The determination of diffusion coefficients in semisolids by Fourier transform infrared (FT-IR) spectroscopy

A Fourier transform infrared (FT-IR) spectrometer with a horizontal attenuated total reflectance (ATR) cell was used to determine the diffusion coefficients of several liquids in two semisolid materials. The experimental setup was that of a system with one open and one closed boundary wherein the open boundary was maintained at constant concentration. While the liquid of interest was diffusing through the film of ointment, the concentration of liquid at the film surface in contact with the ATR crystal was determined at various times by means of IR absorption measurements. The depth of penetration of the IR radiation into the sample was approximately 0.6-0.9 microns at the wavelengths of analysis. Since the ointment thickness was 157 microns, it was reasonable to assume that only the penetrant reaching the lower boundary was being measured. The values of the diffusion coefficients were then calculated using an equation that appropriately modeled the aforementioned conditions. The liquids tested exhibited diffusion coefficients in anhydrous lanolin and in polyethylene glycol ointment that ranged from 0.56 to 7.2 x 10(-7) and 0.68 to 5.7 x 10(-7) cm2/sec, respectively. The expected molecular weight dependency was observed.