Aqueous Colloidal Dispersions Research Articles

Macroporous Morphology Induced by Phase Separation in Sol-Gel Systems Derived from Titania Colloid

ABSTRACTMacroporous titania gels have been prepared by the sol-gel method starting from aqueous colloidal dispersion of titania. The use of titania sol instead of highly reactive titanium alkoxide allows us to control the rate of gelation. Macroporous morphology is formed when the transitional structure of phase separation is fixed as permanent structures by the sol-gel transition. The domain size can be controlled reproducibly by changing the starting composition. The crystal structure of the titania gel is transformed from anatase to rutile through the heat treatment at 900°C, while the macroporous morphology remains unchanged.

Influence of an Enteric Polymer on Drug Release Rates of Theophylline from Pellets Coated with Eudragit® RS 30D

The purpose of this research study was to investigate the influence of an enteric polymer on the drug release properties of theophylline pellets coated with Eudragit® RS 30D. Theophylline pellets were coated with aqueous colloidal dispersions of Eudragit® RS 30D containing various amounts of Eudragit® L 100-55. The effect of storage conditions on the release of drug from coated pellets was determined as a function of the pH of the dissolution medium. The results from the dissolution study showed significant changes in the dissolution rate of theophylline from pellets coated with Eudragit® RS 30D when cured at 40°C for 4 days. No change in the drug release rate was observed when Eudragit® L100-55 was present in the Eudragit® RS 30D dispersion. Increasing the ratio of Eudragit® L100-55 to Eudragit® RS 30D resulted in faster drug release rates from the coated pellets. An increase in the pH of the dissolution medium was found to enhance drug release from the pellets coated with Eudragit® RS 30D containing Eudragit® L 100-55. Theophylline pellets when coated with Eudragit® RS 30D containing the enteric polymer Eudragit® L100-55 demonstrated no aging effects when stored at elevated temperatures. The overcoating of the pellets with Eudragit® RD 100 did not affect the drug release profiles and prevented the particles from agglomerating during curing and storage.

Release of phospholipids from colloidal particles to polymeric surfaces.

This study presents a small-scale polymerization of high molecular weight methyl methacrylate/n-butyl acrylate (MMA/n-BA) colloidal particles that are synthesized in an aqueous environment in the presence of phospholipid hydrogenated soybean phosphatidylcholine (HSPC) molecules that also serve as the particle stabilizing agents. When such particles coalesce to form polymeric films, they release phospholipids, which, in turn, form organized structures near the film-air (F-A) interface. Diffusion and mobility of phospholipid molecules are affected not only by their compatibility with colloidal particles but also by electrolyte environments of colloidal dispersions. When Na(+), K(+), and Ca(2+) counterions are added to MMA/n-BA aqueous colloidal dispersions stabilized with HSPC, and such films are coalesced, different degrees of diffusion of HSPC to the F-A interface exist, depending on the counterion, and conformational changes of HSPC result. For example, in the presence of Ca(2+), HSPC molecules collapse entropically to form random surface layers, as opposed to smaller Na(+) and K(+), which force amphiphilic HSPC ends to align preferentially parallel to the film surface. These studies show that it is possible to design stimuli-response colloidal systems triggered by chemical environments of active molecules on colloidal polymer particles.

Liquid-liquid and liquid-solid phase separation and flocculation for a charged colloidal dispersion.

