Geometrical influences, dispersed-phase

Figure 16.1 Schematic representations of the various geometrical and spatial characteristics of particles of the dispersed phase that may influence the properties of composites (a) concentration, (b) size, (c) shape,...

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

A device consisting of an array of frustum-shaped cells that contain a drug dispersed in a permeable matrix is shown to obey zero-order release kinetics following an initial burst phase. Geometric shapes of dissolving solids or diffusion systems and the constraints of impermeable barriers influence mass transport and can be exploited as in the constant release wedge- or hemispheric-shaped devices. 

The quality of separation of gas-liquid mixture in separators depends on the velocity (flow rate) of gas, thermobaric conditions, physical and chemical properties of the phases, geometrical parameters and, most importantly, dispersiveness of the liquid phase (distribution of drops over sizes and the parameters of this distribution) that goes into the separator together with the gas flow from the delivery pipeline. The key parameter of this distribution is the average size of drops that are formed in the gas flow in the delivery pipeline. Therefore, it depends on the parameters of the pipeline and also on the parameters of the device of preliminary condensation (DPC), which, as a rule, is placed at some distance from the separator (see technological schemes in Chapter 1). Therefore, the efficiency of gas-liquid mixture separation is influenced not only by the parameters of the separator, but also by the particular details of the technological circuit before the separator. 

In this means of interpretation, geometrical factors, including repulsion and dispersion forces and dipolar interactions, dominate. For example, the existence of thermotropic phases of ionic amphiphiles is driven by the formation of strong ion bonding lattices between the head groups, the molecular shape being a secondary factor. Additional groups, that are capable of association can influence the situation dramatically. Thus, the ability of non-ionic amphiphiles to form the anisotropic liquid state must be discussed separately [116]. In par-... 

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