Electrical Double Layer Interaction Research Articles

Effect of nanoparticle aggregation at low concentrations of TiO2 on the hydrophilicity, morphology, and fouling resistance of PES–TiO2 membranes

This paper reports the fabrication and characterization of polyethersulfone–TiO2 (PES–TiO2) nanoparticle composite membranes made from synthesis casting solution consisting of various compositions of polymer solvents (DMF and EtOH) and TiO2 additive. The results also revealed that the membrane permeation and rejection rates, pore size, and porosity were dependent on the TiO2 and EtOH concentrations. Nanoparticles were characterized by zeta potential measurements, TEM observations, and measurement of particle size distributions. Zeta potential measurements in aqueous solution demonstrated that the TiO2 particles size is dominated by electric double layer interactions. Addition of EtOH promotes the increase of the clusters size as consequence of a double effect: reduction of the dielectric constant of solution and the depletion of the suspension field determined by the action of the polymer chains. The observed effects as result of EtOH addition and increase of TiO2 concentration were similar: both procedures provoked an increase of macrovoid dimensions. The modified membranes by TiO2 incorporation showed a structural change from a sponge-like to a finger-like structure. Strong correlations were observed between the hydrophilicity and the permeability of manufactured membranes. The formation mechanism of TiO2-blended membranes was altered, in a similar way, as result of EtOH at different contents of nanoparticles. Fouling resistance of modified membranes was significantly improved showing that EtOH addition is a suitable procedure for the membrane performance improvement. The rejection potential of membranes is hardly affected by the nanoparticles and EtOH incorporation into the polymeric solution.

A General Poisson−Boltzmann Model with Position-Dependent Dielectric Permittivity for Electric Double Layer Analysis

In this paper, we propose a general Poisson-Boltzmann model for electric double layer (EDL) analysis with the position dependence of dielectric permittivity considered. This model provides physically reasonable property profiles in the EDL region, and it is then utilized to investigate the depletion layer effect on EDL structure and interaction near hydrophobic surfaces. Our results show that both the electric potential and the interaction pressure between surfaces decrease due to the lower permittivity in the depletion layer. The reduction becomes more profound at larger variation magnitude and range. This trend is in general agreement with that observed from the previous stepwise model; however, that model has overestimated the influence of permittivity variation effect. For the thin depletion layer and the relative thick EDL, our calculation indicates that the permittivity variation effect on EDL usually can be neglected. Furthermore, our model can be readily extended to study the permittivity variation in EDL due to ion accumulation and hydration in the EDL region.

Dynamic Response of AFM Cantilevers to Dissimilar Functionalized Silica Surfaces in Aqueous Electrolyte Solutions

The dynamic response of an oscillating microcantilever with a gold-coated tip interacting with dissimilar functionalized silica surfaces was studied in electrolyte solutions with pH ranging from 4 to 9. Silica surfaces were chemically modified, yielding dissimilar surfaces with -Br, -NH(2), and -CH(3) functional group terminations. The relative hydrophobicity of the surfaces was characterized by contact angle measurements. The surface charge of the functionalized surfaces was first probed with commonly used static AFM measurements and serves as a reference to the dynamic response data. The amplitude and phase of the cantilever oscillation were monitored and used to calculate the effective interaction stiffness and damping coefficient, which relate to the electrical double layer interactions and also to distance-dependent hydrodynamic damping at the solid/water interface. The data for the dynamic response of the AFM over silica surfaces as a function of chemical functionalization and electrolyte pH show that the effective stiffness has a distinctive dependence on the surface charge of functionalized silica surfaces. The hydrodynamic damping also correlates strongly with the relative hydrophobicity of the surface. The data reported here indicate that interfacial properties can be strongly affected by changing the chemical composition of surfaces.

Exact Analytical Expressions for the Potential of Electrical Double Layer Interactions for a Sphere−Plate System

Exact, closed-form analytical expressions are presented for evaluating the potential energy of electrical double layer (EDL) interactions between a sphere and an infinite flat plate for three different types of interactions: constant potential, constant charge, and an intermediate case as given by the linear superposition approximation (LSA). By taking advantage of the simpler sphere-plate geometry, simplifying assumptions used in the original Derjaguin approximation (DA) for sphere-sphere interaction are avoided, yielding expressions that are more accurate and applicable over the full range of κa. These analytical expressions are significant improvements over the existing equations in the literature that are valid only for large κa because the new equations facilitate the modeling of EDL interactions between nanoscale particles and surfaces over a wide range of ionic strength.

