Acetonitrile cluster solvation in a cryogenic ethane-methane-propane liquid: Implications for Titan lake chemistry.

L René Corrales,David Shalloway,Thomas D Yi,Samantha K Trumbo,David A Usher,Jonathan I Lunine

doi:10.1063/1.4978395

L René Corrales, David Shalloway + Show 4 more

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https://doi.org/10.1063/1.4978395

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Abstract

The atmosphere of Titan, Saturn's largest moon, exhibits interesting UV- and radiation-driven chemistry between nitrogen and methane, resulting in dipolar, nitrile-containing molecules. The assembly and subsequent solvation of such molecules in the alkane lakes and seas found on the moon's surface are of particular interest for investigating the possibility of prebiotic chemistry in Titan's hydrophobic seas. Here we characterize the solvation of acetonitrile, a product of Titan's atmospheric radiation chemistry tentatively detected on Titan's surface [H. B. Niemann et al., Nature 438, 779-784 (2005)], in an alkane mixture estimated to match a postulated composition of the smaller lakes during cycles of active drying and rewetting. Molecular dynamics simulations are employed to determine the potential of mean force of acetonitrile (CH3CN) clusters moving from the alkane vapor into the bulk liquid. We find that the clusters prefer the alkane liquid to the vapor and do not dissociate in the bulk liquid. This opens up the possibility that acetonitrile-based microscopic polar chemistry may be possible in the otherwise nonpolar Titan lakes.

Highlights

Discussions of extraterrestrial life often focus on whether liquid water is available because life on Earth is intimately intertwined with the chemical properties of water
Upon sedimentation into the lakes, these agglomerations might well separate into the individual pure ice through chemical and physical effects
The results suggest that small alkane molecules in liquids at cryogenic temperatures surround small ACN clusters such that ACN aggregates do not separate into individual molecules in this environment

Summary

INTRODUCTION

Discussions of extraterrestrial life often focus on whether liquid water is available because life on Earth is intimately intertwined with the chemical properties of water. Chemistry in Titan’s atmosphere driven by solar-ultraviolet radiation and energetic particles in Saturn’s magnetosphere leads to the formation of a variety of organics, including nitrogen-bearing molecules.. Chemistry in Titan’s atmosphere driven by solar-ultraviolet radiation and energetic particles in Saturn’s magnetosphere leads to the formation of a variety of organics, including nitrogen-bearing molecules.5 Their solvation, possibly as aggregates, in Titan’s seas could be the first step towards the development of an alternative form of life in which the conventional roles of hydrophobic and hydrophilic molecules are reversed.. We explore the solvation behavior of ACN in small alkanes at cryogenic conditions using classical molecular dynamics (MD) simulations Such simulations are well suited for this purpose because they allow the exploration of solvation chemistry at Titan’s cryogenic temperatures (∼90–95 K) and near its ambient pressure (∼146.7 kPa). We used simulations to compute the solvation characteristics of ACN clusters in pure methane and ethane solutions as well as in an ethane-methane-propane (EMP) mixture that is a possible composition of some of the smaller lakes of Titan.

THEORETICAL AND SIMULATION METHODS

MOLECULAR DYNAMICS RESULTS

CONCLUSION