Unlike in water where the hydrophobic effect drives amphiphile self-assembly, aggregation in organic solvents relies on distinct forces such as hydration of polar heads, metal ion complexation, or directional interactions. Well-studied and still actively researched examples include lipophilic amphiphilic extractants that form reverse assemblies via ion binding, and low-molecular-mass organic gelators that self-assemble into extended supramolecular networks often limiting interfacial adsorption despite their amphiphilic nature. This mini-review focuses on these illustrative and timely examples of reverse self-assembly in organic media, highlighting their relevance for applications in separation, materials science, and biotechnology. Emphasis is placed on radiation scattering techniques (DLS, SAXS, SANS) that provide essential insights into the structure and dynamics of these assemblies at nanometer scales, linking supramolecular organization to functional properties.

We report on the synthesis and structural characterization of the polyoxo-20-molybdate wheel, [MoVI 20O60{AsO2(CH3)2}12]12- (Mo20), which can be isolated with an empty cavity (diameter ca. 2 nm) or containing two dimethylarsinate guests, depending on the type of cations used during synthesis. The polyanion Mo20 was characterized by single-crystal XRD, FT-IR, and TGA, and in solution by multinuclear NMR (1H, 13C, 95Mo) and 1H DOSY, as well as x-ray scattering (SWAXS) and mass spectrometry (ESI-MS). We demonstrated a fast, quantitative, and fully reversible transformation of Mo20 to [MoVI 4O12(OH){AsO2(CH3)2}]2- (Mo4) in aqueous solution, whereas Mo20 is cleanly and quantitatively recrystallized from such solution, indicating a unique phenomenon, which is fully supported by all experimental techniques used as well as DFT and MD calculations.

HYPOTHESIS:The adsorption of macro ions onto interfaces covered by neutral surfactants can provide electrostatic stabilization of liquid foams, analogous to conventional ionic surfactants. Here, we investigate how the adsorption of superchaotropic Keggin nano-ions affects both the macroscopic and microscopic structure of liquid foams and their stability. EXPERIMENTS:Foams were prepared using a nonionic surfactant supplemented with either ionic surfactants or nanometer-sized superchaotropic ions at varying molar ratios. Multi-scale foam structural features, including foam height, liquid fraction, bubble size distribution, and thin film thickness, were monitored using an advanced experimental setup combining time-resolved small-angle neutron scattering (SANS) and optical imaging. This approach provides a comprehensive structural picture of the foam and its ageing mechanisms. FINDINGS:Superchaotropic ions stabilize foams as effectively as ionic surfactants, but produce foams with larger initial bubbles due to delayed adsorption at the interface; the resulting quicker liquid flow results in an overall dryer foam. Despite differences in valency and chemistry, the equilibrium foam film thickness converges around 30 nm. Comparison with isolated thin-film measurements suggests that the equilibrium of foam films in three-dimensional foams is governed by additional factors beyond those captured in conventional thin-film experiments. The study demonstrates that nano-ions provide a novel route toward stimuli-responsive foam stabilization, highlights the critical role of interfacial adsorption dynamics in controlling foam properties at all relevant stages, from initial formation to evolution and ultimately collapse, and reveals subtle yet fundamental differences between the behavior of isolated thin films and foam films embedded within three-dimensional foams.

This work investigates the interaction of anionic borate clusters with self-assembled surfactant systems. Anionic borate clusters, such as closo-dodecaborates B12X122- (X = H, Cl, Br, I) and cobaltabisdicarbollide (COSAN-), bind hydrated solutes via a water-mediated (super)chaotropic effect. Cloud point measurements on the micellar phase of the nonionic surfactant C8E4 reveal a continuous series of binding affinity from chaotropic (B12H122-) over superchaotropic (B12Cl122-, B12Br122-) to hydrophobic ions (B12I122-, COSAN), extending seamlessly the Hofmeister series of classical ions and in line with data for ten additional nanometer-sized ions gleaned from the literature. Small-angle X-ray and neutron scattering revealed a structural criterion distinguishing superchaotropic from hydrophobic ions. Superchaotropic ions form a saturated interfacial monolayer between surfactant headgroups. Hydrophobic ions, however, localize in the apolar interior of the micelle, causing a curvature transition into bilayer structures in C8E4, a surfactant system that does not form bilayers on its own, leading to micellar destabilization and phase separation at higher concentrations.

