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)(11)-PO3H2(F(8)H(11)PA), and CH3(CH2)(n)PO3H2 (n = 15 (H(16)PA); n = 21 (H(22)PA)). These compounds show significant differences in film structure and film formation kinetics. Strikingly, the methylene segment of the semifluorinated F(8)H(11)PA SAM never reaches an ordered state even at long assembly times. This contrasts with the ordered chains in equilibrium films from H(16)PA and H(22)PA. We attribute this behavior to steric effects of the fluorocarbon segment and the phosphonic acid headgroup. F(8)H(11)PA 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 F(8)H(11)PA 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 F(8)H(11)PA 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. H(22)PA may also deposit as islands, but in contrast, film formation for H(16)PA probably involves initially disordered chains with higher tilt angles that order and reorient as film assembly proceeds.