Self-organization and pattern formation of janus particles in two dimensions by computer simulations

The phase behavior and associated pattern formation of two-dimensional systems of hard disks decorated with amphiphilic coronae (Janus disks) are studied by means of Monte Carlo computer simulations. A primitive interaction potential that captures the essential interparticle interactions is introduced. Despite its simplicity, the system exhibits a very rich phase polymorphism. Apart from the isotropic phase and depending upon the coronal thickness, the simulated systems self-organize in a number of two-dimensional mesophases of various symmetries exhibiting a variety of novel patterns. The results of these simulations suggest that 2D Janus particles are promising candidates for bottom-up design of precise two-dimensional templates.     

Facile, solution-based synthesis of soft, nanoscale janus particles with tunable janus balance

We present a novel, versatile, and simple solution-based routine to produce soft, nanosized Janus particles with tunable structural and physical properties at high volume yield. This process is based on the cross-linking of compartments within precisely defined multicompartment micelles (MCMs), which are themselves formed by the self-assembly of ABC triblock terpolymers. Therein, the C blocks form the stabilizing corona emanating from B compartments, which in turn reside on an A core. Cross-linking of the B compartments allows to permanently fixate the phase-separated state and dissolution in a good solvent for all blocks breaks up the MCMs into single Janus particles. They now consist of a core of cross-linked B blocks and two phase-separated hemispheres of A and C. The process gives access to unprecedented structural features such as tunable core diameter and control over the Janus balance ranging from dominant A side to equal hemispheres to dominant C side. We demonstrate that this simple one-pot approach can be extended to a range of triblock terpolymers with different block lengths and block chemistries to furnish a library of tailor-made Janus particles with widely tunable physical properties. Such a diversity and simplicity has remained unreachable with our previously developed approach using the controlled cross-linking of bulk morphologies. We show that this new synthetic route can be upscaled to a high volume yield of 10 wt %, thereby enabling large-scale applications. We further demonstrate the effect of the Janus balance on colloidal self-assembly. Janus particles with a dominant hydrophobic and a small hydrophilic patch aggregate into large clusters in water, but merely di- or trimerize in chloroform.     

Electrostatic interaction between soft particles

Electrostatic interaction between two soft particles (i.e., polyelectrolyte-coated particles) in an electrolyte solution is discussed. An approximate analytic expression for the interaction energy between two dissimilar soft spheres is derived by applying Derjaguin's approximation to the corresponding interaction energy between two parallel dissimilar soft plates for the case where the density of fixed charges within the polyelectrolyte layer is low. The obtained expression covers various limiting cases that include hard sphere/hard sphere interaction, spherical polyelectrolyte/spherical polyelectrolyte interaction, soft sphere/spherical polyelectrolyte interaction, soft sphere/hard sphere interaction, and spherical polyelectrolyte/hard sphere interaction.     

Electrostatic interaction between two spherical colloidal particles

Explicit exact analytic expressions are obtained in the form of infinite series for the potential distribution and the potential energy of the electrostatic interaction for the system of two dissimilar spheres in an electrolyte solution on the basis of the linearized Poisson—Boltzmann equation without recourse to Derjaguin's approximation. The leading term of the expression for the interaction energy (the zeroth order approximation) corresponds to the interaction energy that would be obtained if both spheres were ion-penetrable spheres (“soft” spheres). This term is a screened Coulomb interaction due to a simple linear superposition of the unperturbed potentials of the respective spheres, which is proportional to the product of their unperturbed surface potentials. The first-order approximation corresponds to the interaction energy that would be obtained if either sphere were a soft particle (the other being hard). The first-order correction term consists of two sub-terms, each of which is proportional to the square of the unperturbed surface potential of either sphere and does not depend on the unperturbed surface potential of the other sphere, can be interpreted as the interaction between the soft sphere and its image with respect to the hard sphere. This image interaction is attractive if the surface potential of the hard sphere is constant and repulsive if the surface charge density of the sphere is constant. It is shown that Derjaguin's method as well as its extension to the interaction of unequal spheres by Hogg, Healy and Fuerstenau (HHF) is quite a good approximation.     

Electrostatic interactions between amphoteric latex particles and proteins

The electrostatic interactions between amphoteric polymethyl methacrylate latex particles and proteins with different pI values were investigated. These latex particles possess a net positive charge at low pH, but they become negatively charged at high pH. The nature and degree of interactions between these polymer particles and proteins are primarily controlled by the electrostatic characteristics of the particles and proteins under the experimental conditions. The self-promoting adsorption process from the charge neutralization of latex particles by the proteins, which have the opposite net charge to that of the particles, leads to a rapid reduction in the zeta potential of the particles (in other words colloidal stability), and so strong flocculation occurs. On the other hand, the electrostatic repulsion forces between similarly charged latex particles and the proteins retard the adsorption of protein molecules onto the surfaces of the particles. Therefore, latex particles exhibit excellent colloidal stability over a wide range of protein concentrations. A transition from net negative charge to net positive charge, and vice versa (charge reversal), was observed when the particle surface charge density was not high enough to be predominant in the protein adsorption process.     

