Critical Coagulation Concentration Values Research Articles

Impact of electric field on Hofmeister effects in aggregation of negatively charged colloidal minerals

In this study, the aggregation kinetics of negatively charged colloidal minerals in Na++, K++, NH $_{\mathrm {4}}^{\mathrm {+}}$ , Mg2+, Ca2+ and Cu2+ solutions were measured and Hofmeister effects therein were estimated through total average aggregation (TAA) rates and critical coagulation concentration (CCC). Hofmeister effects of TAA rates increased exponentially with the increase in electric field strength, which cannot be explained by the classical theories (i.e., ionic size, hydration and dispersion forces), indicating strong electric field at colloidal surface was an indispensable factor in studying Hofmeister effects. Meanwhile, Hofmeister series of CCC values Na+> K+> NH $_{\mathrm {4}}^{\mathrm {+}}>$ Mg2+> Ca2+> Cu2+ show fine correlation with the polarization of various cations, implying that onic polarization in strong electric field would be responsible for Hofmeister effects in aggregation of colloidal minerals, and the deduction was confirmed by the calculated results of electrostatic interactions between colloidal minerals.

Impact of environmental conditions on aggregation kinetics of hematite and goethite nanoparticles

Hematite and goethite nanoparticles were used as model minerals to investigate their aggregation kinetics under soil environmental conditions in the present study. The hydrodynamic diameters of hematite and goethite nanoparticles were 34.4 and 66.3 nm, respectively. The positive surface charges and zeta potential values for goethite were higher than for hematite. The effective diameter for goethite was much larger than for hematite due to anisotropic sticking of needle-shaped goethite during aggregation. Moreover, the critical coagulation concentration (CCC) values of nanoparticles in solutions of NaNO3, NaCl, NaF, and Na2SO4 were 79.2, 75.0, 7.8, and 0.5 mM for hematite and they were 54.7, 62.6, 5.5, and 0.2 mM for goethite, respectively. The disparity of anions in inducing hematite or goethite aggregation lay in the differences in interfacial interactions. NO3 − and Cl− could decrease the zeta potential and enhance aggregation mainly through increasing ionic strength and compressing electric double layers of hematite and goethite nanoparticles. F− and SO4 2− highly destabilized the suspensions of nanoparticles mainly through specific adsorption and then neutralizing the positive surface charges of nanoparticles. Specific adsorption of cations could increase positive surface charges and stabilize hematite and goethite nanoparticles. The Hamaker constants of hematite and goethite nanoparticles were calculated to be 2.87 × 10−20 and 2.29 × 10−20 J−1, respectively. The predicted CCC values based on DLVO theory were consistent well with the experimentally determined CCC values in NaNO3, NaCl, NaF, and Na2SO4 systems, which demonstrated that DLVO theory could successfully predict the aggregation kinetics even when specific adsorption of ions occurred.

Aggregation and Stability of Reduced Graphene Oxide: Complex Roles of Divalent Cations, pH, and Natural Organic Matter.

The aggregation and stability of graphene oxide (GO) and three successively reduced GO (rGO) nanomaterials were investigated. Reduced GO species were partially reduced GO (rGO-1h), intermediately reduced GO (rGO-2h), and fully reduced GO (rGO-5h). Specifically, influence of pH, ionic strength, ion valence, and presence of natural organic matter (NOM) were studied. Results show that stability of GO in water decreases with successive reduction of functional groups, with pH having the greatest influence on rGO stability. Stability is also dependent on ion valence and the concentration of surface functional groups. While pH did not noticeably affect stability of GO in the presence of 10 mM NaCl, adding 0.1 mM CaCl2 reduced stability of GO with increased pH. This is due to adsorption of Ca(2+) ions on the surface functional groups of GO which reduces the surface charge of GO. As the concentration of rGO functional groups decreased, so did the influence of Ca(2+) ions on rGO stability. Critical coagulation concentrations (CCC) of GO, rGO-1h, and rGO-2h were determined to be ∼ 200 mM, 35 mM, and 30 mM NaCl, respectively. In the presence of CaCl2, CCC values of GO and rGO are quite similar, however. Long-term studies show that a significant amount of rGO-1h and rGO-2h remain stable in Call's Creek surface water, while effluent wastewater readily destabilizes rGO. In the presence NOM and divalent cations (Ca(2+), Mg(2+)), GO aggregates settle from suspension due to GO functional group bridging with NOM and divalent ions. However, rGO-1h and rGO-2h remain suspended due to their lower functional group concentration and resultant reduced NOM-divalent cation bridging. Overall, pH, divalent cations, and NOM can play complex roles in the fate of rGO and GO.

