Interfacial Free Energy of Nucleation for Iron Oxyhydroxide on Mixed Self-Assembled Monolayers

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Abstract

The deposition kinetics of iron oxyhydroxide were studied on self-assembled monolayers with variable density of terminating sulfonate sites. The thin films of FeOOH were formed on the substrates by thermal hydrolysis of millimolar aqueous solutions of Fe(NO3)3 at a pH of approximately 2.0. The terminal site density of sulfonate groups was controlled by selectively sulfonating the vinyl groups of self-assembled monolayers (SAMs) formed from various ratios of vinyl- and methyl-terminated monomers. The percentage composition of the sulfonate sites matched the percentage vinyl composition in the SAM deposition solution. By XPS analysis, iron was shown to rapidly and irreversibly physisorb to the surface in direct proportion to the percentage of sulfonate, with one iron for every two sulfonate groups. The induction time for FeOOH film growth was measured as a function of sulfonate site density. The induction times and, hence, the interfacial free energy for nucleation (IFEN) data suggest two regimes exist for film growth. From 0% to 35% sulfonate density, the IFEN decreased with increasing sulfonate density, while from 35% to 100% the IFEN was independent of sulfonate site density. In contrast the surface composition and its ability to physisorb iron are linearly related to the intended composition. While the sulfonate sites are required for initial physisorption of iron, a simple relationship between sulfonate surface site density and the IFEN does not exist.

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