HYPOTHESIS:Poly(N-isopropylacrylamide) (pNIPAM) microgels are soft particles that adsorb at liquid interfaces and confer emulsion stability against coalescence. Their conformation and interactions at the interface greatly impact the mechanical properties of the interface. In particular, the interfacial elastic modulus increases as the microgel cross-linking density decreases, as a consequence of microgel ability to deform and entangle with neighbors. The purpose of this work is to investigate how these features can be tuned by physical interactions between superchaotropic Keggin nano-ions (POMs) and pNIPAM microgels. EXPERIMENTS:Interactions between polyoxometalates (POMs) and pNIPAM microgels of varying cross-linking densities and sizes are investigated in aqueous suspensions and at liquid/liquid interfaces. The ability of microgels to stabilize oil-in-water emulsions is assessed by evaluating their kinetic stability and flow characteristics, with POMs introduced either before or after emulsification. Cryogenic electron microscopy (cryo-EM) is employed to directly visualize the microgel-stabilized emulsions. The adsorption of microgels, at a model interface and the resulting interfacial elasticity with various POM concentrations, are also studied using the oscillating pendant drop method. FINDINGS:POMs act as physical cross-linkers that promotes microgel deswelling. For large microgels, this effect increases their stiffness. and thus adding POMs reduces the stability of the emulsions. In contrast, small POM-loaded microgels produce highly stable emulsions that resist coalescence under mechanical stress or temperature increase. The same is true for large microgels supplemented in POMs after emulsification. Indeed, POMs enhance interfacial elasticity by promoting both intra-particle and inter-particle crosslinking at the interface. Lastly, by connecting microgel monolayers between neighboring droplet surfaces, POMs promote adhesion between droplets. All levels of the multiscale structure within Pickering emulsions are controlled by interactions between POMs and pNIPAM.

The self-assembly of the cobaltabis(dicarbollide) (COSAN) anionic boron clusters into micelles above a critical micelle concentration (cmc) of 10–20 mM and its behavior as “sticky nano-ions” facilitating controlled protein aggregation have been previously investigated using scattering techniques. These techniques effectively provide average structural parameters but, when applied to colloidal systems, often rely on models assuming polydispersity or anisotropic shapes. Here, we employed sedimentation velocity analytical ultracentrifugation (SV-AUC), which offers the ability to resolve discrete species. We revisited two key questions: (1) the aggregation behavior of COSAN into micelles, a topic still under debate, and (2) the nature of the protein assemblies induced by COSAN, specifically their size/shape distribution and aggregation number. SV-AUC confirms the cmc of COSAN of 16 mM and reveals that COSAN micelles exhibit low aggregation numbers (8 in water and 14 in dilute salt), consistent with recent hypotheses. It shows that COSAN promotes myoglobin aggregation into discrete oligomeric species with well-defined aggregation numbers, such as dimers, tetramers, and higher-order assemblies, depending on the COSAN-to-protein ratio. COSAN binding could be quantified at the lower COSAN/myoglobin ratios. For example, at ratio 5, myoglobin monomer (25

We propose a bimetallic alloy composed of Pt and Ni embedded within laser-induced carbon nanofibers (Pt/Ni-LCNFs) as an enzyme-free transducer for the detection of glucose under physiological pH. Laser exposure on electrospun polyimide nanofibers, embedded with Pt and Ni precursors, facilitated not only the formation of LCNFs but also the generation of Pt/Ni nanoparticles with a radius of approximately 2 nm and a distinctive crystalline structure. X-ray photoelectron spectroscopy revealed the oxidation states of the laser-generated Pt/Ni and confirmed the formation of the Pt/Ni alloy nanocatalysts. Additionally, small-angle X-ray scattering has shown that the graphitic structures of the LCNFs strongly depend on the metal salt concentrations and molar ratio. Pt/Ni-LCNFs were exploited as enzyme-free electrodes for glucose sensing at physiological pH. The presence of Pt in the alloy enabled a low potential (−0.9 V for 20 s) in situ generation of highly localized OH − which facilitated glucose electrooxidation by Ni. Under optimized conditions, Pt/Ni-LCNFs achieved reliable glucose detection in physiological conditions (pH 7.4), with detection limit of 0.3 mM, linearity from 0.1 to 4 mM, and minimal interference from other electroactive species. Self-calibrated data acquisition strategy provided an excellent recovery rate (95 ± 10%) in diluted human serum. Furthermore, unlike enzyme-based sensors, the catalytic activity of Pt/Ni LCNFs was maintained after sterilization, highlighting their robustness and potential in biomedical applications and bioprocess monitoring. Graphical Abstract

