  An important characteristic that determines the behavior of a solute in water
is whether it is hydrophobic or hydrophilic. The traditional classification is
based on chemical experience and heuristics. However, this does not reveal how
the local environment modulates this important property. We present a local
fingerprint for hydrophobicity and hydrophilicity inspired by the two body
contribution to the entropy. This fingerprint is an inexpensive, quantitative
and physically meaningful way of studying hydrophilicity and hydrophobicity
that only requires as input the water-solute radial distribution functions. We
apply our fingerprint to octanol, benzene and the 20 proteinogenic amino acids.
Our measure of hydrophilicity is coherent with chemical experience and,
moreover, it also shows how the character of an atom can change as its
environment is changed. Lastly, we use the fingerprint as collective variable
in a funnel metadynamics simulation of a host-guest system. The fingerprint
serves as a desolvation collective variable that enhances transitions between
the bound and unbound states.
