Is there any formula or estimation for the change of critical micellar concentration (CMC) of detergents in different buffers (with different ionic strengths), since I know that the cmc of detergents decreases drastically in the presence of salt.
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.
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.
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).
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