Specific ion effects on particle aggregation induced by monovalent salts within the Hofmeister series.

Ion specific effects of monovalent salts on charging and aggregation for two types of polystyrene latex particles were investigated by electrophoresis and time-resolved light scattering. The chemical composition of the electrolytes was systematically varied in the experiments. Accordingly, NaH2PO4, NaF, NaCl, NaBr, NaNO3, and NaSCN were used to vary the anions and N(CH3)4Cl, NH4Cl, CsCl, KCl, NaCl, and LiCl for the cations. The salt concentration dependence of the electrophoretic mobilities indicates that the surface charge was screened by the counterions when their concentrations increased. For the SCN(-) ions, adsorption on positively charged particles leads to charge reversal. The aggregation rates are small at low electrolyte concentrations indicating stable dispersions under these conditions, and they increase with the salt concentration. When viscosity corrections are taken into account, no ion specific effects in the fast aggregation regime can be established. The slow and fast aggregation regimes are separated by the critical coagulation concentration (CCC). Within the experimental error, the CCCs are the same in systems containing different co-ions but the same counterions, with the exception of ammonium salts. However, the variation of counterions leads to different CCC values due to specific interaction of the counterions with the surface. These values follow the Hofmeister series for negatively charged sulfate latex particles, while the reversed order was observed for positively charged amidine latex. Comparison between experimental CCCs and those calculated by the theory of Derjaguin, Landau, Verwey, and Owerbeek reveals that variations in the surface charge due to ionic adsorption are mainly responsible for the ion specific effects in the aggregation process.

Effects of humic acid and surfactants on the aggregation kinetics of manganese dioxide colloids

The aggregation of common manganese dioxide (MnO2) colloids has great impact on their surface reactivity and therefore on their fates as well as associated natural and synthetic contaminants in engineered (e.g. water treatment) and natural aquatic environments. Nevertheless, little is known about the aggregation kinetics of MnO2 colloids and the effect of humic acid (HA) and surfactants on these. In this study, the early stage aggregation kinetics of MnO2 nanoparticles in NaNO3 and Ca(NO3)2 solutions in the presence of HA and surfactants (i.e., sodium dodecyl sulfate (SDS), and polyvinylpyrrolidone (PVP)) were modeled through time-resolved dynamic light scattering. In the presence of HA, MnO2 colloids were significantly stabilized with a critical coagulation concentration (CCC) of ∼300 mmol·L−1 NaNO3 and 4 mmol·L−1 Ca(NO3)2. Electrophoretic mobility (EPM) measurements confirmed that steric hindrance may be primarily responsible for increasing colloidal stability in the presence of HA. Moreover, the molecular and/or chemical properties of HA might impact its stabilizing efficiency. In the case of PVP, only a slight increase of aggregation kinetics was observed, due to steric reactions originating from adsorbed layers of PVP on the MnO2 surface. Consequently, higher CCC values were obtained in the presence of PVP. However, there was a negligible reduction in MnO2 colloidal stability in the presence of 20 mg·L−1SDS.

A Modified Method to Calculate Critical Coagulation Concentration Based on DLVO Theory

The critical coagulation concentration (CCC) is defined as the minimum concentration of counterions to induce coagulation of colloidal particles. A modified calculation method was proposed to calculate CCC. Comparing the modified calculation method of CCC with the traditional calculation method, the critical condition of modified calculation method is stricter than traditional calculation method. The critical condition of modified calculation method is the maximum value of interaction force that is zero, and the critical condition of traditional calculation method is the maximum value of interaction energy that is zero. The calculation result of CCC based on interaction force is greater than the calculation value based on interaction energy. The CCC value of modified calculation method can ensure particles to coagulate definitely.

