Critical Coagulation Concentration Values Research Articles

Reciprocal effects of NOM and solution electrolyte ions on aggregation of ferrihydrite nanoparticles

The effects of natural organic matter (NOM) types and electrolyte ions are crucial to the aggregation of ferrihydrite nanoparticles (Fh NPs) in the environment. Dynamic light scattering (DLS) was employed for the aggregation kinetics of Fh NPs (10 mg/L as Fe) in the present study. The critical coagulation concentration (CCC) values of Fh NPs aggregation in NaCl were obtained in the presence of 15 mg C/L NOM as SRHA (857.4 mM) > PPHA (752.3 mM) > SRFA > (420.1 mM) > ESHA (141.0 mM) > NOM-free (125.3 mM), indicating Fh NPs aggregation was inhibited as the above order. Comparatively in CaCl2, the CCC values were measured in ESHA (0.9 mM), PPHA (2.7 mM), SRFA (3.6 mM), SRHA (5.9 mM), NOM-free (76.6 mM), implying NPs aggregation was enhanced following the order of ESHA > PPHA > SRFA > SRHA. To investigate the dominant mechanisms, the aggregation of Fh NPs was comprehensively studied under the effects of NOM types, concentrations (0–15 mg C/L) and electrolyte ions (NaCl/CaCl2 beyond CCC). In NaCl/CaCl2, the low concentration of NOM (<7.5 mg C/L) could accelerate NPs aggregation mainly due to patch-charge attraction. When NOM concentration was high (> 7.5 mg C/L), the inhibition effect on NPs aggregation occurred in NaCl due to steric repulsion, whereas the enhancement effect in CaCl2 of aggregation was dominated by the bridging effect. The results indicated that the effects of NOM types, concentration and electrolyte ions should be carefully considered for the environmental behavior of NPs.

Influence of fulvic acid sub-fractions on aggregation kinetics of graphene oxide in aqueous environments

Fulvic acid (FA) can affect the dispersion of graphene oxide (GO) in aquatic environments, however, the possible mechanisms remain unclear. Dynamic light scattering techniques combined with a multiple regression model were applied to explore the influence of FA sub-fractions (FApH3 − FApH13) on the aggregation kinetics of GO in aqueous environments. The ratios of critical coagulation concentration (CCC) values were CCCNa: CCCMg: CCCLa: CCCCe = 1:2–5.15:3–7.31:3–7.35, which were consistent with the Derjaguin−Landau−Verwey−Overbeek (DLVO) theory and Schulze−Hardy rules. The GO remained stable at pH 3–10 and aggregated at pH < 3 or pH > 10, and its critical coagulation pH values were 1.44 and 12.25 with 10 mM NaCl as background. The CCC values of GO in the presence of FApH3 − FApH13 were greater than those in the absence of FA sub-fractions. The ratios of CCC values of GO (r) increased in the presence of FA sub-fractions in the order of FApH13 > FApH9 > FApH7 > FApH5 > FApH3 and ranged from 1.01 to 2.15 for certain metal ions including Na+, Mg2+, La3+, and Ce3+. The CCC values of GO were significantly related to C, H, O, N, S, H/C, O/C, carboxylic C, and carbonyl C of FA sub-fractions (P < 0.05), respectively, and could be predicted using the multiple linear regression eq. CCC = Z-n (98.959– 60.911 ∗ O/C + 4.799 ∗ O-alkyl C – 0.845 ∗ aromatic C – 6.237 ∗ carbonyl C). The predicted CCC values for GO were within 90 % prediction intervals, and the average error of the CCC values was 3.3 % and R2 = 0.986. This investigation is expected to provide a scientific basis for the transport and ecotoxicity of GO in environments.

Stability of Detonation Nanodiamond Colloid with Respect to Inorganic Electrolytes and Anionic Surfactants and Solvation of the Particles Surface in DMSO–H2O Organo-Hydrosols

In this article, the stability of sols of detonated nanodiamonds, DND, with positively charged colloidal particles, is studied in different salt solutions in water–dimethyl sulfoxide (DMSO) binary systems containing 95 vol.% organic solvent. Additionally, several CCC values are determined in 95 vol.% acetonitrile for comparison. The critical coagulation concentrations (CCC) are determined using the dynamic light scattering technique and the Fuchs function. As coagulators, NaCl, NaBr, NaNO3, NaClO4, Nan-C8H17SO3, and Nan-C12H25OSO3 are used. Comparison of the CCC values in DMSO–H2O and CH3CN–H2O with those obtained in water allows us to make some conclusions. The variations of these values in different solvents are explained in terms of good and poor interfacial solvation of colloidal particles, “structural” contribution to the interparticle interaction energy, lyotropic series for anions, and more or less pronounced adsorption of surfactants. The study of the stability of DND hydrosol in solutions of anionic surfactants with different hydrocarbon tail length demonstrated the crucial role of adsorption in the coagulation process.

