Chromatography Forum

Hi All,

I've read that SDS (sodium dodecyl sulfate) is a popular ion-pair reagent. However, at sufficient concentration (>~8 mM, according to Wikipedia) SDS forms micelles in the mobile phase.

I am wondering about the mode of separation at concentrations above the CMC. Is it IPC, Micellar, or both? Is there a transition above which the IPC mode becomes negligible? Would anyone care to speculate or share experience?

You will always have a little bit of both. In any case, they both go towards the same direction, with MLC providing in general more retention than IPC. MLC provides some advantages in terms of analyte solubilization as now you have introduced a third phase (in addition to mobile and stationary phase) in which your analytes might partition... Propably your loading capacity will be somewhat higher than with IPC.

When working with MLC, temperature and organic concentration must be considered... in general you need to keep organic content in <20-25% (it depends on the hydrophobicity of the organic solvent)...

In short, you will always have the dynamic ion exchange effect of the IPC even when you are working at MLC conditions...

I don't have data to back this up, but I would think that the CMC will change rapidly with increasing concentration of the organic solvent in the mobile phase. A mobile phase whose SDS concentration would be above the CMC in water may be well below the CMC once a bit of organic modifier is added to the mobile phase. Since typical concentrations of an ion-pair reagent are in the range of 10 mM, you may not reach the CMC of SDS in the presence of an organic solvent.

Uwe,

Not quite... according to Fischer and Jandera (J. Chromatogr. B 1996, 681, 3-19) who investigated the CMC of SDS in relation to MeOH, the CMC does not change at all from 0-10% MeOH, then increases slightly (up to 12.5) and the micelle stops forming at 30% MeOH whatever the concentration of SDS... With some variations, this is true for other ion pairing reagents (i.e. the CMC changes a maximum of two fold with increasing concentrations of MeOH, then the micelle disperses at concentratins 10-30% MeOH).

Kostas, thanks for the correction!