Molecular Mechanisms of Solvent-Controlled Assembly of Phosphonate Monolayers on Oxide Surfaces

The explicit role of a series of solvents in octadecylphosphonic acid (ODPA) association and monolayer formation on the (0001) surface of α-aluminum oxide is explored from molecular dynamics simulations. For this purpose, molecule-by-molecule attachment and subsequent relaxation is studied in hexane, THF, and 2-propanol, respectively. From a purely structural viewpoint, the simulations show that the packing and ordering of the resulting SAMs closely resembles that of monolayers initially grown in vacuo, followed by immersion into the solvent afterward. In terms of the formation kinetics, we however find significant dissimilarities which result from solvent structuring at the interface and considerable hindering of surfactant association to the template surface at later stages of SAM growth. This leads to drastic deviation from diffusion-controlled kinetics and calls for a time-dependent picture of SAM formation mechanisms.