Character of Localization and Microenvironment of Solvatochromic Reichardt’s Betaine Dye in Sodium n-Dodecyl Sulfate and Cetyltrimethylammonium Bromide Micelles: Molecular Dynamics Simulation Study

Abstract

The problem of using surfactant micellar aqueous solutions as reaction media centers on estimating the polarity of the micellar pseudophase. The most popular approach is the utilization of solvatochromic dyes. Among the last, the strongest ones are the dipolar pyridinium N-phenolate dyes. The complication of such approach, however, consists in the nonuniform character of the environment of the indicator fixed in the micellar pseudophase. The aim of this study is to reveal the character of localization and orientation of the standard solvatochromic pyridinium N-phenolate dye, 4-(2,4,6-triphenylpyridinium-1-yl)-2,6-diphenylphenolate, the so-called Reichardt's dye, within the micellar pseudophase of an anionic (sodium n-dodecyl sulfate, SDS) and cationic (cetyltrimethylammonium bromide, CTAB) surfactants using MD simulations. The locus and hydration of the dye are found to be dependent on the surfactant nature. New approaches are proposed to quantitatively describe the state of the dye within the pseudophase. The results confirm the experimental data, which indicate the higher polarity of the interfacial region in the case of the SDS micelles. Because this dye is also used as an interfacial acid-base probe, the corresponding study is simultaneously performed for its protonated, i.e., cationic form. The neutral and protonated forms of the dye are found to be localized and hydrated in a different way in both SDS and CTAB micelles. This should be taken into account when using the Reichardt's dye as an acid-base indicator for estimating the electrical surface potential of micelles. The presented approach may be recommended to shed light upon the locus of other solvatochromic and acid-base indicators in micelles and micellar-like aggregates.

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

“…GROMACS 5.0 software [15] was employed. The preparation of initial cells involved several stages and is in detail described in our previous papers [9,10].…”

“…The potential models for the dyes were built in the same way as the model for the standard dye [9,10]. For deriving the atomic point charges, we followed the standard OPLS-AA methodology.…”

“…For a more detailed review on the application and instrumental investigation of these dyes, the reader is addressed to ref. [9,10]. https://doi.org/10.26565/2220-637X-2018-30-03…”

“…In a set of previous papers, we made an attempt to reveal probable location and, thus, the nature of the microenvironment of several dyes within the micelles of cationic and anionic surfactants via molecular dynamics (MD) modeling [9][10][11]. Some most widely used pyridinium N-phenolates were already involved in the research project, such as the standard betaine dye I, 4-(2,4,6triphenylpyridinium-1-yl)-2,6-diphenylphenolate (R 1 = R 2 = R 3 = C 6 H 5 ) and three other compounds with different hydrophilicity/hydrophobicity: II (R 1 = R 2 = C 6 H 5 ; R 3 = H), III (R 1 = R 2 = C 6 H 5 ; R 3 = Cl), and IV [R 1 = R 2 = C 6 H 5 ; R 3 = C(CH 3 ) 3 ] [9,10]. Both colored (phenolate) and colorless (phenolic) forms were considered in micelles.…”

Solvatochromic Reichardt’s Dye in Micelles of Sodium Cetyl Sulfate: Md Modeling of Location Character and Hydration

“…Previously, we have studied the character of location of RD in micelles of sodium n-dodecyl sulfate (or lauryl sulfate), SDS, and n-hexadecyltrimethylammonium (cetyltrimethylammonium) bromide, CTAB, dispersed in water [6]. These surfactants were selected as the most popular ones both in academic research and applied science.…”