Theoretical analysis of factors affecting the formation and stability of multilayered colloidal dispersions

A mathematical analysis of the major factors influencing the formation and stability of colloidal dispersions containing spherical particles surrounded by multilayered polymeric interfacial membranes formed by the layer-by-layer electrostatic deposition technique is carried out. The mathematical model assumes that (i) the colloidal dispersion initially consists of a mixture of electrically charged monodisperse spherical particles and oppositely charged polymer molecules, (ii) the adsorption of polymer molecules to the particle surfaces is diffusion-limited, and (iii) the dominant particle-particle collision mechanism is Brownian motion. This approach was used to produce stability maps that highlight conditions under which bridging flocculation, multilayer formation, or depletion flocculation occurs. The stability maps are derived from calculations of the critical polymer concentrations required to (i) saturate the particle surfaces (C(Sat)), (ii) ensure that polymer adsorption is faster than particle collisions (C(Ads)), and (iii) promote depletion flocculation (C(Dep)). In addition, the influence of interfacial properties on the stability of multilayer colloidal dispersions was assessed by calculating the colloidal interactions between the coated particles (i.e., van der Waals, electrostatic, steric, and depletion). These calculations indicated that the major factors are the interfacial charge and composition rather than the interfacial thickness. This article provides useful insights into the factors affecting the formation of stable multilayer colloidal dispersions.     

Colloid chemistry of clay minerals: the coagulation of montmorillonite dispersions

The transition between stable colloidal dispersions and coagulated or flocculated systems is a decisive process in practical applications of million of tons of bentonites (containing the clay mineral montmorillonite). Dispersion into the colloidal state requires the transformation of the original bentonite into the sodium form, for instance by soda activation. Therefore, we review here the coagulation of sodium montmorillonite dispersions by inorganic and organic cations and the influence of compounds of practical interest such as phosphates, cationic and anionic surfactants, alcohols, betaine-like molecules and polymers like polyphosphates, tannates, polyethylene oxides with cationic and anionic end groups, and carboxy methylcellulose. Typical properties of the sodium montmorillonite dispersions are the very low critical coagulation concentrations, the specific adsorption of counterions on the clay mineral surface, and the dependence of the c_K values on the montmorillonite content in the dispersion. In most cases coagulation occurs between the negative edges and the negative face. The phosphates Na₂HPO₄, NaH₂PO₄ and Na₄P₂O₇ increase the edge charge density and change the type of coagulation from edge (−)/face (−) to face (−)/face (−) with distinctly higher c_K values. Polyanions like polyphosphate and tannate stabilize in the same way. Carboxy methylcellulose causes steric stabilization. Montmorillonite particles with adsorbed betaine-like molecules provide an example of lyosphere stabilization.     

丙烯酸酯共聚物无皂水溶胶稳定性的研究

用溶液聚合法合成了四种AA含量不同的丙烯酸酯共聚物(MMA/BA/HEMA/AA)，通过中和AA使共聚物带有—COO~-能起自乳化作用分散于水中而成为无皂水溶胶．TEM观察表明水溶胶粒子呈球状，单分散性好，粒径随AA含量增加而变小，在30～90 nm范围．用电导滴定法测定水溶胶粒子中—COOH和—COO~-的分布，表明绝大部分—COO~-处于粒子表面，并且随AA含量增加，粒子表面的—COO~-增多，Zeta电位增大，这是导致水溶胶的抗电解质稳定性(以C.C.C.值反映)和贮存稳定性(以表现粘度反映)随AA含量增加而提高的主要原因。     

Stability of non-aqueous dispersions. Paper 7. Electrostatic stabilization of concentrated colloidal dispersions

The stability of colloidal dispersions in apolar media is reviewed beginning with dilute dispersions. A criterion for the definition between dilute and concentrated dispersions is presented followed by a new interpretation of Levine's theory for concentrated colloidal dispersions in apolar media [J. Colloid Interface Sci., 54 (1976) 34]. We conclude that concentrated dispersions containing submicron particles are generally unstable leading to coagulation. Successive coagulations lead to the formation of larger secondary particles, “coagulates”, which are ultimately stable to further coagulation and exist in energetic minima.     

