Pressure-induced ion pairing in MgSO4 solutions: Implications for the oceans of icy worlds

Abstract

At ambient temperature, liquid water transforms from a low-density to a high-density dynamic structure at ~0.2 GPa. The transition persists in electrolyte solutions; however, its effects on solute properties are unknown. We obtained Raman spectra of 0.5-2.0 molal MgSO 4 solutions at 21 °C and 10 -4 to ~1.6 GPa. Above about 0.4 GPa, we observed an increase in the MgSO 4 contact ion pair abundance with pressure, regardless of concentration. This phenomenon contravenes the general rule that dissolved salts dissociate upon compression, and is likely caused by the structural collapse in the solvent with pressure due to increased hydrogen-bond breaking. Increasing ion association in high-pressure aqueous solutions implies that, at a given salinity, high-density water in deep, cold planetary oceans and pore waters will possess lower ionic strength and electric conductivity than previously thought. This behaviour will also lead to higher ocean salinity in the interiors of Pluto and the largest icy moons of Jupiter and Saturn, Ganymede, Callisto, and Titan, or in exoplanet water-worlds, through enhancement of submarine silicate weathering.

Electrical Conductivity of Aqueous Magnesium Sulfate at High Pressure and Low Temperature With Application to Ganymede's Subsurface Ocean

“…However, as pressure increases further but below the solidification pressure, the electrical conductivity decreases with increasing pressure. This is consistent with the low-concentration experimental results of an earlier study (Larionov & Kryukov, 1984), which was interpreted as the result of increasing hydrogen-bond breaking in the solvent upon compression, which in turn improves the ion association and decreases the mobility and thus electrical conductivity (Schmidt & Manning, 2017).…”

“…More importantly, the pressure dependence of conductivity is very consistent. Regardless of the concentration, at low pressure (<200 MPa), the dissociation of MgSO 4 (aq) increases with increasing pressure, leading to an increase in electrical conductivity (Chatterjee et al, 1974; Ritzert & Franck, 1968; Schmidt & Manning, 2017). However, as pressure increases further but below the solidification pressure, the electrical conductivity decreases with increasing pressure.…”

“…Next, we made two assumptions following Sinmyo and Keppler (2016). First, we assume that α is equal to 0.95, which is reasonable in MgSO 4 (aq) (Schmidt & Manning, 2017; Tomšič et al, 2002) at room temperature and below, making the denominator of K expression (Equation ) constant, so that logarithmic K and α can be more easily connected. We take logarithms on both sides of Equation , yielding the following expression: …”

Fluids of the Lower Crust: Deep Is Different

“…In addition, the extent of dissociation of salt solutions increases with rising P and T ( Jahn & Schmidt 2010, Pan & Galli 2016, consistent with inferences from in situ Raman spectroscopy (Frantz 1998, Frantz et al 1994, Schmidt 2014. Recent results suggest that these general trends may break down at the lowest temperatures (Schmidt & Manning 2017). Despite the growing capabilities of experimental and simulation approaches for characterizing the structure and energetics of ion-water interaction in simple systems, understanding of more complex fluids in equilibrium with model or natural mineral assemblages still relies on thermodynamic models of aqueous ions (Dolejš 2013).…”

Magnetic Induction Responses of Jupiter's Ocean Moons Including Effects From Adiabatic Convection

“…Although electrical conductivity generally increases with depth, it begins to decrease at the greatest depths for the warm Ganymede ocean (right‐most curve). This inflection occurs because the ocean achieves GPa + pressures, at which the packing of water molecules begins to inhibit the charge exchange of the dissolved ions (Schmidt & Manning, 2017).…”