We model the intercolloidal interaction by a hard-sphere Yukawa repulsion to which is added the long-range van der Waals attraction. In comparison with the Derjaguin-Landau-Verwey-Overbeek repulsion, the Yukawa repulsion explicitly incorporates the spatial correlations between colloids and small ions. As a result, the repulsive part can be expressed analytically and has a coupling strength depending on the colloidal volume fraction. By use of this two-body potential of mean force and in conjunction with a second-order thermodynamic perturbation theory, we construct the colloidal Helmholtz free energy and use it to calculate the thermodynamic quantities, pressure and chemical potential, needed in the determination of the liquid-liquid and liquid-solid phase diagrams. We examine, in an aqueous charged colloidal dispersion, the effects of the Hamaker constant and particle size on the conformation of a stable liquid-liquid phase transition calculated with respect to the liquid-solid coexistence phases. We find that there exists a threshold Hamaker constant or particle size whose value demarcates the stable liquid-liquid coexistence phases from their metastable counterparts. Applying the same technique and using the energetic criterion, we extend our calculations to study the flocculation phenomenon in aqueous charged colloids. Here, we pay due attention to determining the loci of a stability curve stipulated for a given temperature T0, and obtain the parametric phase diagram of the Hamaker constant vs the coupling strength or, at given surface potential, the particle size. By imposing T0 to be the critical temperature T(c), i.e., setting k(B)T0 (=k(B)T(c)) equal to a reasonable potential barrier, we arrive at the stability curve that marks the irreversible/reversible phase transition. The interesting result is that there occurs a minimum size for the colloidal particles below (above) which the colloidal dispersion is driven to an irreversible (reversible) phase transition.

Effect of plasticizer on the dynamic surface tension and the free volume of Eudragit systems.

The purpose of the present study was to determine the changes of dynamic surface tension of aqueous Eudragit dispersions quantitatively and compare these data with the free volume of the free films formed from several dispersions of different dibutyl sebacate (DBS) concentrations. Eudragit L 30D and Eudragit RL 30D aqueous colloidal polymer dispersions and their cast free films were examined. The concentration of DBS varied in the dispersions from 0 to 20%. The dynamic surface tension of the dispersions were measured by the Du Nouy ring method while the free volume of cast films were determined by positron annihilation spectroscopy. The obtained results show that dynamic surface tension measurements indicate the white point (WP) of Eudragit dispersions by a significant standard deviation increase. This may suggest the applicability of dynamic surface tension measurements for the determination of the WP of polymeric dispersions. A decrease in the WP temperature of Eudragit dispersions with an increasing plasticizer concentration was observed up to the DBS concentration of 10% w/w. Above this concentration neither the WP of the polymer dispersion nor the free volume of the free films of the polymer were changed remarkably.

Use of ion-exchange resins to prepare 100 μm-sized microcapsules with prolonged drug-release by the Wurster process

Ion-exchange resin (IER)–drug complexes were used as core materials to explore their capability to prepare a 100 μm-sized, highly drug-incorporated microcapsule with a prolonged drug release by the Wurster process. Diclofenac sodium was loaded into Dowex 1-X2 fractionated into 200–400 mesh and subsequently microencapsulated with two types of aqueous colloidal polymer dispersion, Aquacoat® or Eudragit® RS30D. The mass median diameter and drug content of the microcapsules thus obtained were 98 μm and 46% with Aquacoat®, and 95 μm and 50% with Eudragit® RS30D, respectively. Each microcapsule was obtained at a product yield of 94%. The rate of drug release from the microcapsules was highly dependent on the encapsulating materials. For the microcapsules coated with Aquacoat®, diclofenac sodium was found to be rapidly released over 4 h, even at a 25 wt% coating level because of cracks on the microcapsule surfaces resulting from the swelling stress of the drug-loaded IER cores. In contrast, significantly prolonged drug-release was achieved in the microcapsules prepared with Eudragit® RS30D: even such a very low coating level as 3 wt% provided an exceptionally prolonged drug-release over 24 h. The results indicated that the use of IER along with a flexible coating material would be a feasible way to prepare a prolonged release type of microcapsules with a diameter of 100 μm and a drug content of more than 50% by the Wurster process.

