Christina McConville: Peritectic phase transition of benzene and acetonitrile into a cocrystal relevant to Titan, Saturn’s moon

Titan, Saturn’s largest moon, is the only body in the solar system known to have stable bodies of liquid—lakes, rivers, and seas—that undergo dynamic processes similar to Earth’s hydrological cycle. To study the potential formation of minerals on Titan, we use a combination of structural characterization methods including high-resolution synchrotron powder X-ray diffraction (PXRD) and differential scanning calorimetry (DSC) to analyze the constituents present on Titan's surface and evaluate their potential for cocrystal formation. Among the compounds detected on the surface of Titan are two common laboratory solvents: benzene and acetonitrile. Here we report the phase diagram of mixtures of acetonitrile and benzene, which features incongruent melting and a peritectic phase transition of solid benzene and liquid acetonitrile into a 1:3 acetonitrile:benzene cocrystal. The crystal structure of this cocrystal was solved and refined from in situ diffraction data using synchrotron radiation. Additionally, to mimic the environment on Titan more accurately, we tested the stability of the structure under liquid ethane. These results provide new insights into the structure and stability of potential extraterrestrial minerals and contribute to a better understanding of the surface composition of Titan.