The effect of block copolymer EPE1100 on the colloidal stability of Mg-Al LDH dispersions

The adsorption isotherm of block copolymer EPE1100 (polyethylene oxide-polypropylene oxide-polyethylene oxide) on the surfaces of Mg-Al LDH particles was determined through a solution depletion method combined with TOC measurement. X-ray diffraction patterns showed that the adsorption of EPE1100 molecules only occurred on the outer surfaces of LDH particles and they did not intercalate into the galleries between the layers. The adsorption of EPE1100 molecules changed the morphology of the particles. The effect of EPE1100 on the colloidal stability of LDH dispersion was investigated from three aspects: after the freezing-melting cycle, after the shearing rotations, and after the addition of electrolyte. The results indicated that the effect of EPE1100 on the colloidal stability of LDH dispersions was strongly related to the state of copolymer adsorption on LDH particle surfaces. It was inferred that the hydrated repulsive force and steric-repulsive force played important roles in determining the stability of the dispersions.     

Colloidal magnesium aluminum hydroxide and heterocoagulation with a clay mineral. II. Heterocoagulation with sodium montmorillonite

Montmorillonite dispersions were completely coagulated ¶by magnesium aluminum hydroxide when the hydroxide mass fraction, χ, was 0.2 or greater. The hydroxide dispersion required only a montmorillonite mass fraction of 0.06 for total coagulation. Thus, heterocoagulates were formed for 0.2<χ<0.94. At an excess of montmorillonite, network formation between the oppositely charged particles led to maxima of the yield value and the storage modulus at 0.4<χ<0.5. At higher hydroxide contents, χ>0.5, both properties decreased steeply, indicating the reduced mechanical stability of the network. Divalent anions at concentrations above 1 mmol/l acted as liquefying agents for the dispersed heterocoagulates. The specific surface area of the freeze-dried dispersions increased to a maximum value at χ = 0.65. The pore size distributions revealed that montmorillonite lamellae and hydroxide particles were not homogeneously distributed.     

Colloidal chemistry as a guide to design intended dispersions of carbon nanomaterials

In this review, some established concepts from Colloidal Science and their application to graphene and carbon nanotubes dispersions in organic or aqueous media are highlighted to rationalize alternatives for some issues in terms of colloidal properties. Recent applications for carbon-based dispersions are presented, as well as van der Waals interactions in carbon materials and strategies to overcome these interactions, such as increasing electrostatic repulsion between dispersed particles, surface functionalization, or adsorption of passivation agents such as macromolecules, which are the basis of many dispersion and exfoliation procedures. The demonstration of how knowledge and fine control of colloidal interactions have been used to overcome several limitations, such as the preparation of stable and concentrated dispersions of carbon materials and keeping appreciable electrical conductivity, is presented. It is also showed that the same knowledge can help the development of more environmentally friendly carbon-based colloids as well as the improvement of similar systems as dispersions of two-dimensional materials.     

Osmotic compression of mixtures of polymers and particles

Aqueous dispersions of nanometric ceria particles have been concentrated through osmotic stress. Mixed dispersions of ceria with small adsorbing macromolecules of poly (vinylpyrrolidone) have been prepared by the same method. The osmotic pressure of pure ceria dispersions results from electrostatic repulsions between particles. The osmotic pressure of dispersions containing a non-saturating amount of PVP is the same as that of pure dispersions, and the colloidal stability is depressed with respect to the pure dispersions. The osmotic pressure of dispersions containing an excess of PVP is increased by the free macromolecules, and the colloidal stability is enhanced. The organization of particles in these dispersions has been examined by small-angle x-ray scattering and cryotransmission electron microscopy. In pure ceria dispersions and in saturated dispersions, a liquid-like short-range order was found; when the concentration is increased, this short-range order follows a three-dimensional swelling law. In dispersions containing a non-saturating amount of PVP, the structure shows an alternance of clusters and voids, and the separations of clusters follow an unusual one-dimensional swelling law.     

Liquid chromatographic determination of carbendazim in the presence of some normal soil constituents with photodiode-array detection

A reversed-phase high-performance liquid chromatographic method was developed for the determination of carbendazim in the presence of some normal soil constituents (kaolinite, montmorillonite and peat). Spiked aqueous soil samples were injected after centrifugation and filtration. Quantitative recoveries were observed and good precision was obtained. The concentration range studied, 1.6716-8.3580 mg/l, is the most suitable for adsorption-desorption studies of carbendazim on soil and soil constituents.     

