The Opposite Effect of Metal Ions on Short-/Long-Range Water Structure: A Multiple Characterization Study

Abstract

Inorganic electrolyte solutions are very important in our society as they dominate many biochemical and geochemical processes. Herein, an in-depth study was performed to illustrate the ion-induced effect on water structure by coupling NMR, viscometer, Raman and Molecular Dynamic (MD) simulations. The NMR coefficient (BNMR) and diffusion coefficient (D) from NMR, and viscosity coefficient (Bvis) from a viscometer all proved that dissolved metal ions are capable of enhancing the association degree of adjacent water molecules, and the impact on water structure decreased in the order of Cr3+ > Fe3+ > Cu2+ > Zn2+. This regularity was further evidenced by Raman analysis; however, the deconvoluted Raman spectrum indicated the decrease in high association water with salt concentration and the increase in low association water before 200 mmol·L−1. By virtue of MD simulations, the opposite changing manner proved to be the result of the opposite effect on short-/long-range water structure induced by metal ions. Our results may help to explain specific protein denaturation induced by metal ions.

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

“…Thus, the outcome from this discussion can be condensed as follows: short-ranged interactions and interactions within the f irst solvation shell are not, and should not be taken as, interchangeable concepts. Consequently, the concluding remark "our results are in agreement with the growing consensus 1,2,22,25,[30][31][32][33][34][35][36][37][38][39][40][41][42] that simple monovalent ions, such as the potassium halide ions discussed in this work, only perturb the structure of water within their first hydration shells" 67 becomes highly questionable as highlighted in SI-1(c) of the Supporting Information. Indeed, by choosing the real-space cutoff σ < r c < nσ ≤ 0.5L, where n = (1,2,3,...) denotes the number of solvation shells included in the determination of the ion−water interaction energy, the calculation described above should provide similar ΔH EPSR ≃ ΔH solv outcomes regardless of the magnitude of n, after choosing the proper real-space convergence parameter α and the associated Fourier-space cutoff k c parameter.…”

“…The above depiction suggests that in order to advance our understanding, and consequently, our ability to interpret the solvation phenomena of interest, we need first to provide ( i ) a definite meaning to (and account for) the microstructural changes of the species environment, ( ii ) the precisely defined descriptors of solvation phenomena, and ( iii ) the thermodynamic manifestation of the solvation behavior of species as well as its link to the solution nonideality. In this regards, we find that a great deal of effort has been invested in the study of electrolyte and nonelectrolyte solutions according to a variety of diffraction and spectroscopic tools to probe locally the solvation environment, including X-ray ( e.g ., EXAFS and XANES), − infrared, , Raman, − NMR, , X-ray absorption, , and femtosecond pump–probe spectroscopies, , neutron diffraction with isotope substitution (NDIS), − hybrid methods involving empirical microstructural refinement of scattering data, − and the molecular simulation interpretation of NDIS data. − …”