Concentration Dependence of Dynamics and Structure among Hydrated Magnesium Ions: An Ultrafast Infrared Study

Abstract

The dynamics of aqueous magnesium chloride solutions, from relatively dilute (0.5 m) to near saturated (4.2 m) concentrations, were investigated using ultrafast two dimensional infrared and polarization selective pump-probe spectroscopies. The experiments were performed on two spectrally distinct nitrile stretch frequencies of the selenocyanate vibrational probe, corresponding to the CN nitrogen lone pair being associated with water and with Mg2+. No chemical exchange of the two species was observed over the experimental time scale (∼100 ps), enabling straightforward analysis of their dynamics. The dynamics reported by the Mg2+-associated peak are slower than those of the water-associated peak, suggesting that the immediate environment of the hydrated Mg2+ is different from the rest of the solution. Notably, the Mg2+-associated peak displays three spectral diffusion time scales, the slowest being ∼30 ps, while the water-associated peak decays as a faster biexponential. From the complete orientational relaxation time and hydrodynamic theory, a magnesium hydration number of six was obtained, which is in good agreement with NMR and X-ray diffraction studies. This hydration number holds for all concentrations until near saturation, when the linewidths and the dynamics deviate from linear trends, indicative of Mg2+ solvation structure changes resulting from a shortage of water molecules needed for full solvation.

Effects of Nanoconfinement on Dynamics in Concentrated Aqueous Magnesium Chloride Solutions

“…Figure shows the area-normalized, background-subtracted linear absorption spectra of SeCN – in confined aqueous MgCl 2 solutions of varying concentrations. As was previously observed in the bulk study of the same aqueous MgCl 2 solutions, there are two peaks centered at ∼2075 and ∼2115 cm –1 . The lower frequency peak (2075 cm –1 ) arises from the CN with its nitrogen lone pair hydrogen bonded to the OD of D 2 O (“D 2 O-associated peak”).…”

“…Linewidth increase can result from inhomogeneous or homogeneous broadening. In the bulk study, the observed line broadening with increasing concentration was attributed to inhomogeneous broadening, which arises from increased variety in solvent configurations . This was supported by the concentration independence of the homogeneous linewidths measured with 2D IR experiments.…”

“…The difference is in the faster t 1 , showing that the more rapidly decaying D 2 O-associated r ( t ) contributes only at early times. In both the bulk and the pore, the Mg 2+ -associated dynamics are slower than the D 2 O-associated dynamics, and all dynamics slow with increasing salt concentration …”

“… a Multiexponential fit parameters to the anisotropy for SeCN – associated with D 2 O in bulk and confined aqueous MgCl 2 solutions of varying concentrations. The A i and t i are the amplitude and time constant of the i th component. b ref . c ref . d Because the pore t 2 values closely correspond to the bulk t 3 values, the pore A 2 and t 2 columns were placed below the bulk A 3 and t 3 columns. e ref . …”

“…The laser system has been discussed in detail . The polarization selective pump–probe (PSPP) , and two-dimensional infrared (2D IR) , vibrational echo experiments, summarized below, were conducted following the methods described in our previous study of the bulk aqueous MgCl 2 solutions …”