On the origin of colloidal particles in the dispersion polymerization of aniline

When aniline is oxidized in an aqueous medium in the presence of a steric stabilizer, colloidal polyaniline (PANI) dispersions are obtained. The generally accepted model of the stabilization assumes that the macromolecules of the water-soluble steric stabilizer are adsorbed at the polymer, precipitating during the dispersion polymerization, and provide steric protection against further aggregation. An alternative mechanism of conducting-polymer particle formation is proposed in the present study. We suggest that the steric stabilizer provides a site for adsorption of oligoaniline initiation centers; subsequent polymerization from anchored centers yields particle nuclei that grow to produce colloidal PANI particles. This hypothesis is based on the observation that the colloidal particles are obtained only in the case where the steric stabilizer is introduced in the early stages of polymerization when aniline oligomers are present in the reaction mixture. If the stabilizer had been added during the growth of PANI chains, colloidal dispersions would not have been produced. The process of particle growth is completely analogous to the formation of conducting PANI films on the surface of microparticles and various materials. There, the polymerization of aniline at the surfaces is preferred to the same process proceeding in the bulk of the reaction mixture. While the films grow at the interfaces with the reaction mixture, the dispersion particles similarly emanate from the stabilizer chains. The particle size, the formation of nonspherical morphologies, the importance of the chemical nature of the stabilizer chains, and the general relation between the conducting-polymer film and particle growth are discussed in the light of the proposed model.     

Production of aqueous colloidal dispersions of carbon nanotubes

Stable homogeneous dispersions of carbon nanotubes (CNTs) have been prepared by using sodium dodecyl sulfate (SDS) as dispersing agent. To our knowledge, it is the first report to quantitatively characterize colloidal stability of the dispersions by UV-vis spectrophometric measurements. When the sediment time reaches 500 h, the supernatant CNT concentration drops as much as 50% for the bare CNT suspension, compared to 15% with the addition of SDS. Furthermore, after 150 h, no precipitation is found for CNT/SDS dispersions, exhibiting an extreme stability. Zeta potential, auger electron microscopy, and FTIR analysis are employed to investigate the adsorption mechanism in detail. It has been concluded that the surfactant containing a single straight-chain hydrophobic segment and a terminal hydrophilic segment can modify the CNTs-suspending medium interface and prevent aggregation over long periods. The morphology of the CNT dispersions is observed with optical microscopy. An intermediate domain of homogeneously dispersed nanotubes exhibits an optimum at 0.5 wt% CNTs and 2.0 wt% SDS.     

Ni–Cu bimetallic colloids prepared in nonaqueous solvents

Colloidal dispersions in nonaqueous media were obtained by simultaneous evaporation of Ni and Cu. The metals were cocondensed at 77 K in an organic matrix of 2-methoxyethanol, 2-propanol or acetone. The metallic dispersions were characterized by transmission electron microscopy, electron diffraction and UV–vis spectroscopy. The stability at room temperature was also measured. The stability of the Ni–Cu bimetallic dispersions is less than that of the Ni and Cu dispersions alone. The presence of more than one phase was observed. It is interesting to note that Ni/Cu ratio does not change the stability of the bimetallic dispersions dramatically. The polarity of the solvent should play a very important role in stabilizing the metal particles by solvatation effects. Transmission electron microscopy studies show the size control effect of Cu with small colloidal size in the bimetal. The electron diffraction studies reveal the presence of amorphous bimetallic particles and particles with crystallinity show typical particles formed for more than one phase (NiO, CuO, Cu, Ni and Cu–Ni). Cu and Ni, for example, are amorphous and crystalline particles, respectively. Differential scanning calorimetry and X-ray spectroscopy characterized the bimetallic solids of Ni–Cu/2-methoxyethanol obtained by evaporation of solvent. The differential scanning calorimetry studies of the solids show transition characteristics of crystalline growth; no glass transitions were observed. The X-ray diffractograms show that the crystallites are too small (less than 100 Å), giving rise to low intensity and wide peaks.     