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.     

Electrostatic interaction between chitosan-modified latex particles and bovine serum albumin

Electrostatic interaction between poly(methyl methacrylate) latex particles with different levels of chitosan modification and bovine serum albumin (BSA) was investigated. The critical flocculation concentration is in the range 5–15 nmol dm⁻³ for these latex products toward added BSA. A series of isothermal equilibrium adsorption experiments shows that the adsorption process is divided into two distinct intervals. Adsorption of BSA on latex particles in intervals I and II is primarily controlled by charge neutralization and hydrophobic interaction, respectively. Intervals I and II can be reasonably described by an empirical parabola equation and the Langmuir isotherm model, respectively. The maximum amount of BSA adsorbed per unit weight of polymer particles was observed at pH ≅ 5. A maximum elution yield of about 80% can be achieved using NaSCN as the elution electrolyte, and NaSCN is more effective in inducing desorption of BSA from the particle surface than NaCl. The chitosan content has very little effect on the interaction between latex particles and BSA. By contrast, the influence of the content of 2,2′-azobis(2-amidinopropane) dihydrochloride, a cationic initiator used in preparing the chitosan-modified latex products, on the BSA adsorption process is significant. Received: 26 March 1999 Accepted in revised form: 3 June 1999     

Electrostatic double-layer interaction between spherical particles inside a rough capillary

Predictions of electrostatic double-layer interaction forces between two similarly charged spherical colloidal particles inside an infinitely long "rough" capillary are presented. A simple model of a rough cylindrical surface is proposed, which assumes the capillary wall to be a periodic function of axial position. The periodic roughness of the wall is characterized by the wavelength and amplitude of the undulations. The electrostatic double-layer interaction force between two spherical particles located axially inside this rough capillary is determined by solving the nonlinear Poisson-Boltzmann equation employing finite element analysis. The effect of surface roughness of the cylindrical enclosure on the interaction force between two particles is extensively studied on the basis of this model. The simulations are carried out for dimensionless amplitudes (amplitude/particle radii) ranging from 0.05 to 0.15 and scaled wavelengths (wavelength/particle radii) ranging from 0.4 to 4.0. The interaction force between the particles is significantly modified by the proximity of the rough capillary wall. Generally, the interaction force for rough capillaries oscillates around the corresponding interaction force in a smooth capillary depending on the magnitudes of the scaled amplitude and wavelength of the roughness. The influence of roughness on the electrostatic interactions becomes more pronounced when the surface potential of the cylinder wall is different from the sphere surface potentials. When the cylinder and the particle surfaces have large potential differences, the axial force experienced by a particle is dominated by the capillary roughness. There are dramatic oscillations of the force, which alternately becomes repulsive and attractive as the particle moves from the crest to the trough of the rough capillary wall. These results suggest that manipulation of colloidal particles in narrow microchannels may be subject to significant force variations owing to the roughness inherent in microfabricated channels etched on metal films.     

Electrostatic effects on nanofiber formation of self-assembling peptide amphiphiles

Self-assembling peptide amphiphile molecules have been of interest to various tissue engineering studies. These molecules self-assemble into nanofibers which organize into three-dimensional networks to form hydrocolloid systems mimicking the extracellular matrix. The formation of nanofibers is affected by the electrostatic interactions among the peptides. In this work, we studied the effect of charged groups on the peptides on nanofiber formation. The self-assembly process was studied by pH and zeta potential measurements, FT-IR, circular dichroism, rheology, atomic force microscopy, scanning electron microscopy and transmission electron microscopy. The aggregation of the peptides was triggered upon neutralization of the charged residues by pH change or addition of electrolyte or biomacromolecules. Understanding the controlled formation of the hydrocolloid gels composed of peptide amphiphile nanofibers can lead us to develop in situ gel forming bioactive collagen mimetic nanofibers for various tissue engineering studies including bioactive surface coatings.     

Film formation with latex particles

The coalescence of latex particles is investigated through small-angle neutron scattering and electron microscopy. The particles are made of a soft polymeric core protected by a hydrophilic membrane, and they are dispersed in water. This dispersion is spread on a substrate, and water is removed to form a dry film. As the membranes of neighboring particles come into contact, they may break up to allow fusion of the particle cores. This is found to occur when the membranes are made of short-chain surfactant molecules; then all hydrophilic material is expelled to the film surface or to large isolated lumps. Alternatively, the membranes may remain until the film is completely dry; this is found to occur when they are made of hydrophilic polymers which are grafted onto the core. Hence, the fusion of particles is controlled by the connectivity of membranes.This work used the neutron beams of ILL in Grenoble and LLB in Saclay     

The formation of penta-twinned gold particles

The formation of gold particles containing five radially arranged twin boundaries (called penta-twinned particles) has been investigated by means of high-resolution transmission electron microscopy. The particles were produced by evaporation of gold in helium at low pressure (0.5 kPa). The processes involved in the formation of the particles are deduced from the experimentally observed size distribution, the characteristic particle morphologies and the frequency of different particle structures. The penta-twinned particles are suggested to be formed by the collision and rearrangement of small single crystals in the helium atmosphere.     

