The effects of added nanoparticles on aqueous kaolinite suspensions. I. Structural effects

The results of an experimental study focused on the effect of added silica nanospheres on the structure of an aqueous suspension of disc-shaped kaolinite particles are presented. In the absence of any additives, kaolinite particles rapidly aggregate and settle. When only nanoparticles were added to a 14% vol. kaolinite suspension, some stabilization was observed, although a thick, fluid-like sediment still formed. Adding both nanoparticles and salt (NaCl or KCl), however, caused the entire suspension to transition into a solid material that was strong enough to actually be sliced. A phase diagram was constructed showing the concentration of salt and nanoparticles needed to produce this transition. With smaller nanoparticles, the transition occurred at much lower nanoparticle volume fractions. Scanning electron micrographs of both the sediment and solid-like material, obtained by cryogenic drying, showed that the latter consisted of a porous, 'sponge-like' structure. The characteristic size of the pores decreased as the number density of the added nanoparticles increased. Although the nanoparticles were not visible in the SEM images, it is believed that they had separated into the pores of the solid-like material. While a similar type of transition could be produced in suspensions containing only the silica nanospheres, the structure and flow behavior of this material were markedly different from that obtained with the added clay.     

Single molecule nanoparticles of the conjugated polymer MEH-PPV, preparation and characterization by near-field scanning optical microscopy

We have developed a straightforward method for producing a stable, aqueous suspension of hydrophobic, fluorescent pi-conjugated polymer nanoparticles consisting primarily of individual conjugated polymer molecules. Features of the method are the facile preparation, purity, unique optical properties, and small size (approximately 5-10 nm) of the resulting nanoparticles. The results of TEM, scanning force microscopy, and near-field scanning optical microscopy of particles cast from the suspension indicate that the particles are single conjugated polymer molecules. The NSOM results yield estimates of the optical cross-sections of individual conjugated polymer molecules. The UV-vis absorption spectra of the nanoparticle suspensions indicate a reduction in conjugation length attributed to deformations of the polymer backbone. Fluorescence spectra of the aqueous nanoparticle suspensions indicate interactions between segments of the polymer chain and intramolecular energy transfer.     

Experimental investigation of nanoparticle dispersion by beads milling with centrifugal bead separation

A new type of beads mill for dispersing nanoparticles into liquids has been developed. The bead mill utilizes centrifugation to separate beads from nanoparticle suspensions and allows for the use of small sized beads (i.e. 15-30 microm in diameter). The performance of the beads mill in dispersing a suspension of titanium dioxide nanoparticle with 15 nm primary particles was evaluated experimentally. Dynamic light scattering was used to measure titania particle size distributions over time during the milling process, and bead sizes in the 15-100 microm range were used. It was found that larger beads (50-100 microm) were not capable of fully dispersing nanoparticles, and particles reagglomerated after long milling times. Smaller beads (15-30 microm) were capable of dispersing nanoparticles, and a sharp peak around 15 nm in the titania size distribution was visible when smaller beads were used. Because nanoparticle collisions with smaller beads have lower impact energy, it was found by X-ray diffraction and transmission electron microscopy that changes in nanoparticle crystallinity and morphology are minimized when smaller beads are used. Furthermore, inductively-coupled plasma spectroscopy was used to determine the level of bead contamination in the nanoparticle suspension during milling, and it was found that smaller beads are less likely to fragment and contaminate nanoparticle suspensions. The new type of beads mill is capable of effectively dispersing nanoparticle suspensions and will be extremely useful in future nanoparticle research.     

Effect of pH on deagglomeration and rheology/morphology of aqueous suspensions of goethite nanopowder

The kinetics of deagglomeration in diluted suspensions of goethite nanopowder, as well as the rheology and morphology of the resulting suspensions, strongly depends on pH. At pH 3, nanopowder can be dispersed as separate nanoparticles, and the resulting suspension is Newtonian, with the viscosity only marginally higher than the viscosity of water. At pH between 5 and 12, nanoparticles tend to reaggregate and form weak aggregates/flocs. Morphology changes from a Newtonian suspension of primary nanoparticles to a non-Newtonian, shear-thinning suspension of large, porous, interconnected flocs with the yield stress reaching a maximum at an isoelectric point. The effect of pH on morphology and rheology is reversible, and as pH is reduced to 3, the suspension becomes Newtonian, with viscosity marginally higher than the viscosity of water. The rheological models based on DLVO theory do not allow prediction of the effect of pH on viscosity and yield stress, but the flow curves of goethite suspensions can be described by a fractal model with five adjustable parameters.     

