Surfactants are amphiphilic molecules containing both hydrophilic and hydrophobic moieties, capable of reducing interfacial tension and forming self-assembled structures such as micelles and microemulsions. Their functionality is due to the synergism between polar headgroups and nonpolar tails, with classification into cationic, anionic, zwitterionic, and nonionic types based on headgroup ionization, directly influencing interfacial behavior, stability, and application.
Cationic surfactants contain positively charged headgroups (e.g., quaternary ammonium, pyridinium salts) that adsorb onto negatively charged interfaces via electrostatic interactions. They are effective in acidic or neutral environments. However, they face compatibility challenges with anionic species due to precipitation risks.
Anionic surfactants contain negatively charged headgroups (e.g., sulfonate, sulfate esters), stabilizing the stability of micelles via charge repulsion. Their high surface activity and strong foamability make them the core component of detergents and emulsifiers. Though their performance is optimal in alkaline conditions, they are susceptible to salting-out or flocculation with multivalent metal ions.
Zwitterionic surfactants integrate both positive and negative charges (e.g., betaines, amino acid derivatives). Their charge states change dynamically with pH, combining cationic and anionic properties. They exhibit low irritation and high biocompatibility across broad pH ranges, thus, they are suitable for drug delivery, albeit with synthetic complexity.
Nonionic surfactants utilize uncharged polar headgroups (e.g., polyoxyethylene chains, polyols) that adsorb at interfaces via hydrogen bonding or dipole interactions.