Self-assembly of a short amphiphile in water controlled by superchaotropic polyoxometalates: H4SiW12O40 vs. H3PW12O40

Nanometric ions, such as polyoxometalates (POMs) or ionic boron clusters, with low charge density have previously shown a strong propensity to bind to macrocycles and to adsorb to neutral surfaces: micellar, surfactant covered water-air and polymer surfaces. These association phenomena were shown to arise from a solvent-mediated effect called the (super-)chaotropic effect. We show here by combining cloud point (CP) measurements, scattering (SAXS/SANS) and spectroscopic techniques (NMR) that Keggin POMs: H4SiW12O40 (SiW) and H3PW12O40 (PW), induce the self-assembly of an organic solvent: dipropylene glycol n-propylether (C3P2), in water. The strong interaction between SiW/PW with C3P2 leads to a drastic increase in the CP, and aqueous solubility, of C3P2, e.g. SiW enables reaching full water-C3P2 co-miscibility at room temperature. At high POM concentrations, SiW leads to a continuous increase of the CP, forming SiW-[C3P2](1-2) complexes, whereas PW produces a decrease in the CP attributed to the formation of nearly dry spherical [PW]n[C3P2]m colloids, with n similar to 4 and m similar to 30. At high C3P2/PW contents, the [PW]n[C3P2]m colloids turn into large interconnected structures, delimiting two pseudo-phases: a PW-C3P2-rich phase and a water-rich phase. It is proposed that the stronger electrostatic repulsions between SiW (4-), compared to PW (3-), prevents the formation of mesoscopic colloids. (c) 2020 Elsevier Inc. All rights reserved.

Automated summary

Nanometric ions such as polyoxometalates and ionic boron clusters have been found to induce the self-assembly of organic solvents in water, significantly increasing their cloud point and aqueous solubility. At high concentrations, these ions can form complexes or colloids with the solvent, leading to the formation of different phases.