Supported lipid bilayer nanosystems: stabilization by undulatory-protrusion forces and destabilization by lipid bridging

Control of the stabilization/destabilization of supported lipid bilayers (SLBs) on nanoparticles is important for promotion of their organized assembly and for their use as delivery vehicles. At the same time, understanding the mechanism of these processes can yield insight into nanoparticle-cell interactions and nanoparticle toxicity. In this study, the suspension/precipitation process of zwitterionic lipid/SiO(2) nanosystems was analyzed as a function of ionic strength and as a function of the ratio of lipid/SiO(2) surface areas, at pH = 7.6. Salt is necessary to induce supported lipid bilayer (SLB) formation for zwitterionic lipids on silica (SiO(2)) (Seantier, B.; Kasemo, B., Influence of Mono- and Divalent Ions on the Formation of Supported Phospholipid Bilayers via Vesicle Adsorption. Langmuir 2009, 25 (10), 5767-5772). However, for zwitterionic SLBs on SiO(2) nanoparticles, addition of salt can cause precipitation of the SLBs, due to electrostatic shielding by both the lipid and the salt and to the suppression of thermal undulation/protrusion repulsive forces for lipids on solid surfaces. At ionic strengths that cause precipitation of SLBs, it was found that addition of excess SUVs, at ratios where there were equal populations of SUVs and SLBs, restored the undulation/protrusion repulsive forces and restabilized the suspensions. We suggest that SUVs separate SLBs in the suspension, as observed by TEM, and that SLB-SLB interactions are replaced by SLB-SUV interactions. Decreasing the relative amount of lipid, to the extent that there was less lipid available than the amount required for complete bilayer coverage of the SiO(2), resulted in precipitation of the nanosystem by a process of nanoparticle lipid bridging. For this case, we postulate a process in which lipid bilayer patches on one nanoparticle collide with bare silica patches on another SiO(2) nanoparticle, forming a single bilayer bridge between them. TEM data confirmed these findings, thus indicating that lipid bridges are composed of half bilayers on adjoining SiO(2) nanoparticles.     

Effect of Surface Charge on a Microsphere on the Stability of a Nanoparticle Suspension

A stable suspension of nanoparticles is generally achieved by modifying the surface properties of the nanoparticles using a surfactant. This is to create a surface charge on the nanoparticles which prevents them from forming aggregates. This phenomenon is very sensitive to changes in the local environment. Conventional optical tweezers though capable of trapping sub-micrometer particles are not known to trap single nanoparticles. However, the stability and dynamics of a suspension of nanoparticles can be probed through the changes in the fluctuations of an optically trapped microsphere that has been added to the suspension. Adding microspheres to the nanoparticle suspension can affect the stability depending on the surface charges the microparticles themselves have. The study reports here on the variation of the dynamics of suspended nanoparticles, which have a positive surface charge, when silica microspheres, which are negatively surface charged, are added to the suspension. With the addition of silica beads, there is agglomeration of the nanoparticles. The dynamics of these agglomerated structures are then probed by measuring the Brownian fluctuations of an optically trapped silica bead. These results are in sharp contrast to those of earlier studies carried out with suspensions of identical nanoparticles but with negative surface charge.     

Thixotropic properties of aqueous suspensions containing cationic starch and aluminum magnesium hydrotalcite-like compound

The rheological properties of aqueous suspensions consisting of cationic starch (CS) and positively charged aluminum magnesium hydrotalcite-like compound (HTlc) were investigated. Special emphasis was placed on the thixotropic phenomena. With the increase of mass ratio (R) of HTlc to CS, the equilibrium viscosity (eta(eq)) and the consistency coefficient (m) values of the suspensions increase in the range of neutral and alkaline pH (higher than 6.5) while decrease in the range of acid pH (lower than 6.5). With the increase of pH value, the eta(eq) and m values of the suspensions in the R range of 0-0.08 studied increase initially and then decrease, appearing a maximum value at about pH 7.41+/-0.25. The CS/HTlc suspensions display viscid character and the yield point of the suspensions was not observed except the suspension with R=0.08 in the pH range of 7.66-9.70, which showed a yield point and viscoelasticity. The CS/HTlc suspensions may display different thixotropic types: negative, complex or positive thixotropy, depending on pH and R value. The thixotropic type of the CS/HTlc suspension may be transformed from negative (pure CS solution), through complex (R=0.02), into positive thixotropy (R=0.05 and 0.08) with the increase of R in the studied R range of 0-0.08, and the thixotropic strength of the suspensions increases initially and then decreases with pH value in the pH range studied. The mechanism of the thixotropic phenomenon is discussed.     

