Wetting of the powder

Surfactants may be included in capsule formulations to increase the wetting of the powder mass and enhance... 

It seems reasonable to conclude, therefore, that dispersibility is related to the wetting of the powder by the liquid rather than to the electrochemical properties of the system. 

The energy dissipated by stirring, which is often used to ensure a satisfactory dispersion and wetting of the powder. 

For the formulation of suspensions, the hydrophobic or hydrophilic solid is dispersed in a aqueous or nonaqueous medium to produce a system that covers a wide particle size range, typically 0.1-5 pm. This process requires the presence of a surfactant (dispersant) that satisfies four criteria (i) wetting of the powder by the Uquid (if) the dispersion of aggregates and agglomerates into single units (iii) comminution of the large particles into smaller units and (iv) stabilisation... 

The adsorption of surfactants at the solid/liquid (S/L) interface determines their efficiency in powder wetting and dispersion. A reduction of the S/L interfacial tension by surfactant adsorption leads to a reduction of the contact angle, which in turn ensures complete wetting of the powder by the Hquid. In addition, the... 

Thus, wetting of the external surface of the powder depends on the liquid surface tension and contact angle. If 0< 90°, cosO will be positive and the work of dispersion negative, such that wetting is spontaneous. The most important parameter that determines wetting of the powder is the dynamic surface tension, dynamic value at shott times). As will be discussed later, depends... 

While tablets and capsules, the two most common solid dosage forms, have their own unique requirements, there are similarities between them. They both require the flow of the correct weight of material into a specific volume, the behaviour of the material under pressure is important and the wetting of the powder is critical for both granulation and subsequent disintegration and dissolution of the dosage form. 

Pigment particles are basically inorganic materials with hydro-phobic surfaces. When brought in contact with water, the pigments do not mix with water unless a surfactant is added to the system. As shown in the Figure 17.16, the first step is the wetting of the powdered material surface in water, with adsorption Step I... 

Capsules disintegrate when the capsule shell dissolves and the powder mixmre is wetted. Hydrophilic excipients promote the wetting of the powder bed (Fig. 4.1). Due to the low compaction of the encapsulated powder, and the easy dissolution of most diluents for capsules, the addition of a disintegrating agent is often not needed for pharmacy preparations. However, when excipients compact easily (e.g. calcium monohydrogen phosphate dihydrate) a disintegrant is recommended. 

Active substances employed in suspensimi eye drops are usually micronised. Polysorbate 80 or 20 may be used for wetting of the powdered active substances. 

Dispersion of the powder in the vehicle. This requires adequate wetting of the powder (external and internal surfaces) by the liquid vehicle, necessitating the use of a wetting/dispersing agent, mostly a nonionic surfactant (such as poly-sorbates) or polymeric surfactant (such as Poloxamer). This process is described in detail in Chapter 7. 

It is clear from equation (3.64) that if 0 < 90° cos 0 is positive and Wd is negative, i.e. wetting of the powder is spontaneous. Since surfactants are added in sufficient amounts (Ydynamk is lowered sufficiently) spontaneous dispersion is the rule rather than the exception. 

The percolation vessel is used for adsorption of liquid on a powder (especially catalyst) [41]. However when a liquid is flowed through a powder, the first interaction that occurs is wetting of the powder followed by adsorption. In order to make distinct these two thermal interactions, a special calorimetric design has been developed with the percolation vessel. The powder is introduced in a metallic cylinder on a sintered metallic section. A first carrier liquid is flowed through the powder to get the wetting (Fig. 2.24). Then the pump is switched to the liquid solute for the adsorption phase. According to the reaction, it will be possible to be back with the first carrier liquid in order to proceed... 

Since the fine dry powder agglomerates spontaneously, and for slip casting It Is necessary to make up a reasonably stable suspension, a dispersion process Is necessary. Parfltt has divided the overall process of dispersion Into three stages wetting of the powder, breaking of the dusters and stabilization. 

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