Analytical Sciences

Conformational Study for Ionic Aggregates of Poly(oxyethylene) Dodecyl Ethers with Ferric-Thiocyanate Anion through the Intermediary of Potassium Ion

Yukio YOKOYAMA,* Hidaka ATSUMI,* Sayaka YOSHIDA,* Naoyuki SONE,* and Kazuyoshi UEDA**

*Department of Analytical Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
**Department of Material Science, Faculty of Engineering, Yokohama National University, Tokiwadai, Hodogaya, Yokohama 240-8501, Japan

A possible formation mechanism of the ionic aggregate of poly(oxyethylene) dodecyl ether nonionic surfactants with a ferric-thiocyanate chromophore is presented, and the dependence of their colorimetric sensitivity on the degree of ethoxylation can be clearly explained through both experimental and theoretical approaches. The stoichiometry of ferric thiocyanate was confirmed to be a monovalent anion comprised of the 1:4 complex, [Fe(SCN)₄]⁻, both by the extractive mole-ratio method and the continuous variation method. The poly(oxyethylene) moiety can form a stable helical conformation by including an alkali cation inside the helix, leading such nonionic surfactants into cationic complexes that can produce ionic aggregates with a bulky anionic complex, such as ferric thiocyanate. The colorimetric sensitivity of the ionic aggregates can depend on the inter-charge distance between the cation and the counter anion. The newly introduced conformational ideas based on experimental results are theoretically supported by CHARMM29 molecular mechanics/dynamics simulations. Molecular dynamics simulations successfully provided the most stable conformation for the K⁺-included heptaoxyethylene dodecyl ether aggregated with the bulky counter anion.

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