In my Diploma Thesis titled "Untersuchungen über die Wechselwirkung zwischen der Mizellkinetik und der Entmischungsdynamik in wässrigen Lösungen des nichtionischen Tensids C7E3 " (title in english: Investigations on interactions between the micelle kinetics and the critical dynamics in aqueous solutions of the nonionic surfactant C7E3) I used ultrasonic absorption spectroscopy and other methods to find out, if well-known chemical processes, namely the monomer exchange process, are influenced by the critical phenomena which appear close to the demixing point in binary fluids.

I will give a short explanation of this topic for interested visitors:

Surfactants are amphiphile molecules, which means, that they consist of both an hydrophilic as well as a hydrophobic part. Whether a molecule or a part of it is hydrophobic or hydrophilic is given by its possibility to form hydrogen bonds with the surrounding water molecules.

Considering C7E3 (Triethylenglykol monoheyptyl ether), the molecule consists of a hydrophilic part, which is build of three ethylenglykol-units and a hydrophobic alkylchain, build from 7 carbon atoms and 15 hydrogen atoms.

Suche amphiphilic molecules, due to their ambivalent character, are able to form a variety of aggregation forms in order to avoid contact of the hydrophobic parts with the surrounding water. In case of C7E3 the molecules form so-called micelles, which are more or less spherical aggregates, consisting of around 80 single molecules. Micellization is the main effect which is used in cleaning agents, as it allows to solubilize substances which could not be washed away with pure water.

As these structures are permanently influenced by thermal movements, impacts, etc., molecules are forced all the time to leave the micelle and other ones will enter. This process is called the monomer exchange process, which is already known since more than 50 years and it has been the subject of numerous investigations so far. The theories on this topic predict a dependency of the processes relaxation time on the surfactant concentration with the reaction constants as the adjustable parameters.

The class of poly(ethylen glykol)alkylether (CiEj), C7E3 being a member of this class, also shows up a miscibility gab in the temperature-concentration plane. For a particular concentration (the critical concentration) that means, that on approaching a certain temperature (the critical temperature) concentration fluctuations with extremely short lifetimes can be found in the liquid. A characteristic byproduct of these concentration fluctuations is the so-called critical slowing down of chemical processes inside of them.

The main intention of my diploma thesis was to investigate this slowing down in the micellar kinetics and to verify the theories, that were originally made for 'simple' binary fluids for a more complex liquid like a micellar solution.