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.     

Gravitational compression dynamics of charged colloidal crystals

We examine the compression of charged colloidal crystals under the influence of gravitational force by monitoring the spatiotemporal variations of Bragg diffraction from the crystal lattice. We use the dilute aqueous dispersions of colloidal silica particles (diameter=216 nm, charge number=733, a particle volume fraction φ=0.06) in the presence of 5-15 μM sodium chloride. The sedimentation profiles of the colloidal crystals along the crystal height are determined by in situ fiber optics reflection spectroscopy. The time evolutions of the sedimentation profiles are calculated by numerical simulations based on a phenomenological continuum model that explicitly incorporates the electrostatic interparticle interactions. The simulation results correctly describe the experiments at sufficiently high ionic strength.     

Stepwise controlled immobilization of colloidal crystals formed by polymer-grafted silica particles

Colloidal crystals formed by polymer-grafted silica particles were immobilized by a stepwise procedure consisting of gelation by radical copolymerization followed by solidification by ring-opening radical polymerization. In the first step, the poly(methyl methacrylate) (PMMA)-grafted silica colloidal crystal suspension was incorporated into the gel without altering the crystal structure by copolymerization of cross-linker, 1,2-dimethylacryloyloxyethane (DME) and methyl methacrylate (MMA). In the second step, ring-opening radical polymerization was performed after substituting the solvent with vinylidene-1,3-dioxolane. By this two-step procedure, the silica particle array of colloidal crystals was immobilized and made into durable material.     

Colloidal crystallization of colloidal silica modified with ferrocenyl group-contained polymers in organic solvents

Surface modification of colloidal silica with ferrocenyl-grafted polymer and colloidal crystallization of the particles in organic solvent were studied. Poly(methyl methacrylate-co-vinylferrocene)-grafted silica never formed colloidal crystals in polar solvent, such as acetone, acetonitrile, ethanol and N,N-dimethylformamide (DMF), while poly(methyl methacrylate-co-ferrocenyl acrylate)-grafted silica gave colloidal crystallization in DMF. The particles prepared by grafting of poly(N,N-dimethylacrylamide-co-vinylferrocene), with vinylferrocene (Vfc) mole fraction of 1/13 and 1/23, were observed to give the crystallization in ethanol and DMF over particle volume fraction of 0.058. Further, silica modified with copolymer of Vfc and N-vinyl-2-pyrrolidone, N-vinylcarbazole or N-isopropylacrylamide formed colloidal crystals in ethanol and DMF. Especially, poly(N-isopropylacrylamide-co-Vfc)-grafted silica, which was composed of the highest mole fraction of vinylferrocene, 1/3, afforded colloidal crystallization in ethanol over particle volume fraction of 0.053. Relatively high polar vinylferrocene copolymer grafting of silica resulted in colloidal polymerization in organic solvents.     

Effects of particle volume fraction on distortion of particle-arrayed structure during immobilization of colloidal crystals formed by poly(methyl methacrylate)-grafted silica in acetonitrile

Changes of particle array structure with particle volume fraction during immobilization of colloidal crystals, formed by poly(methyl methacrylate)-grafted silica in acetonitrile, were investigated. Immobilization of colloidal crystals formed in acetonitrile was carried out by two-step photo-radical copolymerization of methyl methacrylate and ethylene dimethacrylate to make organogel, followed by solidification after exchanging the solvent with methyl methacrylate. Crystallite size in colloidal crystals formed in acetonitrile was mostly unchanged with particle volume fraction in the range of 0.11–0.18, while the size and number of single crystals decreased during gelation. Disordering in particle array in immobilized colloidal crystals in gel and poly(methyl methacrylate) matrix was observed to decrease with increasing particle volume fraction less than 0.18 due to strong electrostatic repulsion between particles.     

Migration and precipitation of soluble species during drying of colloidal films

The evaporation-induced convection resulted in a transport of dissolved species, a water-soluble polymer (carboxymethylcellulose) and dissolved CaCO(3), to the drying front of silica and CaCO(3) dispersions where the material eventually precipitates. Scanning electron microscopy and chemical analysis showed that the concentration of carboxymethylcellulose, CMC, is highest in the centre of the dried silica film and decreases towards the perifery. The colloidal films of the monodisperse silica particles displayed a high degree of structural order even at high concentrations of the non-adsorbed polymer CMC, which suggests that any depletion induced interparticle attraction is insufficient to affect the assembly of the colloidal crystal. The CaCO(3) particles are slightly soluble and we found that rod-like crystals reprecipitated in the centre of the particle films on top of the polyacrylate-coated particles. Addition of CMC disturbs the formation of distinct crystal shapes which was attributed to a complexation of Ca(2+) in solution.     

