Surfactants:
Surfactant as the name indicates, are agents that are used to decrease surface activity (surface tension ) of substances.
When surfactants are dissolved in water they orientate at the surface so that the hydrophobic regions are removed from the aqueous environment.
The reason for the reduction in the surface tension when surfactant molecules adsorb at the water surface is that the surfactant molecules replace some of the water molecules in the surface and the forces of attraction between surfactant and
water molecules are less than those between two water molecules, hence the contraction force is reduced. Surfactants will also adsorb at the interface between two immiscible liquids such as oil and
water and will orientate themselves with their hydrophilic group in the water and their hydrophobic group in the oil.
The reason for the reduction in the surface tension when surfactant molecules adsorb at the water surface is that the surfactant molecules replace some of the water molecules in the surface and the forces of attraction between surfactant and
water molecules are less than those between two water molecules, hence the contraction force is reduced. Surfactants will also adsorb at the interface between two immiscible liquids such as oil and
water and will orientate themselves with their hydrophilic group in the water and their hydrophobic group in the oil.
Types Of Surfactants:
Depending on their charge the surfactants may be:- anionic
- cationic
- ampholytic (zwitterionic)
- non-ionic.
Cationic surfactants
cationic surfactants are so called because of their cationic property.they have a vast pharmaceutical importance.
The quaternary ammonium and pyridinium cationic
surfactants are important pharmaceutically because of their
bactericidal activity against a wide range of gram-positive and
some gram-negative organisms.They may be used on the skin, especially in the cleaning of wounds.
Their aqueous solutions are used for cleaning contaminated utensils.
Anionic surfactants:
These have anionic properties.an example is Sodium Lauryl Sulphate BP which is a mixture of sodium alkyl sulfates, the chief of which is sodium dodecyl sulfate, C12 H25 SO4– Na+ is very soluble in water at room temperature, and is used pharmaceutically as a preoperative skin cleaner, having bacteriostatic action against gram-positive bacteria, and also in medicated shampoos is a component of emulsifying wax.
Non-ionic surfactants:
Sorbitan esters are supplied commercially as Spans and aremixtures of the partial esters of sorbitol and its mono- and
di-anhydrides with oleic acid. They are generally insoluble
in water (low hydrophile–lipophile balance (HLB) value)
and are used as water-in-oil emulsifi ers and as wetting
agents.
surfactants mode of action is illustrated in the following diagram.
