-based dielectrophoresis.

A central challenge in measuring the biophysical properties of cells with electrokinetic approaches is the assignment of these biophysical properties to specific biological characteristics. Changes in the electrokinetic behavior of cells may come from mutations, altered gene expression levels, post-translation modifications, or environmental effects. Here we assess the electrokinetic behavior of chemically surface-modified bacterial cells in order to gain insight into the biophysical properties that are specifically affected by changes in surface chemistry. Using E. coli as a scaffold, an amine coupling reaction was used to covalently attach glycine, spermine, bovine serum albumin (protein), or 7-amino-4-methyl-3-coumarinylacetic acid (fluorescent dye) to the free carboxylic acid groups on the surface of the cells. These populations, along with unlabeled control cells, were subject to electrokinetic and dielectrophoretic measurements to quantify any changes in the biophysical properties upon alteration. The properties associated with each electrokinetic force are discussed relative to the specific reactant used. We conclude that relatively modest and superficial changes to cell surfaces can cause measurable changes in their biophysical properties.

Study of nanostructure of polymer adsorption layers on the particles surface of titanium dioxide

The surface modification of titanium dioxide in aqueous dispersions of specially tailor-made periodic acrylic acid/isobutylene copolymers, poly(acrylic acid)/polystyrene graft copolymers, and hydrophobically modified polyethyleneoxide urethane by ultrasonic treatment was studied. The pigment surface modification by the above copolymers was comparatively investigated regarding conventional adsorption as contrasted to an ultrasonic treatment assisted procedure. The course and efficiency of the polymer adsorption onto the pigment surface was quantified by electrokinetic sonic amplitude measurements. The higher efficiency of the pigment surface coating by the copolymers as achieved by ultrasonic treatment varies with the copolymer architecture and is a consequence of ultrasonically induced pigment surface activation.

Impact of oral astringent stimuli on surface charge and morphology of the protein-rich pellicle at the tooth-saliva interphase.

The proteinaceous pellicle layer, which develops upon contact with saliva on the surface of teeth, is important for the formation of oral biofilms and for the protection of teeth from abrasion and chemically induced erosion. Astringent food ingredients comprising polyphenols, cationic macromolecules, and multivalent metal salts are known to interact with the pellicle. However, astringent-induced changes in the physicochemical properties of the tooth-saliva interphase are not yet completely understood. Here we provide comprehensive insights into interfacial charging, ultrastructure, thickness, and surface roughness of the pellicles formed on the model substrates silicon oxide (SiO2), Teflon® AF, and hydroxyapatite, as well as on bovine enamel before and after incubation with the astringents epigallocatechin gallate, tannic acid, iron(III) salt, lysozyme, and chitosan. Quartz crystal microbalance with dissipation monitoring demonstrated viscous behavior of untreated pellicles formed in vitro on the different materials. Electrokinetic (streaming current) measurements revealed that cationic astringents reverse the charge of native pellicles, whereas polyphenols did not change the charge under physiological pH condition. In addition, transmission electron microscopy and atomic force microscopy showed a concentration-dependent increase in average film thickness and pellicle surface roughness as induced by astringents. These multifaceted alterations of the salivary pellicle may come along with an increase in roughness perceived on the teeth, which is part of the complex sensations of oral astringency.

Structure/Function Analysis of Nonwoven Cotton Topsheet Fabrics: Multi-Fiber Blending Effects on Fluid Handling and Fabric Handle Mechanics.

