Charging and aggregation of negatively charged colloidal latex particles in the presence of multivalent oligoamine cations

Ecotoxicological effects of antibiotic adsorption behavior of microplastics and its management measures

Microplastics adsorb heavy metals and organic pollutants to produce combined pollution. Recently, the adsorption behavior of antibiotics on microplastics has received increasing attention. Exploring the sorption behavior of pollutants on microplastics is an important reference in understanding their ecological and environmental risk studies. In this paper, by reviewing the academic literature in recent years, we clarified the current status of research on the adsorption behavior of antibiotics on microplastics, discussed its potential hazards to ecological environment and human health, and summarized the influence of factors on the adsorption mechanisms. The results show that the adsorption behavior of antibiotics on microplastics is controlled by the physical and chemical properties of antibiotics, microplastics, and water environment. Antibiotics are adsorbed on microplastics through physical and chemical interactions, which include hydrophobic interaction, partitioning, electrostatic interaction, and other non-covalent interactions. Intensity of adsorption between them is mainly determined by their physicochemical properties. The basic physicochemical properties of the aqueous environment (e.g., pH, salinity, ionic strength, soluble organic matter content, and temperature) will affect the physicochemical properties of microplastics and antibiotics (e.g., particle size, state of dispersibility, and morphology), leading to differences in the type and strength of their interactions. This paper work is expected to provide a meaningful perspective for better understanding the potential impacts of antibiotic adsorption behavior of microplastics on aquatic ecology and human health. In the meantime, some indications for future related research are provided.

Highly Uniform Self-Assembly of Gold Nanoparticles by Butanol-Induced Dehydration and Its SERS Applications in SARS-CoV-2 Detection

We report the development of a reproducible and highly sensitive surface-enhanced Raman scattering (SERS) substrate using a butanol-induced self-assembly of gold nanoparticles (AuNPs) and its application as a rapid diagnostic platform for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The butanol-induced self-assembly process was used to generate a uniform assembly of AuNPs, with multiple hotspots, to achieve high reproducibility. When an aqueous droplet containing AuNPs and target DNAs was dropped onto a butanol droplet, butanol-induced dehydration occurred, enriching the target DNAs around the AuNPs and increasing the loading density of the DNAs on the AuNP surface. The SERS substrate was evaluated by using Raman spectroscopy, which showed strong electromagnetic enhancement of the Raman signals. The substrate was then tested for the detection of SARS-CoV-2 using SERS, and a very low limit of detection (LoD) of 3.1 × 10-15 M was obtained. This provides sufficient sensitivity for the SARS-CoV-2 screening assay, and the diagnostic time is significantly reduced as no thermocycling steps are required. This study demonstrates a method for the butanol-induced self-assembly of AuNPs and its application as a highly sensitive and reproducible SERS substrate for the rapid detection of SARS-CoV-2. The results suggest the potential of this approach for developing rapid diagnostic platforms for other biomolecules and infectious diseases.

Effects of ionic valence on aggregation kinetics of colloidal particles with and without a mixing flow

HYPOTHESIS

The classical Schulze-Hardy rule states that the critical coagulation concentration (CCC) of colloidal particles is inversely proportional to the counter-ionic valence at powers ranging from 2 to 6. However, the inverse Schulze-Hardy rule has recently been proposed, suggesting that the CCC can also be inversely proportional to the co-ionic valence. Previous studies on these rules did not consider the effect of flow on aggregation kinetics and the CCC. This study aims to investigate the effect of multivalent counter-ions and co-ions on aggregation kinetics and the CCCs in systems with and without a mixing flow.

EXPERIMENTS

We measured the aggregation rate coefficients of polystyrene sulfate latex particles as a function of the salt concentration with different ionic species. Furthermore, we analyzed these measurements using theoretical models based on hydrodynamic pair-diffusion in a random flow and trajectory analysis in two steady flows. The analysis was conducted using zeta potentials determined through electrophoretic measurements.

