Effect of Co-ions on the Self-Assembly Behavior of Macroions*

Simulating charged macromolecules (macroion) in aqueous solutions is challenging, as such systems usually include complex inter-/intra-molecular interactions. In addition, while the effects of smaller co-ions are known to be important, modeling the behavior of co-ions is difficult. In order to overcome these difficulties, a coarse-grained(CG) model was developed to capture the main features of macroions and compare it with an ideal model system of macroions to validate our method. {Mo₇₂Fe₃₀} polyoxometalates(POMs) is a rigid, hydrophilic, perfectly spherical macroion and known to self-assembly into vesicle-like “blackberry” structure in solutions. The smaller co-ion is K₇[α-PW₁₁O₃₉] (PW₁₁) POMs. The simulation results are found to be consistent with experimental observation—the {Mo₇₂Fe₃₀} can self-assemble into blackberry, while the Keggin co-ions stay well-dispersed in the solution, when they are the only species in the solution, respectively. When the macroion {Mo₇₂Fe₃₀} and the smaller co-ion PW₁₁ co-exist, they co-assemble into a thermodynamically favorable blackberry state that contains both {Mo₇₂Fe₃₀} and PW₁₁. Furthermore, the effect of hydrophobicity of the POMS compared to co-ions and the charge density of co-ions were also simulated and discussed in this presentation.