Greige cotton (GC) has attracted interest in recent years as an eco-friendly, functional fiber for use in nonwoven topsheet materials. GC imparts favorable fluid management and sensorial properties associated with urinary liquid transport and indices related to comfort in wearable incontinence nonwovens. Nonwoven GC has material surface polarity, an ambient moisture content, and a lipid/polysaccharide matrix that imparts positive fluid mechanic properties applicable to incontinence management topsheet materials. However, a better understanding of the connection between functionality and compositional aspects of molecular, mechanical, and material property relations is still required to employ structure/function relations beyond a priori design. Thus, this study focuses on the relation of key indices of material fluid and sensorial functions to nonwoven topsheet composition. Greige cotton, polypropylene, bleached cotton, and polyester fiber blends were hydroentangled at 60, 80, and 100 bar. Greige cotton polypropylene and bleached cotton were blended at ratios to balance surface polarity, whereas low percentages of polyester were added to confer whiteness properties. Electrokinetic and contact angle measurements were obtained for the hydroentangled nonwovens to assess surface polarity in light of material composition. Notably, materials demonstrated a relation of hydrophobicity to swelling as determined electrokinetically by Δζ, ζplateau, and contact angles greater than 90°. Subsequently, three blended nonwoven fabrics were selected to assess effects on fluid management properties including topsheet performance indices of rewet, strikethrough, and fluid handling (rate and efficiency of transport to the absorbent core). These materials aligned well with commercial topsheet fluid mechanics. Using the Leeds University Fabric Handle Evaluation System (LUFHES), the nonwovens were tested for total fabric hand. The results of the LUFHES measurements are discussed in light of fiber contributions. Fiber ratios were found to correlate well with improvement in softness, flexibility, and formability. This study provides insights that improves the understanding of the multifunctional properties accessible with greige cotton toward decisions valuable to selecting greige cotton as an environmentally friendly fiber for nonwoven topsheets.

Novel Concept to Endow Poly(Methyl Methacrylate) Surfaces with Reactive Surface Groups

AbstractThe surface modification of poly(methyl methacrylate) (PMMA) moldings with poly(vinylamine) (PVAm) was performed to produce a novel composite material with a reactive surface, which can be subsequently functionalized to be (bio)compatible. The synthetic potential of the composite material was achieved by endowing the surface with a high density of primary amino groups. To graft the PVAm layer onto PMMA, an interlayer of polymerized dopamine (PDA) was deposited on the PMMA surface. This PDA layer acted as a reactive adhesion promoter. When varying the oxidizing agents, the oxidative polymerization of dopamine was extensively studied concerning deposition rate and homogeneity. Dess‐Martin periodinane possess a mean PDA deposition rate of 14.6 nm⋅h−1 and was the well suited oxidizing agent. The composite surfaces were characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), and X‐ray photoemission spectroscopy (XPS). The phenomenological surface properties were studied by electrokinetic measurements.

Polyelectrolyte Composite Membranes Containing Electrospun Ion-Exchange Nanofibers: Effect of Nanofiber Surface Charges on Ionic Transport.

Poly(vinyl alcohol) (PVA)-based ion-exchange nanofibers (IEX-NFs) and their composite polyelectrolyte membranes were prepared and characterized. The PVA-based NFs are well dispersed and form a three-dimensional network structure in the polymer matrix, Nafion. All of the prepared membranes show a similar ion-exchange capacity of ∼1.0 mmol g-1. The ionic conductivities through the PVA- b-PSS-NF/Nafion composite membranes are superior to that of the Nafion membranes, but the conductivity through the PVA-NF/Nafion composite membrane is half that of the Nafion membrane. Our electrokinetic measurements clearly indicate that a high density of ion-exchange groups on the NF surface results in a continuous ionic transport path in the polymer matrix. In addition, the mechanical strength of all of the NF-composite membranes is improved compared with that of the membranes without NF.

Quartz/Aqueous Electrolyte Solution Interface: Molecular Dynamic Simulation and Interfacial Potential Measurements

In this complementary experimental and theoretical study, we employ surface and electrokinetic potential measurements and equilibrium molecular dynamics (MD) techniques to study the electrical interfacial layer between aqueous solutions of electrolytes and an oxide solid surface. More specifically, we investigate the behavior of a prototypical model system consisting of the (0001) quartz surface in contact with aqueous solutions of alkali metal salts under different conditions. The inner surface potential and electrokinetic ζ-potential were measured by means of single crystal electrodes and via streaming current measurements, respectively. Calculated ζ-potentials allowed us to benchmark MD simulations against experiments, thereby, on the one hand, verifying the validity of our strategy and, on the other hand, enabling a detailed molecular picture of the investigated phenomena and elucidating the role of both water and ions in the formation of the multilayered quartz/aqueous electrolyte interface.

