(266c) Nature-Inspired Hybrid Membranes for Molecular Separations

A mesoporous hybrid membrane has been synthesized for size-exclusive separation of molecules. This membrane is composed of columnar silica structures containing mesopores of a controlled diameter, grown within the channels of an anodic alumina template, which are oriented perpendicularly to the external membrane surface. The mesopores in the silica columns are formed by soft templating, using surfactants, which form micelles, around which the silica grows; removal of the surfactant leads to mesopores.

The fabrication of mesoporous membranes using anodic alumina as a macroporous template facilitates the orientation of the pores, and the control over other geometrical parameters, such as wall thickness, length and diameter. To this end extensive research on the template-directed process using macroporous membranes has been carried out. The nanocomposite formed inside the alumina pores is an assembly of silica and surfactant, with a column diameter equal to the columnar alumina pore diameter. The surfactant-filled nanochannels are oriented parallel to the walls of the columnar alumina pore. For further application as a molecular separator, after removal of the surfactant, it is desirable to produce hybrid membranes in which the mesoporous material completely fills the anodic alumina membrane (AAM) channels to minimize loss of size selective separation and transport of molecules. Additionally, a stronger interaction between the mesoporous material and the alumina channel wall is desired, as shrinkage of the mesostructure is observed in calcined samples, leaving a gap between silica fibers and alumina channel walls. It is also essential to obtain permeable mesostructures such as a hexagonal columnar phase.

In this study, the AAM was loaded with precursor solutions having different surfactant-to-silica ratios and left to dry under ambient condition and controlled humidity. The silica nanocomposites are grown by sol-gel chemistry of silica in confinement and other methods, such as evaporation-induced self-assembly, and electrophoretic deposition. Temperature was found to play a key role in phase formation and, when used, aspiration pressure influenced the type of mesostructure obtained. By functionalizing the mesoporous silica inside the AAM channels, chemical groups could be incorporated, guided by the structure and function of certain biological pores.

Mesoporous hybrid membranes were characterized by scanning electron microscopy (SEM), small-angle X-ray scattering, and nitrogen adsorption and desorption. These provided evidence of pore filling, the nature of structural order and accessible mesoporosity. Functionalized silica nanostructures were characterized by high-resolution transmission electron microscopy (TEM) and size-exclusion permeation experiments. This allowed evaluation of the effective filling and thickness of the mesoporous silica columns.