Anal. Chem. 1999, 71, 4913-4918
Effect of Thiol Chemisorption on the Transport Properties of Gold Nanotubule Membranes Kshama B. Jirage, John C. Hulteen, and Charles R. Martin*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
An electroless gold deposition method was used to deposit Au nanotubules within the pores of a polycarbonate template membrane. Membranes containing Au nanotubules with inside diameters of 2 and 3 nm were prepared for these studies. Thiols were chemisorbed to the inside tubule walls in order to change the chemical environment within the tubules. The effect of the chemical environment within the tubules on the transport properties of the tubule-containing membrane was investigated. Membranes modified with HS-C16H33 preferentially transported hydrophobic permeant molecules. When a homologous series of permeant molecules was used, the most hydrophobic permeant was preferentially partitioned into and transported by the HS-C16H33 derivatized membrane. In addition, the effect of alkyl chain length (R), in a homologous series of thiols R-SH, was investigated. Hydrophobic permeant molecules were preferentially partitioned into and transported by membranes containing the largest alkyl group. In contrast, membranes modified with HS-C2H4OH preferentially transported the more hydrophilic permeant pyridine. Finally, we show here that the HS-C16H33 derivatized membrane can be used to separate hydrophobic species from hydrophilic species. Chemical separations are a cornerstone of modern analytical chemistry. Membrane-based chemical separations represent an evolving separations technology that offers some advantages over competing separations methodologies.1-4 One major advantage is that membrane-based separations are potentially more economical and less energy-intensive than competing separations methods.1-4 However, membranes with higher fluxes and higher transport selectivities will be required if membranes are to make greater inroads into industrial chemical and bioseparations. We have been using gold nanotubule membranes5-7 to investigate how pore size, charge, and chemistry affect transport selectivity in * Corresponding author. Tel.: 970-491-0271. Fax: 970-491-2531. E-mail:
[email protected]. (1) Abelson, P. H. Science (Washington, D.C.) 1989, 224, 1421. (2) Membrane Separation SystemssA Research Needs Assessment; Rep. DE-9001170; U.S. Department of Energy: Washington, DC, April 1990. (3) Spillman, R. W. Chem. Eng. Prog. 1989, 85, 41. (4) Hennis, J. M. S.; Tripodi, M. K. Science (Washington, D.C.) 1983, 220, 11. (5) Jirage, K. B.; Hulteen, J. C.; Martin, C. R. Science (Washington, D.C.) 1997, 278, 655. (6) Hulteen, J. C.; Jirage, K. B.; Martin, C. R. J. Am. Chem. Soc. 1998, 120, 6603. (7) Nishizawa, M.; Menon, V. P.; Martin, C. R. Science (Washington, D.C.) 1995, 268, 700. 10.1021/ac990615i CCC: $18.00 Published on Web 09/25/1999
© 1999 American Chemical Society
membranes. These membranes are prepared by the template method8-10 and contain monodisperse Au nanotubules that span the complete thickness of the membrane. The Au nanotubules can have inside diameters of molecular dimensions (40 h of permeation time. In contrast, a clearly-discernable Ni2+ peak is observed in the permeate solution after 4 h when an unmodified (no thiol) Au nanotubule membrane was used (Figure 4). Figure 4 indicates that because of the hydrophobicity of the C16 thiol, the tubules in the C16-modified membrane are not wetted by water. This suggests that transport of toluene in these membranes occurs via the C16 phase within the tubules. It is also of interest to explore the effect of hydrophobicity of the permeate molecule, within a homologous series, on flux in the C16 modified membrane. Figure 5A shows flux plots for p-cresol, 2,4-dimethylphenol, and 2,4,6-trimethylphenol in a membrane that had i.d. ) 2.0 nm tubules (before chemisorption). The flux increases with increasing molecular weight for the members of this series. Again, this result is anti-intuitive if only the diffusion coefficient for the permeate molecule in the membrane is considered. However, the partition coefficients for these molecules between the aqueous feed solution and the C16 phase would be expected to increase in the order p-cresol < 2,4-dimethylphenol < 2,4,6-trimethylphenol. In analogy to Figure 3, the data in Figure 5 clearly show that transport in this membrane is dominated by the partition term in eq 2. For comparison, Figure 5B shows analogous flux plots for the same series of molecules in an Au nanotubule membrane (i.d. ) 2.0 nm) that was not treated with thiol. While some difference in flux is observed between p-cresol and the two higher molecularweight members of this series, there is essentially no difference in the 2,4-dimethylphenol and 2,4,6-trimethylphenol fluxes. That the fluxes of the larger members of this series are higher in the bare Au nanotubule membrane is again undoubtedly a partitioning effect. There is no doubt that the chemical properties of the nanoscopically confined water in the nanotubules are different than 4916 Analytical Chemistry, Vol. 71, No. 21, November 1, 1999
Figure 5. Flux plots for p-cresol, 2,4 dimethylphenol, and 2,4,6 trimethylphenol in two i.d. ) 2.0 nm Au nanotubule membranes. (a) C16 derivitized membrane. (b) Bare Au (no thiol) membrane.
