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Micelle formation

  1. Feb 26, 2016 #1
    1. The problem statement, all variables and given/known data
    What do we mean by ion-ion repulsion during micelle formation in soaps?

    2. Relevant equations
    None

    3. The attempt at a solution
    I think It is about the positive and Negative charges of water getting repelled by the positive and negative charges of hydrophilic end of soap molecule.This repulsion does not allow the water to dissolve the soap molecules.Hence the soap molecules remain just like a precipitate in a colloidal solution.since ions are responsible for the formation of charges,it is called ion ion repulsion.
     
  2. jcsd
  3. Feb 27, 2016 #2

    Borek

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    Staff: Mentor

    And why not attracted? Water molecule is a dipole, whether it is attracted or repelled is just a matter of orientation.
     
  4. Feb 27, 2016 #3
    I don't know.That is how I imagined it to be.
    I think they can't change their orientation.Maybe they have a particular structure and dislodging themselves out of their position in order to attract is unlikely.
     
  5. Feb 28, 2016 #4

    Borek

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    Staff: Mentor

    There is some ordering, no doubt about it. But it is more about attraction than about repelling.

    I am not entirely sure what the question aims at, especially this "ion-ion" part. Electrostatic forces are definitely involved in keeping the micelles separated, but that's more "micelle-micelle" than "ion-ion". You can break micelles changing pH and charging the hydrophilic groups however, that's definitely where the "ion-ion" repulsion comes into scene - but I feel like that's rather exotic.
     
  6. Feb 29, 2016 #5
    Soap molecules have Na+ and COO- ions in their hydrophilic end.I think since water is a polar solvent it should be capable of dissolving it.But the fact is My book clearly says that soap molecules stand as a precipitate in water.
    So the only key point I am missing out is about how does this ionic ends manage not to dissolve in water molecules though they are capable of doing it.
    One,They might not be able to change their orientation.The Orientation might be in such a way that the Na+ ion is forced towards H+ion and COO- ion is forced towards O- ions.This will make the ions to repel each other.
    Do tell me if I've got something wrong in this,Mr Borek.
    And thanks a lot ! You are an amazing homework helper !
     
  7. Feb 29, 2016 #6

    Borek

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    Staff: Mentor

    Things you wrote about changing orientations and forcing ions to approach ions sound completely off to me.

    Let's say we have a simple soap R-COONa (where R is just some long, aliphatic chain). It will dissociate in water, producing RCOO- and Na+. Fact that the acid is ionized helps to keep it in the solution. However, it is a weak acid, so it will hydrolize:

    RCOO- + H2O ↔ RCOOH + OH-

    This will first - produce some weakly soluble, undissociated acid, second - it will alkalize the solution.

    This is an equilibrium process, it doesn't go to the end.

    Once the pH gets high enough, rest of the acid stays in the solution in the RCOO- form and is capable of creating micelles. This is a separate process (although they do occur at the same time).
     
  8. Mar 1, 2016 #7
    Thank you,Mr Borek
     
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