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

Abstract

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