The decomposition of scheelite in a synergistic H2SO4-H3PO4 mixture is an effective method for extracting tungsten from mining concentrates. The driving force behind the dissolution of scheelite in the H2SO4-H3PO4 mixture is the formation of a soluble Keggin-type polyoxotungstate, H3PW12O40, which prevents the formation of very low solubility tungstic acid. In this work, a multiparametric study of scheelite dissolution kinetics was carried out in a synergistic acid mixture. In particular, the independent contributions of temperature, acid concentration, and W:P molar ratio on scheelite dissolution kinetics and H3PW12O40 formation yield were evaluated. To this end, a method based on the use of small-angle X-ray scattering (SAXS) was developed to identify and quantify H3PW12O40 under different operating conditions. The results provide a better understanding of the stability range of H3PW12O40 in terms of H2SO4 concentration and stoichiometric W:P ratio. These findings led to the selection of optimized, soft-leaching conditions that ensure rapid dissolution of scheelite while avoiding surface passivation by the precipitation of secondary phases. A comparison with speciation calculations using thermodynamic data reported in the literature reveals an absence of a self-consistent thermodynamic dataset. Thus, measuring the concentration of H3PW12O40 in the leachate was necessary to optimize the dissolution conditions. From this perpective, SAXS appears to be a suitable quantitative method.

Liposomes are self‐assembled lipid bilayer nanostructures with an inner aqueous core used for diagnostic signal amplification, drug delivery, and as biomimics. Small molecules and proteins are typically encapsulated. While the lipid composition is used to control liposome and surface characteristics, overlooked is the effect entrapped molecules may have on the outer surface through interface activity. Here, it is demonstrated how different dyes not only distribute between the aqueous core and bilayer but also significantly affect the outer surface chemistry thus influencing interactions with reaction partners and sample matrices. Specifically, IR‐783, sulforhodamine B (SRB), and 1,3,6,8‐pyrenetetrasulfonic acid (PTSA) are encapsulated in liposomes of standard bioanalytical composition. Spectroscopic and small‐angle X‐ray scattering data indicate interactions of IR‐783 with the liposome membrane and its strong influence on the bilayer structure. Increasing SRB concentrations show potential adsorption at liposome bilayer surfaces, whereas PTSA does not interact with the bilayer itself. Surprising is the correlating effect on biological systems discovered through the complement system as a model. Liposomes incubated with serum reveal complement protein interaction with the liposome surface depending on the dye and not only on the lipid composition. This study emphasizes the need for careful selection of both lipid and encapsulant formulations in any biological application.

Boron clusters are applied in medicinal chemistry because of their high stability in biological environments and intrinsic ability to capture neutrons. However, their intermolecular interactions with lipid membranes, which are critical for their cellular delivery and biocompatibility, have not been comprehensively investigated. In this study, we combine different experimental methods – Langmuir monolayer isotherms at the air–water interface, calorimetry (DSC, ITC), and scattering techniques (DLS, SAXS) – with MD simulations to evaluate the impact of closo -dodecaborate clusters on model membranes of different lipid composition. The cluster anions interact strongly with zwitterionic membranes (POPC and DPPC) via the chaotropic effect and cause pronounced expansions of lipid monolayers. The resulting lipid membranes contain up to 33 mol % and up to 52 weight % of boron cluster anions even at low aqueous cluster concentrations (1 mM). They show high (μM) affinity to the hydrophilic-hydrophobic interface, affecting the structuring of the lipid chains, and therefore triggering a sequence of characteristic effects: ( i ) an expansion of the surface area per lipid, ( ii ) an increase in membrane fluidity, and ( iii ) a reduction of bilayer thickness. These results aid the design of boron cluster derivatives as auxiliaries in drug design as well as transmembrane carriers and help rationalize potential toxicity effects.

