Preliminary Studies of Methanol-Water Solutions

Background

Sub-surface oceans have been proposed to exist on many of the large icy moons of the outer solar system. The thickness of a shell and the depth of any such ocean depend on many factors, including the composition and pressure-dependent freezing points of the materials likely to be found in the ocean.

On Enceladus, the observations of water-rich plumes and the measured libration are consistent with a global subsurface ocean under a rather thin ice shell, perhaps less than 10 km thick.

Any subsurface ocean would likely contain impurities, such as ammonia and methanol, that act as powerful antifreeze compounds. Small amounts of methanol may have been detected on the surface of Enceladus as well as in the plume. In addition to being a powerful antifreeze, methanol could also play a role in the formation of methane hydrates

Methanol-Water Phase Diagram

Atmospheric pressure phase diagram for methanol-water solutions, adapted from Kargel (Kargel1992). Data are from Haghighi et al. (2009), Vuillard \& Sanchez (Vuillard1961), Miller \& Carpenter (miller1964).

As a methanol-water mixture is cooled, ice crystals precipitate out until the peritectic point is reached, at a temperature of approximately 171 K and a concentration of 69%. Below the eutectic temperature of 157 K, the system solidifies completely. The eutectic concentration is approximately 88%.

The current experiments have been done at concentrations of 4.9, 9.9, 33.8, and and 75 wt.%.

Experiments

Although the effects of pressure on pure ice and pure methanol have been extensively studied, the behavior of water-methanol mixtures under pressure has not been studied as extensively. In this work, we consider the freezing behavior of methanol-water solutions at low temperatures and moderate pressures such as might be encountered in the icy moons of the outer solar system.

Specifically, we report measurements of the liquidus points for at pressures ranging from 5 to 400 MPa, using simultaneous measurements of pressure, volume, and temperature, coupled with optical images of the sample.

The phase boundaries for pure water {wagner2011,dunaeva2010} and pure methanol {wurflinger1977} are included for comparison. For water-rich solutions, the liquidus trends follow that of pure water, while the eutectic points, and liquidus points for methanol-rich solutions, follow that of pure methanol.

Preliminary results for pressures up to 375 MPa. Transition temperatures for methanol-water mixtures, with lines for water (red) and pure methanol (black) included for comparison.

References

[1] M. Dougherty, et al. (2006) Science 311 (5766):1406. [2] C. Porco, et al. (2006) Science 311 (5766):1393. [3] J. H. Waite, Jr., et al. (2009) Nature 460 (7259):1164. [4] C. J. Hansen, et al. (2011) Geophys Res Lett 38:L11202. [5] H.-W. Hsu, et al. (2015) Nature 519 (7542):207. [6] F. Postberg, et al. (2011) Nature 474 (7353):620. [7] P. C. Thomas, et al. (2016) Icarus 264:37. [8] D. Hogenboom, et al. (1997) Icarus 128 (1):171. [9] R. Hodyss, et al. (2009) Geophys Res Lett 36:L17103. [10] G. McLaurin, et al. (2014) Angewandte Chemie-International Edition 53 (39):10429. [11] J. S. Kargel (1992) Icarus 100:556. [12] G. Vuillard, et al. (1961) Bull Soc Chim France 1877–1880. [13] G. A. Miller, et al. (1964) J Chem & Eng Data 9 (3):371. [14] W. Wagner, et al. (2011) J Phys and Chem Ref Data 40 (4). [15] A. N. Dunaeva, et al. (2010) J Solar System Research 44 (3):202. [16] A. Wurflinger, et al. (1977) J Phys Chem Solids 38:811.

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