Thermal Stability Study of Self-Assembled Monolayers on Mica

In this Letter, we report on the use of atomic force microscopy to study the thermal stability of self-assembled monolayers (SAM) of octadecylphosphonic acid (OPA) grown on mica. The samples were sequentially annealed for 30 min steps at temperatures ranging from 50 to 200 °C. A major change in the SAM morphology takes place at temperatures ∼95 °C, with the disappearance of the original flat-topped and partial-coverage SAM morphology. Concomitantly, the nucleation and growth of OPA precipitates in the investigated temperature range are observed. Two separate regimes are identified: initially, for temperatures up to ∼110 °C, nucleation of OPA precipitates occurs, increasing their number density. Then, for temperatures above ∼10 °C, growth of OPA precipitate size dominates, with the density of precipitates remaining approximately constant. A simple model used to fit the experimental data of both regimes enables an estimate of ∼9 kcal/mol as the energy required for the disorganization (Ed) of OPA molecules on mica.