Surface Active Agents

Surfactants, named for their behavior at interfaces, positively adsorb at the interfaces of two phases, reducing interfacial tension. Their versatility as emulsifiers, detergents, and foaming agents stems from this ability. Surfactants, often termed amphiphiles, share the property of amphipathy, with molecules having both hydrophilic and hydrophobic portions. The hydrophilic part is called the head, and the hydrophobic part, including an elongated alkyl substituent, forms the tail.

Surfactants are classified into four types based on the charge of the polar head group. Anionic surfactants include compounds like carboxylate, sulfate, and polyoxyethylene sulfonate, cationic ones include ammonium, sulfonium, and phosphonium, non-ionic surfactants contain polyoxyethylene alcohol, sulfoxide, and crown ether, and amphoteric surfactants include triglycine, C-betaine, and N-betaine.

Surfactants form micelles (spherical structures formed by amphiphilic molecules in water) when diluted with water above their critical micelle concentration (CMC). The micelle population varies in size and shape, exhibiting polydispersity.

The Hydrophile-Lipophile Balance or HLB quantifies the balance between a surfactant's hydrophilic and hydrophobic parts. HLB values, ranging from 1 to 40, indicate solubility characteristics. Emulsifiers with the same HLB value may differ in solubility, influencing the type of emulsion formed. Griffin's systematic experiments revealed that the HLB values of emulsifier mixtures are additive. Davies contributed to assigning HLB contribution group numbers to functional groups, providing an equation applicable to both ionic and non-ionic surfactants:

HLB = ∑ (hydrophilic group number) - ∑ (hydrophobic group number) + 7.

This equation enables the estimation of a surfactant’s behavior in formulation design based on its molecular structure.