Molecular dynamics and x-ray investigation of an aqueous calcium chloride solution

Abstract

An improved central force model for water recently developed was used to perform a molecular dynamics simulation of a 1.1 m aqueous CaC12 solution at the experimental density at room temperature. The ion-water potentials were derived from ab initio calculations. The solution was simulated for 10 ps at 300 K. A new X-ray scattering study of a CaClz solution was performed at the same concentration and temperature. The structure function and the radial distribution function were evaluated with geometrical models. The comparison between experimental and theoretical results is performed on the level both of the structure function and of the interparticle radial pair distribution functions. More detailed results about the structure of the cationic hydration shell are also presented and compared with results of other recent simulation and neutron diffraction work.

Hydration of Metal and (CH3)4N+ Ions in Solutions of Mixed Salts

“…However, the dipole moments of a few water molecules in the Ca 2+ shell are tilted by about 50 • from the antidipole orientation, as it can be deduced from a shoulder at cos(ϕ) = −0.65. Similar distortion of the antidipole orientation has been observed previously [15,27] in aqueous solution.…”

“…In aqueous solution, the angular distribution exhibits two peaks, centred at 65 • and around 135 • , and only the former peak might be expected for tetrahedral or hexahedral symmetries [15,27]. In Me 4 NCl solutions, as seen from Figure 2, the most probable angles are smaller, about 60 • and 120 • .…”

“…The first shell consists of 11 molecules whereas the second one contains more than 30. In aqueous solution, these shells consist of 10 and 27 water molecules [15,27]. The slight increase of the larger coordination number seems to be in contrast with a decrease of the hydrodynamic radius of Ca 2+ , and in consequence a decrease of the 'dynamic' hydration number of this cation [12].…”

MD Simulation of the Ion Solvation in Methanol-Water Mixtures

“…This model has been frequently employed to simulate aqueous solutions of electrolytes, both in classical (Dietz et al, 1982;Jancso et al 1985;Probst et al, 1985, Probst et al, 1991Hawlicka & Swiatla-Wojcik, 1995, Lavenstein et al 2000Ibuki & Bopp, 2009) and in QM/MM MD (Tongraar & Rode, 2003, Rode et al, 2004, Öhrn & Karlström, 2004Tongraar & Rode, 2005, Payaka et al, 2009Tongraar et al, 2010) simulations. The BJH potential is appropriate to simulate the methanol-water mixtures, because it is fully consistent with the PHH flexible model (Palinkas et al 1987) of the methanol molecule.…”

Local Effects in the X-ray Absorption Spectrum of Salt Water

“…2 Because of the relevance of metal ions in biology, a number of experimental techniques, including X-ray diffraction (XRD), [3][4][5] extended X-ray absorption fine structure (EXAFS) spectroscopy, 6 and neutron diffraction, 7,8 have been used to elucidate the coordination of Ca 2+ , Mg 2+ , Na + , and other cations in water. Both XRD 4 and EXAFS results 9 suggest a coordination number between six and eight for Ca 2+ , while Mg 2+ has been shown by XRD to be coordinated octahedrally with six water molecules, 3 and XRD experiments indicate Na + is coordinated by four to six water molecules. 5 Importantly, first-principles molecular dynamics calculations were shown to be necessary in order to recover experimentally observed coordination numbers for Mg 2+ , 10 Ca 2+ , 11 and Na + .…”