Sign Of Surface Charge Research Articles

Synergy effects of pH and thermal localization on membrane-based salinity gradient energy harvesting

Salinity gradient power generation receives increasing attention due to its availability in nature. However, membrane surface charge is not fixed value in isolation but strongly dependent of various conditions. Here, a coupled ion transport model is proposed to study the synergy effects of pH and thermal localization on membrane-based salinity gradient energy conversion. The research illustrates that the surface charge is regulated by solution environment, membrane properties and thermal conditions. Specially, the surface charge sign can be even reversed at certain case that induces distinct ion transports behaviors. Consequently, Al2O3 membrane achieves the better energy conversion performance at low pH with negative temperature difference. While SiO2 membrane obtains the best output performance at high pH with negative temperature difference. Such is ascribed to pH-regulated surface charge and thermal up-diffusion enhancement. The maximum output power of 10 pW with efficiency of 16 % is achieved for individual membrane channel. As a proof of concept, it shows significant deviations between the single and coupling model on power generation. The findings of this study can help for better understanding the coupling mechanisms of solution, membrane and thermal conditions during the energy conversions, thereby providing a reasonable design route for membrane-based energy devices.

Electrokinetic comparison of CaF2 and AgCl from a MDS perspective

CaF2 has limited electrokinetic sensitivity to fluoride ions in solution and the positive CaF2 surface charge is not reversed at high fluoride ion concentration. Similar insensitivity of surface charge to high fluoride ion concentration has been observed for other alkali and alkaline earth fluoride salts. On the contrary, chloride ions act as potential determining ions for AgCl and reverse the surface charge, as expected. FTIR-IRS results indicate that the AgCl/water interface has a water structure-breaking tendency, whereas the CaF2/water interface has a structure-making tendency. It is speculated that the strong hydrogen bonding between fluoride ions and water prevents the accommodation of excess fluoride ions at surface lattice sites of fluorite, and therefore explains the low sensitivity of the fluorite surface charge sign and magnitude to fluoride ion concentration. However, this speculation has not been examined at the molecular level and is evaluated by molecular dynamics simulations (MDS) in the research presented in this paper. Higher hydrogen bond density at the CaF2 mineral/water interface confirms that the water structure has stronger hydrogen bonding at both the CaF2 mineral surface and with the F− ion in solution. In this way, the interaction of the F− ion with the CaF2 surface is inhibited.

Zeta Potential-Based Control of CdSe/ZnS Quantum Dot Photoluminescence.

The ζ-potential of a colloidal quantum dot (QD) in solution has a strong impact on its photoluminescence emission quantum yield as well as the population lifetime. In this study we show that varying the surface charged groups on CdSe/ZnS QDs allows one to tune the ζ-potential and, with it, to control the quantum yield of emission as well as the recombination dynamics. We infer that the net charge density within the slipping plane around the QD in the solution strongly affects the nonradiative recombination processes, depending on the surface charge sign and value. For zwitterionic surface groups it is possible to tune the ζ-potential and the quantum yield by pH. As a general trend, QDs with zwitterionic surface groups produce a low (absolute) ζ-potential value and exhibit the highest quantum yield. Our results pave the way to, for example, future intracellular, time-resolved pH sensing applications with similar systems.

Water Film Thickness in Unsaturated Porous Media: Effect of Pore Size, Pore Solution Chemistry, and Mineral Type

AbstractThe thickness of the water film on minerals is key for a quantitative understanding of mass transport and reaction in porous media under air entrapment. How the film thickness changes with pore structure, ionic strength, pH, and type of mineral, however, is not fully understood. Here, we present a model to predict the film thickness in porous media under air entrapment. The model numerically solves the Poisson–Boltzmann equation combined with the electrical triple‐layer model to consider the overlapping of electric double layers (EDLs) in the film, where EDLs formed at the mineral/water and air/water interfaces interact with each other. Based on the model, we estimated the film thickness for silica, montmorillonite, calcite, and goethite for various ionic strengths and pH. With decreasing film thickness, the overlapping of EDLs reduces the surface charges but increases the electric potentials of silica/water and air/water interfaces. The predicted film thickness for silica generally agreed with the measured value. The film on calcite and goethite is significantly thinner than that on silica and montmorillonite because the surface charges for calcite and goethite have a sign opposite to that of the air/water interface. Our model shows that the pore radius, ionic strength, pH, and sign of surface charge are the primary factors controlling the film thickness. When the ionic strength and pH are similar to those of rainwater, river water, and groundwater, the typical film thicknesses for silica in silt, sand, and gravel were predicted to be <14 nm, 14–45 nm, and >23 nm, respectively.

