wetting agent

Hydrophobic drug powders can be infuriating: tip them into water and they float on top, clump into lumps, or refuse to sink. A wetting agent is a surfactant added specifically to coax such powders into letting water in, so they disperse and dissolve instead of repelling the liquid.

A wetting agent works by adsorbing at the solid-liquid and liquid-air interfaces, lowering the relevant tensions and reducing the contact angle so that water can creep over the powder surface and displace the trapped air. Surfactants with a hydrophilic-lipophilic balance roughly in the 7-9 range are typical wetting agents; common pharmaceutical choices include polysorbates, sodium lauryl sulfate, and docusate sodium.

Wetting agents are essential in formulating suspensions of poorly wettable drugs, in granulation to ensure uniform liquid distribution, and in dissolution testing where a small amount is sometimes permitted so a hydrophobic drug truly dissolves rather than floating. The honest caveat is restraint: too much surfactant can cause foaming, alter taste, irritate tissue, or even change drug absorption, so the minimum effective level is used, and the choice is constrained by the route of administration.