Effect of surface polarity on water contact angle and interfacial hydration structure

We perform molecular dynamics simulations of water in the presence of hydrophobic/hydrophilic walls at T = 300 K and P = 0 GPa. For the hydrophilic walls, we use a hydroxylated silica model introduced in previous simulations [Lee, S. H.; Rossky, P. J. J. Chem. Phys. 1994, 100, 3334. Giovambattista, N.; Rossky, P. J.; Debenedetti, P. G.; Phys. Rev. E 2006, 73, 041604.]. By rescaling the physical partial atomic charges by a parameter 0 <= k <= 1, we can continuously transform the hydrophilic walls (hydroxylated silica, k = 1) into hydrophobic apolar surfaces (k = 0). From a physical point of view, k is the normalized magnitude of a surface dipole moment, and thus it quantifies the polarity of the surface. We calculate the contact angle of water for 0 <= k <= 1. We find that, at least for the present homogeneous, atomically flat, and defect-free surface model, the magnitude of the surface dipole correlates with the contact angle in a one-to-one correspondence. In particular, we find that polar surfaces with 0 < k <= k(c) = 0.4 are macroscopically hydrophobic; that is, the contact angle is larger than 90 degrees. For the cutoff value k = k(c), the magnitude of the dipole moment of the polar silica surface unit is 41% that of the water molecule dipole moment. We also study the water orientation distributions next to the walls (a microscopic property). We find that these distributions also correlate with the contact angle in a one-to-one correspondence. Thus, the structure of confined water, the surface polarity, and the contact angle are in a direct correspondence to each other, and therefore, each quantifies the hydrophobicity/hydrophilicity of the surface.