An MD simulation study of Reichardt’s betaines in surfactant micelles: Unlike orientation and solvation of cationic, zwitterionic, and anionic dye species within the pseudophase
Abstract
Solvatochromic indicators of the pyridinium N-phenolate series, also known as Reichardt’s betaines, or Reichardt’s dyes, are often used for examining not only pure or mixed solvents, but also various colloidal aggregates, such as surfactant micelles, droplets of microemulsions etc. In order to disclose the locus of these molecular probes within the micellar pseudophase, we recently utilized the molecular dynamics (MD) simulations for the standard dye, i.e. 4-(2,4,6-triphenylpyridinium-1-yl)-2,6-diphenylphenolate, and three other dyes of this family of higher and lower hydrophobicity. Both zwitterionic (colored) and protonated (cationic, colorless) species were involved into the research, as these compounds are also used as acid-base indicators for micellar systems. In the present paper, we extended this investigation further. MD modeling was applied to another three dyes incorporated in sodium n-dodecyl sulfate and cetyltrimethylammonium bromide micelles. The following compounds were examined: (i) the most hydrophobic dye, bearing five tert-butyl groups, 4-[2,4,6-tri(4-tert-butylphenyl)pyridinium-1-yl]-2,6-di(4-tert-butylphenyl)phenolate, (ii) a dye with a hydrocarbon loop around the oxygen atom, 4-(2,4,6-triphenylpyridinium-1-yl)-n-(3,5-nonamethylene)phenolate, and (iii) the dye with additional carboxylate group attached to the phenyl group opposite to the phenol, 4-(4-carboxylatophenyl-2,6-diphenylpyridinium-1-yl)-2,6-diphenylphenol. The orientation and solvation of the cations, zwitterions (both colored and colorless), and the anion of the last-mentioned dye in micelles appeared to be dissimilar, depending on the molecular structure and ionization state. The results were compared with those obtained previously for the standard betaine dye. In some cases, the most probable orientation of the dyes in their colorless form was opposite to that of the standard Reichardt’s dye, i.e., their OH group is directed towards the center of the micelle.
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Citations
Nitroxyl spin probe in ionic micelles: A molecular dynamics study
(2020) Kharkov University Bulletin Chemical Series
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