Another problem with exact recipies found in literature in reference to
fluid surfactants is, that manufacturers sometimes change the
concentration of the surfactant in the solvent (normally water). We have
observed this with Tinovetin JU (Ciba), which has a slightly different
CMC with every batch. Therefore I always measure the CMC for every new
batch: it's time consuming, but at least you know exactly what you have.
Measuring the cmc has to be done separately for any type of water you
are using (deionized, softened, etc.):
The cmc is determined by weighing the weight of droplets at the outlet
of a mounted pipette we call "Bürette" in German. The droplets being in
equilibrium, their surface tension multiplied by their circumference
equals the mass of the droplet multiplied by gravity.
As I am not able to write formulas in e-mail, please see the attached
jpg image file (68 kb) for the formulas, where:

r= Radius of the pipette opening of the burette.
Y0 ( "gamma zero") =The surface tension of pure water = 72,8 dynes/cm2
Y = surface tension of water+surfactant
M'= mass of droplet+surfactant
M= mass of droplet of pure water.

How to proceed:
*Slowly* let drip 50 droplets of pure water and of water with growing
surfactant concentration into a beaker. Each droplet has to be in
equilibrium.
Weigh the 50 drops and calculate Y (surface tension) with the forumla:

Y= (72.8 x M') / M

which is derived from the formulas attached in the image file.
Plot the results in a graph with x-axes = Y (in dynes/cm2); y-axes =
concentration of surfactant (in% or in g/l, depending on how you
calculate your dilutions). This will give you a falling curve which will
at a certain point level out and stabilize. It is here, that you will
find the CMC, because adding more surfactant will no more lower the
surface tension.

For conservation reports and publications it would be helpful to not
only state the amount of surfactant used, but also the measured cmc.
Without this, the information might not be easily transferred to other
peoples work/experience.
For surfactants sold in powdered form, I have until now never  found a
difference between the CMC on the technical data sheet and the one
actually measured by us.

Good luck!
Karin.


--
Karin von Lerber
Prevart GmbH
Oberseenerstr. 93
CH-8405 Winterthur
Tel. +41 (0)52-233 12 54
Fax. +41 (0)52-233 12 57
e-mail: [log in to unmask]
www.prevart.ch