Hydrophobic Colloids Research Articles

Multifunctional Colloids with Reversible Phase Transfer between Organic and Aqueous Media via Layer-by-Layer Assembly

We report the successful multifunctional colloids that enable reversible phase transfer between organic and aqueous phases via layer-by-layer (LbL) assembly. These colloids exhibited a high level of dispersion stability in a variety of solvents ranging from nonpolar to aqueous media, based on the type of outermost layer adsorbed onto the colloids. Hydrophobic nanoparticles (NPs) synthesized using carboxylic acid or ammonium moiety-based ligands (i.e., oleic acid or tetraoctylammonium) in a nonpolar solvent (toluene, hexane, or chloroform) were directly deposited onto dendrimer-coated SiO2 colloids via ligand exchange between the hydrophobic ligands and the amine-functionalized dendrimers in the same organic solvent. Additionally, these hydrophobic NPs were adsorbed onto the colloids forming the densely packed layer structure that could not be easily achieved by conventional electrostatic LbL assembly. The subsequent adsorption of amine-functionalized dendrimers onto hydrophobic NP-coated colloids led to well-dispersed colloids in aqueous media as well as alcohol solvent and possibly induced the deposition of electrostatic LbL-assembled films, such as (cationic AuNP/anionic polyelectrolyte (PE))n or (cationic PE/anionic enzyme)n multilayers. Furthermore, the additional deposition of ligand exchange-induced multilayers (i.e., (dendrimer/hydrophobic NP)n) onto electrostatic multilayer-coated colloids produced colloids with highly dispersible properties in organic media. Given that previous approaches to the reversible phase transfer of colloids have depended on the physicochemical properties of selective ligands under limited and specific conditions, our approach may provide a basis for the design and exploitation of high-performance colloids with tailored functionality in a variety of solvents.

Contribution of different effluent organic matter fractions to membrane fouling in ultrafiltration of treated domestic wastewater

In the present work, effluent organic matter (EfOM) in treated domestic wastewater was separated into hydrophobic neutrals, colloids, hydrophobic acids, transphilic acids and neutrals and hydrophilic compounds. Their contribution to dissolved organic carbon (DOC) was identified. Further characterization was conducted with respect to molecular size and hydrophobicity. Each isolated fraction was dosed into salt solution to identify its fouling potential in ultrafiltration (UF) using a hydrophilized polyethersulfone membrane. The results show that each kind of EfOM leads to irreversible fouling. At similar delivered DOC load to the membrane, colloids present the highest fouling effect in terms of both reversible and irreversible fouling. The hydrophobic organics show much lower reversibility than the biopolymers present. However, as they are of much smaller size than the membrane pore opening, they cannot lead to such severe fouling as biopolymers do. In all of the isolated fractions, hydrophilics show the lowest fouling potential. For either colloids or hydrophobic substances, increasing their content in feedwater leads to worse fouling. The co-effect between biopolymers and other EfOM fractions has also been identified as one of the mechanisms contributing to UF fouling in filtering EfOM-containing waters.

Coagulation by multivalent counterions and the Schulze–Hardy rule

Starting from a number of empirical papers by Matijević et al., the issue of coagulation of hydrophobic colloids by multivalent inorganic counterions is revisited. More recent information is also considered, and some important trends are confirmed and made more quantitative. It can be concluded that the high coagulating power of multivalent ions is mostly not a direct result of their high valence but caused by their propensity of building complexes with water that adsorb specifically. This mechanism requires a revision of the Schulze–Hardy rule interpretations. In addition, this finding helps to define conditions where overcharging by multivalent counterions can be attributed to ion correlations.

