INFRARED ATTENUATED TOTAL-REFLECTION SPECTROSCOPIC INVESTIGATIONS OF THE DIFFUSION BEHAVIOR 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 consi sted of polymer-coated attenuated total reflection (ATR) elements coat ed with different polymers such as low-density polyethylene polyisobut ylene and ethylene/propylene copolymer and was based on the diffusion behaviour of the CHC in different polymer layers. The purpose of the p olymer coating was to enrich chlorinated hydrocarbons and to exclude w ater from the information depth of the penetrating infrared (IR) radia tion. The coated ATR crystals were mounted in a flow-through cell and placed inside the sample chamber of the Fourier transform infrared spe ctrometer equipped with a mercury-cadmium-telluride detector. Measurem ents were performed by filling the cell with aqueous solutions of CHC and recording a series of spectra with constant time offset. The aim o f this study was to investigate the diffusion behaviour and to determi ne the diffusion coefficient of three CHC (monochlorobenzene, tetrachl oroethylene and chloroform) in polymer matrices in order to optimize t he sensor response. For that reason an already existing theoretical mo del was modified. Using this new model numerical simulations of the di ffusion processes occurring in the aqueous and polymer phase with resp ect to different partition coefficients, different diffusion coefficie nts and different thicknesses were performed. A classical Fickian resp onse was found to be the dominant diffusion behaviour observed for the diffusion of chlorinated hydrocarbons into the polymers. This work sh owed the importance to understand the diffusion processes in different polymer materials for optimizing a fiber optic sensing system for CHC s in water.