A DFT study of reaction pathways of NH(3) decomposition on InN (0001) surface

Reaction pathways for complete decomposition of ammonia on the InN (0001) surface are investigated using first principles calculations. We show that while the initial NH(3) decomposition on this surface can proceed by H dissociation, its further decomposition is most favorable by H transfer. The calculated low diffusion barriers for the decomposed species on the surface imply that the metal-organic chemical vapor deposition growth of InN is a reaction-limited process rather than diffusion-limited at low adsorbate coverage.     

Fractal structures of the hydrogels formed in situ from poly(N-isopropylacrylamide) microgel dispersions

Dispersions of poly(N-isopropylacrylamide) (PNIPAM) microgel thermally gel in the presence of inorganic salts. The in situ-formed hydrogels, with a network of soft particles, represent a new type of colloidal gels. Here, their fractal structures were determined by rheological measurements, using the models of both Shih et al. and Wu and Morbidelli. According to the definition of Shih et al., the colloidal PNIPAM gels fall into the strong-link regime. Yet the calculated fractal dimension of the floc backbone, x, yielded unrealistic negative values, suggesting this model is inapplicable for the present system. The Wu-Morbidelli model gives physically sounder results. According to this model, the strengths of the inter- and intrafloc links are comparable, and the in situ-formed gels are in the transition regime. The fractal dimension, d(f), of the hydrogel decreases from ～2.5 to ～1.8 when the heating temperature increases from 34 to 40 °C. The d(f) values suggest different aggregation mechanisms at different temperatures, that is, a reaction-limited one accompanied by rearrangement at low temperature, a typical reaction-limited one at the intermediate temperature, and a diffusion-limited one at high temperature. With increasing salt concentration, the d(f) of the hydrogel decreases from ～2.1 to ～1.7, suggesting the aggregation mechanism changes from reaction-limited to diffusion-limited. The effects of both temperature and salt concentration can be explained by the changes in the interactions among the microgel particles. The thermogellable PNIPAM microgel dispersions may serve as a model system for the study of heat-induced gelation of globular proteins.     

The kinetics of growth of semiconductor nanocrystals in a hot amphiphile matrix

The first comprehensive study on the kinetics of nanocrystal growth in a hot amphiphile medium is presented. An example is given with CdSe semiconductor nanocrystals grown after the injection of precursor (a mixture of Cd- and Se-reagents) in concentrated tri-octylphosphine oxide matrix (heated to more than 300 degrees C). The particle size distribution is reconstructed as a function of time from the absorption and photoluminescence spectra collected during the synthesis process. For this purpose a new expression is used relating the exciton energy due to quantum confinement with the nanocrystal radius. The growth kinetics is considered as a two-stage process in order to describe the time variation of nanoparticle size. During the first stage, called reaction-limited growth, the size of initial nucleus rapidly increases due to a sort of surface reaction exhausting the precursor in the nanoparticle vicinity. The growth in such conditions favors also a remarkable narrowing of the size distribution. The nanocrystal develops further on account of a slow precursor transfer from a distant space driven by the concentration gradient--classical diffusion-limited growth. The width of size distribution also increases proportional to the average particle size. Any growth will stop after the precursor concentration reaches a minimum value defining the limit for the final nanocrystal size in a batch. Solving the kinetic equations for the growth rate in each case of kinetics derives analytical expressions for the mean radius and variance of size distribution. Then the respective expressions are matched in a uniform solution valid during the entire synthesis. The theoretical model is in a good quantitative agreement with the experimental data for independent syntheses. Important characteristic scales of the processes (time-constant and length) and microscopic parameters of the reacting system (interfacial energy and reaction rate constant) are estimated from the data. It turns out that the fast reaction-limited growth is important to obtain well-defined nanocrystals of high optical quality by using less energy, time and consumable. However, to make them reproducibly uniform one should control also the ultra-fast nucleation process preceding the nanocrystal growth, which is still unknown. Nevertheless, our current findings allow the conceptual design of a new continuos-flow reactor for the manufacturing of a large amount of uniform nanocrystals.     

