How Can Talc Surfaces Be Both Hydrophilic and Hydrophobic?

Experiments have shown that individual water molecules adsorb strongly on a talc surface (hydrophilic behavior). However, a droplet of water beads up on the same surface (hydrophobic behavior). To rationalize this dichotomy, we investigate the influence of surface microstructure 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. Because adhesive interactions in talc are strong enough to overcome the unfavorable entropy, 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, we find that 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 lower than the intrusion pressure.