Electrophoretic mobility of a cylindrical colloidal particle in a salt-free medium

Approximate expressions are derived for the electrophoretic mobility of dilute cylindrical colloidal particles in a salt-free medium containing only counterions. The cylinder is assumed to be infinitely long. It is shown that as in the case of a spherical particle, there is a certain critical value of the particle surface charge separating two cases. When the particle surface charge is lower than the critical value (case 1), the electrophoretic mobility increases with increasing particle surface charge per unit length. When the particle surface charge is higher than the critical value (case 2), the mobility becomes constant (for a cylinder in a transverse field) or the increase in the electrophoretic mobility with the particle surface charge becomes suppressed (for a cylinder in a tangential field). These phenomena are caused by the effect of counterion condensation in the vicinity of the particle surface. The critical value of the particle charge is essentially independent of the particle volume fraction phi for the dilute case, unlike the case of a sphere, in which case the critical charge value is proportional to ln(1/phi).     

Effect of anionic surfactants on synthesis and self-assembly of silica colloidal nanoparticles

Effects of the anionic surfactants, sodium dodecyl sulfate and sodium oleate, on the formation and properties of silica colloidal nanoparticles were investigated. At a concentration of approximately 1 x 10(-3) M, adsorption of anionic surfactants increased particle size, monodispersity, and negative surface charge density of synthesized silica particles. As uniformity of particle size and particle-particle interactions increase, colloidal photonic crystals readily self-assemble without extensive washing of the synthesized silica nanoparticles. The photonic crystals diffract light in the visible region according to Bragg's law. The assembled colloidal particle arrays exhibit a face-centered cubic structure in dried thin films. This study offers a new approach for producing ordered colloidal silica thin films.     

Kinetic Analyses of Colloidal Crystallization in a Wide Range of Sphere Concentrations as Studied by Reflection Spectroscopy

The nucleation and growth rates in the colloidal crystallization of silica spheres (103 nm in diameter) from 0.006 to 0.04 in volume fraction (straight phi) have been measured by reflection spectroscopy. Kinetics of the crystallization has been discussed in a wide sphere concentration range (from straight phi=0.0005 to straight phi=0.04) using the data of this work and the previous work (110 nm in diameter) in exhaustively deionized aqueous suspensions. The induction period for nucleation decreases sharply as the sphere concentration increases. The nucleation rate increases substantially from 1x10(-3) to 1x10(7) mm(-3) s(-1) when straight phi increases from 0.0005 to 0.04. The crystal growth process consists of the fast growing step toward metastable crystals (rate v(1)) and slow growth accompanied with the reorientation toward stable ones (rate v(2)). The v(1) values increase first from 5 to 20 μm/s and then turn back to 5 μm/s after passing a maximum. v(1) above straight phi=0.01 remains at 5 μm/s and is insensitive to sphere concentration. The slow step is observed in the high-sphere concentrations only, and v(2) decreases sharply from 3 μm/s to 0.7 nm/s when sphere concentration increases from 0.004 to 0.04 in volume fraction. Importance of the electrostatic intersphere repulsion by overlapping of the electrical double layers and the cooperative and synchronized fluctuation of colloidal spheres in the crystallization processes are supported strongly. Copyright 2000 Academic Press.     

Fabrication of spherical colloidal crystals using electrospray

We demonstrated the use of electrohydrodynamic atomization to prepare uniform-sized emulsion droplets in which equal spheres of silica or polystyrene were dispersed. The size of the emulsion droplets was easily controlled by the electric field strength and the flow rate, independently of the diameter of the nozzles. During the evaporation of solvent in the droplets, spherical colloidal crystals were formed by self-assembly of the monodisperse colloidal spheres. The diameter of the spherical colloidal crystals was in the range of 10-40 microm. Depending on the stability of colloidal particles, the morphology of the self-assembled structure was varied. In particular, silica spheres in ethanol droplets were self-assembled into compactly packed silica colloidal crystals in spherical shapes, whereas polystyrene latex spheres in toluene droplets self-assembled into spherical colloidal crystal shells with hollow cores. The silica colloidal assemblies reflected diffraction colors according to the three-dimensionally ordered arrangement of silica spheres.     

Thermo-sensitive colloidal crystals of silica spheres in the presence of large spheres with poly (N-isopropyl acrylamide) shells

Thermo-sensitive colloidal crystals are prepared simply by mixing colloidal silica spheres and large thermo-sensitive gel spheres. The thermo-reversible change in the lattice spacing of colloidal crystals of monodisperse silica spheres (CS82, 103 nm in diameter) depends on the size of the admixed temperature-sensitive gel spheres. For spheres with sizes less and greater than that of the silica spheres, the lattice spacing upon temperature increase above the lower critical solution temperature of poly(N-isopropyl acrylamide) decreases (cf. Okubo et al. Langmuir 18:6783, 2002) and increases, respectively. A mechanism, which is able to explain these experimental findings, is proposed. Moreover, crystal growth rates and the rigidities of the thermo-sensitive colloidal crystals are studied.     

