Infrared spectroscopy and spectroscopic imaging of n-propyl trichlorosilane monolayer films self-assembled on glass substrates

Abstract

Monolayer films of n-propyl trichlorosilane (PTS) self-assembled on glass substrates were investigated using Fourier transform infrared spectroscopy (FTIR) and planar array infrared spectroscopic imaging (PA-IR spectroscopic imaging) as a function of three different solvents (hexane, benzene and toluene). The alkyl chain of a PTS monolayer has only two conformational isomers, g auche and trans. In this study, we show that the two isomers give rise to significantly different transmission IR spectra in the aliphatic stretching region for both 0° and 15° angles of incidence. The gauche spectrum is dominated by ν(CH 3) contributions, while the trans spectrum has large ν(CH 2) contributions. These observations can be explained in terms of the orientation of the ν a(CH 3) and ν s(CH 3) and the ν a(CH 2) and ν s(CH 2) transition dipole moments. The distinct difference between a g auche and trans spectrum can serve as a probe for conformational order in PTS monolayers. In general, relatively low total integrated absorbance values were associated with disordered films, while relatively high values were associated with ordered films. A plot of the 2925 cm −1/2960 cm −1 peak intensity ratios versus total integrated absorbance suggests the existence of an ordered and disordered phase above and below ∼58% surface coverage, respectively. Under the conditions of this study, the most ordered PTS monolayers films were prepared with hexane while less ordered monolayers were produced using benzene. Only disordered films were produced with toluene. When compared with a previous study (Langmuir (in press)), the relative ability of these three solvents to produce an ordered PTS or n-octadecyl trichlorosilane (OTS) monolayer film follow a similar trend. We also report the first broad band infrared line images of a short chain monolayer film, which were obtained with a PA-IR spectrograph. The line images of PTS monolayers revealed both relatively homogeneous and heterogeneous regions in terms of conformational order.