Counterion effect on α-Keggin polyoxometalates in water: The peculiar role of H+ on their salting-in effect and co-assembly with organics

Hofmeister effects of ions in aqueous solution strongly affect chemical and biological systems. High low charge density anions, such as SO42- SCN- respectively, decrease (salting-out) or increase (salting-in) the solubility organic solutes water. Due to their very charge-density, nanometric e.g. polyoxometalates (POMs), tremendously (highly salting-in) they bind neutral hydrated – a property that is attributed (super-)chaotropic effect. Here, we show salting-out anions can be turned into salting-in presence superchaotropic POM, α-PW12O403-. The effect salts composed SO42-, was investigated on cloud point (CP) an ethoxylated surfactant (C8E4) propoxylated co-solvent (C3P2) SiW12O404- PW12O403- with different counter-cations (H+, Li+, Na+, K+). lead monotonic strong CP-increase regardless counterion, except for combined H+. Indeed, H3PW12O40 shows CP at high POM concentrations. This peculiar behavior formation large H3PW12O40-C3P2 (and H3PW12O40-C8E4) co-assemblies, shown by SAXS. these co-assemblies results from “bridging” H+ lower compared SiW12O404-. addition (basic) leads (i) consumption H+, then (ii) disruption subsequently (iii) CP-increase. In case, this how commonly used become apparently salting-in.