Cluster shape anisotropy in irreversibly aggregating particulate systems

The results for cluster shape anisotropy over a broad range (10)(-3)-10(-1)) of monomer volume fractions, fv values, are presented for both two- (2d) and three-dimensional (3d) simulations of diffusion-limited (DLCA), ballistic-limited (BLCA), and reaction-limited (RLCA) cluster-cluster aggregation classes. We find that all three aggregation classes have different dilute-limit shape anisotropies, with the diffusion-limited model having the largest value of anisotropy and the reaction-limited model having the smallest. The simulation result for the cluster shape anisotropy for each of the three aggregation classes is slightly less than the corresponding prediction of the hierarchial model. In addition, we find excellent agreement between the 2d DLCA simulation results and experimental measurements of shape anisotropy. At late times, shape anisotropy decreases from the dilute-limit value.     

Destabilization of polystyrene latex particles induced by adsorption of polyvinylamine: mass, size and structure characteristics of the growing aggregates

The obviously visible aggregation of suspended colloidal particles resulting from the addition of polyvinylamine to the aqueous dispersion of polystyrene latex particles bearing surface sulfate groups set in with a delay of 24 h. The aggregation mechanisms and the fractal dimension of the aggregates were derived from the variations with time of the weight and number averaged masses of the aggregates as well as of the weight averaged harmonic mean diameter of the size distribution. Since the establishment of starved layers was determined to be relatively fast and to leave the liquid phase free of polymer, the delay for the obvious destabilization was attributed to the reconformation of adsorbed macromolecules that was expected to be extremely slow. This reconformation promoted the emergence of the diffusion-limited aggregation process that accompanies the permanent reaction-limited aggregation process. The fractal dimension of the latex particles/polyvinylamine aggregates was determined to be 2.12.     

Relation between aggregation kinetics and the structure of kaolinite aggregates

Dynamic and static light scattering were applied to the determination of the stability ratio and fractal dimension of kaolinite (KGa-2) at different kaolinite or/and electrolyte concentrations at pH 9.5. Dynamic light scattering was used to measure the kinetics of early stage aggregation to determine the stability ratio, W, as well as the cluster sizes which determine the fractal regime. Static light scattering was used to measure the fractal dimension, D(f). Results show that the two classes of "universality" (Lin et al. Nature 1989, 339, 360) characterizing the diffusion- and reaction-limited regimes of cluster-cluster aggregation do apply to colloidal kaolinite as limit cases when W approximately 1 or W > 100, respectively. In the intermediate regime where 5 < W < 100, the growth of the aggregate radius showed a power-law behavior similar to diffusion-limited cluster aggregation. For the intermediate aggregation regime, a scaling relation between fractal dimension and stability ratio, reflecting a continuous increase in particle packing density in the aggregate as the sticking probability of particles was reduced, was demonstrated.     

Quantifying growth of symmetric and asymmetric lipid bilayer domains

Here, we examine by atomic force microscopy (AFM) the kinetics and morphology of lipid domain growth during lipid phase separation by rapid thermal cooling of fully mixed two-component supported lipid bilayers. At the undercooled temperatures chosen, symmetric 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)-rich domains favored slower reaction-limited growth whereas asymmetric galactosylceramide (GalCer)-rich domains favored faster diffusion-limited growth, indicated by shape factors and kinetic exponents. Because kinetically limited conditions could be accessed, we were able to estimate the activation energy barrier (approximately 16kT) and lateral diffusion coefficient (approximately 0.20 microm2/s) of lipid molecular addition to a growing domain. We discuss these results with respect to transition states, obstructed diffusion, and the necessity for coordinating growth in both leaflets in a symmetric lipid domain.     

A refined algorithm to simulate latex colloid agglomeration at high ionic strength

This study is focussed on the simulation of particle agglomeration at relatively high ionic strength using a refined stochastic algorithm developed in the context of parcel-tracking approaches. For that purpose, experimental data of both diffusion-limited and reaction-limited aggregation of latex particles were obtained using dynamic light scattering techniques for different initial particle sizes (diameters ranging from 24 to 495 nm) and at various chemical conditions (ionic strength between 0.5 and 2 M with NaCl or CaCl\(_2\) solutions). The experimental data collected have been compared to numerical results obtained with the refined parcel-tracking algorithm for particle agglomeration which has been developed. Results show that the evolution of the aggregate diameters over time can be properly captured by the present model with the value of the aggregate fractal dimension that is extracted from experimental data.     