Improved Dissolution and Cytotoxicity of Camptothecin Incorporated into Oxidized-Cellulose Microspheres Prepared by Spray Drying

Oxidized celluloses (OC) containing 7, 13, and 20% carboxylic content (OC-7, OC-13, and OC-20, respectively) have been converted into aqueous colloidal dispersions and used to prepare microspheres of the antineoplastic agent camptothecin (CPT) by spray drying. Plasticizers used were glycerin, polyethylene glycol 400 (PEG-400), and polyethylene glycol 6000 (PEG-6000). Irrespective of the carboxylic content of OC and the nature of plasticizer employed, the size of microspheres varied from 1.25 ± 0.40 to 1.52 ± 0.47 μm. The release studies in pH 7.4 buffer revealed the dissolution of CPT to be faster from the microsphere formulations than from physical mixtures and free CPT. The times to release 50% CPT (T-50%) from microspheres prepared using OC-7, OC-13, and OC-20 were about 31, 37, and 19 h, respectively. The in vitro cytotoxicity results indicated OC-20/CPT microspheres to be more effective than free CPT against human-derived RPMI-8402 lymphoid and THP-1 myeloid leukemia cell lines. The ED50 values for the OC-20/CPT microspheres and free CPT were 1 × 10−5 and 0.25 × 10−1 μg/mL, respectively, against the RPMI-8402 line and 0.5 × 10−2 and 0.75 μg/mL, respectively, against the THP-1 line. The higher activity of OC-20/CPT microspheres compared to that of the free drug is attributed to increased dissolution of CPT from microspheres.

An in situ infrared spectroscopic investigation of adsorption of sodium dodecylsulfate and of cetyltrimethylammonium bromide surfactants to TiO 2, ZrO 2, Al 2O 3, and Ta 2O 5 particle films from aqueous solutions

An in situ infrared spectroscopic study is described of the adsorption of the ionic surfactants, sodium dodecylsulfate (SDS) and cetyltrimethylammonium bromide (CTAB), from aqueous solution to metal oxide particle films. Thin porous metal oxide particle films were prepared by drying of aqueous colloidal dispersions on single internal reflection ZnSe prisms. Analysis of the spectra of SDS in both aqueous solution and adsorbed to positively charged TiO 2, ZrO 2, Al 2O 3 and Ta 2O 5 surfaces indicates that adsorption occurs primarily via electrostatic and hydrogen bonding interactions between the sulfate headgroup and the metal oxide surfaces. The adsorption of CTAB to negatively charged TiO 2 and Ta 2O 5 was observed. The pH dependence of CTAB adsorption to TiO 2 indicated that both electrostatic and hydrophobic interactions are important. Langmuir adsorption constants from isotherm data were similar for adsorption of CTAB on TiO 2 at pH=6.3 and at pH=11.0 indicating that charge effects are not dominant in this pH range. Use of a single internal reflection prism allowed the displacement of interfacial water by surfactant adsorption to be clearly discerned in the spectra.

Colloidal dispersion of magnetite nanoparticles via in situ preparation with sodium polyoxyalkylene di-phosphonates

Hydrosoluble polymers attached to inorganic surfaces allow production of dispersed state particles. Functionalized end-chain polymers were used to prepare stable aqueous magnetite colloidal dispersions. The magnetite nanoparticles were synthesized from coprecipitation of aqueous Fe 3+ and Fe 2+ ions in the presence of sodium polyoxyalkylene di-phosphonates. Transmission electron microscopy (TEM), dynamic light scattering (DLS), pyroanalysis techniques and isotherm determinations are reported to characterize the nanosized particles. Because of the structure of the interface with the polymer tightly anchored to the surface and the biocompatibility of the polyoxyalkylene chain, new developments might be found in the forthcoming time.

Formation of Aqueous Colloidal Dispersions of Exfoliated γ-Zirconium Phosphate by Intercalation of Short Alkylamines

A good exfoliation of layered γ-zirconium phosphate in aqueous medium at room temperature has been obtained by intercalation of short alkylamines such as methyl-, dimethyl- and ethyl-amine. No appreciable exfoliation was observed when butylamine or longer alkylamines were intercalated, while intermediate behavior was obtained with the intercalation of propylamine. Since a good exfoliation of this layered compound is an essential step for its functionalization by topochemical reactions with phosphonic or phosphinic acids, the amine intercalation compounds and the various steps of the intercalation processes were investigated in order to obtain information on the mechanism of the exfoliation and a model of this mechanism is proposed. The use of the colloidal dispersion of exfoliated lamellae for the preparation of auto-consistent and flexible membranes as well as thin supported films of γ-zirconium phosphate is described.

