Synthesis of Calcium Fluoride Ultrafine Particles for the Preparation of Integral Asymmetric Cellulose Acetate/Calcium Fluoride Membranes

The present work reports on the synthesis of cellulose acetate (CA) asymmetric membranes with the incorporation of inorganic fluorides, CaF2 particles. These fillers of polymeric composites can, according to the literature, promote the ordering of the polymer matrix, which can lead to interesting permeation properties. In order to achieve that, fluorite (CaF2 ) fine particles were prepared by a double jet injection method using CaCl2 and NaF solutions as precursors and polyvinylpyrrolidone as a stabilizing agent. The fluorite particles were incorporated in the casting solutions of cellulose acetate/acetone/formamide. Dynamic light scattering measurements showed that formamide acting as the poor solvent in the membrane casting solutions can lead to some agglomeration of the fluorite particles and evident changes in structure and permeation properties. The CA/CaF2 composite membranes were casted by the phase inversion method at different solvent evaporation times. The permeation experiments indicated that the active layer of the membranes prepared with shorter solvent evaporation times is composed of an organic-inorganic network with bigger pores, which allows the permeation of molecules approximately two times the size of those retained by the CA membranes. Further increase in the solvent evaporation time has the opposite effect, as particle agglomerates may be formed.