An experimental study of slip considering the effects of non-uniform colloidal tracer distributions

Various studies have suggested that the no-slip condition may not hold for Newtonian liquids flowing over (for the most part) non-wetting surfaces. This paper describes an experimental study of steady Poiseuille flow at various Reynolds numbers up to 0.12 of four different aqueous monovalent electrolyte solutions through naturally hydrophilic and hydrophobically coated fused-silica channels with a depth of 33 μm. The slip lengths for these flows were estimated using a local method based on a new particle velocimetry technique that determines velocities at three different wall-normal distances within the first 400 nm next to the wall. These results are corrected using direct measurements of the near-wall particle distribution, which is highly non-uniform as expected due to repulsive electric double-layer interactions between the 100 nm tracer particles and the wall. In all cases, the slip lengths were not more than 23 nm and for all but one case, zero within their uncertainties. As illustrated here, the standard assumption of uniformly distributed tracers can significantly increase slip length estimates obtained using local methods and near-wall velocity data.

Effect of active control of electric potential of filter medium on depth filtration

The effect of active control of electric potential of filter medium on depth filtration was explored experimentally. Activated carbon particles were selected as the conductive filter medium. The filter medium potential was controlled with an external DC power supply. Kaolin particles were used as the model suspended solid. The activated carbon and kaolin had negative zeta potential around neutral pH. When the filter medium potential against a counter electrode was greater than or equal to + 0.2 V, rejection rate of particles was 1.8 times higher than that when the potential was less than + 0.2 V. Thus adsorption of particles by interaction of electric double layers was enhanced by maintaining a positive charge on the filter media. Desorption of kaolin trapped on the filter media was also confirmed by changing the filter medium potential from positive to negative. The percentage of kaolin particles desorbed was 11% of the kaolin trapped on the filter media. The desorption rate was not high, but this technique will enhance refreshment of the filter media when combined with back washing.

Acoustic-Wave-Induced Analyte Separation in Narrow Fluidic Confinements in the Presence of Interfacial Interactions

In the present work, we attempt to analyze the influences of acoustic forces, in conjunction with the intrinsic electrokinetic effects as well as the near-wall attractive and repulsive forces, on the transport and size-based separation of charged analytes in a background-pressure-driven flow through a narrow fluidic confinement. By executing a regular perturbation analysis, we establish that the speed of traverse and the extent of spreading (dispersion) of the analyte bands is effectively determined by the ratio of particle to channel heights, channel height relative to the Debye length, and all other significant acoustic and nonacoustic parameters. These factors in tandem may dictate the analyte separation characteristics (quantified by the resolution of separation), in tune with the particular harmonic of the acoustic wave, the strengths of the induced electrical double layer fields, and the van der Waals interaction mechanisms. We quantitatively pinpoint the relationship between the harmonics to be employed and particle sizes to be separated. Our study reveals that there is a critical channel height beyond which the acoustic effects may effectively mask the near-wall interactions and below which the transverse migrative influences induced by the walls may influence the separation characteristics in a rather profound manner. The results implicate an interesting high-efficiency separation regime that can be obtained with a judicious combination of the background flow, energy intensity of the acoustic effects, and induced electrical double layer interactions.

Electric double layers interactions under condition of variable dielectric permittivity

In this paper, a theoretical method has been developed for the electric double layer interaction under condition of the variable dielectric permittivity of water. Using Poisson-Boltzmann equation (PBE), for one plate and two plates having similar or dissimilar constant charge or constant potential, we have investigated the electric double layer potential, its gradient and the disjoining pressure as well as the effect of variation of dielectric permittivity on these parameters. It has been assumed that plates are separated by a specific distance and contain a liquid solution in between. It is shown that reduction of the dielectric permittivity near the interfaces results in compression of electric double layers and affects the potential and its gradient which leads to a decreased electrostatic repulsion. In addition, it is shown that variation of dielectric permittivity in the case of higher electrolyte concentration, leads to a greater change in potential distribution between two plates.