Polarisation‐induced covalent interactions between H+ and surface O atoms promote clay aggregation

AbstractAccumulation of H+ can cause soil acidification, affect soil particle interactions, and alter soil processes. In this study, aggregation kinetics of montmorillonite colloids induced by monovalent ions of Li+ and H+ were determined by dynamic light scattering. The experimental results indicated that the critical coagulation concentration (CCC) value of Li+ was 4.5 times that of H+, indicating that H+ can strongly promote montmorillonite colloids aggregation. Joint analysis of the adsorption force type and energy of Li+ and H+ as well as the infrared spectroscopy of Li+ and H+ montmorillonite indicated that there was only electrostatic adsorption for Li+, while there was not only electrostatic adsorption but also a new type of covalent adsorption for H+: polarisation‐induced covalent adsorption. The total adsorption energy of H+ is thus much higher than that of Li+ on montmorillonite surfaces. Moreover, the polarisation‐induced covalent adsorption energy of H+ increased linearly with increasing electric field strength, which is consistent with the quantum mechanical analysis of the asymmetric orbital hybridisation of surface O atoms on montmorillonite. On this basis, the polarisation‐induced covalent adsorption energy of H+ was employed to predict the CCC value of H+ induced montmorillonite aggregation, and it was 7.40 mmol L−1. The high agreement with experimental results (7.26 mmol L−1) proved that the polarisation‐induced covalent interactions between H+ and surface O atoms promotes montmorillonite aggregation. This polarisation induced covalent effect between H+ and surface O atoms might further affect the formation and stability of soil aggregates and the pore status of soil structure, especially as soil acidification is currently becoming a global issue. The discovery from this study will improve our understanding of the effects of soil acidification on soil properties and processes.Highlights H+ greatly promoted clay aggregation. Polarization‐induced covalent between H+ and surface O presented at clay surface. Polarization‐induced covalent bonding energy increase with increasing electric field. Electrostatic and covalent bonding forces jointly determined clay aggregation.

Functionalization of Pristine, Metallic, and Semiconducting-SWCNTs by ZnO for Efficient Charge Carrier Transfer: Analysis through Critical Coagulation Concentration.

Noncovalent functionalization of single-walled carbon nanotubes (SWCNT) by semiconducting oxides is a majorly sought technique to retain individual properties while creating a synergetic effect for an efficient heterostructure charge transfer. Three types of electronically and optically different SWCNTs: metallic (m), semiconducting (s), and pristine (p) are functionalized by ZnO using a facile sonication method. The physicochemical and morphological properties of the ZnO-functionalized SWCNTs, m-SWCNT+ZnO, s-SWCNT+ZnO, and p-SWCNT+ZnO, are analyzed by advanced characterization techniques. Evidence of charge transfer between SWCNT and ZnO is observed with an increase in charge carrier lifetime from 3.31 ns (ZnO) to 4.76 ns (s-SWCNT+ZnO). To investigate the optimum interaction between SWCNTs and ZnO, critical coagulation concentrations (CCC) are determined using UV–vis absorption spectroscopy for m-SWCNT, s-SWCNT, and p-SWCNT using different molar concentrations of ZnO as the coagulant. The interaction and coagulation mechanisms are described by the modified DLVO theory. Due to the variation in dielectric values and electronic properties of SWCNTs, the CCC values obtained have differed: m-SWCNT (1.9 × 10–4), s-SWCNT (3.4 × 10–4), and p-SWCNT (2 × 10–4). An additional analysis of the aggregates and supernatants of the CCC experiments is also shown to give an insight into the interaction and coagulation processes, explaining the absence of influence exerted by sedimentation and centrifugation.

