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.     

Features of colloidal disperse structure formation in petroleum bitumen

Temperature-modulated differential scanning calorimetry has been employed to analyze the structure-related thermal properties of petroleum bitumen. This method enables one to distinguish between “order–disorder” and glass transitions, thereby making it possible to monitor and identify structure-related phase transformations, the signals from which are invisible or overlapped in the thermograms of conventional differential scanning calorimetry. Bitumen has been shown to be a colloidal disperse system only under certain temperature–time conditions. Its dispersed phase may be represented by aggregates of two types with colloidal sizes. Saturated hydrocarbons form a solid crystalline phase in accordance with the regularities of first-order structural phase transitions and nucleation mechanism of phase separation. Asphaltenes and resins form a solid amorphous phase for a relatively long time as a result of a structural relaxation glass transition by the spinodal mechanism of phase separation.     

Computer simulation on the energy transfer processes in allophycocyanins

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Aggregation processes in self-associating polymer systems: Computer simulation study of micelles in the superstrong segregation regime

We present the results of extensive molecular dynamics and Monte Carlo studies of the self-organization in the solution of short polymer chains with strongly attracting head groups at their end. The formation of micelles (multiplets) is studied in detail. Both two dimensional (2d) and three-dimensional (3d) systems are considered. The off-lattice and lattice models under study incorporate physical factors which control micelle structure and growth in the so-called superstrong segregation regime. These factors include (i) conformational effects associated with short-range excluded-volume interaction between the tails of flexible-chain molecules and (ii) very strong attraction of head groups. Our computer simulations of 3d micelles, constructed a priori from chains with strong attraction of head groups (with the characteristic energy ≈ 10 k_BT), show that size and shape of the micellar core depends crucially on the radius r_c of the interaction of head-groups. If the value of r_c is comparable with chain length, then micelles of nearly spherical shape emerges. The decrease of r_c can induce a sharp polymorphic transition from the micellar core which is spheric in shape to a disk-like (bilayer-shaped) aggregate. Such molecular organization differs from the commonly held notion of a radially symmetric micellar core. On the other hand, these findings fall into line with a recent theory of the super strong segregation regime. When the starting configuration is a random one (i.e., no micelles were a priori formed) the type of final microstructures, emerging as a result of micellization in the superstrong segregation regime, also depends essentially on the radius of head-head attraction. In the case of three-dimensional systems and/or short range attractive potentials we always obtain many small spherically shaped aggregates which, once formed at initial stages of micellization, remain stable for all time scales. Such a behavior is due to both the strong head-head attraction and the screening (repulsive) action of micellar shells creating insurmountable potential barriers. As a result, we deal with kinetically “frozen-in” microstructures which are not reversible and cannot exchange molecules with one another. In dense systems, we observe the formation of a (quasi) periodic pattern of alternating microdomains.     

Computer simulation of non-linear preparative chromatography processes

Summary A theory of non-linear chromatography based on the mass balance equation is presented. The model is a system of partial differential equations and it includes axial dispersion in the mobile phase, flow in the mobile phase and mass transfer between phases. In the case of the injection of a mixture competition between components for places in the stationary or the mobile phase is taken into account. In the paper are discussed: numerical calculations of concentration and volume overloading and the effect of the mutual influence of the components of the injected mixture on the process of mass transfer between phases. Additionally, results of experiments for convex and concave adsorption isotherms are presented. In both cases the results obtained qualitatively confirm the theoretical analyses.     

Computer simulation of the formation of polymer networks

Complex crosslinked polymer structures can be quite easily modeled with the aid of computers. BTOSYM's implementation of an algorithm that has been developed by Eichinger and his co-workers over the last few years is described. This algorithm allows us to model both random (as in sulfur-cured rubber) and site-specific (as in end-linked silicones) crosslinking reactions. The simulation method provides detailed information on gel points, cycle rank, modulus of elasticity and other characteristics of the networks as they are formed. Illustrative results obtained with the program are presented.     

