Mechanisms of self-assembled monolayer desorption determined using in situ atomic force microscopy

We have observed the dissolution of a self-assembled monolayer of octadecylphosphonic acid from a mica surface into organic solvent in real time using atomic force microscopy. Holes in the monolayer are observed to nucleate, grow, and percolate across the sample, leaving isolated monolayer islands that gradually decrease in size. The relative rates of hole growth and hole nucleation suggest that removing a molecule from the monolayer/hole boundary is about 5 x 10(4) times more likely than removing a molecule from within a continuous region of monolayer. The rate of dissolution is increased by flowing solvent through the cell compared to stagnant solvent and increased even further by rapid stirring using the AFM tip. The coverage kinetics can be quantitatively described by a model that incorporates desorption from "hole" regions and diffusive solution-phase transport through a stagnant layer of finite thickness. The coverage kinetics and hole nucleation and growth rates are consistent with a picture of monolayer desorption where molecules detach from the hole/monolayer boundaries and remain adsorbed (at a very dilute density) in the "hole" regions, from which they eventually desorb into the solvent. This is essentially the reverse process as was previously observed for monolayer growth.