Yield stress and zeta potential of washed and highly spherical oxide dispersions — Critical zeta potential and Hamaker constant

The linear relationship between yield stress and the square of the zeta potential based on the yield stress-DLVO force model, was used to determine the zeta potential at the point of transition from flocculated to dispersed state of a range of oxide dispersions. The critical zeta potential at this point for washed α-Al₂O₃, TiO₂ and ZrO₂ dispersions was of magnitude 40, 49 and 52 mV respectively. For highly spherical silica and alumina dispersions, this value was 23 and 38 mV respectively. The square of the critical zeta potential is proportional to the Hamaker constant of the oxide in water when the van der Waals force is the only attractive force in play. Thus the critical zeta potential data obtained allowed the Hamaker constant ratio between the three oxide dispersions to be determined. This ratio between rutile TiO₂/water and α-Al₂O₃/water was 1.50. In comparison, a similar value of 1.46 was obtained for the ratio calculated from Hamaker constant value determined via Lifshitz theory. The ratio between rutile TiO₂/water and ZrO₂/water is ∼ 0.90. Using the Hamaker constant of rutile TiO₂/water of 61.2 zJ as the standard, the Hamaker constant determined by our method is 41 zJ for α-Al₂O₃, 68 zJ for ZrO₂ and 13.6 zJ for silica.     

Critical zeta potential and the Hamaker constant of oxides in water

The critical zeta potential characterises the flocculated-dispersed state transition of a colloidal dispersion. For many colloidal dispersions, yield stress displayed a linear relationship with the square of zeta potential, indicating that they obeyed the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. From this relationship, the critical zeta potential is obtained from the intercept at the zeta potential axis at a yield stress of zero. The critical zeta potential is a measure of the repulsive potential required to exactly counter the maximum attractive potential between particles in dispersion in the flocculated state. When the forces of interaction between particles in the dispersion are only the van der Waals and electrostatic forces, then the critical zeta potential is indirectly a measure of the van der Waals attractive potential and, hence, it may be used to determine the Hamaker constant of solids in water. This potential is proportional to the square root of the solids Hamaker constant in water. At present, only the ratio of Hamaker constant between two oxides was obtained and compared with that obtained by other techniques. These oxides were ultrapure anatase TiO₂ and γ-Al₂O₃, and they displayed a linear relationship between yield stress and the square of zeta potential. At the conductivity (or ionic strength) of about 3000 μS/cm, the critical zeta potential for both TiO₂ and Al₂O₃ is ∼47 and ∼32 mV, respectively. These critical zeta potential data give a value of 2.2 for the ratio of Hamaker constant of anatase TiO₂/H₂O/TiO₂ to γ-Al₂O₃/H₂O/γ-Al₂O₃. This ratio compares well with a value ranging from 1.0 to 2.18 for rutile TiO₂/H₂O/TiO₂ to α-Al₂O₃/H₂O/α-Al₂O₃ where their Hamaker constants were calculated from the Lifshitz theory using full optical spectral data.     

Yield stress of oxide dispersions—intermolecular forces of adsorbed small ionic additives and particle surface roughness

The yield stress‐pH and zeta potential‐pH behaviour of α‐alumina and zirconia dispersions with adsorbed small ionic molecular additives such as phosphate and pyrophosphate were determined. The result for adsorbed citrate was included for comparison. Adsorbed phosphate at high surface coverage increased the maximum yield stress of low surface area α‐Al₂O₃ (AKP30 and AA07) dispersions slightly. This increase is attributed to the intermolecular hydrogen bonding between phosphates adsorbed on interacting particles. With high surface area ZrO₂ (Tosoh) dispersions, however, the adsorbed phosphate decreased the maximum yield stress. This is due to its very rough surface morphology limiting the extent of intermolecular hydrogen bonding between adsorbed phosphate layers. Unlike phosphate, pyrophosphate reduces the maximum yield stress of AKP30 α‐Al₂O₃. This is due to the presence of intramolecular hydrogen bonding, thereby impeding effective bridging. A similar result is observed with citrate. The adsorbed pyrophosphate acts as an effective steric barrier keeping interacting particles further apart, thereby weakening the van de Waals attraction. These dispersions with the presence of non‐DLVO forces, that is bridging and steric, did not affect the linear relationship between yield stress and the square of the zeta potential as predicted by the yield stress–DLVO force model. However the relative importance of these non‐DLVO forces affect the value of the critical zeta potential at the point of transition from flocculated to dispersed state. © 2011 Canadian Society for Chemical Engineering     

