Surface powder wetting

Another minor hazard is that water-soluble ceUulose ether powders form a sUppery surface when wet therefore, spiUs should be cleaned promptly to avoid slipping accidents. 

A low surface tension value correlates with a small contact angle. The binder with the smaller contact angle has improved spreadabiiity and can wet powders more effectively (65,84). A surfactant can also be added to the binder solution to improve wettability, especially for hydrophobic powders, and functions to lower both the surface tension as well as the contact angle of the liquid. If the contact angle, 6, is less than 90, then the powder wetting is spontaneous. However, if the contact angle is closer to 180 then the powder would be considered unwettable by the liquid. The pore space within a particle assembly can be simplistically considered as a model capillary. The capillary pressure, Pc, of a liquid is related to the surface tension by the following equation ... 

For agglomerates (as represented in Figure 11.2), which are found in aU powders, wetting of the internal surface between the particles in the structure requires liquid penetration through the pores. Assuming that the pores behave as simple capillaries of radius r, Ap is given by the following equation ... 

The above molecule has a higher cmc (1 gdm ) than Aerosol OT, but it is less effective in lowering the surface tension of water, reaching a value of 30 mN m at and above the cmc. It is, therefore, not as effective as Aerosol OT for powder wetting. 

Most processes of powder wetting operate under dynamic conditions, and improvements in their efficiency require the use of surfactants that lower the liquid surface tension Yiy under these circumstances. The interfaces involved (particles separated from aggregates or agglomerates) are freshly formed and have only a small effective age of some seconds, or even less than a millisecond. 

Cobalt nanoparticles (surface modified with L-cysteine ethyl ester) ethanol wet are available as 10mn black powder (wet with EtOH) which is best used within 3 months. The material can be easily transferred into stable aqueous suspensions, and can be used as a starting material for surface modifications (e.g. dextran coating) or the preparation of magnetic polymer microspheres. 

Thus, wetting of a solid by a liquid depends on /lv and 0, both of which are reduced by the addition of surfactants. Thus, wetting of a powder by an aqueous surfactant solution in usually spontaneous since 0 is close to zero. This only applies for the external surface. However, wetting of the internal surface requires penetration of the liquid into channels between and inside the agglomerates and this requires a high capillary pressure P, as follows ... 

The ATR method is more efficient for in situ studies in the presence of solvent, which fulfills the function of the immersion medium (Section 2.7.3). For low solution concentrations (<10 " M), a typical experiment will involve (1) spreading a reference paste over the IRE surface (this paste contains pure powder wetted with pure solvent and is spread to a thickness comparable to the depth of penetration of IR radiation beyond the IRE-film interface), (2) scanning the background spectrum, (3) treating the paste directly in the accessory or changing... 

A consequence of this low surface tension is that low energy surfaces such as that of Teflon (critical surface tension = 18.5 dynes/cm) are wetted rather well, and although a strong adhesive bond is not obtained to a Teflon surface, powders of Teflon are readily dispersed into fluoroepoxy liquid to form an extender pigment for the resin. More easily wetted surfaces, such as that of glass, are rapidly and thoroughly covered by fluoroepoxy. 

Sodium chloride can be easily polished between use. The crystal is sanded flat with a flnegrade sandpaper if it is freshly cleaved, scratched, or damaged by water. Two polishing laps are prepared. There are different types, but the wet lap can be simply two thicknesses of fine nylon cloth stretched over a flat surface. The wet lap is wetted with water and sprinkled with a little fine polishing powder such as aluminum oxide or cerium oxide. This is rubbed smooth and all excess water is wiped off. The flat salt plate is mbbed about 20 strokes on the wet lap and then, without delay, is buffed about 7 strokes on the dry lap, which can be simply a layer of diaper cloth held flat. Cesium bromide... 

This description is traditional, and some further comment is in order. The flat region of the type I isotherm has never been observed up to pressures approaching this type typically is observed in chemisorption, at pressures far below P. Types II and III approach the line asymptotically experimentally, such behavior is observed for adsorption on powdered samples, and the approach toward infinite film thickness is actually due to interparticle condensation [36] (see Section X-6B), although such behavior is expected even for adsorption on a flat surface if bulk liquid adsorbate wets the adsorbent. Types FV and V specifically refer to porous solids. There is a need to recognize at least the two additional isotherm types shown in Fig. XVII-8. These are two simple types possible for adsorption on a flat surface for the case where bulk liquid adsorbate rests on the adsorbent with a finite contact angle [37, 38]. 

Froth flotation (qv) is a significant use of foam for physical separations. It is used to separate the more precious minerals from the waste rock extracted from mines. This method reHes on the different wetting properties typical for the different extracts. Usually, the waste rock is preferentially wet by water, whereas the more valuable minerals are typically hydrophobic. Thus the mixture of the two powders are immersed in water containing foam promoters. Also added are modifiers which help ensure that the surface of the waste rock is hydrophilic. Upon formation of a foam by bubbling air and by agitation, the waste rock remains in the water while the minerals go to the surface of the bubbles, and are entrapped in the foam. The foam rises, bringing... 

Wettable powders are prepared by blending the toxicant in high concentration, usually from 15 ndash 95%, with a dust carrier such as attapulgite which wets and suspends properly in water. One to two percent of a surface-active agent usually is added to improve the wetting and suspensibiUty of the powder. Sprays of wettable powders are used widely in agriculture because of their relative safety to plants. 

Fig. 3. Typical design elements for wet-in (a)a spinning disk deagglomerates powder just prior to wet-in (b) a rotor breaks clumps as they wet-in, andcentrifugal force helps submerge nonbuoyant powders and (c) a disk impeller provides a rapidly refreshedHquid surface. In (a) and (c), the system may...

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