CMC concentration (critical micelle concentration)

[Bellico]

How is possible to estimate the cmc on a pure theoretical basis?

I know that:

- The shape of the tenside molecule is cubic and has a volume of 1nm^3

- The amount of dissolved tenside can be neglected compared to the amount of tenside that is absorbed at the surface.

Thank you all,

Bellico 

[DrP]

I think you have to plot a graph with a few points on it and look for the knee.  Plot surface tension or something else dependent upon surfactant concentration vs  surfactant loading - a few low concs and a few higher concs and fine the knee in the curve. 

I don't know if you can calculate it or not from an equation.

I had to look up what a 'tenside' molecule was  -  another way of describing a surfactant. Is this an American term?

[Bellico]

I don't know if it is american.

In our experiment the tenside was SDS.

 

[DrP]

51 minutes ago, Bellico said:

I don't know if it is american.

What country are you working in?

 

52 minutes ago, Bellico said:

In our experiment the tenside was SDS.

Sodium Dodecyl Sulphate?  "The critical micelle concentration (CMC) in pure water at 25 °C is 8.2 mM" - for Sodium Lauryl Sulphate on wiki after searching for CMC of Sodium Dodecyl Sulphate on google.

It would depend on your solvent and the concentration of what is in it and what it is too. It is usually trial and error with surfactants - there is no equation to work it out as far as I know (and if there is I'm not sure it is practical).  

 

[Bellico]

I'm studing in europe, but not UK.

Anyway, solvent was water and concentration of SDS was 0.2M

 

And yes, sodium dodecyl sulphate

 

[DrP]

17 minutes ago, Bellico said:

I'm studing in europe, but not UK.

ahh - that would explain the name of the European company I have dealt with called 'Leuna Tenside'...   which I now know means surfactant.  Must be either a German or a general European thing meaning surfactant.  It would explain why I have never heard of it... I never bought any surfactants from then - just polymers and solvents.

 

17 minutes ago, Bellico said:

Anyway, solvent was water and concentration of SDS was 0.2M

And yes, sodium dodecyl sulphate

well - the internet suggests 8.2mM at 25C for sodium lauryl sulphate in pure water  -  that will give you a ballpark to start then. Will need that graph for an exact measurement I would assume for your system.

What are the units here?  - mM?     Is that supposed to mean mols per litre?

  

Edited April 2, 2019 by DrP

[Bellico]

I've a 0.2 mol/L SDS solution

I've calculated the cmc with my collected data but I need to make a theoretical calculation too... only considering:

- The shape of the tenside molecule is cubic and has a volume of 1nm^3

- The amount of dissolved tenside can be neglected compared to the amount of tenside that is absorbed at the surface.