Dear Atthaphon,

there are some questions to answer, before making a decision.

Do you expect substantial amounts of contamination of compounds of higher surface activity. Any purification steps envisaged?

If you expect relatively high purity related to surface activity, you may try surface tension measurements, maybe pendent drop method starting from higher concentration of a stock solution and then measuring diluted solutions of this.

If there is likely a surface active impurity it would be better to used methods in the bulk solution like measurement with surfactant selective electrodes, conductivity or refractive index measurements, again of a dilution series.

If are are sure about purity, check in the literature the cmc of the surfactants with similar molecular structure (number of carbon atoms in the hydrophobic chains, charge, hydrophilic groups, spacer length. Then measure the surface tension with the sessile drop going down with surfactant concentration.

Use exactly the same procedures as single chain ones. Hence, surface tension, DLS, and ionic conductance (provided they are ionic) suit. In presence of considerable amounts of organic impurities, surface tension is diriment. It shows the presence of a minimum in close proximity of the CMC, whose amplitude is proportional to the amount of organic impurities. Look at an old paper by K.J. Mysels (J. Colloid Interface Sci. 1977?) for more details.

You can mesure it with high precision by potentiometric measurement using surfactant sensitive potentiometric sensor. Performing calibration using standard procedure for electrode response measerement you will obtaine CMC value for the surfactant investigated. First, try the technique with some surfactant for which you know CMC value (from literature), for example - with sodium dodecylsulfate. Good luck!

I agree about the possibilities of impurities, but you may not have enough sample for surface tension or conductivity. If you want to have an idea and you have such a small amount of surfactant, you may want to try 1H NMR. Check this out: 1) Gemini surfactants: a new class of self-assembling molecules (DOI: 10.1021/ja00075a025). 2) Synthesis of new lipophilic ligands and NMR measurement of their critical micellar concentration (CMC), (DOI: 10.1002/bscb.19961050407). Good luck!!!

You guys' answers are very informative and helpful. To be honest, i have totally forgotten about the impurities as the by-products or residues. Even for Sodium Dodecyl Sulfate, i normally recrystallize it before use. So i will think about this for my grmini surfactant as well.

See this: http://pubs.acs.org/doi/pdf/10.1021/ed074p1227

For Gemini Surfactant, see the following classic paper that first coined the term "gemini surfactant" by my postdoc advisor, Prof. Fred Menger: http://pubs.acs.org/doi/pdf/10.1021/ja00004a077

You can use NMR to determine the concentration, and do a serial dilution in the NMR tube- this allows for a very small volume needed, so you can take CMC measurements of a small amount of surfactant. The main problem is that the CMC will be measured either entirely or at least partially (you need at least 10%) in D2O- which does change the CMC slightly.

Tim Davey goes into it a little in this paper http://pubs.acs.org/doi/pdf/10.1021/la971303i

Do not believe that the effect of D2O will be significant relative to effect of contaminants etc. If the cmc is indeed very low you should pre-equilibrate the containers to avoid concentration changes due to ad- and absorption.

There are so many techniques for the cmc determination such as surface tension method, consuctivity as well as fluoroscence quenching technique.

There is a linked, it may be helpfull

http://pubs.rsc.org/en/content/articlepdf/2013/SM/C2SM26926K

The greatest commonly used measurement techniques for studying this area are conductometry, surface tension, UV-Vis spectroscopy and using surfactant selective electrodes.

do it on UV spectrophotometer.

note the abs of solution with varying conc. from 0.001M to 0.01M.

draw the graph and the break point will give u the cmc value of that gemin

do it on UV spectrophotometer.

note the abs of solution with varying conc. from 0.001M to 0.01M.

draw the graph and the break point will give u the cmc value of that gemin

From the calibration graph using surfactant (anionic, cationic or nonionic) sensitive ion selective electrode in area in which you assume the value for its CMC. Procedure for the calibration using potentiometric sensor is standard procedure (Pure and Applied Chemistry, IUPAC), and well known in literature.