Electrophoresis of pH‐regulated particles in the presence of multiple ionic species

An electrophoresis model taking account of the pH‐regulated nature of particles and the presence of multiple ionic species is proposed for arbitrary surface potential and double‐layer thickness. It successfully simulated the electrophoretic behavior of Fe₃O₄ nanoparticles in an aqueous NaCl solution with pH adjusted by HCl and NaOH. The estimated zeta potential is compared with those from the conventional models, Smoluchowski's, Hückel's, and Henry's formulas. Due to the violation of the assumption of low and constant surface potential, these formulas yielded appreciable deviations (e.g., 23–30 mV at pH 9). With the surface charge density measured by titration and the zeta potential by electrophoresis, the true surface potential is estimated through a triple‐layer model. The estimated true potential is typically 1.5–10 times larger than the zeta potential, implying that using the latter in relevant calculations (e.g., stability and critical coagulation concentration) might yield appreciable deviation. © 2013 American Institute of Chemical Engineers AIChE J 60: 451–458, 2014     

Interfacial properties of polyelectrolyte–cellulose systems. II. Electrokinetic properties of cellulose fibers with adsorbed monolayers of cationic polyelectrolyte

Zeta potential measurements by the streaming current method were performed on pulp (DP) fibers with or without irreversibly adsorbed monolayers of cationic polyelectrolyte. Factors affecting the electrokinetic properties of these fibers, such as the amount of adsorbed polymer, the polymer molecular weight (M_n 50,000 and 200,000), ionic strength (10^?5 ～ 10^?2M KCl), and the pH of the streaming medium (KCl solution), were examined. As the amount of adsorbed polymer increased, the negative zeta potential of the fibers decreased until the polarity of the zeta potential was reversed to the positive side. A marked change in the value of zeta potential was not observed when the formation of the saturated monolayer was completed. The zeta potential also varied in proportion to an increase in the amount of polymer adsorbed. Experimental results are interpreted with reference to the origin of the surface charge, the amphoteric nature of the surface, the modes of adsorption, and the adsorbed polymer chain configuration. Possible effects of the adsorbed monolayer formation on the structural change of the electric double layer at the fiber surface are discussed. It is concluded that the formation of a monolayer of cationic polyelectrolytes on the negatively charged cellulose fibers under the condition of k₁ > k₂ (part I) provides a means to arbitrarily control the charge of the fibers until formation of a saturated monolayer.     

Calcium phosphate-mediated surface modification of titanium oxide and its effects on surface potential and fibrinogen adsorption

Surface modification is one of major concerns in biomaterial research for the improvement of biocompatibility. This study examined the effects of calcium phosphate (CaP) formation on TiO₂ particle on the surface properties and adsorption of fibrinogen (FBN). CaP coating was formed by incubating the particles in an ionic solution. The properties of CaP-coated TiO₂ (cpTiO₂) were analyzed using various analytical methods. CaP coating was calcium-deficient showing a highly porous 3D structure and decreased the negativity of zeta potential of the particle. Fibrinogen (FBN) adsorption decreased the negativity of the surface charge of particles dependent on FBN adsorption time and concentration.     

Controlling attractive interparticle forces via small anionic and cationic additives in kaolin clay slurries

Interparticle forces govern slurry behavior in flow, mixing, sedimentation and thickening. This study evaluates the use of small anionic and cationic additives with pH to control the interparticle forces in kaolin slurry via the yield stress parameter. Both phosphate and citrate additives were found to reduce the interparticle attractive force or yield stress in the moderate pH region of 4–12. These relatively low charged additives were unable to impart a sufficiently strong repulsive interparticle force to completely disperse the slurry. Three linear relationships between yield stress and the square of zeta potential were observed in slurry with and without these additives, indicating that the yield stress–DLVO force model is obeyed in each linear region. The mid-range zeta potential region yielded a positive slope which was attributed to heterogeneous charge attraction between clay particles. It is this heterogeneous charge attraction that was weakened by the adsorbed additives. In contrast, cationic Polyethylenimine (PEI) of Mw 70,000 increases the yield stress at all pH level via bridging. Charge reversal was also observed at high PEI concentrations. In two cases, the pH of maximum yield stress and zero zeta potential coincided. A single linear yield stress–zeta potential squared relationship was observed despite particle bridging interaction being the dominant interparticle force.     

