期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：27
页码：8181-8186
DOI：10.1073/pnas.1503302112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceDewetting in hydrophobic confinement plays an important role in diverse phenomena, ranging from protein folding and assembly, to the heterogeneous nucleation of vapor bubbles and superhydrophobicity. Using molecular simulations, we find that dewetting proceeds through the formation of isolated cavities adjacent to one of the confining surfaces. These isolated cavities are stabilized by enhanced water density fluctuations, and their growth is uphill in free energy. Upon growing to a certain size, the isolated cavities transition abruptly into supercritical vapor tubes that span the confined region, and grow spontaneously. Consequently, this nonclassical pathway results in lower free energy barriers than anticipated by macroscopic theory, with important implications for the kinetics of dewetting and hence for water-mediated self-assembly. 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.
