Molecular dynamics simulations on the hydration of fluoroalcohols

Abstract

Molecular dynamics (MD) calculations have been carried out for aqueous solutions of isopropyl alcohol (IPA) and its fluorinated compounds, 1,1,1-trifluoro-2-propanol (TFIPA) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIPA). The simulated systems were canonical ensembles containing 216 molecules in each, one of which was alcohol and the temperature was set to 298.15 K. The MCY (Matsuoka–Clementi–Yoshimine) potential was used for water–water interaction, whereas new potential functions were determined for alcohol–water interactions, on the basis of ab initio molecular orbital calculations on more than 1100 different dimeric configurations for each alcohol–water pair. The static properties of solvent water in the vicinity of each functional group of solute were obtained from MD calculations. It is found that the promotion of water structure and the increase of hydrogen bond between water molecules occurs not only near the fluoroalkyl group but also even near hydroxyl group of fluoroalcohols. Furthermore, the alcohol–water interaction is stronger for fluoroalcohols than for aliphatic alcohols, owing to the electronegativity and the electron withdrawing effect of fluorine atoms. The enthalpies of hydration for fluoroalcohols include the contributions from these features of both water–water and alcohol–water interactions. This is not the case for the hydration of aliphatic alcohols, and is the reason for the lack of regular change of enthalpies of hydration with the degree of the substitution of CH3 by the CF3 group. The hydration of IPA is similar to that of other aliphatic alcohols; hydrophobic hydration near the apolar group and the energetic unstabilization of water near the hydroxyl group are observed.