PEGylation of Superparamagnetic Iron Oxide Nanoparticles with Self-Organizing Polyacrylate-PEG Brushes for Contrast Enhancement in MRI Diagnosis.

For biomedical applications, superparamagnetic nanoparticles (MNPs) have to be coated with a stealth layer that provides colloidal stability in biological media, long enough persistence and circulation times for reaching the expected medical aims, and anchor sites for further attachment of bioactive agents. One of such stealth molecules designed and synthesized by us, poly(polyethylene glycol methacrylate-co-acrylic acid) referred to as P(PEGMA-AA), was demonstrated to make MNPs reasonably resistant to cell internalization, and be an excellent candidate for magnetic hyperthermia treatments in addition to possessing the necessary colloidal stability under physiological conditions (Illés et al. J. Magn. Magn. Mater. 2018, 451, 710–720). In the present work, we elaborated on the molecular background of the formation of the P(PEGMA-AA)-coated MNPs, and of their remarkable colloidal stability and salt tolerance by using potentiometric acid–base titration, adsorption isotherm determination, infrared spectroscopy (FT-IR ATR), dynamic light scattering, and electrokinetic potential determination methods. The P(PEGMA-AA)@MNPs have excellent blood compatibility as demonstrated in blood sedimentation, smears, and white blood cell viability experiments. In addition, blood serum proteins formed a protein corona, protecting the particles against aggregation (found in dynamic light scattering and electrokinetic potential measurements). Our novel particles also proved to be promising candidates for MRI diagnosis, exhibiting one of the highest values of r2 relaxivity (451 mM−1s−1) found in literature.

Interaction of graphene oxide nanoparticles with quartz sand and montmorillonite colloids

ABSTRACTGraphene oxide (GO) nanomaterials are used extensively in a wide range of commercial applications. With GO production growing rapidly, it is expected that GO eventually could reach sensitive environmental systems, including subsurface formations, where montmorillonite, one of the most common minerals, is in abundance. This study examines the interaction of GO with quartz sand and montmorillonite (MMT) colloids at pH = 7, ionic strength IS = 2 mM, and 25°C, under dynamic conditions. Moreover, the effect of pH on MMT kinetic attachment onto quartz sand was investigated. The experimental data suggested that pH affected slightly the attachment of MMT colloids onto quartz sand. GO was attached in greater amounts onto MMT than quartz sand. Also, the attachment of GO onto quartz sand was shown to increase slightly in the presence of MMT colloids. However, when GO and MMT coexisted, the total GO mass attached onto quartz sand, suspended MMT, and attached MMT was increased. Furthermore, the equilibrium attachment experimental data were fitted nicely with a Freundlich isotherm, and the attachment kinetics were satisfactorily described with a pseudo-second-order model. Finally, the extended DLVO (XDLVO) theory was used to quantify the various interaction energy profiles based on electrokinetic and hydrodynamic measurements.

Desiccation-induced alterations in surface topography of thylakoids from resurrection plant Haberlea rhodopensis studied by atomic force microscopy, electrokinetic and optical measurements.

With their ability to survive complete desiccation, resurrection plants are a suitable model system for studying the mechanisms of drought tolerance. In the present study, we investigated desiccation-induced alterations in surface topography of thylakoids isolated from well-hydrated, moderately dehydrated, severely desiccated and rehydrated Haberlea rhodopensis plants by means of atomic force microscopy (AFM), electrokinetic and optical measurements. According to our knowledge, so far, there were no reports on the characterization of surface topography and polydispersity of thylakoid membranes from resurrection plants using AFM and dynamic light scattering. To study the physicochemical properties of thylakoids from well-hydrated H. rhodopensis plants, we used spinach thylakoids for comparison as a classical model from higher plants. The thylakoids from well-hydrated H. rhodopensis had a grainy surface, significantly different from the well-structured spinach thylakoids with distinct grana and lamella, they had twice smaller cross-sectional area and were 1.5 times less voluminous than that of spinach. Significant differences in their physicochemical properties were observed. The dehydration and subsequent rehydration of plants affected the size, shape, morphology, roughness and therefore the structure of the studied thylakoids. Drought resulted in significant enhancement of negative charges on the outer surface of thylakoid membranes which correlated with the increased roughness of thylakoid surface. This enhancement in surface charge density could be due to the partial unstacking of thylakoids exposing more negatively charged groups from protein complexes on the membrane surface that prevent from possible aggregation upon drought stress.

