Interactions of Alkanolamines with Water: Excess Enthalpies and Hydrogen Bonding

We report a transferable force field to describe the interactions of alkanolamines based on the N-C-C-O backbone with water, derived from a comparison with experimental excess enthalpies. This force field is tested on 2-aminoethan-1-ol (MEA), 2-amino-2-methylpropan-1-ol, 2-aminobutan-1-ol (ABU), and 1-aminopropan-2-ol. These alkanolamines are derivatives of MEA obtained by substitution with methyl and ethyl groups on the carbon atoms of the N-C-C-O backbone. A specific cross interaction site corresponding to the hydrogen bond between the hydroxyl group of the alkanolamine and the oxygen atom of water was introduced in order to reproduce quantitatively experimental excess enthalpies. The transferability of this specific site was assessed by predictions on alkanolamines that were not included in the parametrization data set. New data on enthalpy of mixing for ABU with water are reported, since they were not available in the literature. From the molecular simulations, several microscopic quantities of the alkanolamine-water mixtures were analyzed in order to improve our understanding of these systems. The structure of the solvation shells at varying compositions, statistics of hydrogen bonds, conformations, and energy decompositions served as bases for an interpretation of the molecular reasons underlying the behavior of the excess enthalpy. The prominent result is that water-water interactions play a major role in differentiating alkanolamine-water mixtures, which is a manifestation of the hydrophobic effect. Both the structural and energetic effects observed at the molecular level point to phenomena that have strong composition dependence, in particular, the interplay between the intramolecular hydrogen bond in the alkanolamine and the intermolecular hydrogen bonds with water.