  We study by molecular dynamics simulations the wetting/dewetting transition
and the dependence of the free energy on distance between plates that contain
both hydrophobic and hydrophilic particles. We show that dewetting and strength
of hydrophobic interaction is very sensitive to the distribution of hydrophobic
and hydrophilic domains. In particular, we find that plates characterized by a
large domain of hydrophobic sites induce a dewetting transition and an
attractive solvent-induced interaction. On the other hand, a homogeneous
distribution of the hydrophobic and hydrophilic particles on the plates
prevents the dewetting transition and produces a repulsive solvent-induced
interaction. We also present results for a kind of Janus interface in which one
plate consists of hydrophobic particles and the other of hydrophilic particles
showing that the inter-plate gap remains wet until steric constraints at small
separations eject the water molecules. Our results indicate that the Cassie
equation, for the contact angle of a heterogeneous plate, can not be used to
predict the critical distance of dewetting. These results indicate that
hydrophobic interactions between nanoscale surfaces with strong large
length-scale hydrophobicity can be highly cooperative and thus they argue
against additivity of the hydrophobic interactions between different surface
domains in these cases. These findings are pertinent to certain protein-protein
interactions where additivity is commonly assumed.