Polyampholyte gelatin adsorption to colloidal latex: pH and electrolyte effects on acrylic and polystyrene latices.

Polyampholytic gelatin adsorption onto two model, aqueous colloidal dispersions of like size and surface charge density, but varying hydrophobicity are reported. Adsorption isotherms and hydrodynamic layer thickness of the adsorbed gelatin layer are reported for variations of solution pH (above the gelatin IEP) and electrolyte concentration. A van't Hoff analysis of the temperature dependence of the dilute-limiting adsorption equilibrium constants provides evidence for adsorption driven by electrostatic polarization to like-charged surfaces, as well as additional hydrophobic interactions with polystyrene colloidal surfaces. It is demonstrated how these measurements can be used to predict the perikinetic stability of gelatin-coated colloidal latices and, hence, are useful for the rational formulation of dispersions containing polyampholytes.

Effect of Surfactant and Polymer Adsorption on the Viscosity of Aqueous Colloidal Silica Dispersions under Extreme Conditions

The effect of surfactant and polymeric additives on the viscosity behavior and stability of aqueous colloidal dispersions of silica particles under extreme conditions (low pH, high ionic strength) has been investigated. The surfactant and polymer used as dispersing agent were C12TAB, a cationic surfactant, and DarvanC, a commercially available polymer. It was found that the surfactant stabilized dispersions show a lower viscosity and a more uniform resistance to flow than the samples stabilized through electrostatic repulsion or polymer induced forces in the system. Stability analysis through turbidity measurements indicated that the state of the dispersion changes from an unstable regime to a stable regime above a critical concentration of C12TAB in the system. Viscosity measurements as a function of temperature indicates that C12TAB dispersing agent can further improve the flowability of the dispersion at higher temperatures.

Emulsion procedures for thermally reversible covalent crosslinking of polymers

Quaternization and dequaternization of tertiary amine compounds were employed to obtain thermally reversible ionene networks from aqueous colloidal polymer dispersions prepared via emulsion polymerization. Chlorine-functionalized polymers prepared via the emulsion copolymerization of styrene (St), butylacrylate (BA), or both with chloromethylstyrene, and amino-functionalized polymers prepared via the emulsion copolymerization of St, BA, or both with 2-(dimethylamino)ethylacrylate or 4-vinylpyridine, were reacted without polymer separation, with a ditertiaryamine crosslinker and a dihalide crosslinker, respectively, to obtain crosslinked polymers. Crosslinked polymers were also obtained via the reaction of a chlorine-functionalized polymer dispersion with an amino-functionalized polymer dispersion or via the drying of the polymer blend prepared from the two kinds of dispersions. Reactive solubility experiments, flowability investigations (by thermocompression at ca. 215 °C), IR, and 'H NMR analyses of the obtained crosslinked polymers indicated that the generated ionene bridges dequaternized on heating and requaternized on cooling. In comparison with solution crosslinking, no organic solvent was employed, and simple procedures were required for the preparation of the thermally reversible covalent crosslinked polymers.

Macroporous Bulk Gels and Thin Films by Colloidal Templating of Emulsions and Latex Particles

ABSTRACTWe present a new way of making macroporous materials using colloidal suspensions of ceramic nanoparticles. We start from a mondisperse emulsion of 0.5 to 3.0 μm diameter oil droplets suspended in an aqueous colloidal dispersion of 80–100 nm silica particles. Upon evaporation of the water, the silica particles form a particulate gel around the emulsion droplets. Removal of the emulsion droplets by dissolution in alcohol and subsequent drying produces a monolithic solid macroporous body. Bulk gels with volume greater than 1 cm3 could be easily produced with this method. Uniform films with thickness of approximately 10 μm have also been made by dip coating of the emulsion-silica colloid mixture. One can also use polystyrene particles as templates in place of the emulsion droplets. Using this approach, we were able to produce highly ordered macroporous titania and silica ceramics with pore sizes around 500 nm and typical dimensions of 3.0 × 2.0 × 0.5 mm3. These dimensions are much larger than those produced using alkoxides. Lower shrinkage associated with particulate gels reduces cracking and increases the structural integrity of the macroporous body.

