Demonstrating the complementary nature of SANS, SAXS and Wet-STEM on unfrozen samples

Jeremy Causse, Cyril Lavaud, Johann Ravaux, Joseph Lautru and Renaud Podor*

Institut de Chimie Séparative de Marcoule (ICSM), Univ Montpellier, CNRS, CEA, ENSCM, Bagnols-sur-Cèze, France

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Micellar solutions can be used to treat wastewater containing cationic elements that need to be recovered either for valorisation or for immobilisation on solids in a decontamination process. The technique developed by our team uses a poorly water-soluble organic ligand which migrates to the hydrophobic core of the micelles upon complexation with the cationic element [2]. Migration of the resulting complex to the micelle core leads to the formation of complicated mixtures that are difficult to characterize.
The morphology of complex micellar assemblies is generally described using indirect methods such as small-angle X-ray scattering (SAXS) or small angle neutron scattering (SANS) that can yield to ambiguous descriptions of the nano-objects when the sample contains multiple structures or completely unknown structures. We herein propose to go beyond this limit and to directly observe the nano-objects formed in P123 Pluronic solutions with and without a REE/HDEHP complex using Wet-Scanning Transmission Electron Microscopy (wet-STEM) and to compare the results with sample descriptions derived from SAXS and SANS measurements [1].

The results obtained by the different analytical techniques are consistent. Each analytical method provides complementary information. SANS allows us to finely characterize P123 micelles due to the strong contrast between hydrogenated surfactants and deuterated solvent, SAXS allows us to characterize REE/HDEHP complexes due to the high electron density of REE and Wet-STEM allows direct visualization of the mixture in solution.
The compilation of those complementary information provides a fine description of the internal structure of the objects, their compositions and their mutual interactions. The main contributions of wet-STEM microscopy are to describe the nano-objects individually and, in particular, to show that the REE content can vary from 1 to 10 from one micelle to another, to observe the dynamics of the nano-objects in the liquid and to prove the co-existence of nano-objects with very different morphologies in the same system.

[1] J. Causse, C. Lavaud, J. Ravaux, J. Lautru, R. Podor, Characterization of complex micellar systems by Scattering techniques (SAXS and SANS) and wet-scanning transmission electron microscopy (wet-STEM), Colloids and Surfaces A: Physicochemical and Engineering Aspects 682 (2024) 132928.
[2] C. Lavaud, F. Goettmann, A. Grandjean, J. Causse, Simultaneous lanthanides and surfactants micelles removal from aqueous outflows by complexation and sol–gel chemistry, Separation and Purification Technology 145 (2015) 17–23.

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