Modeling the growth processes of polyelectrolyte multilayers using a quartz crystal resonator

The layer-by-layer buildup of chitosan/hyaluronan (CH/HA) and poly(l-lysine)/hyaluronan (PLL/HA) multilayers was followed on a quartz crystal resonator (QCR) in different ionic strengths and at different temperatures. These polyelectrolytes were chosen to demonstrate the method whereby useful information is retrieved from acoustically thick polymer layers during their buildup. Surface acoustic impedance recorded in these measurements gives a single or double spiral when plotted in the complex plane. The shape of this spiral depends on the viscoelasticity of the layer material and regularity of the growth process. The polymer layer is assumed to consist of one or two zones. A mathematical model was devised to represent the separation of the layer to two zones with different viscoelastic properties. Viscoelastic quantities of the layer material and the mode and parameters of the growth process were acquired by fitting a spiral to the experimental data. In all the cases the growth process was mainly exponential as a function of deposition cycles, the growth exponent being between 0.250 and 0.275.     

Taxonomy of nucleation and growth processes in isotropic systems

We construct a taxonomy of transient behavior for one component coupled nucleation and growth based on linear and nonlinear stability analysis of a two species model. The two species are the condensable species and the condensed droplets or particles, respectively. Our model reproduces previously reported asymptotically stable and oscillatory transient nucleation behavior. Additionally, we show previously unreported possibilities of multiple stationary states and combinations of stationary states with limit cycles. We predict that multiple stationary states may be observed under near critical conditions. Estimates of suitable conditions for experimental verification are given for several systems of interest.     

Modeling of self-organization processes in crystal-forming systems: Symmetry and topology code for the cluster self-assembly of crystal structures of intermetallic compounds

We performed the combinatorial and topological modeling of 1D, 2D, and 3D packs of symmetrically connected metal clusters in the form of tetrahedra А₄. Three types of 1D chains with tetrahedral connectivity of 4, 6, and 8 were used to model 2D layers L-1, L-2, and L-3 and 3D frameworks FR-1, FR-2, FR-3, and FR-4. The model structures of the identified suprapolyhedral precursor clusters were used in topological analysis of crystal structures of intermetallic compounds (program package TOPOS and data bases ICSD and CRYSTMET). A match was found between the topological models of tetrahedral 3D frameworks and all types of crystal structures formed in binary systems; in Au–Cu: FR-1 for Cu₃Au-cP4 (Auricupride), Cu₂Au₂-tP2 (Tetraauricupride), CuAu₃-cP4 (Bogdanovite), and Cu_2–x Au_{2 + x} -cF4; in Mg–Cd: FR-3 for Mg₃Cd-hP8, Mg₂Cd₂-oP4, MgCd₃-hP8, and Mg_2–x Cd_{2 + x} -hP2; in Li–Hg: FR-2 for Li₃Hg-cF16 and Li₂Hg₂-cP2 and FR-3 for LiHg₃-hP8; in ternary system Li–Ag–Al: FR-2 for LiAg₂Al-cF16 and Li₂AgAl-cF16; and in quaternary system: framework FR-2 for LiMgPdSn-cF16. Framework FR-4 was identified in ternary intermetallic compounds A(Li₂Sn₂)-tI20, where A = Cu, Ag, Au. The structures of precursor nanoclusters were identified for other most abundant types of crystal structures of intermetallic compounds. For this purpose, we used the algorithms for partitioning the structural graph into nonintersecting cluster substructures and constructed the basal 3D network of the crystal structure in the form of a graph whose nodes correspond to the positions of the centers of precursor clusters. The cluster self-assembly was modeled for the following intermetallic compounds: Mg₂Cu₄-cF24, MgSnCu₄-cF24, (ZrCu)Cu₄-cF24,Mg₂Zn₄-hP12, (CaCu)Cu₄-hP6, Cr₃Si-cP8, Lu₃Co(Fe₃C)-cP16, Ca₂Ge₂(Cr₂B₂)-oC8, Y₂Ni₂(Fe₂B₂)-oP8, AlB₂-hP3, Ca₂Ge-oP12, CaHg₂-hP3, Co₂Ge(Ni₂In)-hP6, Cs₂Hg₄-oI12, Ba₄Po₄-cF8, Mn₅Ge₃-hP16, and NaZn₁₃-cF112. The symmetry and topological code of self-assembly from precursor nanoclusters was reconstituted for all crystal structure types of intermetallic compounds as: primary chain → microlayer → microframework. An abundance frequency analysis of topological and symmetry routes for the generation and evolution of precursor clusters enabled us to elucidate the crystal-formation laws in intermetallic systems on the microscopic level.     

