Generally for surfactants with same hydrophilic head group and varying carbon chain length, the CMC is less for surfactant with larger carbon chain length. What can be the possible reason(s) for greater CMC value for larger carbon chain length as compared to that of smaller carbon chain length ?
Dear Amit,
You are correct that generally speaking a larger hydrophobic group and the same hydrophilic headgroup should give surfactants with lower CMC. Looks like you have found one of the exceptions. CMC is determined by the solubility of the individual surfactant molecules or ions. Therefore a larger hydrophobic group generally lowers the solubility and thus CMC.
In commercial surfactants there is generally a range in compositions and the supplier only tells you the average composition. It could be that the surfactants with the larger alkyl chain contain more short chain alkyls as pollutants, these short alkyl chain species may cause your measurement to be erroneous. Take for example surface tension. The short alkyl chain species will dominate the surface tension (temporary) as they reach the interface faster, next the recorded CMC is that of the short chain species present as pollutants. If you equilibrate the samples longer the longer alkyl chain species will displace the short chain species, but this may take hours..... It could also be that the headgroups are not exactly identical. Suppose the headgroup is ethylene oxide, than also there some distribution in the number of EO groups, and it could be that in the synthesis of the surfactants, the species with the longer alkyl chains are prone to give surfactants with less, or more EO groups. Final point to check is if all alkyl chains are really alkyl and not alkene..... Alkenes have a typical bend in their chain which generally leads to less well fitting tails into a micelle, hence the CMC is higher.
So if it is important to explain your observed behaviour, please check the composition of the surfactant sample in detail. Some HPLC methods will give full resolution of all species present. Next check for double bonds (alkenes) which can be a simple Iodine test or perhaps better Bromine-test. If the CMCs are your own experimental results and you use surface tension, consider aging your solutions for a few hours and remeasure.
I hope this helps, kind regards, Leo
Dear Amit,
You are correct that generally speaking a larger hydrophobic group and the same hydrophilic headgroup should give surfactants with lower CMC. Looks like you have found one of the exceptions. CMC is determined by the solubility of the individual surfactant molecules or ions. Therefore a larger hydrophobic group generally lowers the solubility and thus CMC.
In commercial surfactants there is generally a range in compositions and the supplier only tells you the average composition. It could be that the surfactants with the larger alkyl chain contain more short chain alkyls as pollutants, these short alkyl chain species may cause your measurement to be erroneous. Take for example surface tension. The short alkyl chain species will dominate the surface tension (temporary) as they reach the interface faster, next the recorded CMC is that of the short chain species present as pollutants. If you equilibrate the samples longer the longer alkyl chain species will displace the short chain species, but this may take hours..... It could also be that the headgroups are not exactly identical. Suppose the headgroup is ethylene oxide, than also there some distribution in the number of EO groups, and it could be that in the synthesis of the surfactants, the species with the longer alkyl chains are prone to give surfactants with less, or more EO groups. Final point to check is if all alkyl chains are really alkyl and not alkene..... Alkenes have a typical bend in their chain which generally leads to less well fitting tails into a micelle, hence the CMC is higher.
So if it is important to explain your observed behaviour, please check the composition of the surfactant sample in detail. Some HPLC methods will give full resolution of all species present. Next check for double bonds (alkenes) which can be a simple Iodine test or perhaps better Bromine-test. If the CMCs are your own experimental results and you use surface tension, consider aging your solutions for a few hours and remeasure.
I hope this helps, kind regards, Leo
The micellar formation is explained by the Gibbs energy of cavity formation for the room of surfactants in water. The greater the gain in Gibbs energy of cavity formation, the smaller the CMC. Gibbs energy of cavity formation = мolar volume of surfactant / isothermal compressibility of water.
You have to be more specific, e.g. what is what you measure, what make you believe you have an exception of the general dependence of cmc on alkyl chain length. Do your measurements really determine cmc?
If you go to relatively long chain length you will have very low solubility. Keep in mind that cmc is the concentration of surfactant dissolved in aqueous phase not the initial concentration, which may include surfactant on container walls and surfactant dispersed in aqueous phase.
As expected, the CMC should decrease with increase in the alkyl chain length. For instance, the cmc values in aqueous solution of DTAB, TTAB and CTAB are 0.015, 0.0037, and 0.00091 mol dm-3 respectively. Similar trend is observed for sodium alkylsulfates. The observed increase in CMC with increase in chain length might be as a result of impurity such as hydrotropic substances in the surfactant used. Such may likely have increased the solubility of the surfactant which in turn will lead to delay in the onset of micelle formation with increase in surfactant concentration. If the surfactants used were synthesised by you, then a detailed separation of possible side reaction products will be necessary as suggested by Leo Vleugels above using HPLC.
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