In search of an optimal acid-base indicator for examining surfactant micelles: Spectrophotometric studies and molecular dynamics simulations

Nikolay O. Mchedlov-Petrossyan*, Vladimir S. Farafonov, Tatyana A. Cheipesh, Sergey V. Shekhovtsov, Dmitry A. Nerukh, Alexander V. Lebed

*Corresponding author for this work

Research output: Contribution to journalArticle

Abstract

We report on combined experimental and theoretical investigations of the water/micelle interface of cationic, anionic, zwitterionic, and non-ionic surfactants using a new hydrophobic acid-base indicator 2,6-dinitro-4-n-dodecylphenol. The indices of the so-called apparent ionization constant, pKa app, of the indicator fixed in the micellar pseudophase are determined by the spectrophotometric method. The data allows estimating the Stern layer's electrostatic potential of the ionic micelles Ψ. Molecular Dynamics modeling was used to locate the dye molecule and, in particular, its ionizing group OH→O within the micelles of the studied surfactants. The comparison of the Ψ values estimated using 2,6-dinitro-4-n-dodecylphenol with both our computer simulation and literature experimental results reveals obstacles in monitoring electrical interfacial potentials. In particular, the Ψ values of the surfactant micelles with alkylammonium groups determined via 2,6-dinitro-4-n-dodecylphenol are overestimated. The reason is specific interactions of the indicator anion with the surfactant head groups. For anionic surfactants, however, this indicator is quite suitable, which is confirmed by the location of HA and A equilibrium forms in the pseudophase.

Original languageEnglish
Pages (from-to)97-107
Number of pages11
JournalColloids and Surfaces A: Physicochemical and Engineering Aspects
Volume565
Early online date26 Dec 2018
DOIs
Publication statusPublished - 20 Mar 2019

Bibliographical note

© 2018, Elsevier. Licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International http://creativecommons.org/licenses/by-nc-nd/4.0/

Funding: The authors express their gratitude to the Ministry of Education and Science of Ukraine for the financial support of this study via grant number 0117U004966.

Keywords

  • Acid-base indicator
  • Apparent ionization constant
  • Micelle
  • Molecular dynamics
  • Stern layer
  • Surface potential
  • Surfactant

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