The effect of the salt concentration and counterion valence on the aggregation of latex particles at the air/water interface

An experimental study on colloidal aggregation in two dimensions is presented. This study shows that a high amount of electrolyte concentration is necessary to screen the particle interactions and to induce the aggregation process. Our results indicate that the stability of the colloidal particles, with a diameter of 735 nm, increases when they are trapped at the air-water interface. The reason for this stability is the existence of long-range repulsive interactions between the external parts of the particles that are propagated at the air phase. The subphase electrolyte concentration that separates the slow aggregation rate region from the fast aggregation rate region, the critical coagulation concentration (C.C.C.), has been determined for counterions with a different valence. Two regimes can be distinguished: at low salt concentration the aggregation process becomes slower and the aggregation is reaction limited. At high ionic strength the repulsive interactions between the immersed part of the particles are very weak and the aggregation rate tends to grow. However, because of the aerial repulsive interactions, pure diffusion-limited cluster aggregation is never found.     

How Péclet number affects microstructure and transient cluster aggregation in sedimenting colloidal suspensions

We study how varying the Pe?clet number (Pe) affects the steady state sedimentation of colloidal particles that interact through short-ranged attractions. By employing a hybrid molecular dynamics simulation method we demonstrate that the average sedimentation velocity changes from a non-monotonic dependence on packing fraction φ at low Pe numbers, to a monotonic decrease with φ at higher Pe numbers. At low Pe number the pair correlation functions are close to their equilibrium values, but as the Pe number increases, important deviations from equilibrium forms are observed. Although the attractive forces we employ are not strong enough to form permanent clusters, they do induce transient clusters whose behaviour is also affected by Pe number. In particular, clusters are more likely to fragment and less likely to aggregate at larger Pe numbers, and the probability of finding larger clusters decreases with increasing Pe number. Interestingly, the lifetime of the clusters is more or less independent of Pe number in the range we study. Instead, the change in cluster distribution occurs because larger clusters are less likely to form with increasing Pe number. These results illustrate some of the subtleties that occur in the crossover from equilibrium like to purely non-equilibrium behaviour as the balance between convective and thermal forces changes.     

Predicting the phase diagram of two-dimensional colloidal systems with long-range interactions

The phase diagram of a two-dimensional model system for colloidal particles at the air-water interface was determined using Monte Carlo computer simulations in the isothermic-isobaric ensemble. The micrometer-range binary colloidal interaction has been modeled by hard disklike particles interacting via a secondary minimum followed by a weaker longer-range repulsive maximum, both of the order of kBT. The repulsive part of the potential drives the clustering of particles at low densities and low temperatures. Pinned voids are formed at higher densities and intermediate values of the surface pressure. The analysis of isotherms, translational and orientational correlation functions as well as structure factor gives clear evidence of the presence of a melting first-order transition. However, the melting process can be also followed by a metastable route through a hexatic phase at low surface pressures and low temperatures, before crystalization occurs at higher surface pressure.     

The glass paradigm for colloidal glasses,gels, and other arrested states driven by attractive interactions

For some time, there has existed the idea that dense colloidal systems with repulsive interactions can be interpreted using certain approaches to glass theory. Recent advances in understanding the role of short-ranged attractive interactions in driving another type of ‘glass-transition’ have considerably extended the range of potential applications for such systems. Within this framework, particle gels are now regarded as ‘attractive’ glasses, and for some concentration regimes the details of the density correlation as we approach gellation for a broad range of experimental systems seem to be well described by glass-transition ideas and laws. Initial suggestions that this might be so came from theory, but the close collaboration between theory, simulation, and experimental science has been mutually stimulating. New advances in the theory are now to be expected, and novel systems where the ideas might be applicable are emerging. The exploration of these ideas is still at the beginning, but there is a reasonable expectation that the glass paradigm will be more generally useful in many areas of soft matter and colloid science, perhaps gathering apparently disparate phenomena of particle gellation, polymer gellation, aggregation, and other aspects of ‘solidification’ into a common interpretive scheme.     