Stability and sedimentation of silver nanoparticles in the presence of monovalent, divalent and trivalent electrolyte solutions.

The present study seeks to elucidate the effect of different electrolytes on the stability of citrate-coated silver nanoparticles synthesized by the reduction of the Ag(NH3)(2)(+) complex via Tollens' method. Time-resolved dynamic light scattering technique was used to measure the aggregation behavior of the nanoparticles over a range of electrolyte (NaCl, CaCl₂, and AlCl₃) concentrations. From the aggregation kinetic data, the critical coagulation concentration values were calculated as 0.11, 1.7 × 10⁻³, and 5.06 × 10⁻⁶ M for NaCl, CaCl₂, and AlCl₃, respectively. Supernatant of the nanoparticle suspension in presence of the three electrolytes was collected and measured using inductively coupled plasma mass spectrometry to quantify the sedimentation. It was found that the silver nanoparticle aggregates settled rapidly in the slow aggregation and fast aggregation regimes. Their sedimentation followed a nanoparticle size-dependent manner.

Aqueous stabilization of carbon nanotubes: effects of surface oxidization and solution chemistry

Surface oxygen functional groups can affect the morphological characteristics, aggregation kinetics, and adsorption capacity of multi-walled carbon nanotubes (MWCNTs). However, little is known about the quantitative relationship between oxygen content and the dispersion stability of MWCNTs. To investigate the effects of surface oxidization, MWCNTs were oxidized using concentrated H2SO4/HNO3 acids for 0, 1, 2, 4, and 8 h, respectively. Experimental results showed that the oxygen content of MWCNTs increased with surface oxidization time. Linear correlations were found to exist between the oxygen content, critical coagulation concentration (CCC) for NaCl, and critical coagulation pH values of MWCNTs detected by optical density at 800 nm. The CCC values for CaCl2 increased with oxygen contents for unmodified MWCNTs and lowly oxidized MWCNTs, while which decreased after further increasing the surface oxidization. CCC ratios in the presence of Ca(2+) to Na(+) were consistent with the empirical Schulze-Hardy rule for unmodified MWCNTs and lowly oxidized MWCNTs; however, which were much lower than the expected values for highly oxidized MWCNTs. Fulvic acid can clearly increase the stability of MWCNT suspension with unmodified MWCNTs and lowly oxidized MWCNTs, while it cannot affect the dispersion of highly oxidized MWCNTs. This study implied that the oxidation and presence of fulvic acid will possibly increase the mobility, exposure, bioavailability, and toxicity of MWCNTs.

Colloidal Properties and Stability of Graphene Oxide Nanomaterials in the Aquatic Environment

While graphene oxide (GO) has been found to be the most toxic graphene-based nanomaterial, its environmental fate is still unexplored. In this study, the aggregation kinetics and stability of GO were investigated using time-resolved dynamic light scattering over a wide range of aquatic chemistries (pH, salt types (NaCl, MgCl2, CaCl2), ionic strength) relevant to natural and engineered systems. Although pH did not have a notable influence on GO stability from pH 4 to 10, salt type and ionic strength had significant effects on GO stability due to electrical double layer compression, similar to other colloidal particles. The critical coagulation concentration (CCC) values of GO were determined to be 44 mM NaCl, 0.9 mM CaCl2, and 1.3 mM MgCl2. Aggregation and stability of GO in the aquatic environment followed colloidal theory (DLVO and Schulze-Hardy rule), even though GO's shape is not spherical. CCC values of GO were lower than reported fullerene CCC values and higher than reported carbon nanotube CCC values. CaCl2 destabilized GO more aggressively than MgCl2 and NaCl due to the binding capacity of Ca(2+) ions with hydroxyl and carbonyl functional groups of GO. Natural organic matter significantly improved the stability of GO in water primarily due to steric repulsion. Long-term stability studies demonstrated that GO was highly stable in both natural and synthetic surface waters, although it settled quickly in synthetic groundwater. While GO remained stable in synthetic influent wastewater, effluent wastewater collected from a treatment plant rapidly destabilized GO, indicating GO will settle out during the wastewater treatment process and likely accumulate in biosolids and sludge. Overall, our findings indicate that GO nanomaterials will be stable in the natural aquatic environment and that significant aqueous transport of GO is possible.

