pH- and ionic strength-controlled cation permselectivity in amine-modified nanoporous opal films

The influence of pH and ionic strength on permselective transport in nanoporous opal films prepared from 440 nm silica spheres was investigated by cyclic voltammetry in aqueous and acetonitrile solutions. Three-layer opal films were deposited from a 1.5 wt % colloidal solution of silica spheres onto 25-microm-diameter Pt microdisk electrodes shrouded in glass. The films were chemically modified by immersing them in a dry acetonitrile solution of 3-aminopropyl triethoxysilane. When the surface amino groups of the modified opal films are protonated and there is little or no supporting electrolyte present in solution, the flux of cationic redox species through the opal membrane is blocked because of electrostatic repulsion. The permselectivity is pH-dependent and can be modulated by adjusting the Debye screening length within the nanopores of the opal by changing the ionic strength of the contacting solution.     

pH and ionic strength responsive photonic polymers fabricated by using colloidal crystal templating

In this paper, monodispersed silica particles were synthesized using tetraethoxysiliane hydrolyzing in ethanol by a Stöber–Fink–Bobn method and then self-assembled on cleaning glass slides to form silica colloidal crystals. After photopolymerization of methacrylic acid mixing with ethylene glycol dimethylacrylate and hydrofluoric acid etching, the pH-responsive polymers were obtained with highly 3D-ordered macroporous structures templated by silica colloidal crystals. These polymers films can swell or deswell in response to external stimuli, causing a change of Bragg diffraction to read pH or ionic strength of various solutions by optical signals or electrochemical signals. As an application, they can be used as chemical sensors to detect pH or ionic strength variation of environment.     

Donnan effects in the steady-state diffusion of metal ions through charged thin films

The steady-state diffusion of metals ions through thin films with fixed charged groups was investigated using diffusive gradients in thin films (DGT) measurements. Copolymers of acrylamide and sodium acrylate cross-linked with N,N'-methylenebisacrylamide were used as diffusive gels. The rate of diffusion of cadmium ions through the gels was measured by determining the mass of cadmium bound to a backing chelex resin after a known deployment time. Variation of the ionic strength as well as the fixed charge density and the thickness of the gel layer allowed evaluation of the impact of the Donnan partitioning and the diffusion layer in solution on the observed steady-state flux of ions through the layer. The results underscore that, as the Donnan partitioning increases, the impact of the diffusion layer in solution becomes more significant. At modest Donnan potentials, Donnan partitioning controls the net flux of metal ions, whereas at conditions of increasing Donnan potential, i.e., at decreasing ionic strength, the flux is increasingly limited by diffusion in solution. An analytical expression is developed to describe the influence of Donnan partitioning on the observed steady-state flux of metal ions.     

On the behavior of the electrostatic colloidal interaction in the membrane filtration of latex suspensions

In order to investigate effects of the colloidal interaction in the membrane filtrations, the dead-end ultrafiltration of latex colloids was conducted with fully retentive membranes. Experimental results concerning the permeate flux during the filtration indicate that the void fraction of cake layer increased with the decrease of the ionic strength, due to the expanded Debye double layer thickness around the particles. The concentration dependence of the gradient diffusion coefficient of colloidal particles has been examined as a function of solution ionic strength. The NVT Monte Carlo simulation was applied on the bulk suspension so as to determine the thermodynamic coefficient, and the hydrodynamic coefficient was evaluated from the previously developed relation for an ordered system. The long-range electrostatic interactions between the particles are determined by using a singularity method, which provides accurate solutions to the linearized electrostatic field. The predictions on the variation of concentration polarization layer have been presented, from which we found that both the permeate flux and the particle diffusion are related to determine the concentration distribution above the cake layer.     

