Binary Colloidal System Research Articles

Demixing and nematic behaviour of oblate hard spherocylinders and hard spheres mixtures: Monte Carlo simulation and Parsons–Lee theory

Parsons–Lee approach is formulated for the isotropic–nematic transition in a binary mixture of oblate hard spherocylinders and hard spheres. Results for the phase coexistence and for the equation of state in both phases for fluids with different relative size and composition ranges are presented. The predicted behaviour is in agreement with Monte Carlo simulations in a qualitative fashion. The study serves to provide a rational view of how to control key aspects of the behaviour of these binary nematogenic colloidal systems. This behaviour can be tuned with an appropriate choice of the relative size and molar fractions of the depleting particles. In general, the mixture of discotic and spherical particles is stable against demixing up to very high packing fractions. We explore in detail the narrow geometrical range where demixing is predicted to be possible in the isotropic phase. The influence of molecular crowding effects on the stability of the mixture when spherical molecules are added to a system of discotic colloids is also studied.

Critical conditions of entropy-driving nucleation in binary colloidal system

In a binary colloidal hard-sphere system, due to the entropic depletion interactions, large spheres may be pushed to pack into clusters. If a cluster is taken as a new kind of component, then the binary colloidal system turns to be a ternary one and it therefore can be dealt in the framework of a multi-component system. It is known that, under certain conditions, a cluster can grow larger and larger and turn into a nucleation, even a kind of phase transition will take place. In this paper, the critical conditions, including the critical size of the nucleating cluster and the corresponding volume fraction of the colloidal system, were determined by analyzing entropy variation through the multi-component of Carnahan–Starling (CS) state equation and the principle of entropy maximum. The results obtained in this way are in good agreement with that of the experiments. In addition, the results also show that, when the critical volume fraction of the system increases from 0.18 to 0.19, the cluster’s density ϕ will change from 0.74 to 0.58. Except for the size ratio of large to small spheres, the critical size of the nucleating cluster is almost independent of its density and the volume fraction of the system.

The Effect of Centrifugal Force on the Assembly and Crystallization of Binary Colloidal Systems: Towards Structural Gradients

In this study, the effect of centrifugal force on the assembly and crystallization of binary colloidal mixtures is demonstrated. Monolithic pieces have been prepared which are characterized by a structural gradient along the direction of the centrifugal force. For a given number ratio of monodisperse 154 and 300 nm latex spheres, the absolute latex concentration was varied and with it the sedimentation velocity of the individual particle species. For three different concentrations it has been demonstrated that the structure of the binary colloidal assembly obtained after centrifugation is affected significantly. For the largest initial latex absolute concentrations, the structural variation along the packed latex column in the ultracentrifuge tube is minimal, while a decrease in the absolute concentrations leads to crystalline packing in defined regions of the column. The observation of the thermodynamically favored structure resembling NaCl, but also of the aluminum boride AlB2 analog as well as unordered, glass-like packing depending on the mutual latex concentration under unchanged particle number and size ratio, shows that by centrifugation kinetically favored states can be realized. This result implies that centrifugation of binary latex mixtures is a promising route for investigating the self-organization of binary colloidal systems since the sedimentation velocities of the two particle species are different, and thus the local concentrations and mixing ratios vary continuously also enabling rare packing motifs.

Charge conversion and mass transfer on surface of Al2O3 nanoparticles in Y2O3–Al2O3 colloidal system

The mixing process of Al2O3 and Y2O3 nanopowders in aqueous suspension was studied. Charge conversion on the surface of Al2O3 particles in the Y2O3–Al2O3 colloidal system was observed. Negative charges on the surface of Al2O3 particles became positive due to the assembly of Y2O3 on the surface of Al2O3 nanoparticles in the colloidal system, as shown by zeta potential analysis of Y2O3–Al2O3 colloidal systems with different Y/Al ratios. Rheological properties of the mixed powders slurries with different Y2O3 content were also investigated. XRD peaks of Al2O3 shifted towards smaller angles with increasing amounts of Y2O3 added, implying the encapsulation of Al2O3 by Y2O3via mass transport during the mixing process, as confirmed by HRTEM measurement. A mechanism was proposed for the paragenesis of mineral in this binary colloidal system. Hydrated Y2O3 with higher Ksp attaches and migrates onto the surface of another hydrated sediment Al2O3 with smaller Ksp and opposite surface charge. The mass transfer process between Y2O3 and Al2O3 nanoparticles in this binary mixture suspension is analogous to the Ostwald ripening process. This new growth phenomenon is important for the science of crystal growth as it helps boost the exploration of new mechanisms for crystal growth in various material systems and provide clear evidence for the diffusion growth of crystalline structure in a binary colloidal system.

