X-Ray scattering study of ionic colloidal crystals

The scattering study of ionic colloidal crystals by using one- and two-dimensional ultra-small-angle scattering techniques is reviewed with a special reference to dilute dispersions. Because of large lattice constants of colloidal crystals, ultra-small angle regions need to be covered either by long distance optical systems combined with a synchrotron X-ray source or by adopting the Bonse–Hart optics. The crystal structure, lattice constant, and crystal orientation can be precisely determined.     

Electrostatic interactions of colloidal particles at vanishing ionic strength

Electrostatic interactions of colloidal particles are typically screened by mobile ions in the solvent. We measure the forces between isolated pairs of colloidal polymer microspheres as the density of bulk ions vanishes. The ionic strength is controlled by varying the concentration of surfactant (NaAOT) in a nonpolar solvent (hexadecane). While interactions are well-described by the familiar screened-Coulomb form at high surfactant concentrations, they are experimentally indistinguishable from bare Coulomb interactions at low surfactant concentration. Interactions are strongest just above the critical micelle concentration, where particles can obtain high surface potentials without significant screening, kappaa < 1. Exploiting the absence of significant charge renormalization, we are able to construct a simple thermodynamic model capturing the role of reverse micelles in charging the particle surface. These measurements provide novel access to electrostatic forces in the limit where the particle size is much less than the screening length, which is relevant not just to the nonpolar suspensions described here, but also to aqueous suspensions of nanoparticles.     

Quantifying the effect of ionic strength on colloidal fouling potential in membrane filtration

Ultrafiltration experiments were conducted to study the fouling potential of colloidal suspensions under different ionic strengths and colloid concentrations. A linear relationship was found relating the colloidal fouling potential to the logarithm of the Debye-Huckel parameter, a characteristic for electrical double layers of colloids. This finding provided a useful quantitative linkage between the colloidal fouling potential and the water chemistry. Considering the linear dependence of colloidal fouling potential on the colloid concentration, a bilinear model was proposed to explain the coupling effects of colloid concentration and ionic strength of the suspension on the fouling potential. The model predictions of fouling potential were found to fit accurately with experimentally determined fouling potential values. Further analysis of the model showed that ionic strength can significantly affect colloidal fouling, for example, a 10-fold increase in ionic strength from 0.001 to 0.01 M for a given feed concentration has the same membrane fouling effect as doubling the feed concentration. The model allows for a quick and reliable assessment of fouling potential without even performing any experiments. This could then be used to design the membrane process or pretreatment stages required to mitigate membrane fouling.     

Fabrication of colloidal crystals with tubular-like packings

Methods are reported for the fabrication of colloidal crystal wires with tubular packings. Both free and silica-encased wires have been prepared. Porous silicon membranes are infiltrated with silica spheres, treated with silane, and annealed. After removal of the silicon template, short annealing times were found to result in colloidal crystal wires with varied packing geometries, while repeated annealing cycles produced a thin translucent silica sheath around the wires. Packing in the wires varies with the channel diameter of the Si membrane. The channels used in this study typically produce colloidal crystal wires with six strands, though wires with four to seven strands have been observed. Both chiral and achiral packings are also possible.     

Defects of colloidal crystals

Reasons for the appearance of defects of various types in crystalline colloidal structures formed during the self-organization of the ensembles of spherical nanoparticles are analyzed using lyosols of metal nanoparticles stabilized with polymers as examples. Quantitative characteristic of the degree of imperfection of colloidal crystals is proposed and the procedures for the minimization of the degree of imperfection are discussed. Order-disorder phase transitions of colloidal crystals are studied.     

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.     

Video microscopy of colloidal suspensions and colloidal crystals

Colloidal suspensions are simple model systems for the study of phase transitions. Video microscopy is capable of directly imaging the structure and dynamics of colloidal suspensions in different phases. Recent results related to crystallization, glasses, and 2D systems complement and extend previous theoretical and experimental studies. Moreover, new techniques allow the details of interactions between individual colloidal particles to be carefully measured. Understanding these details will be crucial for designing novel colloidal phases and new materials, and for manipulating colloidal suspensions for industrial uses.     

