Phosphate Solvation in Mixed Organic-Aqueous Solutions: Implications for Marble Conservation Treatments

It has long been suggested that organic solvents disrupt the solvation shell around phosphate ions, which might facilitate calcium phosphate (CaP) nucleation. This would explain recent experimental findings where organic solvents─ethanol, isopropanol, and acetone─produce a denser CaP coating when synthesized on marble for conservation applications. In this work, computational methods are used to investigate the solvation shell of phosphate ions in mixed organic-aqueous solutions. Relative one-to-one interaction energies between the phosphates and solvents were calculated with density functional theory (DFT) and suggest that ethanol and isopropanol could displace water in the hydration sphere of PO₄^3-, HPO₄^2-, and H₂PO₄^-. Classical molecular dynamics simulations with models benchmarked to these DFT interaction energies were then used to investigate solvation under bulk solvent conditions. In mixed organic-aqueous conditions, we find that the behavior in the phosphate solvation shell is dependent on the solvent and protonation state. More specifically, none of the three organic solvents consistently disrupts the hydration shell of the phosphates, which would correlate with the experimental findings. Ultimately, this suggests that the influence of the organic solvent on the solvation shell of the phosphate ions may not contribute significantly to the improved synthesis of CaP.