Binary Colloids Research Articles

Automated Preparation Method for Colloidal Crystal Arrays of Monodisperse and Binary Colloid Mixtures by Contact Printing with a Pintool Plotter

Photonic crystals and photonic band gap materials with periodic variation of the dielectric constant in the submicrometer range exhibit unique optical properties such as opalescence, optical stop bands, and photonic band gaps. As such, they represent attractive materials for the active elements in sensor arrays. Colloidal crystals, which are 3D gratings leading to Bragg diffraction, are one potential precursor of such optical materials. They have gained particular interest in many technological areas as a result of their specific properties and ease of fabrication. Although basic techniques for the preparation of regular patterns of colloidal crystals on structured substrates by self-assembly of mesoscopic particles are known, the efficient fabrication of colloidal crystal arrays by simple contact printing has not yet been reported. In this article, we present a spotting technique used to produce a microarray comprising up to 9600 single addressable sensor fields of colloidal crystal structures with dimensions down to 100 mum on a microfabricated substrate in different formats. Both monodisperse colloidal crystals and binary colloidal crystal systems were prepared by contact printing of polystyrene particles in aqueous suspension. The array morphology was characterized by optical light microscopy and scanning electron microscopy, which revealed regularly ordered crystalline structures for both systems. In the case of binary crystals, the influence of the concentration ratio of the large and small particles in the printing suspension on the obtained crystal structure was investigated. The optical properties of the colloidal crystal arrays were characterized by reflection spectroscopy. To examine the stop bands of the colloidal crystal arrays in a high-throughput fashion, an optical setup based on a CCD camera was realized that allowed the simultaneous readout of all of the reflection spectra of several thousand sensor fields per array in parallel. In agreement with Bragg's relation, the investigated arrays exhibited strong opalescence and stop bands in the expected wavelength range, confirming the successful formation of highly ordered colloidal crystals. Furthermore, a narrow distribution of wavelength-dependent stop bands across the sensor array was achieved, demonstrating the capability of producing highly reproducible crystal spots by the contact printing method with a pintool plotter.

Fluid–fluid demixing transitions in colloid–polyelectrolyte star mixtures

We derive effective interaction potentials between hard, spherical colloidalparticles and star-branched polyelectrolytes of various functionalitiesf and smaller size than the colloids. The effective interactions are based on a Derjaguin-likeapproximation, which is based on previously derived potentials acting betweenpolyelectrolyte stars and planar walls. On the basis of these interactions we subsequentlycalculate the demixing binodals of the binary colloid–polyelectrolyte star mixture,employing standard tools from liquid-state theory. We find that the mixture is indeedunstable at moderately high overall concentrations. The system becomes moreunstable with respect to demixing as the star functionality and the size ratio grow.

Three-particle correlation functions of quasi-two-dimensional one-component and binary colloid suspensions

We report the results of experimental determinations of the triplet correlation functions of quasi-two-dimensional one-component and binary colloid suspensions in which the colloid-colloid interaction is short ranged. The suspensions studied range in density from modestly dilute to solid. The triplet correlation function of the one-component colloid system reveals extensive ordering deep in the liquid phase. At the same density the ordering of the larger diameter component in a binary colloid system is greatly diminished by a very small amount of the smaller diameter component. The possible utilization of information contained in the triplet correlation function in the theory of melting of a quasi-two-dimensional system is briefly discussed.

Study of aluminum powder as transition metal catalyst carrier for CVD synthesis of carbon nanotubes

The possibility of using aluminum powder as transition metal catalyst carrier for CVD growth of carbon nanotubes (CNTs) has been investigated. The fabrication process of Ni/Al catalyst involved the production of binary colloid (Ni(OH) 2/Al) by deposition–precipitation method, followed by calcination in N 2 and reduction in H 2. The nickel particles obtained were with uniform diameters. After the catalytic synthesis at 600 °C a mass of well-graphitized multi-walled CNTs with the diameters in the 10–20 nm range have been obtained, as evidenced by HRTEM. The operated catalytic particle encapsulated in CNT was nickel according to EDX analysis. However, when using pure nickel without aluminum as the catalyst, no CNTs have formed due to the agglomeration of Ni particles. Thus we speculated that the aluminum powder was responsible for the formation and the even dispersion of the nano-scale transition metal particles.

Pattern formation in binary colloids

An experimental and theoretical study of the formation of structures in a binary monolayer of dipolar particles is presented. We construct an experimental technique for an easy to control realization of a binary monolayer where the two components have oppositely oriented dipole moments constrained perpendicular to the plane of motion. The experimental set-up ensures that hydrodynamic effects do not play a crucial role in the structure formation, the particles move deterministically due to the dipole–dipole interaction. At low concentrations cluster–cluster aggregation occurs with chain-like morphologies, while at high concentration the particles self-assemble into various types of binary crystal lattices. We propose a simple theoretical framework to describe the observed structure formation. Based on molecular dynamics simulations and analytic calculations we show that the total concentration of the particles, the relative concentration and the relative dipole moment of the components determine the structure of the dipolar monolayer. The theoretical results are in satisfactory agreement with the experimental findings.

