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.
Highlights
It is well known that salt solution, as compared to pure water, is characterized by four colligative properties, i.e., freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure [1], which makes it used in dialysis, melting snow, etc
The effects of dissolved metal ions on surrounding water molecules have been extensively characterized by Terahertz Time-Domain Spectroscopy (THz TDS), Raman/IR, NMR, Molecular Dynamic (MD) simulations, etc
Ultrafast laser studies have confirmed that the dynamics of the HB network is pretty fast in pure water; the spectral diffusion time for HDO/D2O system and the vibrational lifetime in pure H2O are less than 200 fs [46], and ~200 fs [47], respectively
Summary
It is well known that salt solution, as compared to pure water, is characterized by four colligative properties, i.e., freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure [1], which makes it used in dialysis, melting snow, etc. Kropman and Bakker [7] found the hydrogen-bonding (HB) dynamics of water molecules solvating Cl, Br, Ianions slow compared with pure water, indicating the remarkable influence of center ions on solvating water molecules This group observed significant dependence of vibrational lifetime of hydration water on the nature of metal ions [8], but the enhancement of the HB network upon adding ions is confined within the first solvation shell as indicated by the changes in rotational dynamics [9]. An X-ray diffraction study suggested that the KCl unit in aqueous solution could control 45 water molecules to form rigid spherical structure [10] Another neutron diffraction study provided compelling evidence that Ca2+ could have an impact on the hydrogen structure in the second hydration shell [11]. Ions are very likely to have effects on both the adjacent hydration water and those far away from center ions, which is still waiting for further study
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