Effect of the solute on the micellization process by molecular dynamics simulations

Alcohol molecules are known to change micelle features during the CTAB micellization process. For instance, the presence of ethanol brings about an increase of the critical micelle concentration (cmc) of CTAB and a decrease of the aggregation number. These twoeffects become larger raising the ethanol concentration. Longer chain alchols like octanol or nonanol are known to decrease the cmc and to raise the aggregation number. However, it is not yet experimentally possible to investigate the influences of alcohols on the micelle structure and the behavior of the alcohols itself on an atomistic level. That is why in this work molecular dynamics simulations of CTAB with the three alcohols ethanol, 1-hexanol and 3-hexanol in aqueous solution are carried out with regard to the influences of the alcohols on the micelle structure of a pre-formed CTAB micelle and to the behavior of thealcohols itself. This is done, to my knowledge, for the first time. While all three alcohols in their small concentration do not affect the micelle structure, the high ethanol concentrationbrings about a small swelling of the micelle and higher fluctuations of its structure due to penetration of ethanol into the micelle. However, 1- and 3-ethanol in their high concentrations behave similarly, but compared to ethanol differently. Both medium chain alcohols, which like to reside between the surfactant molecules, tend to form comicelleswith CTAB (act as cosurfactants), although a few alcohol molecules also leave the micelles for a certain time. On the contrary, ethanol acts as a cosolvent, enhancing the solubility ofthe CTAB molecules in water. The simulations indicate that the alcohol concentration should be further raised and the simulation times extended, in order to obtain clearer results, sincethe changes are still rather small and the equilibration time might not be reached yet (200 ns). This is supported by an additional simulation of a CTAB micelle in pure ethanol,which shows clearer results: the micelle already breaks down after a short time.