On the effect of Ca2+ and La3+ on the colloidal stability of liposomes

This work deals with the effect of Ca2+ and La3+ on the colloidal stability of phosphatidylcholine (PC) liposomes in aqueous media. As physical techniques, nephelometry, photon correlation spectroscopy, electrophoretic mobility, and surface tension were used. The theoretical predictions of the colloidal stability of liposomes were followed using the Derjaguin-Landau-Verwey-Overbeek theory. Changes in the size of liposomes and high polydispersity values were observed as La3+ concentration increases, suggesting that this cation induces the aggregation of liposomes. However, changes in polydispersity were not observed with Ca2+, suggesting a coalescence mechanism or fusion of liposomes. The stability factor (W), calculated from the nephelometry measurements indicated that aggregation/fusion occurs at a critical concentration (c.c.) of 0.3 and 0.7 M for La3+ and Ca2+, respectively. To gain a better insight into the interaction mechanism between the liposomes and the studied ions, the interaction between PC monolayers and Ca2+ and La3+ was studied. Changes in the surface area per lipid molecule (A0) in the monolayer at the c.c. values were found for both ions, with a more pronounced effect in the case of Ca2+. This corresponds with a larger reduction of the steric repulsive interaction between the headgroups at the phospholipid membrane (pi(head)). The experimental result validates the hypothesis made on the liposome fusion in the presence of Ca2+ and liposome aggregation in the presence of La3+. These aggregation mechanisms have also been confirmed by transmission electron microscopy.     

Temperature effect on the stability of bentonite colloids in water

The stability of natural bentonite suspensions has been investigated as a function of temperature at pH 9 and ionic strength 10(-3) M. The sedimentation rate of the particles is directly related to their stability. The sedimentation kinetics was determined by examining the variation of particle concentration in solution with time. The observed kinetics for sedimentation is discussed quantitatively in terms of the potential energy between particles. The zeta-potential of the particles was measured and the DLVO theory was used to calculate attractive and repulsive potentials. Experimental observations are consistent with DLVO model predictions and show that the stability of bentonite colloids increases with temperature. Differences with other colloidal systems can be attributed to the temperature dependence of the surface charge of bentonite particles.     

Use of poly(ethylene glycol) to control cell aggregation and fusion

Although poly(ethylene glycol) (PEG) has been widely used as an agent to induce cell aggregation and fusion, the physicochemical principles of its function are only becoming understood recently. PEG has an extremely high affinity for water. The PEG commonly used for these applications is in the molecular weight range of 8000 to 10 000. At low concentrations (0–15 wt.%), PEG in this molecular weight range tends to deplete from cell or lipid surfaces, creating an osmotic gradient which brings cells or lipid vesicles together. The depletion force is measured using a surface force apparatus. The corresponding reduction of surface viscosity is verified by shear viscosity measurements and by vesicle tumbling experiments. At higher concentrations (15–45 wt.%), the extremely high osmotic pressure generated by PEG compresses apposing surfaces of aggregated cells or vesicles to within limits where the membrane is no longer stable, and fusion occurs at point defects. A fusion lumen is formed with the help of cell swelling. If PEG is adsorbed or covalently link to the cell or vesicle surface, the surface force profile becomes entirely repulsive, and aggregation and fusion is inhibited. The repulsion is accountable by steric and electrostatic forces. Therefore, the fusogenic function of PEG can be explained quantitatively by colloidal stability theories.     

