Simulation of structure formation in disperse systems

The main rules governing the redistribution of microparticles in disperse systems were revealed by computer simulation. These rules predetermine the character and properties of the structure being formed, which opens up the possibility to control the properties of disperse materials, including sorbents and catalysts based on high-dispersity solid phases.     

Simulation of ESR spectra of metal nanoparticle aggregates

Aggregates of polydisperse particles characterized by the preset values of the fractal dimensions and prefactor are built with the use of a special algorithm. ESR spectra of such aggregates are calculated in the self-consistent field approximation. It is shown that, even in sufficiently large aggregates with fractal structure, spectra greatly depend on the character of packing of large and small particles in the aggregate.     

Experimental manifestations of the correlation between the local structure of silver nanoparticle aggregates and their absorption spectra

Experimental data testifying in favor of the correlation between electrodynamic characteristics of fractal aggregates and the structure of the local environment of particles are discussed. One of the possible variants of the control of local structure of disordered silver nanoparticle aggregates placed into a polymer matrix via a significant decrease in its volume is proposed and realized. An indirect method for the detection of the variations in the local structure with the use of plasmon absorption spectra is proposed and substantiated. The evolution of local anisotropy of the environment of the particles of loose aggregates during their compaction is studied. Differences in the absorption spectra of silver nanoaggregates in hydrosols with various concentrations of water-soluble polymer that determine the properties of the local structure of aggregates are discussed, these differences being manifested upon the formation of fragments with quasi-ordered arrangement of particles and in the absence of such fragments.     

Structures of DNA-linked nanoparticle aggregates

The room-temperature structure of DNA-linked gold nanoparticle aggregates is investigated using a combination of experiment and theory. The experiments involve extinction spectroscopy measurements and dynamic light scattering measurements of aggregates made using 60 and 80 nm gold particles and 30 base-pair DNA. The theoretical studies use calculated spectra for models of the aggregate structures to determine which structure matches the observations. These models include diffusion-limited cluster-cluster aggregation (DLCA), reaction-limited cluster-cluster aggregation (RLCA), and compact (nonfractal) cluster aggregation. The diameter of the nanoparticles used in the experiments is larger than has been considered previously, and this provides greater sensitivity of spectra to aggregate structure. We show that the best match between experiment and theory occurs for the RLCA fractal structures. This indicates that DNA hybridization takes place under irreversible conditions in the room-temperature aggregation. Some possible structural variations which might influence the result are considered, including the edge-to-edge distance between nanoparticles, variation in the diameter of the nanoparticles, underlying lattice structures of on-lattice compact clusters, and positional disorders in the lattice structures. We find that these variations do not change the conclusion that the room-temperature structure of the aggregates is fractal. We also examine the variation in extinction at 260 nm as temperature is increased, showing that the decrease in extinction at temperatures below the melting temperature is related to a morphological change from fractal toward compact structures.     

Exciton dynamics of GaSe nanoparticle aggregates

Time-resolved and static spectroscopic results on GaSe nanoparticle aggregates are presented to elucidate the exciton relaxation and diffusion dynamics. These results are obtained in room-temperature TOP/TOPO solutions at various concentrations. The aggregate absorption spectra are interpreted in terms of electrostatic coupling and covalent interactions between particles. The spectra at various concentrations may then be interpreted in terms of aggregate distributions calculated from a simple equilibrium model. These distributions are used to interpret concentration-dependent emission anisotropy kinetics and time-dependent emission spectral shifts. The emission spectra are reconstructed from the static emission spectra and decay kinetics obtained at a range of wavelengths. The results indicate that the aggregate z axis persistence length is about 9 particles. The results also show that the one-dimensional exciton diffusion coefficient is excitation wavelength dependent and has a value of about 2 x 10(-5) cm(2)/s following 406 nm excitation. Although exciton diffusion results in very little energy relaxation, subsequent hopping of trapped electron/hole pairs occurs by a Forster mechanism and strongly red shifts the emission spectrum.     

Formation of submicrometer-scale gold nanoparticle aggregates and their self-organization into “supracrystals”

Nearly spherical, submicrometer-scale gold nanoparticle aggregates about 250 nm in diameter are formed directly from an HAuCl₄-p-phenylenediamine aqueous solution at room temperature. The as-formed aggregates were characterized by various techniques, including scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, and their formation mechanism is discussed. It is very interesting that “supracrystals” can be produced with the use of such gold aggregated nanoparticles. The text was submitted by the author in English.     

