Domain Formation and System-Size Dependence in Simulations of Self-Assembled Monolayers

Monte Carlo studies have been made of the role of sample-size effects in the simulation of monolayer systems consisting of long-chain hydrocarbons. The potential parameters were chosen to model the self-assembled monolayers formed by chemisorption of CH₃(CH₂)₁₅SH admolecules on a gold substrate, and results were obtained for three system sizes, N = 30, 90, and 224. Investigation of the translational, orientational, and conformational order within the monolayers reveals a marked dependence on N. In particular, the 224-molecule simulation is characterized by the appearance of well-differentiated domains that are absent in the smaller systems. It therefore appears that the system sizes used in many monolayer simulations (N ≤ 100) are too small to capture certain important structural features of the system of interest. Comparison is made with experimental results and with related molecular-dynamics calculations.