Cluster analysis of the elemental composition of five austrian peats

The elemental composition of peat depends on the plant residues from which the peat was formed. The concentrations of 45 elements were determined for five peat samples by plasma emission spectrometry. Literature data on the composition of three bogs was added. The elemental abundances of six rock or soil categories, the elemental compositions of several peat-forming plants, and the results of the elemental analyses of bog samples, were examined by the SIMCA method. A clear difference was found between fens and raised bogs. Factor analysis of the elemental compositions shows that the concentrations of most of the elements reflect their natural abundance in the surrounding region. Some concentrations are influenced by anthropogenic pollution, e.g. lead, and by plant metabolism. The cluster analysis together with the training sets shows the degree of the deposition of mineral material from adjacent rivers and the type of plant growth which formed the peat.     

Morphology and properties of waterborne adhesives made from hybrid polyacrylic/montmorillonite clay colloidal dispersions showing improved tack and shear resistance

The morphology and adhesive properties of waterborne films from n-butyl acrylate/methyl methacrylate/montmorillonite clay hybrid polymer latexes which were synthesized by miniemulsion polymerization in the presence of a reactive organoclay ((2-methacryloylethyl) hexadecyldimethylammonium modified montmorillonite, CMA16) were investigated. It was found by cryo-TEM analysis that the hybrid dispersions were a mixture of colloidal particles composed of a small fraction of free montmorillonite clay platelets, polymer latex particles, polymer particles to which one or more clay platelets where adhered onto its surface and a fraction of colloidal material consisted of a clay platelet with a polymer lob adhered to either side, in other words hybrid particles with a dumbbell-like morphology. The films made from these waterborne hybrid dispersions presented a homogeneous dispersion of the clay platelets and exfoliated morphology. The shear adhesion failure temperature (SAFT) and shear resistance of the hybrid latex films synthesized with CMA16 were better than those prepared with a commercial clay (Cloisite 30B), but presented a liquid-like probe-tack performance. When allyl methacrylate (AMA) was added in the formulation, SAFT and shear resistance improved, but the film had a very low energy of adhesion due to the excessively crosslinked matrix. In order to reduce crosslink density and thus improve the adhesion energy, small amounts of chain transfer agent, in this case n-dodecyl mercaptan (n-DDM), were used in the miniemulsion polymerization process. Adhesive films made from these waterborne hybrid dispersions showed excellent SAFT and shear resistance, and good energy of adhesion.      

Colloidal stability of magnetic iron oxide nanoparticles: influence of natural organic matter and synthetic polyelectrolytes

The colloidal behavior of natural organic matter (NOM) and synthetic poly(acrylic acid) (PAA)-coated ferrimagnetic (γFe(2)O(3)) nanoparticles (NPs) was investigated. Humic acid (HA), an important component of NOM, was extracted from a peat soil. Two different molecular weight PAAs were also used for coating. The colloidal stability of the coated magnetic NPs was evaluated as a resultant of the attractive magnetic dipolar and van der Waals forces and the repulsive electrostatic and steric-electrosteric interactions. The conformational alterations of the polyelectrolytes adsorbed on magnetic γFe(2)O(3) NPs and their role in colloidal stability were determined. Pure γFe(2)O(3) NPs were extremely unstable because of aggregation in aqueous solution, but a significant stability enhancement was observed after coating with polyelectrolytes. The steric stabilization factor induced by the polyelectrolyte coating strongly dictated the colloidal stability. The pH-induced conformational change of the adsorbed, weakly charged polyelectrolytes had a significant effect on the colloidal stability. Atomic force microscopy (AFM) revealed the stretched conformation of the HA molecular chains adsorbed on the γFe(2)O(3) NP surface at pH 9, which enhanced the colloidal stability through long-range electrosteric stabilization. The depletion of the polyelectrolyte during the dilution of the NP suspension decreased the colloidal stability under acidic solution conditions. The conformation of the polyelectrolytes adsorbed on the NP surface was altered as a function of the substrate surface charge as viewed from AFM imaging. The polyelectrolyte coating also led to a reduction in magnetic moments and decreased the coercivity of the coated γFe(2)O(3) NPs. Thus, the enhanced stabilization of the coated maghematite NPs may facilitate their delivery in the groundwater for the effective removal of contaminants.     

