Asymmetric Hydrogen Bonding and Orientational Ordering of Water at Hydrophobic and Hydrophilic Surfaces: A Comparison of Water/Vapor, Water/Talc, and Water/Mica Interfaces

Interfaces involving aqueous fluid phases play critical roles in natural and technologically important systems, and the atomic scale differences between interfaces involving hydrophobic and hydrophilic substrates are essential to understanding and manipulating their chemical and physical properties. This paper compares computational molecular dynamics results for the atomic density profiles, H-bonding configurations, and orientational ordering of water molecules at three different and illustrative interfaces. These are the free liquid water surface, which can be considered hydrophobic, and the interfaces of liquid water with talc (001) and muscovite (001) surfaces, which are prototypical hydrophobic and hydrophilic inorganic oxide surfaces, respectively. The results clearly demonstrate the importance of substrate structure and composition in controlling interfacial behavior and illustrate the differences between the vapor interface and those involving solids. The atomic density profiles of water at the solid interfaces show substantial layering, with the details related to the composition and crystal structure of the substrate. In contrast, there is no significant layering at the water−vapor interface. Relative to bulk water, the average density of water at the talc (001) surface is reduced about 9−15% within 6−10 Å from the interface. This is equivalent to a depletion layer about 0.8 Å thick with respect to the similar but hydrophilic mica (001) surface. There is no well-defined vaporlike volume for the talc interface however, and the reduced number of water molecules is spread across the interfacial region. For the free liquid water surface, the results show an asymmetric H-bonding environment and charge density oscillations that provide an additional explanation for the previously observed separation of anions and cations at the surfaces of aqueous solutions. Thus, a delicate imbalance between the accepted and donated H-bonds of interfacial water molecules at this surface, and by inference other hydrophobic and hydrophilic surfaces, determines the preference of charged ions for the interface.