Simulation of structure-related mechanical characteristics of disperse systems under dynamic conditions

Results of simulating the structure-related rheological properties of disperse systems under dynamic conditions are reported. Computer-simulation data on the influence of some parameters on the viscosity and structure-related characteristics of dispersions agree with analytical dependences and experimental results. Problems concerning the dynamic equilibrium of dispersions subjected to shear deformation, as well as disturbance of equilibrium, are considered. Results of computer simulating the percolation characteristics of dispersion microstructure under dynamic conditions are reported.     

Computer simulation of the structure formation and disruption processes in the solid-liquid disperse systems

Results of the computer simulation of solid-liquid disperse systems are reported. Main regularities of the formation and disruption of the internal structure of such systems are demonstrated under static and dynamic conditions (including the combination of shear and orthogonal vibrations). The effect of the character of the interaction of dispersed phase particles, as well as of a number of parameters concerning the type and intensity of external actions on the behavior of disperse systems is shown. The possibility of the regulation of the character of microstructure and rheological properties of disperse systems by the combination of shearing and orthogonal vibrations is illustrated.     

Radiation-thermal transformations and distillation of lipids and disperse systems on their basis

The radiation-thermal transformations of rape oil under conditions of postradiation distillation, radiation heating, and electron-beam distillation with auxiliary heating were studied. Diesel hydrocarbons were predominant in the condensate distilled off upon combined heating. The concentration of oxygen atoms decreased as a result of radiation-induced decarboxylation and dehydration processes. The irradiation of oil as a constituent of disperse systems facilitated purposeful deoxygenation, a decrease in the viscosity, and an increase in the yield of the gasoline fraction.     

Photoisomerization of azobenzene derivatives confined in gold nanoparticle aggregates

Photoisomerization is an important reaction that confers photoresponsive functionality on nanoparticles. Although photoisomerization of molecules forming self-assembled monolayers on two-dimensional surfaces or three-dimensional clusters has been studied, a detailed picture of interactions of molecules undergoing isomerization with nanoparticles is not available. In this paper, we report on the photoisomerization of azobenzene derivatives spatially confined in gold nanoparticle (AuNP) aggregates. AuNP aggregates allow us to simultaneously probe the structural changes of molecules via surface-enhanced Raman spectroscopy (SERS) and the accompanying changes in interparticle interactions via surface plasmon couplings. AuNP aggregates are formed by the adsorption of synthesized azobenzene-derivatized sulfides (Az) onto the surfaces of AuNPs. The photoisomerization of the adsorbed Az from trans to cis by excitation at 365 nm causes the AuNPs to move close to each other in the aggregates, leading to a redshift of the surface plasmon coupling band in the UV-vis spectra and a concomitant rise in SERS intensity. SERS spectra reveal that the vibrational modes containing the N=N stretching character redshift upon irradiation, suggesting that the N=N bond is significantly weakened when Az is in the cis form in the AuNP aggregates. The weakening of the N=N bond is attributed to the interaction of the N=N bond, which is more exposed to the outside in the cis conformation, with the nearby AuNPs that have come closer by the isomerization of adsorbed Az. We find that backisomerization from cis to trans occurs much faster in the AuNP aggregates (k = 1.9 × 10(-2) min(-1)) than in solution (k = 1.3 × 10(-3) min(-1)) because of the reduced N=N bond order of cis-Az in the aggregates.     

Surface active properties of polyoxyethylene macromonomers and their role in radical polymerization in disperse systems

Conventional dispersion polymerization and copolymerization of low-molecular weight (conventional) unsaturated monomers allows preparation of monodisperse and micronsize polymer particles. A similar behavior can be found in the surfactant-free dispersion polymerization of non-traditional vinyl monomers, unsaturated macromonomers. The latter systems allow preparation of random, comb-like, star-like and graft copolymers as well. An interesting alternative arises with the use of amphiphilic reactive macromonomers that contain a polymerizable group and aggregate into an organized structure -- a micelle. Under such conditions the high rate of polymerization and ultrafine (microparticles) polymer dispersions are generated. Thus, the surface-active macromonomers promote the formation of micelles and polymer growth within the main reaction loci -- polymer particles. Furthermore, the surface-active compounds can be formed during the copolymerization of hydrophilic macromonomer and hydrophobic low-molecular weight comonomer. The reactive surface-active oligomeric radicals are incorporated into the polymer matrix or the particle surface layer, which prevents them from subsequent migration. Besides, the covalently bound surface-active groups at the particle surface strongly increase the colloidal stability of final polymer dispersion. This article presents a review of the current literature in the field of the surfactant-free dispersion polymerization of the polyoxyethylene unsaturated macromonomers. Besides a short introduction into some kinetic aspects of radical polymerization of traditional monomers in homogeneous and disperse systems, we mainly focus on the organized aggregation of amphiphilic polyoxyethylene macromonomers, the characterization of amphiphilic graft copolymers and their aggregation properties, and radical copolymerization of polyoxyethylene macromonomers. We discuss the birth and growth of chains, the transfer of reaction loci from the continuous phase to polymer particles, the diffusion-controlled termination, association of amphiphilic reaction by-products, the particle growth by agglomeration, the particle nucleation, the deactivation of polymer chain growth and the colloidal stability. Effects of initiator type and concentration, the surface activity of macromonomer, the macromonomer type and concentration, temperature, additives and the type of continuous phase on the kinetics of polymerization, and colloidal parameters of the reaction system are also evaluated. Variation of the polymer coil density, the polymer-polymer interaction, and polymer-solvent interaction with the molecular weight, diluent and method (light scattering, the size exclusion, etc.) are discussed. Polymerization of macromonomers provides regularly branched polymers with varied branching density. Since both the degree of polymerization and the length of branches may be varied, polymeric materials with specific properties can be prepared.     