The intrinsic stiffness of polyglutamine peptides

We have used the method of Trp/Cys contact quenching to measure the rate of contact formation in polyglutamine and find it to be a very stiff peptide. Separation of observed rates into reaction-limited and diffusion-limited rates show that the reaction-limited rates increase (rather than decrease) slightly with length between 4 and 16 amino acids. Using Szabo, Schulten, and Schulten theory, we have modeled the results with a wormlike chain with excluded volume and find the persistence length to be about 13.0 A, much longer than has been observed for other random peptides and unfolded proteins. The preferred extended conformation of polyglutamine could account for a propensity for expanded glutamine stretches to unfold the Huntington's protein and the high propensity to aggregate from a disordered monomer.     

Monitoring gold nanorod synthesis by localized surface plasmon resonance

Surfactants can direct the growth of gold nanoparticles to create anisotropic structures in high yield by simple means, yet the exact roles of surfactants and other reactants are not entirely understood. Here we show that one can exploit the geometrical dependence of the localized surface plasmon resonant extinction spectrum of gold nanorods to monitor their synthesis kinetics. By using quantitative measurements of nanorod extinction cross sections, Gans' theory for the spectral extinction of prolate spheroids can be normalized to provide values for the nanorod length and diameter from extinction spectra measured during growth. The nanorod length growth rate was first observed at 0.15 nm/s and decayed during the growth reaction. The rate dependence on nanorod size did not correspond to any simple reaction-limited or diffusion-limited growth mechanisms.     

Detailed model of the aggregation event between two fractal clusters

A model has been developed for describing the aggregation process of two fractal clusters under quiescent conditions. The model uses the approach originally proposed by Smoluchowski for the diffusion-limited aggregation of two spherical particles but accounts for the possibility of interpenetration between the fractal clusters. It is assumed that when a cluster diffuses toward a reference cluster their center-to-center distance can be smaller than the sum of their radii, and their aggregation process is modeled using a diffusion-reaction equation. The reactivity of the clusters is assumed to depend on the reactivity and number of their particles involved in the aggregation event. The model can be applied to evaluate the aggregation rate constant as a function of the prevailing operating conditions by simply changing the value of the particle stability ratio, without any a priori specification of a diffusion-limited cluster aggregation, reaction-limited cluster aggregation, or transition regime. Furthermore, the model allows one to estimate the structure properties of the formed cluster after the aggregation, based on the computed distance between the aggregating clusters in the final cluster.     

Formation and structure of pachyman aggregates in dimethyl sulfoxide containing water

The aggregation of pachyman, β-(1 → 3)-D -glucan (M_w = 1.68 × 10⁵) from the Poria cocos mycelia, was investigated using static and dynamic laser light scattering (LLS) in dimethyl sulfoxide (DMSO) containing about 15% water, which leads to large aggregates. Both the time dependence of hydrodynamic radius and the angle dependence of the scattering intensity were used to calculate the fractal dimension (d_f) of the aggregates. The aggregation rate and average size of aggregates increase dramatically with increasing the polymer concentration from 1.7 × 10⁻⁴ g/mL to 8.6 × 10⁻⁴ g/mL, and with the decrease of the solvent quality, that is, water content from 13 to 15%. In the cases, the fractal dimensions change from 1.94 to 2.43 and from 1.92 to 2.54, respectively, suggesting that transforms of aggregation processes: a slow process called reaction-limited cluster aggregation (RLCA) to a fast process called diffusion-limited cluster aggregation (DLCA) in different polymer concentrations and water content. The fractal dimensions above 2 of the fast aggregation is larger than the 1.75 predicted for the ideal DLCA model, suggesting that the aggregation involves a restructuring process through the interchain hydrogen bonding interaction. There are no aggregates of pachyman in DMSO without water, but aggregates formed in the DMSO containing 15% water at 25°C as a compact structure. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3201–3207, 1999     

