Large effect of polydispersity on defect concentrations in colloidal crystals

We compute the equilibrium concentration of stacking faults and point defects in polydisperse hard-sphere crystals. We find that, while the concentration of stacking faults remains similar to that of monodisperse hard-sphere crystals, the concentration of vacancies decreases by about a factor of 2. Most strikingly, the concentration of interstitials in the maximally polydisperse crystal may be some six orders of magnitude larger than in a monodisperse crystal. We show that this dramatic increase in interstitial concentration is due to the increased probability of finding small particles and that the small-particle tail of the particle size distribution is crucial for the interstitial concentration in a colloidal crystal.     

Interlayer stacking disorder in zeolite beta family: a Raman spectroscopic study

Raman spectroscopy has been applied to series of BEA- and BEC-type samples differing from each other in size, Si/Al ratio and polymorph percentage in order to analyse the effect of interlayer stacking arrangements on the vibrational modes of zeolite beta. The Raman peaks observed in the spectral range 250-550 cm-1 were assigned to the rings building the basic (001)-layer and to those linking the adjacent layers in zeolite beta. It is shown that the intensity ratio rho between the Raman signals at 314 and 343 cm-1 is most sensitive to the degree of periodicity faults along the c direction. A larger value of rho indicates a larger size of polymorph stacking sequence, i.e. improvement of the stacking faultlessness. The interlayer stacking disorder and the degree of connectivity point defects are higher in nanosized zeolite beta than in micron-sized crystals. The Al content influences the concentration of defected SiOH groups, but is less important for the interlayer stacking sequences in colloidal zeolite beta.     

Experiments on convective in-plane orientation of monolayers

Flow-compressed monolayers of stiff rod-like polymers (poly-glutamates) mixed with oleophilic tracer dyes showed significant in-plane dichroism of dye absorption at the water surface, indicating lateral ordering of polyglutamate-rods more or less “perpendicular” to flow direction. During LB-deposition the lateral orientation of dye- and polymer-molecules was maintained, in-plane orientation of the LB-film was thus depending on substrate-orientation relative to flow-direction. Identical monolayers compressed in a regular Langmuir-trough (with moving barrier) showed no large-scale ordering at the water surface, but during film-transfer polymer matrix and dye were oriented in dipping direction, irrespective of substrate orientation relative to compression direction.     

DEFORMATION OF SOFT COLLOIDAL CRYSTALLINE STRUCTURE — THEORETICAL CONSIDERATIONS AND EXPERIMENTAL EVIDENCES BY SYNCHROTRON SMALL-ANGLE X-RAY SCATTERING ON TENSILE STRETCHED POLYMERIC LATEX FILM

 Films obtained via drying a polymeric latex dispersion are normally colloidal crystalline where latex particles are packed into a face centered cubic (fcc) structure. Different from conventional atomic crystallites or hard sphere colloidal crystallites, the crystalline structure of these films is normally deformable due to the low glass transition temperature of the latex particles. Upon tensile deformation, depending on the drawing direction with respect to the normal of specific crystallographic plane, one observes different crystalline structural changes. Three typical situations where crystallographic c-axis, body diagonal or face diagonal of the fcc structure of the colloidal crystallites being parallel to the stretching direction were investigated. Tilting angle and d-spacing of several crystallographic planes as a function of draw ratio at each situation were derived. Experimental evidences for such relationships were also given by considering in-situ synchrotron small angle X-ray scattering data of a typical latex film during stretching. It turns out that the experimental results are fully in accordance with the mathematical calculations.     

DEFORMATION OF SOFT COLLOIDAL CRYSTALLINE STRUCTURE-THEORETICAL CONSIDERATIONS AND EXPERIMENTAL EVIDENCES BY SYNCHROTRON SMALL-ANGLE X-RAY SCATTERING ON TENSILE STRETCHED POLYMERIC LATEX FILM

Films obtained via drying a polymeric latex dispersion are normally colloidal crystalline where latex particles are packed into a face centered cubic (fcc) structure.Different from conventional atomic crystallites or hard sphere colloidal crystallites,the crystalline structure of these films is normally deformable due to the low glass transition temperature of the latex particles.Upon tensile deformation,depending on the drawing direction with respect to the normal of specific crystallographic plane,one ...     