those of bulk water. For example, the nanoscopically confined water cannot have the extended hydrogen-bonding network of bulk water. The higher fluxes for the larger (more hydrophobic) members of this series undoubtedly reflect the thermodynamic advantage of removing these molecules from bulk water. That the fluxes of these two molecules are roughly the same despite their size difference may reflect the interplay between the partitioning and diffusional terms in eq 2. Hydrophobic/Hydrophilic Transport SelectivitysSingleMolecule Permeation Experiments. We wanted to explore the possibility of using the thiol-modified Au nanotubule membranes to separate molecules on the basis of their chemical nature. The simplest type of separation would be to separate hydrophobic molecules from hydrophilic molecules. To explore this point, the fluxes of a hydrophobic molecule (toluene) and a more hydrophilic molecule (pyridine) in membranes modified with either a hydrophobic thiol (C16) or a hydrophilic thiol (C2H4OH) were investigated. Figure 6A compares flux plots for toluene in a membrane modified with the C16 thiol and a membrane modified with the C2H4OH thiol. The nanotubule i.d. was 3.0 nm for both membranes. The flux of toluene was 1 order of magnitude higher in the C16 membrane than in the C2H4OH membrane (Table 1). Given the discussion above, this preferential transport of toluene in the hydrophobic membrane is not surprising. These data can be expressed in terms of a selectivity coefficient, RC16/C2H4OH, which is the flux of toluene in the C16 membrane divided by the flux in the C2H4OH membrane. For these i.d. ) 3.0 nm nanotubule membranes, RC16/C2H4OH ) 10 (Table 1). Analogous toluene transport data were obtained for the smaller i.d. ) 2.0 nanotubule membranes (Figure 6B). Because of the
Figure 6. Flux plots comparing toluene transport in a C16 membrane and a C2H4OH membrane. Single-molecule permeation experiments. Nanotubule i.d. was (A) 3.0 nm, (B) 2.0 nm.
Figure 7. Flux plots comparing pyridine transport in a C2H4OH membrane and a C16 membrane. Single-molecule permeation experiments. Nanotubule i.d. was (A) 3.0 nm, (B) 2.0 nm.
Table 1. Flux and Selectivity Data for Toluene in the C16 and C2H4OH Thiol Membranesa
Table 2. Flux and Selectivity Data for Pyridine in the C16 and C2H4OH Thiol Membranesa
membrane C16 C2H4OH C16 C2H4OH
nanotube diameter (nm)
flux toluene (moles cm-2 min-1)
3.0 3.0
2.8 ( 0.2 × 10-8 2.6 ( 0.4 × 10-9
2.0 2.0
10-8
2.0 ( 0.1 × 6.1 ( 0.4 × 10-10
RC16/C2H4OH
10
membrane
nanotube diameter (nm)
flux pyridine (moles cm-2 min-1)
RC2H4OH/C16
10-9
C2H4OH C16
3.0 3.0
7.0 ( 0.7 × 4.6 ( 0.5 × 10-10
C2H4OH C16
2.0 2.0
5.6 ( 0.4 × 10-9 1.0 ( 0.2 × 10-10
56
33 a
Standard deviations are from replicate measurements on identical membranes.