Effects of low molecular weight organic acids on aggregation behavior of biochar colloids at acid and neutral conditions

Low molecular weight organic acids (LMWOAs), as active components in the rhizosphere carbon cycling, may influence the environmental behaviors of biochar colloids. This study selected the pine-wood and wheat-straw biochars (PB and WB) as two typical biochars. The effects of typical LMWOAs (oxalic acid, citric acid, and malic acid) on aggregation kinetics of PB and WB colloids were investigated under pH 4 and 6 conditions. Critical coagulation concentrations (CCCs) of both PB and WB colloids were decreased with the LMWOAs regardless of the types of biochar and the solution pH, and the most significant effect occurred in pH 4 due to more LMWOAs sorption on the biochar colloids. The different types of LMWOAs caused various CCCs changes. For example, the CCC values of PB colloids decreased from 75 mM to 56, 52, and 47 mM in the pH 4 NaCl solutions when 1 mM oxalic acid, citric acid, and malic acid were present in the suspensions, respectively. The chemical structure (functional groups) and molecular weight of LMWOAs, solution pH, and the electrophoretic mobility (EPM) of biochar co-influence the interactions between biochar colloids and LMWOAs, thus affecting the stability of biochar colloids in the presence of LMWOAs. The presence of LMWOAs accelerated the aggregation of colloidal biochar by increasing the interaction of surface bridging bonds (hydrogen bonding) and decreasing the repulsive force between colloidal biochar particles. This study showed that LMWOAs could accelerate the aggregation of biochar colloids in acidic or neutral environments and reduce the mobility of biochar colloids in soil rhizosphere.

Divalent cations accelerate aggregation of Black phosphorus nanodots

Black phosphorus (BP) nanodots inevitably interact with electrolyte ions because of their extensive applications in many fields. In biomedical applications, BP-nanodots might encounter various salt ions in body fluids and have a propensity to aggregate, further resulting in an enhanced toxicity risk. In this study, the effects of electrolyte cations (Na+, K+, Ca2+, and Mg2+) on the aggregation kinetics of BP-nanodots were investigated by means of dynamic light scattering (DLS). The critical coagulation concentration values of BP-nanodots were determined to be 242.11 mM NaCl, 95.01 mM KCl, 1.92 mM CaCl2, and 5.88 mM MgCl2. Divalent cations accelerated aggregation of BP-nanodots because of the stronger surface charge screening and their bridging with oxidized phosphorus species (PxOy) available on BP-nanodot surfaces. Furthermore, the divalent cations may interact with PxOy to form insoluble complex coatings on the nanoparticle surface and shield the charge. After modification with PEG, BP-nanodots presented enhanced stability in the presence of 2.5 mM CaCl2. This work is of great significance for understanding the aggregation behaviors of ultrasmall nanoparticles such as BP-nanodots in electrolyte solutions and provides insight into their safety assessment associated with biomedical applications.

Co‐aggregation of mixture components of montmorillonite, kaolinite and humus

AbstractSoil particle aggregation is closely related to soil quality and pollutant migration in the environment. Soil is a naturally complex system with multi‐components, including different organic and inorganic particles. However, current studies with respect to particle aggregation by dynamic light scattering (DLS) have centred on single‐component particles. Therefore, the application of DLS to mixture components is an important issue in soil science research. In this study, montmorillonite (Mont), kaolinite (Kaol) and humic acid (HA) particles were mixed in different mass ratios to form two‐ and three‐mixture components, and then the aggregation kinetics of the mixture components in KNO3 solutions were studied by DLS. The results showed that (a) for any given mixing ratio of two‐ and three‐mixture component particles, there was only one critical coagulation concentration (CCC) value observed for the mixture components; (b) the CCCs of the two‐mixture components aggregation were always between the CCC values of each single component in the mixture, and the CCC values of the three‐mixture components aggregation were always between those of the two‐components aggregation. Based on those experimental results we concluded that, for mixture components aggregation, all the components would participate in the aggregation with equal probability, and the aggregation should be driven by the average DLVO forces of the mixture components. Therefore, for the mixture components of natural soil, the soil particle aggregation would be a co‐aggregation process, and the DLS technique would thus be a useful tool to study the real soil aggregation. Meanwhile, HA would decrease soil particle aggregation by increasing the DLVO/XDLVO repulsive forces, rather than promoting soil particle aggregation.Highlights The aggregation of mixture colloids analysed by DLS is an important issue in soil science research. There was only one CCC for mixture colloids with different colloidal components. The aggregation of mixture colloids (soil) is a co‐aggregation process. HA will decrease soil particle aggregation by increasing the DLVO/XDLVO repulsive forces.