Influence of interfacial water layer on surface properties of silver halides: Effect of pH on the isoelectric point

The hypothesis that pH dependent charge of interfacial water affects electrokinetic charge and electrokinetic potential of hydrophobic colloids, but not the (inner) surface potential was tested. It was found that isoelectric points of silver chloride, bromide and iodide shift to the higher pAg values in the acidic solutions, but that surface potential did not depend on pH. Isoelectric points of water at inert surfaces lie in the range 2<pH<4. In the neutral environment around pH≈7, the interfacial water is negatively charged due to preferential accumulation of OH− ions, with respect to H+ ions, thus increasing the negative electrokinetic charge of silver halide particles. In the acidic region, the isoelectric points of silver halides were shifted to the higher pAg values by 0.5–1.0pAg units depending on the type of silver halide, as well on the ionic strength of the solution. At pH≈3, the interfacial water is almost uncharged so that electrokinetic charge of silver halide particles is due to adsorption of silver and halide ions only. The conclusion was supported by mass titration experiments. Consequently, the electroneutrality points of silver halide surfaces correspond to values obtained in the acidic media. For silver chloride, pAgeln=5.3; for silver bromide, pAgeln=5.5; and for silver iodide, pAgeln=4.8.

Kinetics of Nanochain Formation in a Simplified Model of Amelogenin Biomacromolecules

We show that the kinetics of nanochain formation of amelogenin molecules is well described by a combination of translational and rotational diffusion of a simplified anisotropic bipolar model consisting of hydrophobic spherical colloid particles and a point charge located on each particle surface. The colloid particles interact via a standard depletion attraction whereas the point charges interact through a screened Coulomb repulsion. We study the kinetics via a Brownian dynamics simulation of both translational and rotational motions and show that the anisotropy brought in by the charge dramatically affects the kinetic pathway of cluster formation and our simple model captures the main features of the experimental observations.

Interaction of Monovalent Ions with Hydrophobic and Hydrophilic Colloids: Charge Inversion and Ionic Specificity

Here we study experimentally and by simulations the interaction of monovalent organic and inorganic anions with hydrophobic and hydrophilic colloids. In the case of hydrophobic colloids, our experiments show that charge inversion is induced by chaotropic inorganic monovalent ions but it is not induced by kosmotropic inorganic anions. For organic anions, giant charge inversion is observed at very low electrolyte concentrations. In addition, charge inversion disappears for both organic and inorganic ions when turning to hydrophilic colloids. These results provide an experimental evidence for the hydrophobic effect as the driving force for both ion specific effects and charge inversion. In the case of organic anions, our molecular dynamics (MD) simulations with full atomic detail show explicitly how the large adsorption free energies found for hydrophobic colloids are transformed into large repulsive barriers for hydrophilic colloids. Simulations confirm that solvation free energy (and hence the hydrophobic effect) is responsible for the build up of a Stern layer of adsorbed ions and charge inversion in hydrophobic colloids and it is also the mechanism preventing charge inversion in hydrophilic colloids. Overall, our experimental and simulation results suggest that the interaction of monovalent ions with interfaces is dominated by solvation thermodynamics, that is, the chaotropic/kosmotropic character of ions and the hydrophobic/hydrophilic character of surfaces.

Preparation of hydrophobic nano-silver colloid and aqueous nano-silver colloid by phase transfer

A novel and facile method to prepare hydrophobic nano-silver colloid and aqueous coordinate was proposed. Chemical reduction of silver nitrate in a tributyl phosphate (TBP) solution containing polyvinylpyrrolidone (PVP) was conducted and stable hydrophobic nano-silver/TBP/PVP colloids were obtained. Adopting a phase transfer process, the as-prepared colloids can be diverted into an aqueous system to obtain hydrophilic nano-silver colloids. The resultant materials were characterized by digital camera, TEM, XRD and UV–vis spectra. The results show that the reaction can be accomplished fast at room temperature and the synthesized colloids can remain stable for a long time, and the mean diameter of silver particles is around 10 nm for both nano-silver colloids. The nano-silver crystalline is face-centered cubic (fcc) structure, and the UV–vis absorbance peaks are at 412 and 420 nm for aqueous nano-silver and nano-silver/TBP/PVP colloids respectively.

Ion-induced reversibility in the aggregation of hydrophobic colloids

Experimental evidence of a fully reversible aggregation process due to ionic specificity is presented for the first time. The results indicate the existence of repulsive structural forces between particles coming from the specific accumulation of poorly hydrated ions around hydrophobic surfaces. This result emphasizes the key role that ion-water structure near interfaces plays in Hofmeister effects, as recently suggested by molecular dynamics simulations.

