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Synthesis and Nanofiltration Membrane Performance of Oriented Mesoporous Silica Thin Films on Macroporous Supports M. Kaitlyn Clark Wooten, Venkat R. Koganti, Shanshan Zhou, Stephen E. Rankin,* and Barbara L. Knutson* Department of Chemical and Materials Engineering, University of Kentucky 177 F. Paul Anderson Tower, Lexington, Kentucky 40506-0046, United States S Supporting Information *
ABSTRACT: Silica thin films with accessible hexagonal close-packed (HCP) pores have been deposited on macroporous supports to achieve composite nanofiltration membranes. The properties of these pore channels have been characterized through solvent flux and solute diffusion experiments. A chemically neutral surface (provided by a cross-linked layer of P123 copolymer) for silica thin film synthesis on the alumina macroporous support promotes the alignment of HCP channels vertical to the substrate, where the mesopore templating agent is block copolymer P123 (poly(ethylene glycol)-block-poly(propylene glycol)block-poly(ethylene glycol)). Vertical pore alignment is achieved for thin films (less than ∼100 nm) on a neutral surface and by sandwiching thicker films (∼240 nm) between two chemically neutral surfaces. Solvent flux through the composite membranes is consistent with accessible 10 nm diameter pores. Size selectivity of the membranes is characterized from the permeability of fluorescently tagged solutes (ranging from 4000 to 70 000 Da), where a size cut off occurs at 69 000 Da for the model protein bovine serum albumin. These permeability studies of the nanofiltration membranes serve to demonstrate solute transport in oriented silica thin film membranes and also highlight their versatility for membrane-based separations. KEYWORDS: mesoporous silica, self-assembly, oriented pores, ceramic membranes, size selective
1. INTRODUCTION Porous ceramic membranes are stable in a broad range of operating environments relative to polymers, and their surface chemistry can be tailored to a variety of applications. Hard templating using nanoparticle array pore templates and metal alkoxide precursors to the ceramic film has been reported.1−4 However, the pore sizes are generally limited to 10−100 nm, and achieving a uniform size distribution is challenging.5,6 Membranes synthesized instead by the soft templating of sol− gel metal oxides using surfactants are of interest for separations because of their smaller pore dimensions (2−50 nm) and welldefined pore symmetry and orientation.7−9 Templating of sol− gel oxides is a versatile method to synthesize mesoporous particles, monoliths, and thin films for separations, catalysis, and sensors.10−12 The translation of surfactant-templated thin films to semipermeable membrane applications requires an accessible pore structure. Templates for pore structures include bicontinuous cubic mesophases, 3D disordered wormholelike networks, and hexagonally close-packed (HCP) mesophases, © XXXX American Chemical Society
which are obtained by varying the template, template concentration, and synthesis conditions.2,3,13 The accessibility of the pores in silica thin film membranes with cubic and 3D disordered pore structures on macroscopic supports (primarily alumina) has been demonstrated by gas permeability studies,14−16 pressure-driven solvent flow,17 and the separation of solvents.18,19 For membrane applications requiring limited interactions among solutes within the membrane, the parallel cylindrical channels of 2D HCP pore structures is preferred to the interconnected pores of cubic or 3D disordered mesostructures. An ideal film based on a 2D HCP phase would have no tortuosity and thus permit rapid convection and diffusion of solutes in individual pore compartments. However, the cylindrical pores of HCP structures tend to align parallel to the substrate on which they are deposited due to favorable (either hydrophilic or hydrophobic) interactions between the Received: June 6, 2016 Accepted: August 1, 2016
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DOI: 10.1021/acsami.6b06765 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces surfactant and the support,20−22 making the pores of the silica thin film inaccessible. Strategies to control the pore orientation of 2D HCP pores of thin films include the use of surface modification, surface patterning, external fields, and control of evaporation-induced self-assembly (EISA) parameters, typically resulting in only partial vertical orientation of the porous channels.23−29 In extending these techniques to the synthesis of supported membranes, accessible silica thin films with parallel channels were prepared on alumina supports by the interfacial reaction of sodium fluoride with a silica sol to give partially oriented channels.30,31 The direct synthesis of ordered silica mesopores within the parallel pore channels of a macroporous support has been a more widely investigated approach to forming accessible 2D HCP pores.4,23,26,32 Synthesis conditions leading to silica channels vertical to the substrate (and parallel to the larger pores of the AAO support) have been identified.33−36 However, concentric rings and helical pores are also frequently observed for HCP phases confined in AAO pores. Control over the mesophase orientation depends on the size of the AAO macropores35 but also is influenced by more subtle synthesis parameters such as surfactant concentration and humidity.23,37 Even if the orientation of the HCP channels can be controlled to align them parallel to the AAO macropores, shrinkage of the mesoporous silica away from the alumina walls during drying and calcination can create large-pore defects that prevent their use as membranes.26,36 Investigations in our group demonstrate an alternative approach to vertically aligned 2D-HCP structures: the use of a surface modified to be “chemically neutral” (equally attractive to the head and tail of the surfactant template) to synthesize oxide films with vertically oriented HCP pore arrays.38,39 Using P123 [poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO)-PEO triblock copolymer] as a template, chemically neutral surfaces were created on glass substrates using crosslinked layers of random PEO−PPO or P123 copolymers. Silica thin films templated on a neutral surface (for