The rheology of aqueous alumina suspensions in the presence of hydroxyethylcellulose as binder

The rheological behaviour of aqueous colloidal alumina dispersions stabilised by the dispersant Darvan C, an ammonium salt of polymethacrylic acid, was investigated in presence of hydroxyethyl cellulose (HEC) at room temperature. Measurements were taken under both oscillatory and steady shear. It was found that the polymer induces flocculation of the particles probably due to a depletion mechanism. The flocculation process results in the creation of a particle network, which restricts particle movements and gives rise to a wide distribution of relaxation times. At high HEC concentrations, two sets of relaxation times can be distinguished, one at low and the other at high frequencies. At high solids content and high HEC concentrations, the response of the system becomes predominantly elastic at low frequencies. The behaviour is similar to an entangled network of high molecular weight uncross-linked polymer solution. Thus, HEC causes the formation of ‘topological restraints' comprised of chains of particles that give rise to a wider distribution of relaxation times with significantly lower terminal relaxation times. In addition, similarly to a polymer solution, complete superposition occurs of the apparent and dynamic viscosity in accordance to the Cox–Merz rule. Taking into account the combined data, a Cross type of formulation was found to fit the behaviour of all dispersions that can be represented in terms of a single dimensionless parameter. The behaviour of the adjustable parameters of this empirical model suggests a diffusion controlled cluster–cluster aggregation process that induces a more closed packed particle arrangement at high shear rates. ©

Formation of Thin Transparent Conductive Composite Films from Aqueous Colloidal Dispersions

This investigation examines the percolation and electrical resistivity behavior of films containing electrically conductive antimony-doped tin oxide particles in different polymer matrices. The polymers include gelatin, a soft polymer latex (glass transition temperature Tg room temperature). The films are formed through the evaporation of water from aqueous dispersions of conductive particles and film-forming polymers. The critical volume fraction of conductive particles for conductive network formation was found to be significantly lower when the film former was a polymer latex. This was attributed to differences in the interactions of particles in the colloidal state prior to the point where particles start to touch. In the critical transition region or slightly above the critical transition region, the hard latex provides an order of magnitude improved surface resistivity compared to that of the soft latex. This result was discussed in terms of the po...

PH Effect on an Ionic Ferrofluid: Evidence of a Thixotropic Magnetic Phase

We investigate the stability of aqueous colloidal dispersions of ionic magnetic nanoparticules as a function of pH. The charge of the γ-Fe2O3 particles is modified, and as a consequence, the interparticle interactions may be tuned through pH variations. Scanning the whole pH scale from acidic to alkaline medium, different states are observed for the dispersion: sol, thixotropic gel, and floc (around the point of zero charge). Using the dynamic magnetooptical properties of the magnetic grains, we locally probe the sols and the thixotropic gels. A steep increase of the characteristic time of birefringence relaxation, in the sol phase close to pHGel, marks a divergence of the macroscopic viscosity at the gel point, compatible with a percolation theory. A temporal study of the regeneration of shaken gels reveals that, in these thixotropic systems, the process is dominated by a cluster aggregation on a sample-spanning one, with an energy barrier to overcome, compatible with chemical measurements.

