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large crystals during manufacture. Although the urinary levels of the antibacterial are also lowered by the use of a more slowly dissolving form of the drug, levels are still adequate to produce efficient antibacterial effects.15

1.7 Wetting of powders

Penetration of water into tablets or into granules precedes dissolution. The wettability of the powders, as measured by the contact angle (6) of the substance with water (Fig. 1.17), therefore determines the contact of solvent with the particulate mass. The measurement of the contact angle gives an indication of the nature of the surface. The behaviour of crystalline materials can be related to the chemical structure of the materials concerned, as is shown by the results in Table 1.9 on a series of substituted barbiturates. The more hydro-phobic the individual barbiturate molecules, the more hydrophobic the crystal which

(a)

gL/A

Liquid

Liquid

Figure 1.17 Equilibrium between forces acting on (a) a drop of liquid on a solid surface, and (b) a partially immersed solid.

Table 1.9 Relationship between chemical structure of barbiturates and contact angle (0) with watera

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