Dilution and packing of anionic liquid surfactant in presence of divalent and trivalent counter-ions

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Alkyl ether carboxylates are part of a new class of ionic liquids based on the Concept of Melting Point Lowering due to Ethoxylation (COMPLET) and possess unique aggregation behaviour due to the simultaneous anionic and non-ionic nature of the surfactant anion. Previous publications on H[C8E8c] (Polyoxyethylene(8) octyl ether carboxylic acid) and its monovalent salts have shown a variety of phases in aqueous systems with a transition from small spherical micelles at high water contents towards a phase of interdigitated head groups at low water contents. In the present paper, we examine the shorter homologue H[C8E5c] (Polyoxyethylene(5) octyl ether carboxylic acid) and its di- and trivalent transition and rare earth metal salts using X-ray scattering methods in the small- and wide-angle range (SWAXS). These measurements confirm the presence of spherical micelles in diluted aqueous systems which aggregate and whose head groups interdigitate at higher concentrations of the isotropic, non-birefringent ionic liquid. Swelling experiments yield two-dimensional swelling of the microstructure, and combined with the scattering pattern, the data infer prolate spheroidal direct (o/w) micelles with interdigitated head groups, which align to form short linear chains. These chains are offset to form a localised hexagonal close-like packing of the micelles. Most interesting, within this packing, the metal cations are confined into channels between the micelles in linear chains with ion-ion distances of 0.4–1.1 nm. In particular, in the water-free ionic liquid, this confinement may be responsible for some of the unique properties of these systems. It is suggested that the confinement of the cations leads to strong ion-ion aggregation that could be at the origin of the very peculiar magnetic properties of these simple one-component liquids, which contrasts with classical ferrofluids, in which magnetic nanoparticles are dispersed in another liquid.