Correlation of viscoelastic properties of stable and flocculated suspensions with their interparticle interactions

This review starts with a general introduction on the properties of concentrated suspensions. The distinction between “dilute”, “solid” and “concentrated” suspensions is given in terms of the balance between Brownian, hydrodynamic and interparticle interactions. A section is given on interparticle interactions and their combinations. The four different types of interactions, namely hard-sphere, electrostatic, steric and van der Waals are described. The flocculation of both electrostatically and sterically stabilized suspensions is also discussed. The next section covers the principles of rheological measurements. Transient (static), dynamic (oscillatory), shear wave propagation and steady state measurements are described. The last part of the review deals with the viscoelastic properties of concentrated suspensions. Four different systems were described and examples were given: (a) Suspensions with hard-sphere interactions; (b) Stable systems with soft (electrostatic) interaction; (c) Sterically stabilised systems; (d) Flocculated and coagulated systems. Both weakly and strongly flocculated systems were discussed.In the above review, particular emphasis was given to the relationship between the viscoelastic properties of concentrated suspensions and their interparticle interactions. As far as was possible, the results obtained from rheological measurements were quantitatively analysed in terms of such interparticle forces. The review demonstrated that such correlation is generally followed and this illustrated the powerful use of rheology for studying interparticle interactions.     

Industrial applications of dispersions

In this review some industrial applications of dispersions have been discussed. After a general introduction, some specific topics have been covered. The preparation of dispersions using condensation and dispersion methods was discussed in terms of the various interfacial processes involved such as nucleation and growth, wetting, breaking of aggregates and agglomerates as well as comminution. The process of emulsification (for production of liquid/liquid dispersions) was also analyzed in terms of the interfacial processes such as reduction in interfacial tension, interfacial elasticity and viscosity. The control of the properties of dispersions was described in terms of the interaction forces between the particles or droplets in the system. These interaction forces are governed by the structure and properties of the interfacial region such as double layers, presence of adsorbed surfactant or polymer layers. Four main types of interaction forces may be distinguished : hard-sphere, electrostatic, steric and van der Waals. Combination of these forces lead to three general energy-distance curves that can be used to describe the state of the dispersion (stable, flocculated or coagulated). The various physical states of suspensions and emulsions produced on standing were schematically presented and they could be explained in terms of the energy-distance curves. The flow characteristics (rheology) of dispersions could also be accounted for in terms of the various interaction forces between the particles.Solubilization and microemulsions, which produce thermodynamically stable dispersions, could be described in terms of the balance between the interfacial energy and entropy of dispersion of the system. The driving force for producing such thermodynamically stable systems was the ultra low interfacial tension which could be achieved by using a combination of surfactants. The application of microemulsions in various fields such as solubilization, enhanced oil recovery and energy production was briefly described.The application of dispersions in microncapsulation and slow release was described in terms of interfacial polymerization, coacervation and multiple emulsion formation. These systems find application in medicine, agrochemicals and cosmetics. The application of dispersions in pharmacy and medicine was also described by quoting specific examples such as liposomes (vesicles), nanoparticles and magnetic microspheres. These systems have potential use in targeting delivery of drugs.     

Small-angle static light scattering of concentrated silica suspensions during in situ destabilization

The aggregation of concentrated aqueous silica suspensions is characterized by means of static light scattering. We use an in situ destabilization mechanism based on the enzyme-catalyzed hydrolysis of urea. This method enables us to continuously and homogeneously change the interparticle potential from repulsive to attractive without disturbing the aggregation process. Moreover, our electrostatically stabilized suspensions can be destabilized by two different methods. In the first method, the pH is shifted toward the isoelectric point of the particles ( Delta pH method), thereby leading to a decrease of their surface charge. In the second method, the ionic strength is continuously increased at constant pH ( Delta I method), leading to a compression of the electrical double layer around the charged particles. A laboratory-built flat-cell light-scattering instrument is used, which allows fast data acquisition and an adjustment of the sample cell thickness. To circumvent multiple scattering effects, we use a very small sample thickness ( approximately 13 microm). In addition, the refractive index difference between the aqueous phase and the particles is reduced by adding sucrose to the liquid phase of our suspensions. We are able to characterize the structural changes at the very early stages of the destabilization process, where no significant effects are yet detected in macroscopic rheological measurements. While during the Delta pH destabilization, the scattering curve shows significant changes only after some characteristic delay time, it changes continuously during the Delta I destabilization. The latter is attributed to the formation of a weak pre-gel structure in the suspensions, as a shallow secondary minimum appears in the interparticle potential. Data are evaluated by using a HMSA square-well structure factor model. Results are in good agreement with those predicted from DLVO theory.     