Molecular dynamics simulations on aqueous solution confined in charged nanochannels: asymmetric effect of surface charge

ABSTRACT This work aims at investigating the effect of surface charge sign on static and dynamic properties of aqueous solutions confined in the charged nanochannel. Molecular dynamics simulations of two confinement systems have been performed, in which the nanochannel walls were modelled as the structureless walls with the uniform continuous distribution of surface charge. It is found that the sign of surface charge has a noticeable impact on the aqueous solution in a region close to the wall. The aqueous solution is more strongly layered and less translational and rotational diffusive in the positively charged nanochannel. But, the water molecules are more preferably oriented in the negatively charged nanochannel. The results have been consistently explained by microscopically analysing the forces acting atoms of water molecules and counter-ions, and the contributions into the torque acting on the water molecule.

High-strength paper enhanced by chitin nanowhiskers and its potential bioassay applications

In this paper, nanochitin was used as an alternative natural nanomaterial to combine with cellulose fibers for fabricating high-strength paper. Two typical chitin nanowhiskers having contrasting sign of surface charge were compared to evaluate the enhancement performance on paper in details. The results show that nanochitin with positive charges on the surface has a significant effect on the strength properties of the prepared paper, especially on wet strength. When the dosage of chitin nanowhiskers was 2%, the wet strength index was increased to 2.48 N·m/g, which is important for paper-based analytical devices with the common use in liquid analysis. Typical colorimetric glucose assays were successfully performed, suggesting the improved analytical performance on these prepared paper.

Variable surface charge of humic acid-ferrihydrite composite: Influence of electrolytes on ciprofloxacin adsorption

Antimicrobial compounds are found in a range of environments as pollutants. Here, we evaluated the influence of two common anions, NO3− and PO43-, on ciprofloxacin adsorption on humic acid/ferrihydrite composite (HA-DIG/Fh), synthetic ferrihydrite (Fh), and humic acid (HA-DIG) under controlled pH (7.0), ionic strength (0.1 M) and temperature (25 °C). All materials were characterized by isoelectric point (IEP), while the composite and the iron oxide were characterized by Mössbauer spectroscopy. Kinetic and isotherm adsorption studies were carried out using cyclic voltammetry (in KH2PO4) and square wave voltammetry (in KNO3). The application of kinetic models for both anions revealed Fh to fit to a pseudo second order model (R2 = 0.941); while HA-DIG (R2 = 0.950) and HA-DIG/Fh (R2 = 0.993) were fitted to pseudo first order models. The adsorption results showed a high dependency electrolyte, especially in Fh, where different shape curves (H-type in KNO3 and C-type in KH2PO4) and maximum experimental adsorbed amount Cm were observed. This finding is supported by the distinct IEP values and change in sign of surface charge between the two ions. Finally, results suggest that HA-DIG could be potentially used in environmental remediation to remove antibiotics from natural matrices, though the risk of antibiotic transportation increased with depth in the soil profile.

Reversal of Cation-Specific Effects at the Interface of Mica and Aqueous Solutions

Ion-specific effects are ubiquitous and have gained renaissance over the past few decades while remaining largely elusive. In this work, molecular dynamics simulations have been conducted to investigate the adsorption of different metal ions at the interface of mica and aqueous solutions, and cation-specific effects abide by the sequences of Na+ > K+ > Cs+ and Cs+ > K+ > Na+ for less and more charged surfaces, respectively. Mechanisms for cation-specific effects and reversal of Hofmeister series are then addressed on an atomic level. Hydration effect (i.e., interaction of metal ions with water) is the driving force for less charged surfaces, whereas interaction of metal ions with mica plays a larger role for more charged surfaces, which further result in a reversal of Hofmeister series. Clay minerals generally carry an abundance of negative charges, and the finding that Hofmeister series can be reversed with no change in the sign of surface charges provides new insights about related processes and ion-spe...