Sedimentation of concentrated monodisperse colloidal suspensions: role of collective particle interaction forces

The sedimentation velocities and concentration profiles of low-charge, monodisperse hydroxylate latex particle suspensions were investigated experimentally as a function of the particle concentration to study the effects of the collective particle interactions on suspension stability. We used the Kossel diffraction technique to measure the particle concentration profile and sedimentation rate. We conducted the sedimentation experiments using three different particle sizes. Collective hydrodynamic interactions dominate the particle-particle interactions at particle concentrations up to 6.5 vol%. However, at higher particle concentrations, additional collective particle-particle interactions resulting from the self-depletion attraction cause particle aggregation inside the suspension. The collective particle-particle interaction forces play a much more important role when relatively small particles (500 nm in diameter or less) are used. We developed a theoretical model based on the statistical particle dynamics simulation method to examine the role of the collective particle interactions in concentrated suspensions in the colloidal microstructure formation and sedimentation rates. The theoretical results agree with the experimentally-measured values of the settling velocities and concentration profiles.     

Investigation on the interaction between colloidal gold and human complement factor 4 at different pH by spectral methods

The interaction between colloidal gold and human complement factor 4 (human C4) at different pH was investigated by spectral methods, including absorption and resonance light-scattering spectrometry. According to the changes of color and absorption spectra of colloidal gold solution in presence of human C4, the interaction between colloidal gold and human C4 was quantitatively investigated using a semi-empirical "flocculation parameter". At the same time, the changes of resonance light-scattering spectra and transmission electron microscopy (TEM) images indicate that the aggregation of colloidal gold happens by electrostatic interaction in presence of human C4 in the pH range 5-6. However, the colloidal gold solution remains stable at pH >6 and pH <5 due to the repulsive electrostatic interaction between colloidal gold and human C4. The flocculation parameter is directly proportional to the concentration of human C4 in the range from 9.7 to 233.0 microgl(-1). In addition, the interactions between the colloidal gold and bovine serum albumin (BSA) as well as human serum albumin (HSA) were also investigated using the same methods. It was found that there was no aggregation of colloidal gold in presence of BSA and HSA in the pH range 5-6. However, when the pH of solution is 4, the aggregation of colloidal gold happens. Because BSA and HSA have different structure, the intensity of aggregation of colloidal gold in presence of BSA is greater than that in presence of HSA at pH 4.     

Modelling colloid transport in groundwater; the prediction of colloid stability and retention behaviour

The association of contaminants with mobile colloidal particles present in groundwaters has been recognised as a potentially important mass transfer mechanism for contaminant migration in the environment. To predict the fate of environmental contaminants there is a need to develop numerical models which include colloid-mediated transport. The mobility of groundwater colloids is controlled by their stability towards aggregation and attachment to rock surfaces. For inorganic particles, the conceptual framework for predicting their stability and deposition behaviour is provided by the DLVO theory. However, under conditions unfavourable to coagulation or surface attachment (ie. when particles and surfaces are of like charge) there are significant discrepancies between theory and experimentally measured coagulation and deposition rates.Predictive shortcomings of the DLVO theory arise from the simplicity of the original model, which was formulated for smooth bodies with ideal geometries and uniform surface properties. However, surfaces are by nature rough, non-uniform and heterogeneous in composition. In addition, the theory does not consider the dynamics of particle interactions. Furthermore, the presence of additional forces, which may be either attractive or repulsive, acting at short range, which arise from interactions between surfaces and water, are not accounted for. Significant developments have been made to extend and modify the DLVO model to account for the discrepancies between theory and experiment. In this paper the prediction of colloid stability and deposition behaviour under unfavourable conditions is reviewed. Emphasis is placed on the phenomenological behaviour of inorganic colloids in aqueous systems that may need to be accounted for in a transport model.     

