The dye changes its color in different solvents due to different interactions with solvent molecules. After СМС, the dye is solubilized. Its environment, intermolecular interaction is changing. Solubilization of a dye in the hydrocarbon portion of micelles changes its absorption spectrum by the spectrum characteristic of dye solutions in hydrocarbons. An effective indicator indicating the achievement of CMC can be such a dye whose colored ion has a charge that is opposite in sign to the charge of micelles of surfactants. Thus, it is possible to install CMC of anionic surfactants using cationic dyes (for example, pinacyanoyl chloride). Anionic dyes — eosin, fluorescein, sky blue FF — are used to determine CMC of cationic surfactants.

The dye changes its color in different solvents due to different interactions with solvent molecules. After СМС, the dye is solubilized. Its environment, intermolecular interaction is changing. Solubilization of a dye in the hydrocarbon portion of micelles changes its absorption spectrum by the spectrum characteristic of dye solutions in hydrocarbons. An effective indicator indicating the achievement of CMC can be such a dye whose colored ion has a charge that is opposite in sign to the charge of micelles of surfactants. Thus, it is possible to install CMC of anionic surfactants using cationic dyes (for example, pinacyanoyl chloride). Anionic dyes — eosin, fluorescein, sky blue FF — are used to determine CMC of cationic surfactants.

Hi Khaled,

Check this reference: http://www.journalrepository.org/media/journals/CSIJ_53/2018/Nov/Edbey2442018CSIJ44312.pdf

It can be explained on the basis of bonding behaviour of the dye. if we use different solvent for the same dye, we get different results, may be due to formation of stable positions at ground or excited states. Electrons of the dye in the solution form behave according to the media.

Dear Dr. Khaled Edbey, EBT is a pH indicator that contains acidic SO3H and OH-groups that dissociate in the defined pH range. If such a group, for example OH, enters the pi-system of the indicator, then during dissociation, a change occurs in the maximum and intensity of the absorption of the solution. Such negatively charged groups can electrostatically interact with the surfactant cation forming ion associates R-Surf+ . The sulfo-group first forms an ionic associate, it does not enter the pi-system and only a decrease in absorption occurs. This enhances the acidic properties of the OH group of the EBT, it dissociates and the spectrum shifts bathochromic, and the absorption intensity increases. As a result, an ionic associate is formed with the surfactant cation with the OH group already of 1: 2. OH-group is part of the pi-system, and an additional bathochromic shift and increase in absorption occurs. This electrostatic interaction is also preserved in micelles, but according to the law of action of the masses, the equilibrium shifts more strongly towards the formation of an ionic associate 1:2. The minimum, depending on the concentration of the surfactant cation, is not CMC, but the result of the formation of an ionic association in the 1: 1 sulfo group. The electrostatic nature of changes in the spectrum is confirmed by the fact that the minimum practically does not depend on the nature of the surfactant cation (false CMC is almost the same), but the spectrum of the ion associate 1: 2 depends on the nature of the cation (CP+ or CTMA+). The change (decreasing) in the polarity in the microenvironment of EBT also takes place increasing the stability of the associate that additionally enhanced by hydrophobic interaction. See, please, Fresenius Z Anal Chem (1989) 335:111-116 and cited inside our articles.

Dear Dr. Yuri Mirgorod, Sergei Nikolaevich Shtykov, Zafar Iqbal, Marius Pelmus.

Thank you for your valuable answers.

Thank you Prof. Nirmala S.V.S.G for your interest.

Thank you Marius Pelmus for your answer.