Preparation and characterization of polyelectrolyte-coated gold nanoparticles

Gold nanoparticles of 5 nm diameter, stabilized by 4-(dimethylamino)pyridine (DMAP), were coated with poly(sodium 4-styrene sulfonate) (PSS) via electrostatic self-assembly. The suspension stability, monitored by the gold surface plasmon band (SPB), was studied by varying the pH, the PSS chain length, and PSS concentration. Enhanced stability is obtained at pH 10 (above the pKa of DMAP) when the polymer chain length matches or exceeds the particle circumference. Solid state 13C NMR was used to determine the presence of DMAP and polymers after subsequent deposition of weak and strong polycations: poly(allylamine hydrochloride) (PAH) and poly(diallyldimethylammonium chloride) (PDADMAC). At pH 10, DMAP remains associated with the nanoparticle after the first PSS layer has been formed. When PAH or PDADMAC are subsequently added at pH 4.5, DMAP is expelled, the suspensions remain stable, and zeta potential values indicate complete charge reversal. In the case of PDADMAC, however, the first layer of PSS is not fully retained. When PDADMAC is added at pH 10, DMAP and the first PSS layer are retained but lower zeta potentials and a higher SPB shift indicate a degraded stability. For PAH addition at pH 9.5, both DMAP and PSS are expelled and the suspension becomes unstable. These differences in stability of the multilayer components and the nanoparticle suspension are rationalized in terms of chain flexibility, polymer charge density, and the ability of the polymer functional groups to directly interact with the gold surface.     

Adsorption behavior of oxyethylated surfactants at the air/water interface

The low-shear viscosity eta(0) of colloidal suspensions of acrylic latex or silica in aqueous gelatin has been measured at a temperature above the sol-gel transition. Measurements were made on dilution of a concentrated suspension with water or a gelatin solution. Thus, either the gelatin : colloid ratio was maintained or it was varied at constant aqueous gelatin concentration. Systems were studied with four lime-processed gelatins of different molecular weights at two concentrations of added salt. In addition, the latex particle size and the thickness of the adsorbed gelatin layer were measured by photon correlation spectroscopy (PCS) under dilute conditions. The dependence of the low-shear viscosity eta(0) on particle concentration was exponential and did not follow the well-established Krieger-Dougherty model for simple hard-sphere suspensions over the concentration range studied. A simple phenomenological model, eta(0)=eta(o)10(phi(e)/phi(s)), was found to predict the behavior well. Here, eta(o) is the viscosity of a gelatin solution of the corresponding solution concentration, phi(e) is proportional to the volume fraction of the particles, and phi(s) is a scaling factor, which was determined to have a value of 0.85. With this value of phi(s), the dimensions determined from PCS could be used to predict the viscosity values.     

Negative electrorheological behavior in suspensions of inorganic particles

An investigation is described on the electric-field-induced structures in colloidal dispersions. Both rheological determinations and direct microscopic observations are used with that aim. The starting point of this study is the so-called electrorheological (ER) effect, consisting of the mechanical reinforcing of a fluid or suspension due to formation of chains of molecules or particles after being polarized by the action of the field. One macroscopic manifestation of this phenomenon is the transformation of the fluid from a typically Newtonian behavior to a viscoelastic material, with finite yield stress and high elastic modulus. The systems investigated were suspensions of elongated goethite (β-FeOOH) particles in silicone oils with varying amounts of silica nanoparticles. The results showed the rather unusual behavior known as "negative ER effect", which can be best described by saying that the application of an electric field reduces the yield stress and the elastic modulus, that is, produces destruction of structures rather than their build up. The negative behavior is also found for suspensions of other inorganic powders, including hematite and quartz. On the contrary, the usual positive ER response is found for suspensions of cellulose and montmorillonite clay. The same happens if goethite suspensions are prepared in high volume fractions, high-viscosity fluids, or both. All of the results found are compatible with the so-called interfacial model of electrorheology: the reduction of the yield stress of goethite suspensions when the applied field is high enough is the consequence of particle migration toward the electrodes because of charge injection and subsequent electrophoresis. The migration leaves the gap between the electrodes devoid of particles and explains the decrease in yield stress. The addition of silica nanoparticles contributes to reduce the strength of this effect by hindering the charging and making it necessary to increase the field strength to observe the negative effect. The model appears to also be applicable to cellulose, although the positive response found for such particles is explained by their large size: larger diameters bring about larger attraction forces between particles, leading to a tendency to produce strong aggregates. This is likely to occur in suspensions of colloids which, because of their relatively high electrical conductivity, tend to acquire charge even in such nonpolar liquids as silicone oils.     