扩散模型和凝聚模型耦合作用下胶体凝聚动力学的Monte Carlo模拟研究

以扩散模型(Ds(γ)=D0×sγ)和凝聚模型(Pij(σ)=P0×(i×j)σ)为基础,对胶体体系随时间的演变、团簇大小分布及其标度关系、团簇的重均大小S(t)的变化规律以及模型对最终分形维数的影响四个角度进行了比较研究,发现扩散指数γ0和凝聚概率指数σ0对胶体的凝聚动力学过程有相似的影响.本文在较宽的γ和σ取值范围内,对胶体的凝聚动力学进行了模拟研究,对慢速凝聚向快速凝聚的转化机理作了定量分析,并进一步分析了在团簇-团簇凝聚(CCA)模型下,得到类似扩散置限凝聚(DLA)模型的凝聚体的物理意义,结果表明:(1)γ0代表了体系中团簇或单粒做"定向运动"而非无规则的布朗运动的情况.这种"定向运动"的推动力可能来自于大团簇产生的强"长程范德华力"、"电场力"等,或来自于体系边界处的外力场的作用.(2)当σ0时,体系成为先快后慢的慢速凝聚,这可能对应大团簇为一排斥中心,即胶体颗粒存在"排斥力场"的现象.(3)证实了团簇的重均大小在凝聚过程的早期按指数规律增长,而后期按幂函数规律增长的实验现象.模拟研究还表明,胶体体系的凝聚动力学过程,在σ0时是一个存在正反馈机制的非线性动力学过程,而在σ0时则体现出负反馈的特征.     

Sedimentation of aggregating colloids

We investigate the combined effects of gravity, attractive interactions, and brownian motion in suspensions of colloidal particles and nonadsorbing polymer. Depending on the effective strength of gravitational forces, resulting from a density mismatch between the colloids and the solvent, and the magnitude and range of the depletion interactions induced by the polymer, sedimentation in these suspensions can result in an equilibrium structure or a kinetically arrested state. We employ large-scale molecular dynamics simulations to systematically classify the different regimes that arise as a function of attraction strength and gravitational stress. Whereas strong attractions lead to cluster aggregation and low-density arrested states, moderate attractions can enhance crystallization of the colloidal particles in the sediment. We make direct comparisons to experimental results to infer general conclusions about the mechanisms leading to mechanically stable sediments.     

Stimulant sensitive flocculation and consolidation for improved solid/liquid separation

A novel method of flocculation resulting in both rapid sedimentation and low sediment moisture is described. It relies on changing the inter-particle forces from repulsive to attractive (aggregation and fast settling results) and then back to repulsive (densification of sediment then occurs). The change in inter-particle force is controlled by a stimulus such as pH or temperature. The technique is demonstrated without polymer using the isoelectric point and pH as the stimulus. The polyelectrolyte chitosan produces faster sedimentation and clearer supernatants as a pH sensitive flocculant. Methylcellulose is an effective temperature sensitive flocculant. The sediment bed volumes can be reduced by between 10 and 45% depending upon the conditions which, as yet, have not been fully optimized.     

Role of solvation forces in the gelation of fumed silica-alcohol suspensions

Aggregation and gelation kinetics of fumed silica were investigated by altering the solvent-surface interactions. Native and surface-modified (hydrophobic) fumed silica particles were dispersed in short-chain linear alcohols. Based on the kinetics of aggregation and gelation, we show that the solvent-surface interactions have a tremendous impact on the bulk suspension properties. The gelation kinetics were qualitatively similar in all of the fumed silica-alcohol samples, and the gel times for all the alcohols were captured on a master curve requiring two parameters. The two parameters, the stability ratio and critical volume fraction, describe the two regimes of gelation. At low concentrations, gelation occurs due to aggregation of the particles diffusing over a potential barrier (15-25 kT). The rate of aggregation and time to gelation then scales with the stability ratio. At high particle loadings, gelation occurs at a critical volume fraction due to localization in a secondary minimum with a depth of 3-4 kT. These observations are supported by evidence of hydrogen bonding between the solvent and the particle, creating oscillatory solvation forces that govern the magnitude of these two parameters.     

Aggregation of Charged Particles under Electrophoresis or Gravity at Arbitrary Péclet Numbers

Collision efficiencies are considered for colloidal suspensions of solid spheres moving in a viscous fluid under the influence of electrophoresis or gravity, Brownian motion, and electrostatic and van der Waals forces. The results are compared to those for convection (electrophoresis or gravity) and diffusion (Brownian motion) acting independently. The collision efficiency increases by many orders of magnitude over that predicted by simply adding diffusive and convective efficiencies in a specific parameter regime. This regime occurs when there is a large energy barrier in the interparticle potential, causing a stable region of parameter space if there is no diffusion. Brownian motion alone will only cause small amounts of aggregation under these conditions. However, for electric fields or buoyancy effects which are only slightly too weak to allow particles to overcome the potential barrier, the addition of weak Brownian motion to a system with convection can cause significant numbers of particles to overcome the energy barrier and aggregate. Copyright 2000 Academic Press.     

