Wetting Behavior of Water near Nonpolar Surfaces

We use molecular simulation to study the wetting behavior of water near flat nonpolar surfaces. The interface potential approach is used to capture the evolution of various interfacial properties over a broad range of temperature and substrate strength. Three model substrates are considered: an atomistically detailed face-centered cubic (FCC) lattice, a graphite lattice, and a structureless wall. We first examine the evolution of the contact angle with substrate strength for conditions ranging from near drying to complete wetting at select temperatures. A notable characteristic of all systems is the presence of a surface strength at which the contact angle is independent of temperature. We also identify an analogous point in which the quantity γlv cos θ is temperature invariant. We discuss the relationship between this point and the excess entropy of the solid–liquid interface. We next consider the temperature dependence of interfacial properties. We find the contact angle to decrease with temperature at relatively strong surfaces and increase with the temperature at relatively weak surfaces. The work of adhesion is found to be a useful quantity for describing the interfacial properties of water. The enthalpic and the entropic contributions to the work of adhesion are obtained from its temperature dependence. These properties are found to be directly related to the affinity of water for a solid surface and therefore serve as useful measures of the hydrophobicity of a surface. Finally, we examine the effect of surface strength and temperature on the density depletion associated with water at hydrophobic surfaces. We study various metrics that quantify the density and compressibility of water in the vicinity of hydrophobic surfaces. Comparisons are drawn between the behavior of water and simple nonpolar fluids at solvophobic surfaces.