Unidirectional crystallization of charged colloidal silica due to the diffusion of a base

Dilute aqueous dispersions of charged colloidal silica (particle volume fraction = approximately 0.03-0.04, particle diameter = 110 nm) exhibit unidirectional crystal growth due to the diffusion of a weak base, pyridine (Py). Similar diffusion-crystallization is enabled by a salt of a weak acid and a strong base, sodium hydrogen carbonate (NaHCO3). The resulting crystals consist of columnar (or cubic) crystal grains with a maximum height of a few centimeters and a maximum width of 1 cm. The crystal growth process is attributed to a combination of (i) the diffusion of Py or NaHCO3 accompanied by a charging reaction of the silica particles and (ii) the charge-induced crystallization of the silica colloids. Theoretical growth curves based on the reaction-diffusion model for the case of Py were in good agreement with the observed curves. We also report the immobilization of the resulting large crystals by using a polymer hydrogel matrix.     

Synthesis of Cristobalite Containing Ordered Interstitial Mesopores using Crystallization of Silica Colloidal Crystals

Cristobalite with ordered interstitial dual-sized mesopores was synthesized through the crystallization of silica colloidal crystals composed of monodispersed amorphous silica nanoparticles. An aqueous solution containing both a flux (Na₂O) and a carbon precursor (an aqueous low-molecular weight phenolic resin) was infiltrated into the interstices of silica colloidal crystals. The organic fraction in the nanocomposite was further polymerized and subsequently carbonized in an Ar flow at 750 °C to reinforce the colloidal crystal structure. The thermal treatment resulted in the crystallization of the colloidal crystals into cristobalite while retaining the porous structure. The cristobalite-carbon nanocomposite was calcined in air to remove the carbon and create interstitial ordered mesopores in the cristobalite. The surfaces of crystalline mesoporous silica are quite different from those of various ordered mesoporous silica with amorphous frameworks; thus, the present findings will be useful for a precise understanding and control of the interfaces between the mesopores and silica networks.     

Novel and simple route to fabricate 2D ordered gold nanobowl arrays based on 3D colloidal crystals

In this study, we present a new method to fabricate large-area two-dimensionally (2D) ordered gold nanobowl arrays based on 3D colloidal crystals by wet chemosynthesis, which combines the advantages of a very simple preparation and an applicability to "real" nanomaterials. By combination of in situ growth of gold nanoshell (GNSs) arrays based on three-dimensional (3D) colloidal silica crystals, a monolayer ordered reversed GNS array (2D ordered GNS array) was conveniently manufactured by an acrylic ester modified biaxial oriented polypropylene (BOPP). 2D ordered gold nanobowl array with adjustable periodic holes, good stability, reproducibility, and repeatability could be obtained when the silica core was etched by HF solution. The surface-enhanced Raman scattering (SERS) enhancement factor (EF) of this 2D ordered gold nanobowl array could reach 1.27 × 10(7), which shows high SERS enhancing activity and can be used as a universal SERS substrate.     

Formation of colloid crystals from polymer-modified monodisperse colloidal silica in organic solvents

The formation of colloid crystals from monodisperse and polymer-modified silica particles in organic solvents was investigated. Maleic anhydride–styrene copolymer-modified silica formed crystals in polar organic solvents, which dissolve the copolymer, while the original colloidal silica formed crystals in organic solvents which were miscible with water. The critical volume fraction in the crystal formation of the polymer-modified silica was lower than that from the unmodified silica in the same solvent. Polystyrene- and poly(methyl methacrylate)-modified silica particles also crystallized in organic solvents, but the features of the formation were different from those of poly(maleic anhydride-styrene)-modified particles. Received: 19 September 1998 Accepted in revised form: 1 January 1999     

Fabrication of binary colloidal crystals and non-close-packed structures by a sequential self-assembly method

Binary colloidal films of polystyrene (PS) spheres and silica spheres were fabricated with a sequential growth method using differently sized colloidal particles. In particular, we demonstrate the structures formed by a silica monolayer growing on top of a PS monolayer and a silica multilayer growing on top of a PS monolayer. By removal of the bottom PS layers, non-close-packed hexagonal, pentagonal, and square silica arrays were obtained at the original silica/PS interface. The possible formation mechanism of the non-close-packed structure was discussed, which may be used to explain how 3D colloidal crystals grow on patterned substrates.     