Ion Specific Effects on the Stability of Halloysite Nanotube Colloids-Inorganic Salts versus Ionic Liquids.

Charging and aggregation processes were studied in aqueous dispersions of halloysite nanotubes (HNTs) in the presence of monovalent inorganic electrolytes and ionic liquid (IL) constituents. The same type of co-ion (same sign of charge as HNT) was used in all systems, while the type of counterions (opposite sign of charge as HNT) was systematically varied. The affinity of the inorganic cations to the HNT surface influenced their destabilizing power leading to an increase in the critical coagulation concentration (CCC) of HNT dispersions in the Cs+ < K+ < Na+ order. This trend agrees with the classical Hofmeister series for negatively charged hydrophobic surfaces. For the IL cations, the CCCs increased in the order BMPY+ < BMPIP+ < BMPYR+ < BMIM+. An unexpectedly strong adsorption of BMPY+ cations on the HNT surface was observed giving rise to charge neutralization and reversal of the oppositely charged outer surface of HNT. The direct Hofmeister series was extended with these IL cations. The main aggregation mechanism was rationalized within the classical theory developed by Derjaguin, Landau, Verwey, and Overbeek, while ion specific effects resulted in remarkable variation in the CCC values. The results unambiguously proved that the hydration level of the surface and the counterions plays a crucial role in the formation of the ionic composition at the solid–liquid interface and consequently, in the colloidal stability of the HNT particles in both inorganic salt and IL solutions.

Surface-Engineered Nanomaterials in Water: Understanding Critical Dynamics of Soft Organic Coatings and Relative Aggregation Density

Inorganic-organic nanocomposites, typically as an inorganic core with surface organic coating(s), have received interest as potential platform materials for sensor, catalyst, sorbent, and environmental applications, among others. Here, we describe the critical role of organic surface coatings with regard to the colloidal stability of engineered manganese oxide nanoparticles (MnxOy NPs). Specifically, we prepared libraries of monodisperse MnxOy NPs with a serial selection of surface coatings (stearic acid (SA), oleic acid (OA), poly(maleic anhydride-alt-1-octadecene) (PMAO), linear polyethyleneimine (LPEI), and multibranched polyethyleneimine (BPEI)), which were chosen based on comparable structure(s) and functional group(s). We systematically evaluated the role of surface organic coatings via critical coagulation concentrations (CCCs), which were compared with theoretical calculations (Schulze-Hardy rule). Through a newly developed light scattering protocol, we observed that the effective density of nanoclusters can exceed NPs' primary (bulk) density depending on the open space(s) within organic coatings. Interestingly, PMAO-coated NPs were more stable at the point of zero charge (PZC) than at neutral pH (pH 7), despite the loss of effective surface charge potential. CCC was 334 mM in NaCl and 1.5 mM in CaCl2 at pH 7, compared to CCC values of 807 mM in NaCl and 210 mM in CaCl2 at PZC. This increase in stability is due to polymer (re)configuration (at PZC), which was further confirmed with a quartz crystal microbalance-based technique to evaluate surface-based polymer dynamics. Taken together, this work quantifies the role of organic coating dynamics, including structure, grafting density, and configuration on the colloidal stability of organic-coated NPs.

Aggregation kinetics of fragmental PET nanoplastics in aqueous environment: Complex roles of electrolytes, pH and humic acid.

The aggregation kinetics of fragmental polyethylene glycol terephthalate (PET) nanoplastics under various chemistry conditions in aqueous environment were firstly investigated in this work. The aggregation of PET nanoplastics increased with increasing electrolyte concentrations and decreasing solution pH, which became stronger with the presence of divalent cations (e.g. Ca2+ and Mg2+) than that of monovalent cations (e.g. Na+ and K+). The effect of cations with the same valence on the aggregation of PET nanoplastics was similar. The measured critical coagulation concentrations (CCC) for PET nanoplastics at pH 6 were 55.0mM KCl, 54.2mM NaCl, 2.1mM CaCl2 and 2.0mM MgCl2, which increased to 110.4mM NaCl and 5.6mM CaCl2 at pH 10. In addition, the aggregation of PET nanoplastics was significantly inhibited with the presence of humic acid (HA), and the CCC values increased to 558.8mM NaCl and 12.3mM CaCl2 (1mgL-1 HA). Results from this study showed that the fragmental PET nanoplastics had the quite higher CCC values and stability in aqueous environment. In addition, the aggregation behaviors of PET nanoplastics can be successfully predicted by the Derjguin Landau Verwey Overbeek (DLVO) theory.

