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.

The extent of wetting by water is dependent on the hydrophillicity of the materials. If the material is more hydrophilic it finds less difficulty in wetting by water. Inability of wetting reflects the higher interfacial tension between material and liquid. The interfacial tension must be reduced so that air is displaced from the solid surface by liquid.

Wetting agents are surfactants that lower the interfacial tension and contact angle solid particles and liquid vehicle. Non-ionic surfactants are most commonly used as wetting agents in pharmaceutical suspension. Non-ionic surfactants having HLB value between 7-10 are best as wetting agents.


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, also causes stability problem. The high HLB surfactants are also foaming agents; however, foaming is an undesirable property during wetting of suspension for formulation. The concentration used is less than 0.5 %.

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.

Ionic surfactants are not generally used because they are not compatible with many adjuvant and causes change in pH.


Surfactants 

Surfactants decrease the interfacial tension between drug particles and liquid and thus liquid is penetrated in the pores of drug particle displacing air from them and thus ensures wetting. Surfactants in optimum concentration facilitate dispersion of particles.

Disadvantages of surfactants are that they have foaming tendencies and bitter in taste. Some surfactants such as polysorbate 80 interact with preservatives such as methyl paraben and reduce antimicrobial activity.

All surfactants are bitter except Pluronics and Poloxamers. Polysorbate 80 is most widely used surfactant both for parenteral and oral suspension formulation. Polysorbate 80 is adsorbed on plastic container decreasing its preservative action. Polysorbate 80 is also adsorbed on drug particle and decreases its zeta potential. This effect of polysorbate 80 stabilizes the suspension.

Polysorbate 80 advantages are :
  • non-ionic surfactant means no change in pH of medium
  • No toxicity. Safe for internal use.
  • Less foaming tendencies, should be used at concentration less than 0.5%.
  • Compatible with most of the adjuvant.

Hydrophilic Colloids

Hydrophilic colloids coat hydrophobic drug particles in one or more than one layer. This will provide hydrophillicity to drug particles and facilitate wetting. They cause deflocculation of suspension because force of attraction is declined. e.g. acacia, tragacanth, alginates,
guar gum, pectin, gelatin, wool fat, egg yolk, bentonite, Veegum, Methylcellulose etc.

Solvents

The most commonly used solvents used are alcohol, glycerin, polyethylene glycol and polypropylene glycol. The mechanism by which they provide wetting is that they are miscible with water and reduce liquid air interfacial tension. Liquid penetrates in individual particle and facilitates wetting.

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