Energy of interaction in colloids and its implications in rheological modeling

This work deals with the problem of deriving theoretical connections between rheology and interparticle forces in colloidal suspensions. The nature of interparticle forces determines the colloidal structure (crystalline order due to long range repulsive forces, flocculation due to attractive forces, etc.) and hence, the flow behavior of suspensions. The aim of this article is to discuss how these interactions enter the modeling of rheometric functions, in particular, the shear viscosity. In this sense, the main interactions commonly appearing in colloids are reviewed, as well as the role they play in phase transition behavior. Then, a series of approaches relating the interaction potential to viscosity is examined. The results of applying these models to experimental data are also discussed. Finally, examples of viscosity modeling for different interaction potentials are given, by using the structural model proposed previously by the authors. The possibility of relating the flow behavior of colloidal suspensions to the interaction between particles offers new perspectives for the study and technical applications of these systems.     

The influence of interparticle surface forces on the coagulation of weakly magnetic mineral ultrafines in a magnetic field

In this paper, it is shown that the coagulation of dispersions of weakly magnetic mineral ultrafines (such as hematite and chromite) in an external magnetic field can be described theoretically by invoking interparticle forces. Essentially, coagulation occurs when the short-range London—van der Waals interactions and the long-range magnetic forces outweigh the stabilizing electric double layer repulsion. From classical colloid chemistry theory, we have calculated the various components of the potential energy for different-sized particles at a series of ionic strengths and magnetic field intensifies. Principles governing the stability of the suspensions were derived and the computations lead to the establishment of criteria which can be used to predict the stability of the suspensions of weakly magnetic oxide mineral ultrafines in a “wet magnetic separation process”.

Experimentally, the magnetic-field induced coagulation of ultrafines of natural hematite and chromite in aqueous suspensions at moderate ionic strength was investigated using a laboratory-scale electromagnetic solenoid. The experimental results relate the coagulation process (as determined by magnetosedimentation analysis) to particle size, slurry pH and the external magnetic field. In the magnetic fields, maximum coagulation occurred near the pH of the point of zero charge (pH_PZC) of the minerals (where the electrostatic double layer repulsion was reduced to a minimum) enabling the particles to enter the “primary minimum” energy sink. In contrast, in cases where the electrostatic repulsion was not suppressed, the long-range magnetic forces enabled coagulation to occur in the “secondary minimum”. This caused the formation of chains which appeared to be relatively stable at enhanced rates of settling. The experimental results could be interpreted from a theoretical analysis of the interparticle forces controlling the process.     

Colloido-chemical characteristics of zinc oxide in electrolyte solutions

The electrosurface characteristics (total surface charge, electrokinetic potential, and positions of the point of zero charge and isoelectric point) of zinc oxide have been comprehensively studied as depending on pH and background electrolyte (NaCl) concentration. The constants of surface reactions and the adsorption potentials of potential determining of background electrolyte ions have been obtained in terms of the 2pK-model. The aggregation stability of aqueous zinc oxide suspensions has been studied in a wide range of concentrations of 1: 1 and 1: 2 electrolytes, and the threshold of sol coagulation has been found. Within the framework of the DLVO theory, the interaction energy between ZnO particles has been calculated as a function of the interparticle distance. It has been demonstrated that the experimental data can be qualitatively described in terms of the classical DLVO theory, which takes into account only ion-electrostatic and molecular (dispersion) forces of interparticle interaction.     