The structure and melting transition of two-dimensional colloidal alloys

We study theoretically the structure and melting transition of two-dimensional (2D) binary mixtures of colloidal particles interacting via a dipole–dipole potential. Using a lattice sum method, we find that at zero temperature (T = 0) the system forms a rich variety of stable crystalline phases whose structure depends on the composition and dipole moment ratio. Using Monte Carlo (MC) simulations, we also find that the melting temperature of the different T = 0 structures is a very strong and non-monotonic function of composition. For example, from a direct analysis of the radial distribution function vs.temperature, we find that the melting temperature of hexagonal AB2 and AB6 phases is three orders of magnitude higher than that of hexagonal AB5. Finally the melting transition for our binary colloidal system is found to proceed via at least two stages for hexagonal AB2 and AB6 and at least three stages for hexagonal AB5 and is thus much richer compared to the melting transition of 2D one component colloidal systems.

Hydrogen-Bond-Induced Heteroassembly in Binary Colloidal Systems

A new binary colloidal system is designed with each colloidal component being able to exclusively interact with the colloids of the second component, respectively. The colloids are based on cross-linked polystyrene and polymerized by means of surfactant-free emulsion polymerization with either 4-hydroxyl styrene or 4-vinyl pyridine as comonomers. The comonomers are selected to decorate the colloids with complementary H-bond donors or acceptors. Characterization of the colloids by light scattering in CHCl(3) results in particle radii covering a regime of 130 nm < R < 270 nm and indicates a narrow particle size distribution for all prepared samples. The colloids are slightly swollen compared to their state in H(2)O. The corresponding size ratio expressed as the radius of the small colloids divided by the radius of the large colloids in CHCl(3) extends over a regime of 0.52 < R(S)/R(L) < 0.85. IR analysis indicates strong hydrogen bonds between a colloidal component and the complementary molecular functions, that is, phenol or pyridine. If both colloidal components are directly combined as suspension in CHCl(3) a fast heteroaggregation is observed. This aggregation slows down and becomes accessible to an analysis by means of time-resolved static light scattering if the suspensions get sufficiently dilute. Fast aggregation can be totally inhibited if capping agents like phenol to cap the 4-vinyl pyridine functions are added as highly concentrated solutions. Dried samples of those binary suspensions equilibrated with an appropriate amount of phenol show the first indication for an ordered binary assembly.

Study on crystallization kinetics of binary alloys through model colloidal mixtures

Since the discovery of amorphous alloys, extensive attentions have been paid to understand the mechanism of glass-forming. The structural studies on its underlying mechanism have met challenge on direct structural characterization. It has been shown that the phase behavior of colloids dispersed in a solvent is thermodynamically equivalent to that of atoms and small molecules, however, colloids can be studied with optical microscopy due to their relatively large size. In this work, we use a binary colloidal model system with the particle size ratio comparable to atomic ratio of reported binary bulk metallic glasses to study the topological effect on crystallization and glass-forming ability of binary metallic alloys (Cu–Hf and Cu–Zr systems). The crystallization kinetics and structure of the colloid system were studied by real-time optical examination and light scattering technique. The results exhibit that there are two confined regions in the mixing ratio (composition) range in the colloid system with an enhanced glass-forming ability and retarded crystallization kinetics. The agreement between results of the model system and the experimental data on the binary bulk metallic glass formation suggests that purely topological factor plays an important role in determining the glass-forming ability.

