Surfactant is a general name for substances that absorb to surfaces or interfaces to reduce surface or interfacial tension. These agents aid wetting and dispersion of hydrophobic active pharmaceutical ingredients and they usually act by reducing the interfacial tension between solids and liquids in suspensions. There are several general classes of surfactants used in pharmaceutical suspensions and they include:
- Anionic surfactants
- Cationic surfactants
- Amphoteric surfactants
- Non-ionic surfactants
Anionic surfactants used in pharmaceutical suspensions
Anionic surfactants are organic salts which dissociate at high pH to form a long-chain anion with surface activity. These surfactants contain carboxylate, sulfonate, or sulfate groups.
Anionic surfactants e.g. sodium lauryl sulfate and docusate sodium are used in pharmaceutical suspensions to lower the surface tension between the suspended agent and the suspending medium. However, anionic surfactants can also affect the lower gastrointestinal tract and act as laxatives above certain concentrations. They are thus restricted to uses requiring low concentrations, e.g. to aid wetting of hydrophobic surfaces, including powders, and as solubilizing agents.
Anionic surfactants are ionic in nature, and therefore, there is the potential for interaction and incompatibility with other charged species within the suspension.
Read Also: Surfactants: Types and Applications
Cationic surfactants used in pharmaceutical suspensions
Cationic surfactants are positively charged substances that are generally used as preservatives in pharmaceutical formulations because of their antimicrobial properties, rather than surfactants at relatively low concentrations per se, e.g. benzalkonium chloride. They may also be used in medicated throat preparations, e.g. cetylpyridinium chloride.
Many cationic surfactants can irritate the mucosa at higher concentrations. The phosphatides, e.g., phosphatidylcholine is one group of cationic surfactants that is well tolerated. These are found in lecithin; a component of cell walls, that is obtained from, e.g., eggs, egg yolks, or soybeans.
One disadvantage lecithin has as a surfactant is the potential for interaction and incompatibility with other charged species in the suspension since it is ionic.
Amphoteric surfactants used in pharmaceutical suspensions
When a single surfactant molecule exhibit both anionic and cationic dissociations it is called amphoteric or zwitterionic surfactant. This is the case of synthetic products like betaines or sulfobetaine and natural substances such as amino acids and phospholipids. They are mainly used in the development and manufacture of suspensions used as topical lotions and hair shampoos, and not for internal medicines.
Non-ionic surfactants used in pharmaceutical suspensions
Non-ionic surfactants are the largest group of surfactants used in the formulation of pharmaceutical suspensions. These surfactants are nonelectrolytes; that is, their hydrophilic groups do not ionize at any pH value.
There are several different types of nonionic surfactants available. Since many of the surfactants in this group are esters, they are usually susceptible to hydrolysis under conditions of high or very low pH. The final choice of non-ionic surfactant depends on a variety of factors, but chief among them is the Hydrophilic–lipophilic balance (HLB) value and their chemical compatibility with other components of the formulation.
Examples of substances under this group of surfactants are Polyoxyethylene sorbitan fatty acid esters (Polysorbate, Tween®), Polyoxyethylene 15 hydroxy stearate (Macrogol 15 hydroxy stearate, Solutol HS15®), Polyoxyethylene castor oil derivatives (Cremophor® EL, ELP, RH 40), Polyoxyethylene stearates (Myrj®), Sorbitan fatty acid esters (Span®), Polyoxyethylene alkyl ethers (Brij®), and Polyoxyethylene nonylphenol ether (Nonoxynol®).
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