Influences of perfluorooctanoic acid on the aggregation of multi-walled carbon nanotubes

The aggregation of multi-walled carbon nanotubes (MWCNTs) in the aqueous phase not only inhibits their extensive utilization in various aspects but also dominates their environmental fate and transport. The role of surfactants at low concentration in the aggregation of MWCNTs has been studied, however the effect of perfluorinated surfactants at low concentration is uncertain. To understand this interfacial phenomenon, the influences of perfluorooctanoic acid (PFOA), and sodium dodecyl sulfate (SDS) as a control, on MWCNT aggregation in the aqueous phase were examined by the UV absorbency method. Influences of pH and cationic species on the critical coagulation concentration (CCC) value were evaluated. The CCC values were dependent on the concentration of PFOA, however a pronounced effect of SDS concentration on the CCC values was not observed. The CCC values of the MWCNTs were 51.6 mmol/L in NaCl and 0.28 mmol/L in CaCl2 solutions, which suggested pronounced differences in the effects of Na+ and Ca2+ ions on the aggregation of the MWCNTs. The presence of both PFOA and SDS significantly decreased the CCC values of the MWCNTs in NaCl solution. The aggregation of the MWCNTs took place under acidic conditions and was not notably altered under neutral and alkaline conditions due to the electrostatic repulsion of deprotonated functional groups on the surface of the MWCNTs.

Stability of zirconia sol in the presence of various inorganic electrolytes

Zirconia sol was prepared from zirconium oxychloride solutions by forced hydrolysis at 102?C. The prepared sol consisted of almost spherical, monoclinic, hydrated zirconia particles 61 nm in diameter. The stability of zirconia sol in the presence of various inorganic electrolytes (LiCl, NaCl, KCl, CsCl, KBr, KI, KNO3, and K2SO4) was studied by potentiometric titration method. Dependence of the critical concentration of coagulation (CCC) on the dispersion pH was determined for all studied electrolytes. The critical coagulation concentration values, for all investigated electrolytes, are lower at higher pH. These values for all 1:1 electrolytes are equal in the range of experimental error. For a given pH value, CCCs of K2SO4 are 3-4 orders of magnitude lower than the corresponding values for 1:1 electrolytes.

Spheres vs. rods: The shape of gold nanoparticles influences aggregation and deposition behavior

The influence of shape on nanomaterial aggregation and deposition was systematically studied with poly-acrylic acid (PAA) coated uniform-sized gold nanospheres (AuNSs) and nanorods (AuNRs). Time resolved dynamic light scattering was employed to study their aggregation kinetics in a wide range of mono- and di-valent electrolyte conditions. Results indicated that PAA coated AuNSs have higher aggregation propensity compared to anisotropic PAA coated AuNRs, as observed through critical coagulation concentration (CCC). The CCC values were estimated as 50mM NaCl and 1.8mM CaCl2 for AuNS, which showed substantial increase to 250mM NaCl and 7mM CaCl2 for anisotropic AuNRs. Though electrokinetic behavior showed similar surface potential for the spherical and rod-shaped materials, the geometric differences between the samples have likely resulted in unique conformation of the PAA coatings, leading to different magnitudes of steric hindrances and hence yielding the observed aggregation behavior. The deposition kinetics was monitored using the quartz crystal microbalance with dissipation technique. AuNRs showed relatively slower deposition compared to AuNSs for low electrolytes concentrations. With the increase in electrolyte concentration, the differences in deposition rates between spheres and rods diminished. The results from this study showed that the shape of nanomaterials can influence interfacial properties and result in unique aggregation and deposition behavior under typical aquatic conditions.