Stability of silicon and titanium carbide suspensions in electrolyte, poly(ethylene oxide), and PEO-surfactant solutions

It has been shown that the coagulation values of counterions for SiC and TiC suspensions with particle radius from 0.5 to 5 microm obey a z(2.5-3.5) law and there is an insufficient change in the critical concentration of 1-1 electrolytes (CCE) when the surface potential of particles increases more than two times. Also, the CCE values hardly depend on the position of counterions in the lyotropic sequence. This is explained by aggregation of SiC and TiC particles at a secondary minimum, which is proved by calculations of the potential curves of interparticle interactions using the DLVO theory. The adsorption of poly(ethylene oxide) on the surfaces studied does not cause--in contradiction to dispersions with smaller particles--an unlimited growth in the stability of suspensions. This is due to the aggregation of large particles with adsorbed PEO, as in polymer-free dispersions, under barrierless conditions in which the coordinates of the secondary minimum are determined by superposition of molecular attractive forces and steric repulsive forces of adsorbed polymeric chains, without a contribution from the electric repulsion term. PEO-anionic surfactant complexes possess higher stabilizing capacity compared to the individual components of the mixture. Our results show that the adsorbed polymer layers may hinder the aggregation both in the primary and in the secondary minimum for not very large particles only, the critical size of which depends on the dispersed phase nature and the molecular mass of the polymer.     

Direct surface force measurements of polyelectrolyte multilayer films containing nanocrystalline cellulose

Polyelectrolyte multilayer films containing nanocrystalline cellulose (NCC) and poly(allylamine hydrochloride) (PAH) make up a new class of nanostructured composite with applications ranging from coatings to biomedical devices. Moreover, these materials are amenable to surface force studies using colloid-probe atomic force microscopy (CP-AFM). For electrostatically assembled films with either NCC or PAH as the outermost layer, surface morphology was investigated by AFM and wettability was examined by contact angle measurements. By varying the surrounding ionic strength and pH, the relative contributions from electrostatic, van der Waals, steric, and polymer bridging interactions were evaluated. The ionic cross-linking in these films rendered them stable under all solution conditions studied although swelling at low pH and high ionic strength was inferred. The underlying polymer layer in the multilayered film was found to dictate the dominant surface forces when polymer migration and chain extension were facilitated. The precontact normal forces between a silica probe and an NCC-capped multilayer film were monotonically repulsive at pH values where the material surfaces were similarly and fully charged. In contrast, at pH 3.5, the anionic surfaces were weakly charged but the underlying layer of cationic PAH was fully charged and attractive forces dominated due to polymer bridging from extended PAH chains. The interaction with an anionic carboxylic acid probe showed similar behavior to the silica probe; however, for a cationic amine probe with an anionic NCC-capped film, electrostatic double-layer attraction at low pH, and electrostatic double-layer repulsion at high pH, were observed. Finally, the effect of the capping layer was studied with an anionic probe, which indicated that NCC-capped films exhibited purely repulsive forces which were larger in magnitude than the combination of electrostatic double-layer attraction and steric repulsion, measured for PAH-capped films. Wherever possible, DLVO theory was used to fit the measured surface forces and apparent surface potentials and surface charge densities were calculated.     

DLVO AND NON-DLVO SURFACE FORCES AND INTERACTIONS IN COLLOIDAL DISPERSIONS

Some aspects of DLVO and non-DLVO forces in colloidal systems are over-viewed. The influence of long range interactions on some kinetic properties of dispersions, as Brownian diffusion, is discussed. It is shown, both theoretically and experimentally, that the electrostatic repulsion increases the collective diffusivity. The film stratification and oscillatory structure forces in colloidal suspensions are considered within the framework of an uniform approach The presence of small colloidal species (e. g. micelles or polymer molecules) may lead to several maxima and minima in the disjoining pressure isotherm. The particular case of interacting emulsion droplets is examined accounting for the interfacial deformability. The droplet deformation acts as a soft repulsion but affects also the remaining contributions to the interaction energy due to changes of the droplet shape. A general procedure for calculating the inter-droplet interaction energy, as well as the equilibrium film radius and thickness in a doublet of droplets, is suggested. The energy of interaction between charged colloidal particles, due to correlations of the density fluctuations in the electric double layer is also studied. It is found that this effect may lead to attraction greater than the van der Waals contribution, especially when multivale counter ions are present.     