Methane production and carbon mineralisation of size and density fractions of peat soils

For the purpose of characterisation of soil organic matter breakdown, soil organic matter is often divided into different fractions, each with its own decomposition rate. Thus far, no attempts had been made to quantify the methane production capacity of individual soil fractions. We aimed to improve our understanding of CH 4 emissions from peat soils by studying the CH 4 production capacity of individual soil fractions in parallel with their carbon mineralisation capacity. Samples from two wet grasslands on peat soil (0–60 cm) were fractionated into different size and density fractions (>2.0 mm; 0.25–2.0 mm, light; 0.25–2.0 mm, heavy; 150–250 μm, light; 150–250 μm, heavy; <150 μm) using sieves and Ludox, an aqueous colloidal dispersion of silica particles. The individual fractions were rather similar with respect to C mineralisation capacity and C-to-N ratio, but not with respect to CH 4 production capacity. C mineralisation capacity ranged from 10 to 100 μmol CO 2-C g −1 C d −1 and decreased with depth. CH 4 production capacity ranged from 0 to 150 nmol CH 4-C g −1 C d −1. We found that significant CH 4 production only occurred for fractions with a large particle size; the fraction >2.0 mm contributed 90% to total CH 4 production capacity. Furthermore, CH 4 production capacity strongly decreased with depth; the layer 0–5 cm contributed 70% to total CH 4 production capacity. This indicates that in the wet peat soils recent plant residues are a major substrate for methanogens.

Depletion flocculation of particulate systems induced by hydroxyethylcellulose

The flocculation behaviour of three different aqueous colloidal dispersion systems is examined in the presence of soluble hydroxyethyl cellulose (HEC) molecules by means of rheology. One alumina and two polystyrene dispersions were used for comparison with particle solid contents varying from 20 to 50% (v/v). All dispersions were sterically stabilised by either chemical or physical adsorption of an appropriate polymeric dispersant. Addition of HEC (MW range 170 000–1 000 000) followed stabilisation within a concentration range of 0.2–1% (w/solvent volume). Both continuous and oscillatory shear measurements were carried out and the yield value and elastic modulus have been chosen as providing a good indication of the degree of aggregation. The variation of the above parameters is examined with respect to HEC and solid contents as well as to the polymer molecular weight and the flocculation mechanism and aggregation kinetics are discussed. It is suggested that particles aggregate as a result of a depletion flocculation mechanism. According to this mechanism, above a critical concentration (critical flocculation point), polymer is depleted from the interparticle space after a particle collision which gives rise to an osmotic pressure gradient with respect to the bulk of the solution and flocculation starts taking place. All systems seem to exhibit characteristics of a fractal based structure above a certain solid volume fraction The differences found in the aggregation kinetics of the ceramic dispersion compared to the two polystyrene dispersions were attributed to the significant degree of pre-shearing of the ceramic dispersion. In addition, some form of interaction between the dispersant and the polymer was suggested in the case of chemically grafted polystyrene dispersions. Generally, the aggregation process can be explained with existing depletion flocculation theories.

Physical and Chemical Properties of Magnetite and Magnetite-Polymer Nanoparticles and Their Colloidal Dispersions

The properties of polymer-coated magnetite nanoparticles, which have the potential to be used as effective magnetic resonance contrast agents, have been studied. The magnetite particles were synthesized by using continuous synthesis in an aqueous solution. The polymer-coated magnetite nanoparticles were synthesized by seed precipitation polymerization of methacrylic acid and hydroxyethyl methacrylate in the presence of the magnetite nanoparticles. The particle size was measured by laser light scattering. It was shown that the particle size, variance, magnetic properties, and stability of aqueous magnetite colloidal dispersion strictly depend on the nature of the stabilizing agent. The average hydrodynamic radius of the magnetite particles was found to be 5.7 nm in the stable aqueous colloidal dispersion. An inclusion of the magnetite particle into a hydrophilic polymeric shell increases the stability of the dispersion and decreases the influence of the stabilizing agent on the magnetic and structural properties of the magnetite particles as was shown by X-ray diffraction and Mössbauer and IR spectroscopy, as well as by vibrating sample magnetometry. The variation in the polymeric shell size and the polymer net density can be useful tools for evaluation of the polymer-coated magnetite particles as effective contrast agents.