Fabrication and Optical Modeling of Micro‐Porous Membranes Index‐Matched with Water for On‐Line Sensing Applications

Membranes made of perfluorinated materials having a refractive index close to that of water enable straightforward detection of water solutes at the interface, hence providing a novel sensing tool for fluidic systems. Here it is reported the production by non-solvent induced phase separation and characterization of microporous membranes made of Hyflon AD, an amorphous copolymer of tetrafluoroethylene. Their optical turbidity when soaked in liquids having different refractive indices is studied and the data are interpreted with an optical model linking scattered light and structural features of the membrane. The average polymer and pore chord lengths obtained in this way scale with the filtering pore size measured by capil-lary flow porometry. Then, the optical response of the membranes when soaked in aqueous surfactant solutions is investigated and the effects of the molecular adsorption on the internal membrane surface is success-fully interpreted by an extension of the model. Overall, the results show that index-matched membranes can be designed and produced to provide a simple optical detection of the composition of the soaking liquid or to monitor the initial stage of membrane fouling.