Electrokinetic Properties Research Articles

Enabling the characterization of the nonlinear electrokinetic properties of particles using low voltage.

Insulator-based electrokinetically driven microfluidic devices stimulated with direct current (DC) voltages are an attractive solution for particle separation, concentration, or isolation. However, to design successful particle manipulation protocols, it is mandatory to know the mobilities of electroosmosis, and linear and nonlinear electrophoresis of the microchannel/liquid/particle system. Several techniques exist to characterize the mobilities of electroosmosis and linear electrophoresis. However, only one method to characterize the mobility of nonlinear electrophoresis has been thoroughly assessed, which generally requires DC voltages larger than 1000 V and measuring particle velocity in a straight microchannel. Under such conditions, Joule heating, electrolysis, and the DC power source cost become a concern. Also, measuring particle velocity at high voltages is noisy, limiting characterization quality. Here we present a protocol-tested on 2 μm polystyrene particles-for characterizing the mobility of nonlinear electrophoresis of the liquid/particle system using a DC voltage of only 30 V and visual inspection of particle dynamics in a microchannel featuring insulating obstacles. Multiphysics numerical modelling was used to guide microchannel design and to correlate particle location during an experiment with electric field intensity. The method was validated against the conventional characterization protocol, exhibiting excellent agreement while significantly reducing measurement noise and experimental complexity.

Dynamic Light Scattering of DNA-Ligand Complexes.

Dynamic light scattering (DLS) enables the characterization of sizes and electrokinetic properties of colloids, polymers, and macromolecules. DNA is a charged semiflexible polyelectrolyte that is condensed or compacted by counterions, proteins, and other condensing agents in processes such as chromosome compaction and gene therapeutic applications. DNA condensation is closely related to charge screening since packaging requires effective neutralization of its surface negative charges. In this chapter, we describe in detail the protocol for DLS DNA-ligand complexes. As an example, we describe data for the condensation of DNA by chitosan and the measurement of size, zeta potential, and electrophoretic mobility of the DNA-ligand complex by DLS.

INFLUENCE OF CLAY MINERAL ADDITIVES ON THE PROPERTIES OF COMPOSITE WATER–COAL FUEL.

The study aimed to examine the electrokinetic and rheological properties of composite water-coal fuel (CWCF) based on anthracite and P-grade coal in the presence of clay mi­nerals. The influence of clay mineral additives on the electrokinetic and rheological properties of CWCF was studied. The clay minerals used were montmorillonite, illite, and kaoli­nite from Ukrainian deposits. The fuel was prepared based on P-grade coal and anthracite. The share of the solid phase (coal) in the suspensions was 62 %. According to the particle size distribution, the bulk of the coal has a size of 30–110 μm with a bimodal distribution. To obtain the CWCF, the coal powder was homogenized with the dispersion medium by wet grinding in a ball mill for 20–25 minutes. The dispersion medium was distilled water. Wet grinding was carried out until particles < 0.25 mm in size were obtained. We obtained a CWCF with a concentration of 62% of the so­lid phase of coal. Additions of chemicals were not practiced to study the effect of clay mine­rals in a pure experiment. Clay minerals were added at a concentration of 0.5–3 % by weight of the CWCFs. Their rheological data were measured using a Rheotest-2 rheometer at a temperature of 20 ºC and the shear-stress/shear-rate data for the CWCF covered a range of 0.1 to 470 s-1. The sedimentation stability of St was stu­died at the time of stratification of the CWCF sample in a measuring cylinder. After preparation, a 10-mL sample of the CWCF was placed in a 25-mL glass cylinder. St is equal to the time of the beginning of the suspension stratification in days. It was established that ξ - the potential of CWCF particles with the addition of clay mine­rals shifts to the negative region and reaches – 45–50 mV at pH>8. Montmorillonite was found to be most effective additive for controlling the properties of the CWCF. Systems with clay mineral additives demonstrate a pseudoplastic type of flow in the range of shear rates γ 0–100 s-1 and a Newtonian type of flow at higher γ. According to their effect on the increase in apparent viscosity, clay mine­rals are arranged in the following order: kaolinite, illite, and montmorillonite. The thixotropic properties of the studied systems provide complete restoration of the properties of the CWCF when the shear rate is reduced to 0–40 s-1. Sedimentation stability in the presence of illite and montmorillonite additives reaches 8–14 days. In contrast to illite and montmorillonite, kaolinite has a very weak effect on the properties of CWCF.

