Unraveling Molecular Packing and Hydrogen-Bond Networks in Ethaline–Propanol Mixtures: Insights from Volumetric Behavior

The volumetric behavior of two deep eutectic solvent mixtures, Ethaline + 1-propanol and Ethaline + 2-propanol, was systematically investigated over the temperature range 303–333 K at atmospheric pressure. Densities were measured across the full composition range, and derived volumetric properties were analyzed to reveal the underlying molecular interactions governing mixing. Both systems exhibit contraction upon mixing, indicating the dominance of attractive forces and efficient structural packing. The degree of contraction increases slightly with temperature and is consistently greater for the 2-propanol system, where the more compact secondary alcohol fits more efficiently within the Ethaline network despite its weaker hydrogen-bonding ability. The data further show that, in Ethaline-rich mixtures, alcohol molecules are strongly solvated by the choline chloride–ethylene glycol matrix, while in alcohol-rich regions, Ethaline pseudomolecules become the solvated species surrounded by propanol molecules. This dual solvation behavior reflects a delicate balance between hydrogen bonding, molecular geometry, and packing efficiency. The results provide the first detailed volumetric description of Ethaline–propanol systems and establish structure–property relationships valuable for the design of deep eutectic solvent-based separation and reaction media.