Effects of the dielectric discontinuity on the counterion distribution in a colloidal suspension

We introduce a new method for simulating colloidal suspensions with spherical colloidal particles of dielectric constant different from the surrounding medium. The method uses an approximate calculation of the Green function to obtain the ion-ion interaction potential in the presence of a dielectric discontinuity at the surface of the colloidal particle. The method is very accurate and is orders of magnitude faster than the traditional approaches based on series expansions of the interaction potential.     

Effect of counterion geometry on cationic micelles

The critical micelle concentration, aggregation number, and binding properties of decyltrimethylammonium ion micelles are determined as a function of counterion geometry using the dianions of phthalic, isophthalic, and terephthalic acids. Light-scattering methods were used to secure the CMC and AN data whereas binding data were obtained with a specific ion electrode. It is shown that the differences among the micellar parameters are quite small relative to counterion effects induced by variations in polarizability and hydrophobicity. The data are used as evidence for a disorganized micelle surface containing water-filled grooves and fatty patches which do not discriminate among various geometric dispositions of the carboxylates about the aromatic ring.     

Crystallization of colloidal crystal on hydrogel surface

A dip-coating method to fabricate wet and dry type of colloidal crystal films was developed. The wet type of colloidal crystal film was fabricated by lifting an agarose-hydrogel-coated substrate out of an aqueous suspension containing monodisperse polymer spheres and the dry type of colloidal crystal film was derived by following desiccation of the wet film. Monodisperse spheres formed ordered structures in the both type of the films, which contributed sharp reflection peaks. Brilliant colors were observed when the reflection peaks fell in the visible region. Formation mechanism of the colloidal crystal and their optical properties were discussed.     

Unexpected effect of light on colloidal crystal spacing

Colloidal crystals were formed from microsphere suspensions via a simple and novel approach using gel beads. The microspheres self-assembled not only around each bead but also between beads in an ordered pattern. The crystals shrunk under incident light, with the effect of blue (wavelength 450 to 500 nm) being the most profound. The results shed new light on the fundamental issue of self-assembly and colloid science.     

Enzymatically active colloidal crystal arrays

We report the construction of three-dimensional (3D) colloidal crystal arrays (CCA) and hollow colloidal crystal arrays (HCCA) derived from the self-assembly of polyelectrolytes (PE)-coated polystyrene (PS) particles and their use as models of high surface area systems to immobilize peroxidase (POD). POD molecules could infiltrate into the deep layers of CCA and HCCA through their interconnected pores and strongly adsorbed at the PE shell of the colloidal particles. And the total enzyme loading amount and bioactivities increased linearly with the thickness of the CCA till ca. 10 mum. Compared with flat substrates with the same geometrical area, CCA and HCCA exhibit much higher enzyme loading abilities (approximately 43 and 53 times respectively) and the resulting bioactivities (approximately 35 and 41 times respectively) due to their inherently higher surface area and 3D interconnected porous structures. In addition, HCCA could load approximately 30% more POD than CCA because some POD molecules could infiltrate into the interior of the hollow capsule under salt condition.     

Dry etching of colloidal crystal films

Two types of non-close-packed colloidal crystal films were prepared by etching the films made of polystyrene nanospheres using a hyperthermal neutral beam of oxygen gas. Etching without sintering above glass transition temperature of the polymer particles resulted in the non-close-packed structure of the nanospheres, in which polystyrene nanospheres in different lattice planes touched each other due to the reduction in the size of the nanospheres that occurred during the etching process. In contrast, a different non-close-packed structure with inter-connecting networks between etched nanospheres was generated by annealing of the colloidal crystal and a subsequent etching process. The photonic bandgap could be tuned during this dry etching of colloidal photonic crystals. This connected open structure could be used as a template for a silica inverse opal by chemical vapor deposition. An alternative dry etching process, reactive ion etching, mainly affected the morphology of particles near the top surface, and only a slight change in the stop band position of the colloidal crystal film was observed.     