Conventional optical microscopy of colloidal suspensions

Optical microscopy can resolve detail at the larger end of the colloidal length scale, and to image suspensions at an individual particle level of resolution would allow the investigation of local behaviour in a way denied to the established scattering techniques. However, to achieve high-contrast single-particle resolution in dense suspensions that are thick enough to show behaviour the same as would be expected in the bulk is not a trivial exercise. We build on established advanced techniques of the conventional (i.e. non-confocal) light microscopy of phase objects to develop a suitable experimental protocol. Furthermore, we demonstrate the effectiveness of this protocol by means of an 'atlas' of the hard-sphere crystalline solid (where random stacking results in many complex facets), which should serve as a compendium for future study.     

A new multiphasic buffer system for sodium dodecyl sulfate-polyacrylamide gel electrophoresis of proteins and peptides with molecular masses 100,000-1000, and their detection with picomolar sensitivity.

A novel multiphasic buffer system for high resolution sodium dodecyl sulfate-polyacrylamide gel electrophoresis of dansylated and nondansylated proteins/peptides in the relative molecular mass (Mr) range of 100,000-1000 is described. The system, based on Jovin's theory of multiphasic zone electrophoresis, allows complete stacking and destacking of proteins/peptides within the above Mr range. The buffer system uses Bicine and sulfate as trailing and leading ion, respectively, and Bistris and Tris as counter ions in the stacking and separating phase, respectively. Through selection of two different counter ions--the characteristic feature of the present ionic system--the stacking limits of a multiphasic buffer system can be further widened, thus making it applicable to gel electrophoresis of a larger spectrum of rapidly migrating species, such as sodium dodecyl sulfate-proteins/peptides and nucleic acids, than has been possible previously. Highly sensitive detection methods for proteins as well as for polypeptides down to approximately Mr 1000 are described. Dansylated proteins/peptides were detected by their fluorescence either directly within the gel or following electroblotting into anion-exchange or polyvinylidene difluoride membranes. The latter procedure resulted in detection sensitivities of approximately 1 ng. Nondansylated proteins/peptides were either detected within the gel by colloidal Coomassie staining or by electroblotting into polyvinylidene difluoride membranes, followed by colloidal gold staining. Prior to both staining procedures the proteins/peptides were pretreated with glutardialdehyde in the presence of borate at near neutral pH values to generate protein/peptide polymers of poor solubility. For a given pH the efficiency of the latter procedure was significantly influenced by the nature of the buffer ion used in the fixation buffer. In contrast to conventional fixation procedures even small polypeptides (Mr 1000) were immobilized and approximately 15 ng and 0.75 ng could be detected after colloidal Coomassie and colloidal gold staining, respectively.     

Diffusing wave spectroscopy study of the colloidal interactions occurring between casein micelles and emulsion droplets: comparison to hard-sphere behavior

Understanding the underlying processes that govern interparticle interactions in colloidal systems is fundamental to predicting changes in their bulk properties. In this paper we discuss the colloidal behavior of casein micelles and protein-stabilized fat globules individually and in a mixture. The colloidal interactions were observed by transmission diffusing wave spectroscopy. The turbidity parameter, l*, and the diffusion coefficients of the samples studied were measured experimentally and compared to the theoretically calculated parameters for a hard-sphere system. The light scattering properties of casein micelles (volume fraction phi = 0.1-0.2) dispersed in milk permeate showed no deviation from the theoretically predicted model. Whey protein isolate (WPI)-stabilized emulsions (phi = 0.025-0.1) prepared either in milk permeate or in 5 mM imidazole buffer at pH 6.8 showed a behavior identical to that of the hard-sphere model. Similarly to the WPI-stabilized fat globules, the sodium caseinate (NaCas)-stabilized emulsions (phi = 0.025-0.1) prepared in milk permeate also showed resemblance to the theory. In contrast, NaCas-stabilized emulsions prepared in 5 mM imidazole buffer exhibited some discrepancy from the theoretically calculated parameters. The deviation from theory is attributed to the enhanced steric stabilization properties of these droplets in a low ionic strength environment. When recombined milks made from concentrated milk and WPI- and NaCas-stabilized droplets prepared in permeate (phi = 0.125-0.2) were studied, the experimental data showed a significant deviation from the theoretical behavior of a hard-sphere model due to mixing of two different species.     

Steric stabilization of spherical colloidal particles: Implicit and explicit solvent

We present the results of Monte Carlo simulations and density functional theory treatment of interactions between spherical colloidal brushes both in implicit (good) solvent and in an explicit polymeric solution. Overall, theory is seen to be in good agreement with simulations. We find that interactions between hard-sphere particles grafted with hard-sphere chains are always repulsive in implicit solvent. The range and steepness of the repulsive interaction is sensitive to the grafting density and the length of the grafted chains. When the brushes are immersed in an explicit solvent of hard-sphere chains, a weak mid-range attraction arises, provided the length of the free chains exceeds that of the grafted chains.     

