  While individual water molecules adsorb strongly on a talc surface
(hydrophilic behavior), a droplet of water beads up on the same surface
(hydrophobic behavior). To rationalize this dichotomy, we investigate the
influence of the microscopic structure of the surface and the strength of
adhesive (surface-water) interactions on surface hydrophobicity. We show that
at low relative humidity, the competition between adhesion and the favorable
entropy of being in the vapor phase determines the surface coverage. However,
at saturation, it is the competition between adhesion and cohesion (water-water
interactions) that determines surface hydrophobicity. The adhesive interactions
in talc are strong enough to overcome the unfavorable entropy, and water
adsorbs strongly on talc surfaces. However, they are too weak to overcome the
cohesive interactions, and water thus beads up on talc surfaces. Surprisingly,
even (talc-like) surfaces that are highly adhesive, do not fully wet at
saturation. Instead, a water droplet forms on top of a strongly adsorbed
monolayer of water. Our results imply that the interior of hydrophobic zeolites
suspended in water may contain adsorbed water molecules at pressures much
smaller than the intrusion pressure.
