Selective Monovalent Cation Association and Exchange around Keplerate Polyoxometalate Macroanions in Dilute Aqueous Solutions

Abstract

The interaction between water-soluble Keplerate polyoxometalate {Mo(72)Fe(30)} macroions and small countercations is explored by laser light scattering, anomalous small-angle X-ray scattering (ASAXS), and isothermal titration calorimetry (ITC) techniques. The macroions are found to be able to select the type of associated counterions based upon the counterions' valence state and hydrated size, when multiple types of additional cations are present in solution (even among different monovalent cations). The preference goes to the cations with higher valences or smaller hydrated sizes if the valences are identical. This counterion exchange process changes the magnitude of the macroion-counterion interaction and, thus, is reflected in the dimension of the self-assembled {Mo(72)Fe(30)} blackberry supramolecular structures. The hydrophilic macroions exhibit a competitive recognition of various monovalent counterions in dilute solutions. A critical salt concentration (CSC) for each type of cation exists for the blackberry formation of {Mo(72)Fe(30)} macroions, above which the blackberry size increases significantly with the increasing total ionic strength in solution. The CSC values are much smaller for cations with higher valences and also decrease with the cations' hydrated size for various monovalent cations. The change of blackberry size corresponding to the change of ionic strength in solution is reversible.

Expanding the Schulze–Hardy Rule and the Hofmeister Series to Nanometer‐Scaled Hydrophilic Macroions

“…[9] Examples include inorganic molecular clusters such as polyoxometalates( POMs), [10] [a] Dr. Similar behavior was also observed for otherc olloidal systems.…”

“…[9] All the slopes of the best-fit lines at different concentrationsw ere close to 2.5 ( Figure 1a nd Table S1). Rb + and Cs + ions can strongly bind to {Mo 72 Fe 30 }a nd precipitate them;t herefore, the data for these cations were excluded from the linear regression.…”

Electronic and relativistic contributions to ion-pairing in polyoxometalate model systems

“…4 Aqueous solubility and ion-pairing is extremely complex because many phenomena are involved including the lattice energy of the crystallized or precipitated solid, the hydration sphere of both the cation and anion, the sum of ions present in solution, and pH effects if the cation and/or anion is a polyatomic oxo-ion. [5][6][7] Alkali salts of polyoxometalates (POMs), the early d 0 Group V/VI metal-oxo clusters, are ideal for probing both ion-association [8][9][10][11][12] and crystal growth mechanisms, [13][14][15][16][17][18] and these two phenomena are intimately related. The large ion size and heavy metals of the POMs scatter X-rays strongly.…”

Anomalous small-angle X-ray scattering from charged soft matter

“…In addition to synthetic and bio-polymers, ASAXS has been recently used to probe cation binding efficiency on to Keplerate polyoxometalate blackberry-like supermolecular structures [32]. This study confirmed that the less hydrated monovalent cations can bind stronger to a polyoxometalate macroion and reduce the critical salt concentration required for the formation of self-assembled colloidal structure.…”