In this study, we explore the superchaotropic effect of various polyoxometalate or boron cluster nano-ions on hydrophilic neutral surfaces. Nano-ions, characterized by low charge densities, exhibit strong adsorption on non-ionic hydrophilic surfaces like PEGylated micelles. This adsorption phenomenon was attributed to the enthalpically favorable dehydration of nano-ions, the so-called superchaotropic effect. Here, we investigate the adsorption of three nano-ions, alpha-SiW12O404-, alpha-PW12O403-, and B12I122-, with decreasing charge density or increasing superchaotropicity (or hydrophobicity), on hydrophilic solid surfaces, PEGylated gold nanoparticles, and PEGylated gold-coated quartz crystal. Solid surfaces are devoid of hydrophobic regions, enabling the study of the subtle nuance between hydrophobic and superchaotropic effects. Unlike adsorption on PEGylated micelles, the adsorption constant decreases with a reduced charge density, aligning with the well-established principle that hydrophobic ions do not adsorb on hydrophilic surfaces. This research improves our understanding of the subtle difference between superchaotropic and hydrophobic effects in nano-ion adsorption phenomena. Read this Article

In living systems, protein assemblies have essential functions, serving as structural supports, transport highways for molecular cargo, and containers of genetic material. The construction of protein assemblies, which involves control over space and time, remains a significant challenge in biotechnology. Here, we show that anionic boron clusters, 3,3'-commo-bis[closo-1,2-dicarba-3-cobaltadodecaborane] (COSAN-), and halogenated closo-dodecarboranes (B12X12 2-, X=H, Cl, or I), described as super-chaotropic nano-ions, induce the formation of 2D assemblies of model proteins, myoglobin, carbonic anhydrase, and trypsin inhibitor. We found that the nano-ion concentration reversibly controls the size of the protein assemblies. Furthermore, the secondary structures of the proteins are only slightly affected by assembly formation. For myoglobin, the formation of these assemblies even prevents temperature denaturation, highlighting a preservation effect of nano-ions. Our study reveals that inorganic boron-based nano-ions act as a reversible molecular glue for proteins, providing a potential starting point for the further development of controlled protein assemblies.

The self-assembly of organic amphiphilic species into various aggregates such as spherical or elongated micelles and cylinders up to the formation of lyotropic hexagonal or lamellar phases results from cooperative processes orchestrated by the hydrophobic effect, while those involving ionic inorganic polynuclear entities and nonionic organic components are still intriguing. Herein, we report on the supramolecular behavior of giant toroidal molybdenum blue-type polyoxometalate, namely, the {Mo-154} species in the presence of n-octyl-beta-glucoside (C8G1), widely used as a surfactant in biochemistry. Structural investigations were carried out using a set of complementary multiscale methods including single-crystal X-ray diffraction analysis supported by molecular modeling, small-angle X-ray scattering and cryo-TEM observations. In addition, liquid NMR, viscosimetry, surface tension measurement, and isothermal titration calorimetry provided further information to decipher the complex aggregation pathway. Elucidation of the assembly process reveals a rich scenario where the presence of the large {Mo-154} anion disrupts the self-assembly of the C8G1, well-known to produce micelles, and induces striking successive phase transitions from fluid-to-gel and from gel-to-fluid. Herein, intimate organic-inorganic primary interactions arising from the superchaotropic nature of the {Mo-154} lead to versatile nanoscopic hybrid C8G1-{Mo-154} aggregates including crystalline discrete assemblies, smectic lamellar liquid crystals, and large uni- or multilamellar vesicles where the large torus {Mo-154} acts a trans-membrane component.

Supramolecular polymers built from stimuli-responsive host-guest interactions represent an attractive way of tailoring smart materials. Herein, we exploit the chaotropic effect of polyoxometalates and related host-guest properties to design unconventional polymer systems with reversible redox and thermo-responsive sol-gel transition. These supramolecular networks result from the association of cyclodextrin-based oligomers and Keggin-type POMs acting as electro-active crosslinking agents. The structure and the dynamics of such self-assembly systems have been investigated using a multiscale approach involving MALDI-TOF, viscosity measurements, cyclic voltammetry, 1H-NMR (1D and DOSY), and Small-Angle X-ray Scattering. Our results reveal that the chaotropic effect corresponds to a powerful and efficient force that can be used to induce responsiveness in hybrid supramolecular oligomeric systems. The chaotropic behavior of polyoxometalates is exploited as an operating principle to promote a reversible and thermo-responsive sol-gel transition of cyclodextrin-based polymer where supramolecular networking results from the host-guest interactions between cyclodextrin-based oligomers and Keggin-type POMs acting as electro-active crosslinking agents. This study reveals that the chaotropic effect corresponds to an effective force able to induce responsiveness into supramolecular materials. image

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.

