Monolayers and Corrosion

SFG can provide considerable information regarding the buried interface that is of central importance to corrosion inhibition processes. A model system in this respect is the monolayer of benzotriazole (BTA) that forms beneath a thick multilayer of the same molecule on Cu. Two SFG studies have examined this system thus far [125, 126]. In the study by Schultz et al., SFG showed that BTA forms a relatively well-ordered monolayer on Cu(lOO) between -0.7 and -tO.2 V, while on Cu(lll) this order is only present at high potential. Titration with Cl showed that the monolayer was destabilized at lower Cl concentrations than those needed to destabilize the polymeric and somewhat more inaccessible multilayer. Work performed by Romero et al. using 5-methylbenzotriazole on Cu(poly) show that the 5-methylbenzotriazole is stable on the surface with no orientation changes with potential [125]. Similarly to the system studied by Schultz, the degree of preferential ordering of BTA on Cu(lll) seems to be less than that on the Cu(poly) surface. 

The results presented above show the huge potential of SFG for obtaining information from systems in the electrochemical environment To date, only a few systems have been examined, with the usual suspects - CO and other small mole- 

These advantages relating to SFG come with a corresponding cost regarding (a) instrumental complexity and (b) complications due to data analysis. For the former of these cost areas, the appearance of commercially available laser systems is a welcome improvement For the latter, future attention will have to be directed at developing methodology to cleanly separate resonant and non-reso-nant components. The use of DFG and/or tunable visible sources may prove quite helpful in this regard. A detailed theoretical framework for SFG in the electrochemical environment will have to describe relative phase factors of different resonances with each other and will be necessary to obtain quantitative information from the measurement. 

Fifteen years ago, STM and AFM were poised to move into the realm of electrochemistry. Their introduction - leveraged by the wide availability of instrumentation - certainly enlivened the field, such that it is difficult to find an electrochemical program today not influenced by these microscopies. While the vibrational spectroscopic information available from SFG is perhaps somewhat less unique than the structural information available from STM and AFM, this technique is ready today to continue to make important contributions to the study of the electrified soHd/Hquid interface. 

2 Guyot-Sionnest, P. Tadjeddine, A. Chem. Phys. Lett. 1990, 172, 341. 

---