Yield Stress of Concentrated Zirconia Suspensions: Correlation with Particle Interactions

The presence of a sufficient concentration of solid particles in a solution gives rise to a large increase in its viscosity and, more importantly, to significant deviations with respect to its original Newtonian behavior. Different rheological techniques are available to characterize such deviations, but the simplest one, obtention of steady-state rheograms, is already extremely useful with that purpose. In this work, this technique is applied to suspensions of zirconia particles, both synthesized with spherical geometry and commercial. The sigma(shear stress)-gamma;(shear rate) curves show that the suspensions are nonideal plastic, thus exhibiting a finite yield stress, sigma(0), and a shear-thinning flow. It is through sigma(0) that a connection can be established between steady-state rheological behavior and interaction energy between particles, since sigma(0) can be estimated as the maximum attractive force between particles multiplied by the number of bonds per unit area between a given particle and its neighbors. Having an experimental determination of sigma(0), the verification of its relation with the attractive forces requires estimation of the potential energy of interaction between any pair of particles. Two approaches will be considered: one is the classical DLVO model, in which the potential energy, V, is the sum of the van der Waals (V(LW)) and electrostatic (V(EL)) contributions. The second approach is the so-called extended DLVO theory, in which the acid-base interaction V(AB) (related to the hydrophilic repulsion or hydrophobic attraction between the particles) is considered in addition to V(LW) and V(EL). The three contributions can be calculated as a function of the interparticle distance if the particle-solution interface is characterized from both the electric and the thermodynamic points of view. The former is carried out by means of electrophoretic mobility measurements and the latter by contact angle determinations for three probe liquids on zirconia powder layers. Comparison between measured and calculated sigma(0) values was carried out for suspensions of spherical, monodisperse ZrO(2) particles, with volume fraction of solids, straight phi, ranging between 4.6 and 21.7%, in 10(-3) M NaCl solutions. In the case of commercial particles, the effects of both NaCl concentration (10(-5) to 10(-1) M) and volume fraction (3.5 to 21%) were investigated. It is found that the classical DLVO theory cannot be used to predict the yield stress when [NaCl]=10(-5) M, since the high zeta potentials and thick double layers never yield partial differential V/ partial differential R>0 (the interaction is repulsive for all distances) in such a case. A similar problem was encountered in 10(-1) M solutions, but now because V is always attractive, and no maximum force can be found. On the contrary, the extended DLVO model always yield physically reasonable sigma(0) values (coincident with those deduced from the classical approach when calculation is possible in the latter case). The comparison with experimental data shows that theory clearly underestimates sigma(0) by one order of magnitude or even more. The possible role of particle aggregation in this underestimation is discussed in terms of the scaling behavior of sigma(0) as a function of straight phi. Copyright 2000 Academic Press.     

Influence of Temperature on Stability of Electrostatically Stabilized Alumina Suspensions

Pure electrostatically stabilized aqueous alumina suspensions were prepared at various solid loadings in order to study the influence of temperature on the surface charge properties and rheology. Surface charge density at various temperatures was measured through potentiometric titration, and the analysis of the potentiometric data was accomplished using the constant capacitance surface model. Calculations of the pair-wise interaction potential between charged colloidal spheres dispersed in water were then carried out using conventional DLVO theory and a software package Stabil45, taking into account the temperature dependence of surface charge density and dielectric constant. The results showed that increasing temperature leads to a gradual diminution of alumina surface ionization, dielectric constant, and a total energy barrier for coagulation. The enhancement of the coagulation rate with increasing temperature leads to a gradual increase of both relative apparent viscosity and thixotropy. Copyright 2000 Academic Press.     