Colloidal stability of cellulose nanocrystals in aqueous solutions containing monovalent, divalent, and trivalent inorganic salts

Aggregation kinetics and surface charging properties of rod-like sulfated cellulose nanocrystals (CNCs) have been investigated in aqueous suspensions containing monovalent, divalent, or trivalent inorganic salts. Electrophoresis and time-resolved dynamic light scattering (DLS) were used to characterize the surface charge and colloidal stability of the CNCs, respectively. The surface charge and aggregation kinetics of the sulfated CNCs were found to be independent of solution pH (pH range 2–10). For the monovalent salts (CsCl, KCl, NaCl, and LiCl), the critical coagulation concentration (CCC) followed the order of Cs+ < K+ < Na+ < Li+, which follows the direct Hofmeister series, indicating specific interaction of the cations with the CNCs surface. The experimental aggregation kinetics of CNCs were in very good agreement with predictions based on the classic Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. A Hamaker constant of 3.6 × 10−20 J for the CNCs in aqueous medium was derived, for the first time, from the colloidal stability curves with monovalent salts. This value is consistent with a previous value determined by direct force measurements for cellulose surfaces in aqueous solutions. For the divalent salts (MgCl2, CaCl2, and BaCl2), the CCC values followed the order Mg2+ > Ca2+ > Ba2+, which is in the reverse order of the counterion ionic size. For the trivalent salts (LaCl3, AlCl3, and FeCl3), the CNCs suspension was destabilized much more effectively. The observed complex stability curves with AlCl3 and FeCl3 are attributed to charge neutralization and charge reversal imparted by the adsorption of aluminum and ferric hydrolysis species on the CNC surface. The significant charge reversal induced by the ferric hydrolysis species led to the restabilization of suspensions. Our results on the colloidal stability of CNCs are of central importance to the nanotechnology and materials science communities working on various applications of CNCs.

A Simple Method to Determine Critical Coagulation Concentration from Electrophoretic Mobility

Critical coagulation concentration (CCC) is a key parameter of particle dispersions, since it provides the threshold limit of electrolyte concentrations, above which the dispersions are destabilized due to rapid particle aggregation. A computational method is proposed to predict CCC values using solely electrophoretic mobility data without the need to measure aggregation rates of the particles. The model relies on the DLVO theory; contributions from repulsive double-layer forces and attractive van der Waals forces are included. Comparison between the calculated and previously reported experimental CCC data for the same particles shows that the method performs well in the presence of mono and multivalent electrolytes provided DLVO interparticle forces are dominant. The method is validated for particles of various compositions, shapes, and sizes.

Aggregation behavior of graphitic C3N4 nanosheets in aqueous environment: Kinetics and mechanisms

The aggregation behaviors of graphitic carbon nitride (g-C3N4) nanosheets under various electrolytes and pH conditions were systematically investigated. The aggregation of g-C3N4 nanosheets was significantly enhanced with increasing electrolyte concentrations. The divalent electrolytes (CaCl2 and MgCl2) were more effective than monovalent electrolytes (NaCl and KCl) in promoting the aggregation of g-C3N4 nanosheets. At the same valence, cations with higher atomic weight were more effective in enhancing the aggregation of g-C3N4 nanosheets. The measured critical coagulation concentrations (CCC) of g-C3N4 nanosheets were 4.7 mM KCl, 9.2 mM NaCl, 1.0 mM CaCl2 and 1.9 mM MgCl2 at pH 6.0, which were lower than some of other two-dimensional nanoparticles. The CCC values of g-C3N4 nanosheets were decreased to 5.5 mM NaCl at pH 2.0, but increased to 29.0 mM NaCl and 2.1 mM CaCl2 at pH 10.0, indicating that the aggregation degree of g-C3N4 nanosheets was decreased with increasing pH. The Fe/Al hydrated complexes generated at the specific pH inhibited the aggregation of g-C3N4 nanosheets and enhanced the stability. Overall, findings from this study demonstrated that the electrolytes and pH conditions played important and combined roles on the aggregation of g-C3N4 nanosheets. In addition, the aggregation behaviors of g-C3N4 nanosheets could be well predicted with the DLVO theory.