Interaction and structure of surfaces coated by poly(vinyl amines) of different line charge densities

Interactions between preadsorbed films of poly(vinyl amine) (PVA) of two different line charge densities on silica substrates were studied with the colloidal probe technique based on the atomic force microscope (AFM). The preadsorbed films were prepared by adsorption of PVA from a pH 4 solution without any added salt. The highly charged PVA adsorbs in a flat configuration and in laterally heterogeneous layers, while the more weakly charged PVA analog adsorbs in thicker and more homogeneous films. As revealed by reflectivity measurements, such preadsorbed PVA films are stable in polyelectrolyte-free solutions. However, force measurements with the colloidal probe reveal that their interactions depend strongly on the ionic strength. Upon approach, interactions are dominated by electrostatic diffuse layer overlap forces. Both PVA films have very similar diffuse layer charge densities of about 1.5 mC/m2. Since these values are substantially lower than what would be expected from the total charge of the adsorbed polyelectrolytes measured by reflectivity, we infer that coadsorption of anions represents the principal mechanism in charge neutralization. Upon retraction, the adhesion between the films is dominated by bridging forces due to single polymer chains. Such bridging adhesion becomes progressively important with increasing ionic strength, whereby their range and frequency increase. The work of adhesion due to bridging is about 0.3 mN/m. At low ionic strengths, the films behave differently. While the highly charged PVA shows unspecific adhesion at small distances, the more weakly charged PVA analog shows few adhesion events occurring at long distances.     

Modification of Surface Forces by Metal Ion Adsorption

Abstract

Sorption of ions may lead to variations in interparticle forces and, thus, changes in the stability of colloidal particles. Chemical interactions between metal ions and colloidal particles modify the molecular structure of the surface, the surface charge, and the electrical potential between colloidal particles. These modifications to the surface and to the electrical double layer due to metal ion sorption are reflected in the interaction force between a particle and another surface, which is measured in this study by atomic force microscopy (AFM). Specifically, AFM is used to investigate the sorption of copper ions from aqueous solutions by silica particles. The influence of metal ion concentration and solution ionic strength on surface forces is studied under transient conditions. Results show that as the metal ion concentration is decreased, charge reversal occurs and a longer period of time is required for the system to reach equilibrium. The ionic strength has no significant effect on sorption kinetics. Furthermore, neither metal concentration nor ionic strength exhibits any effect on sorption equilibria, indicating that for the experimental conditions used in this study, the surface sites of the silica particle are fully occupied by copper ions.     

Influences of colloidal stability and electrokinetic property on electrodialysis performance in the presence of silica sol

Negatively charged silica sol is known to lead to fouling of anion exchange membranes during electrodialysis (ED) as a result of its deposition on the membrane surface. It is known that the fouling potential is related to the physical and electrochemical properties of the silica particles as well as those of the anion exchange membranes. In this study, the properties of the silica sol were characterized in terms of its particle size, turbidity, and zeta potential in order to predict their effects on the electrodialysis performance. In the stability of colloidal particles, the critical coagulation concentrations of silica sol were determined as functions of ionic strength, cation species, and solution pH. In the electrodialysis of NaCl solution containing silica sol with various concentrations of CaCl(2), the colloidal behavior related to deposition and transport was examined during and after electrodialysis. The electrodialysis experiments clearly showed that the deposition and transport of silica sol during electrodialysis were related to the colloidal stability of dispersion.     

Effect of introduced charge in cellulose gels on surface interactions and the adsorption of highly charged cationic polyelectrolytes

The interaction between cellulose surfaces in aqueous solution has been measured using colloidal probe microscopy. Cellulose thin films with varying charge through carboxyl group substitution were used in this study with the surface forces fit to DLVO theory. It was found that the surface potential increased, as expected, with increasing carboxyl substitution. Furthermore, for a given degree of substitution, the surface potential increased as a function of increasing pH. At low pH, the surface forces interaction were attractive and could be fit to the non-retarded Hamaker equation using a constant of 3 x 10(-21) J. At pH greater than 5, the force interactions were monotonically repulsive, regardless of the ionic strength of the solution for all charge densities of the cellulose thin films. The adsorption of polyDADMAC to these charged cellulose films was also investigated using the quartz crystal microbalance. It was found that for the low charge film, a low surface excess of PDADMAC was sensed and that the adsorbed conformation was essentially flat. However for the higher charged cellulose film, a spontaneous de-swelling was observed resulting in no possibility of quantitatively determining the sensed mass using QCM.     

