Structure and Dynamics of Spherical and Rodlike Alkyl Ethoxylate Surfactant Micelles Investigated using NMR Relaxation and Atomistic Molecular Dynamics Simulations

Published

October 29, 2019

Author(s)

Allison Talley Edwards, Abdolreza Javidialesaadi, Kathleen Weigandt, George Stan, Charles D. Eads

Abstract

Predicting and controlling the properties of amphiphile aggregate mixtures requires understanding the arrangements and dynamics of the constituent molecules. To explore these topics, we study molecular arrangements and dynamics in alkyl ethoxylate nonionic surfactant micelles by combining NMR relaxation measurements with large-scale atomistic molecular dynamics simulations. We calculate parameters that determine relaxation rates directly from simulated trajectries, without introducing specific function forms. NMR relaxation rates, which depend on relative motions of interaction atom pairs, are influenced by wide distributions of dynamic time scales. We find that relative motions of neighboring atom pairs are rapid and liquid like, but are subject to structural constraints imposed by micelle morphology. Relative motions of distant atom pairs are slower than nearby atom pairs because changes in distances and angles are smaller when the moving atoms are further apart. These slow relative motions contribute to predominantly negative cross relaxation rates. For spherical micelles, but not for cylindrical micelles, cross relaxation rates are positive only for surfactant tail atoms connected to the hydrophilic head group. This effect is related to the lower packing density of these atoms ar the hydrophilic-hydrophobic boundary in spherical vs. cylindrical arrangements, with correspondingly rapid and less constrained motion of atoms at the boundary.

Citation

Langmuir

Volume

35

Issue

43

Pub Type

Journals

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Keywords

Micelles, NMR, Dynamics, Structure, Molecular Simulations

Citation

Edwards, A. , Javidialesaadi, A. , Weigandt, K. , Stan, G. and Eads, C. (2019), Structure and Dynamics of Spherical and Rodlike Alkyl Ethoxylate Surfactant Micelles Investigated using NMR Relaxation and Atomistic Molecular Dynamics Simulations, Langmuir, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=928058 (Accessed January 19, 2025)