Dynamic segregation

B.A. Pint, Experimental observations in support of the dynamic segregation theory to explain the reactive element effect. Oxidation of Metals, 45, 1/2, 1-31 (1996). 

In the case of alumina scales formed on alumina-forming materials, the presence of the garnet phases is explained by the diffusion of RE from the underlying metal to the oxide surface via the oxide grain boundaries. This theory, called dynamic segregation , has been proposed by Pint and is based on ihe oxygen potential gradient across the oxide scale [128], The consequences of the diffusion of the RE towards the gas-oxide interface are, on the one hand, to suppress the diffusion of metal ions, and on the other hand, to form a mixed Al-RE oxide at the oxide surface. 

Pint B A (1996), Experimental Observations in Support of the Dynamic Segregation Theory to Explain the Reactive Element Effect, Oxid Met, 45, 1-37. 

In order to segregate the theoretical issues of condensed phase effects in chemical reaction dynamics, it is usefiil to rewrite the exact classical rate constant in (A3.8.2) as [5, 6, 7, 8, 9,10 and U]... 

The primary mechanisms responsible for most particle segregation problems are sifting, particle velocity, air entrainment, particle entrainment, and dynamic effects (14). 

Dynamic Effects. Particles often differ in their residence, inertia, and other dynamic characteristics which can cause them to segregate, particularly when they are forming a pile such as when charged into a bin or discharged from a chute. 

While thin polymer films may be very smooth and homogeneous, the chain conformation may be largely distorted due to the influence of the interfaces. Since the size of the polymer molecules is comparable to the film thickness those effects may play a significant role with ultra-thin polymer films. Several recent theoretical treatments are available [136-144,127,128] based on Monte Carlo [137-141,127, 128], molecular dynamics [142], variable density [143], cooperative motion [144], and bond fluctuation [136] model calculations. The distortion of the chain conformation near the interface, the segment orientation distribution, end distribution etc. are calculated as a function of film thickness and distance from the surface. In the limit of two-dimensional systems chains segregate and specific power laws are predicted [136, 137]. In 2D-blends of polymers a particular microdomain morphology may be expected [139]. Experiments on polymers in this area are presently, however, not available on a molecular level. Indications of order on an... 

Figure 3.2 Our approach to surmounting the metabolome obstacles of chemical complexity and dynamic range employs sequential extraction followed by parallel analyses. Segregation of the metabolome into subclasses helps minimize chemical interferences, while parallel analyses help to visualize a greater portion of the metabolome.

Dynamics of an Equalisation Basin 560 Dimensionless Kinetics in a Batch Reactor 235 Batch Reactor with Complex Reaction Sequence 240 Single Solute Batch Extraction 442 Mixing and Segregation 394... 

As briefly discussed in Section 1.2, chemical-reaction engineers recognized early on the need to predict the influence of reactant segregation on the yield of complex reactions. Indeed, the competitive-consecutive and parallel reaction systems analyzed in the previous section have been studied experimentally by numerous research groups (Baldyga and Bourne 1999). However, unlike the mechanical-engineering community, who mainly focused on the fluid-dynamics approach to combustion problems, chemical-reaction... 

At the mesoscopic scale, interactions between molecular components in membranes and catalyst layers control the self-organization into nanophase-segregated media, structural correlations, and adhesion properties of phase domains. Such complex processes can be studied by various theoretical tools and simulation techniques (e.g., by coarse-grained molecular dynamics simulations). Complex morphologies of the emerging media can be related to effective physicochemical properties that characterize transport and reaction at the macroscopic scale, using concepts from the theory of random heterogeneous media and percolation theory. 

In this section, we describe the role of fhe specific membrane environment on proton transport. As we have already seen in previous sections, it is insufficient to consider the membrane as an inert container for water pathways. The membrane conductivity depends on the distribution of water and the coupled dynamics of wafer molecules and protons af multiple scales. In order to rationalize structural effects on proton conductivity, one needs to take into account explicit polymer-water interactions at molecular scale and phenomena at polymer-water interfaces and in wafer-filled pores at mesoscopic scale, as well as the statistical geometry and percolation effects of the phase-segregated random domains of polymer and wafer at the macroscopic scale. 

Theoretical calculations [43] based on first principles molecular dynamics discussed in Sect. 3.2.6 have suggested that Mg Al LDHs are most stable for n = 3 (i.e. x = 0.25) and indeed many minerals, including hydrotalcite itself, have this stoichiometry [4]. It has been reported that the synthesis of LDHs (with benzoate or terephthalate anions in the interlayers) from solutions containing Mg/Al = 2, leads to LDHs having the same composition when the synthesis is carried out at moderate temperatures but LDHs with Mg/Al = 3 (plus AlOOH) when the reaction is carried out under hydrothermal conditions [44]. It was proposed that the latter ratio represents the thermodynamically most favorable product. A similar observation has been reported [45] for solutions with Ni VPe = 2, where hydrothermal preparation led to segregation of an LDH with Ni VPe = 3 and Ni Fe 204. An attempt to synthesize a Co sAl LDH resulted in partial oxidation of the Co and formation of a Co o.yCo o.s LDH with complete migration of Al " from the layers to generate interlayer aluminum oxy-species [46]. 

The combination of careful chemical synthesis with NSE and SANS experiments sheds some light on the fast relaxation processes observed in the collective dynamics of block copolymers melts. The results reveal the existence of an important driving force acting on the junction points at and even well above the ODT. Modelling the surface forces by an expression for the surface tension, it was possible to describe the NSE spectra consistently. The experimental surface tension agrees reasonably well with the Helfand predictions, which are strictly valid only in the strong-segregation hmit. Beyond that, these data are a first example for NSE experiments on the interface dynamics in a bulk polymer system. 

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