Ultrasonic Velocities,Densities,Viscosities,Electrical Conductivities,Raman Spectra,and Molecular Dynamics Simulations of Aqueous Solutions of Mg(OAc)_2 and Mg(NO_3)_2:Hofmeister Effects and Ion Pair Formation

摘要

The ultrasonic velocities,densities,viscosities,and electrical conductivities of aqueous solutions of magnesium nitrate and magnesium acetate have been measured from dilute to saturation concentrations at 0<= t/ deg C <= 50.The temperature derivative of the isentropic compressibility,k_s,became zero at 2.28 and 2.90 mol kg~(-1) for Mg(OAc)_2 and Mg(NO_3)_2 solutions,respectively,at 25 deg C.The total hydration numbers of the dissolved ions were estimated to be,respectively,24.3 and 19.2 at these concentrations.Differences in K_S for various M~(2+) salts,using the present and literature data,correlated with reported M~(2+)-OH_2 bond lengths and to a lesser extent with cationic charge densities (ionic radii).The influence of anions on K_S appears to follow the Hofmeister series and also correlates approximately with the anionic charge density.Substantial differences between Mg(OAc)_2(aq) and Mg(NO_3)_2(aq) occur with respect to their structural relaxation times (derived from compressibility and viscosity data) and their electrical conductivities.These differences were attributed to a much greater ion association in Mg(OAc)_2 solutions.Raman spectra recorded at 28 deg C confirmed the presence of various types of contact ion pairs including mono- and bidentate complexes in Mg(OAc)_2(aq).In Mg-(NO_3)_2(aq),only noncontact ion pairs appear to be formed even at high concentrations.The experimental results are supported by molecular dynamics simulations,which also reveal the much stronger tendency of OAc~- compared to NO_3~- to associate with Mg~(2+) in aqueous solutions.The simulations also allow an evaluation of the ion-ion and ion-water radial distribution functions and cumulative sums and provide a molecular picture of ion hydration in Mg(OAc)_2(aq) and Mg(NO_3)_2(aq) at varying concentrations.