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

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 (CH 3 CN) 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.

Properties and Behavior of the Acetonitrile–Acetylene Co-Crystal under Titan Surface Conditions

“…In contrast to nonpolar hydrocarbons like acetylene, nitriles have lower solubility in methane and ethane. , Nitriles, such as acetonitrile, would not readily dissolve, but instead would become residual grains that could be liberated from surrounding materials following dissolution. Molecular dynamic simulations support this theory, suggesting that acetonitrile forms clusters when exposed to liquid methane, ethane, and propane, primarily due to dipole–dipole interactions, and that larger clusters are preferred over smaller clusters . This suggests that deposits of pure acetonitrile might form at the base of a Titan lake or river delta as these clusters grow and precipitate, and over time such deposits could be entrained within the crust of Titan.…”

“…Molecular dynamic simulations support this theory, suggesting that acetonitrile forms clusters when exposed to liquid methane, ethane, and propane, primarily due to dipole−dipole interactions, and that larger clusters are preferred over smaller clusters. 75 This suggests that deposits of pure acetonitrile might form at the base of a Titan lake or river delta as these clusters grow and precipitate, and over time such deposits could be entrained within the crust of Titan. In this scenario, acetonitrile would form a lag deposit and could be transported by typical overland clastic transport processes such as eolian movement or fluvial transport.…”

Distributed Charge Models of Liquid Methane and Ethane for Dielectric Effects and Solvation

“…We expect these new DC models to be useful in the study of solvation and assembly of polar and charged solutes in liquid methane and ethane, both of which require a description of dielectric response 23,77,[97][98][99] . In particular, there is great interest in understanding chemistry that could be occurring in the liquid hydrocarbon lakes on the surface of Titan.…”

“…The existence of liquid reservoirs on Titan's surface, combined with its rich atmospheric chemistry, has led many to hypothesize that the hydrocarbon lakes could harbor prebiotic chemistry and even non-aqueous life [14][15][16][17][18] . However, any such chemistry would be vastly different than similar processes in aqueous environments, and a fundamental, molecular-scale understanding is necessary, beginning with characterizing solvation in methane and ethane 15,[19][20][21][22][23] . Such a microscopic picture of cryogenic hydrocarbon solutions can be provided by molecular simulations, but there remains a need to make these simulations efficient and predictive.…”

Titan: Earth-like on the Outside, Ocean World on the Inside

“…Theoretical investigations are exploring both the possibilities for lipid membrane-like structures in low temperature environments and whether cell membranes are even necessary (Palmer et al 2017;Stevenson et al 2015;Rahm et al 2016;Sandström & Rahm 2020). Laboratory and theoretical models are revolutionizing our understanding of the possible conditions within Titan's lakes and seas (Cordier & Carrasco 2019;Luspay-Kuti et al 2015;Cordier et al 2012Cordier et al , 2016Cordier et al , 2017Cordier & Liger-Belair 2018;Hodyss et al 2013;Corrales et al 2017;Malaska et al 2017;Hartwig et al 2018;Czaplinski et al 2019Czaplinski et al , 2020Farnsworth et al 2019). Employing these new findings to constrain the habitability potential of Titan's liquid hydrocarbons requires both determining the composition of Titan sediments-as the Dragonfly mission's plans to do by exploring at a portion of one of Titan's low-latitude dune fields-and monitoring the composition, physical conditions, and seasonal evolution of Titan's polar lakes and seas with future missions.…”