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

Abstract

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(CH2)8CH3−HS(CH2)9CH3 and HS(CH2)8CH3−HS(CH2)10CH3 alkanethiol mixtures in solutions at 300 and 373 K. The canonical ensemble simulations were performed at 300 K for a fixed composition of HS(CH2)9CH3−HS(CH2)9+ n CH3 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.

Phase transition behavior of two-component viologen adsorption layers on a HOPG electrode surface

“…In this case, two distinct reductive desorption peaks are observed in the voltammogram. Similar phase separation due to the alkyl chain length difference has been demonstrated by a Monte-Carlo calculation by Shevade and coworkers [4]. As an intriguing case, 2-aminoethanethiol and 2-mercaptoethane sulfonic acid form a stable SAM of 1:1 composition on a Au electrode surface irrespective of the solution composition due to strong lateral electrostatic interaction, demonstrating an ideal nonideality [5].…”

Formation of viologen radical cation condensed phase through two-dimensional molecular organization process on an HOPG electrode surface in binary viologen solutions

“…The well-mixing into a unique binary phase has frequently been observed, for example, for the SAMs of two alkanethiol molecules with a minor difference of alkyl chain length on a Au electrode [21]. coworkers [34]. Importantly, it was found that the phase separated state observed in…”

Selective adsorption of alkyl thiols on gold in different geometries

“…The adsorption selectivity for NCs is almost identical to that in the pure solvent. This is quite different for Au(111), where the longest alkyl thiol is preferentially adsorbed [11]. This is due to differences in interaction strength between the surfactants themselves, see Fig.…”