Effect of Surface Hydrophilicity on the Interfacial Properties of a Model Octane-Water-Silica System

We use molecular simulation to study the impact of substrate hydrophilicity on the wetting properties of water at a hydroxylated β-cristobalite (111) surface in a mother n-octane liquid. We employ the silica model introduced by Lee and Rossky [Lee, S. H.; Rossky, P. J. J. Chem. Phys. 1994, 100, 3334-3345]. The hydrophilicity of the substrate is tuned by scaling the magnitude of partial charges placed on atoms within the first layer of the silica substrate (modification of the substrate polarity). An interface potential approach is used to compute the contact angle of a water droplet at the substrate over a wide range of hydrophilicities at temperatures of 300 and 400 K. Our results illustrate an anomalous trend in the contact angle of water. For relatively hydrophobic surfaces, the system behaves as is expected, with increases in substrate polarity resulting in a decrease in the contact angle. However, at a sufficiently high substrate polarity, the value of the contact angle reaches a minimum, with further increases in polarity leading to an increase in the contact angle. The amount of water adsorbed at the interface changes abruptly in the vicinity of the extremum point. From a macroscopic perspective, a monolayer-thick precursor film surrounds the droplet at sufficiently high substrate polarity. We link the evolution of these macroscopic properties to the manner in which water organizes at the β-cristobalite (111) surface. The structure is characterized via analysis of configurational snapshots, density profiles, in-plane radial distribution functions, and molecular orientation probability distributions. The analysis reveals that water molecules reside within the hollows of the β-cristobalite (111) surface at sufficiently high substrate polarity. As the polarity increases, the interfacial water molecules progressively adopt a more planar structure and limit the number of hydrogen bonds that they form with water in subsequent interfacial layers. This evolution of the interfacial water structure leads to an effective weakening of the water-substrate interaction and corresponding increase in the water contact angle.