Vaitheeswaran Subramanian, Yin Hao, Rasaiah Jayendran C
Department of Physics and Astronomy and Department of Chemistry, University of Maine, Orono, Maine 04469, USA.
J Phys Chem B. 2005 Apr 14;109(14):6629-35. doi: 10.1021/jp045591k.
Molecular dynamics simulations of water at 298 K and 1 atm of pressure are used to investigate the electric-field dependence of the density and polarization density of water between two graphite-like plates of different sizes (9.8 x 9.2 and 17.7 x 17.2 A) in an open system for plate separations of 8.0, 9.5, and 16.4 A. The interactions with water were tuned to "hard-wall-like" and "normal" C-O hydrophobic potentials. Water between the larger plates at 16.4 A separation is layered but is metastable with respect to capillary evaporation at zero field (Bratko, D.; Curtis, R. A.; Blanch, H. W.; Prausnitz, J. M. J. Chem. Phys. 2001, 115, 3873). Applying a field decreases the density of the water between the plates, in apparent contradiction to thermodynamic and integral equation theories of bulk fluid electrostriction that ignore surface effects, rendering them inapplicable to finite-sized films of water between hydrophobic plates. This suggests that the free energy barrier for evaporation is lowered by the applied field. Water, between "hard-wall-like" plates at narrower separations of 9.5 A and less, shows a spontaneous but incomplete evaporation at zero field within the time scale of our simulation. Evaporation is further enhanced by an electric field. No such evaporation occurs, on these time scales, for the smaller plates with the "hard-wall-like" potential at a separation of 8.0 A at zero field, signaling a crossover in behavior as the plate dimension decreases, but the water density still diminishes with increasing field strength. These observations could have implications for the behavior of thin films of water between surfaces in real physical and biological systems.
在开放系统中，对尺寸不同（9.8×9.2 和 17.7×17.2 Å）的两块类石墨板之间的水进行了 298 K 和 1 atm 压力下的分子动力学模拟，以研究板间距为 8.0、9.5 和 16.4 Å 时水的密度和极化密度对电场的依赖性。与水的相互作用被调整为“类硬壁”和“正常”的 C - O 疏水势。在 16.4 Å 间距的较大板之间的水是分层的，但在零场下相对于毛细管蒸发是亚稳的（布拉特科，D.；柯蒂斯，R. A.；布兰奇，H. W.；普劳斯尼茨，J. M.《化学物理杂志》2001 年，115 卷，3873 页）。施加电场会降低板间水的密度，这显然与忽略表面效应的体相流体电致伸缩的热力学和积分方程理论相矛盾，使这些理论不适用于疏水板之间有限尺寸的水膜。这表明施加的电场降低了蒸发的自由能垒。在 9.5 Å 及更小的更窄间距的“类硬壁”板之间的水，在我们模拟的时间尺度内，在零场下会自发但不完全蒸发。电场会进一步增强蒸发。在这些时间尺度上，对于零场下 8.0 Å 间距的具有“类硬壁”势的较小板，不会发生这种蒸发，这表明随着板尺寸减小行为发生了转变，但水的密度仍会随着场强增加而减小。这些观察结果可能对实际物理和生物系统中表面之间的水薄膜行为有影响。
