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In recent years the improvement in Fourier transform infrared instrumentation has led to a considerable growth in the use of infrared microscopy as an analytical tool. Many of these applications have concentrated on the spatial resolving power of the microscope for the analysis of small samples or localised contamination within a material. Recently we have been looking at some broader applications of the microscope, in particular, with respect to the non-destructive analysis of bulk polymeric materials and to its use for reflection/absorption measurements of monomolecular films on aqueous substrates. When radiation impinges on a surface the reflected light is generally considered to consist of two components. The first is termed diffuse reflectance and the second Fresnel or specular reflectance. In the case of diffuse reflectance the reflectivity is given by f(R∞) = (1-R∞)2/R∞=k/s, where R∞ is the reflectance from an infinitely thick layer and k and s are related to the absorption and scattering within the sample. Here the term k, related to the absorption, contains molecular information about the material in question. When specular reflectance dominates, the recorded spectrum bears no resemblance to an absorption spectrum but has the appearance of a first derivative spectrum. The reflectivity at normal incidence is given by R = (m-1)2/(m+1)2, where m is the complex refractive, index m = n - ik, and where n is the real component, k the imaginary component or the extinction coefficient and i is √-1 Hence the recorded reflectivity spectrum contains molecular information about the material under investigation. This molecular information can be extracted using the Kramers-Kronig transformation. The experimental system used for this study consisted of a Perkin-Elmer model 1760-X Fourier transform infrared spectrometer. A Spectra-Tech IR Plan microscope, equipped with a narrow band MCT detector and a x15 objective, was fitted to the external port. Data storage and manipulation was carried out using a 7700 series data station. For our liquid monolayer studies the samples were spread on the water surface (minimum resistivity 15 MRcm-1) from heptane solutions using a microsyringe.
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