Synthesis and electrorheological behavior of sterically stabilized polypyrrole-silica-methylcellulose nanocomposite suspension

Various polypyrrole (PPy)-silica-methylcellulose nanocomposite particles were synthesized by suspension polymerization in the presence of silica nanoparticles controlling the ratio of pyrrole, silica, and methylcellulose during the polymerization. The electrorheological (ER) and dielectric properties of the sterically stabilized PPy-silica-methylcellulose nanocomposite suspensions were investigated. The ER response increases with the increase in the silica/pyrrole ratio. The ER behavior also depends on the methylcellulose amount during the polymerization. The yield stress initially increases with the methylcellulose amount, passes through a maximum, and then decreases with the methylcellulose amount. The dielectric constants and dc conductivities of the PPy-silica-methylcellulose nanocomposite particles and the dielectric properties of their suspensions indicate that the increased ER response arises from the enhanced interfacial and particle polarization which depends on the silica/pyrrole ratio and the methylcellulose amount during the polymerization.     

AFM colloidal forces measured between microscopic probes and flat substrates in nanoparticle suspensions

Colloidal forces between atomic force microscopy probes of 0.12 and 0.58 N/m spring constant and flat substrates in nanoparticle suspensions were measured. Silicon nitride tips and glass spheres with a diameter of 5 and 15 mum were used as the probes whereas mica and silicon wafer were used as substrates. Aqueous suspensions were made of 5-80 nm alumina and 10 nm silica particles. Oscillatory force profiles were obtained using atomic force microscope. This finding indicates that the nanoparticles remain to be stratified in the intervening liquid films between the probe and substrate during the force measurements. Such structural effects were manifested for systems featuring attractive and weak repulsive interactions of nanoparticles with the probe and substrate. Oscillation of the structural forces shows a periodicity close to the size of nanoparticles in the suspension. When the nanoparticles are oppositely charged to the probes, they tend to coat the probes and hinder probe-substrate contact.     

Interface-induced anisotropy and the nematic glass/gel state in jammed aqueous Laponite suspensions

Aqueous suspensions of Laponite, a system composed of disklike nanoparticles, are found to develop optical birefringence over several days, well after the suspensions solidified because of jamming. The optical anisotropy is particularly enhanced near the air-Laponite suspension interface over length scales of several millimeters, which is beyond 5 orders of magnitude larger than the particle length scale, suggestive of large-scale ordering influenced by the interface. The orientational order increases with time and is always greater for higher concentration of salt, higher concentration of Laponite, and higher temperatures of the suspension. Although weakly birefringent, Laponite suspensions covered by paraffin oil do not show any enhancement in optical anisotropy near the interface compared to that in the bulk. We suggest that the expedited structure formation near the air interface propagating progressively inside the sample is responsible for the observed behavior. We discuss the observed nematic ordering in the context of glass-like and gel-like microstructure associated with aqueous Laponite suspensions.     

Specific ion effects: Interaction between nanoparticles in electrolyte solutions

Dependence of colloidal interactions on salt identity, observed frequently in experiments, can be accounted for once ion specific non-electrostatic forces are included in the theory. Ability to predict the effect of added salt on the phase diagram of colloid dispersions is essential for the design of processes involving nanocolloids. The Ornstein–Zernike equation with hypernetted chain closure approximation provides a viable first estimate for the potential of mean force between ionized nanoparticles like alumina aggregates in aqueous electrolytes subject to dispersion interactions with hydrated simple ions. Calculated potentials of mean force enable the prediction of osmotic second virial coefficients and phase diagrams showing a dramatic dependence on ion type. The choice of salt therefore provides an efficient, non-intrusive way to tune the phase behavior of nanoparticle dispersions.     

Steric repulsion as a way to achieve the required stability for the preparation of ionic liquid-based ferrofluids

With this work we would like to emphasize the necessity of steric repulsion to stabilize novel ionic liquid-based ferrofluids. For this purpose, we prepared a suspension of magnetite nanoparticles coated with a double layer of oleic acid, dispersed in 1-ethyl-3-methylimidazolium ethylsulphate ([EMIM][EtSO(4)]). For comparison, a suspension of bare magnetite nanoparticles in [EMIM][EtSO(4)] was also prepared. The stability of these suspensions was checked by magnetic sedimentation and centrifugation processes. Furthermore, their yield stress was measured as a function of the applied magnetic field, which gave additional information on their stability. The results of these experiments showed that the suspension of bare nanoparticles was rather unstable, whereas the suspension of double layer coated nanoparticles gave rise to a true (stable) ferrofluid.     

