Hydration Effects on Ion Solvation Shell Structure and Cation Associations in ssDNA/ssRNA-Ionic Liquid Electrolytes

Abstract The effect of hydration on the solvation shell structure and ion association was investigated in biopolymer-based ionic liquid electrolytes composed of single-stranded DNA (ssDNA) or RNA (ssRNA) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]). All-atom molecular dynamics simulations were performed using the OL15/OL3 nucleic acid force fields, an optimized OPLS-AA model for [BMIM][PF6], and the TIP3P water model. Radial distribution functions g(r) and coordination numbers CN(r) reveal that BMIM+ and PF6– ions form compact and symmetric coordination shells with CN(r) ≈ 5.0 at the lowest hydration levels. Increasing water concentration weakens this association, reducing CN(r) to ∼4.16 at 25 wt % water, indicative of water-mediated screening and the disruption of extended ion–ion correlations. In contrast, BMIM+-phosphate (PO4–) interactions remain weak (CN(r) < 1.1) across all hydration levels, as Na+ counterions preferentially neutralize phosphate sites. Analysis of ion-association probability distributions P(n) further shows that the broad, high-n tails observed at low hydration collapse into narrow peaks centered near n = 0–1 as water content increases, signifying the breakdown of multi-ion clusters into isolated ion pairs and free ions. Although modest sequence-dependent variations in coordination are observed, the overall hydration-dependent trends in solvation-shell structure and ion association remain qualitatively consistent across the nucleic-acid sequences. Collectively, these results elucidate the molecular mechanisms by which hydration modulates local ion-polymer correlations and dynamic clustering in hybrid biopolymer-ionic liquid electrolytes, providing microscopic insights relevant to ionic conductivity and charge transport in hydrated soft materials.