Three-dimensional colloidal crystals with a well-defined architecture

Monodisperse silica spheres with diameters of 220-1100 nm were prepared by hydrolysis of tetraethyl orthosilicate (TEOS) in an alcoholic medium in the presence of water and ammonia. By grafting vinyl or amino groups onto silica surfaces using the coupling agents allyltrimethoxysilane and aminopropyltriethoxysilane, respectively, amphiphilic silica spheres were obtained and could be organized to form a stable Langmuir film at the air-water interface. The controlled transfer of this monolayer of particles onto a solid substrate gave us the ability to build three-dimensional regular crystals with a well-defined thickness and organization. These colloidal crystals diffract light in the UV, the visible, and the near-infrared (NIR) spectral regions, depending on the size of the silica spheres and according to Bragg's law. The depth of the photonic stop band can be tuned by varying the number of deposited layers of particles. By using successive depositions, we could prepare multilayered films with silica spheres of different sizes. The thickness of each slab in the binary crystals can be tuned at the layer level, while the crystalline order of each layer is well preserved.     

Microcontact printing of colloidal crystals

Patterned two-dimensional (2D) colloidal crystals have been transferred by a modified mucp technique that was based on the use of polymer film as "glue" to provide an efficient interaction between the microsphere "ink" and substrate. The versatility of this method has been demonstrated by the patterning of colloidal crystal on a nonplanar substrate and heterogeneously structured colloidal crystal film. The table of contents graphic shows an SEM image of the ordered parallel lines of 2D colloidal crystals on a polymer-coated glass tube with a 3.7 mm radius of curvature.     

Electro-optic effects of colloidal crystals

Many kinds of electro-optic effects of colloidal crystals are observed and discussed on the basis of the fundamental properties of colloidal crystals themselves. Several electro-optic effects of colloidal crystals have been found by the authors mainly by use of light-scattering, reflection- and transmitted-light intensity measurements in an electric field, (a) waveform deformation, (b) phase-shift effects, (c) second-order harmonics generation, (d) self-resonance frequency generation (characteristic frequency and harmonic oscillation), (e) peak wavelength-shift effects and (f) waveform transformation. These electro-optic responses are explained successfully by the resonance-, visco-elastic- and structural relaxation-parameters of colloidal crystals.     

Sintered silica colloidal crystals with fully hydroxylated surfaces

Silica colloidal crystals require multiple processing steps before they are useful materials in analytical applications, such as chemical separations, microarrays, sensors, and total internal reflection microscopy. These chemical processing steps include calcination, sintering, surface rehydroxylation, and chemical modification, but these steps have not been fully characterized for colloidal crystals. Silica particles of nominally 200 nm in diameter were prepared, and FTIR, SEM, UV-visible spectroscopy, and refractive index measurements were used to study the changes in chemical composition, particle size, and particle density throughout the process. The final material is shown to be a durable, crack-free crystal of solid particles bearing a fully hydroxylated surface of silanols, which can then be chemically modified.     

Madelung-like attractions in colloidal crystals

The Debye-Hückel (Derjaguin-Landau-Verwey-Overbeek) two-sphere interaction potential is discussed in relation to the dissociative electrical double layer (DEDL) theory. The DEDL theory provides new electrostatic models to investigate the origin of attractions in colloidal crystals. Three Maxwellian models of two, three, and four interacting spheres are suggested. It is estimated that at least four spheres are needed to obtain Madelung-like attractions that are brought about by co-ion exclusion.     

Simultaneous growths of gold colloidal crystals

Natural systems give the route to design periodic arrangements with mesoscopic architecture using individual nanocrystals as building blocks forming colloidal crystals or supracrystals. The collective properties of such supracrystals are one of the main driving forces in materials research for the 21st century with potential applications in electronics or biomedical environments. Here we describe two simultaneous supracrystal growth processes from gold nanocrystal suspension, taking place in solution and at the air-liquid interface. Furthermore, the growth processes involve the crystallinity selection of nanocrystals and induce marked changes in the supracrystal mechanical properties.     