Colloidal Behaviors of Two-Dimensional Titanium Carbide in Natural Surface Waters: The Role of Solution Chemistry

Although two-dimensional titanium carbide (Ti3C2Tx MXene) has emerged as a shining star material in various communities, its environmental behaviors and fate remain unknown. Herein, the colloidal properties and stability of Ti3C2Tx MXene are explored in aquatic systems for the first time, considering the roles of solution chemistry conditions (e.g., pH, ionic types, and strength). It was found that pH had no effect on the stability of Ti3C2Tx in the range of 5.0-11.0, whereas ionic valence and concentrations displayed significant effects on the aggregation behavior of Ti3C2Tx. By employing time-resolved dynamic light scattering measurements, the critical coagulation concentration (CCC) value of Ti3C2Tx was determined to be 12 mM for NaCl. The divalent cations Ca2+ and Mg2+ exhibited higher destabilizing capacity to Ti3C2Tx, as evidenced by the lower CCC values (0.3 and 0.4 mM for CaCl2 and MgCl2, respectively) and faster coagulation rates. Long-term stability studies implied that Ti3C2Tx MXene was less likely to be transported over long distances in the synthetic or natural waters. These findings provided significant insights into the fate and transport of Ti3C2Tx in the aquatic environment.

Aggregation and stability of nanoscale plastics in aquatic environment

The widespread use and release of plastics in nature have raised global concerns about their impact on public health and the environment. While much research has been conducted on macro- and micro-sized plastics, the fate of nanoscale plastics remains unexplored. In this study, the aggregation kinetics and stability of polyethylene and polystyrene nanoscale plastics were investigated over a wide range of aquatic chemistries (pH, salt types (NaCl, CaCl2, MgCl2), ionic strength) relevant to the natural environment. Results showed that salt types and ionic strength had significant effects on the stability of both polyethylene and polystyrene nanoscale plastics, while pH had none. Aggregation and stability of both polyethylene and polystyrene nanoscale plastics in the aquatic environment followed colloidal theory (DLVO theory and Schulze-Hardy rule), similar to other colloidal particles. The critical coagulation concentration (CCC) values of polyethylene nanoscale plastics were lower for CaCl2 (0.1 mM) compared to NaCl (80 mM) and MgCl2 (3 mM). Similarly, CCC values of polystyrene nanospheres were 10 mM for CaCl2, 800 mM for NaCl and 25 mM for MgCl2. It implies that CaCl2 destabilized both polyethylene and polystyrene nanoscale plastics more aggressively than NaCl and MgCl2. Moreover, polystyrene nanospheres are more stable in the aquatic environment than polyethylene nanoscale plastics. However, natural organic matter improved the stability of polyethylene nanoscale plastics in water primarily due to steric repulsion, increasing CCC values to 0.4 mM, 120 mM and 8 mM for CaCl2, NaCl and MgCl2 respectively. Stability studies with various water conditions demonstrated that polyethylene nanoscale plastics will be fairly stable in the natural surface waters. Conversely, synthetic surface water, wastewater, seawater and groundwater rapidly destabilized polyethylene nanoscale plastics. Overall, our findings indicate that significant aqueous transport of nanoscale plastics will be possible in natural surface waters.

Effects of salt ions and heating on the behaviour of whey protein particle dispersions

