Nature of flocculation and tactoid formation in montmorillonite: the role of pH.

The dissolution and swelling properties of montmorillonite at different pH have been studied, using small angle X-ray scattering (SAXS), imaging and osmotic stress methods combined with Monte Carlo simulations. The acidity of montmorillonite dispersions has been varied as well as the counterions to the net negatively charged platelets. At low pH, Na montmorillonite dissolves and among other species Al(3+) is released, hydrated, polymerized and then it replaces the counterions in the clay. This dramatically changes the microstructure of Na montmorillonite, which instead of having fully exfoliated platelets, turns into a structure of aggregated platelets, so-called tactoids. Montmorillonite dispersion still has a significant extra-lamellar swelling among the tactoids due to the presence of very small nanoplatelets.

Reactivity, swelling and aggregation of mixed-size silicate nanoplatelets.

Montmorillonite is a key ingredient in a number of technical applications. However, little is known regarding the microstructure and the forces between silicate platelets. The size of montmorillonite platelets from different natural sources can vary significantly. This has an influence on their swelling behavior in water as well as in salt solutions, particularly when tactoid formation occurs, that is when divalent counterions are present in the system. A tactoid consists of a limited number of platelets aggregated in a parallel arrangement with a constant separation. The tactoid size increases with platelet size and with very small nanoplatelets, ∼30 nm, no tactoids are observed irrespectively of the platelet origin and concentration of divalent ions. The formation and dissociation of tactoids seem to be reversible processes. A large proportion of small nanoplatelets in a mixed-size system affects the tactoid formation, reduces the aggregation number and increases the extra-lamellar swelling in the system.

Equation of state of colloidal dispersions.

We present a comparison of experimentally and theoretically determined osmotic pressures for various colloidal dispersions. Experimental data is collected from several different silica and polystyrene dispersions. The theoretical pressure determinations are based on the primitive model combined with the cell model, and the physical quantities are calculated exactly using Monte Carlo simulations in the canonical and grand canonical ensemble. The input to the simulations in terms of colloidal particle size, surface charge density, and so forth are taken directly from experiments, and the approach does not contain any adjustable parameters. The agreement between theory and experiment is very good without any fitting parameters, showing that the simplifications behind the primitive model and the cell model are physically sound. The results reveal a surprising correspondence between the equations of state in spherical and planar geometries, indicating that the particle shape is of secondary importance in dispersions dominated by repulsive interactions. For one of the silica dispersions, we have also investigated how various monovalent counterions influence the swelling properties. Within experimental error, we are unable to detect any ion specificity, which is further support for the theoretical models used.

Ca/Na montmorillonite: structure, forces and swelling properties.

Ca/Na montmorillonite and natural Wyoming bentonite (MX-80) have been studied experimentally and theoretically. For a clay system in equilibrium with pure water, Monte Carlo simulations predict a large swelling when the clay counterions are monovalent, while in presence of divalent counterions a limited swelling is obtained with an aqueous layer between the clay platelets of about 10 A. This latter result is in excellent agreement with X-ray scattering data, while dialysis experiments give a significantly larger swelling for Ca montmorillonite in pure water. Obviously, there is one "intra-lamellar" and a second "extra-lamellar" swelling. Montmorillonite in contact with a salt reservoir containing both Na(+) and Ca(2+) counterions will only show a modest swelling unless the Na(+) concentration in the bulk is several orders of magnitude larger than the Ca(2+) concentration. The limited swelling of clay in presence of divalent counterions is a consequence of ion-ion correlations, which reduce the entropic repulsion as well as give rise to an attractive component in the total osmotic pressure. Ion-ion correlations also favor divalent counterions in a situation with a competition with monovalent ones. A more fundamental result of ion-ion correlations is that the osmotic pressure as a function of clay sheet separation becomes nonmonotonic, which indicates the possibility of a phase separation into a concentrated and a dilute clay phase, which would correspond to the "extra-lamellar" swelling found in dialysis experiments. This idea also finds support in the X-ray scattering spectra, where sometimes two peaks corresponding to different lamellar spacings appear.

Dips and rims in dried colloidal films.

