What is CMC (critical micelle concentration) of surfactant exactly

Let me ask my question in an example SLS surfactant.

CMC (critical micelle concentration) of SLS surfactant is 185mg/L. Now

Does it mean each 185mg of surfactant in 1L water form 1 micelle?
For example 1850mg surfactant form 10 micelles in 1L water.

Or up to 185mg surfactant will stay as free surfactant molecule and more than that will form micelles.
For example in 200mg SLS, 185mg will be free and 15mg will form micelles. 

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Comments

  • PharmaPharma Member, Pharmacist
    CMC is the concentration above which a surfactant is no longer soluble (and the water/air interface becomes saturated as a side-effect) but starts to form micelles. The CMC is the lowest concentration at which a surfactant actually behaves as surfactant and not just a dissolved compound (the amount at the water/air interface is actually not considered in 'real' CMC though several methods for the determination of CMC automatically/unwillingly include that effect as well). In case of SLS: If you use it below 185 mg/l, it won't foam and it won't clean. Sure, surface tension and wettability are already lower than plain water but that's not enough for most applications. Even more, CMC is utterly useless in most everyday situations, especially in complex surfactant blends. There is however a tendency that surfactants with a low CMC perform better; it's neither a direc correlation nor scientific in any regard, it's just a trend when looking at commercially available surfactants.
    CMC gives neither an indication on micelle size nor the number of molecules within a micelle. Again, the tendency is that lower CMC mean smaller micelles with less molecules per micelle (= aggregation number). These values also depend on the amount of added salt and other ingredients.
  • @Pharma thanks 😊 
    Some more questions.

    1. In a flat jar a solution would have more water/air interface than the same amount in a thin bottle. Does it mean they need different amount for surfactant to saturate the water/air interface and so different CMC?

    2. Why CMC is utterly useless in most everyday situations, especially in complex surfactant blends?

    3. Does added salt effect micelle size or the number of molecules within a micelle? If yes how and what it does?

    4. What factors (adding or removing some ingredients or special process) effect micelle size and the number of molecules within a micelle?


  • PharmaPharma Member, Pharmacist
    1. No, the amount of molecules within the water/air interface is too small to change anything. Also, the used concentration is commonly way above CMC.
    2. Once you mix two surfactant, the resultin apparent CMC is unknown. And, as said, most everyday situations deal with concentrations well above CMC.
    3. Yes, both. Read a book.
    4. Many. Read a book.
    Recommended online literature: CLICK
  • @Pharma thanks a lot for comments and link. 

    Just one more question 😃

    Does salt (sodium chloride or ammonium chloride) increase the micelle size or decrease it? 
  • PharmaPharma Member, Pharmacist
    Increase. That's why shampoo thickens with salt ;) .
  • AbdullahAbdullah Member
    edited September 14
    @Pharma that is good. 

    Some more questions 😊

    1. If we pass the salt curve and extra salt decrease the viscosity, does it mean it is reducing the micelle size?

    2. If we add salt to surfactant system with some decyl Glucoside and the salt doesn't increase the viscosity, does it mean the salt isn't increasing the micelle size in this system or it is increasing the micelle size but not viscosity?

    For example, SLES, CAPB, decyl Glucoside at 10/1/2 active surfactant, salt doesn't increase the viscosity. Does it mean it doesn't increase the micelle size?

    3. Does increasing the micelle size reduce the irritation of shampoo or increase it? 
  • PharmaPharma Member, Pharmacist
    1. It means you shift to another type of emulsion and the end result would be inverse micelles.
    2. Salt only works well on ionic emulsifiers. It would also work (regarding changing the system, not necessarily changing it to a higher viscosity) with nonionics but takes more salt, too much to be of any use.
    The increase in viscosity has to do with the kind of micelles which are formed, not just their size. For exampel worm-like of sheet-like emulsifier arrangements are more viscous than drop-like ones.
    3. I don't think that micelle size has anything to do with skin irritation.
  • @Pharma which type of emulsion is better? Micelle or inverse micelle? 
  • PharmaPharma Member, Pharmacist
    Depends on the intended use of your product.
  • @Pharma shampoo and body wash
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