Bridging Structure, Dynamics, and Thermodynamics: An Example Study on Aqueous Potassium Halides

Abstract

Aqueous salt systems are ubiquitous in all areas of life. The ions in these solutions impose important structural and dynamic perturbations to water. In this study, we employ a combined neutron scattering, nuclear magnetic resonance, and computational modeling approach to deconstruct ion-specific perturbations to water structure and dynamics and shed light on the molecular origins of bulk thermodynamic properties of the solutions. Our approach uses the atomistic scale resolution offered to us by neutron scattering and computational modeling to investigate how the properties of particular short-ranged microenvironments within aqueous systems can be related to bulk properties of the system. We find that by considering only the water molecules in the first hydration shell of the ions that the enthalpy of hydration can be determined. We also quantify the range over which ions perturb water structure by calculating the average enthalpic interaction between a central halide anion and the surrounding water molecules as a function of distance and find that the favorable anion−water enthalpic interactions only extend to ∼4 Å. We further validate this by showing that ions induce structure in their solvating water molecules by examining the distribution of dipole angles in the first hydration shell of the ions but that this perturbation does not extend into the bulk water. We then use these structural findings to justify mathematical models that allow us to examine perturbations to rotational and diffusive dynamics in the first hydration shell around the potassium halide ions from NMR measurements. This shows that as one moves down the halide series from fluorine to iodine, and ionic charge density is therefore reduced, that the enthalpy of hydration becomes less negative. The first hydration shell also becomes less well structured, and rotational and diffusive motions of the hydrating water molecules are increased. This reduction in structure and increase in dynamics are likely the origin of the previously observed increased entropy of hydration as one moves down the halide series. These results also suggest that simple monovalent potassium halide ions induce mostly local perturbations to water structure and dynamics.

The ability of trimethylamine N-oxide to resist pressure induced perturbations to water structure

“…We attempt to monitor water–water hydrogen bond interaction energies, TMAO–water hydrogen bond interaction energies, water–water dipole angle distributions, TMAO oxygen–water dipole angle distributions, and TMAO methyl–water dipole angle distributions through a custom analysis routine 56 , 57 , 74 .…”

“…To gain experimentally constrained atomistic level structural information on water and aqueous TMAO under pressure, we employ neutron diffraction and the computational method of empirical potential structure refinement (EPSR). H/D isotopic substitution can be exploited to provide detailed insight into hydrogen bonding in the system 73 and, when combined with structural refinement, can give a rich and detailed picture of particular microenvironments in aqueous solutions which are inaccessible to other ensemble averaged experimental techniques but are certainly necessary to fully understand the restructure effects of pressure and TMAO 56 , 57 , 74 .…”

On the Elusive Links between Solution Microstructure, Dynamics, and Solvation Thermodynamics: Demystifying the Path through a Bridge over Troubled Conjectures and Misinterpretations

“…For that purpose, we start with the statement “Our data shown in Table 1 shows that the enthalpy of hydration, calculated by only considering structural data on water molecules in the first hydration shell of an ion, is in close agreement with the experimental enthalpy of hydration. [70,74] This demonstrates that by only considering short-ranged interactions in aqueous solutions of potassium halides, we can recapture bulk experimental properties” where the underline is ours. After a careful reading of the described procedure for the calculation of the hydration enthalpy Δ H EPSR in ref , we find that the observed “close agreement” Δ H EPSR ≃ Δ H solv in Table 1 is not (and cannot be taken as) a proof for the solvation quantities of ions being the result of the interactions within their first solvation shell alone, as we discuss next.…”

“…[70,74] This demonstrates that by only considering short-ranged interactions in aqueous solutions of potassium halides, we can recapture bulk experimental properties” where the underline is ours. After a careful reading of the described procedure for the calculation of the hydration enthalpy Δ H EPSR in ref , we find that the observed “close agreement” Δ H EPSR ≃ Δ H solv in Table 1 is not (and cannot be taken as) a proof for the solvation quantities of ions being the result of the interactions within their first solvation shell alone, as we discuss next.…”

“…Summarizing, the verification of Δ H EPSR ≃ Δ H solv in Table 1 of ref (or from any other source for that matter) does not constitutes a validation of the statement “ that by only considering short-ranged interactions in aqueous solutions of potassium halides, we can recapture bulk experimental properties ”; it is rather an indication of a proper handling of the ion–water Coulombic interactions and resulting assessment of the internal energy. The quoted mischaracterization results from the imprecise language, i.e ., the use of “ short-ranged interactions” as equivalent to “ enthalpy of hydration, calculated by only considering structural data on water molecules in the first hydration shell of an ion ”.…”