Electrokinetic Measurements at High Electrolyte Concentrations

The method of capillary electrokinetics has for the first time been used to measure streaming current at high electrolyte concentrations. It has been shown that the streaming current is proportional to the applied pressure. At an electrolyte concentration of 1 M, the thickness of the diffuse layer is comparable with the size of a water molecule (0.3 nm); i.e., there is almost no diffuse layer. The existence of the streaming current in this case indicates that there are no hydrodynamically immobile layers near a smooth solid surface.

Lysozyme as a flocculant-inducing agent improving the silica removal from aqueous solutions - A turbidimetric study

In this paper, the lysozyme (LSZ) adsorption impact on the silica suspension stability was established. In other words, the stabilization/destabilization mechanism of the SiO2/LSZ system was explained based on the adsorption, electrokinetic and stability measurement results. Lysozyme adsorbs on the silica surface in the whole pH range. This process contributes to the changes in silica surface charge and zeta potential values. The lysozyme addition influences the system stability too. At pH 7.6 and 9, a large decrease in the silica suspension stability was found. It is connected with the neutralization of solid negative charge by the positively charged macromolecules. As a result, large aggregates can be formed, which is highly desirable in the silica removal procedure.

Synthesis of sodium polyacrylate copolymers as water-based dispersants for ultrafine grinding of praseodymium zirconium silicate

A series of sodium polyacrylate copolymers (i.e., poly acrylic acid-co-itaconic acid-co-2-acrylamide-2-methylpropanesulfonic acid or PSIA with different molecular weights and monomer molar ratios) were synthesized based on acrylic acid-co-itaconic acid (PAI) as water-based dispersants for ultrafine grinding of praseodymium-doped zirconium silicate (Pr-ZrSiO4). The particle size/size distribution, suspension rheology and dispersant adsorption were analyzed by laser particle size analysis, rheometry, Fourier transform infrared (FTIR) spectroscopy, thermogravimetry (TG) and electrokinetic potential measurement, respectively. Compared to the dispersant PAI, the use of the dispersant PSIA1 with a greater molecular weight in the Pr-ZrSiO4 suspension ground for 45 min at pH values 7.0–8.0 can give a finer product with the narrower particle size distribution. The dispersant PSIA1 used in ultrafine grinding has a suitable viscosity of the suspension and generates more hydrogen bonds on the particle surface, and sulfonic acid group in the dispersant PSIA1 can provide a greater charge density than carboxylic acid groups in the dispersant PAI. In addition, the total potential energies between the particles in suspension with different copolymer dispersants at different pH values were also calculated. It is indicated that the effective ultrafine grinding of the particles is related to the total interparticle potential energy provided by the dispersant PSIA1 at pH 7.0–8.0.

Hysteresis Charges in the Dynamic Enrichment and Depletion of Ions in Single Conical Nanopores

AbstractInhomogeneous distribution of electrolyte ions at solid–solution interfaces at the nanometer scale is known to contribute to several emerging transport phenomena discovered from various nanopores or nanochannels. The topic has attracted extensive research effort, aiming to achieve better energy harvesting, storage, conversion, desalination and separation, sample concentration, stochastic sensing, and so on. In this report, dynamic ion concentration polarization inside charged conical nanopores, one of the two characteristics to describe the electrical double layer (EDL) structure (charge distribution and potential profile), is quantified. The hysteresis in the ion transport or redistribution in single nanopores is determined from the pinched current–potential loops by time‐dependent electrokinetic measurements. The extent of polarization in ion concentrations over different timescales, or residual charges during the transition from one ion distribution/EDL structure to another, is quantitated from the area enclosed in the hysteresis current loops. The ratio of the residual charges during the high/low conductivity states is correlated with the current ratio at the threshold potentials (rectification factor), which, in turn, serves as a convenient parameter to estimate the charge transport hysteresis. By adjusting the solution pH and ionic strength, and under different potential sweeping rates, the dynamics of ion enrichment or depletion is elucidated, respectively, and successfully described with a simple capacitive charging model. Taken together with our previous reports of the memristive ion transport in conical nanopores/nanopipettes and the physical meaning of the unique non‐zero cross‐point potential, a comprehensive view of the charge redistribution and EDL dynamics at nanoscale interfaces is offered.