Stability of binary colloids: kinetic and structural aspects of heteroaggregation processes.

This review reports on recent advances in our knowledge about the stability of binary colloids. We focus not only on experimental results but also discuss theoretical and simulation studies regarding kinetic and structural aspects of heteroaggregation processes arising in such systems. In the first part of this work, heteroaggregation of oppositely charged particles is reviewed. When the interactions are short ranged, binary diffusion-limited cluster-cluster aggregation takes place. In this case, the short time behavior of the system follows the Hogg, Healy and Fuerstenau (HHF) theory. At long times, however, stable aggregates may form and remain in the system. Furthermore, cluster discrimination is observed, clusters that differ only by one constituent particle were found to behave quite differently. When the range of the interactions is increased, the latter effects become more pronounced. The fractal dimension of heteroaggregates is, in general, smaller than the values reported for fast and slow homoaggregation processes. In some cases, even values close to unity were obtained. This means that heteroaggregates have an open branched structure that may approach a chain-like morphology. In the second part of this work, we briefly discuss similar effects arising in heteroaggregation phenomena due to differences in particle size and chemical composition. The third part of this review tackles recent developments in the field of equilibrium phase diagrams of binary colloids. In the last section, the relatively small number of papers about heteroaggregation processes in two-dimensional systems is also discussed.

Fabrication of Superhydrophobic Surfaces from Binary Colloidal Assembly

In this work, superhydrophobic surfaces were derived from binary colloidal assemblies. CaCO(3)-loaded hydrogel spheres and silica or polystyrene ones were consecutively dip-coated on silicon wafers. The former assemblies were recruited as templates for the latter self-assembly. Due to the hydrophilicity difference between silicon wafers and CaCO(3)-loaded hydrogel spheres, the region selective localization of silica or polystyrene spheres leads to irregular binary structures with a hierarchical roughness. The subsequent modification with low surface energy molecules yields a superhydrophobic surface. The heating treatment may largely enhance the mechanical stability of the resulting binary structures, which allows regeneration of the surface superhydrophobicity, providing a good durability in practice.

Anomalous behavior of the depletion potential in quasi-two-dimensional binary mixtures

We report an experimental determination of the depletion interaction between a pair of large colloid particles present in a binary colloid mixture that has a high density of large particles and is tightly confined between two parallel plates, as a function of the small colloid particle density. The bare interaction between the large particles in the one component large colloid suspension, and the effective potential between the large particles in the binary colloid suspension represented as a pseudo-one-component fluid, were obtained by inverting the Ornstein-Zernike equation with the hypernetted chain closure. The depletion interaction is defined by subtracting the bare potential from the effective potential at fixed large colloid density. We find that the depletion potential in the quasi-two-dimensional (Q2D) system is purely attractive and short ranged as described by Asakura-Oosawa model. However, the depth of the depletion potential is found to be almost an order of magnitude larger than the counterpart depletion potential predicted for the same density and diameter ratio in a three-dimensional system. Although it is expected that the confining walls in the Q2D geometry enhance the excluded volume effects that generate entropic attraction, the observed enhancement is much larger than predicted for a Q2D binary mixture of hard spheres. We speculate that this anomalously strong confinement-induced depletion potential is a signature of characteristics of the real confined binary colloid mixture that are not included in any extant theory of the depletion interaction, specifically the omission of the role of the solvent in those theories. One such characteristic could be differential wall or particle wetting that generates a wall induced one-particle effective potential that confines the centers of the small particles to lie closer to the midplane between the walls than expected from the wall separation and the direct particle-wall interaction, thereby enhancing the depletion interaction.

Depletion interaction in a quasi-two-dimensional binary colloid mixture: Monte Carlo simulations

We report the results of extensive calculations of the depletion interaction between the large hard spheres in a quasi-two-dimensional (q2D) binary mixture of large and small hard spheres. Two definitions of the depletion interaction have been examined, and the dependencies of both on the large and small sphere densities, and the confining wall separation have been explored. The results of the simulations show that the depletion interaction is enhanced relative to its magnitude in a three-dimensional binary mixture with the same density, composition and sphere diameter ratio and that it has a complex dependence on the large sphere-large sphere separation. There are qualitative differences between the properties of q2D and mathematical 2D systems that are relevant to the interpretation of experimental data.

Structure formation in binary colloids.