Dependence of fragmentation behavior of colloidal aggregates on their fractal structure

The fragmentation dynamics of aggregate of non-Brownian particles in shear flow is investigated numerically. The breakup behaviors of aggregates having the same connectivity but the different space-filling properties are examined. The Lagrangian particle simulation in a linear flow field is performed. The effect of surrounding fluid on the motion of multiple particles is estimated by Stokesian dynamics approach. The inter-particle force is calculated from the retarded van der Waals potential based on the Lifshitz theory. The results obtained in this work indicate that the fragmentation behavior of colloidal aggregates depends on their fractal structure. However, if the resultant aggregate size is smaller than the critical one, the fragmentation behavior shows the universality regardless of their original structure. Furthermore, the restructuring of aggregate in shear flow and its effect on the fragmentation process are also discussed.     

Kinetics of the crystallization of nanostructured disperse systems

The kinetics of the formation of crystalline colloidal structures upon the self-organization of ensembles of spherical nanoparticles in model metal lyosols is studied by the Brownian dynamics using realistic potentials of interparticle interaction. The effect of single factors (parameters of dispersed phase particles and characteristics of dispersion medium), as well as their set and arbitrary combinations on the rate of crystallization of sols and regularities of this process are analyzed.     

Hydrodynamic method for regulating pH during electrical purification of natural disperse systems from heavy metals

The conditions for the hydrodynamic regulation of pH in disperse systems, which influence the surface charge of disperse particles, as well as the solubility of salts of heavy metals and their electrostatic adsorption and electromigration mobility are theoretically substantiated. Experimental verification of the developed theoretical model performed based on the example of a kaolinite dispersion confirmed the validity of the employed theoretical assumptions. The developed method makes it possible to remove cadmium ions almost completely over 24 h, which is, in principle, impossible without pH regulation even after a long-term treatment.     

Simulation of electrical percolation in disperse systems at a small difference between specific conductivities of components

The possibility of development of electrical percolation upon variations in the contents of components in a disperse system has been studied in terms of Bruggeman’s ideas of an effective medium and the continual model of the electrical conductivity of composite materials. The case of a small difference between the specific conductivities of a dispersed phase K_m and a dispersion medium K_d has been considered. It has been shown that the dependence of electrical conductivity K of a disperse system on volume fraction p of a wellconducting component may exhibit an inflection in the vicinity of the p* value, which depends on the K_m/K_d ratio. Different methods for determining the p* value have been described. At low ratios between the conductivities of a dispersed phase and a dispersion medium, the inflection is very weakly pronounced. However, the representation of the K(p) dependences in semilog coordinated substantially facilitates its identification.     

Stimuli-responsive disassembly of nanoparticle aggregates for light-up colorimetric sensing

Controlled assembly of nanomaterials has been the focus of much research. In contrast, controlled disassembly has not received much attention, even though both processes have been shown to be important in biology. By using a Pb2+-dependent RNA-cleaving DNAzyme, we demonstrate here control of the disassembly of gold nanoparticle aggregates in response to Pb2+. In the process, we show that nanoparticle alignment plays an important role in the disassembly process, with the tail-to-tail configuration being the most optimal, probably because of the large steric hindrance of other configurations. The rate of disassembly is significantly accelerated by using small pieces of DNA to invade the cleaved substrate of the DNAzyme. Investigation of such a controlled disassembly process allows the transformation of previously designed "light-down" colorimetric Pb2+ sensors into "light-up" sensors.