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:  CF<sub>3</sub>(CF<sub>2</sub>)<sub>7</sub>(CH<sub>2</sub>)<sub>11</sub>PO<sub>3</sub>H<sub>2</sub> (F<sub>8</sub>H<sub>11</sub>PA), and CH<sub>3</sub>(CH<sub>2</sub>)<i><sub>n</sub></i>PO<sub>3</sub>H<sub>2</sub> (<i>n</i> = 15 (H<sub>16</sub>PA); <i>n</i> = 21 (H<sub>22</sub>PA)). These compounds show significant differences in film structure and film formation kinetics. Strikingly, the methylene segment of the semifluorinated F<sub>8</sub>H<sub>11</sub>PA SAM never reaches an ordered state even at long assembly times. This contrasts with the ordered chains in equilibrium films from H<sub>16</sub>PA and H<sub>22</sub>PA. We attribute this behavior to steric effects of the fluorocarbon segment and the phosphonic acid headgroup. F<sub>8</sub>H<sub>11</sub>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<sub>8</sub>H<sub>11</sub>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 <i>T</i><sub>c</sub> formalism advanced by Rondelez and co-workers (<i>Langmuir</i> <b>1994</b>, <i>10</i>, 4367−4373). We also suggest that the accelerated film formation exhibited by F<sub>8</sub>H<sub>11</sub>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<sub>22</sub>PA may also deposit as islands, but in contrast, film formation for H<sub>16</sub>PA probably involves initially disordered chains with higher tilt angles that order and reorient as film assembly proceeds.