Efficient surfactants Krister Holmberg SEPAWA Nordic 2019 Malmö, 2019-05-16

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:

Efficiency and effectiveness

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

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

How to improve the efficiency?

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

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

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

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: 48 oC C.P. of C13E6 with decyl glucoside added: 37oC C.P. of C13E6 with toluene sulfonate added: 9 oC

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

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

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

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.

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

Gemini surfactants are very efficient

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 12-4-12

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

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

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

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)

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)