Efficient surfactants

1. Efficientsurfactants Krister Holmberg SEPAWA Nordic 2019 Malmö, 2019-05-16

2. Surfactant unimers may: Surfactantshave a driving force to get outofwater adsorb at interfaces aggregate into micelles crystallize air oil water or solid

3. Efficiency and effectiveness Efficiency Effectiveness

4. Efficiency and effectiveness Efficiency Effectiveness Highefficiencymeansthat less surfactant is need to cover the surfaces

5. Exampleof the importanceofsurfactantefficiency • 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 dispersedphase, i.e. 0.1 % surfactant based on the dispersedphase, is enough to cover the drop surface

6. How to improve the efficiency?

7. 1. Increasing the lengthofhydrophobictailof the surfactantleads to higherefficiency. However … • the surfactantmay be difficult ta handledue to poorwatersolubility • the Krafft pointmaybecometoohigh, particularly for ionicsuractants • the cloudpoint for nonionicsurfactantsmaybecometoolow

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. Surfactantscarrying the polyoxyethylenechainmayphaseseparate at elevatedtemperature, causingclouding

10. A cosurfactant, or a hydrotrope, may be added to raise the cloudpoint (C.P.) of a hydrophobicnonionicsurfactant • C.P. of C13E6: <0 oC • C.P. of C13E6 with butyl glucosideadded: 9 oC • C.P. of C13E6 withoctylglucosideadded: 48 oC • C.P. of C13E6 withdecylglucosideadded: 37oC • C.P. of C13E6 withtoluenesulfonateadded: 9 oC

11. 2. Nonionic and zwitterionicsurfactantsaremoreefficientthananionic and cationicsurfactants – the CMC is typically 100 timeslower for a nonionicthan for an ionicsurfactant Adsorption at a hydrophobicsurface An adsorption isotherm

12. For ionicsurfactantsthere is an entropypenaltycaused by the accumulationofcounterionsabove the self-assembledlayerofsurfactantions Low entropy High entropy Real intermediate situation

13. 3a. Mixturesof an anionic and a cationicsurfactantcan be veryefficient (a catanionicsurfactant) The CMC ofsuchmixturesbecomesverylowbecausethere is no need for counterionsabove the mixed surfactantlayer – no counterionentropypenalty

14. 3b. Mixturesof an anionicsurfactant and a zwitterionicsurfactantcan be equallyefficient The driving force for formation of mixed micellesconsistingofanionic and cationicsurfactant is very strong. The same principleholds for amidobetaines and for amineoxidesurfactants.

15. 4. A geminisurfactant is muchmoreefficientthan the correspondingmonomericsurfactant

16. Gemini surfactantsareveryefficient

17. Protectionof mild steel to 1 M HCl for 4 hours. The gemini is cationic 12-4-12

18. 5. Polymericsurfactantsareefficient – buttheyalsohaveshortcomings

19. Polymer adsorption is practicallyirreversible – butslow Polymers diffuse slowly – the diffusion coefficientof a sphere is inversely proportional to the radius

20. The higher the molecularweight the stronger is the adsorption and the higheramount is adsorbed Poly(vinyl alcohol) on polystyrene

21. The slow diffusion ofpolymericsurfactants makes themunsuitable for dynamicprocessessuch as • emulsification • foaming • wetting • However, theyareveryefficient in stabilizingnewlycreated interfaces, such as oil-water (emulsions) and air-water (foams)

22. Thus, thereare different options for obtainingefficientsurfactants • Increase the lengthof the hydrocarbontail • Usenonionicinsteadofionicsurfactants • Usemixturesofanionic and cationic (or certainzwitterionic) surfactants • Usegeminisurfactants • Usepolymericsurfactants (surfaceactive polymers)