Hydrogen‐Bond Structure and Low‐Frequency Dynamics of Electrolyte Solutions: Hydration Numbers from ab Initio Water Reorientation Dynamics and Dielectric Relaxation Spectroscopy

Abstract

We present an atomistic simulation scheme for the determination of the hydration number (h) of aqueous electrolyte solutions based on the calculation of the water dipole reorientation dynamics. In this methodology, the time evolution of an aqueous electrolyte solution generated from ab initio molecular dynamics simulations is used to compute the reorientation time of different water subpopulations. The value of h is determined by considering whether the reorientation time of the water subpopulations is retarded with respect to bulk-like behavior. The application of this computational protocol to magnesium chloride (MgCl 2) solutions at different concentrations (0.6-2.8 mol kg À 1) gives h values in excellent agreement with experimental hydration numbers obtained using GHz-to-THz dielectric relaxation spectroscopy. This methodology is attractive because it is based on a well-defined criterion for the definition of hydration number and provides a link with the molecular-level processes responsible for affecting bulk solution behavior. Analysis of the ab initio molecular dynamics trajectories using radial distribution functions, hydrogen bonding statistics, vibrational density of states, water-water hydrogen bonding lifetimes, and water dipole reorientation reveals that MgCl 2 has a considerable influence on the hydrogen bond network compared with bulk water. These effects have been assigned to the specific strong Mg-water interaction rather than the Cl-water interaction.

Molecular dynamics simulations of heterogeneous hydrogen bond environment in hydrophobic deep eutectic solvents

“…The eight DESs contain a considerable portion of unsaturated hydrogen bond donors and acceptors not forming hydrogen bonds. As shown in Table 4, the highest ratio of N HB / N HBA and N HB / N HBD is 0.65, much less than the typical value for water (~0.8–0.9) 52 . In the Men‐Lid21 system, only less than 20% of the hydrogen bond donors and acceptors form hydrogen bonds, while most of them are unsaturated.…”

Water Induced Structural Transformations in Flexible Two-Dimensional Layered Conductive Metal–Organic Frameworks

“…We formulated TCFs in terms of the reorientation of the unit vector that lies along one of the OH bonds of a water molecule: where P 2 is the Legendre polynomial of order 2. To determine water reorientation relaxation times (τ reor ), we fitted the correlation function ensemble averaged over all water molecules over time to a biexponential function of the form The final reorientation relaxation times were then calculated from the weighted average of the fitting parameters: The correlation functions were also fitted to a triexponential function of a similar form which yielded similar trends. For example in the case of the Cu MOF with n = 216 water molecules, a biexponential fit yielded a τ reor value of 34.0 ps, and a triexponential fit resulted in a similar value of 36.2 ps.…”

“…where P 2 is the Legendre polynomial of order 2. To determine water reorientation relaxation times (τ reor ), we fitted the correlation function ensemble averaged over all water molecules over time to a biexponential function 75 of the form…”

“…For example in the case of the Cu MOF with n = 216 water molecules, a biexponential fit yielded a τ reor value of 34.0 ps, and a triexponential fit resulted in a similar value of 36.2 ps. The first 100 fs which corresponds to the fast librational motion of the water molecules 75 was excluded from these fits (see Figure S12). Table 1 summarizes the calculated relaxation times and diffusion coefficients for five different water contents in both Cu 3 (HHTP) 2 and Co 3 (HHTP) 2 2D MOFs; the corresponding experimental and simulated data for bulk water are also given for comparison.…”

Fluorescence of KCl Aqueous Solution: A Possible Spectroscopic Signature of Nucleation

“…The organization of ions in electrolyte water solutions has been the subject of intensive investigations both experimentally and by computer simulations in order to elucidate the structure and dynamics of the water molecules in the immediate vicinity of the ions as well as their long range effect. ,− Solvated ions of opposite charge can form ion pairs and clustering even at a low concentration; in particular, three types of ion pairs have been described, depending on the number of water molecules that separate the two ions: contact ion pairs (CIPs) where the cation and anion are in contact, solvent-shared ion pairs (SIPs), or solvent separated ion pairs (SSIPs) where one or two water molecules separate the two ions.…”