Particle interactions and rheological effects in kaolinite suspensions

Particle interactions in kaolinite suspensions are modelled by representing the edge face of a kaolinite platelet as cylinder and flat plate, respectively. Computations of total energy of interaction show that at pH 6,7 and 8 and in the presence of 10^?4 ?10^?I M NaCl both face-edge and edge-edge modes of interaction are likely. Rheological parameters for flocculated suspensions (extrapolated shear stress and plastic viscosity) for dilute sodium kaolinite suspensions (volumetric concentration 0.02) are interpreted in terms of the proposed interaction models.     

In situ observation of gamma-Fe2O3 nanoparticle adsorption under different monolayers at the air/water interface

We studied the adsorption of gamma-Fe 2O 3 nanoparticles from an aqueous solution under different charged Langmuir monolayers (stearic acid, stearyl alcohol, and stearyl amine). The aqueous subphase was composed of a colloidal suspension of gamma-Fe 2O 3 nanoparticles. The average hydrodynamic diameter of the nanoparticles measured by dynamic light scattering measurements was 16 nm. The observed zeta potential of +40 mV (at pH 4) results in a long-term stability of the colloidal dispersion. The behavior of the different monolayer/nanoparticle composites were studied with surface pressure/area (pi/ A) isotherms. The adsorption of the nanoparticles under the different monolayers induced an expansion of the monolayers. These phenomena depended on the charge of the monolayers. After the Langmuir/Blodgett transfer on glass substrates, the nanoparticle/monolayer composite films were studied by means of UV-vis spectroscopy. The spectra pointed to increasing adsorption of the nanoparticles with increasing electronegativity of the monolayers. On the basis of these results, we studied the in situ adsorption of nanoparticles under the different monolayers by X-ray reflectivity measurements. Electron density profiles of the liquid/gas interfaces were obtained from the X-ray reflectivity data. The results gave clear evidence for the presence of electrostatic interaction between the differently charged monolayers and the positively charged nanoparticles. While the adsorption process was favored by the negatively charged stearic acid monolayer, the positively charged layer of stearyl amine prevented the formation of ultrathin nanoparticle layers.     

NMR studies into colloidal stability and magnetic order in fatty acid stabilised aqueous magnetic fluids

We report the physico-chemical characterisation of fatty acid stabilised aqueous magnetic fluids, which are ideal systems for studying the influence of nanoparticle aggregation on the emergent magnetic resonance properties of the suspensions. Stable colloids of superparamagnetic magnetite, Fe(3)O(4), nanoparticle clusters in the 80 to 100 nm size range were produced by in situ nanoparticle growth and stabilisation, and by suspending pre-formed nanoparticles. NMR relaxation analysis shows that the magnetic resonance properties of the two types of suspension differ substantially and provides new insights into how the relaxation mechanisms are determined by the organisation of the nanoparticles within the clusters.     

Micro-structure differences in kaolinite suspensions

SEM observations of the aqueous suspensions of kaolinite from Birdwood (South Australia) and Georgia (USA) show noticeable differences in number of physical behaviour which has been explained by different micro-structure constitution. Birdwood kaolinite dispersion gels are observed at very low solid loadings in comparison with Georgia KGa-1 kaolinite dispersions which remain fluid at higher solids loading. To explain this behaviour, the specific particle interactions of Birdwood kaolinite, different from interaction in Georgia kaolinite have been proposed. These interactions may be brought about by the presence of nano-bubbles on clay crystal edges and may force clay particles to aggregate by bubble coalescence. This explains the predominance of stair step edge-edge like (EE) contacts in suspension of Birdwood kaolinite. Such EE linked particles build long strings that form a spacious cell structure. Hydrocarbon contamination of colloidal kaolinite particles and low aspect ratio are discussed as possible explanations of this unusual behaviour of Birdwood kaolinite. In Georgia KGa-1 kaolinite dispersions instead of EE contact between platelets displayed in Birdwood kaolinite, most particles have edge-to-face (EF) contacts building a cardhouse structure. Such an arrangement is much less voluminous in comparison with the Birdwood kaolinite cellular honeycomb structure observed previously in smectite aqueous suspensions. Such structural characteristics of KGa-1 kaolinite particles enable higher solid volume fractions pulps to form before significantly networked gel consistency is attained.     

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.