Ion-Ion Repulsion in Micelle Formation: Understanding Soaps

[Docscientist]

Homework Statement

What do we mean by ion-ion repulsion during micelle formation in soaps?

Homework Equations

None

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.
[/B]

 

[Borek]

positive and Negative charges of water getting repelled by the positive and negative charges of hydrophilic end of soap molecule

And why not attracted? Water molecule is a dipole, whether it is attracted or repelled is just a matter of orientation.

 

[Docscientist]

And why not attracted? Water molecule is a dipole, whether it is attracted or repelled is just a matter of orientation.

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.

 

[Borek]

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.

 

[Docscientist]

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.

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 !

 

[Borek]

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).

 

[Docscientist]

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).

Thank you,Mr Borek