A theoretical study of the structure formation observed very recently [W. D. Ristenpart, I. A. Aksay, and D. A. Saville, Phys. Rev. Lett. 90, 128303 (2003)] in binary colloids is presented. In our model solely the dipole-dipole interaction of the particles is considered, electrohydrodynamic effects are excluded. Based on molecular dynamics simulations and analytic calculations we show that the total concentration of the particles, the relative concentration, and the relative dipole moment of the components determine the structure of the colloid. At low concentrations the kinetic aggregation of particles results in fractal structures which show a crossover behavior when increasing the concentration. At high concentration various lattice structures are obtained in a good agreement with experiments.

Computer Simulation of Selective Aggregation in Binary Colloids

The morphology of clusters formed by selective aggregation of binary colloids is studied in a two-dimensional Monte Carlo simulation for a large range of number fractions (200:1, 100:1, 10:1, 2:1). We find remarkable similarity in morphology to those observed in experiments, from the formation of closed "micelles" to large branched clusters. Quantitative studies of the fractal dimension, kinetics, and cluster size distribution are also carried out and compared with diffusion-limited cluster aggregation and reaction-limited cluster aggregation models.

Structure and phase transitions in confined binary colloid mixtures

We report the results of a study of crystallization in quasi-two-dimensional binary mixtures of large and small colloids. The experiments sample the parameter spaces of colloid particle diameter ratio, large particle density, and small particle packing fraction. The depletion potential between the large particles, induced by the presence of the small particles in the system, affects the density at which the large particles undergo a liquid-to-solid freezing transition. For systems with a large to small particle diameter ratio of 4.6, the addition of small particles increases the large particle liquidus transition density, a seemingly counterintuitive result given that the depletion potential is purely attractive when the small particle packing fraction is low. When the large to small particle diameter ratio is 8.8, the same trend in the large particle liquidus transition density is seen, but to a lesser extent. The other system properties for the system with diameter ratio 8.8 show the same trends as for the system with diameter ratio 4.6. Liquid–liquid phase separation is observed for binary mixtures with diameter ratios of 20 and 40. Although the particles used in our experiment can be well modeled as hard spheres, our results cannot be readily explained by extant descriptions of the depletion interaction developed for three-dimensional binary hard sphere mixtures. Inversion of the pair correlation functions obtained from our measurements yields a depletion interaction that is much stronger than predicted for the same densities and diameter ratio in a three-dimensional hard sphere mixture. Our results imply that the depletion interaction is strongly dependent on the degree of confinement of the system.

Kinetics of Pyrene Fluorescence Quenching in the Presence of N,N-Dimethylaniline on the Surface of Titania Silica Colloids

A study was carried out on pyrene (Py) fluorescence quenching in the presence of N,N-dimethylaniline (DMA) on the surface of titania–silica colloids. Py fluorescence quenching is a function of the TiO2 content in the binary colloids. The coadsorption of DMA molecules is accompanied by an increase in the Py fluorescence intensity due to the competitive adsorption of amine on the surface. Calculation of the formal kinetics of Py fluorescence quenching indicates the existence of two groups of sites differing in adsorption activity within each group.

Model colloid–polymer mixtures in porous matrices: density functional versusintegral equations

We test the accuracy of a recently proposed density functional (DF) for afluid in contact with a porous matrix. The DF was constructed in thespirit of Rosenfeld’s fundamental measure concept and was derived forgeneral mixtures of hard core and ideal particles. The required doubleaverage over fluid and matrix configurations is performed explicitly. As anapplication we consider a model mixture where colloids and matrix particles arerepresented by hard spheres and polymers by ideal spheres. Integrating overthe degrees of freedom of the polymers leads to a binary colloid–matrixsystem with effective Asakura–Oosawa pair potentials, which we treatwith an integral-equation theory. We find that partial pair correlationfunctions from both theories are in good agreement with our computersimulation results, and that the theoretical results for the demixing binodalscompare well, provided the polymer-to-colloid size ratio, and hence the effectof many-body interactions neglected in the effective model, is not toolarge. Consistently, we find that hard (ideal) matrix–polymer interactionsinduce capillary condensation (evaporation) of the colloidal liquid phase.

Electrochemical stripping detection of DNA hybridization based on cadmium sulfide nanoparticle tags

We report on the detection of DNA hybridization in connection to cadmium sulfide nanoparticle tracers and electrochemical stripping measurements of the cadmium. A nanoparticle-promoted cadmium precipitation is used to enlarge the nanoparticle tag and amplify the stripping DNA hybridization signal. In addition to measurements of the dissolved cadmium ion we demonstrate solid-state measurements following a ‘magnetic’ collection of the magnetic-bead/DNA-hybrid/CdS-tracer assembly onto a thick-film electrode transducer. The new protocol combines the amplification features of nanoparticle/polynucleotides assemblies and highly sensitive stripping potentiometric detection of cadmium, with an effective magnetic isolation of the duplex. The low detection limit (100 fmol) is coupled to good reproducibility (RSD=6%). Prospects for using binary inorganic colloids for multi-target detection are discussed.

