Wetting of powders

Penetration of water into tablets or into granules precedes dissolution. The wettability of 

Rgure 1.17 Equilibrium between forces acting on solid. 

The type of wetting in which a liquid spreads over the surface of the solid is referred to as spreading wetting. The tendency for spreading may he quantified in terms of the spreading coefficient S, where 

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Pre-wetting of powders or fibrous solids. This extends to wet sweeping (if vacuum cleaning is impractical). 

To rate the wetting of powders by water i.e. their hydrophilicity. A definite value can be assigned even in cases where the contact angle is zero. ... 

Lubricants act by interposing an intermediate layer between the tablet constituents and the die wall, to prevent adherence of the granules to the punch faces and dies. Thus, they ensure smooth ejection of the tablet from the die. In addition, many lubricants also enhance the flow properties of the granules. Stearic acid and its magnesium and calcium salts are widely used. The most effective lubricants, such as magnesium stearate, are very hydrophobic and can also prevent wetting of powders and hence retard dissolution (Figure 6.9). 

Lessons learned from paints and inks in achieving complete wetting of powders and separation of particles to micron levels with dispersion stability to forestall settling and caking are quite applicable to aqueous and nonaque-ous flowables. 

The first is the pressure compensation method, originally developed by Bartell and Walton. The principle is that it is not the capillary pressure that is measured (as the weight of a liquid column) but the pressure required to compensate for it, that is the pressure to keep the height inside the capillary at the same level as outside. This principle has found application in the measurement of the wetting of powders, to which we shall return in secs. 5.4i. 

Denoyel, R., Beurroies, I., and Vincent, D. (2000). Microcalorimetric methods for studying vapour adsorption and wetting of powders. J. Thermal Anal. Calorimetry, 70, 483-92. 

The wetting of powders by liquids is very important in their dispersion, for example in the preparation of concentrated suspensions. The particles in a dry powder form either aggregates or agglomerates, as illustrated in Eigure 18.6. 

The phase that is similar to the stabilizing reagent tends to become a dispersion medium, as clearly seen in the example of emulsions stabilized by finely dispersed powders. Such stabilization is possible under the condition of a finite selective wetting of powder, i.e. at finite values of contact angle, 0° < 0<180°. The powders are able to stabilize the phase that poorer wets the particles, while the liquid that is more similar to the powder becomes the dispersion medium. The reasons of such behavior are apparent from Fig. VIII-12. If water droplets covered by hydrophobic powder, such as e.g. carbon black, are placed into hydrocarbon phase (oil), the layer of carbon black due... 

The abundance of new material on the theory and practice of adhesive interaction has made it necessary to eliminate from the present edition a number of topics in particle adhesion. These matters have either been examined by the author in more detail in other monographs or have acquired independent importance. The wetting of powders, for example, was discussed in detail in [8]. 

A slow and continuous binding liquid addition is a prerequisite for obtaining a power consumption that is suitable for use in process control of wet agglomeration (118). Five characteristic phases of power consumption are identifiable (Fig. 4). In the first phase, a wetting of powder mixture takes place without any observable rise in the power consumption. 

Wetting of powders is an important prerequisite for dispersion of powders in liquids, i.e. preparation of suspensions. It is essential to wet both the external and internal surfaces of the powder aggregated and agglomerates. Suspensions are applied in many industries such as paints, dyestuffs, printing inks, agrochemicals, pharmaceuticals, paper coatings, detergents, etc. 

P. M. Heertjes and W. C. Witvoet, Some aspects ofthe wetting of powders. Powder Technol, 3, 339 (1969/1970). 

Several interfacial phenomena may be considered when dealing with colloidal dispersions (i) Charge separation and formation of electrical double layers, (ii) Wetting of powders and the role of surfactants, (iii) Adsorption of surfactants and polymers at the solid/liquid and liquid/liquid interfaces. 

Acceleration of mixing through, for example, better and fester wetting of powders... 

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