Wetting of Powders by Liquids

Smolders [14] suggested the following relationship for change of 0 with C, 

Wetting of powders by liquids is very important in their dispersion, e.g. in the preparation of concentrated suspensions. The particles in a dry powder form either 

It is essential in the dispersion process to wet both external and internal surfaces and displace the air entrapped between the particles. Wetting is achieved by the use of surface active agents (wetting agents) of the ionic or nonionic type that can diffuse quickly (i.e. lower the dynamic surface tension) to the solid/liquid interface and displace the air entrapped by rapid penetration through the channels between the particles and inside any capillaries . For wetting of hydrophobic powders into 

A useful concept for choosing wetting agents of the ethoxylated surfactants is the hydrophilic-lipophilic balance (HLB) concept. 

Most wetting agents of this class have an HLB number in the range 7-9. 

The process of wetting of a solid by a liquid involves three types of wetting adhesion wetting, W immersion wetting, Wj and spreading wetting, Wg. In every step. Young s equation can be applied  

Wetting and dispersion depends on the liquid surface tension and 0, the contact angle between the Hquid and solid. W, W and Wg are spontaneous when 0 90°, and Wd is spontaneous when 0 = 0. Since surfactants are added in sufficient amounts ( dynamic lowered sufficiently), spontaneous dispersion is the rule rather than the exception. 

Wetting of the internal surface requires penetration of the liquid into chaimels between and inside the agglomerates, a process which is similar to forcing a Hquid through fine capillaries. In order to force a liquid through a capillary with radius r, a pressure p is required that is given by Rideal [7] and Washburn [8], 

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To measure the site energy distribution or other surface properties of powders by measuring heats of immersion as a function of the amount of preadsorbed wetting liquid. Heats of immersion of the partly covered surfaces reveal the site energy distributions. For acid sites on cracking catalysts, for example, adsorbates of different basicity can be used to develop a topographical map of the surface activity. 

PREFERENTIAL. Descriptive of the selectivity of action, either chemical or physiochemical. exhibited by a substance when in contact with two other substances it may be due either to chemical affinity or to surface phenomena. An example of a preferential chemical combination is that of hemoglobin with carbon monoxide, with which it unites 200 times as readily as it does with oxygen when expose to a mixture of the two. Such phenomena as adsorption, corrosion, and the wetting of dry powders by liquids are other examples,... 

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. 

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... 

A typical plot of torque versus amount of liquid (di-butyl-phthalate, DBT) added is given in Fig. 15 for a very porous sodium carbonate powder mixture. The increase in measured torque at around 25% by volume DBT and again at around 65 %, can easily be seen (the final decrease in torque upon the formation of the wet cake is not shown). These two points correspond to the condition under which the continuous network of bridges forms, at about 25% liquid present, and the formation of the dry paste, at around 65%, where enough binder is available to fill most internal voids. The amount of liquid in Fig. 15 is given as a fraction of the total volume of powder, instead... 

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