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

Abstract

We study molecular arrangements and dynamics in alkyl ethoxylate nonionic surfactant micelles by combining high field (600 and 700 MHz) NMR relaxation measurements with large-scale atomistic molecular dynamics simulations. For spherical micelles, but not for cylindrical micelles, cross relaxation rates are positive only for surfactant alkyl tail atoms connected to the hydrophilic head group. All cross relaxation rates are negative for cylindrical micelles. This effect is reproducible either by changing composition (ratios of the nonionic surfactants) or changing temperature of a single surfactant in order to change the micelle shape. We validate the micelle shape by SANS and use the results as a guide for our simulations. We calculate parameters that determine relaxation rates directly from simulated trajectories, without introducing specific functional forms. Results indicate that relative motions of nearby atoms are liquid-like, in agreement with 13C T1 measurements, though constrained by micelle morphology. Relative motions of distant atoms have slower components because the relative changes in distances and angles are smaller when the moving atoms are further apart. The slow, long-range motions appear to be responsible for the predominantly negative cross relaxation rates observed in NOESY spectra. The densities of atoms from positions 1 and 2 in the boundary region are lower in spherical micelles compared to cylindrical micelles. Correspondingly, motions in this region are less constrained by micelle morphology in the spherical compared to the cylindrical cases. The two effects of morphology lead to the unusual occurrence of positive cross relaxation involving positions 1 and 2 for spheres. File list (2) download file view on ChemRxiv NMR_MD_Nonionic_UCincinnati.pdf (2.58 MiB) download file view on ChemRxiv SI_NMR_MD_Nonionic_UCincinnati.pdf (261.49 KiB)

Studying the “Rigid–Flexible” Properties of Polymeric Micelle Core-Forming Segments with a Hydrophobic Phthalocyanine Probe Using NMR and UV Spectroscopy

“…In the field of dispersed systems, T 1 and/or T 2 relaxation time measurements of macromolecules and colloids have been employed to investigate the motion of the associated water in poly(styrene sulfonate) and poly(allylamine) hydrochloride multilayers, 35 the dependence of proton T 2 relaxation times of particular fragments (amino acid residues) in gelatin on the ratio of κ-carrageenan to gelatin, 41 the dispersion state of cellulose nanocrystals [in the presence of carboxymethylcellulose and poly(ethylene oxide) polymers], 42 as well as the structure, shape, and dynamics of nonionic, PEG-ylated surfactant micelles. 31 Most often, the aforementioned studies involved the analysis of only water molecules, enabling to gain information about the solvated or hydrated domains. On the other hand, such an approach is available only for hydrophilic/amphiphilic microenvironments such as the outer corona or the interior of liposomes/polymersomes but not for the hydrophobic domains with limited mobility (like non-water soluble polymers or their derivatives).…”

“…Although the term “rigid-flexible” polymers most often refers to biopolymers like proteins, it may be also used to describe the structure of micro-heterogeneous systems like self-assembled synthetic block copolymers. 27 − 31 The aforementioned subdomains are hardly detectable via conventional methods as X-ray diffraction or DSC but may be thoroughly studied by means of combined probing and high-resolution NMR. Generally, the miscibility of polymers as well as the entrapment of low-molecular weight molecules (e.g., drugs and chemical probes) in a polymeric matrix is more possible in the amorphous part, especially the flexible one, because co-crystallization, although probable, is uncommon.…”

“…26 The aforementioned approach of NMR relaxometry is particularly useful for structure analysis of certain polymers and their blends [polycaprolactone/poly(vinyl alcohol) 27 and starch/polycaprolactone, 28 nanostructured inorganic (SiO 2 /TiO 2 ) hybrids for drug (anti-HIV inhibitor—nevirapine) delivery applications 29 and amorphous solid dispersions, 30 as well as aggregates of amphiphilic poly(ethylene oxide) derivatives. 31 For the direct investigation of the interactions between block copolymer fragments and the payload of the NOE, including basic measurements, such as 1D NOE or 2D NOESY, is a crucial analytical method. 32 NOE methodologies have been used to study self-organization in dynamic or static systems such as PMs (including hydrophobic/amphiphilic payload loci of solubilization), 33 , 34 polyelectrolyte multilayers on silica, 35 clusters of protic ionic liquids and water, 36 and self-assembled structures of amphiphilic star-shaped polymers.…”

Strong Synergistic Molecular Interaction in Catanionic Surfactant Mixtures: Unravelling the Role of the Benzene Ring

“…Isothermal titration calorimetry (ITC) and proton nuclear magnetic resonance ( 1 H NMR) and two-dimensional nuclear Overhauser enhancement spectroscopy (2D NOESY) are powerful techniques used to study the interactions of mixed surfactant systems. − ITC is employed to measure the binding thermodynamic of polymer–surfactant complexation, hydrogel formation, polymer aggregation, micellization, and guest–host complexation. Additionally, it can also be used to study the dissociation of micelles into monomers or the formation of micelles with increasing surfactant concentration.…”