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 ...
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, alpha-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 SiW(12)O(40)(4-)and PW(12)O(40)(3-)with different counter-cations (H+, Li+, Na+, K+). SiW12O40 4-and PW(12)O(40)(3-)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. (C) 2022 Elsevier B.V. All rights reserved.
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