We describe a spatial pattern arising from the nonuniform evaporation of a colloidal film. Immediately after the film deposition, an obstacle is positioned above its free surface, minimizing evaporation at this location. In a first stage, the film dries everywhere but under the obstacle, where a liquid region remains. Subsequently, this liquid region evaporates near its boundaries with the dry film. This loss of water causes a flow of liquid and particles from the center of the obstructed region to its periphery. The final film has a dip surrounded by a rim whose diameter is set by the obstacle. This turns out to be a simple technique for structuring films of nanometric thickness.

Rheology and phase behavior of dense casein micelle dispersions.

Casein micelle dispersions have been concentrated through osmotic stress and examined through rheological experiments. In conditions where the casein micelles are separated from each other, i.e., below random-close packing, the dispersions have exactly the flow and dynamic properties of the polydisperse hard-sphere fluid, demonstrating that the micelles interact only through excluded volume effects in this regime. These interactions cause the viscosity and the elastic modulus to increase by three orders of magnitude approaching the concentration of random-close packing estimated at C(max) approximately 178 g/l. Above C(max), the dispersions progressively turn into "gels" (i.e., soft solids) as C increases, with elastic moduli G(') that are nearly frequency independent. In this second regime, the micelles deform and/or deswell as C increases, and the resistance to deformation results from the formation of bonds between micelles combined with the intrinsic mechanical resistance of the micelles. The variation in G(') with C is then very similar to that observed with concentrated emulsions where the resistance to deformation originates from a set of membranes that separate the droplets. As in the case of emulsions, the G(') values at high frequency are also nearly identical to the osmotic pressures required to compress the casein dispersions. The rheology of sodium caseinate dispersions in which the caseins are not structured into micelles is also reported. Such dispersions have the behavior of associative polymer solutions at all the concentrations investigated, further confirming the importance of structure in determining the rheological properties of casein micelle systems.

Phase transition pathways for the production of 100 nm oil-in-water emulsions.

Oil/water emulsions can be produced through phase inversion, by adding water to a reverse water/oil microemulsion. According to small angle neutron scattering experiments and visual observations performed during phase inversion, the stages of this process are as follows: (i) upon water addition, the microemulsion gives way to a highly swollen lamellar phase; (ii) the transient lamellar phase breaks up to yield an array of droplets; (iii) the droplets loses the correlations of the lamellar phase. This emulsion is already present less than one minute after the initial addition of water, and it reaches the final size distribution in one hour. The final population of oil droplets is homogenous with a mean diameter below 100 nm.

Interaction of nanometric clay platelets.

The free energy of interaction between two nanometric clay platelets immersed in an electrolyte solution has been calculated using Monte Carlo simulations as well as direct integration of the configurational integral. Each platelet has been modeled as a collection of charged spheres carrying a unit charge the face of a platelet contains negative charges, and the edge, positive charges. The calculations predict that a configuration of "overlapping coins" is the global free energy minimum at intermediate salt concentrations (10-100 mM). A second weaker minimum, corresponding to the well-known "house of cards" configuration, also appears in this salt interval. At low salt concentrations the electrostatic repulsion dominates, while at intermediate concentrations electrostatic interactions alone can create a net attraction between the platelets. At sufficiently high salt content (>200 mM), the van der Waals interaction takes over and the net interaction becomes attractive at essentially all separations. From the calculated free energy and its derivative, we can derive a yield stress and elasticity modulus in fair agreement with experiment. The roughness of the platelets affects the quantitative behavior of the free energy of interaction but does not alter the results in a qualitative way. From the variation of the free energy of interaction, we would tentatively describe the phase behavior as follows: At low salt, the interaction is strongly repulsive and the dispersion should appear as a solid ("repulsive gel"). With increasing salt concentration, the repulsion is weakened and a liquid phase appears ("sol"). A further increase of the salt content leads a second solid phase ("attractive gel") governed by attractive interactions between the platelets. Finally, at sufficiently high salinity, the clay precipitates due to van der Waals forces.

How to concentrate nanoparticles and avoid aggregation?

