INTERFACIAL FREE-ENERGY OF NUCLEATION FOR IRON OXYHYDROXIDE ON MIXED SELF-ASSEMBLED MONOLAYERS

The deposition kinetics of iron oxyhydroxide were studied on self-asse mbled monolayers with variable density of terminating sulfonate sites. The thin films of FeOOH were formed on the substrates by thermal hydr olysis of millimolar aqueous solutions of Fe(NO3)(3) at a pH of approx imately 2.0. The terminal site density of sulfonate groups was control led by selectively sulfonating the vinyl groups of self-assembled mono layers (SAMs) formed from various ratios of vinyl- and methyl-terminat ed monomers, The percentage composition of the sulfonate sites matched the percentage vinyl composition in the SAM deposition solution. By X PS analysis, iron was shown to rapidly and irreversibly physisorb to t he surface in direct proportion to the percentage of sulfonate, with o ne iron for every two sulfonate groups. The induction time For FeOOH f ilm growth was measured as a function of sulfonate site density. The i nduction times and, hence, the interfacial Gee energy for nucleation ( IFEN) data suggest two regimes exist for film growth. From 0% to 35% s ulfonate density, the IFEN decreased with increasing sulfonate density , while from 35% to 100% the IFEN was independent of sulfonate site de nsity. In contrast the surface composition and its ability to physisor b iron are linearly related to the intended composition. While the sul fonate sites are required for initial physisorption of iron, a simple relationship between sulfonate surface site density and the IFEN does not exist.