Pathways to dewetting in hydrophobic confinement

Liquid water can become metastable with respect to its vapor in hydrophobic confinement. The resulting dewetting transitions are often impeded by large kinetic barriers. According to macroscopic theory, such barriers arise from the free energy required to nucleate a critical vapor tube that spans the region between two hydrophobic surfaces—tubes with smaller radii collapse, whereas larger ones grow to dry the entire confined region. Using extensive molecular simulations of water between two nanoscopic hydrophobic surfaces, in conjunction with advanced sampling techniques, here we show that for intersurface separations that thermodynamically favor dewetting, the barrier to dewetting does not correspond to the formation of a (classical) critical vapor tube. Instead, it corresponds to an abrupt transition from an isolated cavity adjacent to one of the confining surfaces to a gap-spanning vapor tube that is already larger than the critical vapor tube anticipated by macroscopic theory. Correspondingly, the barrier to dewetting is also smaller than the classical expectation. We show that the peculiar nature of water density fluctuations adjacent to extended hydrophobic surfaces—namely, the enhanced likelihood of observing low-density fluctuations relative to Gaussian statistics—facilitates this nonclassical behavior. By stabilizing isolated cavities relative to vapor tubes, enhanced water density fluctuations thus stabilize novel pathways, which circumvent the classical barriers and offer diminished resistance to dewetting. Our results thus suggest a key role for fluctuations in speeding up the kinetics of numerous phenomena ranging from Cassie–Wenzel transitions on superhydrophobic surfaces, to hydrophobically driven biomolecular folding and assembly.The favorable interactions between two extended hydrophobic surfaces drive numerous biomolecular and colloidal assemblies (1–5), and have been the subject of several theoretical, computational, and experimental inquiries (6–22). Examples include the association of small proteins to form multimeric protein complexes, of amphiphlic block copolymers, dendrimers, or proteins to form vesicular suprastructures, and of patchy colloidal particles into complex crystalline lattices (23–27). When two such hydrophobic surfaces approach each other, water between them becomes metastable with respect to its vapor at a critical separation, d_c, that can be quite large (8, 9, 28–30). For nanometer-sized surfaces at ambient conditions, d_c is proportional to the characteristic size of the hydrophobic object, whereas for micron-sized and larger surfaces, d_c ～ 1?μm (29, 30). However, due to the presence of large kinetic barriers separating the metastable wet and the stable dry states, the system persists in the wet state, and a dewetting transition is triggered only at much smaller separations ( ～ 1 nm) (13, 22, 28, 30).To uncover the mechanism of dewetting, a number of theoretical and simulation studies have focused on the thermodynamics as well as the kinetics of dewetting in the volume between two parallel hydrophobic surfaces that are separated by a fixed distance, d < d_c (8, 10–16, 18–21). These studies have highlighted that the bottleneck to dewetting is the formation of a roughly cylindrical, critical vapor tube spanning the region between the surfaces (11, 14, 15). A barrier in the free energetics of vapor tube formation as a function of tube radius is also supported by macroscopic interfacial thermodynamics, wherein the barrier arises primarily from a competition between the favorable solid–vapor and unfavorable liquid–vapor surface energies (Eq. 1 and Fig. 1). Thus, the classical mechanism for the dewetting transition prescribes that a vapor tube that spans the volume between the two surfaces must first be nucleated, and if the vapor tube is larger than a certain critical size, it will grow until the entire confined volume is dry (9).Open in a separate windowFig. 1.(A–C) Simulation snapshots of water (shown in red/white) in confinement between two square hydrophobic surfaces (shown in cyan) of size L = 4 nm that are separated by a distance of d = 20 Å; configurations highlighting (A) the liquid basin, (B) a cylindrical vapor tube of radius, r, that spans the confined region, and (C) the vapor basin are shown. In the front views, only one of the confining surfaces is shown. (D) Macroscopic theory predicts a free energetic barrier to vapor tube formation (Eq. 1), suggesting that a vapor tube larger than a critical size must be nucleated before dewetting can proceed.Although it has been recognized that water density fluctuations must play a crucial role in nucleating vapor tubes (14, 15), the precise mechanism by which these tubes are formed is not clear. To understand how vapor tubes are formed and to investigate their role in the dewetting process, here we use molecular simulations in conjunction with enhanced sampling methods (31, 32) to characterize the free energetics of water density fluctuations in the region between two nanoscopic hydrophobic surfaces. Such a characterization of water density fluctuations in bulk water and at interfaces has already provided much insight into the physics of hydrophobic hydration and interactions (5, 13, 31–44). In particular, both simulations and theory have shown that the likelihood of observing low-density fluctuations adjacent to extended hydrophobic surfaces is enhanced relative to Gaussian statistics (13, 31, 36–38, 42). Further, the intricate coupling between enhanced solvent fluctuations and dewetting kinetics has been highlighted by both coarse-grained (45–47) and atomistic simulations (48–51).Here we show that such enhanced water density fluctuations influence the pathways to dewetting in hydrophobic confinement by stabilizing isolated cavities adjacent to one of the confining surfaces with respect to vapor tubes. As the density in the confined region is decreased, the stability of isolated cavities relative to vapor tubes also decreases, and at a particular density, isolated cavities abruptly transition to vapor tubes. Surprisingly, for d?d_c, that is, separations for which dewetting is thermodynamically favorable, we find that the nascent vapor tubes formed from the isolated cavities are already larger than the corresponding critical vapor tubes predicted by classical theory. Because the newly formed vapor tube is supercritical, it grows spontaneously. Importantly, because the formation of this supercritical vapor tube involves a nonclassical pathway that circumvents the critical vapor tube altogether, the process entails a smaller free energetic cost. Our results thus point to smaller kinetic barriers to dewetting than predicted by macroscopic theory.