Magnetic 3-D ordered macroporous silica templated from binary colloidal crystals and its application for effective removal of microcystin

Magnetic 3-D ordered macroporous siliceous materials were conveniently fabricated by combination of a simple co-sedimentation of binary colloidal system of PMMA spheres and magnetite nanoparticles and the infiltration of silica precursors. The SEM and TEM observations indicate that the obtained magnetic 3-D ordered macroporous silica materials have face-centered cubic ( fcc) packing structure with ordered macropore of ∼200 nm and interconnected windows of about 50 nm. Magnetic property characterization shows that the magnetic 3-D ordered macroporous silica materials possess high magnetization (19.2 emu/g) and superparamagentism. By utilization of their ordered macroporous structure with large entrances and good affinity for biomacromolecules as well as high magnetization, a fast and efficient removal of microcystin in water with high removal efficiency (>93%) and removal capacity (3.3 ng MC/mg) was achieved with the help of an applied magnetic field.

Structure of binary colloidal systems confined in a quasi-one-dimensional channel

The structural properties of a binary colloidal quasi-one-dimensional system confined in a narrow channel are investigated through modified Monte Carlo simulations. Two species of particles with different magnetic moment interact through a repulsive dipole-dipole force are confined in a quasi-one-dimensional channel. The impact of three decisive parameters (the density of particles, the magnetic-moment ratio, and the fraction between the two species) on the transition from disordered phase to crystal-like phases and the transitions among the different mixed phases are summarized in a phase diagram.

An experimental study on colloidal crystals formed in two-component dispersion of charged particles

Characterization of colloidal crystals composed of two types of polystyrene particles of different sizes (94 nm and 141 nm respectively) at different number ratios is studied by means of Kossel diffraction and reflection spectra technique. The results showed the dependence of the mean interparticle distance (D0) and crystal structure on the number ratio of the two types of particles. The formation time of crystals lengthens as the number ratio of the two components approaches 1∶1. When the number ratio (9 4nm∶141 nm) decreases, the mean interparticle distance (D0) becomes larger. This study also shows that when the above number ratio equals 5:1, superlattice structure appears in the binary colloidal system.

Use of Parsons-Lee and Onsager theories to predict nematic and demixing behavior in binary mixtures of hard rods and hard spheres

Parsons-Lee and Onsager theories are formulated for the isotropic-nematic transition in a binary mixture of hard rods and hard spheres. Results for the phase coexistence and for the equation of state in both phases for mixtures with different relative sizes and composition are presented. The two theories explain correctly the general behavior observed in experiments and computer simulations for these fluids. In particular, the theory accounts for the destabilization of the nematic phase when spherical or globular macromolecules are added to a system of rodlike colloids, and the entrance of the system into a demixed regime at high volume fractions of the spherical particles. Upon demixing a nematic state rich in rods coexists in equilibrium with an isotropic state much more diluted in the rodlike component. Onsager theory fails on quantitative grounds for aspect ratios of the rodlike molecules smaller than 100, and in the cases where the molar fractions of spheres becomes close to unity. On the contrary, the Parsons-Lee approximation remains accurate down to aspect ratios as small as 5. The spinodal analysis indicates that the isotropic-isotropic and nematic-nematic coexistences become feasible for sufficiently large spheres and long rods, respectively. The latter type of coexistence interferes partially with the isotropic-nematic coexistence regime of interest to the present work. Overall, the study serves to rationalize and control key aspects of the behavior of these binary nematogenic colloidal systems, which can be tuned with an appropriate choice of the relative size and molar fractions of the particles.

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.

Optical travelator: transport and dynamic sorting of colloidal microspheres with an asymmetrical line optical tweezers

We report the transport, funnelling and dynamic sorting of colloidal microspheres in an aqueous suspension using line optical tweezers with asymmetrical intensity profiles. The line tweezers readily trapped and propelled the microspheres along the length of the line tweezers. Using this simple technique, transporting and funnelling of microspheres within a microscopic region were demonstrated. To illustrate the dynamic particle-sorting capability of the line tweezers, a binary colloidal system comprising of microspheres with diameters of 1.1 μm and 3.2 μm were driven past the line tweezers by electrophoresis. As the optical trapping force is dependent on the size of the microspheres, the line tweezers was able to change the path of the larger spheres while exerting little influence on the smaller spheres thus sorting the two types of microspheres. At optimized laser power and flow rate of microspheres, sorting efficiency greater than 90% has been achieved.

