Aggregation Patterns in Low- and High-Charge Anions Define Opposite Solubility Trends

Abstract

Molecular dynamics simulations in aqueous solution reveal the existence of two distinct patterns of aggregation in low and high charge density Lindqvist-type polyoxometalates (POMs). Our results indicate the presence of contact and solvent-shared ion pairs and specific and preferential interactions of alkalis with POMs. Highly charged POMs are capable of breaking apart the Li+ and Cs+ solvation shell, thus enhancing the formation of long-lived alkali–POM contact ion pairs, where alkalis act as an electrostatic “glue” forming large oligomers. Stronger ion pair interactions for Li+ than for Cs+ promote lower solubility for Li+ than for Cs+, evoking anomalous solubility trends. Lower charge density POMs are not capable of disrupting the Li+ solvation shell and only solvent-shared ion pairs are formed, whereas for Cs+, contact ion pairs exist. The large number of oxygen atoms in the POM surface enhances the hydrogen bonds between POM and water, thus promoting aggregation. In this case, aggregation follows normal solubility trends. Thus, aggregation depends on the strength of ion pair interactions, the capacity of POM to disrupt alkali’s solvation shell, and the contact surface area between the solvent and POM.

Exploring the self-assembly of dumbbell-shaped polyoxometalate hybrids, from molecular building units to nanostructured soft materials

“…We also show that these supramolecular systems are prone to aggregation according to the solvent composition and demonstrate that the difference in the nature of the POM, the charge of the metal linker or the solvent composition considerably affects the supramolecular organization, which in some cases lead to the formation of molecular gels retaining the initial nanostructuration. Since in previous studies some of us and other authors have shown that molecular dynamics (MD) simulations provided unique insight into the origin of POMs/counter-cation interactions, [27][28][29][30][31][32] we show here that the solvent plays a key role by modulating ionpairing and thus self-assembly.…”

Aqueous solutions of super reduced polyoxotungstates as electron storage systems

“…The trend at lower concentration is similar to that observed for the previously reported Li-{P 2 W 18 } 16 , however the trend breaks down above 5 mM as the 10 mM solution (20.7 e -) did not show an increase in performance over the 5 mM one (20.7 e -). Considering the effect of pH, 33 we dissolved the cluster in 0.2 M H 2 SO 4 which led to an increase in the solutions storage capacity to 22.1 e -(a comparison of 10 mM K-{P 5 W 30 } in H 2 O, 0.2 M H 2 SO 4 and 0.2 M Li 2 SO 4 can be seen in Figure S18), however upon increasing the concentration further to 25 mM and 50 mM the performance decreased to 17.2 and 8.5 erespectively. A representative 23 ereduction/reoxidation curves of a 10 mM solution of K-{P 5 W 30 } in 0.2 M H 2 SO 4 with the best performance in this group is shown in Figure 2b.…”

“…[18][19][20] {P 8 W 48 } is a wheel-shaped polyoxotungstate which is stable in pH range 1 to 8; it is also a potential candidate for multielectron storage. 21 Previous studies show that the electrochemistry of POMs is dependent on the molecular charge, charge density, 22 cation type, and cation/anion size. 23,24 Therefore, as {P 5 W 30 } and {P 8 W 48 } have bigger size and higher charge density than {P 2 W 18 }, the effect of these factors on the electron storage ability suggest that they should be great candidates to test our hypothesis to assess how many electrons can be stored in a single cluster molecule.…”

Redox Flow Batteries: Electrolyte Chemistries Unlock the Thermodynamic Limits

“…The highest solubility was observed for POM with H + . The smaller cations with larger solvation spheres will hinder the formation of contact ion pairs with the low charge‐density [P 2 W 18 O 62 ] 6− anions in solution, leading to higher aqueous solubility [124,125] …”

“…The smaller cations with larger solvation spheres will hinder the formation of contact ion pairs with the low charge-density [P 2 W 18 O 62 ] 6À anions in solution, leading to higher aqueous solubility. [124,125] Anthraquinone derivatives have often been studied as active species for anolytes for aqueous RFBs. [126] The low water solubility of the conjugated structure has been strategically tackled with molecular engineering approaches.…”