Anomalous electrokinetics at hydrophobic surfaces: Effects of ion specificity and interfacial water structure

We study the electrokinetic transport behavior of water molecules and ions in hydrophobic graphene nanochannels with variable surface charge densities as well as the interfacial water structure based on detailed molecular dynamics simulations. The interfacial water structure, described by the water density, hydrogen bonding, diffusion, distribution of the OH bond and dipole orientations, is strikingly influenced by the surface charge sign and density. We find anomalous electrokinetic effects which are related to the distribution of counterions close to the surface, ion-specific effects and the interfacial water structure. On a negatively charged graphene layer, the attraction of Na+ ions towards the surface enhances the interfacial friction. In contrast, if the surface is positively charged, high surface charge density triggers an anomalous enhancement of electroosmotic flow, accompanied by an abrupt change of the interfacial water structure. At high surface charge densities, the mobility of the interfacial water at the positively charged surfaces is suppressed more strongly compared to the negatively charged surface. Our results reveal new electrokinetic phenomena by the comparison of negatively and positively charged surfaces.

Boron Nitride Nanoporous Membranes with High Surface Charge by Atomic Layer Deposition.

In this work, we report the design and the fine-tuning of boron nitride single nanopore and nanoporous membranes by atomic layer deposition (ALD). First, we developed an ALD process based on the use of BBr3 and NH3 as precursors in order to synthesize BN thin films. The deposited films were characterized in terms of thickness, composition, and microstructure. Next, we used the newly developed process to grow BN films on anodic aluminum oxide nanoporous templates, demonstrating the conformality benefit of BN prepared by ALD, and its scalability for the manufacturing of membranes. For the first time, the ALD process was then used to tune the diameter of fabricated single transmembrane nanopores by adjusting the BN thickness and to enable studies of the fundamental aspects of ionic transport on a single nanopore. At pH = 7, we estimated a surface charge density of 0.16 C·m-2 without slip and 0.07 C·m-2 considering a reasonable slip length of 3 nm. Molecular dynamics simulations performed with experimental conditions confirmed the conductivities and the sign of surface charges measured. The high ion transport results obtained and the ability to fine-tune nanoporous membranes by such a scalable method pave the way toward applications such as ionic separation, energy harvesting, and ultrafiltration devices.

Asymmetry-induced electric current rectification in permselective systems.

For a symmetric ion permselective system, in terms of geometry and bulk concentrations, the system response is also symmetric under opposite electric field polarity. In this work we derive an analytical solution for the concentration distribution, electric potential, and current-voltage response for a four-layered system comprised of two microchambers connected by two permselective regions of varying properties. It is shown that any additional asymmetry in the system, in terms of the geometry, bulk concentration, or surface charge property of the permselective regions, results in current rectification. Our work is divided into two parts: when both permselective regions have the same surface charge sign and the case of opposite signs. For the same sign case we are able to show that the system behaves as a dialytic battery while accounting for field-focusing effects. For the case of opposite signs (i.e., bipolar membrane), our system exhibits the behavior of a bipolar diode where the magnitude of the rectification can be of order 10^{2}-10^{3}.

Topography-driven bionano-interactions on colloidal silica nanoparticles

We report here that the surface topography of colloidal mesoporous silica nanoparticles (MSNs) plays a key role on their bionano-interactions by driving the adsorption of biomolecules on the nanoparticle through a matching mechanism between the surface cavities characteristics and the biomolecules stereochemistry. This conclusion was drawn by analyzing the biophysicochemical properties of colloidal MSNs in the presence of single biomolecules, such as alginate or bovine serum albumin (BSA), as well as dispersed in a complex biofluid, such as human blood plasma. When dispersed in phosphate buffered saline media containing alginate or BSA, monodisperse spherical MSNs interact with linear biopolymers such as alginate and with a globular protein such as bovine serum albumin (BSA) independently of the surface charge sign (i.e. positive or negative), thus leading to a decrease in the surface energy and to the colloidal stabilization of these nanoparticles. In contrast, silica nanoparticles with irregular surface topographies are not colloidally stabilized in the presence of alginate but they are electrosterically stabilized by BSA through a sorption mechanism that implies reversible conformation changes of the protein, as evidenced by circular dichroism (CD). The match between the biomolecule size and stereochemistry with the nanoparticle surface cavities characteristics reflects on the nanoparticle surface area that is accessible for each biomolecule to interact and stabilize any non-rigid nanoparticles. On the other hand, in contact with variety of biomolecules such as those present in blood plasma (55%), MSNs are colloidally stabilized regardless of the topography and surface charge, although the identity of the protein corona responsible for this stabilization is influenced by the surface topography and surface charge. Therefore, the biofluid in which nanoparticles are introduced plays an important role on their physicochemical behavior synergistically with their inherent characteristics (e.g., surface topography).