Stability of polydimethylsiloxane-magnetite nanoparticle dispersions against flocculation: interparticle interactions of polydisperse materials

The colloidal stability of dispersions comprised of magnetite nanoparticles coated with polydimethylsiloxane (PDMS) oligomers was investigated theoretically and experimentally. Particle-particle interaction potentials in a theta solvent and in a good solvent for the PDMS were predicted by calculating van der Waals, electrostatic, steric, and magnetic forces as functions of interparticle separation distances. A variety of nanoparticle sizes and size distributions were considered. Calculations of the interparticle potential in dilute suspensions indicated that flocculation was likely for the largest 1% of the population of particles. Finally, the rheology of these complexes over time in the absence of a solvent was measured to probe their stabilities against flocculation as neat fluids. An increase in viscosity was observed upon aging, suggesting that some agglomeration occurs with time. However, the effects of aging could be removed by exposing the sample to high shear, indicating that the magnetic fluids were not irreversibly flocculated.     

Aggregation Stability of Aqueous Dispersions of Microcrystalline Cellulose: pH Dependence

The aggregation stability of aqueous dispersions of microcrystalline cellulose (MCC) was studied by the flow ultramicroscopy in a wide range of pH (1–11). The calculations of the molecular and ion-electrostatic components of the interparticle interaction energy, which were performed according to the DLVO theory with and without allowance for the particle conductivity, demonstrated that, in most cases, the loss of the aggregation stability can not be explained without taking into account the concept of additional attraction forces between the MCC particles. It was assumed that such forces could be attributed to the dipole–dipole interactions or hydrogen bonding between hydrated particles.     

Small-angle neutron scattering study of concentrated colloidal dispersions: the interparticle interactions between sterically stabilized particles

Small-angle neutron scattering (SANS) was used to investigate the interparticle interactions in concentrated dispersions of colloidal silica stabilized either by steric or by electrostatic repulsive interactions. In 10 mM NaCl, an adsorbed PEO layer is required to prevent flocculation, and particles are stabilized by steric repulsions. The adsorbed layer was made invisible to neutrons by contrast matching with the aqueous continuous phase. Dispersions of the same particles at the same concentrations but in the absence of added salt and adsorbed PEO were also studied. In both cases, the SANS spectra of concentrated dispersions show a peak at low Q, which is due to interparticle interactions: a structure factor. The SANS data can be described rather well by a homogeneous spherical form factor and a structure factor based on the Hayter-Penfold/Yukawa potential model. The steric potential was compared to the electrostatic potential obtained by fitting the SANS data of the bare silica dispersions. The steric potential shows a greater dependence on the particle volume fraction, which we ascribe to the penetration and compression of the adsorbed PEO layer as the particles approach.     

Effect of polycarboxylate ether comb-type polymer on viscosity and interfacial properties of kaolinite clay suspensions

The interactions between kaolinite clay particles and a comb-type polymer (polycarboxylate ether or PCE), so-called PCE super-plasticizer, were investigated through viscosity and surface forces measurements by a rheometer and a Surface Forces Apparatus (SFA). The addition of PCE shows a strong impact on the viscosity of concentrated kaolinite suspensions in alkaline solutions (pH=8.3) but a weak effect under acidic conditions (pH=3.4). In acidic solutions, the high viscosity measured is attributed to the strong electrostatic interaction between negatively charged basal planes and positively charged edge surfaces of clay particles. Under the alkaline condition, the suspension viscosity was found to first increase significantly and then decrease with increasing PCE dosages. The results from surface forces measurement show that PCE molecules at low dosages can bridge the kaolinite particles in the concentrated suspensions via hydrogen bonding, leading to the formation of a kaolinite-PCE "network" and hence an increased suspension viscosity. At high PCE dosages, clay particles are fully covered by PCE molecules, leading to a more dispersed kaolinite suspensions and hence lower suspension viscosity due to steric repulsion between the adsorbed PCE molecules. The insights derived from measuring viscosity and interfacial properties of kaolinite suspensions containing varying amount of comb-type super-plasticizer PCE at different pH provide the foundation for many engineering applications and optimizing industrial processes.     

