Probably it is to do with charge. I guess you are dealing with ionic surfactants. When salts are present, the dissociated salt ions approach the charged surfactant and screen it, allowing them to come together more easily.
Yes, I am using the cationic surfactant ( cetyltrimethylammonium bromide). The standard free energies of micelle formation of cetyltrimethylammonium bromide in pure water and in the presence of NaCl and KCl are -24.80 kJ/mol, -24.82 kJ/mol and -25.06 kJ/mol respectively. Why is the value of the standard free energies of micelle formation of cetyltrimethylammonium bromide in the presence of NaCl is high in comparison to KCl?
That is a good question. I am not sure but having a look online it seems that it might be related to Fajans rules: http://en.wikipedia.org/wiki/Fajans%27_rules, which say that Potassium Chloride should have a more ionic character.
Apparently Potassium dodecyl sulfate is a better cleaner than Sodium dodecyl sulfate because of this rule, so I would imagine this is the reason. I hope somebody can correct me if I am wrong.
Dear Dr. Bhattarai,
When you add electrolyte, you increase the ionic strength so the diffuse electric layer electroneutralizes your micelle much more efficiently. In pure water, the charged surfactants in your micelle repulse each other strongly, and the added salt decrease this repulsion through Debye-Hueckel screening, thus decreesing the energy for micelle formation.
There is also a well-known ion specific effect due to the specific ion-micelle interactions such as van der Waals, hydration and other forces. You can check the Adv. Colloid Interface Sci. paper or chapter 2 of Romsted book in my contributions for more detailed information and formulas to calculate the electrolyte effect. Check also the classics - the works of Corrin and Harkins, Shinoda etc.
I like your answer very much.I have data of the standard free energies of micelle formation of cetyltrimethylammonium bromide in pure water and in the presence of NaCl and KCl are -24.80 kJ/mol, -24.82 kJ/mol and -25.06 kJ/mol respectively. Why is the value of the standard free energies of micelle formation of cetyltrimethylammonium bromide in the presence of NaCl is high in comparison to KCl?
Dear Dr. Bhattarai,
I am not sure how is your free energy defined. RTlncmc? And what are the concentrations of the salt? The coion has in general little importance, and sometimes the difference is related to salting out coefficients, other times - to the adsorption of electrolyte. The small difference between pure water and NaCl solution is quite spurious, I should say...
free enegry is RTlncmc. Both the salts (NaCl and KCl) have a concentration of 0.01 M.
Dear Bhattarai,
I find these numbers strange. I just looked at several data sets; here what I am finding:
CTAB / pure water / 25oC: cmc = 0.9-0.95 mM, RTln(cmc/[M]) = -17200-17400 J/mol
When 10 mM NaCl or KCl are added, using eq. 2.145 from the Romsted chapter I told you about, I get cmc = 0.4 mM and RTln(cmc/[M]) = -19400 J/mol.
There can be a small difference between NaCl and KCl, but it should be something like 0.4 vs. 0.41...
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