Kinetics of Self-Assembled Monolayer Growth Explored via Submonolayer Coverage of Incomplete Films

Abstract

We have studied the growth kinetics of self-assembled monolayers of octadecylphosphonic acid on mica by examining films removed from solution before completion. Atomic force microscope (AFM) images of the quenched films showed islands approximately 2 nm high indicating a growth mechanism consisting of nucleation, growth, coalescence, etc. of dense submonolayer islands. This was consistent with previous in situ AFM studies. Infrared spectroscopy data were consistent with well-ordered alkyl chains and indicated only one type of chain conformation, implying that the areas between islands were bare, not covered with loosely packed disordered molecules. The surface coverage of the submonolayer islands was extracted from AFM images as a function of immersion time for solution concentrations ranging from 0.02 to 2 mM. These data were compared to two common models for adsorption kineticsdiffusion-limited kinetics and adsorption-limited (Langmuir) kinetics. The previously reported quasi-Langmuir−Blodgett deposition process occurring during removal was also taken into account in this modeling. The data were completely inconsistent with the adsorption-limited kinetics and were in reasonable agreement with the functional form for diffusion-limited kinetics. However, the diffusion parameters extracted from the latter fits were much smaller than expected for molecular diffusion in solution and did not scale correctly with solution concentration. We conclude that the island growth is limited by a process that obeys diffusion-like kinetics but not by actual solution diffusion.