Interpreting the Dynamic Interaction between a Very Small Rising Bubble and a Hydrophilic Titania Surface

We interpret recent measurements of time-varying interference fringe intensities observed between rising bubbles (diameter 15−120 μm) and a horizontal hydrophilic titania plate to determine the rate of approach of such bubbles to the surface. The bubbles remain spherical because of high Laplace pressures and small buoyancy forces so the approach velocity remains in the Stokes flow regime. The rate of approach of the bubble to the titania plate is controlled by buoyancy force and surface forces such as van der Waals and electric double layer interactions together with a hydrodynamic force arising from the drainage of the thin water film between the bubble and the titania surface. The dynamics are found to be consistent with the no-slip hydrodynamic boundary condition at the surface of the bubble and at the titania plate. However, far from the titania plate the terminal velocities of the rising bubbles suggests that the bubble surfaces are fully mobile with zero tangential stress.

Energy Transfer through Streaming Effects in Time-Periodic Pressure-Driven Nanochannel Flows with Interfacial Slip

We analytically investigate the prospect of using electrokinetic phenomena to transfer hydrostatic energy to electrical power with high energy transfer efficiencies, by exploiting time periodic pressure-driven flows in narrow fluidic confinements. An expression for the energy transfer efficiency for such pulsating pressure-driven flows is derived by considering wall-slip effects due to hydrophobic interactions, strong electrical double layer interactions in the confined flow passages, possibilities of exploring the regimes of large wall potentials, and the adverse consequences of the finite conductance of the Stern layer. It is revealed from our studies that high-frequency pressure pulsations may be employed in practice to improve the concerned energy transfer efficiency to a considerable extent, instead of using a steady-state pressure field. Such favorable effects are found to be best exploited by utilizing "slipping" electro-hydrodynamics in thick electrical double layers in the presence of high surface potentials.

Low Viscosity Highly Concentrated Injectable Nonaqueous Suspensions of Lysozyme Microparticles

Subcutaneous injection of concentrated protein and peptide solutions, in the range of 100-400 mg/mL, is often not possible with a 25- to 27-gauge needle, as the viscosity can be well above 50 cP. Apparent viscosities below this limit are reported for suspensions of milled lysozyme microparticles up to nearly 400 mg/mL in benzyl benzoate or benzyl benzoate mixtures with safflower oils through a syringe with a 25- to 27-gauge needle at room temperature. These apparent viscosities were confirmed using a cone-and-plate rheometer. The intrinsic viscosity regressed from the Kreiger-Dougherty model was only slightly above the Einstein value of 2.5, indicating the increase in viscosity relative to that of the solvent was caused primarily by the excluded volume. Thus, the increases in viscosity from electrical double layer interactions (electroviscous effects), solvation of the particles, or deviations of the particle shape from a spherical geometry were minimal, and much smaller than typically observed for proteins dissolved in aqueous solutions. The small electroviscous effects are expected given the negligible zeta potential and thin double layers in the low dielectric constant organic solvent. The suspensions were resuspendable after a year, with essentially constant particle size after two months as measured by static light scattering. The lower apparent viscosities for highly concentrated protein suspensions relative to protein solutions, coupled with these favorable characteristics upon resuspension, may offer novel opportunities for subcutaneous injection of therapeutic proteins.

Interaction of Electrical Double Layers Between Oppositely Charge Particles in Variable-Charge Soils as Related to Source to Salt Adsorption

The interaction of electrical double layers between oppositely charged particles of variable-charge soils collected from southern China was investigated by means of column leaching experiments. The results showed that cations and anions of indifferent electrolytes such as NaCl and NaNO3 were simultaneously adsorbed by some of the soils during leaching experiments. The number of the ions released was shown to be less than the number of the ions adsorbed. This probably led to the decrease in the electrical conductivity (EC) of the leachate at the initial stage of the leaching experiments. When the soil with adsorbed salt was washed with distilled water, both cations and anions adsorbed were immediately removed from the soil particle surfaces. This phenomenon is attributed to the overlapping of the diffuse layers of the electrical double layers between positively charged iron (Fe)/aluminum (Al) oxides and negatively charged phyllosilicates of the variable-charge soils. The extent of interaction between oppositely charged particles was shown to be directly related to the EC and the content of free Fe/Al oxides in the soils. The lower soil EC and higher content of soil free Fe/Al oxides appeared to cause a stronger interaction between oppositely charged particles.

