Infrared attenuated total reflection spectroscopic investigations of the diffusion behaviour of chlorinated hydrocarbons into polymer membranes

The response behaviour of a sensor for chlorinated hydrocarbons (CHC) in water was evaluated practically and theoretically. The sensor consisted of polymer-coated attenuated total reflection (ATR) elements coated with different polymers such as low-density polyethylene polyisobutylene and ethylene/propylene copolymer and was based on the diffusion behaviour of the CHC in different polymer layers. The purpose of the polymer coating was to enrich chlorinated hydrocarbons and to exclude water from the information depth of the penetrating infrared (IR) radiation. The coated ATR crystals were mounted in a flow-through cell and placed inside the sample chamber of the Fourier transform infrared spectrometer equipped with a mercury-cadmium-telluride detector. Measurements were performed by filling the cell with aqueous solutions of CHC and recording a series of spectra with constant time offset. The aim of this study was to investigate the diffusion behaviour and to determine the diffusion coefficient of three CHC (monochlorobenzene, tetrachloroethylene and chloroform) in polymer matrices in order to optimize the sensor response. For that reason an already existing theoretical model was modified. Using this new model numerical simulations of the diffusion processes occurring in the aqueous and polymer phase with respect to different partition coefficients, different diffusion coefficients and different thicknesses were performed. A classical Fickian response was found to be the dominant diffusion behaviour observed for the diffusion of chlorinated hydrocarbons into the polymers. This work showed the importance to understand the diffusion processes in different polymer materials for optimizing a fiber optic sensing system for CHCs in water.