The effect of cations on the aggregation of commercial ZnO nanoparticle suspension

Nanoscale ZnO materials have been largely used in many products due to their distinct properties. However, ZnO nanoparticles (NPs) are hazardous to human health and the ecosystem. The characteristics and the stability of ZnO NPs are relevant to their fate in the environment and their potential toxicities. In this study, a stable commercial ZnO NP suspension was chosen to investigate its aggregation under various salt additions. Different concentrations of NaCl, KCl and CaCl2 were chosen to represent various environmental conditions. Under pH 8–9, the surface charge of commercial ZnO NPs was negative. The behavior of the stabilized ZnO NPs in water was affected by ionic combinations and ionic strength; that is, divalent cations were more effective than monovalent ones in promoting aggregation formation. The attachment efficiencies of ZnO aggregates were calculated based upon the aggregation kinetics. The critical coagulation concentration values for this commercial ZnO NPs were higher than previous reported for ZnO NPs, indicating this ZnO NP could be stable in the aquatic environment and might have increased hazardous potentials. Based upon the Derjaguin–Landau–Verwey–Overbeek theory, interactions between ZnO NPs in the presence of different ions were evaluated to illustrate the aggregation mechanism. Our results indicated that critical ionic type and concentration promote the aggregation of stable ZnO NPs. These understandings also can facilitate the design of the precipitation treatment to remove NPs from water.

Effects of inorganic and organic anions on the stability of illite and quartz soil colloids in Na-, Ca- and mixed Na–Ca systems

The colloidal behavior of soil mineral colloids (quartz and illite) in Na, Ca and mixed Na–Ca systems was systematically investigated. The critical coagulation concentrations (CCCs) were determined for Na+ and Ca2+ solutions containing anions of a different nature (Cl−, SO42− and ubiquitous soil organic anions formate and oxalate) at pH 5.5 and pH 8.5. Higher CCC values in the presence of Na+ at pH 8.5 were related to a lower screening effect of monovalent cations and the ionization properties of the soil mineral colloids. Attraction energy between colloids was experimentally scaled at CCC from zeta potential and aggregation kinetics measurements. In the Na-system, a higher stability for colloidal illite and quartz particles suspensions in the presence of oxalate anions confirmed surface complexation at an acidic pH. In the Ca-system, slight CCC variations in illite suspensions in the presence of Cl−, SO42−, formate and oxalate supported attractive ion–ion correlation forces between equally highly charged clay mineral colloids. It was shown that the CCCs of mixed Na–Ca systems close to soil chemical conditions can be calculated using the CCCs for colloidal illite and quartz particles in pure Na and Ca systems at pH 5.5 and pH 8.5. Sensitive dispersion effects due to adsorbed oxalate can be analyzed as a function of the molar ratios of calcium to sodium in solution.

An approach for the critical coagulation concentration estimation of polydisperse colloidal suspensions of soil and humus

The critical coagulation concentration (CCC) is considered as one of the most important parameters to evaluate the particles aggregation and sedimentation behaviors in the environment. Even though there are a few methods for its measurement, each method has its limitations especially as those methods to be applied to polydisperse systems like soil. Thus, the purpose of this research is to establish a new and more reliable method for its determination. Two types of polydisperse colloidal materials were adopted: soil and humus in the experimental studies. The dynamic light scattering technique was employed to determine the effective hydrodynamic diameter of the particles or aggregates changing with time under different pH and electrolyte concentrations of CaCl2 or KCl. In addition, the fractal dimension of aggregates was also detected with the static light scattering technique. A new aggregation rate total average aggregation rate (TAA rate) was defined firstly. We found the defined TAA rate increased linearly with the increase of electrolyte concentration at electrolyte concentrations lower than the CCC value, and the TAA rate stayed approximately constant at electrolyte concentrations higher than the CCC value; an intersection point of the two straight lines therefore could be observed, and the electrolyte concentration at the intersection point will theoretically be the CCC value. The experimental results for the two materials under different pH conditions indeed meet those theoretical predictions, which imply that the CCC value can be determined through the measurement of the TAA rate under different electrolyte concentrations. The comparison of the CCC values obtained between our new method and the widely applied stability ratio method showed that the new method was much better than the stability ratio method for the two polydisperse materials. In addition, the static light scattering measurements also showed that the variations of the fractal dimensions of the aggregates with electrolyte concentrations could be well explained by the CCC values obtained by the new method, which once again verified the applicability of the new method. A new theory and a corresponding new method for the CCC estimation, which can be applied to polydisperse colloidal suspensions, were developed, and the new method has been demonstrated to be much better than the widely applied method for polydisperse materials in environment.

