Stability of Water Confined between Supported Self-Assembled Monolayers

We present a thermodynamic argument showing that the evaporation and condensation free-energy barriers of water confined between two hydrophobic self-assembled monolayers (SAMs) vary more gradually with the SAM hydrophobicity as compared to the case of water confined between two bare hydrophobic surfaces (no SAMs). We validate our theory by calculating the free-energy profiles of water confined between two SAMs and between two bare surfaces of different hydrophobicities. An implication of our findings is the existence of three regimes of stability of confined water as a function of the hydrophobicity of the SAMs. In comparison to bare planar surfaces with no SAMs, the highly hydrophobic SAMs act to stabilize the liquid state, whereas weakly hydrophobic SAMs stabilize the vapor state of confined water. For intermediate hydrophobicities, the SAMs reduce both the evaporation and the condensation free-energy barriers. These results imply that the effects of SAM hydrophobicity on the behavior of confined water are nontrivial and richer than previously thought.