(500a) Ion Pair Association in Aqueous Metal Sulfates and Its Interpretation From NDIS by Molecular Simulation

The hydration behavior of multi-charged anions plays a significant role in biological systems and separation processes involving the Hofmeister series. Ionic species in aqueous environment induce strong microstructural changes typically described in terms of local water-density perturbation around the ions as the consequence of the ion-water (and eventually ion-ion) interactions not present in the pure solvent, i.e., the difference of strength between the solvent-solvent and the ion-solvent interactions macroscopically manifested as solution nonideality (1).

An early view of the structure of aqueous electrolyte solutions introduced the concept of structure-maker and structure-breaker ions, introduced as a measure of the order/disorder induced on the surrounding solvent relative to that of the pure solvent. This picture also provides the idea of the ion solvation (hydration) numbers as the estimated number of solvent (water) molecules coordinated with the ion under consideration, a concept that has allowed a simplified interpretation and modeling of these systems. From a basic microstructural viewpoint solvation numbers are directly linked to the number of coordinated solvent molecules around the species under consideration, consequently, the best approach to their assessment is through the determination of the corresponding ion-solvent radial distribution function. Among others, neutron diffraction with isotope substitution (NDIS) is a versatile tool for the determination of the microstructure of aqueous electrolytes, e.g., the ion-water correlations by the first-order difference method. The suitability of this method depends on the ions having different isotopic coherent scattering lengths while the successful implementation and the outcome of the experiments depends strongly on the interpretation of the shape, location, and resolution of scattering peaks, and in particular, on de-convoluting the peak overlapping associated with ion-pairing (2). From a strictly microstructural viewpoint, the formation of a contact ion pair in aqueous solutions of a salt (MXn) is a penetration of solvation shell of cation M into that of anion X. In terms of NDIS data, this formation leads to an overlapping of the M-X with either the H-M (H-X) or O-M (O-X) contributions to the corresponding total structural factors, and represented as a distortion of the normal shape of either the first or the second peak of the first-order difference of the neutron-weighted distribution functions in heavy water (3). In this context molecular-based simulation can help in the interpretation of the diffraction data, because it provides the unambiguous link between the system structure under study and the corresponding neutron-weighted distribution functions, consequently, simulation offers a route for the unambiguous test of consistency and/or accuracy for the methods and hypothesis used in the extraction of structural information from diffraction experiments.

Here we address the hydration behavior of aqueous lithium, nickel, and ytterbium sulfates at ambient conditions according to the relevant radial distributions functions and the resulting first-order difference for the sulfur-site neutron-weighted distribution functions generated by isothermal-isobaric molecular dynamics simulation (4). We focus on the partial contributions to the neutron-weighted distribution functions, identify the main contributing peaks of the corresponding radial distribution functions, and highlight the effect of the contact ion-pair configuration on the resulting water's hydrogen coordination around the sulfate's sulfur site. For that purpose, we also assessed the extent of the ion-pair formation according to Poirier-DeLap formalism to gain insights into the significant increase of the ion-pair association exhibited by these salts with cation charge.

Cited References: (1) A. A. Chialvo and J. M. Simonson, Journal of Chemical Physics 119, 8052 (2003). (2) A. A. Chialvo and J. M. Simonson, Journal of Chemical Physics 124 (15) (2006). (3) A. A. Chialvo and J. M. Simonson, Molecular Physics 100, 2307 (2002). (4) A. A. Chialvo and J. M. Simonson, Collec. Czech. Chem. Commun. 75 (4), 405 (2010).