  The structure and dynamics of aqueous solvation of ethanol and ethylene are
studied by DFT-based Car-Parrinello molecular dynamics. We did not find an
enhancement of the structure of the hydrogen bonded network of hydrating water
molecules. Both ethanol and ethylene can easily be accommodated in the
hydrogen-bonded network of water molecules without altering its structure. This
is supports the conclusion from recent neutron diffraction experiments that
there is no hydrophobic hydration around small hydrophobic groups. Analysis of
the electronic charge distribution using Wannier functions shows that the
dipole moment of ethanol increases from 1.8 D to 3.1 D upon solvation, while
the apolar ethylene molecule attains an average dipole moment of 0.5 D. For
ethylene, we identified configurations with $\pi$-H bonded water molecules,
that have rare four-fold hydrogen-bonded water coordination, yielding
instantaneous dipole moments of ethylene of up to 1 D. The results provide
valuable information for the improvement of empirical force fields, and point
out that for an accurate description of the aqueous solvation of ethanol, and
even of the apolar ethylene, polarizable force fields are required.