Influence of composite flocculants on electrokinetic properties and electroosmotic vacuum preloading behavior of sludge

In recent years, dredging projects in rivers and lakes have generated large volumes of sludge that exhibit high water content and low permeability. This dredged sludge needs to be treated quickly to reduce its volume, thus reducing transportation costs and environmental impacts. Flocculation combined with electroosmotic vacuum preloading is a new technology for dewatering sludge. At present, the influence of flocculants on the electrokinetic properties of sludge has not been thoroughly studied, and composite forms of these have not been applied in electroosmotic vacuum precompression. Therefore, inorganic flocculant and organic flocculant were combined to form a composite flocculant, which was used in electroosmosis vacuum preloading to increase the water discharge effect of dredged sludge. Based on analyzing the mechanism of flocculants, two kinds of composite flocculants, PAC-APAM and FeCl3-APAM, were configured, and the optimal ratio of inorganic flocculant and organic flocculant in the composite flocculant was determined with a settling column test. The properties, including pore size distribution, electric conductivity, and electroosmotic permeability coefficient, of the sludge mixed with flocculants PAC, FeCl3, APAM, PAC-APAM, and FeCl3-APAM were analyzed by NMR tests and Miler Soil Box tests. In addition, a model test of electroosmotic vacuum preloading treatment of dredged sludge was conducted, and the influences of different types of flocculants on the surface settlement, water discharge, current, pore water pressure, and shear strength of the sludge were compared and analyzed. The results showed that the composite flocculants suggested in this study were able to increase the electroosmotic permeability coefficient of sludge more significantly than single-type flocculants. In situations where electroosmotic vacuum preloading combined with composite flocculants was used to treat sludge, water discharge, and consolidation settlement were larger, and the shear strength of the treated sludge was higher. Compared with FeCl3-APAM, PAC-APAM showed better performance in electroosmotic vacuum preloading.

Development of calcium phosphate-chitosan composites with improved removal capacity toward tetracycline antibiotic: Adsorption and electrokinetic properties

Two eco-friendly and highly efficient adsorbents, namely brushite-chitosan (DCPD-CS), and monetite-chitosan (DCPA-CS) composites were synthesized via a simple and low-cost method and used for tetracycline (TTC) removal. The removal behavior of TTC onto the composite particles was studied considering various parameters, including contact time, pollutant concentration, and pH. The maximum TTC adsorption capacity was 138.56 and 112.48 mg/g for the DCPD-CS and DCPA-CS, respectively. Increasing the pH to 11 significantly enhanced the adsorption capacity to 223.84 mg/g for DCPD-CS and 205.92 mg/g for DCPA-CS. The antibiotic adsorption process was well-fitted by the pseudo-second-order kinetic and Langmuir isotherm models. Electrostatic attractions, complexation, and hydrogen bonding are the main mechanisms governing the TTC removal process. Desorption tests demonstrated that the (NH4)2HPO4 solution was the most effective desorbing agent. The developed composites were more efficient than DCPD and DCPA reference samples and could be used as valuable adsorbents of TTC from contaminated wastewater.

Predicting streaming potential in reactive media: the role of pore geometry during dissolution and precipitation

SUMMARY Dissolution and precipitation processes modify the structure of the porous media at microscale which significantly affects the macroscopic properties of the media. These variations in the pore geometry result in changes in the hydraulic properties that control the groundwater flow, and also modify the electrokinetic properties associated to the displacement of electrical charges carried by the flow which originates the streaming potential. Under the hypothesis of a uniform dissolution or precipitation of the pores and based on the effective excess charge density approach, we present a physically based theoretical model for estimating the effective excess charge density as a function of time. The model is based on the assumption that the pore structure can be represented by an ensemble of capillary tubes with a smooth periodic variation of their radius and a fractal pore size distribution. The analytical expressions obtained to describe the effective excess charge density depend on the chemical parameters of the fluid and the petrophysical properties of the medium. In addition, the periodic variations assumed in the pore geometry represent a more realistic description of a porous medium than considering the pores as constant radii capillaries. These irregularities allow us to include the hysteresis phenomenon in the electrokinetic properties. The expressions of the proposed model have been tested with experimental data consisting of sets of effective excess charge density-effective saturation, permeability-effective saturation, porosity-time and permeability-time values. In all cases, the model is able to satisfactorily reproduce the behaviour of the data.