Inward-growing self-assembly of colloidal crystal films on horizontal substrates

Colloidal crystal films have been fabricated on solid substrates with a horizontal deposition method. Scanning electron microscope images showed that the colloidal crystal films exhibit ordered face-centered cubic structures in large domains. Optical measurements demonstrated the presence of photonic band gap along the crystallographic [111] direction. The fabrication method described in this paper allows one to rapidly fabricate colloidal crystal films of different thicknesses, which can be controlled by varying colloidal suspension concentration or volume. In addition, the method also works well for growing colloidal crystal films on a hydrophilic solid substrate with a rough surface. Furthermore, the fabrication of colloidal crystal heterostructures has been demonstrated. An inward-growing mechanism responsible for self-assembly of colloidal spheres on horizontal substrates has been proposed to interpret the observed experimental results.     

Nonspherical colloidal crystals fabricated by the thermal pressing of colloidal crystal chips

Nonspherical colloids and their ordered arrays may be more attractive in applications such as photonic crystals than their spherical counterparts because of their lower symmetries, although such structures are difficult to achieve. In this letter, we describe the fabrication and characterization of colloidal crystals constructed from nonspherical polyhedrons. We fabricated such nonspherical colloidal crystals by pressing spherical polymer colloidal crystal chips at a temperature slightly lower than the glass-transition temperature (T(g)) of these polymer colloids. During this process, the polymer microspheres were distinctively transformed into polyhedrons according to their crystal structures, whereas the long-range order of the 3D lattice was essentially preserved. Because a working temperature lower than T(g) effectively prevented the colloidal crystals from fusing into films, the spherical colloidal crystals were transformed greatly under pressure, which lead to obvious change in the optical properties of colloidal crystals. Besides their special symmetry and optical properties, these nonspherical colloidal crystals can be used as templates for 2D or 3D structures of special symmetry, such as 2D nano-networks. We anticipate that this fabrication technique for nonspherical colloidal crystals can also be extended to nonspherical porous materials.     

Liquid crystal phases of charged colloidal platelets

The liquid crystal phase behavior of a suspension of charged gibbsite [Al(OH)3] platelets is investigated. By variation of the ionic strength, we are able to tune the effective thickness-to-diameter ratio of the platelets in suspension. This enables us to experimentally test the liquid crystal phase transition scenario that was first predicted a decade ago by computer simulations for hard platelets (Veerman, J. A. C.; Frenkel, D. Phys. Rev. A 1992, 45, 5632), that is, the isotropic (I) to nematic (N) and isotropic to columnar (C) phase transitions in one colloidal suspension. In addition to the shape-dependent thermodynamic driving force, the effect of gravity is important. For example, a biphasic (I-N) suspension becomes triphasic (I-N-C) on prolonged standing. This effect is described by a simple osmotic compression model.     

Influence of the counterion and co-ion diffusion coefficient values on some dielectric and electrokinetic properties of colloidal suspensions

The dependences of the conductivity increment, the electrophoretic mobility, and the permittivity increment on the counterion diffusion coefficient value were numerically determined. The use of the network simulation method made it possible to solve the governing equations for the whole range of counterion and co-ion diffusion coefficients and for very low frequencies, despite the far-reaching field-induced charge density outside the double layer. Calculations performed for different zeta potential and electrolyte concentration values show that increasing the counterion mobility, while keeping constant the electrolyte solution conductivity and the kappa a values, strongly increases the conductivity increment, barely affects the electrophoretic mobility, and strongly decreases the permittivity increment. The numerical results are discussed and compared to analytical predictions derived from the Shilov-Dukhin model, which generally leads to a good agreement, at least for high kappa a and moderate zeta.     

Facile fabrication of mechanochromic-responsive colloidal crystal films

This paper presents a simple approach to fabricate a reversible mechanochromic-responsive crystal film based on the room-temperature film-formation of monodisperse polymer latex by the aid of nanosilica particles. In this approach, when the "soft" colloidal polymer spheres were blended with colloidal silica particles and then cast on a substrate, followed by drying at room temperature for self-assembly, an elastic crystal film was directly obtained. This crystal film has not only reversible and repeatable mechanochromic-responsive property, but also tunable color and peak position covering almost entire visible spectral region, depending upon the sizes of polymer spheres and strains. This optical response is attributed to the variation of lattice spacing during deformation.     

