(705c) Properties of Aqueous Cluster-Ions and Their Relation to Single-Ion Hydration Thermodynamics – Computer Simulation Study

Experimental and computational studies of small clusters comprised of an ion and solvent (water) molecules are a rich source of information about the details of ion solvation thermodynamics, which is otherwise experimentally inaccessible in bulk solution conditions. In particular, cluster-ion data from electrospray mass spectrometry experiments have been frequently used as a basis for determination of bulk single-ion hydration enthalpies and Gibbs free energies using the cluster pair approximation (CPA) of Tissandier et al.¹ However, our recent molecular simulation studies suggest that the assumptions underlying the CPA are incorrect² and, consequently, the currently accepted values of single-ion hydration thermodynamic properties^1,3 are not reliable. Estimates based on our current results indicate that the errors of the CPA-based properties can reach up to 40 kJ/mol for hydration enthalpies.

In light of these findings, we use a combination of classical and quantum mechanical Monte Carlo simulations to evaluate the impact of factors, such as polarization and charge penetration effects, on the resulting ion hydration in small water clusters.⁴ These computational studies rooted in cluster-ion experimental data will ultimately lead to more accurate estimation of single-ion solvation thermodynamics without relying on extra-thermodynamic assumptions.

1)   Tissandier M.D. et al “The Proton’s Absolute Aqueous Enthalpy and Gibbs Free Energy of Solvation from Cluster Ion Solvation Data” J. Phys. Chem. A 1998, 102, 7787-7794

2)   Vlcek L., Chialvo A.A., Simonson J.M. “Correspondence Between Cluster-Ion and Bulk Solution Thermodynamic Properties: On the Validity of Cluster-Pair-Based Approximation” J. Phys. Chem. A 2013,  DOI: 10.1021/jp408632e.

3)   Marcus Y. “Thermodynamics of Solvation of Ions. 5. Gibbs Free Energy of Hydration at 298.15 K” J. Chem. Soc., Faraday Trans. 1991, 87, 2995-2999.

4)   Vlcek L., Uhlik, F., Moucka F., Nezbeda I., Chialvo, A.A., “Properties of cluster-ions and water clusters from classical simulations, quantum chemistry, and experiments” J. Phys. Chem. A, (in preparation).