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 electrical conductivities of aqueous solutions of magnesium nitrate and magnesium acetate have been measured from dilute to saturation concentrations at 0 < or = t/degrees C < or = 50. The temperature derivative of the isentropic compressibility, kappa(s), became zero at 2.28 and 2.90 mol kg(-1) for Mg(OAc)2 and Mg(NO3)2 solutions, respectively, at 25 degrees C. The total hydration numbers of the dissolved ions were estimated to be, respectively, 24.3 and 19.2 at these concentrations. Differences in kappa(s) 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 kappa(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(NO3)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 degrees 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 concentrations. The experimental results are supported by molecular dynamics simulations, which also reveal the much stronger tendency of OAc- compared to NO3- to associate with Mg2+ 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(NO3)2(aq) at varying concentrations.

Micro‐Hydration of the MgNO3+ Cation in the Gas Phase

“…[7][8][9][10][11][12][13][14][15] for large values of n. [16,17] Further, magnesium ions have been discussed in the context of the possible formation of contact ion pairs in aqueous solution. [18][19][20] Nitrate is of particular interest as a counterion, because it can either bind as a mono or bidentate ligand, but as a weakly coordinating anion, it may also promote the formation of ion pairs in solution. [21,22] Further, the structures and vibrational spectra of a large number of nitrato complexes in the condensed phase have been characterized in detail.…”

Ab initio investigation on the ion‐associated species and process in Mg(NO3)2 solution

“…28 The vibrational spectroscopy is also an effective technique to investigate the associated species in nitrate solutions and many reports were presented. 15,[29][30][31][32][33][34][35][36][37][38][39][40] However, there is no consistent opinion for the existence and structure of the associated species. Peleg observed the Raman spectra of Mg(NO 3 ) 2 -H 2 O system in various concentrations and phase states.…”

Mapping the Free Energy of Lithium Solvation in the Protic Ionic Liquid Ethylammonuim Nitrate: A Metadynamics Study

“…[23][24][25][26][27] Moreover,w hen other inorganic salts, such as Mg(NO 3 ) 2 ,C a(NO 3 ) 2 ,a nd Al(NO 3 ) 2 ,a re dissolved into PILs, the metal cations( Mg 2 + ,C a 2 + ,a nd Al 3 + )a re more likely to reside in polar domains. [14,15,72,100,101] This arrangement is also favoredb yL i + ,w hich is found close to the polar groups and is strongly associated with the nitrate ions. [43,47] Also in EAN, the lithium ion is knownt ob es olvated by af irst shell of nitrate ion, which screen it from the resto ft he liquid.…”