Electrostatic energy of vacancy formation in crystals of polar molecules

An exact expression is derived for the electrostatic energy of a crystal of polar polarizable molecules containing an unrelaxed vacancy. The energy exceeds twice the electrostatic binding energy per molecule L by a polarization energy term, when changes in dielectric response due to the vacancy are ignored. The electrostatic energy of a molecule at a crystal surface is estimated from surface and bulk polarization energy measurements. It can be as high as 1.5 L. making the electrostatic energy of vacancy formation significantly less than L. the value for non-polarizable molecules. This may help to explain anomalous results for certain plastic crystals.     

Thermodynamics of the adsorption-accompanied formation of new-phase particles

A thermodynamic model was proposed to describe the process of the formation of inorganic nanoparticles in a solution containing well-adsorbable organic compounds. An equation for the critical nucleus size was derived and analyzed. It was shown that intense adsorption of organic compounds leads to a strong decrease in the critical nucleation size of the forming nanoparticles.     

Single-particle tracking of janus colloids in close proximity

We describe algorithms and an experimental method based on differential interference contrast microscopy to discriminate optically anisotropic colloidal spheres under situations where diffraction owing to their close proximity causes overlapping images. The data analysis is applied to modulated optical nanoprobes (MOONs) that are coated with metal on one hemisphere. These methods enable single-particle tracking of rotation in addition to translation not only in concentrated suspensions but also in dilute suspensions when particles come into transient hydrodynamic contact. An illustrative example is given.     

Spontaneous formation of platinum particles on electrodeposited palladium

Pt particles have been spontaneously formed on the electrochemically deposited Pd layer on ITO substrate. SEM reveals that the Pt particles spontaneously formed on the Pd surface are uniformly distributed. The as-prepared material (denotes as Pt–Pd/ITO) as electrode shows a higher activity for ethanol oxidation than that of Pd/ITO. The mechanism is tentatively explained as that the H dehydrogenated from ethanol on Pt can efficiently spillover to the underneath Pd, resulting in an enhanced kinetics. The rapid removal of H on the Pt active sites accelerates the further adsorption of ethanol and dehydrogenation (oxidation). This work demonstrates a strategic method to spontaneous prepare small particles on the reductive species-containing substrates. The metal ion with a higher standard potential than that of hydrogen is theoretically possible to be spontaneously reduced to metal on hydrogenated Pd.     

Preparation and formation mechanism of wood-block-like calcite particles

Pure calcite crystal with different morphologies such as wood-block and spherical aggregates were prepared by a precipitation reaction in the presence of citric acid. The as-prepared products were characterized with scanning electron microscope (SEM) and X-ray diffraction (XRD). The results showed that citric acid obviously influenced the formation of precipitates and the morphology of final products. The formation mechanism of wood-block-like particles was proposed according to theoretical deduction and the proposed growth mode.     

Hydroplasticization effect in structured latex particles film formation

Series of emulsion copolymers with structured particles were synthesized comprising copolymerized acrylic or methacrylic acid in the outer layer. All samples were based on particles, containing identical cores slightly crosslinked by allyl methacrylate and variable shells, weight ratio core/shell being constant 1/1. Each sample contained 10 wt% HEMA in the shell to achieve the film crosslinkability. In both series samples with different hardness and polarity (varible styrene/butyl acrylate ratio) of the shell layer were prepared. It was shown that the extent of particle swelling and hydroplasticization depends not only on the content of dissociated carboxylic groups, but also on the composition and crosslinking of the rest of polymer chain i.e. on its polarity and rigidity and on the origin of carboxylic groups. The effect of dissociated carboxylic groups on lowering the minimum film forming temperature was much more pronounced if the polymer chains were more polar.     

Network formation during vinyl polymerization via inorganic particles

Methyl methacrylate containing a small amount of 4-methacryloxyethyl trimellitate anhydride (1.0–1.5 wt %) yielded an insoluble network when polymerized in the presence of various inorganic powders, such as lithium aluminum silicate. This unexpected result was obtained for polymerization initiated either by exposure to γ-rays or by heating with azobisisobutyronitrile. In contrast, polymerization in the absence of inorganic powder gave the expected soluble products. Therefore, it is concluded that the inorganic particles play a role in network formation. In order to account for network formation even in a supernatant layer of clear monomer, i.e., above the centrifuged sedimentation volume of the powder, it is suggested that the monomer reacts on the surface of particles to form a diffusive crosslinking agent.