Good question. To my knowledge there is no formula to do that. The reason is that different detergents respond differently to ionic strength. As you correctly noted, CMC will go down as ionic strength increases, but how much it goes down varies depending on the detergent. For example, at 0-50 mM ionic strength, the CMC of SDS is 0.2-0.3%, whereas at 100-200 mM the CMC is .03-.06%. In contrast, the CMC of octyl glucoside, MEGA 9, and others don't decrease much at all when ionic strength is increased from 0-50mM to 100-200mM, and the CMC of CHAPS only decreases by about 2-fold. These values are taken from a pamphlet published more than 20 years ago by Calbiochem entitled 'A Guide to the Properties and Uses of Detergents in Biology and Biochemistry.' In my lab our 'rule of thumb' is to use detergents in buffers that have ionic strength no less than 100mM.

Thank you so much for your input!

Agree. Very good question. Usually the CMC of surfactants is measured in water, while their activity in various solutes. I mention this problem in recently submitted ms. Due to interactions of tested biosurfactnat with cell culture medium components, CMC measured in culture medium was about 2-fold higher than previously measured in water.

Hi,

an interesting piece of literature is also chapter 34 in Methods in Enzymology Vol 463, written by Dirk Linke. However, even in this overview on detetrgents it is stated that their application other than in routine protocols ("where they just work" [citation from the abstract]) the biochmical application of different detergents will stay subject to testing despite available physico-chemical data.

I hope that at least the given citation will be of interst/help.

btw does anybody understand why Daniel Hardy´s answer got a down vote? I cannot find anything wrong about it.

Note: I do neither want to know who it was nor why exactly:

I am just curious to learn if there is anything about it that could be of general interst to understand the biochmeical application of different detergents under varying conditions.

May be because he gives CMCs in % which I have never seen. It is always given in molar, and correctly so. % (wt or vol) gives no clue to the molarity.

Most of the cmc values I have seen were in % (wt/vol) in the literature (now they also write the cmc in molar concentration as well). I do not think it is that big of a deal, since as a scientist, you can always use a little math with the molecular weight already given :)

I think to be more accurate, we will use Isothermal Titration Calorimetry to calculate the specific cmc value for the detergent in our case.

Thank you so much for all the inputs! I appreciate everyone paying attention and contribute!

this was exactly why I was not very botherd by the % - I must admit though that I am "only" a biologist and working with organic chemists and people alike, I have learned that our "inprecise" handling of concentrations as for example w/v or v/v and of course including % is not very well appreciated in those disciplines (for a reason as I will quickly add here, becasue they do have a point in that).

Toan's question was about a formula, not about CMC values for individual detergents. I listed some CMC concentrations only to illustrate why no generally useful formula exists. The concentrations were given in 'percent' because that's how detergent solutions are typically made and used in biochemistry labs all around the world. Those who work much with detergents would know that SDS and CHAPS are solids (as are most laundry and dishwasher detergents), so the concentrations had to be weight %.

While it's true that CMC values are routinely listed in mM, such values are meaningless for most people who use detergents in their research. In fact, the Calbiochem booklet where I got the values (which the company in turn had pulled from published literature) lists the CMCs in mM. I converted to percent so people would be able to relate the values to their practical experience. For example, from the values for SDS the many millions of people who do SDS-PAGE can readily see why the detergent is used at 0.1% in gels run with 125mM Tris-Cl in the resolving gel and with a ~200mM Tris-glycine running buffer.

Hernan's point about interaction of different cations with ionic detergents is easily appreciated if you've ever done your laundry in 'hard' and 'soft' water. Hard water contains divalent cations, primarily Ca and Mg, that precipitate the anionic detergent molecules. That's why detergents formulated for hard water contain phosphates, and why with hard water you have to use more detergent.

As to the down vote, having dealt with incompetent reviewers of manuscripts and grant applications for nearly 30 years, I can say that it's a complete waste of time to try to figure out what's going on inside some people's heads. Indeed, if your work doesn't evoke some resistance, you may not be doing your job as a scientist. Our purpose is to disturb the universe. If we do that, it's inevitable that we'll disturb some individual people along the way.