1. Efficient surfactants
Krister Holmberg
SEPAWA Nordic 2019
Malmö,

2. Surfactants have a driving force to get out of water
solid or
water
oil
air
adsorb at interfaces
aggregate into micelles
crystallize
Surfactant unimers may:

3. Efficiency and effectiveness

4. Efficiency and effectiveness
High efficiency means that less surfactant is need to cover the surfaces

5. Example of the importance of surfactant efficiency
An emulsion with 50 % dispersed phase and drops of 2 mm size
Total area of a drop: 13 mm2 = 13x 106 nm2
Area per surfactant at interface: 1 nm2
Number of surfactants at one drop surface: 13x 106
Corresponds to 2x10-17 mol = 4x10-15 g surfactant
Volume of liquid in one drop: 4 mm3 = 4x10-12 cm3 =
4x10-12 ml = 4x10-15 l
1 gram surfactant per liter dispersed phase, i.e. 0.1 % surfactant based on the dispersed phase, is enough to cover the drop surface

6. How to improve the efficiency?

7. 1. Increasing the length of hydrophobic tail of the surfactant leads to higher efficiency. However …
the surfactant may be difficult ta handle due to poor water solubility
the Krafft point may become too high, particularly for ionic suractants
the cloud point for nonionic surfactants may become too low

8. The Krafft point, or Krafft temperature, is where the curve for unimer solubility vs. temperature and the curve for CMC vs. temperature intersect

9. Surfactants carrying the polyoxyethylene chain may phase separate at elevated temperature, causing clouding

10. A cosurfactant, or a hydrotrope, may be added to raise the cloud point (C.P.) of a hydrophobic nonionic surfactant
C.P. of C13E6: <0 oC
C.P. of C13E6 with butyl glucoside added: 9 oC
C.P. of C13E6 with octyl glucoside added: oC
C.P. of C13E6 with decyl glucoside added: oC
C.P. of C13E6 with toluene sulfonate added: oC

11. 2. Nonionic and zwitterionic surfactants are more efficient than anionic and cationic surfactants – the CMC is typically 100 times lower for a nonionic than for an ionic surfactant
Adsorption at a hydrophobic surface
An adsorption isotherm

12. For ionic surfactants there is an entropy penalty caused by the accumulation of counterions above the self-assembled layer of surfactant ions
Low entropy
High entropy
Real intermediate
situation

13. 3a. Mixtures of an anionic and a cationic surfactant can be very efficient (a catanionic surfactant)
The CMC of such mixtures becomes very low because there is no need for counterions above the mixed surfactant layer – no counterion entropy penalty

14. 3b. Mixtures of an anionic surfactant and a zwitterionic surfactant can be equally efficient
The driving force for formation of mixed micelles consisting of anionic and cationic surfactant is very strong.
The same principle holds for amidobetaines and for amine oxide surfactants.

15. 4. A gemini surfactant is much more efficient than the corresponding monomeric surfactant

16. Gemini surfactants are very efficient

17. Protection of mild steel to 1 M HCl for 4 hours
Protection of mild steel to 1 M HCl for 4 hours. The gemini is cationic

18. 5. Polymeric surfactants are efficient – but they also have shortcomings

19. Polymer adsorption is practically irreversible – but slow
Polymers diffuse slowly – the diffusion coefficient of a sphere is inversely proportional to the radius

20. Poly(vinyl alcohol) on polystyrene
The higher the molecular weight the stronger is the adsorption and the higher amount is adsorbed
Poly(vinyl alcohol) on polystyrene

21. The slow diffusion of polymeric surfactants makes them unsuitable for dynamic processes such as
emulsification
foaming
wetting
However, they are very efficient in stabilizing newly created interfaces, such as oil-water (emulsions) and air-water (foams)

22. Thus, there are different options for obtaining efficient surfactants
Increase the length of the hydrocarbon tail
Use nonionic instead of ionic surfactants
Use mixtures of anionic and cationic (or certain zwitterionic) surfactants
Use gemini surfactants
Use polymeric surfactants (surface active polymers)