We investigated the aggregation stability of whey protein isolate (WPI) particles in the presence of different salts (NaCl, MgCl2, and AlCl3) and the heat stability of dispersions in the presence of protein (native WPI) and polysaccharides (dextran and gum Arabic, GA). WPI particles coagulated either at pH values close to the iso-electric point of proteins or at high salt concentrations. Critical coagulation concentration (CCC) for different salts decreased with increasing valence of ions. CCC values for colloidal WPI particle dispersions closely followed the Schulze-Hardy rule, which is derived from the DLVO theory; however, deviations from the theory, particularly in the case of multivalent ions, showed that the specific ion adsorption also played a role. To investigate the heat stability of WPI particles, particle size and viscosities of dispersions containing different biopolymers were measured. Using dynamic viscosity of dilute dispersions, with the assumption of no particle aggregation after heating, swelling ratio (SR) of particles was estimated. Increasing concentrations of biopolymers decreased the SR of particles, and the presence of GA totally inhibited the swelling. Shear viscosity of concentrated dispersions showed an increase and shear-thickening behaviour after heating, when native WPI or dextran was present, whereas viscosity did not change when GA was present in the continuous phase. At low volume fractions, swelling was more likely to occur; whereas at high volume fractions, possibly both swelling and aggregation occurred. In addition, concentration and type of biopolymer were found to be important for the heat stability of dispersions.

Colloidal properties and stability of UV-transformed graphene oxide in aqueous solutions: The role of disorder degree

Motivated by the use of ultraviolet (UV) radiation in the disinfection processes of drinking and waste water treatment plants, this study explores the colloidal properties and stability of UV irradiated graphene oxide (GO) by using the batch technique and time-resolved dynamic light scattering over a wide range of salt types (NaCl, MgCl2, and CaCl2) and ionic strength relevant to engineered and natural systems. The results show that the UV irradiation time has an important impact on the physicochemical properties of GO and consequently on its colloidal properties and stability. The aggregation kinetics, attachment efficiency and critical coagulation concentration (CCC) values of UV irradiated GO are obtained for the first time. By correlating CCC values with physicochemical properties, we find that the disorder degree plays a more important role in colloidal properties and stability of UV irradiated GO than oxygen containing functional groups. The findings are valuable for environmental fate assessments on various families of functionalized GO.

Dispersion, sedimentation and aggregation of multi-walled carbon nanotubes as affected by single and binary mixed surfactants.

Two commonly used dispersants, octyl phenol ethoxylate (Triton X-100) and sodium dodecyl sulfate (SDS), were employed to explore the effects of single or mixed surfactants on the dispersion, sedimentation and aggregation of multi-walled carbon nanotubes (MWCNTs). Non-ionic surfactant TX100 showed much superior capability to anionic surfactant SDS in dispersing MWCNTs due to the benzene ring structure in its tail group. The addition of SDS reduced the adsorption of TX100 on the surface of MWCNTs and the consequent suspension of MWCNTs. The dispersing ability of TX100–SDS binary mixture was between those of individual SDS and TX100. The introduction of SDS greatly retarded the sedimentation and aggregation of suspended MWCNTs. The critical coagulation concentration (CCC) values of suspended MWCNTs dispersed by TX100 (2000 mg l−1), SDS (2000 mg l−1) and TX100–SDS (2000 mg l−1 of each component) were 48.6, 398 and 324 mM, respectively, for Na+ treatments. The CCC values were much lower for Ca2+ treatments, which were 30.4 and 32.1 mM, respectively, for MWCNTs dispersed by TX100 and TX100–SDS mixture. Overall, these results demonstrated that although the introduction of SDS did not improve the ability of TX100 in suspending MWCNTs, the suspensions exhibited more stable properties than those dispersed by TX100 alone. Our findings have important implications for the design of surfactant mixtures and the prediction of the behaviour and fate of MWCNTs in the water environment.

Specific ion effects of Cu2+, Ca2+ and Mg2+ on montmorillonite aggregation

Differences in ion-surface reactions among various divalent cations may result in large differences in clay particle interaction behaviors. Specific ion effects of Cu2+, Ca2+ and Mg2+ in montmorillonite (Mt) aggregation were studied by laser light scattering. The aggregation rate in the presence of Cu2+ was much higher than that in the presence of Ca2+ and Mg2+. The critical coagulation concentrations (CCCs) were Cu2+ (0.650 mM)<<Ca2+ (2.38 mM)<Mg2+ (7.99 mM). The differences in aggregation rates and CCC values were attributed to non-electrostatic interaction forces between the cations and the clay mineral surface. These non-electrostatic forces generated from (i) non-classical polarization of cations in the strong electric field near the clay mineral surface; and (ii) the quantum-mechanical interaction between the strongly polarized cations, especially for the d-type orbital electronic configuration of Cu2+, and the strongly polarized surface oxygen atoms. It was assumed that a new, polarization-induced covalent bond formed between Cu2+ and the surface O.