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

Abstract

Nanometer-sized anions (nano-ions) like polyoxometalates and boron clusters exhibit so-called superchaotropic behavior, which describes their strong binding to hydrated non-ionic matter in water. We show here that nano-ions, at millimolar concentrations, dramatically enhance the viscosity and induce gelation of aqueous solutions of non-ionic cellulose ethers (CEs), a class of widely utilized polymers known for their thickening and gel-forming ability. These phenomena arise from an interplay of attractive forces and repulsive electrostatic forces between CE-chains upon nano-ion binding. The attractive forces manifest themselves as aggregation of CE-chains into a physically crosslinked polymer network (gel). In turn, the electrostatic repulsions hamper the viscosity increase and gelation. Superchaotropic nano-ion binding emerges as a novel and general physical crosslinking motif for CE-solutions and exceeds by far the conventional thickening effects of classical salts and ionic surfactants.

When Ions Defy Electrostatics: The Case of Superchaotropic Nanoion Adsorption

“…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 ,,, ).…”

“…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 ,,, ). In most of these latter works, ,,,, 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).…”

Stabilization of Guest Molecules inside Cation‐Lidded Cucurbiturils Reveals that Hydration of Receptor Sites Can Impede Binding

“…In full accordance, the change in counterion from Cl − to SO 4 2− , a strong salting‐out reagent, also caused no significant changes in methane affinity at the same cation concentration (see SI, Figure S5, Table S5). In general, salting‐out type Hofmeister effects become only sizable at larger (molar) concentrations for conventional salts, while only very large (superchaotropic) anions can cause reductions in guest binding due to competitive binding [24] or precipitation [25] …”

Dilute polyelectrolyte solutions: recent progress and open questions

“…658 In addition, Collins’ concept of matching water affinities 659 and ion-pair formation can be extended to soft matter interfaces: Malikova et al 660 proposed that there can be a hydration match between counter-ions that favors ion pairing on polyelectrolytes. These solvent-mediated effects are well-known and have been studied for a variety of soft matter systems, 213–215,661 but the physical origin of the observed behaviour remains controversial. 662 Neutral polymers in solutions of ionic liquids are known to display associative properties, 663–665 a phenomenon often attributed to interactions between the ionic liquid ions and the dipoles along the polymer backbone.…”