Effects of chain length and unsaturation of surface coatings on 5CB monolayers at the metal surfaces: a molecular dynamics study

Molecular dynamics simulations were used to examine the orientational, structural, and dynamic properties of 4-cyano-4′-pentylbiphenyl (5CB) monolayers on surface-functionalized Au and Ag substrates. The surfaces were coated with saturated and unsaturated molecules of different chain lengths to clarify how the coating structure influences the behavior of 5CB molecules. The results show that the orientational order of 5CB strongly depends on the surface coating. In saturated systems, increasing chain length decreases the second-rank order parameter, whereas in unsaturated systems, the order parameter does not show a systematic dependence on chain length. Tilt-angle and density-profile analyses indicate that 5CB molecules reorient toward a tilted, nearly planar alignment, with short saturated coatings promoting closer CN–COOH contact and stronger attachment of 5CB molecules to the surface molecules. Diffusion is dominated by lateral motion in the xy-plane, and the highest mobility is observed for the systems coated with a 15‑carbon saturated chain. Hydrogen-bond and interaction-energy analyses reveal that short saturated chains favor hydrogen bonding, whereas long saturated chains strengthen the total interaction through enhanced van der Waals contributions. These findings show that the interfacial behavior of 5CB monolayers can be effectively tuned by the chain length and unsaturation degree of the surface molecular coating.