Wetting agents:

The degree of wettability depends on the affinity of drugs for water and whether the solids are hydrophilic of hydrophobic. Hydrophilic solids are easily wetted by water and can increase the viscosity of aqueous suspensions. Hydrophobic solids repel water but can be wetted by non-polar liquids. Hydrophilic solids usually can be incorporated into suspensions without the use of wetting agent. The majority of drugs in aqueous suspension are, however, hydrophobic. These are extremely difficult to suspend and frequently float on the surface of water and polar liquid due to entrapped air and poor wetting. Wetting agents are surfactants that lower the interfacial tension and contact angle solid particles and liquid vehicle. The usual concentration of surfactant varies from 0.05 to 0.5% and depends on the solids content intended for suspension. The use of surfactants as wetting agents will also retard crystal growth. On the other hand, employing surfactants at conc. less than 0.05% can result incomplete wetting and greater than 0.5% may solubilize ultra-fine particles and lead eventually to changes in particle size distribution and crystal growth. The high HLB surfactants are also foaming agents; however, foaming is an undesirable property during wetting of suspension for formulation. Polysorbate-80 is still the most widely used surfactant for suspension formulation because of its lack of toxicity and compatibility with most formulation ingredients. The rate of wetting is often determined by placing measured amount of powder on the undisturbed surface of water containing a given conc. of surfactant and measuring the time required to completely wet and sink the powder.