Dynamic Forces between Bubbles and Surfaces and Hydrodynamic Boundary Conditions

A bubble attached to the end of an atomic force microscope cantilever and driven toward or away from a flat mica surface across an aqueous film is used to characterize the dynamic force that arises from hydrodynamic drainage and electrical double layer interactions across the nanometer thick intervening aqueous film. The hydrodynamic response of the air/water interface can range from a classical fully immobile, no-slip surface in the presence of added surfactants to a partially mobile interface in an electrolyte solution without added surfactants. A model that includes the convection and diffusion of trace surface contaminants can account for the observed behavior presented. This model predicts quantitatively different interfacial dynamics to the Navier slip model that can also be used to fit dynamic force data with a post hoc choice of a slip length.

Effect of Solution Concentration, Surface Bias and Protonation on the Dynamic Response of Amplitude-Modulated Atomic Force Microscopy in Water

The dynamic response of amplitude-modulated atomic force microscopy (AM-AFM) is studied at the solid/water interface with respect to changes in ionic concentration, applied surface potential, and surface protonation. Each affects the electric double layer in the solution, charge on the tip and the sample surface, and thus the forces affecting the dynamic response. A theoretical model is developed to relate the effective stiffness and hydrodynamic damping of the AFM cantilever that is due to the tip/surface interaction with the phase and amplitude signals measured in the AM-AFM experiments. The phase and amplitude of an oscillating cantilever are measured as a function of tip-sample distance in three experiments: mica surface in potassium nitrate solutions with different concentrations, biased gold surface in potassium nitrate solution, and carboxylic acid-terminated self-assembled monolayers (SAMs) on gold in potassium nitrate pH buffers. Results show that, over the range where the higher harmonic modes of the oscillation are negligible, the effective stiffness of the AFM cantilever increases to a maximum as the tip approaches the surface before declining again as a result of the repulsive electrical double layer interaction. For attractive electrical double-layer interactions, the effective stiffness declines monotonically as the tip approaches the surface. Similarly, the hydrodynamic damping of the tip increases and then decreases as the tip approaches the solid/water interface, with the magnitude depending on the species present in the solution.

Transport and Separation of Charged Macromolecules under Nonlinear Electromigration in Nanochannels

In this work, we theoretically investigate the implications of nonlinear electrophoretic effects on the transport and size-based separation of charged macromolecules in nanoscale confinements. By employing a regular perturbation analysis, we address certain nontrivial features of interconnection among wall-induced transverse migrative fluxes, electrophoretic and electroosmotic transport, confinement-induced hindered diffusive effects, and hydrodynamic interactions in detail. We demonstrate that there occurs an optimal regime of influence of the nonlinear electrophoretic effects, within which high values of separation resolution may be achieved. This size-based optimal regime, however, can be effectively exploited only for nanochannel flows, as attributed to the strong electric double layer interactions prevalent within the same.

Electrical double layers’ interaction between oppositely charged particles as related to surface charge density and ionic strength

Phyllosilicates with net negative surface charge and Fe/Al oxides with net positive surface charge co-exist in variable charge soils and the interaction between these oppositely charged particles affects the stability of mixed colloids, aggregation, and even the surface chemical properties of variable charge soils. The overlapping of the diffuse layers of electrical double layers between kaolinite and goethite, hematite, gibbsite and γ-Al 2O 3 was investigated in this article based on the change of zeta potential and electrokinetic charge density induced by the addition of Fe/Al oxides. The results indicated that in the range of pH from 3 to 7, the kaolinite is negatively charged and the Fe/Al oxides are positively charged, and the overlapping of the diffuse layers led to the increase in zeta potential and the decrease in surface charge density of binary-system containing kaolinite and Fe/Al oxides compared with the single kaolinite system. The presence of gibbsite resulted in the strongest interaction of electrical double layers between this and kaolinite, followed by goethite and hematite. The interaction extent of electrical double layers between kaolinite and Fe/Al oxides increased with the amount of Fe/Al oxides added. The interaction also resulted in the increase in IEP of the binary-system containing kaolinite and Fe/Al oxides. The increase in ionic strength led to the decrease in the thickness of colloid diffuse layer and thus resulted in the reduction of overlapping of the diffuse layers between kaolinite and Fe/Al oxides.

