Surface Thermodynamics: Roles of Surfactants and solutes

Effect of surfactant on interfacial tension

A surfactant or a surface active agent is an organic molecule which is amphiphilic in nature, that is, contains two distinct parts.

           1) Hydrophilic part (the head group)       

           2) Hydrophobic part (the tail group)       

Fig 6.1: Schematic of a surfactant molecule

The hydrophilic (water-loving) head is attracted to water molecules while the lipophilic (fat-loving) tail attaches itself to oil and grease, repelling water. An effective surfactant is usually considered to be one with a hydrophobic tail > 8 Carbon atoms.

While surfactants may be further classified as nonionic (eg. Fatty alcohols), cationic (eg. Cetyl trimethyl ammonium bromide or CTAB), anionic (eg. Sodium dodecyl sulfate or SDS) or zwitterionic (Cocamidopropyl hydroxysultaine) based on the charge of the head group let us focus here on its effect on surface tension that renders it invaluable to us.