The structure of concentrated NiCl2aqueous solutions: what is molecular simulation revealing about the neutron scattering methodologies?

An analysis is made of adequacy and limitations of the neutron weighted ion distribution function for the determination of the hydration ion structure in hydrothermal solutions. Our analysis indicates that the coordination number based on the O—Ni2+ interactions is unambiguously defined by the first peak of the neutron weighted cation distribution function G Ni(r), but that the corresponding H—Ni+2 and H—Cl− coordination numbers may be ill-defined due to the occurrence of Ni+2-Cl− ion pairing. For the system considered in this work, this effect contributes about 1.5 units to the H—Ni−2 and 0.85 units to the H—Cl− coordination numbers, respectively, for a 3.9 M NiCl2 aqueous solution under ambient conditions. A comparison under ambient conditions between the most reliable NDIS data on Ni2+ hydration and our simulation results suggests that the present intermolecular potential models underestimate the O-Ni+2 coordination numbers by about 1.5 units, and it might indicate the need for a reparametrization of the current ion-water intermolecular potentials. The hydration structure exhibits practically no temperature dependence for the isochore studied (1.356 g cm−3) and composition. The Ni+2-Cl− ion pair formation appears to affect the location of the shoulder in the neutron weighted distribution functions G Ni(r) and G Cl(r), although it does not affect the magnitude of the ion—water coordination.