FINDINGS

Although the trajectory analysis underestimates the CCCs, the hydrodynamic pair-diffusion model can capture the shift of critical coagulation concentrations in the mixing flow to higher values than those in Brownian aggregation and also shows a better agreement with the experimental results. This result suggests that combining random flow and Brownian diffusion is crucial for developing a consistent framework for predicting both Brownian aggregation and aggregation in a mixing flow.

Aggregation of Halloysite Nanotubes in the Presence of Multivalent Ions and Ionic Liquids

Colloidal stability was investigated in two types of particle systems, namely, with bare (h-HNT) and polyimidazolium-functionalized (h-HNT-IP-2) alkali-treated halloysite nanotubes in solutions of metal salts and ionic liquids (ILs). The valence of the metal ions and the number of carbon atoms in the hydrocarbon chain of the IL cations (1-methylimidazolium (MIM+), 1-ethyl-3-methylimidazolium (EMIM+), 1-butyl-3-methylimidazolium (BMIM+), and 1-hexyl-3-methylimidazolium (HMIM+)) were altered in the measurements. For the bare h-HNT with a negative surface charge, multivalent counterions destabilized the dispersions at low values of critical coagulation concentration (CCC) in line with the Schulze-Hardy rule. In the presence of ILs, significant adsorption of HMIM+ took place on the h-HNT surface, leading to charge neutralization and overcharging at appropriate concentrations. A weaker affinity was observed for MIM+, EMIM+, and BMIM+, while they adsorbed on the particles to different extents. The order HMIM+ < BMIM+ < EMIM+ < MIM+ was obtained for the CCCs of h-HNT, indicating that HMIM+ was the most effective in the destabilization of the colloids. For h-HNT-IP-2 with a positive surface charge, no specific interaction was observed between the salt and the IL constituent cations and the particles, i.e., the determined charge and aggregation parameters were the same within experimental error, irrespective of the type of co-ions. These results clearly indicate the relevance of ion adsorption in the colloidal stability of the nanotubes and thus provide useful information for further design of processable h-HNT dispersions.

An improved step-by-step airflow/paper-based colorimetric method for highly selective determination of halides in complex matrices

An improved step-by-step colorimetric method for determination of halides has been developed. The method is based on successive selective oxidation of iodide, bromide and chloride into corresponding free halogens, their extraction by airflow and colorimetric detection with different paper test-strips. This procedure can be performed in a single analyzed solution and possesses high selectivity and good sensitivity due to the extraction step. Three types of paper test-strips were examined: paper modified with tetramethylammonium iodide and starch, paper modified with methyl orange and paper modified with silver triangular nanoplates. Limits of detection for iodide, bromide and chloride are 0.01, 0.02, and 0.04 mg L^-1 respectively in case of the last mentioned paper. The method was applied to the analysis of samples having complex matrices, such as various seafood, preserves, bread, and natural waters, showing good accuracy of the analysis with recoveries of 95-105% and relative standard deviations not higher than 6%.

Strength of Flocs Formed by the Complexation of Lysozyme with Leonardite Humic Acid

Aggregation and aggregates properties of natural organic and nanosized macromolecules such as humic substances and proteins are crucial to explore so-called colloid-mediated transport and the fate of substances in soil and water environments. Therefore, the aggregation and dispersion, charging, and floc strength of lysozyme (LSZ)–leonardite humic acid (LHA) flocs were experimentally investigated. The experiments were performed in different salt concentrations and LSZ to LHA mass ratios as a function of pH. We obtained the stronger flocs at pH 4.4, where the isoelectric point (IEP) of the complex with the mass ratio 2.5 was confirmed. Thus, the aggregation of LSZ–LHA flocs is mainly caused by charge neutralization. We obtained the floc strength of 4.7 nN around IEP at low salt concentration of 3 mM, which was stronger than 2.8 nN in high salt concentration of 50 mM. The effect of salt concentration can be rationalized by charge-patch attraction at low salt concentration. With increasing mass ratio, the IEP shifted to higher pH. This is due to the increase in positive charge from LSZ in the mixture. The effect of the LSZ to LHA mass ratio on the maximum strength was weak in the range studied.

