Solution science impacts a wide range of scientific inquiries including catalysis, supramolecular chemistry, biology, and geoscience. Gaining a comprehensive understanding of solution behavior is key to improving and expanding the applications of solutions.
One poorly understood but critical aspect of solution behavior is the interaction between macroions and their counterions because experimental determination of this process has been very challenging. Macroion charge is screened by the surrounding electrical double layer: the Stern layer (strongly bound, immobilized counterions) and the diffuse layer (loosely associated counterions and co-ions). The thickness of the diffuse layer can be modeled by a characteristic length-Debye length, which is related to the counterion distribution and is critical to understanding various charge-regulated process in solutions, as it gives the decay of electrical potential from the charged surface.
Employing the anomalous small-angle x-ray scattering (ASAXS) facility at NSF’s ChemMatCARS at the Argonne Advanced Photon Source, the authors of this paper have demonstrated that the ASAXS technique can be a powerful tool to directly measure the counterion distribution and further understand the counterion-macroion interaction. These ASAXS results provide an accurate number and distribution of counterions, acting as a good model for further theoretical studies. They also provide a plausible explanation on the puzzle that some macroions have higher solubility with divalent counter-ions than with monovalent ones.
Jiahui Chen, Mrinal K. Bera, Hui Li, Yuqing Yang, Xinyu Sun, Jiancheng Luo, Jessi Baughman, Cheng Liu, Xuesi Yao, Steven S. C. Chuang, and Tianbo Liu “Accurate Determination of the Quantity and Spatial Distribution of Counterions around a Spherical Macroion,” Angew. Chem. Int. Ed. 2021, 60, 5833–5837. https://doi.org/10.1002/anie.202013806