Size effects in electronic and breakdown processes during barrier electric discharge in disperse systems

The differences in the breakdown characteristics of barrier electric discharge (BED) in air and disperse systems (air + ZrO₂) at 77 and 300 K are determined by polarization, plasma-forming medium charge deposition on the ZrO₂ surface, and surface effects on the duration and mechanism of electron avalanches changing with the sizes of air voids between oxide surfaces (E/P ∼ const, T ∼ const). Original Russian Text ? M.M. Aliev, E.A. Zelenkova, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 5, pp. 978–984.     

Computer simulation of the adsorption of acetylene by disperse aqueous medium. IR spectra

Adsorption of acetylene molecules by water clusters at T 230 K was studied by the method of molecular dynamics. Addition of already two C₂H₂ molecules to (H₂O) _n clusters (10 ≤ n ≤ 20) makes them thermodynamically unstable. With an increase in the acetylene concentration in the disperse aqueous system, the IR absorption by the cluster system in the frequency range 0 ≤ ω ≤ 1000 cm^?1 increases. Depending on the number of C₂H₂ molecules per water cluster, the IR reflection by cluster systems can either increase or decrease. The power of the thermal radiation emitted by the clusters considerably increases after the adsorption of C₂H₂ molecules and grows with an increase in the acetylene concentration in the disperse aqueous system.     

Computer model of the formation of self-organizing systems

A model of a simple self-organizing system is investigated. Mechanisms that are important to a molecular-biological understanding of evolution are studied with the aid of the model.     

Simulation of the growth of nanoparticle aggregates reproducing their natural structure in disperse systems

A three-dimensional continuum model of the generation of aggregates of spherical particles is developed that allows us to reproduce natural conditions of structure formation in real disperse systems with an arbitrary viscosity of a dispersion medium. The exchange between translational and rotational degrees of freedom for aggregates, subaggregates, and single particles at an arbitrary interparticle interaction potential, as well as at the preset functions of the distribution over particle sizes and thicknesses of the adsorption layer, is taken into account in the model. The interaction potential of the particles includes electrostatic (among them, screened), elastic, and van der Waals interactions and allows for the effect of external electric, magnetic, and gravitational fields. Structural and statistical characteristics of aggregates formed at different stages of particle coagulation are studied, and the quasi-ordering of single subaggregates in the elastic adsorption layer of particles is revealed. The model can be applied to the study of optical absorption by large aggregates of nanoparticles with natural structure and the kinetics of sol aggregation as functions of the properties of the adsorption layer of particles and the action of external physical factors.     

Computer simulation and the dielectric constant of polarizable polar systems

A theory presented recently, allowing the static and frequency-dependent dielectric constant of polar systems to be calculated in computer simulations with arbitrary boundary conditions, is extended to the polarizable case. For modified dipolar interactions not explicitly depending upon time, the resulting equations are found to be of the generalized Fröhlich-Glarum type.     

Atomistic simulation of solid-liquid coexistence for molecular systems: application to triazole and benzene

A method recently developed to rigorously determine solid-liquid equilibrium using a free-energy-based analysis has been extended to analyze multiatom molecular systems. This method is based on using a pseudosupercritical transformation path to reversibly transform between solid and liquid phases. Integration along this path yields the free energy difference at a single state point, which can then be used to determine the free energy difference as a function of temperature and therefore locate the coexistence temperature at a fixed pressure. The primary extension reported here is the introduction of an external potential field capable of inducing center of mass order along with secondary orientational order for molecules. The method is used to calculate the melting point of 1-H-1,2,4-triazole and benzene. Despite the fact that the triazole model gives accurate bulk densities for the liquid and crystal phases, it is found to do a poor job of reproducing the experimental crystal structure and heat of fusion. Consequently, it yields a melting point that is 100 K lower than the experimental value. On the other hand, the benzene model has been parametrized extensively to match a wide range of properties and yields a melting point that is only 20 K lower than the experimental value. Previous work in which a simple "direct heating" method was used actually found that the melting point of the benzene model was 50 K higher than the experimental value. This demonstrates the importance of using proper free energy methods to compute phase behavior. It also shows that the melting point is a very sensitive measure of force field quality that should be considered in parametrization efforts. The method described here provides a relatively simple approach for computing melting points of molecular systems.     