Modeling of ion processes in atmospheric pressure matrix-assisted laser desorption/ionisation

Atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) has proven a convenient and rapid method for ion production in the mass spectrometric analysis of biomolecules. This technique, like other atmospheric pressure ionization methods, suffers from ion loss during ion transmission from the atmosphere into the vacuum of the mass spectrometer. In this work, a simple model describing ion formation and ion motion towards the inlet capillary of the mass spectrometer is described. Both the gas flow and electric field near the MALDI plate were numerically calculated using the boundary element method (BEM). The ions were moving along with the gas flow and drifting in the electric field in accordance with their ion mobility properties. The ion signal dependence on an electric field strength obtained in the proposed model correlates well with experimental results.     

Uncovering molecular processes in crystal nucleation and growth by using molecular simulation

Exploring nucleation processes by molecular simulation provides a mechanistic understanding at the atomic level and also enables kinetic and thermodynamic quantities to be estimated. However, whilst the potential for modeling crystal nucleation and growth processes is immense, there are specific technical challenges to modeling. In general, rare events, such as nucleation cannot be simulated using a direct "brute force" molecular dynamics approach. The limited time and length scales that are accessible by conventional molecular dynamics simulations have inspired a number of advances to tackle problems that were considered outside the scope of molecular simulation. While general insights and features could be explored from efficient generic models, new methods paved the way to realistic crystal nucleation scenarios. The association of single ions in solvent environments, the mechanisms of motif formation, ripening reactions, and the self-organization of nanocrystals can now be investigated at the molecular level. The analysis of interactions with growth-controlling additives gives a new understanding of functionalized nanocrystals and the precipitation of composite materials.     

Modeling of thermal processes in high pressure liquid chromatography: I. Low pressure onset of thermal heterogeneity

Heat due to viscous friction is generated in chromatographic columns. When these columns are operated at high flow rates, under a high inlet pressure, this heat causes the formation of significant axial and radial temperature gradients. Consequently, these columns become heterogeneous and several physico-chemical parameters, including the retention factors and the parameters of the mass transfer kinetics of analytes are no longer constant along and across the columns. A robust modeling of the distributions of the physico-chemical parameters allows the analysis of the impact of the heat generated on column performance. We developed a new model of the coupled heat and mass transfers in chromatographic columns, calculated the axial and radial temperature distributions in a column, and derived the distributions of the viscosity and the density of the mobile phase, hence of the axial and radial mobile phase velocities. The coupling of the mass and the heat balances in chromatographic columns was used to model the migration of a compound band under linear conditions. This process yielded the elution band profiles of analytes, hence the column efficiency under two different sets of experimental conditions: (1) the column is operated under natural convection conditions; (2) the column is dipped in a stream of thermostated fluid. The calculated results show that the column efficiency is remarkably lower in the second than in the first case. The inconvenience of maintaining constant the temperature of the column wall (case 2) is that retention factors and mobile phase velocities vary much more significantly across the column than if the column is kept under natural convection conditions (case 1).     

Modeling high pressure reactivity in unsaturated systems: application to dimethylacetylene

A general model is introduced to study pressure-induced reactivity on unsaturated systems in the condensed state. The model is applied here to dimethylacetylene (DMA) in the solid phase II (C/2m) because it has been proposed that two DMA molecules can react to form tetramethyl-cyclobutadiene (TMCBD). The proposed reaction process has been modeled by studying the structural and electronic changes undergone by two DMA molecules as they approach each other preserving the crystal symmetry of phase II. Both monodeterminantal (MP2 and DFT) and multideterminantal (CASSCF and MRMP2) methodologies were used to check the reliability of our model in predicting the reactivity of the system under compression. In all cases, structural results are in agreement with low-temperature diffraction experiments for the solid phase II. Our model indicates that DMA is expected to form the TMCBD dimer at intermolecular distances close to 2 A. This value is in excellent agreement with previous calculations on the existence of long carbon-carbon bonds.     

High pressure synthesis,crystal growth and magnetic properties of TiOF

Polycrystalline samples of TiOF have been prepared at 1300 °C and 8 GPa, with small single crystals grown at the same conditions. The crystal structure remains tetragonal rutile-type down to at least 90 K (space group P4₂/mnm, a = 4.6533 (2) Å and c = 3.0143 (2) Å at 90 K) and the Ti(O,F)₆ octahedra are slightly compressed, consistent with Jahn-Teller distortion of 3d¹ Ti³⁺. Diffuse scattering reveals disordered structural correlations that may arise from local cis-order of oxide anions driven by covalency. TiOF is paramagnetic down to 5 K and observation of a small paramagnetic moment and a substantial Pauli term indicates that the d-electrons are partially delocalised.     