Modification of stability properties of yttrium(III) oxide particles by simultaneous adsorption of poly(acrylic acid) and poly(ethylene glycol) for possible applications in wastewater treatment

The adsorption, electrokinetic and stability properties of yttrium(III) oxide—Y2O3 in mixed solutions of macromolecular compounds were investigated. The interfacial behavior of poly(acrylic acid)-PAA and poly(ethylene glycol)—PEG in single and binary systems was examined in the pH range 3–10. The polymers used were characterized by different ionic nature—the PAA is an anionic polymer, whereas PEG belongs to the group of non-ionic polymers. Based on the results obtained, the most probable mechanisms for the binding of PAA and PEG macromolecules on the yttrium(III) oxide surface were proposed. In addition, the analysis of adsorption and electrokinetic data enabled explanation of the obtained changes in the stability of Y2O3 suspensions without and in the presence of PAA or/and PEG, as well as determination of the specific stabilization-destabilization mechanisms of the studied systems. It was shown that yttrium(III) oxide modification by mixed adsorption layers of both polymers with different ionic character changes considerably the surface and stability properties of the examined solid suspensions.Graphical

Electrokinetic properties of healthy and β-thalassemia erythrocyte membranes under in vitro exposure to static magnetic field

Introduction: The current understanding of the biological impacts of a static magnetic field (SMF) is restricted to the direct interactions of the magnetic field with biological membranes. The electrokinetic (zeta) potential is an electrochemical property of erythrocyte surfaces which was negatively charged in physiological media after SMF exposure (0.1‒2.0 T).Methods: The novel data about electrokinetic parameters of the erythrocytes is determined by microelectrophoresis after SMF-exposure in norm and heterozygous β-thalassemia. The methods of light scattering, lipid peroxidation, fluorescence microscopy are used.Results: The electrokinetic potential of erythrocytes in norm is increased after SMF intensities due to enhanced negatively exposed charges on the outer surface of the membrane accompanied by an increase in light scattering where changes in cell morphology are observed. Conversely, a decrease in the zeta potential of β-thalassemia erythrocytes upon SMF-treatment was determined because of the reduction in the surface electrical charge of the membranes, where a significant decrease in light scattering at 1.5 T and 2.0 T was recorded. Exposure to SMF (0.5–2.0 T) was associated with an increase in the malondialdehyde content in erythrocytes. Biophysical studies regarding the influence of SMF on the electrostatic free energy of cells shows an increase in negative values in healthy erythrocytes, which corresponds to the implementation of a spontaneous process. This is also the process in β-thalassemia cells after SMF exposure with lower negative values of free electrostatic energy than erythrocytes in norm.Discussion: The effect of static magnetic field (SMF 0.1–2.0 T) on the electrokinetic and morphological characteristics of erythrocytes in norm and β-thalassemia is determined and correlated with the increase/reduction in surface charge and shrinkage/swelling of the cells, respectively. Lipid peroxidation of healthy and β-thalassemia erythrocytes caused an enhancement of lipid peroxidation because of the higher concentrations of TBARS products in cellular suspension. SMF (0.1‒2.0 T) altered the spontaneous chemical processes with negative values of electrostatic free energy of erythrocytes in norm and β-thalassemia accompanied by a lower FITC-Concanavalin A binding affinity to membrane receptors (SMF 2.0 T). The electrokinetic properties of human erythrocytes in norm and β-thalassemia upon SMF treatment and their interrelationship with the structural-functional state of the membrane were reported. The presented work would have future fundamental applications in biomedicine.

Competitive adsorption between sodium citrate and naphthenic acids on alumina surfaces: Experimental and computational study

The adsorption of naphthenic acids (NAs) on clay surfaces induces an increase in surface hydrophobicity, which is detrimental to crude oil recovery and froth quality. Sodium citrate (Na3Cit), with more functional groups than NAs, is expected to compete with NAs for adsorption on mineral surfaces. The adsorption of NAs on alumina surfaces was quantified in the presence and absence of Na3Cit at pH 8 ± 0.1. An increase in hydrophobicity of the alumina surface was observed after the adsorption of NAs without Na3Cit. However, adding Na3Cit significantly reduced NAs adsorption and protected the surface wettability. The viscoelasticity and electrokinetic properties of adsorbed materials indicate Na3Cit was preferentially covered on alumina surfaces and repelled NAs by its negative charges. The competition between Na3Cit and NAs for adsorption sites was explained using ab initio Car-Parrinello molecular dynamics (CPMD) simulation and periodic plane-wave density functional theory (DFT) to calculate inner-sphere adsorption and outer-sphere adsorption, respectively. Favourable adsorption of sodium citrate than NAs representatives on aluminum (hydr)oxide cluster and the basal and edge planes of gibbsite were observed from the perspective of free energy barrier and adsorption energy, respectively.