The effect of inorganic ions on the aggregation kinetics of lab-made TiO2 nanoparticles in water

Metal oxide nanoparticles released from nanotechnology-based industries have attracted more attention recently because of their potentially hazardous effect to humans and the ecosystem. The aggregation of nanoparticles has a great influence on their fate in the environment and is relevant to their potential toxicities. In this study, we synthesized stable TiO2 nanoparticle suspension and investigated their distinct aggregation kinetics in the presence of various inorganic ions. Different concentrations of salts (NaCl, CaCl2 and Na2SO4) were chosen to represent various environmental solutions. The particle size of TiO2 nanoparticles increased when ionic concentrations went up. The stability of TiO2 nanoparticles in water is also affected by combinations of salts and ionic strength. For our lab-made TiO2 nanoparticles, it was easier to form aggregates in the presence of anions than in the presence of cations. Furthermore, divalent ions were better than monovalent ions in promoting aggregation formation. The attachment efficiencies of TiO2 aggregates were constructed based upon the aggregation kinetics. The critical coagulation concentration values for our lab-made TiO2 nanoparticles were higher than previously reported. The aggregation kinetics fit the Derjaguin–Landau–Verwey–Overbeek theory well; that is, divalent anions neutralized the nanoparticle surface charge and promoted aggregate formation more efficiently than the monovalent ones did. Our results indicated that the ionic concentration and ionic type required for the aggregation of TiO2 NPs are critical. These factors should be taken into account for interpreting the behavior of NPs and for developing strategies to remove NPs from the aquatic environment.

Aggregation of stabilized TiO2 nanoparticle suspensions in the presence of inorganic ions

The present study aims to evaluate the effect of inorganic ions on the aggregation kinetics of stabilized titanium dioxide (TiO(2) ) nanoparticle (NP) suspension, an NP mode widely used in consumer goods and in aquatic environments. The point of zero charge of stabilized TiO(2) NPs was approximately pH 6.5. The particle size of the stabilized TiO(2) NP suspensions increased with the increase in salt concentrations. The additional salts caused the shift of zeta potentials of TiO(2) suspensions to a lower value. The TiO(2) NPs aggregated more obviously in the presence of anions than cations, and the effect of divalent anions was larger than that of monovalent anions. The critical coagulation concentration (CCC) values for commercial TiO(2) NP suspensions with positive surfaces were estimated as 290 and 2.3 meq/L for Cl(-) and SO 42-, respectively. These CCC values of stabilized TiO(2) NP suspensions are higher than those of TiO(2) NP powders, indicating greater stability of the commercial stabilized TiO(2) NP suspensions. The effects of commercial TiO(2) NP suspensions still need to be explored and defined. Derjaguin-Landau-Verwey-Overbeek (DLVO) analysis can explain the aggregation behaviors of stabilized TiO(2) NP suspensions. Such an understanding can facilitate the prediction of NP fate in the environment.

