DEVELOPMENT OF SORET FORCED RAYLEIGH SCATTERING METHOD TO MEASURE MASS DIFFUSION COEFFICIENT OF METHANOL AQUEOUS SOLUTIONS IN POLYMER ELECTROLYTE MEMBRANE - REDUCTION OF SCATTERING LIGHT BY HETERODYNE DETECTION USING OPTICAL PHASE MODULATOR -

摘要

In the present study, the Soret forced Rayleigh scattering (SFRS) apparatus has been developed to measure the mass diffusion coefficient of methanol aqueous solutions in polymer electrolyte membrane (PEM). In this method, the concentration gradient in solutions can be created directly by interference of two heating laser beams. Hence, it is expected that the mass diffusion coefficient of the solutions in PEM can be measured without the influence of the interface between the bulk solutions and solutions in PEM. Also, the diffusion length is in micrometer order, it enables to high-speed (～ 100 ms) measurement. In order to apply SFRS to measure the mass diffusion in PEM, one of the most critical problems to be overcome is the influence of the coherent scattered light from the PEM. The unwanted scattering light interferes with the diffracted light signal; therefore, the decay curve of signal contains two time constants. This influence may lead us to miscalculate the diffusion coefficient. In the case when the diffracted light intensity is smaller than the scattering light, the heterodyne detection using reference light have an advantage over homodyne detection. Therefore, we have reconstructed the SFRS apparatus using the heterodyne detection. For evaluation of the new apparatus, the mass diffusion coefficient of 80 wt% methanol aqueous solutions at room temperature is measured. The deviation of the measured value by heterodyne detection from the literature data determined by the conventional measurement technique is+9 %, whereas the deviation of the measured value by previous apparatus with the homodyne detection from the literature data was+42%.