Aqueous Colloidal Stability Evaluated by Zeta Potential Measurement and Resultant TiO2 for Superior Photovoltaic Performance

Controlling the morphological structure of titanium dioxide (TiO₂) is crucial for obtaining superior power conversion efficiency for dye‐sensitized solar cells. Although the sol–gel‐based process has been developed for this purpose, there has been limited success in resisting the aggregation of nanostructured TiO₂, which could act as an obstacle for mass production. Herein, we report a simple approach to improve the efficiency of dye‐sensitized solar cells (DSSC) by controlling the degree of aggregation and particle surface charge through zeta potential analysis. We found that different aqueous colloidal conditions, i.e., potential of hydrogen (pH), water/titanium alkoxide (titanium isopropoxide) ratio, and surface charge, obviously led to different particle sizes in the range of 10–500 nm. We have also shown that particles prepared under acidic conditions are more effective for DSSC application regarding the modification of surface charges to improve dye loading and electron injection rate properties. Power conversion efficiency of 6.54%, open‐circuit voltage of 0.73 V, short‐circuit current density of 15.32 mA/cm², and fill factor of 0.73 were obtained using anatase TiO₂ optimized to 10–20 nm in size, as well as by the use of a compact TiO₂ blocking layer.     

Effects of Poly(acrylic acid) on Rheological and Dispersion Properties of Aqueous TiO2 Suspensions

The poly(acrylic acid) was used as dispersant to prepare aqueous TiO₂/poly(acrylic acid) suspensions. The poly(acrylic acid) was adsorbed on the surface of the TiO₂ particles. The zeta potential of the TiO₂ particles in TiO₂/poly(acrylic acid) suspensions was higher than that of the TiO₂ particles in TiO₂ suspensions, and the zeta potential of the TiO₂ particles increased with increasing poly(acrylic acid) content. At the same shear rate, the viscosity of TiO₂/poly(acrylic acid) suspensions was lower than that of TiO₂ suspensions, and the liquidity was improved. The dispersion of TiO₂ particles in TiO₂/poly(acrylic acid) suspensions was improved compared with that of TiO₂ particles in TiO₂ suspensions.     

Effect of PAA-g-MPEO Comb Polymer on TiO2 Suspensions

A comb PAA-g-MPEO was prepared. The PAA-g-MPEO underwent two thermal degradation steps. First, the decarboxylation and the dehydration of the carboxyl in the PAA backbone took place in the range of 204–313°C; the weight loss in the range of 313–450°C was from decomposition of MPEO chains and the survivor of the decomposed PAA backbone. By adding comb PAA-g-MPEO as disperser to TiO₂ suspensions, TiO₂/PAA-g-MPEO suspensions were prepared. The PAA-g-MPEO was adsorbed onto the surfaces of the TiO₂ particles, and the zeta potential of TiO₂/PAA-g-MPEO suspensions was higher than that of TiO₂ suspensions. Compared with that of TiO₂ suspensions, the viscosity of TiO₂/PAA-g-MPEO suspensions decreased, and the viscosity further decreased with increasing PAA-g-MPEO content. After adding PAA-g-MPEO, the size distribution of the TiO₂ particles in the suspensions became narrow and the average diameter of the TiO₂ particles decreased due to electrostatic repulsions and the steric hindrances of the PAA-g-MPEO.     