Effects of floc aluminum on activated sludge characteristics and removal of 17-α-ethinylestradiol in wastewater systems

The effects of floc aluminum (Al) on activated sludge performance and 17-α-ethinylestradiol (EE2) removal were studied using bench-scale activated sludge systems. The results showed that higher Al-fed activated sludge led to better settling, dewatering, and effluent quality with better EE2 removal. EE2 concentrations in the effluent revealed correlations with effluent suspended solids and large particulate/colloidal effluent biopolymer (protein + polysaccharide). Furthermore, a significant correlation existed between effluent proteins and EE2 for all size fractions, indicating that hydrophobic proteinaceous colloids provide binding sites for EE2 and washout together into the effluent. These results suggest that aluminum plays a crucial role in bioflocculation of activated sludge and the efficacy of flocculation influences the removal of endocrine disrupting compounds (EDCs) from wastewater treatment systems.

Correction to Polysilazane-Induced Aggregation of Hydrophobic Silver Colloids

Correction to Polysilazane-Induced Aggregation of Hydrophobic Silver Colloids

Effect of pH on Electrocoagulation of Bentonite Suspensions in Batch Using Iron Electrodes

The electrocoagulation of a synthetic wastewater has been studied in this work. The electrochemical process was carried out in a batch electrochemical cell equipped with iron electrodes without agitation. Bentonite suspensions were used as a model of wastes polluted with colloids, as clays behave as hydrophobic colloids in water. The results obtained were useful to clarify the mechanisms that are involved in the electrocoagulation of this kind of waste and also to study the influence of pH in the process. Two primary coagulation mechanisms can explain the experimental behavior of the system: at acid pH the neutralization of the superficial charges of the clays and at alkaline pH the enmeshment of the kaolin particles into a sweep floc. At neutral pH, the formed cations (Fe2+ and Fe3+) neutralize colloidal particles and conduct to the hydroxides (Fe(OH)2(s) and Fe(OH)3(s)) which adsorbate colloids and enhance sweep flocculation.

Polysilazane-Induced Aggregation of Hydrophobic Silver Colloids

Adsorbing polymers such as polysilazanes induce irreversible coagulation of hydrophobic silver colloids in nonpolar solvents. This is accompanied by broadening of the surface plasmon resonance (SPR) peak. A method to analyze the coagulation kinetics based on von Smoluchowski's theory utilizing the SPR change is described. The approach allows evaluating extinction spectra of aggregates of small sizes. A model for polymer adsorption kinetics in combination with a modified bridging efficiency model explains the observed coagulation inhibition over time in terms of macromolecules adsorption, spreading, and mutual repulsion.

Synthesis, Surface Modifications, and Size‐Sorting of Mixed Nickel–Zinc Ferrite Colloidal Magnetic Nanoparticles

We report on the spontaneous covalent growth of monomolecular adlayers on mixed nickel-zinc nanoferrite colloidal suspensions (ferrofluids). Synthesized nanoparticles were subjected to surface modification by means of acid chloride chemistry, leading to the formation of covalent bonds between the hydroxy groups at the nanoparticle surface and the acid chloride molecules. This procedure can be easily tailored to allow for the formation of adlayers containing both hydrophobic and hydrophilic regions stacked at predetermined distances from the magnetic core, and also providing the nanoferrites with functional carboxy groups capable of further modifications with, for example, drug molecules. Here, fluorophore aminopyrene molecules were bound to such modified nanoferrites through amide bonds. We also used the same chemistry to modify the surface with covalently bound long-chain palmitoyl moieties, and for comparison we also modified the nanoferrite surface by simple adsorption of oleic acid. Both procedures made the surface highly hydrophobic. These hydrophobic colloids were subsequently spread on an aqueous surface to form Langmuir monolayers with different characteristics. Moreover, since uniformity of size is crucial in a number of applications, we propose an efficient way of sorting the magnetic nanoparticles by size in their colloidal suspension. The suspension is centrifuged at increasing rotational speed and the fractions are collected after each run. The mean size of nanoferrite in each fraction was measured by the powder X-ray diffraction (PXRD) technique.