Most of the methods that are used to produce pharmaceutical suspensions of nanoparticles for drug-targeting yield suspensions having a low content in drug carriers. This can be a dramatic limitation when the volume of suspension that would have to be administered in vivo to reach therapeutic concentrations of the drug is much above the acceptable range. Concentrating the drug-carrier suspension by centrifugation, lyophilization and evaporation is often inapplicable because aggregates are formed. Here we present a simple method that is able to increase the concentration of nanoparticle suspensions without forming aggregates. It consists in a dialysis of the suspensions against a polymer solution. This causes an osmotic stress, which produces a displacement of water from the nanoparticle suspension towards the counter-dialysing solution. Various types of nanoparticle suspensions can be concentrated in near equilibrium conditions, and the result is controlled and reproducible. Concentration factors up to 50 were obtained in a few hours at room temperature. The original characteristics of the nanoparticles were fully preserved in the concentrated dispersion.

Restructuring of colloidal cakes during dewatering.

Aqueous suspensions of aggregated silica particles have been dewatered to the point where the colloidal aggregates connect to each other and build a macroscopic network. These wet cakes have been compressed through the application of osmotic pressure. Some cakes offer a strong resistance to osmotic pressure and remain at a low volume fraction of solids; other cakes yield at low applied pressures, achieving nearly complete solid/liquid separation. We used small angle neutron scattering and transmission electron microscopy to determine the processes by which the particles move and reorganize during cake collapse. We found that these restructuring processes follow a general course composed of three stages: (1) at all scales, voids are compressed, with large voids compressed more extensively than smaller ones; the local order remains unchanged; (2) all voids with diameters in the range of 2-20 particle diameters collapse, and a few dense regions (lumps) are formed; and (3) the dense lumps build a rigid skeleton that resists further compression. Depending on the nature of interparticle bonds, some cakes jump spontaneously into stage 3 while others remain stuck in stage 1. To elucidate the relation between bond strength and compression resistance, we have constructed a numerical model of the colloidal network. In this model, particles interact through noncentral forces that are produced by springs attached to their surfaces. Networks made of bonds that break upon stretching evolve through a plastic deformation that reproduces the three stages of restructuring evidenced by the experiments. Networks made of bonds that are fragile jump into stage 3. Networks made of bonds that can be stretched without breaking evolve through elastic compression and restructure only according to stage 1.

The role of interparticle forces in colloidal aggregates: local investigations and modelling of restructuring during filtration.

Industrial solid-liquid separation processes, such as pressure filtration or membrane processes, involve the application of pressure to suspensions. In response, some water is extracted, the suspension volume is reduced, and the dispersed aggregates start to form a network. In recent works, we aimed to make a prediction for the response of aggregates to stress which occurs during a filtration. We chose model systems made of aggregated silica nanoparticles. Some of these systems offer a strong resistance to applied stresses, and retain their permeability; others yield and collapse. We used small angle neutron scattering by which we can locally quantify the particle distribution withi the network to determine the processes by which particles reorganise during collapse: we found that reordering processes at the scale of 1 to 10 particle diameters control the course of collapse and the loss of permeability. Finally we constructed a numerical model for describing the processes by which colloidal aggregates are compressed. This model predicts that the response of such networks to pressure follows some scaling laws, which depend only on the elastic vs. dissipative nature of interparticle bonds.

Recombination of nanometric vesicles during freeze-drying.

Concentrated dispersions of nanometric lipid vesicles (mean diameter 20 nm) in water/maltose solutions have been freeze-dried and then redispersed in water, yielding again dispersions of lipid vesicles. At each stage of the freeze-drying process, the organization of the vesicles in the dispersion and their size distribution were examined through small-angle neutron scattering and gel permeation chromatography. It was found that the osmotic deswelling of the vesicles caused them to recombine into larger vesicles. A single burst of recombination events occurred when the maltose concentration in the aqueous phase rose above 100 g/L. The final vesicle population was monopopulated, with a central diameter about twice as large as that of the original dispersion.

Controlling the cohesion of cement paste.

