Using Nanoscopic Hosts, Magnetic Guests, and Field Alignment to Create Anisotropic Composite Gels and Aerogels

Abstract

Anisotropic composite architectures of vastly different length scales are organized upon nanogluing magnetic microparticulate guests (ca. 10−15 μm iron powder) into a guest−host network by gelation of silica sol in the presence of an externally applied magnetic field. The magnetic-field-induced structure of the guest (long needle-like alignment) is frozen in place by the silica nanoscale network created by gelation. No organization is observed in composites in which the silica sol undergoes gelation in the absence of an external magnetic field or when the field is applied to the iron-silica composite after gelation. Aged iron−silica composite gels that are dried supercritically yield highly porous aerogels that retain both their structural integrity and the anisotropic alignment of the magnetic guest, unlike gels dried to form xerogels in which the partial collapse of the porous network destroys the integrity of the monoliths, reducing them to powder. Aged iron−silica composite gels may also be treated with a solution of Na2Cr2O7/HCl to dissolve the iron guest, thereby producing nanostructured mesoporous gels with anisotropic macroporosity.