Farafonov V. Localization and hydration of organic dyes in surfactant micelles by molecular dynamics simulations

The thesis is devoted to determining the effect of surfactant nature and protonation state on localization, orientation, and hydration of molecules of organic dyes in neutral and ionized states, adsorbed on surfactant micelles, by means of computational chemistry methods. Properties of surfactant micelles without dyes were examined and all atom potential models in frameworks of common force fields were developed, as well. On the example of sodium dodecyl sulfate (SDS) it was shown that the behavior of surfactant monomers during simulation can be determined not only by the parameters of head group atoms, counter-ion, and water, but by the ones of the hydrocarbon tail, as well. The standard force field parameters, derived for the lower alkanes, lead to computational artifacts when micelles with aggregation numbers >100 are simulated. The improved parameters, derived for higher alkanes and lipids, allows obtain correct results. For a series of ionic surfactant micelles, it was determined that the water concentration in surface layers of these micelles is similar that indicates the weak impact of the charge sign and nature of head group and the length of hydrocarbon radical on this quantity. The approach for calculating the microscopic characteristics (localization, orientation and microenvironment composition) of organic dye molecules adsorbed on surfactant micelles was developed. It was applied to a series of solvatochromic and acid-base indicator dyes in micelles of commonly used cationic, anionic and zwitter-ionic surfactants. Analysis of characteristics of a molecule of the standard solvatochromic Reichardt’s dye adsorbed by SDS and CTAB micelles showed that the localization of the molecule is similar in both micelles. Consequently, the difference between the polarity parameters of these surfactant solutions is a result of interactions between the dye molecule and surfactant head groups. The comparison of characteristics of neutral and ionized forms for a series of acid-base indicator dyes allowed assessing the appropriateness of each dye for its use in the indicator method of determination of micelle surface electrostatic potential. It was shown that for both cationic and anionic micelles, the smallest difference in localization and microenvironment of two forms takes place for 4-n-dodecyl-2,6-dinitrophenol. In CTAB micelles only this also takes place for the standard Reichardt’s dye, and in SDS micelles only for 4-n-heptadecyl-7-aminocoumarin.