Measurement of interaction forces between lignin and cellulose as a function of aqueous electrolyte solution conditions

The interaction between a lignin film and a cellulose sphere has been measured using the colloidal probe force technique as a function of aqueous electrolyte solution conditions. The lignin film was first studied for its roughness and stability using atomic force microscopy imaging and quartz crystal microbalance measurements, respectively. The film was found to be smooth and stable in the pH range of 3.5-9 and in ionic strengths up to and including 0.01 M. This range of ionic strength and pH was hence used to measure the surface force profiles between lignin and cellulose. Under these solution conditions, the measured forces behaved according to DLVO theory. The force-distance curves could be fitted between the limits of constant charge and constant potential, and the surface potential of the lignin films was determined as a function of pH. At a pH greater than 9.5, a short range steric repulsion was observed, indicating that the film was swelling to a large extent but did not dissolve. Thus, lignin films prepared in this manner are suitable for a range of surface force studies.     

Controlled association in suspensions of charged nanoparticles with a weak polyelectrolyte

The properties of high-pH suspensions of mixtures of silica with low-molecular-weight samples of the water-soluble polymer polyethylenimine (PEI) have been studied. At pH > 10 and low ionic strength, silica nanoparticles are stabilized by a negative surface charge, and PEI has only a very low positive charge. The adsorption of PEI induces a localized positive charge on the segments of polymer closest to the silica surface. The parts of the molecule furthest away from the surface have little charge because of the high pH of the medium. The polymer-covered particle remains negatively charged, imparting some electrostatic stabilization. Suspensions of silica and low-molecular-weight PEI are low-viscosity fluids immediately after mixing, but aggregation occurs leading to the eventual gelation (or sedimentation at lower concentrations) of these mixtures, indicating colloidal instability. The gelation time passes through a minimum with increasing surface coverage. The rate of gelation increases exponentially with molecular weight: for molecular weight > or = 10,000 Da PEI, the instability is so severe that uniform suspensions cannot be produced using simple mixing techniques. The gelation rates increase rapidly with temperature, ionic strength, and reduction in pH. The rate of gelation increases with increasing particle concentration at low surface coverage but decreases at high coverage as a consequence of a small increase in pH. Gels are broken by application of high shear into aggregates that re-gel more rapidly than the original discrete coated particles.     

Mechanical characterization of Ormosil films on plastics: Young's modulus determination by three points bending

A protective layer has been deposited to improve the scratching properties of polycarbonate organic glasses. The starting solution consists of a mixture of organosilicon compounds, silicon alkoxides or silica colloidal solutions. The deposition is carried out using the dip coating technique. The thickness is about 2–5 µm for the film and 1 mm for the substrate.Young's moduli of the films are obtained by a new three points bending apparatus. Young's modulus of coating depends both on silica content and on the nature of the silica used as a filler.     

Electrochemical modeling of the silica nanoparticle-biomembrane interaction

The interaction of amorphous colloidal silica (SiO(2)) nanoparticles of well-defined sizes with a dioleoyl phosphatidylcholine (DOPC) monolayer on a mercury (Hg) film electrode has been investigated. It was shown using electrochemical methods and microcalorimetry that particles interact with the monolayer, and the electrochemical data shows that the extent of interaction is inversely proportional to the particle size. Scanning electron microscopy (SEM) images of the electrode-supported monolayers following exposure to the particles shows that the nanoparticles bind to the DOPC monolayer irrespective of their size, forming a particle monolayer on the DOPC surface. A one-parameter model was developed to describe the electrochemical results where the fitted parameter is an interfacial layer thickness (3.2 nm). The model is based on the adsorptive interactions operating within this interfacial layer that are independent of the solution pH and solution ionic strength. The evidence implies that the most significant forces determining the interactions are van der Waals in character.     

