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

• Hydrolysis of α-Keggin polyoxometalates is inhibited by superchaotropic effect. • Counter-ion effects of α-Keggin polyoxometalates strongly impact their co-assemblies with non-ionic surfactant/hydrotropes. • Peculiar role of H + as co-assembly promotor leads to phase demixion (salting-out) • Titration of H + counter-ions gives a way to control the formation and disruption of co-assembly by acid-base reaction. • classical salting-out (basic) anions on H 3 PW 12 O 40 -surfactant/hydrotrope co-assemblies leads to a strong salting-in effect. Hofmeister effects of ions in aqueous solution strongly affect chemical and biological systems. High and low charge density anions, such as SO 4 2- 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-PW 12 O 40 3- . The effect of salts composed of salting-out anions, e.g. SO 4 2- , was investigated on the cloud point (CP) of an ethoxylated surfactant (C 8 E 4 ) and a propoxylated co-solvent (C 3 P 2 ) in the presence of SiW 12 O 40 4- and PW 12 O 40 3- with different counter-cations (H + , Li + , Na + , K + ). SiW 12 O 40 4- and PW 12 O 40 3- lead to a monotonic strong CP-increase regardless of the counterion, except for PW 12 O 40 3- combined with H + . Indeed, H 3 PW 12 O 40 shows a CP decrease at high POM concentrations. This peculiar behavior is attributed to the formation of large H 3 PW 12 O 40 -C 3 P 2 (and H 3 PW 12 O 40 -C 8 E 4 ) 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 PW 12 O 40 3- compared to SiW 12 O 40 4- . The addition of (basic) salting-out anions leads to (i) the consumption of H + , then to (ii) the disruption of the large H 3 PW 12 O 40 -C 3 P 2 (and H 3 PW 12 O 40 -C 8 E 4 ) 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.