Two-dimensional colloidal crystal in nonlinear Poisson-Boltzmann model

A two-dimensional hexagonal colloidal crystal of charged particles obeying the general nonlinear Poisson-Boltzmann equation is studied by the numerical method. Force constants and pressure in a system, as well as elastic constants of a crystal, are calculated on the basis of the solutions of the equation. Calculation procedures are described briefly and numerical data are reported. The effect of nonlinearity of charge distribution on the manifestation of many-body interactions and on the validity of the approximation of interaction of the nearest neighbors is considered.     

Growth dynamics of self-assembled colloidal crystal thin films

A theoretical and experimental analysis of the growth dynamics of colloidal crystal films deposited by evaporation induced self-assembly is herein presented. We derive an expression for the film growth velocity from which we obtain an equation that describes the evolution of the forming crystal thickness with time. Its validity is confirmed by comparison to the experimental profiles of a large number of films grown under different conditions. We find that, on top of the already reported linear increase in film width over long distances in the growth direction, periodic variations of the friction force at the meniscus give rise to short-range thickness fluctuations that are the main source of spatial inhomogeneities observed in these lattices. The key parameters that determine the period and the intensity of these fluctuations are identified.     

Gravity-induced liquid crystal phase transitions of colloidal platelets

The influence of gravity on a suspension of sterically stabilized colloidal gibbsite platelets is studied. An initially isotropic-nematic biphasic sample of such a suspension develops a columnar phase on the bottom on prolonged standing. This phenomenon is described using a simple osmotic compression model. We performed Monte Carlo simulations of cut spheres with aspect ratio L/D=1/15 and took data from the literature to supply the equations of state required for the model. We find that the model describes the observed three-phase equilibrium quite well.     

Effect of counterion substitution on the viscosity anomaly in AOT microemulsions

AOT-based water-in-oil microemulsions display an anomalous maximum in the viscosity with X, the water to surfactant ratio. Several explanations for this phenomenon have been offered. In this work, we investigate viscosity and droplet interactions in Ca(AOT)(2)/water/n-decane and KAOT/water/n-decane microemulsions and compare our results with the commonly studied NaAOT/water/oil system. The Ca(AOT)(2) system demonstrates a maximum in relative viscosity and droplet attraction near X=15, similar to the NaAOT system, although the maximum occurs at a higher value of X in the Ca(AOT)(2) system. By contrast, the viscosity and interparticle interactions in the KAOT system do not strongly depend on the amount of water in the system. We attribute the differences in behavior between the two systems to different hydration characteristics of the counterion, and we believe that our results are consistent with a previously proposed model that attributes interdroplet attractions to charge fluctuations and surfactant exchange. Our findings support the connection between the viscosity anomaly and interparticle interactions.     

Centimeter-scale colloidal crystal belts via robust self-assembly strategy

Centimeter-scale poly(acrylic acid-co-DVB80) (PAA) 3D colloidal crystal belts were prepared via a novel robust vertical deposition technique based on negative pressure and curvature substrate of the glass vial. The formation of PAA colloidal crystal belts was investigated. The results indicated that curvature could control the dimension of PAA colloidal crystal belts. Well-controlled negative pressure resulted in rapid fabrication of well-defined PAA colloidal crystal belts. Curvature substrate of glass vial could distribute shrinking stress in the process of drying of colloidal films. Strong hydrogen bonding interactions among carboxyl groups on the surface of PAA colloidal particles was responsible for PAA colloidal crystal belts with closed-packing characteristics.     

Multistep crystal nucleation: a kinetic study based on colloidal crystallization

Crystallization via an amorphous precursor, the so-called multistep crystallization (MSC), plays a key role in biomineralization and protein crystallization. MSC has attracted much attention in the past decade, but a quantitative understanding of it has so far not been available. The major challenge is that the kinetics governing the nucleation of crystals occurring in the metastable amorphous precursor remains unclear. In this study, the kinetics of MSC is addressed experimentally. Most importantly, a mathematical method is developed to calculate the local nucleation rate of the crystals in the amorphous precursor, which is not accessible to conventional methods. This local nucleation rate is critical to the understanding of MSC, but it has never been dealt with experimentally because of the difficulties of in situ observation. With the local crystal nucleation rates, the supersaturation for crystallization and the crystal-liquid interfacial free energy in the amorphous precursor are evaluated.