Aggregation of Colloidal Particles in the Presence of Hydrophobic Anions: Importance of Attractive Non-DLVO Forces

Aqueous suspensions of amidine latex (AL) and sulfate latex (SL) particles containing sodium tetraphenylborate and NaCl are studied with electrokinetic and time-resolved light-scattering techniques. In monovalent salt solutions, AL is positively charged, whereas SL is negatively charged. Electrophoretic mobility measurements demonstrate that adsorption of tetraphenylborate anions leads to a charge reversal of AL particles. At higher concentrations, both types of particles accumulate negative charge. For AL particles, the charge reversal leads to a narrow fast aggregation region and an intermediate regime of slow aggregation. For SL particles, the intermediate slow regime is also observed. These aspects can be explained with classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). Another regime of fast aggregation is observed at intermediate concentrations, and the existence of this regime can be rationalized by an additional attractive non-DLVO force. We suspect that this additional force is caused by surface charge heterogeneities.

Aggregation Behavior of Dissolved Black Carbon: Implications for Vertical Mass Flux and Fractionation in Aquatic Systems

The fluvial export of dissolved black carbon (DBC) is a major land-ocean flux in the global black carbon cycle, affecting the size of refractory carbon pool in the oceans. The aggregation behavior of DBC is a significant determinant of its transport and vertical mass flux. In this study, the aggregation kinetics and interaction energy of DBC leached from biochar were investigated. DBC was mainly stabilized by hydration force and underwent structural compacting in divalent cation solutions. Na⁺ and Mg²⁺ had limited impact on the colloidal stability of DBC due to the strong hydration of these cations. Ca²⁺ and Ba²⁺ readily destabilized DBC by forming inner-sphere complexes, reducing its hydrophilicity. Consistently, charge reversal of DBC was observed with high concentrations of Ca²⁺ and Ba²⁺. Simulated sunlight exposure led to photo-oxidation of DBC, increasing its colloidal stability. DBC behaved nonconservatively in laboratory mixing experiments using estuary water samples due to aggregation/sedimentation; while model aquatic humic acid behaved conservatively. Our results infer that there is a vertical mass flux of DBC and possible fractionation from the dissolved organic matter pool in the fluvial and estuarine systems, which have been overlooked in efforts to determine global carbon budgets and associated climate change implications.

Colloidal Stability in Asymmetric Electrolytes: Modifications of the Schulze-Hardy Rule

The Schulze-Hardy rule suggests a strong dependence of the critical coagulation concentration (CCC) on the ionic valence. This rule is addressed theoretically and confronted with recent experimental results. The commonly presented derivation of this rule assumes symmetric electrolytes and highly charged particles. Both assumptions are incorrect. Symmetric electrolytes containing multivalent ions are hardly soluble, and experiments are normally carried out with the well-soluble salts of asymmetric electrolytes containing monovalent and multivalent ions. In this situation, however, the behavior is completely different whether the multivalent ions represent the counterions or co-ions. When these ions represent the counterions, meaning that the multivalent ions have the opposite sign than the charge of the particle, they adsorb strongly to the particles. Thereby, they progressively reduce the magnitude of the surface charge with increasing valence. In fact, this dependence of the charge density on the counterion valence is mainly responsible for the decrease of the CCC with the valence. In the co-ion case, where the multivalent ions have the same sign as the charge of the particle, the multivalent ions are repelled from the particles, and the surfaces remain highly charged. In this case, the inverse Schulze-Hardy rule normally applies, whereby the CCC varies inversely proportional to the co-ion valence.

Heteroaggregation of oppositely charged particles in the presence of multivalent ions

Time-resolved dynamic light scattering is used to measure absolute heteroaggregation rate coefficients and the corresponding stability ratios for heteroaggregation between amidine and sulfate charged latex particles.