Determination of colloid size by 2-D optical detection of laser induced plasma

A new method is presented for the size determination of aquatic colloids, which is based on the selective plasma generation (breakdown) on individual particles by a focused laser pulse. For this purpose, the breakdown events of particles are monitored by 2-D optical detection instead of commonly used photoacoustic detection. The length of the breakdown volume determined for the 2-D projection of spatially distributed plasma events generated on colloidal particles is proportional to the particle size. Based on this fact, an average diameter of a given colloidal dispersion can be determined. The method is calibrated by well-characterised hard-sphere polystyrene reference particles of different diameters.     

Conformal solutions: which model for which application?

Shukla, K.P., Luckas, M., Marquardt, H. and Lucas, K., 1986. Conformal solutions: which model for which application? Fluid phase Equilibria, 26: 129–147.Various forms of conformal solution theories are discussed and compared to computer simulations for Lennard-Jones mixtures under rather different conditions. It is found that the simple VDW1-theory is applicable when the ratio of the energy parameters is 1.5, and the ratio of the size parameters is 1.1. The same region of applicability is found for the Mean Density Approximation (MDA). Two models using explicitly known properties of the hard-sphere mixture, i.e., the Hard-Sphere Expansion theory (HSE) and a particular version of the Hard-Sphere Perturbation theory (WCA-LL-GH), have also been tested. The HSE is not complete unless a consistent method is found to calculate the hard-sphere diameter. The overall best results are found for WCA-LL-GH. While WCA-LL-GH turns out to be limited in applicability to size ratios smaller than 1.3 and appears to be inaccurate for particular cases, it is definitely the best model available today.     

Melt processing of hexa-peri-hexabenzocoronene on the water surface

A discotic polycyclic aromatic hydrocarbon, hexa-peri-hexabenzocoronene, was oriented by slow cooling from the isotropic phase on a water surface as a film. For melt processing at low temperatures, an HBC derivative with long swallow-tailed alkyl side chains was chosen. The supramolecular organization in the resulting thin layer was investigated by electron microscopy. In high-resolution mode, the structural study showed large domains in which the columnar structures were oriented uniaxially with an edge-on arrangement of the hydrophobic molecules. The length of the stacks exceeded several hundred nanometers without obvious defects. The small-area analysis by TEM allowed the direct visualization of individual packed molecules. Electron diffraction revealed a high in-plane order of the columnar superstructures in which the discs were tilted by ca. 40° with respect to the stacking direction. This is the first example of a discotic system melt processed on the water surface yielding a pronounced order.     

Fabrication of 2D ordered films of tobacco mosaic virus (TMV): processing morphology correlations for convective assembly

Biological colloids, and in particular viruses, have demonstrated substantial potential as scaffolds for nanoparticle arrays. However, the large-area, low-cost, and rapid assembly of viruses, such as by traditional colloidal processing techniques, is not well-established. Systematic exploration of processing space (virus concentration, assembly speed, and substrate surface energy) for the convective assembly method enables the fabrication of films of rod-shaped viruses (tobacco mosaic virus, TMV) with a high degree of long-range order. Monolayer assemblies several centimeters in length are comprised of TMV aligned parallel to the direction of assembly. Increasing TMV concentration and reducing assembly speed resulted in well-ordered viral layering ( N = 2 to N = 12); however, the top virus layer exhibits varying degrees of in-plane disorder.     

Three-dimensional structure and defects in colloidal photonic crystals revealed by tomographic scanning transmission X-ray microscopy

Self-assembled colloidal crystals have attracted major attention because of their potential as low-cost three-dimensional (3D) photonic crystals. Although a high degree of perfection is crucial for the properties of these materials, little is known about their exact structure and internal defects. In this study, we use tomographic scanning transmission X-ray microscopy (STXM) to access the internal structure of self-assembled colloidal photonic crystals with high spatial resolution in three dimensions for the first time. The positions of individual particles of 236 nm in diameter are identified in three dimensions, and the local crystal structure is revealed. Through image analysis, structural defects, such as vacancies and stacking faults, are identified. Tomographic STXM is shown to be an attractive and complementary imaging tool for photonic materials and other strongly absorbing or scattering materials that cannot be characterized by either transmission or scanning electron microscopy or optical nanoscopy.     