Theoretical Interpretation of pH and Salinity Effect on Oil-in-Water Emulsion Stability Based on Interfacial Chemistry and Implications for Produced Water Demulsification

The petroleum industry produces thousands of barrels of oilfield waters from the initial stage driven by primary production mechanisms to the tertiary stage. These produced waters contain measurable amounts of oil-in-water emulsions, the exact amounts being determined by the chemistry of the crude oil. To meet strict environmental regulations governing the disposal of such produced waters, demulsification to regulatory permissible levels is required. Within the electric double layer theory, coupled with the analytical solutions to the Poisson–Boltzmann Equation, continuum electrostatics approaches can be used to describe the stability and electrokinetic properties of emulsions. In the literature, much of the surface charge density and zeta potential relationship to emulsion stability has been confined to systems with less salinity. In this paper, we have exploited the theoretical foundations of the electric double layer theory to carry out theoretical evaluations of emulsion salinity based on zeta potential and surface charge density calculations. Most importantly, our approaches have enabled us to extend such theoretical calculations to systems of the higher salinity characteristic of oil-in-water emulsions found in oilfield-produced waters, based on crude oil samples from the literature with varying surface chemistry. Moreover, based on the definition of acid crude oils, our choice of samples represents two distinct classes of crude oils. This approach enabled us to evaluate the stability of emulsions associated with these produced oilfield waters in addition to predicting the potential of demulsification using demulsifiers. Given that the salinity range of this study is that encountered with the vast majority of produced oilfield waters, the findings from our theoretical predictions are perfect guides as far as emulsion stability is concerned.

Active Cellulose Acetate/Chitosan Composite Films Prepared Using Solution Blow Spinning: Structure and Electrokinetic Properties.

Cellulose acetate (CA), a very promising derivative of cellulose, has come into the focus of research due to its highly desired good film-forming ability for food packaging applications. Frequently, this derivative is used in combination with other compounds (polymers, nanoparticles) in order to obtain active materials. Here, we report the preparation of thin films made of cellulose acetate loaded with chitosan (CS) using the solution blow spinning (SBS) method. Films are prepared by SBS processing of the polymers mixture solution, considering the following variables: (i) the concentration of cellulose acetate and chitosan in the solution and (ii) the solvent system consisting of acetic or formic acid. The prepared materials are characterized in terms of physical properties, roughness (optical profilometer), porosity, wettability (contact angle measurements), chemical structure (Fourier transform infrared spectrometry), and electrokinetic properties (zeta potential). SBS enables the preparation of CA/CS films with high water vapor permeability, high porosity, and also higher water contact angle compared with pure CA films. The electrokinetic properties of composites are influenced by the inclusion of chitosan, which causes a shift of the isoelectric point (IEP) towards higher pH values, but the magnitude of the shift is not in correlation with chitosan concentration. Adsorption kinetic studies using bovine serum albumin (BSA) as a model protein reveal that chitosan modified cellulose acetate films manifest low affinity towards proteins that suggests prevention of biofilm formation on its surface.

Induced charge electro-osmotic mixing performance of viscoelastic fluids in microchannels with an electrically conductive plate

Micromixers have important applications in lab-on-a-chip, biomanufacturing, and chemical engineering. In this study, a micromixer with a conductive barrier plate based on the induced charge electro-osmosis is proposed. The Oldroyd-B constitutive model was chosen to characterize the flow characteristics of viscoelastic fluids, and the Poisson–Boltzmann model was used to characterize the electrokinetic properties. The effects of the installation of the conductive plate, the concentration of the polymer, and the shape of the conductive plate on the mixing were studied based on the finite volume method. The mixing efficiency of the viscoelastic fluids is 78.3% when a non-conductive plate is placed in the micromixer. However, placing a conductive plate increases the mixing efficiency to 89.8%. As the polymer concentration increases, the mixing efficiency increases, which is attributed to the elastic instability. As the curvature of the conductive plate increases from 0° to 360°, the mixing efficiency of the Newtonian fluid increases by 2.82%, while that of the polyacrylamide solutions at concentrations of 100 and 250 ppm increases by 5.31% and 1.97%, respectively.

