Stability of Water Confined between Supported Self-Assembled Monolayers

Abstract

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.

Understanding the “Berg limit”: the 65° contact angle as the universal adhesion threshold of biomatter

“…49,50 However, the cavitation transition of larger systems is associated with the nucleation of a vapor bubble that spans across both surfaces. The free energy required to nucleate such a bridging bubble scales quadratically with separation, DW* B g w D 2 , 37,45,46,51,52 although linear terms stemming from line tension may become important at small separations. 48,52 This free energy represents the kinetic barrier the system needs to overcome in order to cavitate.…”

“…The free energy required to nucleate such a bridging bubble scales quadratically with separation, DW* B g w D 2 , 37,45,46,51,52 although linear terms stemming from line tension may become important at small separations. 48,52 This free energy represents the kinetic barrier the system needs to overcome in order to cavitate. On experimental time scales, a macroscopic system is capable of overcoming a barrier of the order of DW* B 10-100k B T, where k B T is the thermal energy (with k B being the Boltzmann constant).…”

Reversible Surface Energy Storage in Molecular-Scale Porous Materials

“…In contrast to monodisperse samples, this feature leads to multi-step liquid uptake and expulsion processes or continuous transitions with finite slopes of intrusion and expulsion branches constituting the wetting/dewetting cycle. In view of weak correlations [41][42][43] between intrusion/expulsion events in distinct pores, results for a spectrum of pore sizes should enable predictions for water content in a polydisperse absorbent as a linear superposition of fractional contributions from pores of different sizes. In a general case, a normalized pore size distribution function p(h), where p(h)dh represents the fraction of pores of widths between h and h + dh, determines the volume change of the bulk phase associated with the absorption of N( P b ) molecules in the material at pressure P b , ∆V(P b ) = −V(P b )N(P b ), where V(P b ) is the partial molar volume in bulk water at pressure P b , N(P b ) = ∞ 0 N(P b , h)p(h)dh, and N(P b , h) denotes the average number of absorbed molecules inside a pore of width h at bulk pressure P b.…”