Freezing of Binary Colloidal Systems for the Formation of Hierarchy Assemblies

Cryo-etch scanning electron microscopy (cryo-etch SEM) of aqueous gels composed of colloidal silica nanoparticles in the 1−40 nm range and liposomes of ∼200 nm gave unique morphologies. The aqueous gels are frozen at subcooled liquid nitrogen and fractured to obtain a fresh surface. High-vacuum sublimation of ice from the freshly exposed surface (etching) results in the formation of a hierarchy assembly, characterized by granular fences composed of colloidal silica and liposomes surrounded by empty areas in which amorphous ice originally resided. The biocompatible character of this ice segregation induced self-assembly (ISISA) process that allows for the preservation of the structural integrity of liposomes within the assembly is demonstrated by fluorescence anisotropy performed at the binary colloidal aqueous gels and differential scanning calorimetry and electron microscopy at the hierarchy assembly. The resulting assembly shows an interesting dual character, with one colloidal entity supporting the structure (e.g., silica) and the other providing functionality (e.g., liposomes).

Molecular dynamics simulation study of binary fullerene mixtures

We report constant-pressure molecular dynamics (MD) simulations of binary ${\mathrm{C}}_{60}∕{C}_{n}$ fullerene-mixtures ($n=70$, 76, 84, 96) modeled in terms of a spherically symmetric two-body potential. By starting from a liquid configuration of the system, we cool mixtures down to freezing and beyond, until room temperature is reached, in order to verify the formation of solid solutions, namely, of configurations characterized by a unique crystalline lattice whose sites are randomly occupied by the two component fullerene species. We first explore the entire concentration range of the ${\mathrm{C}}_{60x}∕{\mathrm{C}}_{70(1\ensuremath{-}x)}(0lxl1)$ mixture and find fairly good agreement with experimental data, exhibiting partial reciprocal solubility of the two components into each other with an immiscibility gap at intermediate compositions. In fact, the system we simulate forms substitutional solid solutions over a wide range of concentrations except for $0.3\ensuremath{\leqslant}x\ensuremath{\leqslant}0.5$; over such an interval, it turns out that the initially liquid mixture can be supercooled down to relatively low temperatures, until eventually a glassy phase is formed. The study is then extended to fullerene mixtures of molecular diameter ratio $\ensuremath{\alpha}={\ensuremath{\sigma}}_{{\mathrm{C}}_{60}}∕{\ensuremath{\sigma}}_{{\mathrm{C}}_{n}}$ smaller than in ${\mathrm{C}}_{60}∕{\mathrm{C}}_{70}$ (where $\ensuremath{\alpha}=0.93$), as is the case for ${\mathrm{C}}_{60}∕{\mathrm{C}}_{76}$ $(\ensuremath{\alpha}=0.89)$, ${\mathrm{C}}_{60}∕{\mathrm{C}}_{84}(\ensuremath{\alpha}=0.85)$ and ${\mathrm{C}}_{60}∕{\mathrm{C}}_{96}$ $(\ensuremath{\alpha}=0.79)$. The effect of the size mismatch between the two species is dramatic: The solid immiscibility region rapidly expands even upon a tiny reduction of $\ensuremath{\alpha}$, with formation of an amorphous phase at sufficiently low temperature, as found for the ${\mathrm{C}}_{60}∕{\mathrm{C}}_{70}$ mixture. For the smallest $\ensuremath{\alpha}({\mathrm{C}}_{60}∕{\mathrm{C}}_{96})$ cocrystallization of the two components turns out to be forbidden over the whole concentration axis. A mapping of the MD evidences of the fullerene mixtures' phase behavior onto the phase diagram of binary hard-sphere mixtures (determined by other authors) turns out to be worthwhile and enlightnening. In particular, size ratio effects and the onset of glassy phases emerge in qualitative good agreement with such studies, and with results of phase coexistence calculations in model binary colloidal systems.

