Chemical transport selectivity

Molecular Eiltration and Chemical Transport Selectivity in the Au Nanotubule Membranes 30... 

B. Chemical transport selectivity 42 Nanomaterials in Secondary Battery Research and Development 48... 

V. MOLECULAR FILTRATION AND CHEMICAL TRANSPORT SELECTIVITY IN THE AU NANOTUBULE MEMBRANES... 

The idea of using membranes to filter molecules on the basis of size is not without precedent. Dialysis is used routinely to separate low molecular weight species from macromolecules [105]. In addition, nanofiltration membranes are known for certain small molecule separations (such as water purification), but such membranes typically combine both size and chemical transport selectivity and are particularly designed for the separation involved. Hence, in spite of the importance of the concept, synthetic membranes that contain a collection of monodisperse, molecule-sized pores that can be used as molecular filters to separate small molecules on the basis of size are currently not available. 

We demonstrated above that these Au nanotubule membranes can show charge-based transport selectivity, and we have now shown that these membranes can also have molecular size-based selectivity. The next step was to attempt to introduce chemical transport selectivity. 

The size-based selective separation has been in use in the form of dialysis for a long time. The nanofiltration membranes demonstrated here combine both size and chemical transport selectivity and are selective for the particular separation involved. The tubules are prepared as bottleneck tubules as shown in Figure 20.5. The diameter of the tubule may be manipulated, depending on the preparation conditions, resulting in applications based on size-based separations, pH switch-able ion-transport selectivity, manipulation of potential dependent fluxes etc. [75, 95, 96]. 

Hulteen, J.C., Jirage, K.B., Martin, C.R. Introducing chemical transport selectivity into gold nanotubule membranes. J. Am. Chem. Soc. 120, 6603-6604, 1998. 

Hyperpolarized 129Xe not only allows for sufficient signal intensity for chemical shift selective gas phase MRI, it also provides the means for a unique type of contrast. The imaging contrast derived from the transport of hyperpolarized gases into the material can be utilized to obtain snapshots of the gas flow and diffusion into porous samples. In this context, it is important to appreciate that Figures... 

The work discussed above shows that the Au nanotubule membranes can have one important type of transport selectivity—charge-based selectivity. It occurred to us that because the Au nanotubules can be of molecular dimensions, these membranes might show molecular size-based transport selectivity as well [72]. Finally, the thiol chemisorption chemistry introduced above provides a route for introducing chemically based transport selectivity [85]. Hence, the Au nanotubule membranes should, in principle, be able to show all three of the important transport selectivity paradigms—... 

In addition to the transport selectivities based on molecular charge or size described above, chemical interactions between the membrane material and the molecule to be transported can also strongly influence the rate and selectivity of transport. The introduction of chemically based transport selectivity was accomplished by chemisorbing thiols (RSH) to the Au tubule surfaces [113]. Membranes derivatized with two different R groups—the hydrophobic R = -CigHjj and the more hydrophilic (2)R = -C2H4-OH— were prepared. The rate and selectivity of transport in these membranes is dramatically altered by the chemical identity of the R group. 

Chemical compatibility is another important factor to reckon with in material selection as in the case of butterfly valves which are frequently used in industrial fluid flow systems where the valves are operated to load and unload different kinds of fluids for shipment or storage. These applications include tank trailers for transportation of fluids from one location to another as well as fluid flow pipelines. A typical butterfly valve usually incorporates a resilient rubber seal component which the valve pivots to seal off the flow lines and when opened allows the fluid to be conveyed to the flow line. The chemicals transported in tanks using butterfly valves are often caustic or acidic. These chemicals corrode the pivot area of the valve making its function ineffective. The rubber gasket material prevents leakage while the valve chamber is protected by a chemically resistant rubber material. 

A follow-up study demonstrated that Qi phases further enhance the performance of LLC-BR composite membranes in both water transport and harmful chemical vapor rejection [170]. A cross-linkable, gemini phospho-nium amphiphile (Fig. 23) was blended with BR and cross-linked to form films exhibiting a Qi-phase nanostructure. Materials with a Qi-phase showed 300 times greater water vapor permeability and 500 times greater permeability selectivity for water/CEES than pure cross-linked BR. Furthermore, these Qi-phase composite films were far superior to their Hu and L analogues in both water vapor permeability and water/CEES transport selectivity. Further studies were planned to process thinner films as well as test their rejection properties against other types of chemical agents. 

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