Effect of Magnetite Content and Specificity of Nickel(II) Ions on Electrokinetic Properties of Composites Based on Porous Silica Particles

Effect of Magnetite Content and Specificity of Nickel(II) Ions on Electrokinetic Properties of Composites Based on Porous Silica Particles

Development of super-hydrophobic surface using Tio2 to reduce adverse effects of environmental pollutants on buildings

The development of self-cleaning surfaces in recent years has been significant in preserving buildings from environmental contaminants and improving visual durability. Titanium dioxide (or titania), a photocatalyst, is used with cement to improve aesthetic durability and de-pollution properties. Super hydrophilic materials were developed by covering glass, ceramic tiles, or polymers with the semiconducting photocatalyst titanium dioxide (TiO2). Grease, grime, and organic pollutants disintegrate when TiO2 is exposed to light and are simply swept away by water (rain). Ink and vehicle smoke are utilised to evaluate the self-cleaning properties of TiO2-containing concretes, and the oxidation of carbons (COx), gaseous atmospheric pollutants responsible for acid rain and photochemical smog, is explored using nano titanium dioxide. The research allowed for the identification of photo-oxidation mechanisms of stain and COx on TiO2-containing cement, as well as parallels and differences with pure unsupported TiO2. Engineering TiO2 particles with certain crystallite sizes and modifying the photocatalyst electrokinetic properties by changing the overall pKa of the TiO2 surface have both been shown to be efficient approaches for controlling the attractive features of ion-ion correlation forces. The effects on applications in a concrete building, specifically on the exterior surface, to improve overall photocatalytic activity are reviewed. The experimental data on photocatalytic performances measured for regular and nano-coated samples are examined regarding photocatalyst qualities and the effect of cement's chemical environment on titania particles. The research allowed for the identification of photo-oxidation mechanisms of stain and COx on TiO2-containing cement, as well as parallels and differences with pure unsupported TiO2.

Temperature controlled adsorption of polyethyleneimine on the sepiolite surface

The effects of temperature (15, 25 and 35 °C) on the adsorption properties of polyethyleneimine (PEI) on the sepiolite (Sep) surface and some properties of the Sep/PEI suspensions including their stability were studied. It was shown that the increase in temperature caused larger adsorption of PEI macromolecules on the Sep surface, which was the effect of the spacious PEI conformation at higher temperatures. Stability of the Sep/PEI suspension was also better when temperature was higher due to effective electrosteric stabilization. The obtained results indicate how the temperature influences the textural and electrokinetic properties of the Sep/PEI systems. The following analytical methods: UV–Vis spectrophotometry, XRD, nitrogen adsorption-desorption method, elemental analysis, SEM-EDX, FT-IR, TG-DSC, viscometry and zeta potential measurements were used in the performed research. The presented findings can be successfully used to create hybrid materials dedicated to water and wastewater treatment as well as in the gas capture processes.

Physico-Chemical Properties of Laponite®/Polyethylene-oxide Based Composites.

This review aims to provide a literature overview as well as the authors' personal account to the studies of Laponite® (Lap)/Polyethylene-oxide (PEO) based composite materials and their applications. These composites can be prepared over a wide range of their mutual concentrations, they are highly water soluble, and have many useful physico-chemical properties. To the readers' convenience, the contents are subdivided into different sections, related with consideration of PEO properties and its solubility in water, behavior of Lap systems(structure of Lap-platelets, properties of aqueous dispersions of Lap and aging effects in them), analyzing ofproperties LAP/PEO systems, Lap platelets-PEO interactions, adsorption mechanisms, aging effects, aggregation and electrokinetic properties. The different applications of Lap/PEO composites are reviewed. These applications include Lap/PEO based electrolytes for lithium polymer batteries, electrospun nanofibers, environmental, biomedical and biotechnology engineering. Both Lap and PEO are highly biocompatible with living systems and they are non-toxic, non-yellowing, and non-inflammable. Medical applications of Lap/PEO composites in bio-sensing, tissue engineering, drug delivery, cell proliferation, and wound dressings are also discussed.