Diffusion-limited hyperbranched polymers with substitution effect

Highly branched structure has the essential influence on macromolecular property and functionality in physics and chemistry. In this work, we proposed a diffusion-limited reaction model with the consideration of macromolecular unit relaxations and substitution effect of monomers to study the structure of hyperbranched polymers prepared by slow monomer addition to a core molecule. The exponential relationship (R(g) ～ N(λ)) between the radius of gyration R(g) and the degree of polymerization N, was systematically analyzed at various branching degrees. It is shown that the effective exponent λ(eff) decreases at lower N and but increases toward that of diffusion-limited aggregation (DLA) clusters (λ(DLA) = 0.4) with the degree of polymerization increasing. The substitution effect of monomers in reaction strongly influences the evolution pathway of λ(eff). With the static light scattering technique, the fractal property of internal chains was further calculated. A general law about the radial distribution of the units of diffusion-limited hyperbranched polymers was found that, at smaller reactivity ratio k(12), the radial density of all monomer units D(A) declines from the center region to the peripheral layer revealing the dense core structure; however, at larger k(12), the density distribution shows a loose-dense-loose structure. These structural characteristics are helpful to deeply understand the property of hyperbranched polymers.     

Aggregation and deposition kinetics of fullerene (C60) nanoparticles

The aggregation and deposition kinetics of fullerene C60 nanoparticles have been investigated over a wide range of monovalent and divalent electrolyte concentrations by employing time-resolved dynamic light scattering (DLS) and quartz crystal microbalance (QCM), respectively. Aggregation kinetics of the fullerene nanoparticles exhibited reaction-limited (slow) and diffusion-limited (fast) regimes in the presence of both electrolytes, having critical coagulation concentrations (CCC) of 120 and 4.8 mM for the monovalent (NaCl) and divalent (CaCl2) salts, respectively. The measured stability ratios of the aggregating fullerene nanoparticles were in very good agreement with Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, with a derived Hamaker constant of 6.7 x 10-21 J for the fullerene nanoparticles in aqueous medium. For the deposition kinetics studies, the rate of fullerene nanoparticle deposition increased with increasing electrolyte concentrations, as was indicated in the aggregation kinetics results. However, at electrolyte concentrations approaching or exceeding the CCC, the rate of deposition dropped sharply due to significant concurrent aggregation of the fullerene nanoparticles. The deposition of the fullerene nanoparticles was further shown to be mostly irreversible, with immediate detachment of the nanoparticles observed only when exposed to a solution of high pH.     

Interfacial nematodynamics of heterogeneous curved isotropic-nematic moving fronts

The early stages of liquid crystal phase ordering upon thermal quenches of isotropic phases into unstable and metastable temperature ranges is studied using two-dimensional (2D) computational solutions of the governing Landau-de Gennes (L-dG) equations for low molar mass nematic liquid crystals and analysis based on the corresponding interfacial nematodynamic model. The early phase ordering stage, for both unstable and metastable quenches of the isotropic phase, is shown to lead to highly textured nematic spherulites through a mechanism of interfacial defect nucleation. The underlying mechanisms of interface-driven texturing are elucidated using complementary 2D computational parametric studies of the bulk L-dG equation and analysis of the IN model. It is shown that for highly curved nanodomains and realistic elastic anisotropy, sharp interfacial transitions between uniaxial and biaxial states arise and are resolved by interfacial defect nucleation, which upon subsequent migration into the spherulite's interior leads to strong texturing. This paper shows that texture formation in the early stages of phase ordering is interface driven, and due to low interface tension, elastic anisotropy, and large curvature. Interfacial defect shedding in highly curved, low tension, anisotropic interfaces is a significant defect nucleation mechanism that needs to be taken into account when considering texturing processes.     