Anisotropic crystal growth in aerosil/polymethylmethacrylate systems dispersed in dimethacrylate monomers

Complex organized crystals were shown to grow within the settled phase composed of aggregated aerosil/polymethylmethacrylate or aerosil/poly(cetyl vinyl ether-maleic anhydride) resulting from the supply of the corresponding homopolymer or the alternated copolymer to the aerosil initially dispersed in a mixture of dimethacrylate monomers. The basic suspending phase was realized by mixing 1,3-butanediol dimethacrylate and bisphenol A dimethacrylate. The crystal nucleation and growth were both extremely slow processes that developed over months. Small and large crystals were realized usually displaying the shape of hexagonal platelets. The dose of polymer initially supplied to the system exerted a structuring role in the crystalline organization. DCS determinations showed the enthalpy of fusion to depend on the polymer dose and molecular weight. AFM showed the basal and lateral phases to grow with developing steps of two different thicknesses. The monomer ordering was evidenced by the fact that the polymerization shrinkage only affected the sequence of monomers lying parallel to the basal phases, the polymerization of monomers along the perpendicular axis being prevented from any shrinkage.     

Mechanisms of heterogeneous crystal growth in atomic systems: insights from computer simulations

In this paper we analyze the atomic-level structure of solid/liquid interfaces of Lennard-Jones fcc systems. The 001, 011, and 111 faces are examined during steady-state growth and melting of these crystals. The mechanisms of crystallization and melting are explored using averaged configurations generated during these steady-state runs, where subsequent tagging and labeling of particles at the interface provide many insights into the detailed atomic behavior at the freezing and melting interfaces. The interfaces are generally found to be rough and we observe the structure of freezing and melting interfaces to be very similar. Large structural fluctuations with solidlike and liquidlike characteristics are apparent in both the freezing and melting interfaces. The behavior at the interface observed under either growth or melting conditions reflects a competition between ordering and disordering processes. In addition, we observe atom hopping that imparts liquidlike characteristics to the solid side of the interfaces for all three crystal faces. Solid order is observed to extend as rough, three-dimensional protuberances through the interface, particularly for the 001 and 011 faces. We are also able to reconcile our different measures for the interfacial width and address the onset of asymmetry in the growth rates at high rates of crystal growth/melting.     

Polydispersity in liquid crystal systems

Here we discuss the statistical mechanics of polydisperse liquid crystal systems. Three different kinds of liquid crystal systems are treated: nematic order in thermotropic Maier-Saupe-like systems and in lyotropic Onsager-like rod systems, and smectic order in a perfectly aligned hard rod fluid. In the first two cases we calculate the broadening of the isotropic-nematic transition. In the last case the suppression of smectic order is dealt with. We discuss the relationship between real systems and the models discussed in the paper.     

Modeling of protonation processes in acetohydroxamic acid

This work presents a theoretical study of acetohydroxamic acid and its protonation processes using ab initio methodology at the MP2(FC)/cc-pdVZ level. We find the amide form more stable than the imidic tautomer by less than 1.0 kcal mol(-)(1). For comparison with the experimental data, a three-dimensional conformational study is performed on the most stable tautomer (amide). From this study, the different barriers to rotation and inversion are determined and the intramolecular hydrogen bond between the OH group and the carbonyl oxygen is characterized. The electrostatic potential distribution shows three possible sites for electrophilic attack, but it is shown that only two of them, the carbonyl oxygen and the nitrogen atoms, are actual protonation sites. The protonation energy (proton affinity) is obtained from the results of the neutral and charged species. Proton affinities for the species charged on the carbonyl oxygen and the nitrogen atoms are estimated to be 203.4 and 194.5 kcal mol(-)(1), respectively. The development of a statistical model permits the quantification of DeltaG (gas-phase basicity) for the two protonation processes. In this way, the carbonyl oxygen protonated form is found to be more stable than that of the nitrogen atoms by 8.3 kcal mol(-)(1) at 1 atm and 298.15 K, due to the enthalpic contribution. As temperature increases, the proportion of the nitrogen protonated form increases slightly.     

Dynamic processes in ferroelectric liquid crystal filled cells

The switching process in a ferroelectric liquid crystal cell is of great interest. The precise way in which the optic tensor structure reorientates during switching between states is, however, difficult to determine. Here we consider the use of guided modes and surface plasmon-polaritons as techniques for the investigation of this. It is seen that because of the nature of the dynamic processes the guided mode data is inconclusive, but surface plasmon-polariton data show the surface reorientation mechanism.     

Dynamic processes in ferroelectric liquid crystal filled cells

Abstract

The switching process in a ferroelectric liquid crystal cell is of great interest. The precise way in which the optic tensor structure reorientates during switching between states is, however, difficult to determine. Here we consider the use of guided modes and surface plasmon-polaritons as techniques for the investigation of this. It is seen that because of the nature of the dynamic processes the guided mode data is inconclusive, but surface plasmon-polariton data show the surface reorientation mechanism.     

