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


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.

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Divisions: College of Engineering & Physical Sciences > Systems analytics research institute (SARI)
Funding Information: 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 . Appendix A
Additional Information: © 2018, Elsevier. Licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International 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.
Uncontrolled Keywords: Acid-base indicator,Apparent ionization constant,Micelle,Molecular dynamics,Stern layer,Surface potential,Surfactant,Surfaces and Interfaces,Physical and Theoretical Chemistry,Colloid and Surface Chemistry
Publication ISSN: 1873-4359
Last Modified: 15 Apr 2024 07:30
Date Deposited: 14 Jan 2019 11:11
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Related URLs: http://www.scop ... tnerID=8YFLogxK (Scopus URL)
https://www.sci ... 7606?via%3Dihub (Publisher URL)
PURE Output Type: Article
Published Date: 2019-03-20
Published Online Date: 2018-12-26
Accepted Date: 2018-12-23
Authors: Mchedlov-Petrossyan, Nikolay O.
Farafonov, Vladimir S.
Cheipesh, Tatyana A.
Shekhovtsov, Sergey V.
Nerukh, Dmitry A. (ORCID Profile 0000-0001-9005-9919)
Lebed, Alexander V.

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