Determination of Intermolecular Distances in Dilute Solutions of Macroions and Their Direct Correlations with Self-Assembly and Macrophase Transitions

Here, this work demonstrates new attempts to determine the intermolecular distances between charged macroionic solutes in their dilute solutions, which regulate the solute’s microphase (self-assembly) and macrophase transitions. Small-angle X-ray scattering (SAXS) and analytical ultracentrifuge (AUC) techniques were applied to determine the intermolecular distances for the 2.42 nm-sized, spherical uranyl peroxide molecular cluster {U₆₀} in their self-assembled states in dilute aqueous solution and concentrated phases, respectively. The counterion-mediated attraction among {U₆₀} leads to characteristic, inter-{U₆₀} distances in solutions, which increase gradually with decreasing the strength of introduced counterions (e.g., lower valency). The gradual increment of inter-{U₆₀} distance demonstrates a nice correlation with the macroscopic phase transitions of {U₆₀}, from single crystals to concentrated fluids containing rigid 2-D sheets, then to dilute solutions containing a small amount of standalone, floating 2-D sheets of {U₆₀}, and finally to dilute solutions containing a limited amount of self-assembled single-layered, spherical blackberry structures of different sizes from the bending of more flexible 2-D sheets.. Authors: Xiao, Kexing [Univ. of Akron, OH (United States)] (ORCID:000000023911342X); Zhou, Yifan [Univ. of Akron, OH (United States)] (ORCID:0000000283612189); Szymanowski, Jennifer E. S. [University of Notre Dame, IN (United States)]; Xu, Xiaohan [Univ. of Akron, OH (United States)] (ORCID:0000000235000815); Chodankar, Shirish [Brookhaven National Laboratory (BNL), Upton, NY (United States). National Synchrotron Light Source II (NSLS-II)]. DOE Contract: SC0012704. Subjects: 36 MATERIALS SCIENCE; cations; counterions; oligomers; phase transitions; solution chemistry