Computer simulation of the atomic structure of regenerated cellulose

The atomic structure of amorphous sulfate hardwood pulp obtained via regeneration in a dimethyl acetamide–LiCl solution is studied with the use of X-ray diffraction and computer simulation. When dimethyl acetamide appears within a cellulose chain, it becomes swollen. A chosen cluster has a complex structure and contains two cellulose chains II distorted by twisting and bending and two chains deformed by dimethyl acetamide molecules. A cluster with the formula unit (C₆O₅H_8.5Li_1.5) ? (H₂O)_2.8 is obtained after relaxation with the addition of water. The uncertainty-profile factor for the model and experimental curves of X-ray scattering intensity distribution I(s) is 8%, and the distribution curve of s-weighted interference function H(s) calculated for the model best fits the experimental curve.     

Vinylogous anionic processes in the formation and interconversion of tetracyclic ring systems

Tandem oxy-Cope and transannular vinylogous aldol reactions and/or vinylogous retro-aldol, conjugate addition, and transannular vinylogous aldol reactions transformed some tricyclic vinyl enones into fused tetracycles under basic conditions. Mesylates derived from similar tetracyclic products underwent efficient skeletal reorganization via transannular ring-opening but then different modes of transannular ring-closure upon treatment with tert-butoxide.     

Computer simulation of network formation via crosslinking copolymerization

The topology of networks formed by radical crosslinking copolymerization is strongly dependent on the type of crosslinker chosen. By a series of computer simulations, the influence of the relative reactivities of crosslinker and monomer on the network properties was investigated. Basic condition of the simulation is the postulation of a diffusion process control of the polymerization reaction. The resulting networks are analyzed with respect to cycle rank, loop formation, dangling ends, sol fraction and crosslinker molecules reacted only by one double bond. The results of the computer simulations indicate that the reactivity of the crosslinker decisively affects the properties of the network. Particularly the cycle rank, which determines the elastic properties of the network, is influenced strongly by the reactivity of the crosslinker. A change of the cycle rank of approximately one decade was observed. The influence of the reactivity was found to be most important in the region where the reactivity of the crosslinker is less than the reactivity of the monomer.     

Computer simulation of interfacial polarization in stratified dielectric systems

Time and frequency dependent dielectric properties of bilayer dielectrics are treated with components showing dielectric relaxation. The time dependent behaviour can be calculated using the Boltzmann superposition principle. The exact integro-differential equations are derived, and the boundary conditions discussed. The analytical formulae resulting for the frequency domain are an extension of those published for bilayer dielectrics with nonrelaxing components. For the time domain the numerical solution is preferred because the analytical solution is very complicated. After a short discussion of the numerical method some examples are presented, and the effect of various component parameters on the dielectric properties is studied.The main conclusion is that two types of interfacial polarization mechanisms must be distinguished: the classical ohmic, and the dielectric. The latter is the consequence of the polarizational surface charge density, and influences strongly the observed relaxation strengths of the components.Presented at the 20. Jahrestagung Organische Festkörper, July 1983, Brandenburg, German Democratic Republic     

Computer simulation of the structure of the electrochemical double layer

We analyze and compare the structure of the electrochemical double layer obtained from molecular dynamics simulations of concentrated aqueous NaCl and CsF solutions near a model electrode. The electrode is modeled as a corrugated external potential in conjunction with the image charge model. Calculations are performed for uncharged electrodes and for electrodes carrying positive or negative surface charges.