Inversion of Hofmeister Series by Changing the Surface of Colloidal Particles from Hydrophobic to Hydrophilic

In this paper, we demonstrate that in addition to the nature of the ions the nature of the surface is also of vital importance in order to elucidate the origin of Hofmeister effects. Specifically, we show that for the solid/water interface when the surface turns from hydrophobic to hydrophilic, an inversion in the Hofmeister series occurs. Results were recorded from colloidal-stability experiments performed with seven different anions located at different positions in the Hofmeister series and working with four colloidal systems that varied in their surface-charge sign and hydrophobic/philic degree. A mechanism based on the structural modifications that ions and surfaces induced in water is proposed to explain all these results. The existence of hydration forces on hydrophilic systems enables us to explain the data and to reinforce our arguments concerning the relevance of considering the water structure around both the ions and the interfaces on Hofmeister effects.

Analysis of effect of electrolyte types on electrokinetic energy conversion in nanoscale capillaries

An analytical study on the effect of electrolyte types on the electrokinetic energy conversion is presented using nanoscale cylindrical capillary, which is either positively or negatively charged. The sign of surface charge determines the role and concentration magnitude of ions in the capillary and the energy conversion performance. Our study shows that the electrokinetic energy conversion performance (maximum efficiency, pressure rise and streaming potential) are approximately identical for 1:1 (KCl), 2:1 (CaCl2) and 3:1 (LaCl3) electrolytes when capillary is positively charged. For negatively charged capillary, energy conversion performance degrades significantly with the increase of counter-ion valence. For both positively and negatively charged capillaries, higher maximum efficiency can be resulted in low bulk concentration and surface charge density regimes. However, high maximum pressure rise generation for the pumping is found in the low bulk concentration and high surface charge density regimes. For the electric power generation, higher maximum streaming potential is found when both bulk concentration and surface charge density are low.

Zeta potential study of thermally treated dolomite samples in electrolyte solutions

In this paper, electrokinetic properties of raw dolomite and thermally treated dolomite have been investigated as function of concentrations dolomite of various electrolytes such as NaCl, KCl, MgCl 2, BaCl 2, AlCl 3 and Na 2HPO 4. The dolomitic solids such as D-600, D-800, D-900 and D-1000 were obtained further to calcinations at 600, 800, 900 and 1000 °C, respectively, during 2 h. From the electric potential results, it was found that the zeta potential of dolomite is not affected by the solid concentration. Isoelectric point of dolomite was determined at pH 6.8; H + and OH − are potential determining ions of dolomite. The monovalent cations are indifferent electrolytes for dolomitic solids samples, whereas MgCl 2 and BaCl 2 reverse the sign of surface charge of dolomite, while Na 2HPO 4 is potential determining ion of D-800, D-900 and D-1000.

Understanding the Role of Surface Charges in Cellular Adsorption versus Internalization by Selectively Removing Gold Nanoparticles on the Cell Surface with a I2/KI Etchant

This Letter presents a new method for differentiating the Au nanospheres attached to the cell surface from those being internalized into the cells. We introduced an etching solution based on I2 and KI that can selectively dissolve the Au nanospheres on the cell surface within a short period of time. The advantage of this etchant is its low toxicity to the cells because it is capable of etching away a relatively large amount of Au nanospheres at a low molar concentration. By combining with quantitative elemental analysis, we found that the deposition of Au nanospheres on the surface of cancer cells was highly dependent on the sign of surface charges on the Au nanospheres. In addition, by fitting the uptake data with a kinetic model, we were able to derive the overall and internalization rate constants for Au nanospheres and both of them were found to be governed by the surface charges on Au nanospheres.

