Computer Simulation Insights into the Chemical Origins of Life: Can HCN Enrichment on Interstellar Amorphous Ice Be a Starting Point?

The adsorption of HCN at the surface of low-density amorphous (LDA) ice and its dissolution in the bulk LDA phase is studied by grand canonical Monte Carlo (GCMC) simulations at the temperatures of 50, 100, and 200 K, characteristic of different domains of the interstellar medium (ISM). Dissolved and adsorbed molecules are distinguished using the identification of the truly interfacial molecules (ITIM) method. The results reveal that the adsorption is monomolecular and the adsorption monolayer is only partially saturated at the point of condensation of HCN. The surface coverage corresponding to the saturated adsorption monolayer is estimated to be 9.8 ± 0.3 μmol/m², providing a better estimate for this quantity than the crude approximation used in evaluating certain experiments. For the entropy of the condensed (glassy) phase of HCN, the simulations provide the value of 17.37 J/(mol K). The adsorption isotherms deviate considerably from the Langmuir shape, revealing that non-negligible interaction occurs between the adsorbed HCN molecules. The adsorption is found to be primarily governed by the dipolar interactions both between the surface water and adsorbed HCN molecules and between HCN neighbors within the adsorption layer. The heat of adsorption at infinitely low surface coverage is estimated to be −49.4 ± 3.9 kJ/mol. Further, the isosteric heat of adsorption at finite coverages is calculated in the entire range of surface coverages. In clear contrast with the adsorption, the dissolution of the HCN molecules remains ideal up to the point of condensation. This indicates that, in spite of the surprisingly large HCN concentrations reached, the HCN–HCN interaction is negligible in the bulk LDA phase. Further, contrary to its adsorption, the dissolution of HCN in LDA ice turns out to be an endothermic process. Finally, our results concerning the adsorption of HCN are not incompatible with the possibility of the oligomerization reaction of the HCN molecules, leading to the prebiotic formation of certain building blocks of biological macromolecules, under interstellar conditions at the LDA surface. Further, assuming that upon approaching the Earth, the transformation of the LDA phase with increasing temperature to liquid water goes through the thermodynamically stable crystalline (I_h) ice phase, which does not dissolve HCN molecules, the presumed expulsion of HCN to the ice surface could provide an additional window of opportunity for their oligomerization.