Tailoring the surface properties of nanotube membranes for controlled separations

Author: Leonora Velleman

Velleman, Leonora, 2011 Tailoring the surface properties of nanotube membranes for controlled separations, Flinders University, School of Chemical and Physical Sciences

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Membrane-based separation is a rapidly developing technology which offers many advantages over other separation techniques. However, existing membrane technology requires further research into improving efficiencies which involves flux enhancement, improved selectivity, sufficient long term stability and anti-fouling properties. The fabrication of membrane materials capable of performing highly controlled molecular separations can be achieved by developing nanoporous materials with controllable structural, physical and chemical properties. Recently there has been increased interest in the functionalisation of membrane surfaces in order to enhance the stability and transport properties of membranes. However, current research into the characterisation of functional layers within porous materials is lacking. Further insight into how surface modifications may impact the transport properties of porous membranes is essential for the development of membrane materials. This thesis presents an approach for tailoring porous materials with surface functionalities and controlling pore architecture to provide controlled transport properties. Membranes such as polycarbonate and porous alumina membranes were used in these studies due to their ordered pore architectures. Further structural modification of the membranes was carried out in order to reduce the pore diameter of the membranes. Pore size reduction was achieved using two methods; electroless deposition of gold and atomic layer deposition (ALD) of silica. The pore size of the membranes was altered systematically by adjusting the number of ALD cycles or by adjusting gold deposition time. The surface properties of the membranes were tailored in order to provide controlled molecular transport. It is important to determine how surface modifications may impact the transport properties of porous membranes in order to devise more efficient separation processes. Desired chemical properties were imparted to the membranes by modifying the membrane surfaces with self assembled monolayers (SAMs). Predominantly, hydrophobic SAMs were used as it presented a simple technique to demonstrate changes to the transport properties of membranes due to introduced surface functionalities. The transport properties of fluoro-derivatised membranes (1H,1H,2H,2H-perfluorodecanethiol) towards hydrophobic and hydrophilic molecules was compared with a membrane modified with an analogous alkanethiol; 1-decanethiol to demonstrate the influence that a slight variance in surface modification can have on the transport properties of the membrane. The effects of the controlled positioning of functional groups on the transport properties of the membrane were investigated. Several hybrid membrane structures based on polycarbonate membranes were created in which gold was deposited on different areas on the membrane; on one of the membrane interfaces, within the pores of the membrane and completely coating all surfaces of the membrane. Gold-thiol chemistry was exploited in which the thiols only assembled on the gold coated regions of the membrane thus providing controlled positioning of functional regions. Lastly, silica coated PA membranes were functionalised with perfluorodecyldimethylchlorosilane (PFDS) to demonstrate that the transport and selectivity properties of silica composite PA membranes can be varied by functionalisation using silane chemistry. The investigation of the coverage and reproducibility of SAMs within porous matrices is of utmost importance in the design of filtration membranes and sensing platforms. The surface enhanced Raman scattering (SERS) effect was employed to confirm and characterise the formation of SAMs of 3-mercaptobenzoic acid (mMBA) on the surfaces of the gold coated alumina membranes. To explore more sophisticated surface functionalisation, stimuli responsive membranes were produced. The transport properties of the gold nanotube membranes were controlled through the reversible switching of adsorbed fluorinated azobenzene layers. The fluorinated, hydrophobic end group of the azobenzene chain produces a transition between hydrophobic and less hydrophobic surface properties when switching from the trans to the cis state. The selective mediation of a hydrophilic probe dye across the membrane was investigated.

Keywords: nanotube,electroless gold deposition,selective transport,membrane
Subject: Nanotechnology thesis

Thesis type: Doctor of Philosophy
Completed: 2011
School: School of Chemical and Physical Sciences
Supervisor: Prof. Joseph G. Shapter