Ion specific effects on the stability of layered double hydroxide colloids

Positively charged layered double hydroxide particles composed of Mg(2+) and Al(3+) layer-forming cations and NO3(-) charge compensating anions (MgAl-NO3-LDH) were synthesized and the colloidal stability of their aqueous suspensions was investigated in the presence of inorganic anions of different charges. The formation of the layered structure was confirmed by X-ray diffraction, while the charging and aggregation properties were explored by electrophoresis and light scattering. The monovalent anions adsorb on the oppositely charged surface to a different extent according to their hydration state leading to the Cl(-) > NO3(-) > SCN(-) > HCO3(-) order in surface charge densities. The ions on the right side of the series induce the aggregation of MgAl-NO3-LDH particles at lower concentrations, whereas in the presence of the left ones, the suspensions are stable even at higher salt levels. The adsorption of multivalent anions gave rise to charge neutralization and charge reversal at appropriate concentrations. For some di, tri and tetravalent ions, charge reversal resulted in restabilization of the suspensions in the intermediate salt concentration regime. Stable samples were also observed at low salt levels. Particle aggregation was fast near the charge neutralization point and at high concentrations. These results, which evidence the colloidal stability of MgAl-NO3-LDH in the presence of various anions, are of prime fundamental interest. These are also critical for applications to develop stable suspensions of primary particles for water purification processes, with the aim of the removal of similar anions by ion exchange.

Charging and aggregation of latex particles in aqueous solutions of ionic liquids: towards an extended Hofmeister series

Ion specific effects govern the aggregation of latex particles in aqueous solutions of ionic liquids.

Nanometer-ranged attraction induced by multivalent ions between similar and dissimilar surfaces probed using an atomic force microscope (AFM)

Forces between similar and dissimilar surfaces are quantified and a short-ranged attraction can be identified.

Electrostatic interaction between nonuniformly charged colloids: experimental and numerical study

The influence of the surface charge distribution on the interaction between nanosized particles in water is reported. The distribution of charges at the surface of initially neutral microemulsion droplets has been modulated by additions of various oligomeric cationic surfactants. The osmotic compressibility of the doped microemulsions was measured by light and small-angle neutrons scattering and reveals that the overall effective interaction induced by the ionic groups is repulsive. However, particular charge distributions decrease the osmotic compressibility much less than others. Independent measurements of the activity of the bromide counterions with specific electrodes evidence a significant decrease in the effective charge, which, however, cannot account for the osmotic compressibility in the framework of the primitive model. The q dependence of the structure factor reveals an attractive contribution over a short distance. Numerical studies assign this attractive contribution to the overlap of hydration shells that are extended as a result of the charge localization.

Aggregation of negatively charged colloidal particles in the presence of multivalent cations

The aggregation and charging behavior of sulfate and carboxyl latex particles in the presence of different multivalent salts was studied. Time-resolved light scattering and electrophoresis are the main experimental techniques used. In particular, the influence of the type of counterion is investigated. The main conclusion is that the valence of the counterion is highly relevant in determining the aggregation behavior, whereas its chemical nature is rather unimportant. Multivalent ions of higher valence destabilize the suspensions more effectively, in particular, by shifting the critical coagulation concentration (CCC) to lower values. This behavior reflects the classical Schulze-Hardy rule. Comparison with literature data reveals that the presently investigated systems behave similarly to the ones described earlier, but the observed dependence on valence is weaker than in some other systems. Moreover, we observe a slowdown of the aggregation at high electrolyte concentrations. This slowdown can be explained by the greater viscosity of the electrolyte solutions under these conditions.

Probing titanate nanowire surface acidity through methylene blue adsorption in colloidal suspension and on thin films

The interaction of the cationic dye methylene blue (MB) with titanate nanowires (TiONWs) was investigated in different pH environments using visible spectroscopy and electrophoresis on thin films as well as in aqueous suspension. The surface charge of the TiONWs depends on the pH and ionic strength leading to positive charge under acidic and negative under alkaline conditions. The TiONWs have the same adsorption capacity on films and in suspensions at neutral pH while under alkaline conditions they are able to adsorb significantly more MB in suspension due to their higher surface area. Detailed adsorption studies in water revealed that dye cations form monomers, dimers and larger aggregates of H-type (face-to-face) on the TiONW films. The results indicate that below pH = 4.0 the TiONWs' external surface consists of Brøntsted acid sites capable of protonating MB. It was suggested that reversible indicator role of MB molecule dimers probes the TiONW surface acidity (Brøntsted sites).