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Hypothesis: In a recent paper, we determined the phase behavior of an aqueous solution of octyl ether octaethylene oxide carboxylic acid ([H + ][C8E8c - ], Akypo™ LF2) and with partial replacement of H + by Na + and Ca 2+ . It was found that even the neat surfactants are liquid at room temperature and that they form only direct micelles for any aqueous content and over large temperature ranges. The aim of the present work was to find an explanation for the clouding in these systems as well as for the coacervation observed at very low surfactant content. We expected that very similar phase diagrams would be found for a full replacement of H + by the mentioned ions., Experiments: We established the respective phase diagrams of the above-mentioned salts in water and determined the structures of the occurring phases in detail with small-and wide-angle X-ray scattering, small-angle neutron scattering, dynamic light scattering, heat flux differential scanning calorimetry, as well as surface tension, ESI-MS, and NMR experiments., Findings: To our surprise, we discovered a new type of nematic phase between an isotropic and a hexagonal phase. Based on the complete description of all occurring phases both in the acidic and the charged surfactant systems, we were able to design a coherent and unified picture of all these phases, including the auto-coacervation at low surfactant concentration, the non-conventional clouding at high temperatures, the unusual liquid crystalline phases in a small domain at high surfactant concentrations, and the L β phase at low temperatures and at very low water content. It turned out that all phenomena are a consequence of the subtle interplay between a) the packing constraint due to the very large head-group, b) the relatively small hydrocarbon chain and c) the tunable electrostatic interactions versus entropy., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Inc. All rights reserved.)

The ternary catanionic system octylammoniumoctanoate/octane/water is studied by combined SANS, light scattering, conductivity, and phase diagram approach in the water-poor microemulsion region. The sphere-to-cylinder growth and branching depends on the concentration, the water-to-surfactant ratio, and the temperature. The unidimensional growth leads to a network of interconnected wormlike micelles. Like most studied linear nonionic surfactants, in this true catanionic system at equimolarity of anionic and cationic surfactant, the curvature toward water increases with temperature, making connections between cylinders less frequent.

Revisiting aggregation of extractant molecules into water-poor mixed reverse micelles, we propose in this paper to identify the thermodynamic origins of synergy in solvent extraction. Considering that synergistic extraction properties of a mixture of extractants is related to synergistic aggregation of this mixture, we identify here the elements at the origin of synergy by independently investigating the effect of water, acid, and extracted cations. Thermodynamic equations are proposed to describe synergistic aggregation in the peculiar case of synergistic solvent extraction by evaluating critical aggregation concentration (CAC) as well as specific interactions between extractants due to the presence of water, acid and cations. Distribution of two extractant molecules in the free extractants and in reverse micelles was assessed, leading to an estimation of the in-plane interaction parameter between extractants in the aggregates as introduced by Bergström and Eriksson. Based on this model, we study the N,N′-dimethyl-N,N′-dioctylhexylethoxymalonamide (DMDOHEMA) and di(2-ethylexyl) phosphoric acid (HDEHP) mixture and show that adding nitric acid enhances synergistic aggregation at the equimolar ratio of the two extractants and that this configuration can be related to a favored enthalpy of mixing. [ABSTRACT FROM AUTHOR]

Abstract: This work is a review of the effect of different monovalent anions (belonging to the Hofmeister series) on a single model amphiphile, phosphatidylcholine molecules in contact with water, in three different geometries, Langmuir monolayers, bilayers and micelles, which differ by area per molecule and radius of curvature. The zwitterionic lipids DPPC (1,2-dipalmitoyl-phosphatidylcholine) and DPC (Dodecylphosphocholine) have been used to form the three geometries. The salt solutions used were NaCl, NaBr, NaNO3, NaI, NaBF4, NaClO4 and NaSCN. Various interfacial and bulk experimental techniques were used in order to study the effect of the anions on the amphiphiles in the three geometries. The interfacial techniques used were surface pressure–area isotherms, Brewster angle microscopy (BAM), Grazing incidence X-ray diffraction (GIXD), infrared reflection absorption spectroscopy (IRRAS). The bulk sensitive techniques used were small-angle X-ray scattering (SAXS), osmotic stress (OS) and light scattering. The experimental results obtained from the three model systems indicate that the interfaces become charged through lipid-ion association. The results were fitted with appropriate theoretical models to provide “binding” or “partitioning” constants of anions in the headgroup region of the lipid molecules. The theoretical analysis strongly suggests that the anions do not associate with the lipid molecules through a local chemical reaction, but rather they partition inside the disordered lipid interfaces. [Copyright &y& Elsevier]