Unraveling aqueous alcohol freezing: new theoretical tools from graph theory to extract molecular processes in MD simulations.

Ice clouds in the upper troposphere are crucial for regulating Earth's climate by affecting stratospheric humidity and the global radiative balance. A key aspect of cloud formation is heterogeneous ice nucleation, influenced by the surface properties of aerosol particles, particularly those with chemical groups capable of hydrogen bonding with water. Short-chained alcohols, such as 1-pentanol and 3-hexanol, which readily accumulate at the liquid-vapor interface, are of particular interest due to their potential impact on ice nucleation, despite their role in freezing processes being underexplored. To address this gap, molecular dynamics (MD) simulations combined with topological graph analysis (GT) were used to investigate the onset of water-alcohol surface freezing at temperatures ranging from 283 K to 192 K. Both macroscopic properties, like surface tension and solubility, and microscopic properties, including the incorporation of alcohols within the 2D-film of surface water, were analyzed. The results indicate that adsorbed films of 1-pentanol and 3-hexanol significantly influence the onset of surface freezing, with 1-pentanol forming more organized and efficiently packed surface layers compared to 3-hexanol, thus reducing surface tension more effectively. The novel application of topological graph analysis based on the representation of intra- and inter-molecular interactions in a graph revealed the insertion of alcohol molecules into the collective hydrogen-bonded 2D network at the water surface, promoting the enhanced formation of six-membered H-bonded rings at lower temperatures. This effect was particularly pronounced with 1-pentanol, which proved more efficient than 3-hexanol in facilitating the creation of ice-like structures-a critical precursor to ice formation. These findings offer valuable insights into the processes governing cloud formation and ice nucleation, with significant implications for understanding climate science and cloud dynamics.