Structure and Rheology of Simulated Gels Formed from Aggregated Colloidal Particles

Soft, elastic, solvent-rich materials made from networks of aggregated colloidal particles are called particle gels. The networks may be regarded as being permanent or transient depending on whether the short-range attractive forces between the particles arise from strong irreversible bonding or weak reversible interactions. Understanding the relationships between the interparticle forces and the structure and rheology of particle gels is a challenging problem. This article shows how useful insight is being provided by Brownian dynamics simulations involving systems of spherical particles interacting with a combination of bonded and nonbonded interparticle potentials. Copyright 2000 Academic Press.     

Electrostatic stabilization in non-aqueous media

This paper discusses electrostatic stabilization of dispersions in non-aqueous media. It begins with the theory of repulsion, with particular attention to the roles of the dielectric constant and ionic strength of the liquid medium. Results for flat plates and spheres are compared. Methods are reviewed to measure the dielectric and electric parameters in non-aqueous media, in conjunction with procedures to determine the mechanical properties of electrostatically stabilized, concentrated suspensions.From theoretical considerations it appears that the extent of the electrostatic stabilization in non-aqueous media is extremely sensitive to the dielectric constant ϵ of the liquid, affecting stability in particular through the degree of dissociation of the stabilizing electrolyte. It is essential that, besides the presence of charge on the particles, there also are certain levels of ions in the solution to ensure a sufficient force of repulsion. It is expedient to distinguish three regimes of ϵ: (a) ϵ ⩾ 11, the (semi-)polar range, where systems can be charge-stabilized more or less as in aqueous systems, (b) the low-polar regime (5 ⩽ ϵ ⩽ 11), where electrostatic stabilization is possible provided some dissociated electrolyte is present and (c) the apolar range (ϵ⩽ 5), where screening is exclusively determined by the polarization of the solvent, and where electrostatic stabilization may be more problematic.Concentrated dispersions of solids in liquid nonionic carriers with dodecyl-benzene sulphonic acid (HDBS) as the stabilizer arc good models for the ‘low polar’ category, as detailed experimental data illustrate. In such media HDBS creates a ζ-potential, and enhances the dielectric constant and the ionic strength of the continuous phase. Especially when attraction between suspended salt particles in liquid nonionics is weak, electrostatic stabilization is easily achieved. This is particularly noticed in the rheology of the concentrated suspensions of some salts.     

Electrical double-layer effects on the deposition of zeolite A on surfaces

Zeolite particles formed from an aluminosilicate solution possess a negative surface charge due to the substitution of aluminum atoms into a SiO4 tetrahedral structure making it difficult to form a continuous layer in solution. The particle interactions with surfaces and each other can be studied using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The interaction energy between zeolite-zeolite and zeolite-substrate on various materials can be estimated in this fashion. The zeolite LTA particles show a stronger repulsion interaction on all substrates and on the each other as compared to the ZSM-5 particles. This repulsive energy also increases as the particles size increases. This results in the formation of conglomerate in the solution rather than forming an adhered layer on the substrate.     

Rheology and UV protection properties of suspensions of fine titanium dioxides in a silicone oil

Ultrafine particles of titanium dioxide (TiO2) are very attractive as a UV protection ingredient in cosmetic products. The UV-scattering behavior of TiO2 suspensions in a silicone oil are studied in relation to rheological properties. To control the dispersion stability of suspensions, two types of polyether-modified silicones are used as dispersants. When the suspensions are prepared with branch-type dispersants in which the polyether groups are incorporated as side chains along the backbone, the flow is shear-thinning even at low shear rates. The appearance of plateaus in the frequency-dependence curves of storage modulus implies the solidlike responses. On the other hand, the suspensions prepared with linear conformation dispersants, in which the silicone group and polyether group are alternately repeated in one long chain, are Newtonian at low shear rates. The suspensions are regarded as liquids, because the storage modulus decreases rapidly in the low-frequency region. The suspension rheology is strongly associated with flocculated structures that are primarily controlled by the interparticle attractions. The differences in rheological behavior can be explained by the differences in the adsorbed conformation of dispersant silicones. From optical measurements, it is confirmed that UV scattering increases with decreasing flocculation degree. Therefore, good agreement is established between rheological properties and UV protection ability.