Study on Interaction Between Two Parallel Plates with Iteration Method in Functional Theory

By introducing the functional theory into the calculation of electric double layer (EDL) interaction, the interaction energies of two parallel plates were calculated respectively at low, moderate, and high potentials. Compared with the results of two existing methods, Debye-Huckel and Langmuir methods, which are applicable just to the critical potentials and perform poorly in the intermediate potential, the functional approach not only has much simpler expression of the EDL interaction energy, but also performs well in the entire range of potentials.

Electrokinetic separation of charged macromolecules in nanochannels within the continuum regime: Effects of wall interactions and hydrodynamic confinements

In this paper, a detailed continuum-based theoretical model is proposed to investigate the effects of near-wall potentials and hydrodynamic confinement on separation of charged macromolecules in channels of nanoscale dimensions. These wall effects are primarily confined within a few nanometers from the channel wall, and hence have negligible influences in the conventional electrokinetic separation methods that are routinely performed in microchannels. However, in nanochannels, their zone of influence becomes significant in comparison to the channel height, thereby inducing certain nontrivial effects on the resultant separation characteristics. By executing a regular perturbation analysis, it is established that depending on the macromolecular size relative to the channel height and the extent of electrical double layer (EDL) interactions, the wall forces decide the speed of traverse and the extent of spreading (dispersion) of the macromolecular bands. These factors combine together to finally decide the separation characteristics (quantified by the resolution of separation) of the charged macromolecules in nanochannels. It is demonstrated that because of the near-wall effects, macromolecular pairs with less disparities in sizes give rise to higher values of resolution. Moreover, the wall-induced influences are shown to magnify the resolution for any given pair of macromolecules in the nanofluidic systems, thereby signifying greater separation efficiency.

Electric double layer and electrostatic interaction of hydrophobic particles

An hypothesis regarding the impact of water density near hydrophobic surfaces on the electrostatic component of their interaction was offered. A theoretical model of the electric double layer and the interparticle interaction under conditions of the variable density and, consequently, variable dielectric permittivity of water has been developed. It was shown that reduction of the dielectric permittivity near interfaces determined by their hydrophobicity resulted in compression of double electrical layers and weakening of their overlapping. This, in its turn, results in reduction of the electrostatic repulsion of hydrophobic disperse particles as compared with nonhydrophobic ones.

Selective adsorption of bacterial cells onto zeolites

Zeolites adsorb microbial cells on their surfaces and selective adsorption for specific microorganisms was seen with certain zeolites. Tests for the adsorption ability of zeolites were conducted using various established microbial cell lines. Specific cell lines were shown to selectively absorb to certain zeolites, species to species. In order to understand the selectivity of adsorption, we tested adsorption under various pH conditions and determined the zeta-potentials of zeolites and cells. The adsorption of some cell lines depended on the pH, and some microorganisms were preferentially adsorbed at acidic pH. The values of zeta-potentials were used for calculating the electric double layer interaction energy between zeolites and microbial cells. There was a correlation between the experimental adsorption results and the interaction energy. Moreover, we evaluated the surface hydrophobicity of bacterial cells by using the microbial adherence to hydrocarbon (MATH) assay. In addition, we also applied this method for zeolites to quantify relative surface hydrophobicity. As a result, we found a correlation between the adsorption results and the hydrophobicity of bacterial cells and zeolites. These results suggested that adsorption could be explained mainly by electric double layer interactions and hydrophobic interactions. Finally, by using the zeolites Na-BEA and H-Y, we succeeded in clearly separating three representative microbes from a mixture of Escherichia coli, Bacillus subtilis and Staphylococcus aureus. Zeolites could adsorb each of the bacterial cell species with high selectivity even from a mixed suspension. Zeolites can therefore be used as effective carrier materials to provide an easy, rapid and accurate method for cell separation.