Aggregation of nanosized colloidal silica in the presence of various alkali cations investigated by the electrospray technique

The slow aggregation process of a concentrated silica dispersion (Bindzil 40/220) in the presence of alkali chlorides (LiCl, NaCl, KCl, RbCl, and CsCl) was investigated by means of mobility measurements. At intervals during the aggregation, particles and aggregates were transferred from the liquid phase to the gas phase via electrospray (ES) and subsequently size selected and counted using a scanning mobility particle sizer (SMPS). This method enables the acquisition of particle and aggregate size distributions with a time resolution of minutes. To our knowledge, this is the first time that the method has been applied to study the process of colloidal aggregation. The obtained results indicate that, independent of the type of counterion, a sufficient dilution of the formed gel will cause the particles to redisperse. Hence, the silica particles are, at least initially, reversibly aggregated. The reversibility of the aggregation indicates additional non-DLVO repulsive steric interactions that are likely due to the presence of a gel layer at the surface. The size of the disintegrating aggregates was monitored as a function of the time after dilution. It was found that the most stable aggregates were formed by the ions that adsorb most strongly on the particle surface. This attractive effect was ascribed to an ion-ion correlation interaction.     

Settling of paraffin crystals in cooled middle distillate fuels

The mechanism of action of additives that control the sedimentation of paraffin crystals after their crystallization in model diesel oil has been studied by means of a new experimental approach. The chemical analysis of the crystals and detailed measurements of the sedimentation phenomenon give new insights into this complex process. Thus, the wax antisettling additives used for preventing wax crystal sedimentation adsorb at the surfaces of wax particles and provide them with enhanced colloidal stability. The settling rate is not related to the size of the crystals or the viscosity of the liquid medium, but to the ability of the additives to prevent the aggregation of wax crystals. The reported methodology makes it possible to investigate the fundamental mechanisms, but also to evaluate structure-activity relationships of the various additives used in the petroleum industry.     

Emergence of convective flows during diffusional mass transfer in ternary gas systems: The effect of component concentrations

Specific features of diffusional mass transfer in ternary gas mixtures and their relation to the concentration of the densest component in a mixture are studied experimentally under isothermic conditions. At certain compositions of the gas mixture, unique diffusion regimes develop in the system that are characterized by a considerable rate of mass transfer and are attributed to the presence of concentration convection. We show that transitions from the diffusional and convective regimes can be predicted using the stability theory extended to isothermal mixing in ternary gas systems. Theoretical predictions are compared to experimental data.     

Specific cation adsorption on protein-covered particles and its influence on colloidal stability

Protein coated particles present an anomalous colloidal stability at high ionic strength when the classical theory (DLVO) predicts aggregation. This observed deviation from DLVO behaviour appears for electrolyte concentrations above some critical bulk value. As we have suggested in previous publications the existence of an additional short-range repulsive 'hydration force' due to specific hydrated cation adsorption could explain this anomalous stability. The overlap of the hydration layers when two particles approach should provoke this repulsive force. New evidence of this mechanism has been observed when electrophoretic mobilities of protein-carrying latex particles were measured at various concentrations of sodium and calcium chloride. In the latter case a sign reversal of zeta-potential was found, probably due to the specific adsorption of Ca(2+) ions on protein molecules. The adsorption increases with the medium pH. These results have been analyzed following the treatment proposed by Ohshima and co-workers for large charged colloidal particles coated with a layer of protein. This study shows an increase in the positive fixed-charge density on the protein caused by the adsorption of cations.