Electrophoretic Deposition and Characterization of Er-Doped Bi2O3 Cathode Barrier Coatings on Non-Conductive Ce0.8Sm0.2O1.9 Electrolyte Substrates

In this study, the formation of thin-film barrier coatings based on a highly conductive Bi1.60Er0.4O3 (EDB) solid electrolyte on supporting Ce0.8Sm0.2O1.9 (SDC) electrolyte substrates was implemented for the first time using electrophoretic deposition (EPD). The electrokinetic properties of EDB-based suspensions in a non-aqueous dispersion medium of isopropanol modified with small additions of polyethyleneimine (PEI, 0.26 g/L) and acetylacetone (0.15 g/L), as well as in a mixed isopropanol/acetylacetone (70/30 vol.%) medium, were studied. The dependences of the thickness of the EDB coatings on voltage and deposition time were obtained using deposition on a model Ni foil electrode. Preliminary synthesis of a conductive polypyrrole (PPy) polymer film was used to create surface conductivity on non-conductive SDC substrates. The efficiency of using a modified dispersion medium based on isopropanol to obtain a continuous EDB coating 12 μm thick, sintered at a temperature of 850 °C for 5 h, is shown. The microstructure and morphology of the surface of the EDB coating were studied. A Pt/SDC/EDB/Pt cell was used to characterize the coating’s conductivity. The EPD method is shown to be promising for the formation of barrier coatings based on doped bismuth oxide. The developed method can be used for creating cathode barrier layers in SOFC technology.

From Raw to Finished Cotton—Characterization by Interface Phenomena

Abstract Interface phenomena that occur at the solid–liquid interface, such as wettability, adsorption, and particle aggregation, depend on the kind and magnitude of the solid surface free energy and electrokinetic properties found in water solutions. These phenomena are crucial for textile dyeing, finishing, and care. They characterize the material surface and change with different material pretreatment and finishing. In this paper, electrokinetic potential, isoelectric point, point of zero charge, a specific amount of surface charge and surface free energy of raw, enzymatically scoured, bleached, and finished cotton fabrics were investigated. Electrokinetic potential was measured by a streaming potential method and a specific quantity of surface charge by the back-titration method. For determination of the solid surface free energy components, the thin-layer wicking and contact-angle methods were used. On the basis of these results, components of solid surface free energy were calculated and discussed.

Carbon Adsorbents Obtained from Pistachio Nut Shells Used as Potential Ingredients of Drinking Water Filters.

Water resources are increasingly degraded due to the discharge of waste generated in municipal, industrial and agricultural areas. Therefore, the search for new materials enabling the effective treatment of drinking water and sewage is currently of great interest. This paper deals with the adsorption of organic and inorganic pollutants on the surface of carbonaceous adsorbents prepared by thermochemical conversion of common pistachio nut shells. The influence of the direct physical activation with CO2 and chemical activation with H3PO4 on parameters, such as elemental composition, textural parameters, acidic-basic character of the surface as well as electrokinetic properties of the prepared carbonaceous materials was checked. The suitability of the activated biocarbons prepared as the adsorbents of iodine, methylene blue and poly(acrylic acid) from the aqueous solutions was estimated. The sample obtained via chemical activation of the precursor turned out to be much more effective in terms of all the tested pollutants adsorption. Its maximum sorption capacity toward iodine was 1059 mg/g, whereas in relation to methylene blue and poly(acrylic acid) 183.1 mg/g and 207.9 mg/g was achieved, respectively. For both carbonaceous materials, a better fit to the experimental data was achieved with a Langmuir isotherm than a Freundlich one. It has also been shown that the efficiency of organic dye, and especially anionic polymer adsorption from aqueous solutions, is significantly affected by solution pH and temperature of the adsorbate-adsorbent system.

Modeling Zeta Potential for Nanoparticles in Solution: Water Flexibility Matters.

Nonequilibrium molecular dynamics simulations were performed to study the electrokinetic properties of five mainstream TIPxP water models (namely, TIP3P-FB, TIP3Pm, TIP4P-FB, TIP4P-Ew, and TIP4P/2005) in NaCl aqueous solutions in the presence of a negatively charged TiO2 surface. The impact of solvent flexibility and system geometry on the electro-osmotic (EO) mobility and flow direction was systematically assessed and compared. We found that lack of water flexibility decelerates the forward EO flow of aqueous solutions at moderate (0.15 M) or high (0.30 M) NaCl concentrations, in some special cases to such an extent that EO flow reversal occurs. Zeta potential (ZP) values were then determined from the bulk EO mobilities using the Helmholtz-Smoluchowski formula. The straight comparison against available experimental data strongly suggests that water flexibility improves the ZP determination of NaCl solutions adjacent to a realistic TiO2 surface under neutral pH conditions.