The main source of cohesion in cement paste is the nanoparticles of calcium silicate hydrate (C-S-H), which are formed upon the dissolution of the original tricalcium silicate (C(3)S). The interaction between highly charged C-S-H particles in the presence of divalent calcium counterions is strongly attractive because of ion-ion correlations and a negligible entropic repulsion. Traditional double-layer theory based on the Poisson-Boltzmann equation becomes qualitatively incorrect in these systems. Monte Carlo (MC) simulations in the framework of the primitive model of electrolyte solution is then an alternative, where ion-ion correlations are properly included. In addition to divalent calcium counterions, commercial Portland cement contains a variety of other ions (sodium, potassium, sulfate, etc.). The influence of high concentrations of these ionic additives as well as pH on the stability of the final concrete construction is investigated through MC simulations in a grand canonical ensemble. The results show that calcium ions have a strong physical affinity (in opposition to specific chemical adsorption) to the negatively charged silicate particles of interest (C-S-H, C(3)S). This gives concrete surprisingly robust properties, and the cement cohesion is unaffected by the addition of a large variety of additives provided that the calcium concentration and the C-S-H surface charge are high enough. This general phenomenon is also semiquantitatively reproduced from a simple analytical model. The simulations also predict that the affinity of divalent counterions for a highly and oppositely charged surface sometimes is high enough to cause a "charge reversal" of the apparent surface charge in agreement with electrophoretic measurements on both C(3)S and C-S-H particles.

Hydration of an amphiphilic excipient, Gelucire 44/14.

The incorporation of drugs into Gelucires has been reported to increase the dissolution rate of poorly soluble drugs, often leading to improved drug bioavailability. In pharmaceutical applications, it is important to know how the excipient interacts with the drug, and how the mixture behaves during manufacturing, storage as well as during administration. The uptake of water by an amphiphilic excipient, Gelucire 44/14, has been investigated in two ways: storage in humid air and addition of liquid water. During exposure to humid air, the uptake goes in stages that correspond to the dissolution of the components of the excipient, starting with the most hydrophilic ones: glycerol, then polyethylene glycol (PEG), PEG esters (PEG monolaurate and PEG dilaurate), and finally glycerides (trilaurin). At each stage, the remaining crystals are in equilibrium with an interstitial solution made of water and the dissolved components. In this range of hydrations, the total uptake is close to the sum of the equilibrium hydrations of the components. In the pharmaceutical formulation, the active ingredient could dissolve in the liquid phase. At larger hydrations, obtained through addition of liquid water, the state of Gelucire 44/14 differs from those of its components. Gelucire 44/14 forms a lamellar phase and this phase melts at 30 degrees C whereas the pure PEG esters form hexagonal and cubic mesophases. The cubic mesophases do not melt until the temperature exceeds 40 degrees C. At body temperature, all crystals in Gelucire 44/14 melt to an isotropic fluid as soon as the total water content exceeds 5%. Therefore the formulation of amphiphilic excipients can be optimized to avoid the formation of mesophases that impede dissolution of the excipient at body temperature.

Onset of cohesion in cement paste.

It is generally agreed that the cohesion of cement paste occurs through the formation of a network of nanoparticles of a calcium-silicate-hydrate ("C-S-H"). However, the mechanism by which these particles develop this cohesion has not been established. Here we propose a dielectric continuum model which includes all ionic interactions within a dispersion of C-S-H particles. It takes into account all co-ions and counterions explicitly (with pure Coulomb interactions between ions and between ions and the surfaces) and makes no further assumptions concerning their hydration or their interactions with the surface sites. At high surface charge densities, the model shows that the surface charge of C-S-H particles is overcompensated by Ca2+ ions, giving a reversal of the apparent particle charge. Also, at high surface charge densities, the model predicts that the correlations of ions located around neighboring particles causes an attraction between the particle surfaces. This attraction has a range of approximately 3 nm and a magnitude of 1 nN, values that are in good agreement with recent AFM experiments. These predictions are stable with respect to small changes in surface-surface separation, hydrated ion radius, and dielectric constant of the solution. The model also describes the effect of changes in cement composition through the introduction of other ions, either monovalent (Na) or multivalent (aluminum or iron hydroxide).

Small-angle neutron scattering study of particle coalescence and SDS desorption during film formation from carboxylated acrylic latices.

