Effect of the Packing Density on the Surface Hydrophobicity of ω-Functionalized (-CF₃, -CH₃, -OCH₃, and -OH) Self-Assembled Monolayers: A Molecular Dynamics Study

The surface properties of self-assembled monolayers (SAMs) are generally determined by the terminal functional groups. However, variations in the chain flexibility or packing density of SAMs significantly affect the surface properties as well. In this study, we investigated the effect of the packing density on the surface hydrophobicity/hydrophilicity of ω-functionalized (-CF3, -CH3, -OCH3, and -OH) SAMs using molecular dynamics simulations. The surface roughness and chain flexibility of these SAMs commonly increase with a decreasing packing density. The increase in the chain flexibility caused an enhancement in the surface hydrophobicity, regardless of the terminal groups. For SAMs with the CF3 terminal, only the fluorocarbon segments were exposed to the surface at any packing density; thus, the surface roughness and chain flexibility were the only parameters that affected the surface hydrophobicity. Conversely, the surface (exposed) segments of the other SAMs were alternating depending on the packing density, altering the surface-water interfacial energies, which also contributed to a variation in the surface hydrophobicities. Therefore, the variation in the surface hydrophobicity of the SAMs due to the packing density was well characterized by considering the chain flexibility and exposed surface segments.