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.

Nitroxyl spin probe in ionic micelles: A molecular dynamics study

“…The COO group in total is negatively charged, hence, it repels anionic headgroups and attracts the cationic ones. This effect was previously observed by us for acid-base and solvatochromic probes [12][13][14], where hydroxyl and carboxyl groups in neutral and, even more, anionic protolytic forms were strongly screened by cationic headgroups and, thus, had limited amount of water molecules around. The remaining 6% of the difference correspond to the increased fraction of hydrocarbon, which consequently is attributed to the slightly deeper immersion into micelles (Fig.…”

“…Simulations were run for 40 ns, the first 10 ns of the resulting trajectories were treated as equilibration and not used in calculations. The multistage procedure of preparation of initial configurations is described in detail in our previous papers [13]. Three 11 ns simulations in pure water were also performed, the first 1 ns was omitted in calculations as equilibration.…”

“…It allows direct observation of the microscopic details of the system, which are unachievable for most experimental means. Hence, the MD method was actively applied firstly to individual micelles [9,10], and then to micelles with solubilized molecular probes, including the fluorescent [11], acid-base [12], and solvatochromic [13,14] ones. Yet, for the group of spin probes such studies still are rather scarce [15][16][17], albeit they are numerous for lipid bilayers [18,19] and proteins [20,21].…”

Solvatochromic and Acid–Base Molecular Probes in Surfactant Micelles: Comparison of Molecular Dynamics Simulation with the Experiment

“…In total, seventeen molecular probes were examined mainly in SDS and CTAB micellar solutions. The results of our research group have been reported in seventeen articles from 2016 to the present [53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69]. We thought it is expedient to summarize the data obtained and the regularities found.…”