Detachment of colloids from a solid surface by a moving air–water interface

Colloid attachment to liquid–gas interfaces is an important process used in industrial applications to separate suspended colloids from the fluid phase. Moving gas bubbles can also be used to remove colloidal dust from surfaces. Similarly, moving liquid–gas interfaces lead to colloid mobilization in the natural subsurface environment, such as in soils and sediments. The objective of this study was to quantify the effect of moving air–water interfaces on the detachment of colloids deposited on an air-dried glass surface, as a function of colloidal properties and interface velocity. We selected four types of polystyrene colloids (positive and negative surface charge, hydrophilic and hydrophobic). The colloids were deposited on clean microscope glass slides using a flow-through deposition chamber. Air–water interfaces were passed over the colloid-deposited glass slides, and we varied the number of passages and the interface velocity. The amounts of colloids deposited on the glass slides were visualized using confocal laser scanning microscopy and quantified by image analysis. Our results showed that colloids attached under unfavorable conditions were removed in significantly greater amounts than those attached under favorable conditions. Hydrophobic colloids were detached more than hydrophilic colloids. The effect of the air–water interface on colloid removal was most pronounced for the first two passages of the air–water interface. Subsequent passages of air–water interfaces over the colloid-deposited glass slides did not cause significant additional colloid removal. Increasing interface velocity led to decreased colloid removal. The force balances, calculated from theory, supported the experimental findings, and highlight the dominance of detachment forces (surface tension forces) over the attachment forces (DLVO forces).

Charged Colloidal Particles and Small Mobile Ions near the Oil-Water Interface: Destruction of Colloidal Double Layer and Ionic Charge Separation

We study suspensions of hydrophobic charged colloids in a demixed oil-water solvent with salt by means of a modified Poisson-Boltzmann theory, taking into account image-charge effects and partitioning of the monovalent ions. We find that the ion's aversion for oil can deform the double layers of the oil-dispersed colloids, which qualitatively affects the colloidal density profiles. The same theory also predicts crystallization of colloid-free micron-sized water-in-oil droplets at water volume fractions as small as approximately 10(-3) in a narrow range of the oil-dielectric constant. These findings explain recent observations by M. E. Leunissen et al. [Proc. Natl. Acad. Sci. U.S.A. 104, 2585 (2007)10.1073/pnas.0610589104].

Study on the macromolecular coagulant PEX which traps heavy metals

Coagulation is one of the most important methods for water treatment, but the main objects of coagulation are only the hydrophobic colloids and suspended particles. In order to remove both soluble heavy metal species and insoluble substances efficiently by coagulation, sodium xanthogenate group was grafted to polyethyleneimine. This new kind of coagulant can be called PEX. The efficiency of PEX was tested by using Ni 2 + , Cu 2 + or Cr 6 + and kaoline as the targets. The experiment proved that PEX was an amphoteric polyelectrolyte and its isoelectric point was pH 3.0. When the pH of water sample is lower, PEX is electrically positive, therefore the colloidal substances with negative charges can be coagulated by it, but the cationic Ni 2 + ion cannot be removed very well. When the pH of water sample is higher, the positive charge of PEX decreases, but more xanthogenic acid groups ionize to the negative xanthogenate radical, so that the turbidity removal decreases, and the Ni 2 + removal increases.

Clay Swelling and Colloid Stability. Martin Smalley. CRC Press, Taylor and Francis Group, (2006), 272 pp. [ISBN 0-8493-8079-0] Price $139.95.

