Simulation of Mixed Self-Assembled Monolayers on Gold: Effect of Terminal Alkyl Anchor Chain and Monolayer Composition.

Self-assembling monolayers provide a reproducible synthetic microenvironment for tethering lipid bilayers to incorporate proteins and lay the ground for numerous applications in nanotechnology and biomedical engineering. Although the structure of single-component monolayers is well investigated, there is far less insight into the molecular behavior at the interface of mixed monolayers at different mole fractions. Here, we present and apply a novel procedure to simulate and analyze multicomponent self-assemblies of alkanethiols over a wide range of mole concentrations of anchoring compounds. In particular, the structural features of monolayers consisting of a matrix compound and either a short (C8) or a long (C16) anchor compound on Au(111)-like surfaces were investigated first using coarse-grained and subsequently full-atomistic molecular dynamics simulations. Different scenarios of spatial distributions (random vs clustering) of anchoring molecules on flat surfaces were probed. The results of the simulations are in excellent agreement with the experimental data from ellipsometry and infrared reflection absorption spectroscopy. For short anchoring molecules, a random spatial distribution in the matrix is obtained. At low, experimentally relevant anchor compound mole fractions < 0.1, only for long-chain (C16)-terminal alkyls, phase segregation and self-association of the anchoring molecules can be observed, which are also seen in experiment.