Powders surface interaction

Surface Area Determination The surface-to-volume ratio is an important powder property since it governs the rate at which a powder interacts with its surroundings. Surface area may be determined from size-distribution data or measured directly by flow through a powder bed or the adsorption of gas molecules on the powder surface. Other methods such as gas diffusion, dye adsorption from solution, and heats of adsorption have also been used. It is emphasized that a powder does not have a unique surface, unless the surface is considered to be absolutely smooth, and the magnitude of the measured surface depends upon the level of scrutiny (e.g., the smaller the gas molecules used for gas adsorption measurement the larger the measured surface). 

Let us consider two hypothetical phases in our composite, A and B, without specifying their physical state. They conld be a polymer melt and a glass fiber reinforcement during melt infiltration processing, a metal powder and ceramic powder that are being snbjected to consolidation at elevated temperatnre and pressure, or two immiscible polymer melts that will be co-extruded and solidified into a two-phase, three-dimensional object. In any case, the surface that forms between the two phases is designated AB, and their individual surfaces that are exposed to their own vapor, air, or inert gas (we make no distinction here) are labeled either A or B. The following three processes are defined as these surfaces interact and form ... 

Present data illustrate the technique for an in situ determination of surface areas. Related methods had been applied primarily to the study of site distributions in clay minerals, particularly by Russian workers (66), and they were used by Bergmann and O Konski in a detailed investigation of the methylene blue-montmorillonite system (3). In fact, changes in electronic spectra arising from surface interactions received sufficient attention in the past to warrant their review by A. Terenin (65). Most of these investigations involved transmittance spectra but new techniques in reflection spectrophotometry and applications of the Kubelka-Munk relation have facilitated the quantitative evaluation of spectra in highly turbid media (35, 69, 77). Thus, in agreement with the work of Kortiim on powders and anhydrous dispersions (31, 32, 33), our results demonstrate the applicability of the Kubelka-Munk function... 

The dispersion stability, rheology, and consolidation of numerous aqueous and non-aqueous Si3N4 suspensions have been studied extensively [251-257]. Recently a novel class of dispersants for Si3N4 powders in non-aqueous media has been designed and its interactions with the powder surface have been characterised systematically on the basis of surface chemistry and fundamentals of colloidal stabilisation [255, 258]. 

At their melting temperature these powders can be dissolved in the epoxy monomers and they are able to react and participate in the cross-linking reaction through the amide groups (Lennon et al., 2000). For this reason, the cure cycle must be selected in order to keep the polyamide particles below their melting point (in the range 170°C or 220°C, depending on the type of polyamide used), and thus keep their initial shape and size. But in some cases a partial dissolution of the powder surface can improve the particle-polymer network interactions. 

Microporous and, particularly, ultramicropous membranes are more difficult to characterize. Different procedures based on the low-pressure part of the N2 adsorption isotherm have been proposed [36], but they often require knowledge of the shape of the pores and of gas-surface interaction parameters which are not always available. Small angle X-ray scattering (SAXS) is another technique which is well suited to micro-porous powders, but difficult to execute in the case of composite layers, as in microporous membranes. Xenon-129 NMR has recently been proposed [37] for the characterization of amorphous silica used in the preparation of microporous membranes, but the method requires further improvement. Methods based on permeability measurements appear to be limited by the lack of understanding of the mass transport mechanisms in (ultra)microporous systems. 

Unfortunately microcalorimetry experiments for the heat of wetting are difficult to perform due to the care that must be taken to keep the powder surface free from adsorbed impvuities. As a result an approximate method based on an infrared band shift caused by the interaction of the solvent with the solid surface has been developed. Drago often used spectroscopic shifts,, of the OH stretching frequency of phe-... 

Powders generally pack rather densely, particularly in comparison to small-size substrate, and the interaction of luminous gas (for deposition) with powder surface is limited to more or less the top powder layer. Consequently, the mixing by tumbling action is important to treat powder uniformly. The mixing along the axis of tube is also an important factor. If dilferent color powders are placed in different... 

Malghan. S.G., Dispersion of 813X4 powders Surface chemical interactions in aqueous media, Colloids Surf., 62, 87, 1992. 

The Impact of Water Adsorption on the Energetics of Surface Interactions of Powders of Different Crystal Forms... 

For relatively reactive surfaces, specific ion pairs may interact to produce particularly good adsorption to a powder surface. An ion pair may even be formed in situ, particularly if in a low-dielectric medium. Some surfaces are actually heterogeneous and can have both basic and acidic sites, especially near the isoelectric point. Although hydrogen bonding is weak, it is particularly important for polymeries which may have multiple anchoring. 

The detained analysis of ESR powder spectra, as presented in this paper, has two advantages. (1) Reliable parameters of the paramagnetic species are obtained, which may be subjected to profound theoretical analysis. In that way a cluster-surface interaction may be established. (2) Quantitative measurements of overlapping signals are possible, as demonstrated for Na-clusters in this paper. This is important chemical information as only the metallic clusters are catalytically active [22],... 

We wish to report some work on the adsorption of acid and alkali-precursor gelatins, the water-soluble products of collagen. The study of both types allowed us to determine whether the differences exhibited by the two kinds in solution are reflected in the adsorbed state, and offers some insight into the state of flexible molecules at interfaces. The use of two dissimilar adsorbents, glass and stainless steel powders, should yield information on segment-surface interactions during the adsorption process. Some effects of added calcium ions were also studied since the Ca-ions of hydroxyapatite in bone and teeth are intimately related to collagen. 

For structuring, the IL has to be immobilised. This can be done using i.e. zeolitic structures or molecular sieves. It is obvious that with increasing surface area of the solid phase, the motion of the liquid and the proton transport will be hindered. From polymerisation experiments it is known that the stiffening of polymers by cross-linking can be compared with the polymer-surface interaction. Electrode surfaces and solids such as silica, carbon black or cathode powder also stiffen the polymer [52]. This can be explained by different transport properties at the interfaces. As a consequence it must be expected that at the surface of the added particles the ionic liquid will behave in a different way than in the immobilised liquid phase. 

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