Luzar Alenka, Bratko Dusan
Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284, USA.
J Phys Chem B. 2005 Dec 1;109(47):22545-52. doi: 10.1021/jp054545x.
Measured forces between apolar surfaces in water have often been found to be sensitive to exposure to atmospheric gases despite low gas solubilities in bulk water. This raises questions as to how significant gas adsorption is in hydrophobic confinement, whether it is conducive to water depletion at such surfaces, and ultimately if it can facilitate the liquid-to-gas phase transition in the confinement. Open Ensemble molecular simulations have been used here to determine saturated concentrations of atmospheric gases in water-filled apolar confinements as a function of pore width at varied gas fugacities. For paraffin-like confinements of widths barely exceeding the mechanical instability threshold (spinodal) of the liquid-to-vapor transition of confined water (aqueous film thickness between three and four molecular diameters), mean gas concentrations in the pore were found to exceed the bulk values by a factor of approximately 30 or approximately 15 in cases of N2 and CO2, respectively. At ambient conditions, this does not result in visible changes in the water density profile next to the surfaces. Whereas the barrier to capillary evaporation has been found to decrease in the presence of dissolved gas (Leung, K.; Luzar, A.; and Bratko, D. Phys. Rev. Lett. 2003, 90, 065502), gas concentrations much higher than those observed at normal atmospheric conditions would be needed to produce noticeable changes in the kinetics of capillary evaporation. In simulations, dissolved gas concentrations corresponding to fugacities above approximately 40 bar for N2, or approximately 2 bar for CO2, were required to trigger expulsion of water from a hydrocarbon slit as narrow as 1.4 nm. For nanosized pore widths corresponding to the mechanical instability threshold or above, no significant coupling between adsorption layers at opposing confinement walls was observed. This finding explains the approximately linear increase in gas solubility with inverse confinement width and the apparent validity of Henry's law in the pores over a broad fugacity range.
尽管气体在大量水中的溶解度较低，但人们经常发现水中非极性表面之间测量到的力对暴露于大气气体很敏感。这就引发了一些问题，比如气体吸附在疏水限制环境中有多重要，它是否有助于此类表面的水耗尽，以及最终它是否能促进限制环境中的液-气相转变。本文使用开放系综分子模拟来确定在不同气体逸度下，充满水的非极性限制环境中大气气体的饱和浓度与孔径的函数关系。对于宽度仅略超过受限水的液-气转变的机械不稳定性阈值（旋节线）（水膜厚度在三到四个分子直径之间）的类石蜡限制环境，发现孔中的平均气体浓度在氮气和二氧化碳的情况下分别比大量水中的值高出约30倍或约15倍。在环境条件下，这不会导致表面附近水密度分布出现明显变化。虽然已发现溶解气体的存在会降低毛细管蒸发的势垒（Leung, K.; Luzar, A.; and Bratko, D. Phys. Rev. Lett. 2003, 90, 065502），但要使毛细管蒸发动力学产生显著变化，所需的气体浓度要比正常大气条件下观察到的高得多。在模拟中，对于氮气，需要对应于逸度高于约40 bar的溶解气体浓度，对于二氧化碳，则需要高于约2 bar的溶解气体浓度，才能触发水从仅1.4 nm宽的烃狭缝中排出。对于对应于机械不稳定性阈值或更高的纳米级孔径，在相对的限制壁处的吸附层之间未观察到明显的耦合。这一发现解释了气体溶解度随反限制宽度近似线性增加以及亨利定律在很宽的逸度范围内在孔中的明显有效性。