Formation, properties and stability of silver nanoparticle monolayers at PDADMAC modified polystyrene microparticles

A facile self-assembly procedure was developed for the preparation of silver nanoparticle shells at polystyrene microparticles (PSMPs) modified by poly(diallyldimethylammonium chloride) (PDADMAC). Initially, physicochemical properties of microparticles, silver particles and (PDADMAC) were determined for various ionic strength and pHs. Then, PDADAMAC adsorption on the PSMPs was studied in situ via electrokinetic measurements. The influence of polyelectrolyte bulk concentration on electrokinetic properties of PDADMAC-coated microparticles for different ionic strength was investigated. The coverage of PDADMAC monolayer was estimated applying an electrokinetic model. The stability of PDADMAC-modified microparticles under controlled conditions of pH and ionic strength was also evaluated. It was confirmed that under acidic conditions (pH 4–5.6) the microparticles were stable for at least two weeks. The modified microparticles were used as substrate for silver nanoparticle immobilization carried out under a diffusion controlled self-assembly process. The coverage of nanoparticles was precisely determined using electrophoretic mobility measurements, the concentration depletion method involving atomic force microscopy and via direct scanning electron microscopy imaging of particles. A large stability of the silver particle monolayers was confirmed. These monolayers of well-defined coverage and acid-base properties can be used for controlled delivery of silver nanoparticles.

Tubular and Spherical SiO₂ Obtained by Sol Gel Method for Lipase Immobilization and Enzymatic Activity.

A wide range of hybrid biomaterials has been designed in order to sustain bioremediation processes by associating sol-gel SiO2 matrices with various biologically active compounds (enzymes, antibodies). SiO2 is a widespread, chemically stable and non-toxic material; thus, the immobilization of enzymes on silica may lead to improving the efficiency of biocatalysts in terms of endurance and economic costs. Our present work explores the potential of different hybrid morphologies, based on hollow tubes and solid spheres of amorphous SiO2, for enzyme immobilization and the development of competitive biocatalysts. The synthesis protocol and structural characterization of spherical and tubular SiO2 obtained by the sol gel method were fully investigated in connection with the subsequent immobilization of lipase from Rhizopus orizae. The immobilization is conducted at pH 6, lower than the isoelectric point of lipase and higher than the isoelectric point of silica, which is meant to sustain the physical interactions of the enzyme with the SiO2 matrix. The morphological, textural and surface properties of spherical and tubular SiO2 were investigated by SEM, nitrogen sorption, and electrokinetic potential measurements, while the formation and characterization of hybrid organic-inorganic complexes were studied by UV-VIS, FTIR-ATR and fluorescence spectroscopy. The highest degree of enzyme immobilization (as depicted from total organic carbon) was achieved for tubular morphology and the hydrolysis of p-nitrophenyl acetate was used as an enzymatic model reaction conducted in the presence of hybrid lipase–SiO2 complex.

Electrical Double-Layer Expansion Impact on the Oil–Quartz Adhesion for High- and Low-Salinity Brines

Expanding the electrical double layer of minerals through low-salinity brine injection has been suggested as a possible enhanced oil recovery mechanism. To investigate this theory, we measured the ζ potential of different minerals, namely, sands from Fontainebleau, Ottawa, and Landes, a sample from a sandstone outcrop, and one crude oil. ζ-potential measurements can be used to quantify the surface charges of materials; therefore, experiments associated with this technique were performed to predict the behavior of repulsion or attraction between materials in different salinity and pH conditions. We showed that there is no significant difference between the ζ potentials of the tested materials. We did, however, observe that the different sands and sandstone have dissimilar adhesion behaviors. No correlation was found between the electrokinetic measurements performed on the minerals and their response to crude oil introduced into the system. The adhesion results obtained for the Landes and Ottawa sands were ...