Pyrolysis, crystallization, and sintering of mesostructured titania thin films assessed by in situ thermal ellipsometry

In-situ thermal ellipsometric analysis is used to elucidate new and fine-scale details on the thermally driven densification, pyrolysis, crystallization, and sintering of dense and ordered mesoporous titania thin films prepared by evaporation-induced self-assembly. The role of the heating schedule, initial film thickness, nature of the substrate and templating agent, solution aging, and presence of water and other additives in the calcination environment is examined. Each of these parameters is shown to have unique and often substantial effects on the final film structure, while the technique itself provides detailed insight into the chemical origin and evolution of these effects. In-situ monitoring and control over the governing chemical processes, such as high-temperature adsorption phenomena that impact nanocrystal growth, is also demonstrated. The evolution of both the porosity and chemical processes occurring inside these materials are evaluated, including extraction of kinetic parameters for the pyrolysis of the template and crystallization of the matrix walls. The latter is shown to be strongly dependent on the presence of mesoscale ordering with ordered cubic films indicating a 1D diffusion-limited crystallization process and dense films following a 3D diffusion-limited process. Less well-ordered mesoporous films, despite similarities in pore volume and pore size distributions, are kinetically more reminiscent of dense films in terms of crystallization. In-situ thermal ellipsometry, by detailing the evolution of the thermally driven chemistry and ceramization that dictate the final film properties, provides immensely important insight into the synthesis and optimization of advanced functional materials based on titania and other metal oxide thin films.     

Growth of Fractal Aggregates in the Intermediate Regime of Particle Accommodation

The growth dynamics of fractal aggregates was studied within the framework of continuum model in the self-consistent mean field approximation. The regime that is intermediate between the diffusion-limited aggregation and reaction-limited aggregation was considered. The dependence of aggregate fractal dimension on the attachment probability of particles during their collisions with an aggregate was obtained. In the limiting cases, the values of fractal dimension coincide with those determined earlier. The domain of the values of attachment probability was revealed where several regions characterized by their own values of fractal dimension were specified in the structure of growing cluster. Physical nature of the emergence of various regions in the aggregate structure was discussed.     

Rheology and stability of oil-in-water nanoemulsions stabilised by anionic surfactant and gelatin 2) addition of homologous series of sugar-based co-surfactants

The oil-in-water emulsions used in silver-halide photographic coatings are stabilised with anionic surfactants and made in the presence of excess gelatin, which acts as an electrosteric stabilising agent and continuous phase viscosifier. The oil droplet sizes are close to 100 nm but the adsorbed gelatin increases the effective volume of the droplets significantly. These nanoemulsions are manufactured and coated at temperatures in excess of 40 degrees C, where gelatin adopts a random coil structure. At oil concentrations above 15% by volume, the emulsions are viscoelastic liquids with a high low-shear viscosity and strong shear-thinning. The viscosity and shear-thinning can be decreased by reducing the adsorption of gelatin, which can be achieved by addition of nonionic surfactants. This is a rheological study of the effects of adding novel, nonionic sugar-based surfactants on the rheology of photographic nanoemulsions, with additional measurements of static and dynamic surface tension. These surfactants have two sugar (gluconamide) heads and either one or two alkyl tails. Homologous series of each type of sugar surfactant were investigated over a wide range of alkyl tail length. The optimum surfactant choice for commercial applications depends not only on rheological effects but also on ease of synthesis, purification and dissolution, and of course, cost. The dynamic surface tension of the emulsion containing the anionic-nonionic surfactant mixture must also be compatible with the multilayer coating process.     

Associative and segregative phase separations of gelatin/kappa-carrageenan aqueous mixtures

The effects of ionic strength, temperature, and pH on the phase separation behavior of type B pigskin gelatin/sodium-type kappa-carrageenan aqueous mixtures were investigated. Depending on the different combinations of temperature and sodium chloride (NaCl) concentration, the mixtures showed compatible, associative, and segregative phase separation behaviors. Additionally, a coexistence of associative and segregative (associative-co-segregative) phase separations was expected at low temperature and low NaCl concentration. These different phase separation events were observed using confocal scanning laser microscopy. Moreover, it was found that the segregative phase separation when alone is induced by the ordering of kappa-carrageenan chains, while that in the coexistence region is induced by the ordering of gelatin chains. pH had a significant effect on the associative phase separation, resulting in morphologies changing from compatible solution to liquid coacervate and further to solid precipitate with decreasing pH. These were attributed to the dramatic changes of the charge density of amphoteric gelatin during the pH decrease.