Aggregation kinetics of SDBS-dispersed carbon nanotubes in different aqueous suspensions

The aggregation kinetics of single-walled (SWNTs) and multi-walled (MWNTs) carbon nanotubes dispersed using sodium dodecylbenzene sulfonate (SDBS) were investigated using time-resolved dynamic light scattering (DLS), in the presence of several electrolytes, and humic acid (HA). The effects of changes in the concentration of monovalent (NaCl, 5–200mmol/L) and divalent (CaCl2, 0.2–5mmol/L) electrolytes and the solution pH (3–10) on the aggregation behavior were evaluated. The results showed that the carbon nanotubes (CNTs) could be effectively suspended in aqueous solution using the surfactant (SDBS), and that increased electrolyte concentrations induced aggregation. At a solution pH of 6.3±0.2 (without HA), the critical coagulation concentration (CCC) values of NaCl were calculated as 90 and 120mmol/L for SWNTs and MWNTs, respectively; the values for CaCl2 were calculated as 0.82 and 2.6mmol/L, respectively. Increases in the solution pH from 3 to 10 led to a significant decrease in CNT aggregation, indicating the presence of functional groups on the CNTs’ surface. However, the impact of HA on the aggregation of CNTs in suspension depended on the type of the electrolyte that it was exposed to. In a NaCl solution, the stability of the CNTs was increased. For HA concentrations of 1, 5, and 20mg/L, the CCC values of NaCl for the MWNTs increased to 130, 160, and 240mmol/L, respectively. The effect was attributed to the steric repulsion induced by the adsorbed HA molecules. However, this effect was negligible in CaCl2 solutions. The results of this study elucidated the effects of surfactants, solution chemistry, and HA on the aggregation kinetics of CNTs, all of which may have significant influences on the behavior of CNTs in the environment.

Multi-salt coagulation of soft pitch colloids

The dynamic coagulation of colloidal pitch was quantified under orthokinetic conditions. The effects of single salt, multiple salt and cation valency on the stability of the colloidal pitch were investigated as a function of salt concentration. Critical coagulation concentrations (CCCs) were determined for a range of individual cations. The CCC in the presence of a number of divalent or trivalent cations was investigated to gain an understanding of the effect of multiple salts normally found in industrial systems.Electrostatic destabilisation of wood resin colloids by a single salt is strongly influenced by salt valency (z) and mostly independent of the individual cation (at constant z). Addition of a second cation to solution resulted in a decrease in the CCC for both calcium and aluminium ions in the presence of sodium ions. The decrease in the CCC for the wood resin colloids was non-linear and showed restabilisation of the colloids above the CCC, unlike the effects observed for a single salt. Comparison of the experimental CCC results with the DVLO theory indicates that a higher Hamaker constant than reported for the interaction of abietic acid with talc in water (used here as a model interaction) is needed. This suggests that the aggregation process of the wood resin colloids involves a stronger interaction than abietic acid with talc. Experimental CCC values for divalent and trivalent ions were also lower than those predicted theoretically. To obtain agreement between theory and experimental results, reduced Stern potentials and higher Hamaker constants were required, suggesting specific ion adsorption of the multivalent cations is occurring to reduce surface charge of the colloid.

Effects of material properties on sedimentation and aggregation of titanium dioxide nanoparticles of anatase and rutile in the aqueous phase

This study investigated the sedimentation and aggregation kinetics of titanium dioxide (TiO 2) nanoparticles with varying material properties (i.e., crystallinity, morphology, and chemical composition). Used in the study were various types of commercially available TiO 2 nanoparticles: three spherical anatase (nominal diameters of 5, 10, and 50 nm) and two rutile nanoparticles (10 × 40 and 30 × 40 nm). The 50 nm anatase and 10 × 40 nm rutile showed higher stability in deionized water and 5 mM NaCl solutions at pH 7 than the 5, and 10 nm anatase nanoparticles in sedimentation experiments. In aggregation experiments, critical coagulation concentration values for the 50 nm anatase were the highest, followed by the 10 × 40 nm rutile and the 5 nm anatase nanoparticles in NaCl and CaCl 2 solutions. The aggregation kinetics was fitted reasonably well with the Derjaguin–Landau–Verwey–Overbeek (DLVO) equations for the TiO 2 nanoparticles tested. Results showed that crystallinity and morphology are not influential factors in determining the stability of TiO 2 nanoparticle suspensions; however, the differences in their chemical compositions, notably, the varying concentrations of impurities (i.e., silicon and phosphorus) in the pristine materials, determined the surface charge and therefore the sedimentation and aggregation of TiO 2 nanoparticles in the aqueous phase.