Array patterns of binary colloidal crystals

Binary colloidal crystals (BCCs) were prepared from two kinds of poly(styrene-methyl methacrylate-acrylic acid) colloids approximately 190 and approximately 380 nm in diameter by the codeposition method. A variety of array patterns of BCCs were observed and characterized by AFM and SEM. The significance of these colloidal arrays in crystallography has been discussed preliminarily.     

Self-assembly patterning of silica colloidal crystals

We developed a self-assembly process of silica particles to fabricate desired patterns of colloidal crystals having high feature edge acuity and high regularity. A micropattern of colloidal methanol prepared on a self-assembled monolayer in hexane was used as a mold for particle patterning, and slow dissolution of methanol into hexane caused shrinkage of molds to form micropatterns of close-packed SiO2 particle assemblies. This result is a step toward the realization ofnano/micro periodic structures for next-generation photonic devices by a self-assembly process.     

Assembling and manipulating two-dimensional colloidal crystals with movable nanomagnets

We study crystallization of paramagnetic beads in a magnetic field gradient generated by one-dimensional nanomagnets. The pressure in such a system depends on both the magnetic forces and the hydrodynamic flow, and we estimate the flow threshold for disassembling the crystal near the magnetic potential barrier. A number of different defects have been observed which fluctuate in shape or propagate along the crystal, and it is found that the defect density increases away from the nanomagnet. We also study the melting of the crystal/fluid system after removal of the nanomagnet and demonstrate that the bond-oriental order parameter decreases with time. The nanomagnet can be moved in a controlled manner by a weak external magnetic field, and at sufficiently large driving velocities we observe self-healing crack formation characterized by a roughening of the lattice as well as gap formation. Finally, when confined between two oscillating nanomagnets, the colloidal crystal is shown to break up and form dipolar chains above a certain oscillation frequency.     

From colloids in liquid crystals to colloidal liquid crystals

ABSTRACT

Liquid crystals in combination with nanoparticles are a fascinating topic of research, because of the wealth of aspects and questions to study. These range from simple effects of nanoparticles on phase transitions and phase diagrams, to the tuning of physical properties, adding of novel functionalities, all the way to the formation of spontaneous order by nanoparticles themselves and the possibilities that templating has for future materials design and applications. This article intends to provide a flavour of the multiplicity, variety and diversity that these thermotropic and lyotropic systems have to offer in the area of materials development, which we believe will become increasingly important, especially for switchable non-display applications and nanotechnology. It is not intended to provide a conclusive overview, which would be a presumptuous attempt considering the limited space available, but rather to place our own work into a wider context and to point out some more recent developments and trends in liquid crystal – nanoparticle dispersions.     

High-speed electroseparations inside silica colloidal crystals

Crystals made from 200 nm silica colloids are hardened and chemically modified with chlorodimethyloctadecylsilane for use in electrically driven, reversed-phase separations. A van Deemter plot reveals extremely narrow peak widths for the separation of a cationic hydrophobic dye, DiI, with both the A and C terms 10-fold smaller than those for a conventional HPLC column. Electrically driven separations are demonstrated to be achieved in less than 10 s for three dyes differing in hydrophobicity and also for three peptides differing in electrophoretic mobility. The results show that these media are promising for high-speed separations.     

Preparation of colloidal crystals with polyhedral building blocks through post-polymerization

An alternative approach was adopted to prepare colloidal crystal with polyhedral building blocks. First, monodisperse polystyrene particles that contained about 30% wt of monomer were obtained by emulsifier-free emulsion polymerization at 38 °C. These monomer-containing particles were used to prepare colloidal crystal on the surface of dispersion, before the spherical particles in the colloidal crystals underwent deformation between two quartz plates at 75 °C for 40 min by interfacial tensions, and finally the deformed particles were frozen through post-polymerization.