Dear Myriam,

✪ It appears that the confidence is rather low about the optimal concentration, mainly upon the following paper, that would make it an ongoing question.

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https://arxiv.org/pdf/1406.2113.pdf

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✪ you may also check out with Aksay's team whom they are in this game for several years.

https://www.princeton.edu/cbe/people/faculty/aksay/

Best,

J.

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http://pubs.acs.org/doi/abs/10.1021/j100811a503

http://www.academia.edu/3887198/Adsorption_of_sodium_dodecyl_sulfate_on_functionalized_graphene_measured_by_conductometric_titration

http://www.cellulosechemtechnol.ro/pdf/CCT1(2015)/p.69-80.pdf

https://www.princeton.edu/~cml/assets/pdf/pu_13_xxhsieh.pdf

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Thank you Mr. J. Rafiee for your help .

First you should keep in mind that optimum surfactant concentration means the concentration in equilibrium in aqueous phase when consider the relation to cmc. The more surfactant adsorbed the larger the difference between initial and equilibrium concentration. This obviously depends on ratio graphite to solution and degree of delamination.

Keeping this in mind one should expect an optimum close to cmc. If concentration is several times cmc, one has to expect depletion induced by micelles. For kinetic reasons in addition starting concentration should be above cmc to ensure fast diffusion to newly exposed surfaces generated by delamination.