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

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: HSiWO (SiW) and HPWO (PW), induce the self-assembly of an organic solvent: dipropylene glycol n-propylether (CP), in water. The strong interaction between SiW/PW with CP leads to a drastic increase in the CP, and aqueous solubility, of CP, e.g. SiW enables reaching full water-CP co-miscibility at room temperature. At high POM concentrations, SiW leads to a continuous increase of the CP, forming SiW-[CP] complexes, whereas PW produces a decrease in the CP attributed to the formation of nearly "dry" spherical [PW][CP] colloids, with n ~ 4 and m ~ 30. At high CP/PW contents, the [PW][CP] colloids turn into large interconnected structures, delimiting two pseudo-phases: a PW-CP-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.