Coagulation kinetics of amorphous colloidal silica suspensions with and without hydroxypropyl cellulose polymer

The coagulation rate constant of submicron silica has been measured as a function of solution pH, salt concentration and hydroxypropyl cellulose (HPC) polymer concentration. Results show that the colloidal stability of silica is dominated by the cation concentration in the presence of salt in the pH range 3–9.5. The stability increases as cation concentration decreases. At low salt concentration and a minimum colloid stability was found in the intermediate pH range 4–8. These results show that differences in the literature values of the critical coagulation constant by relative light-scattering experiments can be explained by the use of the coagulation rate constant analysis. When HPC polymer was present in the solution, the colloid stability of the silica increased. The adsorption of polymer stabilizes the silica suspensions, both at low pH near the isoelectric point and at high ionic strength where it coagulates without the polymer. A monolayer coverage was necessary to provide steric stabilization. At 10^–3 M KCl a smaller equilibrium concentration of HPC in solution is needed to give monolayer coverage and steric stabilization than at 1 M KCl and pH 4.2.     

Polycondensation kinetics of orthosilicic acid in a hydrothermal solution

The polycondensation kinetics of orthosilicic acid in hydrothermal solutions was studied experimentally at different temperatures, pH, and ionic strengths. As a result of the mathematical processing of experimental data, equations approximating the time dependence of the polycondensation process and the dependence of the polycondensation rate constant on the solution ionic strength were obtained. Experimental data on the kinetics of the reverse process of dissolving colloidal silica were obtained at different temperatures and pH.     

Structural evolution of colloidal crystals with increasing ionic strength

We have directly observed the structural evolution of colloidal crystals as a function of increasing ionic strength using confocal scanning laser microscopy. Silica colloids were sedimented onto a glass substrate in deionized water to create large, single domain crystals. The solution ionic strength was then increased by one of three methods of controlled electrolyte addition: (1) direct injection of electrolyte solutions, (2) single step diffusion of electrolyte solutions through a dialysis membrane, and (3) multiple step diffusion of electrolyte solutions of increasing ionic strength through a dialysis membrane. During direct injection of electrolyte solutions, initially large, single domain colloidal crystals were shear melted and then evolved into polycrystalline structures at low ionic strengths and gels at higher ionic strengths. Diffusion of electrolyte solutions though dialysis membranes in a single step produced gradient-driven transport that also melted initial single domain crystals to yield polycrystalline and gel structures similar to the injection approach. Interestingly, the multistep diffusion of several electrolyte solutions through dialysis membranes facilitated retention of large, single domain crystals even as particles came into adhesive contact. This was achieved by reducing the contraction rate of the crystalline lattice to allow sufficient time for diffusion-limited configurational rearrangements to occur within the evolving structure. These mechanically robust, single domain colloidal crystals may find important applications as templates for photonic materials and sensors.     

Electroosmotically enhanced mass transfer through polyacrylamide gels

We present an internal pumping strategy to enhance solute fluxes in polymer gels. The method is based on electroosmotic flow driven by an electric field applied across a gel that has been doped with charged colloidal inclusions. This work is motivated by the need to enhance the transport in gel-based biosensor devices whose response dynamics are often mass transfer limited. In this case, polyacrylamide gel slabs were doped with immobilized, charged silica colloids, and the flux of a fluorescent tracer was measured as a function of applied field strength, the volume fraction and size of the colloidal silica inclusions, and the bulk electrolyte composition. Significant flux enhancements were achieved with applied electric currents on the order of a few mA. Control experiments indicated that the flux enhancement was not due to any distortion of the gel diffusional properties in response to the presence of the inclusions. At a constant inclusion volume fraction, the electroosmotic solute flux enhancement was strongest for the smallest particle sizes that provide the highest total surface area, consistent with the electroosmotic mechanism whereby fluid flow is generated along the solid/liquid interface.     