Effects of comb copolymer PAA-g-MPEO on rheological and dispersion properties of aqueous CaCO<Subscript>3</Subscript> suspensions

Summary The comb copolymer poly(acrylic acid) (PAA) grafted methoxyl poly(ethylene oxide) (MPEO) (PAA-g-MPEO) as dispersant was used in aqueous CaCO₃ suspensions. The PAA-g-MPEO was adsorbed onto CaCO₃particle surfaces due to the electrostatic attraction. The adsorbed amount increased with increasing PAA-g-MPEO content. The CaCO₃ adsorbed PAA-g-MPEO displayed negative zeta potential. The zeta potential was more negative with increasing PAA-g-MPEO content. Addition of PAA-g-MPEO, the conductivity of aqueous CaCO₃ suspensions decreased firstly, and then increased with increasing PAA-g-MPEO content. Compared to that of aqueous CaCO₃ suspensions, the viscosity of aqueous CaCO₃/PAA-g-MPEO suspension reduced remarkably, and the liquidity of the suspensions was improved. The dispersion of CaCO₃ particles in aqueous CaCO₃/PAA-g-MPEO suspensions was significantly improved due to electrostatic repulsions and steric hindrance between CaCO₃ particles adsorbed PAA-g-MPEO.     

Pristine point of zero charge (p.p.z.c.) and zeta potentials of boehmite’s nanolayer and nanofiber surfaces

The pristine point of zero charge (p.p.z.c) and zeta potential as a function of pH of boehmite oxide/hydroxide (α-Al₂O₃·H₂O) have been determined for three filter media. The active component in the first two filter media is boehmite nanofibers, only 2 nm in diameter and about 300 nm long. Boehmite nanofibers create high zeta potential (ζ_true≥46 mV) in aqueous solutions in the pH range of 3–8. The p.p.z.c. values were determined to be 11.60 ± 0.15 for nanofibers grafted onto microglass fibers and 11.40 ± 0.15 for agglomerated nanofibers. In the third filter media, a boehmite nanolayer in the form of monocrystalline oxide/hydroxide with a thickness of approximately 1.2 nm is electroadhesively deposited onto siliceous support material with large surface area of about 50 m²/g, therefore forming a highly electropositive composite of boehmite nanolayer on the second highly electronegative solid. Boehmite’s oxide-hydroxide nanolayer surface creates high zeta potential (ζ_true≥50 mV) in aqueous solutions in the pH range of 3–8. The p.p.z.c. value was determined to be 11.38 ± 0.15. The reported values are within accuracy, but they are much higher than the values reported in the literature. X-ray powder diffraction data were supplemented by microscopy, infrared spectroscopy in order to characterize fully synthetic boehmite surfaces.     

Immobilized titanium dioxide/powdered activated carbon system for the photocatalytic adsorptive removal of phenol

Titanium dioxide (TiO₂) and powdered activated carbon (PAC) were fabricated via a layer by layer arrangement on a glass plate using a dip-coating technique for the photocatalytic-adsorptive removal of phenol. Thinner TiO₂ layer coated on PAC sub-layer has larger surface area and better phenol removal than the thicker TiO₂ layer. The system obeyed the Langmuir isotherm model, which exhibited a homogeneous and monolayer adsorption with a maximum capacity of 27.8 mg g^-1. The intra-particle diffusion was the rate-limiting step as the linear plot crossed the origin, while the adsorption was unfavorable at elevated temperature. Under light irradiation, the TiO₂/PAC system removed phenol two-times more effectively than the TiO₂ monolayer due to the synergistic effect of photocatalysis by TiO₂ top layer and adsorption by PAC sub-layer. The COD removal of phenol was rapid for 10mg L^-1 of concentration and under solar light irradiation. It was shown that the PAC sub-layer plays a significant role in the total removal of phenol by providing the adsorption sites and slowing down the recombination rate of charge carriers to improve the TiO₂ photocatalytic oxidation performance.     

Interfacial properties of polyelectrolyte-cellulose systems. IV. Electrokinetic properties of carboxymethylcellulose fibers with adsorbed multilayers of cationic polyelectrolyte

Zeta potential measurements by the streaming current method were performed on carboxymethylcellulose (CMC) with and without irreversibly adsorbed multilayers of cationic polyelectrolyte. Factors affecting the electrokinetic properties such as the amount of adsorbed polymer, polymer molecular weights (M_n 50,000 and 200,000), ionic strength (10^?5 ～ 10^?2M KCl), and pH of the streaming medium (KCl solutions) were examined. The negative zeta potential of CMC decreased to the point of monolayer formation and increased from that point to the saturated multilayer formation. The polarity of the zeta potential was negative throughout every adsorption stage. The negative zeta potential increase was attributed to: (a) binding of anions (Cl^? and OH^?) to the outermost layer of the multilayer from KCl solutions and (b) change in chemical potentials of counterions in a diffuse double layer due to expansion of the double layer in the presence of the adsorbed multilayer on CMC. The results suggest that the carboxyl groups under the monolayer are undetectable electrokinetically; however, the negative charge, due to unneutralized carboxyl groups under the monolayer, appears to cause further adsorption forming a saturated multilayer. When the effect of unneutralized carboxyls of CMC are shielded at higher levels of adsorption, the outermost layer of the multilayer becomes a potential-determining layer.     

