Phase Behavior of Mixed Self-Assembled Monolayers of Alkanethiols on Au(111):  A Configurational-Bias Monte Carlo Simulation Study

We report a configurational-bias Monte Carlo simulation study to investigate the effect of chain length, system size, and temperature on the preferential adsorption and phase behavior of mixed self-assembled monolayers (SAMs) of alkanethiols on Au(111). Simulations were performed in both semigrand canonical and canonical ensembles. The semigrand canonical simulations were carried out at conditions corresponding to low concentration of HS(CH 2 ) 8 CH 3 −HS(CH 2 ) 9 CH 3 and HS(CH 2 ) 8 CH 3 −HS(CH 2 ) 10 CH 3 alkanethiol mixtures in solutions at 300 and 373 K. The canonical ensemble simulations were performed at 300 K for a fixed composition of HS(CH 2 ) 9 CH 3 −HS(CH 2 ) 9+ n CH 3 mixed SAMs, where n = 1, 2, ..., 10. Continuous and discrete models were used in these simulations for describing the alkanethiol chains, and an analytical potential was used to model their interactions with the gold surface. Results show the preferential adsorption of long chains on the surface over in the solution and the occurrence of phase segregation when the two components in the mixed SAMs differ by more than three carbon atoms at a fixed surface composition (1:1). Calculated translational, orientational, and conformational orders of the mixed SAMs reveal a more pseudocrystalline structure consisting of closely packed and inclined chains at 300 K than at 373 K. The effect of system size on various properties was also studied.