Four monodisperse core-shell latices were synthesized for small-angle neutron scattering (SANS) studies, differing by the acrylic acid content in the particle shell (1 or 4 wt%) and the T(g) of the acrylic core (around -40 or 10 degrees C). In a first part, the coalescence kinetics of the surfactant-free latices were studied. It was shown that coalescence was hindered by an increase in the acrylic acid content of the shell, pH of the latex, and Tg of the core. These results could be interpreted in terms of chain mobility in the shell and in the core. Upon coalescence, the hydrophilic phase was segregated in spherical, polydisperse domains with an average diameter of 110 nm. In a second part, labeled SDS was used to follow desorption of the surfactant during film formation. It was shown that desorption occurred early in the film formation process when the latex still contained around 20% of water. A small fraction of the surfactant remained irreversibly adsorbed at the particle surface.

Surface Complexation of Calcite by Carboxylates in Water.

Small molecules that have two carboxylic functions can adsorb from water onto calcite. The adsorption site is a -Ca+ site. The mechanism of adsorption is a complexation of the -Ca+ site by the two carboxylates, similar to the solution complexation of Ca++ ions. The complex has a ring structure where the two carboxylates are joined on one side by the -Ca+ ion and on the other by the n CH2 groups of the small molecule. Five-bond rings (n = 0) are the most stable, followed by six-bond rings (n = 1) and seven-bond rings (n = 2). Five-bond rings can also be formed with one carboxylate and one hydroxyl group (this is the case for alpha-hydroxycarboxylates) or with one enolate and one hydroxyl group (catechol). The sequence of binding strengths is enolate > carboxylate > hydroxyl; it matches the sequence of complexation efficiencies of these groups in solution and their characters as electron donors toward the metal cation. Copyright 1999 Academic Press.

Transfer of Oil between Emulsion Droplets

A contrast matching technique was used to determine the exchange of oils between emulsion droplets having different refractive indexes. Emulsions of tetradecane and 1-bromo tetradecane in water were made separately in a high-pressure homogenizer, then mixed and equilibrated at rest. It was found that droplets exchanged oil molecules through the continuous phase, in a process similar to Ostwald ripening. Emulsions of hexadecane and 1-bromo hexadecane were also mixed; at rest, no exchange of oil took place. These mixtures were subsequently recirculated in the high-pressure homogenizer. Exchange of oil occurred as a result of droplet coalescence in the homogenizer. Two regimes were found, "surfactant-poor" and "surfactant-rich." In the "surfactant-poor" regime, recoalescence took place at all values of the pressure used in the homogenizer. In the "surfactant-rich" regime, recoalescence took place only if the pressure was at least equal to that used originally to make the emulsion. These results demonstrate that the size of the emulsion droplets made in a high-pressure homogenizer results from a succession of fragmentation and recoalescence processes. Possible mechanisms preventing recoalescence are discussed.

Structures of nanoparticles prepared from oil-in-water emulsions.

Hydrophobic substances were dissolved in an organic solvent and emulsified with an aqueous solution at very high shear. Droplets of very small sizes (50-100 nm) were obtained by using surfactants which were combinations of lecithins and bile salts. After emulsification, the organic solvent was removed by evaporation, yielding stable dispersions of solid particles. The sizes, shapes, and structures of the particles were examined through quasi-elastic light scattering, small-angle neutron scattering and cryotransmission electron microscopy. Cholesterol acetate particles stabilized by lecithin and bile salts were found to be platelets of 10-20 nm thickness and 80 nm diameter. Cholesteryl acetate particles stabilized with POE-(20)-sorbitan monolaurate were dense spherical globules of diameter 100 nm. Particles with a composition similar to the endogenously occurring, lipoprotein, LDL, were large spherical globules studded with small vesicles. The subsequent evolution of the cholesteryl acetate dispersion upon aging was examined. There was no transfer of cholesteryl acetate between particles nor to large crystals. However, some aggregation of the particles was observed when the volume fraction of the particles in the aqueous dispersion exceeded 0.05. Thus, the structure of the nanoparticles obtained through deswelling of emulsion droplets changes according to the nature of the emulsifiers and to the composition of the hydrophobic substances which they contain.