Silica nanofibrous membranes for the separation of heterogeneous azeotropes

Nanofibrous materials produced through electrospinning are characterized by a high porosity, large specific surface area, and high pore interconnectivity and, therefore, show potential for, e.g., separation and filtration. The development of more inert nanofibers with higher thermal and chemical resistance extends the application field to high-end purification. Silica nanofibrous membranes produced by direct electrospinning of a sol–gel solution without a sacrificing carrier, starting from tetraethoxysilane, meet these challenging requirements. After electrospinning the membrane is highly hydrophobic. Storage under dry conditions preserves this property. Oppositely, a superhydrophilic membrane is obtained by storage under high humidity (month scale). This switch is caused by the reaction of ethoxy groups, present due to incomplete hydrolysis of the precursor, with moisture in the air, resulting in an increased amount of silanol groups. This transition can be accelerated to hour scale by applying a heat treatment, with the additional increase in cross-linking density for temperatures above 400 °C, enabling applications that make use of hydrophobic and hydrophilic membranes by tuning the functionalization. It is showcased that upon designing the water repellent or absorbing nature of the silica material, fast gravity-driven membrane separation of heterogeneous azeotropes can be achieved.