Fig. 5. 2D representation of the variation of the spectra prior to and following the injection of Allura red solution. A brighter color corresponds to a larger intensity. The time of injection at 400 s is indicated with an arrow. The integrated area of the 1040 cm− 1 band of Allura red was used for plotting the SNDX-275 transient in Fig. 6 and Fig. 7.Figure optionsDownload full-size imageDownload as PowerPoint slide
Fig. 6. Variation of the characteristic band intensity for Allura red after solute injection for a bare sensor (no film) showing the delay due to the unstirred layer.Figure optionsDownload full-size imageDownload as PowerPoint slide
Fig. 7. Variation of the intensity of the relevant IR band over time after injection for the 4 solutes diffusing across the PVA film and best fits to Eq. (8) with and without time delay.Figure optionsDownload full-size imageDownload as PowerPoint slide
Representative results showing evolution of the characteristic bands over time following solute injection are displayed in Fig. 7. They were fitted with Eq. (6) (zero gap) with and without extra time delay tdelay due to the unstirred layer, which ultimately yields D of the solutes within the film. Noteworthy, the maximal time of experiment was limited to about 2000 s because of the possible drift of the instrument's optical alignment. Therefore, experiments for most slowly diffusing solute were terminated before the intensity saturated and absorbance at saturation A∞ was made a fitting parameter. Nevertheless, this should not compromise the accuracy of the fitted diffusivity, as in the cases where the intensity saturated, the difference between the fitted A∞ and actual A∞ was insignificant.