Binary hard-sphere crystals with the cesium chloride structure.

The possibility of binary hard-sphere colloids crystallizing with the cesium chloride (CsCl) structure was examined experimentally using poly (methyl methacrylate) particles dispersed in organic solvents. Towards this end, two dispersions were prepared that contained particles with a radius ratio alpha=R(B)/R(A), where A refers to the large particles and B the small, of 0.736. This is close to the optimum ratio of 0.732 at which this structure is predicted to form as determined by space-filling calculations. The phases found within the binary mixture were examined using laser light crystallography and scanning electromicroscopy, and some were shown to have the CsCl structure. Over a period of time some of the CsCl crystals disappeared indicating that they were metastable and that this structure may not be the most enduring phase at this size ratio.

Entropic Attraction and Repulsion in Binary Colloids Probed with a Line Optical Tweezer

The long-range entropic forces that arise between two micrometer-sized colloidal spheres in a fluid of much smaller colloidal spheres were directly measured using a line-scanned optical tweezer. This new technique allowed us to measure the functional form of the potential with sub- ${k}_{B}T$ energy and $15\mathrm{nm}$ spatial resolution. At the lowest small sphere concentrations, the potential was monotonically attractive, while at higher concentrations an oscillatory potential was observed, due to the liquid structure of the small spheres. Surprisingly, the large spheres came together only rarely at the higher concentrations, suggesting a new means for stabilizing suspensions using entropy alone.

Sorption and desorption of triadimefon by soils and model soil colloids.

Sorption-desorption of the azole fungicide triadimefon [1-(4-chlorophenoxy)-3,3-dimethyl-1-(1H-1,2, 4-triazol-1-yl)-2-butanone] on eight soils and a series of single, binary, and ternary model soil colloids was determined using the batch equilibration technique. Regression analysis between Freundlich sorption coefficients (K(f)) and soil properties suggested that both clay and organic C (OC) were important in triadimefon sorption by soils, with increasing importance of clay for soils with high clay and relatively low OC contents. Triadimefon sorption coefficients on soil were not significantly affected by the concentration of electrolyte or the presence of soluble soil material in solution, but they were highly dependent on the soil:solution ratio due to the nonlinearity of triadimefon sorption on soil. Freundlich sorption isotherms slopes were very similar for all soils (0.75 +/- 0.02). Desorption did not greatly depend on the concentration at which it was determined and showed higher hysteresis for more sorptive soils. Results of triadimefon sorption on model sorbents supported that both humic acid and montmorillonite-type clay constituents contribute to triadimefon retention by soil colloids.

Charge-Stabilized Liquidlike Ordered Binary Colloidal Suspensions. 1. Ultra-Small-Angle X-ray Scattering Characterization

In this and the following paper we present scattering studies on binary colloidal mixtures made of charge-stabilized polystyrene (PS) and perfluorinated (PFA) particles with diameters σ = 79 and 162 nm, respectively. Both colloidal species were mixed to well-defined compositions. The total concentration was about 9 vol % fraction. By using ultra-small-angle X-ray scattering, we directly obtained the partial scattering intensities of only the PFA particles in liquidlike ordered suspensions. Furthermore, after dividing intensities by the PFA particle form factor PPFA(Q), we got the partial structure factor SPFA-PFA(Q) without any additional treatment. The experimental results are compared with theoretical predictions obtained from the pure repulsive DLVO potentials and the hypernetted chain (HNC) integral equation, as applied to charged colloidal mixtures. It is shown that all measured intensities and extracted structure factors are in good agreement with the theoretical results.

Charge-Stabilized Liquidlike Ordered Binary Colloidal Suspensions. 2. Partial Structure Factors Determined by Small-Angle Neutron Scattering

In this paper we present small-angle neutron scattering (SANS) studies on liquidlike ordered binary colloidal suspensions. Using perfluorinated (σPFA = 162 nm) and polystyrene (σPS = 79 nm) particles, we prepared samples made of the same colloids as in the preceding paper. All system parameters of the mixtures were as before. Via neutron contrast variation we directly obtained all three partial intensities IPFA-PFA(Q), IPFA-PS(Q), and IPS-PS(Q) and partial structure factors SPFA-PFA(Q), SPFA-PS(Q), and SPS-PS(Q). The experimental results are compared with theoretical predictions for binary colloidal mixtures, based on the pure repulsive part of the DLVO potential. Within the hypernetted chain (HNC) closure an accurate agreement is achieved. The particle correlation in the mixtures is clarified by means of the partial distribution functions gij(r) and the one-component effective pair potentials (r) and (r). From this we deduce for the examined suspensions a partial clustering or local demixing by attractive interaction contributions of depletion origin.