Surface Force of Polystyrene Latex Particles in Aqueous Anionic Amphipathic Solutions

The surface forces between a polystyrene particle (negatively charged surface) of latex and a flat silica plate for several concentrations of aqueous solution of anionic amphipathic molecules (sodium dodecylsulfonate) were investigated using an atomic force microscope (AFM) colloidal probe method. In the lower concentration region approximately 1–2 mM, the surface force showed general repulsive profiles according to normal Derjaguin–Landau–Verwey–Overbeek (DLVO) theory between the surfaces having the same sign of surface charge under an electrolyte aqueous solution. While in the higher concentration region at less than the critical micelle concentration (cmc), the surface force showed attractive profiles. Findings suggest that the anionic amphipathic molecules adsorbed to the polystyrene particle in the higher concentration region and induced changes in the surface morphology and properties of the surface region. The change in the surface morphology is believed to be one of the origins of the attractive interaction.

Water Contact Angle On Polyelectrolyte‐Coated Surfaces: Effects of Film Swelling and Droplet Evaporation

The wettability by water of polyelectrolyte mono‐ and multilayers with different polycations or polyanions as the exposed layer is studied by contact angle measurements using the sessile drop technique. Measurements in a water‐saturated atmosphere and in ambient conditions [∼40% relative humidity (r.h.)] are made to study the effect of swelling and evaporation on the contact angle. It is found that these effects depend on the outermost layer of the polyelectrolyte coating. For a coating with Poly(styrene sulfonate) (PSS) as the exposed layer the water contact angle also indicates a change in the swelling state when the pre‐equilibrated film is directly in contact with liquid water. Droplet evaporation from polyelectrolyte coatings leads to a linear decrease of the contact angle with time. A significant enhancement of the droplet evaporation rate is found for a coating with Poly(diallyldimethylammonium bromide) (PDADMAC) as the exposed layer, which is attributed to the existence of a surface energy related driving force to the swelling of this polyelectrolyte coating. The studies show that a highly sophisticated interplay between hydrophobic backbone and charge density determines the wetting behavior, irrespective of the sign of surface charge.

CREEPING MOTION OF A BUBBLE WITH FIXED SURFACE CHARGE IN A UNIFORM ELECTRIC FIELD

Approximate solutions of the shape function and the electrophoretic movement of a bubble in an aqueous medium with ionized detergent and a small number of electrolytes under the influence of uniform electric field are derived. Specifically, a bubble that has the interface saturated and densely packed with detergent molecules is considered, and the charges were deemed to be fixed provided the position change among the detergent molecules is hindered relative to the free molecule. The surface tension at the interface is weakened with the addition of the detergent, and the deformation can be significant. The variation of the shape of the bubble was represented by the Legendre polynomial and the perturbed velocity profile was used to derive the drag force for the deformed bubble. The speed and size of the bubble was assumed to be very small so that the flow around it is in the creeping flow regime. The shape of the bubble with fixed surface charge undergoes a prolate deformation with axial symmetry around the axis parallel to the direction of the electric field and exhibits asymmetry with respect to the plane that goes through the center of the bubble and is perpendicular to the axis. But this asymmetry is not significant unless the surface charge density and the electric field are very large. Only symmetric deformation governs the movement, and, consequently, the drag force is not dependent on the sign of surface charge. As the bubble becomes more slender with deformation, the movement is enhanced.

Surface charge retention and enhanced polarization effect on ferroelectric thin films

The authors report surface charge retention and enhanced ferroelectric polarization effect on ferroelectric thin films by Kelvin probe force microscope (KFM). Since high electric field is induced at scanning probe microscope tip during domain switching and the sign of surface charge induced by this high electric field is always opposite to the sign of induced ferroelectric polarization charge, the surface potential of the KFM signal exhibits the relative potential difference between the potential of the ferroelectric polarization charge and the potential of the surface charge. After the discharge process of surface charge, the enhanced contribution from the ferroelectric polarization charge was observed. Similarly, the reduction of surface charge was observed as time passed and the surface potential indicating the ferroelectric polarization charge was enhanced due to this retention property of surface charge.