The DLVO (Derjaguin-Landau-Verwey-Overbeek) theory, formulated by Derjaguin and Landau (1941) and later independently by Vervey and Overbeek (1948), laid the foundations for our understanding of particle interactions in colloidal lyophobic dispersions and their kinetic stabilities. The configuration of the electrical double layers had been described previously by the Gouy-Chapman model with refinements later by many other investigators. In this model the counterions were reduced to point charges, liquid media were represented by a uniform dielectric constant, and colloidal particles by flat and uniformly charged large surfaces. The particle interactions in DLVO theory were assumed to be essentially controlled by two opposing forces, the screened Coulombic repulsion and by the London-van der Waals attraction. These oversimplifications have caused several problems in the application of the DLVO theory since its inception. For the last 50 years, various refinements and corrections were proposed to explain these discrepancies or to reconcile the theory with the observed data. This monograph calls an end to these corrections for the DLVO theory both on experimental and theoretical grounds. It is, therefore, a very important publication. It will be beneficial to the reader if I present this review in two parts: Part 1 presents the author’s own experimental data collected over the years that led him down the path for the formulation of a new theory for the stability of hydrophobic colloids. Chapters 1–7 are devoted to this section which is the essence of the monograph. Part 2 discusses the configurations of counterions and water molecules in the interfaces between macro-ions i.e. charged platy colloidal particles in Chapters 8–10. The effects of a special polymer, PEO, on the stability of n -C4H9NH+3-vermiculite dispersion, on its interlamellar separation, and the stacking order and crystallinity of the vermiculite layers are examined in Chapters …

Electron Transfer Behavior of Monolayer Protected Nanoclusters and Nanowires of Silver and Gold

Understanding the electron transfer behavior of nanometer sized, both metallic and semiconducting particles and wires is important due to the fundamental interest in size and shape dependent electronic properties and also because of its applications in nano-electronic devices like single electron transistors and molecular switches. Monolayer protected nanoclusters enable one simple and elegant method of synthesis of these types of metallic and semiconducting materials using interfacial chemistry as has been successfully used in several applications ranging from catalysis to molecular electronics. The success of this type of nanostructured materials is due in part to the well known protecting/stabilizing action of the ligands (also known as surface passivating/capping agents), which facilitate the synthesis and processing of these hydrophobic colloids in solution form. The present article discusses the electron transfer behavior of silver nanowires and nanoparticles with varied sizes. In particular, we have investigated the electrochemical properties of silver nanowires (diameter 70 nm, length several micrometers) and compared with the behavior of similar relatively larger sized nanoparticles (size 40 nm). A critical analysis of the redox behavior of silver nanowires and nanoparticles is presented in aqueous medium under various electrolytic conditions along with a comparison of analogous properties of smaller sized (2-7 nm) silver and gold nanoclusters.

Studies on the Preparation and Characterization of Monodisperse Polystyrene Latices. VI. Preparation of Zwitterionic Latices

The preparation of zwitterionic latex particles is reported by using a mixed anionic and cationic initiator system without requiring surface-active agents. Isoelectric points were found from microelectrophoresis experiments and were in the pH range of 3.5-5. Close to the isoelectric point, the latices coagulated as expected, and good stability was achieved outside this narrow range. This range of stability was in good agreement with predictions from current theory. Redispersion after coagulation was found to be difficult as was expected for a hydrophobic colloid. The electrokinetic behavior did not result in the maximum in zeta potential at an electrolyte concentration of 1 mM unlike the situation for other hydrophobic polystyrene latex particles, and hence these systems may be even better models for other colloidal studies.

Continuous Electrocoagulation of Synthetic Colloid-Polluted Wastes

The electrocoagulation of a synthetic wastewater has been studied in this work. The electrochemical process was carried out in a continuous single-flow electrochemical cell equipped with aluminum electrodes. Kaolin suspensions were used as a model of wastes polluted with colloids, as clays behave as hydrophobic colloids in water. The results obtained were useful to clarify the mechanisms that are involved in the electrocoagulation of this kind of waste and also to study the influence of the different operation conditions in the process. It has been found that the more important variables in the process were the aluminum concentration generated in the system and the pH. The concentration of aluminum generated in the electrochemical cell was always over the expected value (superfaradaic efficiencies) due to the important contribution of the chemical dissolution of the electrodes. This chemical dissolution of the electrodes depended strongly on the pH. A larger concentration of aluminum in the waste did not result in greater process efficiencies. For acidic pHs, a small concentration of aluminum achieved good coagulation efficiencies (80% removal of turbidity), while for alkaline pHs neither high nor low concentrations of aluminum yielded good coagulation results. For pHs close to neutral, a large concentration of aluminum was required to achieve good results. Two primary coagulation mechanisms can explain the experimental behavior of the system: at acid pH the neutralization of the superficial charge of the clays and at neutral pH (and also at high concentrations of aluminum) the enmeshment of the kaolin particles into a sweep floc.