A Two-Step Intermolecular Interaction Of Molecular Macroions With Multivalent Counterions

Area of Science:

• Colloid and Surface Chemistry
• Supramolecular Chemistry
• Materials Science

Background:

• Macroion-counterion interactions govern the behavior of hydrophilic macroions in solution.
• Understanding these interactions is key to controlling macroion self-assembly and macroscopic properties.
• Uranyl peroxide molecular clusters (U60) serve as model hydrophilic macroions.

Purpose of the Study:

• To investigate the interaction mechanism between uranyl peroxide molecular clusters (U60) and multivalent counterions.
• To elucidate the structural and dynamic changes occurring during macroion-counterion complexation.
• To correlate these interactions with observed phase transitions in U60 solutions.

Main Methods:

• Isothermal titration calorimetry (ITC) to quantify binding thermodynamics.
• Light and X-ray scattering to determine size and aggregation state.
• Electron microscopy to visualize nanoscale structures.

Main Results:

• A two-step interaction process was identified for U60 with multivalent counterions.
• This involves initial ion-pairing with partial hydration shell disruption, followed by strong U60-U60 attraction.
• Formation of large nanosheets with significant hydration shell alteration was observed.

Conclusions:

• Multivalent counterions induce distinct self-assembly pathways in U60 solutions compared to monovalent ions.
• The observed macroion-counterion interactions directly correlate with microscopic and macroscopic phase transitions.
• This study provides insights into controlling the assembly and properties of nanoscale materials.