Exact solution of a RNA-like polymer model on the Husimi lattice

We investigate a two-tolerant polymer model on the square Husimi lattice, which aims at describing the properties of RNA-like macromolecules. We solve the model in a numerically exact way, working out the grand-canonical phase diagram, both with and without taking into account the stacking effect. Besides a nonpolymerized phase, we observe two different polymerized phases characterized by a lower or higher density of doubly visited lattice bonds. The system exhibits three qualitatively different regimes, as a function of the monomer chemical potential. Below some T1 temperature and above some T2 temperature, the transition to the nonpolymerized phase is continuous, whereas, in the (T1,T2) temperature range, the transition is first order. In the dilute-solution limit, the high temperature regime corresponds to a swollen ("coil") state, the intermediate regime to a moderately collapsed ("molten") state, with a small fraction of paired segments, and the low temperature regime to an almost fully paired ("native") state. The molten state ends in a tricritical (Theta-like) transition at high temperature and in a critical end point at low temperature. Upon increasing the stacking energy parameter, the temperature range of the molten state turns out to be progressively reduced but never completely removed.     

Stacking fault structure in shear-induced colloidal crystallization

We report measurements of the spatial distribution of stacking faults in colloidal crystals formed by means of an oscillatory shear field at a particle volume fraction of 52% in a system where the pair potential interactions are mildly repulsive. Stacking faults are directly visualized via confocal laser scanning microscopy. Consistent with previous scattering studies, shear orders the initially amorphous colloids into close-packed planes parallel to the shearing surface. Upon increasing the strain amplitude, the close-packed direction of the (111) crystal plane shifts from an orientation parallel to the vorticity direction to parallel the flow direction. The quality of the layer ordering, as characterized by the mean stacking parameter, decreases with strain amplitude. In addition, we directly observe the three-dimensional structure of stacking faults in sheared crystals. We observe and quantify spatial heterogeneity in the stacking fault arrangement in both the flow-vorticity plane and the gradient direction, particularly at high strain amplitudes (gamma> or =3). At these conditions, layer ordering persists in the flow-vorticity plane only over scales of approximately 5-10 particle diameters. This heterogeneity is one component of the random layer ordering deduced from previous scattering studies. In addition, in the gradient direction, the stacking registry shows that crystals with intermediate global mean stacking probability are comprised of short sequences of face-centered cubic and hexagonal close-packed layers with a stacking that includes a component that is nonrandom and alternating in character.     

Hard-sphere perturbation theories applied to concentrated colloidal dispersions

The application of hard-sphere perturbation theory to monodisperse colloidal dispersions is examined in detail. Osmotic pressures and radial distribution functions are calculated by these theories and compared with exact Monte Carlo results to allow a critical assessment of the accuracy of the theories. For most conditions encountered in practical colloidal dispersions the predictions of zero- and firstorder perturbation theories are very accurate.     

Computational simulation of the molecular ordering of 3,6-bis(4-butylphenyl)[1,2,4,5]tetrazine at the phase transition temperatures

Computational simulation of 3,6-bis(4-butylphenyl)[1,2,4,5]tetrazine was carried out. Based on the energy of intermolecular interactions of dimers was determined the probability of each configuration (stacking, in-plane and terminal). It was revealed that in the 3,6-bis(4-butylphenyl)[1,2,4,5]tetrazine dimers the sliding of the molecules relative to each other is energetically allowed in the narrow range of distances that provides retaining orientation of the molecules in the mesophase. The relationship between the translational rigidity of the molecules in the dimer and nematic properties was revealed.     

Small-angle Hv light scattering from deformed spherulites with orientational fluctuation of optical axes

Mathematical evaluation was done for small-angle light scattering from disordered spherulites under Hv polarization conditions. The calculation was carried out for a two-dimensional deformed spherulite whose major optical axes are oriented at 0 or 45° with respect to the radial direction. The calculated results were compared with the scattering patterns observed for polypropylene (PP) spherulites, whose optical axes are oriented parallel to the radial direction, and poly(butylene terephthalate) (PBT) spherulites, whose optical axes are oriented at 45° with respect to the radial direction. The degree of disorder for PBT was much larger than that for PP. By selecting a parameter associated with the degree of disorder of the optical axes with respect to the radial direction, the patterns calculated as a function of draw ratios were in good agreement with the observed patterns, which changed from four leaves to streaks extended in the horizontal direction. Through a series of observed and calculated patterns, it turns out that an increase in the disorder under the deformation process occurs drastically even for perfect spherulites in an undeformed state.