Coating of Methyltriethoxysilane—Modified Colloidal Silica on Polymer Substrates for Abrasion Resistance

Organic-inorganic hybrid composites were prepared by the sol-gel method for the hard coating agent of transparent plastic, and their abrasion resistance, optical and surface characteristics were evaluated. Methyltriethoxy-silane (MTES) and colloidal silica were used as starting materials. The addition of MTES to colloidal silica enabled the formation of dense thin films with very smooth surface on the substrates. The thin films were strongly adhered to the substrates without primer treatment. The abrasion resistance increased with the increase in the ratio of MTES to the colloidal silica. Optimal amount of MTES for the hard coating agent was 100 wt% to the colloidal silica. The addition of curing catalyst, tetramethylammonium formate was found to be very effective to enhance the adhesion strength of coating agent to the substrates and reduced curing time.     

Sorption of iron(III) in ionic and colloidal states on iron(III) oxide deposited on a silica gel sufrace

The sorption of iron(III) in ionic and colloidal states on iron(III) oxide deposited on a silica gel surface has been studied as a function of pH of aqueous solution by batch equilibrations. The behaviour of Fe³⁺ and colloidal Fe(III) on the sorbent column has also been investigated. Conditions for sorption of iron from aqueous solutions are given. The colloidal iron(III) can be quantitatively separated from Fe³⁺ on the sorbent column under given experimental conditions.     

From colloidal stability in ionic liquids to advanced soft materials using unique media

Owing to their fascinating properties, ionic liquids (ILs) are now receiving a great deal of attention as alternatives to organic solvents and electrolyte solutions and as synthetic and dispersion media for colloidal systems. Colloidal stability is an essential factor in determining the properties and performance of colloidal systems combined with ILs. The remarkable properties of ILs primarily originate from their highly ionic nature. Although such high ionic strength often causes colloidal aggregation in aqueous and organic suspensions, some colloidal particles can be well suspended in ILs without any stabilizers. In the first part of this article, we focus on recent experiments conducted to investigate the colloidal stability of bare and polymer-grafted silica nanoparticles and on the surface force between silica substrates and ILs. Three different repulsions between colloidal particles (i.e., electrostatic, steric, and solvation forces) are also highlighted, after which a possible interpretation of the results in terms of the stabilization mechanism in ILs both in the presence and in the absence of stabilizers is proposed. The latter part of this article provides an overview of our recent studies on colloidal soft materials with ILs. On the basis of the dispersed states of the silica colloids in ILs, two different soft materials, a colloidal gel and a colloidal glass in ILs, were fabricated. The relationship between their functional properties, such as ionic transport, rheological properties, and optical properties, and the microstructure of the colloidal materials is also described.     

Evaporation‐Driven Self‐Assembly of Colloidal Silica Dispersion: New Insights on Janus Particles

The evaporation driven self‐assembly of novel colloidal silica Janus particles was evaluated by scanning electron microscopy in comparison to unfunctionalized silica particles. The cyclodextrin‐ and azobenzene‐modified compound was obtained utilizing Pickering emulsion approach, in which the particles were immobilized on solidified wax droplets and subsequently functionalized. Silica particles were modified with 3‐aminopropyl trimethoxysilane and afterward reacted with tosyl‐β‐CD or phenylazo(benzoic acid), respectively. Mesoscopic structures of the colloidal dispersions, as dried films from aqueous solution, have been investigated by scanning electron microscopy and dynamic light scattering. Interestingly, it has been observed that the Janus particles show a significantly different evaporation‐induced assembly than the unmodified particles.