Accurate Determination of the Quantity and Spatial Distribution of Counterions around a Spherical Macroion

Abstract

The accurate distribution of countercations (Rb+ and Sr2+) around a rigid, spherical, 2.9‐nm size polyoxometalate cluster, {Mo132}42−, is determined by anomalous small‐angle X‐ray scattering. Both Rb+ and Sr2+ ions lead to shorter diffuse lengths for {Mo132} than prediction. Most Rb+ ions are closely associated with {Mo132} by staying near the skeleton of {Mo132} or in the Stern layer, whereas more Sr2+ ions loosely associate with {Mo132} in the diffuse layer. The stronger affinity of Rb+ ions towards {Mo132} than that of Sr2+ ions explains the anomalous lower critical coagulation concentration of {Mo132} with Rb+ compared to Sr2+. The anomalous behavior of {Mo132} can be attributed to majority of negative charges being located at the inner surface of its cavity. The longer anion–cation distance weakens the Coulomb interaction, making the enthalpy change owing to the breakage of hydration layers of cations more important in regulating the counterion–{Mo132} interaction.

Abnormal Association between Metal−Organic Cages and Counterions Regulated by the Hydration Shells

“…In other words, Cage 3 , which is poorly hydrated and less charge‐accessible, interacts more strongly with counterions than that of Cage 4 , confirming that when charges are buried inside the MOCs, the one with thinner hydration shell show stronger interaction with counterions. What's more, we also observed closer association between Mo 132 and Rb + than that between Mo 132 and Sr 2+ , [11] showing similar phenomenon as here. As most charges on Mo 132 are located inside its skeleton, the hydration shell of the cations, rather than electrostatic interaction, dominates the macroion‐cation interaction.…”

“…Usually, highly charged ions exhibit strong interactions with the opposite‐charged ions, although the their thick hydration shells impede the interaction at the same time [10] . However, our group also reported the opposite trend as weaker association between macroions and divalent cations comparing with monovalent cations, which is due to the unique charge distribution of the macroion‐charges located inside its skeleton [11] …”

“…[10] However, our group also reported the opposite trend as weaker association between macroions and divalent cations comparing with monovalent cations, which is due to the unique charge distribution of the macroion-charges located inside its skeleton. [11] MetalÀ organic cages (MOCs) can serve as good models to study the macroion-counterion interactions due to their welldefined, rigid molecular structures and positive charges from transition-metal ions. [12] What's more, gaining more understanding of counterion-MOC interaction can largely facilitate the development of MOC-based catalysts.…”

Counterion-Induced Aggregation of Metallacarboranes

“…New findings opened a question about the localization of counterions in the small aggregates considering COSAN as the low coordinating ion. − Thus, the driving force for the cation binding should be long-range and strong enough to keep counterions within the aggregate in analogy to POMs. − Furthermore, the aggregation number range for the small aggregates is strictly confined around pentamers in agreement with SAXS data. − , The nearly temperature- and concentration-independent COSAN aggregation number strongly disagrees with the known behavior of surfactants. Thus, the classical hydrophobic effect is not likely to be the main driving force. ,, …”

Two GiantCalixarene‐LikePolyoxoniobate Nanocups {Cu12Nb120} and {Cd16Nb128} Built from Mixed Macrocyclic Cluster Motifs

“…Polyoxometalates (POMs) have grown as an important branch of inorganic chemistry due to their esthetically appealing structures and potential applications in catalysis, host-guest chemistry, sensing and so on. [5] POMs with different composition, sizes, and shapes have been demonstrated to be important units for constructing a variety of nanoscale molecules, such as POM-based cages, macrocycles and composite nanoclusters. [6][7][8] In contrast, the use of POMs as building units to make cup-shaped molecules is still a virgin land waiting for exploration.…”