Convectively assembled asymmetric dimer-based colloidal crystals

Monolayer films from polystyrene asymmetric dimer colloidal particles were formed on a silicon substrate using a heat assisted vertical deposition technique. In dilute particle suspensions of systematically varied concentrations, the system maximizes the packing efficiency within a thin meniscus region. Structures with positional order and orientational order in and out of the substrate plane were observed in surface and cross-sectional scanning electron microscopy (SEM) images. The confining effect of the meniscus height drove the formation of the resulting oblique and hexagonal lattices with controlled orientation. The crystals exhibited features similar to the planes of the boron nitride and zinc sulfide atomic structures. The diffraction properties of both colloidal crystal structures were demonstrated via selected area diffraction for laser light in the visible region.     

Colloidal crystals formed by aqueous suspensions of monodispersed silica, polystyrene, and poly(methyl methacrylate) colloidal spheres

Colloidal crystals consisted of silica, polystyrene, and poly(methyl methacrylate) monodispersed suspensions; deionized sufficiently in water at the same condition; were formed; and their properties were compared changing sphere diameter and volume fraction systematically. The size of these colloidal crystals was maximized at their critical sphere concentration irrespective of their sphere size. The Bragg peak wavelengths of these colloidal crystals were uniquely determined only by the sphere diameter and volume fraction for all kinds of colloidal spheres used in this work. The larger the sphere volume fraction, the larger the crystal growth rates, and there were no significant differences among the colloidal spheres. The rigidity of colloidal crystals increased in proportion to the number density of spheres. Consequently, the crystallization mechanism and properties of colloidal crystals formed by these spheres are not dependent on the kind of spheres, but they are dependent only on the sphere diameter and number density.     

Colloidal crystals of core-shell type spheres with poly(styrene) core and poly(ethylene oxide) shell

Elastic modulus and crystal growth kinetics have been studied for colloidal crystals of core–shell type colloidal spheres (diameter = 160–200 nm) in aqueous suspension. Crystallization properties of three kinds of spheres, which have poly(styrene) core and poly(ethylene oxide) shell with different oxyethylene chain length (n = 50, 80 and 150), were examined by reflection spectroscopy. The suspensions were deionized exhaustively for more than 1 year using mixed bed of ion-exchange resins. The rigidities of the crystals range from 0.11 to 120 Pa and from 0.56 to 76 Pa for the spheres of n = 50 and 80, respectively, and increase sharply as the sphere volume fraction increase. The g factor, parameter for crystal stability, range from 0.029 to 0.13 and from 0.040 to 0.11 for the spheres of n = 50 and 80, respectively. These g values indicate the formation of stable crystals, and the values were decreased as the sphere volume fraction increased. Two components of crystal growth rate coefficients, fast and slow, were observed in the order from 10⁻³ to 10¹ s⁻¹. This is due to the secondary process in the colloidal crystallization mechanism, corresponding to reorientation from metastable crystals formed in the primary process and/or Ostwald-ripening process. There are no distinct differences in the structural, kinetic and elastic properties among the colloidal crystals of the different core–shell size spheres, nor difference between those of core–shell spheres and silica or poly(styrene) spheres. The results are very reasonably interpreted by the fact that colloidal crystals are formed in a closed container owing to long-range repulsive forces and the Brownian movement of colloidal spheres surrounded by extended electrical double layers, and their formation is not influenced by the rigidity and internal structure of the spheres.     

Inverse opal photonic crystal of chalcogenide glass by solution processing

Chalcogenide opal and inverse opal photonic crystals were successfully fabricated by low-cost and low-temperature solution-based process, which is well developed in polymer films processing. Highly ordered silica colloidal crystal films were successfully infilled with nano-colloidal solution of the high refractive index As(30)S(70) chalcogenide glass by using spin-coating method. The silica/As-S opal film was etched in HF acid to dissolve the silica opal template and fabricate the inverse opal As-S photonic crystal. Both, the infilled silica/As-S opal film (Δn ~ 0.84 near λ=770 nm) and the inverse opal As-S photonic structure (Δn ~ 1.26 near λ=660 nm) had significantly enhanced reflectivity values and wider photonic bandgaps in comparison with the silica opal film template (Δn ~ 0.434 near λ=600 nm). The key aspects of opal film preparation by spin-coating of nano-colloidal chalcogenide glass solution are discussed. The solution fabricated "inorganic polymer" opal and the inverse opal structures exceed photonic properties of silica or any organic polymer opal film. The fabricated photonic structures are proposed for designing novel flexible colloidal crystal laser devices, photonic waveguides and chemical sensors.