Lasing in micro-grating liquid crystal systems

A waveguide lasing effect has been observed and investigated in a dye-doped layer of a nematic liquid crystal (NLC) between two quartz plates. One of the plates has an electrode micro-grating, which allows (i) creating the feedback, (ii) guiding a part of the lasing emission into the quartz substrate and (iii) modulating the NLC refractive index by an electric field. At 0 V, a single Transverse Magnetic mode (TM)-polarised mode lasing has been observed. The emission goes out from the edge of the quartz plate in a narrow angular sector (±1.5°) at an angle of about 67.0° with respect to the NLC layer normal. At voltage applied, a number of additional lasing modes propagating at the same angle, but located at different wavelengths, appear. The experimental results are interpreted on account of numerical simulations based on the finite difference time domain method.     

Modeling crystal growth from solution with molecular dynamics simulations: approaches to transition rate constants

The feasibility of using the molecular dynamics (MD) simulation technique to study crystal growth from solution quantitatively, as well as to obtain transition rate constants, has been studied. The dynamics of an interface between a solution of Lennard-Jones particles and the (100) face of an fcc lattice comprised of solute particles have been studied using MD simulations, showing that MD is, in principle, capable of following growth behavior over large supersaturation and temperature ranges. Using transition state theory, and a nearest-neighbor approximation growth and dissolution rate constants have been extracted from equilibrium MD simulations at a variety of temperatures. The temperature dependence of the rates agrees well with the expected transition state theory behavior.     

Modeling and simulation of stationary catalytic processes

Generalized models for steady state catalytic processes are presented in matrix form. Multistep reaction rate control is assumed. Numerical algorithms for solving of the created linear and nonlinear equation systems are developed and tested. Four examples are considered: an Eley–Rideal-mechanism, a Langmuir–Hinshelwood mechanism, a dual route, dual site mechanism, and a monomolecular decomposition with steady state multiplicity. The overall reaction rates are simulated as a function of the reactant concentrations. A maximum reaction rate is obtained in the case of a Langmuir–Hinshelwood mechanism (example 2), the location of the rate maximum in the concentration domain is shifted towards the concentration of the reactant with the lowest adsorption constants. An Eley–Rideal mechanism (example 1) has always monotonously increasing rate curves. In the case of steady state multiplicity (example 4) all steady states could be simulated with the proposed algorithm. The computation of reaction rate surfaces is important in investigating the behavior of complicated catalytic systems (e.g., systems with multistep rate control and/or steady state multiplicity), in planning of experiments and in chemical reactor simulation.     

Relaxation processes in semidilute solutions of polymers in liquid crystal solvents

We investigate the relaxation phenomena in a polymer (polystyrene)/liquid crystal (4-cyano-4'-n-octyl-biphenyl) system, in its homogeneous isotropic phase near the isotropic-isotropic, isotropic-nematic, and isotropic-smectic coexistence curve, using both polarized and depolarized photon correlation spectroscopy (PCS). We study this system for different polystyrene molecular weights (4750, 12 500, and 65 000 g/mol), different compositions (50, 40, 30, and 10% polystyrene (PS) by weight), and different temperatures close to phase boundaries. First of all, we determine the phase diagrams of this system for the different molecular weights. The shape of the phase diagrams strongly depends on the molecular weight. However, in all cases, at low temperatures, these systems separate into an almost pure liquid crystalline (LC) phase and polystyrene-rich phase. PCS measurements show that the relaxation processes in the homogeneous phase are not affected by the proximity of the nematic, or smectic, boundaries (even at a temperature of 0.1 degrees C above the phase separation in two phases). In polarized PCS experiments, we always see three relaxation processes well separated in time: one, very fast, with a relaxation time of the order of 10(-5) s; a second one with a relaxation time within the range 10(-2)-10(-3) s; and a last one, very slow, with a relaxation time of the order of 1 s. Both the fast and slow modes are independent of the wave vector magnitude, while the intermediate relaxation process is diffusive. In depolarized PCS experiments, the intermediate mode disappears and only the fast and slow relaxation processes remain, and they are independent of the magnitude of the wave vector. The diffusive mode is the classical diffusive mode, which is associated with the diffusion of polymer chains in all polymer solutions. The fast mode is due to the rotational diffusion of 4-cyano-4'-n-octyl-biphenyl (8CB) molecules close to polystyrene chains (transient network). Finally, we assign the slowest mode to reorientational processes of small aggregates of PS chains that are not dissolved in 8CB.