Prof. Wojciech Marczak
(Institute of Occupational Medicine and Environmental Health, Sosnowiec, Poland)

Molecular clusters due to hydrogen bonding in aqueous solutions of cyclic amines

Dissolution of pyridine, piperidine, pyrrolidine and their methyl-substituted derivatives in water leads to microheterogeities on the nanometer-order length scale, that are manifested in SANS curves. Most probably, molecular clusters are formed in liquid aqueous solutions due to hydrogen bonds between the water molecules in amine hydrates rather than due to solute-solute interactions. No such effects occur in the mixtures of the amines with methanol, that supports the idea of active role of water solvent in the aggregation. Another question are contributions in thermodynamic functions due to specific interactions, van der Waals forces, and the size and shape of the molecules. Limiting partial molar enthalpies of solution of the amines in water and methanol and vice versa can be assessed from theoretically calculated molecular interaction energies using a simple “chemical reaction” model. Nearly quantitative agreement of the theoretical and empirical enthalpies can be achieved for the solutions of water and methanol in the amines and for the amines in methanol. An empirical hydrophobic hydration term must be considered for the amines in water. Thus, water-rich mixtures clearly stand apart from the other compared systems. The structuredness of aqueous solutions results from the cooperativity of hydrogen bonds.