Effects of Fluorination on Self-Assembled Monolayer Formation from Alkanephosphonic Acids on Aluminum:  Kinetics and Structure

We have used infrared spectroscopy, ellipsometry, and contact angle measurements to study self-assembled monolayer (SAM) formation on aluminum native oxide from three alkanephosphonic acids:  CF₃(CF₂)₇(CH₂)₁₁PO₃H₂ (F₈H₁₁PA), and CH₃(CH₂)_nPO₃H₂ (n = 15 (H₁₆PA); n = 21 (H₂₂PA)). These compounds show significant differences in film structure and film formation kinetics. Strikingly, the methylene segment of the semifluorinated F₈H₁₁PA SAM never reaches an ordered state even at long assembly times. This contrasts with the ordered chains in equilibrium films from H₁₆PA and H₂₂PA. We attribute this behavior to steric effects of the fluorocarbon segment and the phosphonic acid headgroup. F₈H₁₁PA represents an amphiphile in which bulky head and tail groups prevent an interposed hydrocarbon segment from ordering. For all three phosphonic acids, negative peaks attributed to loss of Al−OH groups in the infrared spectra of the monolayers are consistent with a condensation reaction between the acids and surface hydroxyls to form bound aluminophosphonate salts. With respect to kinetics, our results indicate that F₈H₁₁PA approaches its equilibrium film structure considerably faster than the hydrocarbon phosphonic acids. We interpret the structural dependence of film formation kinetics in terms of the T_c formalism advanced by Rondelez and co-workers (Langmuir 1994, 10, 4367−4373). We also suggest that the accelerated film formation exhibited by F₈H₁₁PA may be due to chain entanglement and solubility effects, to the extent that this species may self-assemble as islands of approximately vertically oriented chains which fill in as coverage increases. H₂₂PA may also deposit as islands, but in contrast, film formation for H₁₆PA probably involves initially disordered chains with higher tilt angles that order and reorient as film assembly proceeds.