Anomalous stability of aqueous boehmite dispersions induced by hydrolyzed aluminium poly-cations

Aqueous dispersions of colloidal boehmite rods turn into strong gels when the concentration of (1–1) electrolyte concentrations becomes exceeds 50 mM. However, after addition of aluminium chlorohydrate (ACH) the rods remain stable up to salt concentrations as high as 2 M. Moreover the boehmite-ACH dispersions with an aspect ratio of 19 quickly separate into an isotropic and a liquid crystal nematic phase above a typical threshold concentration of 2 v/v%. It is known that ACH forms polynuclear cations at mild acidic conditions. The anomalous stability as encountered in these dispersions is explained by assuming that these hydrolyzed poly-cations cause a shift of the charge carrying surface.     

Effect of the ionic surfactant concentration on the stabilization/destabilization of polystyrene colloidal particles

One of the most interesting properties of the surfactants is that they are able to alter the stability of colloidal dispersions. Despite its great industrial relevance, only a few works analyze the colloidal stability of these systems at high surfactant concentrations (well above the critical micelle concentration (CMC)). In the present work, the colloidal stability of polystyrene particles is studied under a wide range of ionic surfactant concentrations. The effects of the surface charge of the latex particles (evaluating both sign and value), and surfactant type (cationic or anionic) have been examined. Colloidal stability data have been gathered by monitoring aggregation using a nephelometric technique. As will be shown, it is possible to reach different stability regimes using the same colloidal system just by changing the surfactant concentration. Independently of the sign of both the surfactant and the surface, the destabilization of the system consistently takes place above certain surfactant concentration due to a depletion effect from non-adsorbed micelles. This destabilization can be predicted by adding to the DLVO interaction energy a new contribution addressing the force between two spherical particles in the presence of non-adsorbing spherical macromolecules.     

Redispergation of TiO2 particles in aqueous solutions

The colloidal stability of TiO2 dispersions in aqueous solutions was studied. Aqueous solutions of ATLAS G-3300 (1.57 x 10(-3) mol/l), TRITON X-100 (5 x 10(-5) mol/l), and PMAA (4 x 10(-6) and 5.81 x 10(-3) mol/l) have been used as medium for redispergation of TiO2 particles. Stability of dispersions was investigated at different pH values by two different methods. By using analytical centrifuge the sedimentation velocity of TiO2 particles was directly measured and by means of light scattering the particle size of dispersed particles has been monitored. Combination of these two methods allowed determination of the aggregation degree of TiO2 particles as well as structure of the aggregates formed in aqueous phase. It has been found that redispergation process does not provide complete separation of virgin TiO2 particles. Even in the case of stable dispersions some aggregates were found, which consisted of 2-4 virgin TiO2 particles. With increasing colloidal stability of dispersions aggregates appear to be spherically shaped. In the system where TRITON X-100 was used, formation of secondary aggregates by fusion of primary ones was observed.     

Effects of glucose and mannose on the flocculation behavior of Saccharomyces cerevisiae at different life stages

The temporal flocculation behavior of Saccharomyces cerevisiae at different life stages is investigated using glucose and mannose as the different carbon sources, and the temporal variations of cell size, zeta potential and stability ratio of cell suspension are measured. It is found that the largest cell size and the lowest stability ratio of cell suspension occurred at the middle period of the exponential growth phase independent of carbon sources. The colloidal aspect was analyzed by using the DLVO theory, and indicated that the gravitational force plays a major role in determining the flocculation behavior of yeast cells.     

The influence of micelle formation on the stability of colloid surfactant mixtures

The stability of colloidal dispersions can be severely affected by the presence of surfactants. Because surfactants can adsorb at colloidal surfaces as well as form micelles, one can expect an interplay between both phenomena. Using grand-canonical coarse-grained Monte Carlo simulations on surfactant solutions confined between two surfaces, we investigate how adsorption and micelle formation affects the effective interaction between two colloidal particles, and hence, the stability of the colloidal dispersion. For solvophilic colloidal surfaces, we observe a short-ranged oscillatory solvation pressure that is hardly affected by the presence of surfactants in the system. The effective surface-surface interaction, however, reveals a decrease in solvophilic stabilization as a function of surfactant chemical potential. For solvophobic surfaces, we find that the capillary evaporation observed in a confined pure solvent, is counteracted by the addition of surfactants. Around the critical micelle concentration (CMC), the surface-surface interaction even becomes repulsive, enhancing stabilization of the colloidal dispersion. In contrast, the formation of micelles at concentrations above the CMC causes an additional depletion effect, resulting in an effective attraction, which in turn can destabilize a colloidal dispersion.