Solubility of tholins in hexane, acetonitrile, and acrylonitrile: Implications for Titan’s lakes

Aerosols formed in the atmosphere of Titan sediment and accumulate on the surface where they may interact with the liquid hydrocarbons and organics composing the lakes and seas. In this study, Titan tholins made from nitrogen and methane (95/5) exposed to a cold plasma discharge have been mixed with acetonitrile, acrylonitrile, hexane and a saturated solution of acetonitrile in hexane, under an inert atmosphere at room temperature. The liquid phase has been monitored through time (days to weeks) using gas chromatography coupled to mass spectrometry. Fourier transform infrared spectroscopy has been used to characterize tholins before and after solubilization events, in both the near and mid infrared regions. Infrared spectra do not show differences induced by the interaction of tholins with the liquid phases used in this work. Gas chromatograms indicate that many organics are extracted from our tholins in acetonitrile, with the characterization of nitriles, amines and other nitrogen bearing molecules. Among the three other solvents tested, only acrylonitrile shows limited solubilization of organics from tholins, but no structure could be identified. Pure hexane fails to solubilize any organics detectable in our conditions, as expected by its nonpolar nature. Despite the high solubility of our tholins in acetonitrile, no organics are solubilized when this molecule is saturated in hexane. The positive results obtained with polar solvents (acetonitrile and acrylonitrile) and the negative results obtained with nonpolar liquid phases (hexane with or without acetonitrile) are consistent with previous studies. However, these studies also showed that the composition of tholins is highly dependent on the conditions used for their synthesis, which cannot capture the complexity of Titan’s atmosphere where many processes co-exist. This suggests that Titan haze could contain a rich reservoir of organics that, given the geological timescales available, would make the presence of dissolved organics in the lakes and seas a real possibility.