Synergy between salt and sugar Nanometric ions like Keggin polyoxometalates are revealed to induce the cross-linking and hydrogel formation of polymeric sugars, namely non-ionic cellulose ethers, as reported by Max Hohenschutz, Walter Richtering, and co-workers in their Communication (e202210208). This novel gelation phenomenon exceeds the thickening effects of typical additives like classical salts and surfactants by orders of magnitude and is due to the superchaotropicity of the nano-ions, which drives them out of the water bulk onto the polymers.

Hypothesis: Solubilization of hydrophobic compounds in water is commonly performed by using organic solubilizers such as hydrotropes, surfactants, co-solvents, and macrocycles to form host-guest com-plexes. 3,30-commo-bis[closo-1,2-dicarba-3-cobaltadodecaborane] derivatives (COSANs) are fully inor-ganic and non-amphiphilic ionic boron clusters with nanometric size (nano-ions) showing superchaotropic properties as they strongly bind to neutral organic molecules. Therefore, we expect COSANs to act as solubilizers of sparingly water-soluble molecules, but with a mechanism different from all other organic solubilizers known so far. Experiments: The aqueous solubilization efficiency of COSANs towards butanol was evaluated by deter-mining phase diagrams and comparing them to classical solubilizers. Nanostructuration of the mixture was studied using UV spectroscopy, small-angle X-ray, and neutron scattering with contrast variation.Findings: COSANs act as efficient aqueous solubilizers of medium-chain alcohols (0.6 < log P < 1.5). Unlike surfactants, COSAN is an efficient solubilizer in its monomeric state, at concentrations well below its crit-ical aggregation concentration. Solubilization by COSAN takes place with a bi-dimensional anisotropic growth of COSAN/butanol co-assemblies, whereas solubilization by surfactant occurs via an isotropic swelling of micelles. Appealingly, COSANs/2-butanol co-assemblies efficiently solubilize more hydropho-bic compounds with log P values up to around 6, offering new opportunities in many applied fields.(c) 2023 Elsevier Inc. All rights reserved.

Nanometer-sized anions, like polyoxometalates and borate clusters, bind to nonionic hydrated matter driven by the chaotropic effect, which arises from the favorable dehydration of the ions. Herein, we evaluate the adsorption and activity coefficient of the superchaotropic Keggin polyoxometalate SiW12O404- (SiW) on nonionic surfactant (C8E4) micelles by modeling small-angle X-ray and neutron-scattering spectra. Neither hard sphere nor electrostatic repulsion models reproduce the experimental activity coefficient of adsorbed SiW ions on the micelles. However, the activity and binding of SiW on the micelles is well-described by a Langmuir adsorption isotherm. These results imply that adsorbed SiW ions are non-interacting and "create" around themselves adsorption sites on the micelle. The temperature dependence of the adsorption constant showed that the SiW adsorption is enthalpically driven and entropically unfavorable, in line with the typical chaotropic thermochemical signature. The adsorption enthalpy can be split into an electrostatic term and a water-recovery term to evaluate and qualitatively predict the superchaotropicity of a nanoion.

Ion flotation is a separation technology for recovering ions from dilute aqueous solutions. All ion flotation processes known so far make use of ionic foaming agents (surfactants) to selectively extract ions of opposite charge by electrostatic interactions. Recently, it was shown that nanometric-sized ions (nano-ions) with low charge density, such as certain polyoxometalates (POMs), strongly adsorb to neutral hydrated surfaces. In this study, we provide proof-of-concept for a method that we call "superchaotropic ion flotation," which uses nonionic surfactant foams to selectively recover and separate superchaotropic POMs from other ions, including non-superchaotropic ions. Specifically, we investigated the extraction of isopolyoxomolybdates, formed by pH and concentration variation of molybdate aqueous solutions, using foams produced with a commercial polyethoxylated surfactant (BrijO10). The speciation of polyoxomolybdates was investigated by Raman spectroscopy and small angle X-ray scattering (SAXS). SAXS has also confirmed the superchaotropic behaviour of the polyoxomolybdates species via the characterization of their strong adsorption on surfactant micelles. The flotation recovering is high and maximal at pH 1, a pH for which Mo36O1128  , an anion with a low charge density, is the predominant molybdate species. It was found that the surfactant has a strong influence on the molybdate speciation. It was also demonstrated that molybdate may be separated from tungstate at pH 2 by superchaotropic ion flotation. In conclusion, we propose a novel ion separation method via non-ionic surfactant aqueous foams and we show that the superchaotropic effect makes it possible to foresee various applications in separation science.