Ion aggregation in high salt solutions. VI. Spectral graph analysis of chaotropic ion aggregates

Carrying out molecular dynamics simulations and graph theoretical analyses of high salt solutions, and comparing numerically calculated vibrational spectroscopic properties of water with femtosecond IR pump-probe experimental data, we have recently found that ions in high salt solutions can form two morphologically different ion aggregate structures. In the cases of NaCl solutions, Na⁺ and Cl^- tend to form compact cluster-like ion aggregate in high NaCl solutions. In contrast, K⁺ and SCN^- form spatially extended network-like ion aggregates that also exhibit a percolating network behavior. Interestingly, a variety of graph theoretical properties of ion network in high KSCN solutions were found to be very similar to those of water H-bonding network. It was shown that spatially extended ion networks in high KSCN solutions are completely intertwined with water H-bonding networks, which might be the key to understand the high solubility of thiocyanate salts in water. Here, we further consider two salts that have been extensively studied experimentally by using femtosecond IR pump-probe technique, which are NaClO₄ and NaBF₄. Note that ClO₄^- and BF₄^- are well-known chaotropic ions that have been believed to behave as water structure breaker. To understand how such chaotropic ions affect water H-bonding structure, we carried out spectral graph analyses of molecular dynamics simulation data of these aqueous solutions. Graph spectra and degree distribution of ion aggregates formed in high NaBF₄ and NaClO₄ solutions show that these chaotropic anions also have a strong propensity to form ion networks. The fact that salts containing chaotropic ions like SCN^-, BF₄^-, and ClO₄^- have very high solubility limits in water could then be related to our observation that these chaotropic anions with counter cations in high salt solutions are capable of forming intricate ion networks intertwined with water H-bonding networks. We anticipate that the present graph theoretical analysis method would be of use in further studying both various anomalous behaviors of interfacial water and fundamental physical chemistry of mixing and salt solubility in water.