Pártay Lívia, Jedlovszky Pál, Vincze Arpád, Horvai George
Department of Colloid Chemistry, Eötvös Lorand University, Pazmany Péter stny. 1/a, H-1117 Budapest, Hungary.
J Phys Chem B. 2005 Nov 3;109(43):20493-503. doi: 10.1021/jp0534885.
Monte Carlo simulation of the vapor-liquid interface of water-methanol mixtures of different compositions, ranging from pure water to pure methanol, have been performed on the canonical (N, V, T) ensemble at 298 K. The analysis of the systems simulated has revealed that the interface is characterized by a double layer structure: methanol is strongly adsorbed at the vapor side of the interface, whereas this adsorption layer is followed at its liquid side by a depletion layer of methanol of lower concentration than in the bulk liquid phase of the system. The dominant feature of the interface has been found to be the adsorption layer in systems of methanol mole fractions below 0.2, and the depletion layer in systems of methanol mole fractions between 0.25 and 0.5. The orientation of the molecules located at the depletion layer is found to be already uncorrelated with the interface, whereas the methanol molecules of the adsorption layer prefer to align perpendicular to the interface, pointing straight toward the vapor phase by their methyl group. Although both the preference of the molecular plane for a perpendicular alignment with the interface and the preference of the methyl group for pointing straight to the vapor phase are found to be rather weak, the preference of the methyl group for pointing as straight toward the vapor phase as possible within the constraint imposed by the orientation of the molecular plane is found to be fairly strong. One of the two preferred orientations of the interfacial water molecules present in the neat system is found to disappear in the presence of methanol, because methanol molecules aligned in their preferred orientation can replace these water molecules in the hydrogen-bonding pattern of the interface.
在298K的正则（N，V，T）系综下，对从纯水到纯甲醇的不同组成的水 - 甲醇混合物的气液界面进行了蒙特卡罗模拟。对模拟系统的分析表明，该界面具有双层结构特征：甲醇强烈吸附在界面的气相一侧，而在其液相一侧接着是甲醇浓度低于系统本体液相的耗尽层。已发现，在甲醇摩尔分数低于0.2的系统中，界面的主要特征是吸附层；在甲醇摩尔分数介于0.25和0.5之间的系统中，主要特征是耗尽层。位于耗尽层的分子的取向已被发现与界面不再相关，而吸附层的甲醇分子更倾向于垂直于界面排列，通过其甲基直接指向气相。虽然发现分子平面与界面垂直排列的偏好以及甲基指向气相的偏好都相当弱，但在分子平面取向所施加的约束范围内，甲基尽可能直接指向气相的偏好被发现相当强。在纯系统中存在的界面水分子的两个优选取向之一，在甲醇存在时会消失，因为以其优选取向排列的甲醇分子可以在界面的氢键模式中取代这些水分子。