Synthesis of thin alpha alumina platelets with a large aspect ratio

Since α-Al₂O₃ platelets with small thickness and large aspect ratio are needed in various applications, it is of great importance to control their morphology. In the present work, we successfully fabricated α-Al₂O₃ platelets with small average thickness below 100 nm and large average aspect ratio above 100 by molten-salt method, using Al₂(SO₄)₃ as the raw material, eutectic mixtures of Na₂SO₄ and K₂SO₄ as the molten salt as well as TiO₂ and SiO₂ as the co-dopants. The effects of calcination temperature, seeds amount and seeds morphology on the final morphology of α-Al₂O₃ platelets were investigated. The growth mechanism of the α-Al₂O₃ platelets in molten salt was also discussed. A new growth mechanism of α-Al₂O₃ platelets in molten salt has been proposed, which is different from previous mechanism but can explain the two different phenomena caused by seeds with different sizes and quantity.     

Polyacrylate‐core/TiO2‐shell nanocomposite particles prepared by in situ emulsion polymerization

We report the preparation of polyacrylate‐core/TiO₂‐shell nanocomposite particles through in situ emulsion polymerization in the presence of nano‐TiO₂ colloid obtained by the hydrolysis of titanium tetrachloride. The resultant colloidal system can be stable for months without any precipitation. In a typical sample, the diameter of nanocomposite particles was about 150 nm, and the thickness of TiO₂‐shell was 4–10 nm. Only cetyltrimethylammonium bromide was employed to provide the latex particles with positive charge, which was enough for the formation of fine TiO₂ coatings. Three initiators were tested. Ammonia persulfate was the most suitable one, because the cooperative effect was formed by the negatively charged TiO₂ particles and the terminal anionic group (SO₄^2?, the fraction of Ammonia persulfate) of the polymer chain on the surface of latex particles to maintain the stability of nanocomposite system. The pH value played a vital role in obtaining a tight TiO₂ coating. Transmission electron microscopy, X‐ray diffraction and Atomic force microscopy were used to characterize this nanocomposite material. It was found that rutile and anatase coexisted in the nanocomposite film. This may suggest a potential application in the field of photocatalytic coating. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1466–1470, 2006     

Optimizing graphene-TiO2 interface properties via Fermi level modulation for photocatalytic degradation of volatile organic compounds

The interfacial interaction between graphene and semiconductors significantly affects volatile organic compound (VOC) adsorption and charge separation. Herein, we optimize the interaction between TiO₂ and reduced graphene oxide (rGO) by reducing the Fermi level of GO with Cu²⁺. The surface photocurrent (SPC) and surface photovoltage (SPV) of the prepared materials were tested under different atmospheres to study the effect of enhanced interfacial interactions on charge separation. The results revealed that Cu²⁺ treatment tended to induce the TiO₂ nanoparticles to form a thicker and more uniform layer on the rGO surface. The composition of TiO₂ nanoparticles and rGO generated a 2D arranged porous structure that had both hydrophobic rGO and hydrophilic TiO₂ as the pore walls. The amount of adsorbed toluene for the optimized TiO₂-graphene (t-TiO₂/rGO) was 1.21-fold higher than that for TiO₂/rGO. The SPC and SPV results showed that the optimized contact between TiO₂ and rGO significantly enhanced photogenerated electron mobility and toluene-induced hole utilization. Given the advantages in adsorption and charge separation, the photocatalytic reaction rates of t-TiO₂/rGO were 1.47-fold and 1.91-fold higher than those of TiO₂/rGO and porous TiO₂, respectively.     

