Currently small molecules are separated from one another on the basis of differences in bulk macroscopic properties, such as boiling point or solubility in a given solvent. Separations based on molecular size could be very useful for separating a smaller molecule from a slightly larger one with similar physical and chemical properties. Such separations could be much more complete than those currently in use, and might be achieved in a single step.

Until recently, only very large molecules could be separated by membranes solely on the basis of size (polycarbonate membrane that had pores 30 nm in diameter, small enough to prevent very large molecules from passing through

Martin’s group found that they could shrink the size of these pores by using a nonelectrical gold-plating procedure, where gold ions in solution are chemically (rather than electrochemically) reduced at the surface of the membrane (2). The gold is deposited on the inside of the pores, gradually decreasing their width. This method was used to make tubules with diameters as small as 0.6 nm (1).

The width of the tubules can be controlled by varying the time of the plating reaction, so that if the reaction is continued long enough, the pores fill in completely. In addition, the structure of the tubules can be controlled by varying the pH. Raising the pH increased the rate of the reaction; moreover, at elevated pH the reaction occurred preferentially on the faces of the membrane, so the gold surface film partially extended over the opening of the tubule. Under these conditions “bottleneck” tubules are formed (see illustration below).

Membranes containing both regular and bottleneck tubules were used to filter small molecules. Three pairs of different-sized molecules were used to test the filtering ability of the tubules. In each case a solution containing equal concentrations of the two different-sized molecules was passed through the membrane. Each solution was tested with both conventional and bottleneck membranes, using various tubule diameters. In experiments where mixtures of methyl viologen and Ru(bpy)₃Cl₂ (see structure at left) were filtered through tubules with a diameter of 5.5 nm, methyl viologen passed through the membrane 50 times faster than Ru(bpy)₃Cl₂.

When the diameter of the nanotubules was decreased to 0.6 nm, methyl viologen still passed through the membrane, but no detectable amounts of Ru(bpy)₃Cl₂ did so. Bottleneck tubules with an opening of the same diameter gave similar results, but the methyl viologen passed through the membrane 200 times faster than through a conventional tubule. On this basis, bottleneck tubules would be preferable for separations.

Nanotubule-based filtration membranes could have many future applications for chemical separations, particularly when only small amounts of materials are involved. Since the size of the nanotubule can be controlled, one could custom design the membrane for individual separations. A further refinement is that nanotubules can be modified to carry out separations based on chemical properties (3). For example, monolayers containing nonpolar functional groups like -CH₃ can be adsorbed onto the gold surface via thiol (-SH) groups. (The thiol groups interact with gold as well as other heavy metals such as mercury. For an example of mercury-thiol adsorption see Microporous Silica.) When these nonpolar groups are adsorbed on the gold surface, the tubule becomes selective for nonpolar substances. With polar groups like -OH attached to the thiol monolayer, the tubule becomes selective for polar substances. Furthermore, if an electric potential is placed across the tubule, it can be made selective for either positive or negative ions (4). Nanotubule-based filtration membranes may be useful, not only for separations, but also as models for cells in biological systems, where membranes are capable of very selective separations.

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