Understanding Organic Co-crystals Through Trends in Thermodynamic Properties Using Simulations and Cryogenic Experiments

On organic-rich worlds such as Titan, the largest moon of Saturn, certain compounds may form co-crystals—organic solids consisting of two or more neutral molecules incorporated throughout a unique crystalline structure in a specific stoichiometric ratio. Under the cryogenic conditions on the surface of Titan, weaker intermolecular forces become more important, resulting in co-crystal lattice energies that are often more stable than the pure, producing unique physical and chemical properties. While the properties of the pure constituents are generally well understood, mixtures remain less explored, leaving it unclear under what conditions co-crystals form, destabilize, or undergo phase transitions.

In this report, we present computational analyses and initial laboratory observations of previously identified co-crystals. By replicating Titan surface interactions of binary mixtures, we observe how they may change when present in pure and complex mixtures of organic and aqueous ices. Comparing the thermodynamic properties of co-crystals with those of their pure components provides a foundation for understanding these materials. Density Functional Theory (DFT) and Molecular Dynamics (MD) simulations offer a deeper insight into the structural properties and molecular interactions taking place within these organic ice mixtures. Thermodynamic calculations for benzene:ethane and acetonitrile:propane co-crystals reveal energy profiles distinct from those of their pure counterparts. The band structures of the co-crystals compared to the pure solids give insight into how these materials are able to form. Gaining a better understanding of these mixtures and their properties can provide insight into the composition and evolution of surface features, such as rivers, lakes, seas, and mountains of icy worlds.