Ultrasonic Velocities, Densities, Viscosities, Electrical Conductivities, Raman Spectra, and Molecular Dynamics Simulations of Aqueous Solutions of Mg(OAc)2 and Mg(NO3)2: Hofmeister Effects and Ion Pair Formation

Abstract

The ultrasonic velocities, densities, viscosities, and elec. conductivities of aq. solns. of magnesium nitrate and magnesium acetate have been measured from dil. to satn. concns. at 0 ≤ t/°C ≤ 50. The temp. deriv. of the isentropic compressibility, κs, became zero at 2.28 and 2.90 mol kg-1 for Mg(OAc)2 and Mg(NO3)2 solns., resp., at 25 °C. The total hydration nos. of the dissolved ions were estd. to be, resp., 24.3 and 19.2 at these concns. Differences in κs for various M2+ salts, using the present and literature data, correlated with reported M2+-OH2 bond lengths and to a lesser extent with cationic charge densities (ionic radii). The influence of anions on κs appears to follow the Hofmeister series and also correlates approx. with the anionic charge d. Substantial differences between Mg(OAc)2(aq) and Mg(NO3)2(aq) occur with respect to their structural relaxation times (derived from compressibility and viscosity data) and their elec. conductivities. These differences were attributed to a much greater ion assocn. in Mg(OAc)2 solns. Raman spectra recorded at 28 °C confirmed the presence of various types of contact ion pairs including mono- and bidentate complexes in Mg(OAc)2(aq). In Mg(NO3)2(aq), only noncontact ion pairs appear to be formed even at high concns. The exptl. results are supported by mol. dynamics simulations, which also reveal the much stronger tendency of OAc- compared to NO3- to assoc. with Mg2+ in aq. solns. The simulations also allow an evaluation of the ion-ion and ion-water radial distribution functions and cumulative sums and provide a mol. picture of ion hydration in Mg(OAc)2(aq) and Mg(NO3)2(aq) at varying concns.