Planar Array Infrared Spectroscopic Imaging of Monolayer Films in the 3200−2800 cm-1 Region

Abstract

The ability of a planar array infrared (PA-IR) spectrograph to obtain spatially resolved infrared spectra (infrared line images) of a monolayer film with reasonable signal-to-noise-ratios is demonstrated, and the first infrared line images of a monolayer film ever recorded are reported. In this context, octadecyltrichlorosilane (OTS) monolayer films self-assembled on glass substrates were investigated as a function of three different solvents (hexane, benzene, and toluene). It is shown that the PA-IR line images could be used to investigate surface coverage and conformational order as a function of spatial location. A frequency versus absorbance relationship was observed that can be interpreted as a well-defined correlation between conformational order and surface coverage. The plot of this relationship, which looks similar to a phase diagram containing a first-order phase transition, is in excellent agreement with that reported recently (Liu, Y.; Wolf, L. K.; Messmer, M. C. Langmuir 2001, 17, 4329) by sum frequency generation (SFG). On the basis of the results from this study and the SFG results, it appears that conformational order and surface coverage of self-assembled OTS monolayer films on glass can be treated as two dependent thermodynamic state functions. In addition, these results suggest that different solvent systems can be exploited to tailor self-assembled monolayer films in terms of surface coverage and conformational order.