smaller tubule diameter, the toluene fluxes in these membranes are lower than in the analogous i.d. ) 3.0 nm membranes (Table 1). However, higher C16 vs C2H4OH selectivity (RC16/C2H4OH ) 33) is observed in these smaller i.d. nanotubule membranes. This increase in selectivity is largely due to the drop in flux of toluene in the C2H4OH membrane (Table 1). Because the partitioning advantage is diminished in this more hydrophilic membrane, flux is retarded by the smaller tubule i.d. Identical transport studies were done with the more hydrophilic permeant molecule pyridine. Figure 7A shows the flux plots for the i.d. ) 3.0 nm membranes treated with either the C16 or C2H4OH thiols. The key observation is that the selectivity pattern for the more hydrophilic pyridine is opposite of that for the more hydrophobic toluene, i.e., pyridine is preferentially transported in the C2H4OH membrane (Figure 7) whereas toluene is preferentially transported in the C16 membrane (Figure 6). This preferential transport of pyridine in the C2H4OH membrane can be quantified by defining a selectivity coefficient, RC2H4OH/C16, which
15
a Standard deviations are from replicate measurements on identical membranes.
is the flux of pyridine in the C2H4OH membrane divided by the flux of it in the C16 membrane. For these i.d. ) 3.0 nm nanotubule membranes RC2H4OH/C16 ) 15 (Table 2). Figure 7B shows the analogous pyridine flux plots for the i.d. ) 2.0 nanotubule membranes. Again, the flux of pyridine is higher in the C2H4OH membrane than in the C16 membrane. Furthermore, as was observed in the toluene transport studies (Table 1), the selectivity of pyridine transport is higher in the smaller i.d. ) 2.0 nanotubule membranes (RC2H4OH/C16 ) 56, Table 2). As noted above, the increased selectivity in the toluene case was largely due to decreased flux in the “nonpreferred” C2H4OH membrane. The same effect is observed in the pyridine casesthe increased selectivity is largely due to the decreased flux in the “nonpreferred” C16 membrane. In this case, the decreased flux is due to the thermodynamic penalty paid by the pyridine molecule when it leaves the aqueous feed solution and enters the water-free tubules in the C16 membrane. Analytical Chemistry, Vol. 71, No. 21, November 1, 1999
4917
CONCLUSIONS
Figure 8. Flux plots showing toluene and pyridine transport in an i.d. ) 2.0 nm, C16 nanotubule membrane. Two-molecule permeation experiment.
Hydrophobic/Hydrophilic Transport SelectivitysTwoMolecule Permeation Experiments. The single-molecule permeation experiments discussed above suggest that the thioltreated membranes can be used to separate hydrophobic species from hydrophilic species. To test this, two-molecule permeation experiments were conducted in which the feed solution contained both pyridine and toluene. Figure 8 shows flux plots for toluene and pyridine from such a two-molecule experiment on a C16 derivatized, i.d. ) 2.0 nm nanotubule membrane. The flux of toluene through this hydrophobic membrane is 165 times higher than the flux of pyridine. (Toluene flux ) 1.6 × 10-8 mol min-1 cm-2; pyridine flux ) 9.7 × 10-11 mol min-1 cm-2.) These data show that this membrane can provide excellent selectivity for separation of hydrophobic from hydrophilic species. As would be expected from the single-molecule permeation experiments, an i.d. ) 3.0 nm C16 membrane provided higher fluxes, but lower selectivity (toluene flux was 40 times higher than pyridine flux).
4918 Analytical Chemistry, Vol. 71, No. 21, November 1, 1999
These studies have shown that the transport properties of the Au nanotubule membranes can be varied by chemisorbing thiols (R-SH) to the Au tubule walls. The transport properties vary in a chemically rational way with the chemical nature of the R group. An analogy was drawn between the transport properties of these membranes and the retention characteristics of reversed-phase HPLC columns. This analogy exists because in both cases alkyl groups are used to control the chemistry, and in both cases partitioning of molecules into the alkyl phase plays an important role in determining the chemical behavior of the system. A C16 derivatized membrane showed excellent selectivity for the separation of toluene from pyridine. This can undoubtedly be generalized to other hydrophobic vs hydrophilic species. In our previous work, we showed that the Au nanotubule membranes can be ideally cation permselective or ideally anion permselective.7 In addition, we have shown that these membranes can cleanly separate small molecules on the basis of molecular size.5 These results, combined with the present study, show that, with the Au nanotubule membranes, one can independently adjust the size-based, charge-based, and chemical-nature-based transport selectivity. Hence, these membranes are ideal model systems for understanding how chemistry, electrostatics, and sterics affect transport in membranes. ACKNOWLEDGMENT This work was supported by the Office of Naval Research and the National Science Foundation.
Received for review June 9, 1999. Accepted July 30, 1999. AC990615I
