Surfactant

1. Active on surface

2. Have tendency to adsorb at surfaces and interfaces:
- reduce the surface tension of the phase
- reduce the interfacial tension between 2 phases

3. The tendency depends on nature of the surfactant and the phase

4. Surface active:
- characteristic of liquid (water) due to cohesion action of liquid molecules
- storage energy at the surface
- energy arises due to presence of asymmetric forces on the surface

5. Surface tension:
- molecules in the interior experience attractive forces from neighboring molecules (surround on all sides)
- molecules on the surface have neighboring molecules only from one side (side facing the interior) - thus experience attractive force that tend to pull them into the interior


6. Assymmetric force
- surface of liquid will rearrange until the least number of molecules are present on the surface → minimizing surface area
- surface molecules will pack close together than the rest of the molecules in the interior → surface molecules will be more ordered and resistant to molecular disruption (surface seems to have a skin)

7. Prerequisites of surfactants:
- better surfactant should have higher tendency of surface adsorption
- good surfactant should have low solubility in the bulk phase(s)

8. Surfactant must have amphiphilic structures

9. Classification:

- Anionic surfactants (-)
- Cationic surfactants (+)
- Zwitterionic (-+)
- Nonionic (neutral)

10. Commonly used surfactants:
- Sodium lauryl sulfate (-)
- Quaternary and pyridinium cationic surfactant (+)
- Sorbitan esters - span (nonionic)
- Polysorbates - tween (nonionic)
- Poloxamers (nonionic)
- Lecithin (nonionic)