Stability of kaolinite dispersions in the presence of sodium and aluminum ions

The stability of colloidal kaolinite dispersions in the presence of NaCl and AlCl₃ was studied by measuring turbidity, electrophoretic mobility and adsorption. The kaolinite particles coagulated at pH 2.5–3.5 and were dispersed at pH >4.5. These results well obeyed the classic DLVO theory if the mean zeta potential of the kaolinite particles in aqueous solutions was taken into account in the computation of potential energy of electrical double layer repulsion, which suggests that the kaolinite particles might coagulate in the same way as normal colloidal particles. The kaolinite particles in aqueous aluminum salt solution only coagulated at a medium AlCl₃ concentration, and formed a stable dispersion at a high salt concentration. This is caused by Stern-layer adsorption of hydrolyzed aluminum species, probably adsorbed on the kaolinite surfaces through hydrogen bonds between the hydroxyl groups of the aluminum species and the oxygen atoms on the kaolinite surfaces.     

Enhanced photocatalytic degradation of tetramethylammonium on silica-loaded titania

Photocatalytic degradation (PCD) of tetramethylammonium (TMA) in water was studied using both pure TiO₂ and silica-loaded TiO₂ (Si–TiO₂). Use of Si–TiO₂ catalyst prepared from commercial TiO₂ powder by a simple method developed in this work enhanced the PCD rate of TMA considerably. The Si/Ti atomic ratio of 18% was found to be an optimum in photoactivity and the calcined sample was more efficient than the uncalcined one. Several factors were noted to be responsible for the higher photoefficiency of Si–TiO₂ catalyst. Si–TiO₂ calcined at 700 °C did not show any sign of change in the crystalline structure from that of uncalcined pure TiO₂. The increased thermal stability of Si–TiO₂ enabled the bulk defects to be removed at high temperatures without forming the inactive rutile phase, which may partly contribute to the higher photoactivity. The most outstanding characteristics of Si–TiO₂ is its surface charge modification. Loading silica on to a titania surface made the surface charge highly negative, which was confirmed by zeta potential measurements. The enhanced electrostatic attraction of cationic TMA onto the negatively charged Si–TiO₂ surface seems to be the main reason for the enhanced photoactivity of Si–TiO₂. As a result of this surface charge change, the TMA PCD rate with Si–TiO₂ exhibited a maximum around pH 7 whereas the PCD with pure TiO₂ was minimized at pH 7. The X-ray photoelectron spectroscopic analysis showed the formation of SiO_x on the TiO₂ surface but the diffuse reflectance UV spectra indicated no significant difference in the band gap transition between pure TiO₂ and Si–TiO₂. In addition, the diffuse reflectance IR spectra showed the presence of more surface OH groups on Si–TiO₂ than on pure TiO₂, which may also contribute to the higher photoactivity of Si–TiO₂ through generating more OH radicals upon UV illumination.     

Study on corrosion resistance and roughness of micro-plasma oxidation ceramic coatings on Ti alloy by EIS technique

Micro-plasma oxidation (MPO) technique has been developed quickly in recent years. The produced ceramic coatings are reported to possess fine properties and promising application prospects in many fields. The aim of this work is to study the corrosion resistance and the roughness of the micro-plasma oxidation ceramic coatings on Ti alloy by electrochemical impedance spectroscopy (EIS) technique. Compound ceramic coatings were prepared on Ti-6Al-4V alloy by pulsed bi-polar micro-plasma oxidation in NaAlO₂ solution. The phase composition and element distribution in the coating were investigated by X-ray diffractometry and electron probe micro-analyzer. EIS of the coatings was measured through CHI604 electrochemical analyzer in 3.5% NaCl solution. The ceramic coating is composed of a large amount of Al₂TiO₅ and a little α-Al₂O₃ and rutile TiO₂. The coating is of double-layer structure with the loose outer layer and the dense inner layer. The thickness of the coatings is reduced when the working frequency or the cathode pulse current density is increased, while the thickness is increased when the frequency or the anode current density is increased. The established “equivalent circuit” of the coatings is consistent with the double-layer structure. The electric charge transfer resistance (R_t) in the equivalent circuit can be used to assess the corrosion resistance of the coatings, which is consistent with the result of the polarizing curves test. And the empirical exponent (n₁) of the constant phase element (Q₁) in the equivalent circuit can be used to assess the surface roughness of the coatings, which is consistent with the surface SEM analysis of the coatings.