Binary colloidal systems in two-dimensional circular cavities: Structure and dynamics

We study the melting behavior of a binary system (R = 4.5 μm, r = 2.8 μm) of paramagnetic colloidal spheres in two-dimensional (2D) circular cavities. A repulsive interaction between the particles is caused by an external magnetic field B that induces magnetic dipole moments perpendicular to the sample plane. By means of video microscopy, we investigate the positions of the particles and their trajectories. For small interaction strengths, we observe a completely liquid phase where large and small particles diffuse across the entire system. With increasing B the larger particles become—due to their larger magnetic moment—localized and form a stable structure while the smaller particles behave still as a liquid. For even higher magnetic fields, the small particles also become increasingly localized and preferentially arrange as interstitial sites between the structures formed by the large particles. We present a systematic study of this rather complex multi-stage melting process which strongly depends on the particle numbers of large and small particles.

Synthesis of colloidal aluminosilicate for light-scattering investigations

To develop a binary colloidal system with a slight index of refraction mismatch suitable for light scattering studies, pure silica particles synthesized by the method of Stöber were mixed with aluminosilicate colloids synthesized using a novel approach. With this, index-matching for one component allowed extraction of the spatial distribution of the other. In addition, it was observed that by varying the solvent, interactions between colloids could be tuned from purely repulsive to weakly attractive.

Absolute Heteroaggregation Rate Constants by Multiangle Static and Dynamic Light Scattering

The analysis of early-stage heteroaggregation (or heterocoagulation) in binary colloidal systems composed of oppositely charged latex particles of different size in the submicrometer size range with multiangle static and dynamic light scattering is presented. The solution conditions were adjusted to exclude any significant homoaggregation. The apparent rate obtained from static light scattering mostly strongly decreases with increasing scattering angle regardless of the number fractions, while the rate from dynamic light scattering varies in a more complicated manner. The light scattering data could be interpreted quantitatively on the basis of the Rayleigh−Gans−Debye approximation. The values of absolute heteroaggregation rate constants were found to be the same within experimental error when evaluated from static or dynamic light scattering, and they were independent of the mixing ratio. The average hydrodynamic radius of the doublet obtained from the dynamic light scattering was in good agreement with theoretical estimates based on an exact hydrodynamic treatment at low Reynolds numbers. A simple formula is proposed to estimate the hydrodynamic radius of the asymmetric particle doublet, and this formula is shown to agree well with experimental data and with theory. The new conclusion from this study is that multiangle dynamic light scattering represents the method of choice for the determination of absolute heteroaggregation rate constants.

Detection of the heterocoagulation-stability transition in binary colloidal systems by electrophoretic light scattering Silica-silica/aminopropyl system

Electrophoretic light scattering is shown to detect the heterocoagulation-stability transition, induced by a variation of pH, in a model binary colloidal (silica-silica/aminopropyl) system. For stable mixtures of silica, bimodal zeta-potential distributions have been obtained with peak positions corresponding to those of monomodal zeta-potential distributions due to the separate silicas, while for heterocoagulated mixtures, only monomodal distributions appeared with peak positions between those of separate silicas. The isoelectric point was found at pH 3.7 and 6.25 for silica and silica/aminopropyl respectively, and the heterocoagulation-stability transition was predicted at pH 6.6. This is in accordance with the DLVO theory.

Use of binary organic colloidal systems in gas chromatography

A new kind of sorbent, a colloidal organic system on a solid support which possesses many qualities essential for the analysis of highly polar compounds is proposed. The main advantage of such sorbents lies in the possibility of controlling their selectivity over wide limits by changing the composition of the binary phase, by changing the column temperature and by changing the surface area of the colloid particles. The influence of adsorption factors on the retention of solutes by these systems is examined, the chromatographic properties (conventional polarity, efficiency and thermal stability) of the stationary phases under review are evaluated and specific examples of their practical application in gas chromatography are given.