Cationic surfactants are normally compound which employ a positive charge based on a nitrogen atom; typical exams are amine and quaternary ammonium compound. Quaternary ammonium compounds have a better functional range than amines as they can retain their positive change over a wide pH range, amines are unable to retain their charge a higher pH (2). Quaternary ammonium compounds have bactericidal properties acting against cell membranes of mostly gram positive bacteria but also some gram negative bacteria. These properties lend them well to application such as the cleaning of open wounds and cleaning of equipment used in pharmaceutical applications (3).
Zwitterionic surfactants can hold both a negative and a positive charge depending on the pH of the solution in which it is employed, the positive charge is almost exclusively associated with a nitrogen atom in an ammonium group and the negative charge is usually held on a carboxylate, sulphate or sulphonate group. As is the case with cationic if the ammonium group is quaternary it is able to retain its charge over a large pH range. When at the isoelectric point each of the charged groups will be fully ionised and due to this net neutral charge the zwitterionic surfactants will have similar properties to that of a non ionic surfactant (2). Zwitterionic surfactants are employed in dermal applications such as shampoos and other cosmetics.
Non ionic surfactants are commonly based around fatty acids, fatty alcohols, fatty amines, and alkly phenols with the addition of a polyoxyethylene group as a hydrophilic portion (2). An examples of a non ionic surfactant are spans, these sorbitan esters have a low solubility in water and are frequently employed as emulsifiers in water in oil emulsions and as wetting agents. Conversely tweens are polysorbates that are miscible in water and therefore used as emulsification agents in oil in water emulsions (3).
As previously stated surfactants are used to reduce the surface tension at the surface or interfaces of liquids. Surface tension is a result of the inner attractive forces of the molecules within the liquid the imbalance of these forces results in a contraction of the liquids at the surface or interface. Due to the characteristics of a surfactant having hydrophobic and hydrophilic regions they are drawn to this interface by the hydrophobic group in an aqueous matrix and by the hydrophilic group in oil. When positioned at the interface the attractive forces between the surfactant molecule and the water molecules is less than of two water molecules thus reducing the surface tension at the interface. As the concentration of surfactant increases the surface tension at the interface decreases up to the point that the interface is then saturated with surfactant molecules, beyond this point any increase in the concentration of surfactant will caused the surfactant molecules in the liquid to converge in on one another. The coming together of these surfactant molecules will form spherical shapes, arranged in an aqueous medium with the hydrophobic groups all facing inwards away from the medium, these aggregations of surfactant molecules are known as micelles. The point at which these are formed is the point at which the increase in concentration of surfactant no longer has a significant effect on the surface tension. This point is known as the critical micelle concentration (2).
Solubilisation is an application of surfactants in pharmaceuticals that exploits the characteristics of micelle formation at the higher concentration of surfactants in solution. The principle of solubilisation is based on the adsorption of an insoluble drug molecule into the structure of the micelle. With the insoluble drug now surrounded by the protruding hydrophilic regions of the micelle this enables it to be taken up into aqueous solution. This is very useful for formulation of otherwise insoluble drugs, soaps can be used to solubilise compounds such as iodine to form micelles, and resulting solutions are called iodophors. Iodophor solutions display a marked increase in stability when compared to standard iodine solutions and are useful as detergents for instruments as corrosion is reduced. This mechanism is extremely useful in formulation of steroid ophthalmic products as steroids have a low solubility in aqueous solution, one of the critical attributes of eye drops is to be clear and particle free. Solubilisation by micelles solves this difficulty in formulation of