Can Polyoxometalates Be Considered as Superchaotropic Ions?

Abstract

In this work we have studied the influence of different anionic species, including typical Hofmeister ions (citrate 3and SCN-), hydrophobic ions (tetraphenyl borate, Ph4B-) and polyoxometallic clusters (Keggin-type POM, PW 3-), on the electrophoretic mobility of colloidal particles of different nature, charge and wettability. We focused the attention on the adsorption properties of PW 3in comparison with Ph4Bbecause both species have previously termed as super-chaotropes. However, their different structural properties make it difficult to reconcile the same classification for both anions. We found that the influence of Ph4Bis alike and surpasses the characteristic effect of chaotropic anions (SCN-), in particular interacting more strongly with hydrophobic interfaces. On the contrary, we observed clear POM adsorption on the different particles investigated, regardless of their charge or wettability. These observations are complemented by direct adsorption measurement combining Quartz Crystal Microbalance and Atomic Force Microscopy, confirming the strong and irreversible self-assembly of POM on different substrates. Our results indicates that polyoxometallic anionic clusters should better be described as small colloids instead of as large anions. The exceptional adsorption properties of POMs clusters on any kind of interface suggest that their behavior goes beyond the classical Hofmeister effects and, therefore, these species should not be included in the Hofmeister series as super-chaotropic anions.

Exploring the impact of sodium salts on hydrotropic solubilization

“…23 Whilst both contributions can best be rationalised in terms interfacial accumulation or depletion, it is important to emphasise that as fully inorganic anions do not possess hydrophobic moieties (by common definition), their mechanism of action can differ from that of classical hydrotropes. [24][25][26] In the last decade, ionic liquids (ILs) were shown to be a promising class of ionic hydrotropes, since both parts of their structure (cationic and anionic) if properly selected, can play an important role to increase the solubility of hydrophobic compounds in aqueous solution. 23,27 The very driving force behind the melting point decrease in ILs, namely the poor intermolecular packing resulting from the introduction of bulky and flexible -typically organic -cations presenting a low charge density, confers ILs the required structural characteristics to act as hydrotropes.…”

When Ions Defy Electrostatics: The Case of Superchaotropic Nanoion Adsorption

“…10,13,14 The superchaotropicity of nanoions has opened pathways to the formulation of novel materials such as association complexes with nonionic polymers, 15−17 decorated micelles with nonionic surfactants, 10,13 colloidal aggregates with short-chain amphiphiles, 18 hierarchical supramolecular structures, 19 and decorated latex colloids. 20 The interaction of (super)chaotropic ions with soft interfaces is commonly found to follow a Langmuir adsorption isotherm (classical chaotropes 7,9,21,22 and superchaotropic nanoions 10,14,15,17 ). In most of these latter works, 7,9,10,14,15 the Langmuir isotherm model was used to fit the evolution of the cloud point (CP), i.e., the liquid−liquid phase-separation temperature of aqueous solutions containing nonionic hydrated species: polyethoxylated surfactants, thermosensitive polymers, and proteins, upon addition of salt (ions).…”

“…Chaotropic anions in the Hofmeister series such as I – , NO 3 – , and SCN – are characterized by low charge densities and high polarizabilities. , The hydration water around a chaotropic ion exhibits high diffusion dynamics, high structural entropy, and low hydration enthalpy with respect to bulk water. , As a consequence, chaotropic ions tend to strip off part of their hydration shells leading to faint water/air surface activity , and weak interactions with nonionic interfaces as provided by polymers, surfactant self-assemblies, or the protein backbone . In extension to classical chaotropic ions, nanometric anions (nanoions), such as boron clusters and polyoxometalates (POMs), were shown to exhibit much stronger chaotropic, so-called superchaotropic, behavior related to their very low charge densities. − These superchaotropic ions associate in water with nonionic soft matter at micro- to millimolar concentrations in contrast to the adsorption of classical chaotropes that occurs in the molar range. ,, The superchaotropicity of nanoions has opened pathways to the formulation of novel materials such as association complexes with nonionic polymers, − decorated micelles with nonionic surfactants, , colloidal aggregates with short-chain amphiphiles, hierarchical supramolecular structures, and decorated latex colloids . The interaction of (super)­chaotropic ions with soft interfaces is commonly found to follow a Langmuir adsorption isotherm (classical chaotropes ,,, and superchaotropic nanoions ,,, ).…”

Superchaotropic Nano‐ion Binding as a Gelation Motif in Cellulose Ether Solutions

“…[2] In water, due to their very low charge densities, nano‐ions are weakly hydrated and exhibit a strong tendency to co‐assemble with non‐ionic matter. [ 1b , 3 ] Co‐assembly of nano‐ions was thus shown to occur with a host of diverse non‐ionic solutes including polymers, [4] surfactants,[ 1a , 3b ] hydrotropes, [5] and aromatic molecules [6] as well as with interfaces as provided by nanoparticles[ 2b , 7 ] or non‐ionic surfactant‐covered water surfaces. [ 3b , 8 ] The chaotropic effect, an enthalpic, water‐mediated driving force that arises from the favorable release of hydration water of the nano‐ion and the solute upon contact formation, lies at the bottom of these nano‐ion/solute co‐assembly phenomena.…”