Contents lists available at ScienceDirect Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa Solvatochromic betaine dyes of different hydrophobicity in ionic surfactant micelles: Molecular dynamics modeling of location character Vladimir S. Farafonov, Alexander V. Lebed, Nikolay O. Mchedlov-Petrossyan ⁎ Department of Physical Chemistry, V. N. Karazin Kharkov National University, 61022, Kharkov, Ukraine GRAPHICAL ABSTRACT ARTICLE INFO Keywords: Betaine dye Solvatochromism Micelle Localization Molecular dynamics ABSTRACT In this paper, the locus and hydration character of three solvatochromic pyridinium-N-phenolate dyes in ionic surfactant micelles were examined using molecular dynamics modeling. These dyes, also called ‘betaine dyes’, are of various hydrophobicity due to different substituents in 2,6-positions of the phenolate moiety. They are as follows: 4-(2,4,6-triphenylpyridinium-1-yl)phenolate, 2,6-dichloro-4-(2,4,6-triphenylpyridinium-1-yl)phenolate, and 2,6-di-tert-butyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate; the abbreviations are BD-H, BD-Cl, and BD- tBu, respectively. The results were compared with those obtained previously with the so-called Reichardt’s standard betaine dye, 4-(2,4,6-triphenylpyridinium-1-yl)-2,6-diphenylphenolate, or BD-Ph. The aggregates of widely used ionic surfactants, sodium dodecyl sulfate, SDS, and cetyltrimethylammonium bromide, CTAB, were selected as the most typical micellar media. The results of modeling shed some light on the state of the solva- tochromic dipoles within the micellar pseudophase. Though the location of the dyes is rather similar, with the phenolate oxygen atom directed toward the Stern layer, the more hydrophobic dyes, BD-tBu and BD-Ph, ap- peared to be immersed somewhat deeper as compared with BD-H and BD-Cl. The average compositions of the local microenvironments of the dye molecules and the hydration of the oxygen atom were estimated. The last finding confirms the higher values of the E T parameter of the interfacial micellar region of SDS as against those of CTAB, in line with experimental data. The versatile information concerning the dye locus was compared with the NMR data available in the literature. Also, the same studies were performed with the protonated colorless forms of the betaine dyes, keeping in mind that these compounds are also often used as acid-base indicators in micellar media. Noteworthy, for all of the dyes under study, and in both micelles, the protonated colorless forms are oriented somewhat different as compared with the corresponding colored dipolar molecules. This should be taken into account when utilizing these compounds as interfacial pH-probes, in particular, for estimating the surface electrical potential of micelles and other colloidal species. https://doi.org/10.1016/j.colsurfa.2017.11.046 Received 18 September 2017; Received in revised form 12 November 2017; Accepted 16 November 2017 ⁎ Corresponding author. E-mail address: mchedlov@univer.kharkov.ua (N.O. Mchedlov-Petrossyan). Colloids and Surfaces A 538 (2018) 583–592 Available online 20 November 2017 0927-7757/ © 2017 Elsevier B.V. All rights reserved. T