Langmuir 1998, 14, 6545-6549
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Insight into the Permeation Selectivity of Calix[n]arene-Based Langmuir-Blodgett Films: Importance of Headgroup Association and the Solid Phase Robert A. Hendel,†,‡ Lan-hui Zhang,† Vaclav Janout,† Mark D. Conner,† James T. Hsu,‡ and Steven L. Regen*,† Departments of Chemistry and Chemical Engineering, and the Zettlemoyer Center For Surface Studies, Lehigh University, Bethlehem, Pennsylvania 18015 Received June 3, 1998 This paper reports the design and synthesis of 5,11,17,23,29,35-hexaacetyl-37,38,39,40,41,42-hexakis(1-hexadecyloxy)calix[6]arene (2) and 5,11,17,23-tetraamideoxime-25,26,27,28-tetrakis(1-hexadecyloxy)calix[4]arene(3), plus a detailed comparison of the membrane properties of corresponding LangmuirBlodgett films with those assembled from 5,11,17,23,29,35-hexaamideoxime-37,38,39,40,41,42-hexakis(1hexadecyloxy)calix[6]arene (1a). Surface pressure-area isotherms that were recorded for 1a and 2 over a pure water subphase at 25 °C showed significant hysteresis that was eliminated after two compressionexpansion cycles; the limiting area in both cases was ca. 162 Å2/molecule. In sharp contrast, 3 showed no hysteresis at 25 °C and a limiting area of 107 Å2/molecule. Careful examination of the dependence of surface viscosity on temperature has provided compelling evidence for a solid-analogous to liquid-analogous phase transition occurring at 31 and 28 °C for monolayers of 1a and 2, respectively. Permeation measurements that were made across Langmuir-Blodgett (LB) films [four monolayers supported on poly[1-(trimethylsilyl)-1-propyne] with respect to helium and nitrogen] revealed high selectivity in the case of 1a and much lower selectivity with 2 and 3 at ambient (21-23 °C) temperatures. The implications of these findings, in terms of the further development of LB films as membranes for gas separations, are briefly discussed.
Introduction One of the long-standing debates concerning LangmuirBlodgett (LB) films has centered on their potential usefulness as membranes for molecular separations.1 This original concept was, in fact, first suggested by Katharine Blodgett in her early pioneering studies.2 Since that time, the majority of the literature reports that have examined this possibility for gas separations have proven disappointing. In particular, membrane selectivities have generally obeyed Graham’s law, indicating that large defects within the film serve as the primary pathway for diffusion; in some instances, however, molecular separations based on solubility differences have been observed.3a-o † ‡
Only a few reports have appeared in which separations have been achieved via a molecular sieving mechanism;3p,4 e.g., composite membranes made from calixarene-based surfactants such as 1a-c deposited on poly[1-(trimethylsilyl)-1-propyne], PTMSP.4
Department of Chemistry. Department of Chemical Engineering.
(1) Ulman, A. An Introduction To Ultrathin Organic Films: From Langmuir-Blodgett To Self-Assembly; Academic Press:New York, 1991. (2) Blodgett, K. D., U.S. Patent 2,220,860,1940. (3) (a) Rose, G. D.; Quinn, J. A. J. Colloid Interface Sci. 1968, 27, 193. (b) Albrecht, O.; Laschewsky, A.; Ringsdorf, H. Macromolecules 1984, 17, 937. (c) Higashi, N.; Kunitake, T.; Kajiyama, T. Polym. J. 1987, 19, 289. (d) Stroeve, P.; Coelho, M. A. N.; Dong, S.; Lam, P.; Coleman, L. B.; Fiske, T. G.; Ringsdorf, H.; Schneider, J. Thin Solid Films 1989, 180, 291. (e) Stroeve, P.; Spooner, G. J. R.; Bruinsma, P. J.; Coleman, L. B.; Erdelen, C.; Ringsdorf, H. Am. Chem. Soc., Symp. Ser. 1990, 447, 177. (f) Stroeve, P.; Bruinsma, P. J. Thin Solid Films 1994, 244, 958. (g) Gaines, G. L., Jr.; Ward, W. J., III. J. Colloid Interface Sci. 1977, 60, 210. (h) Heckmann, K.; Strobl, C. H.; Bauer, S. Thin Solid Films 1983, 99, 265. (i) Miyashita, T.; Konno, M.; Matsuda, M.; Saito, S. Macromolecules 1990, 23, 3531. (j) Bruinsma, P.; Sturesson, C.; Spooner, G.; Coleman, L.; Stroeve, P. Polym. Prepr. 1991, 242. (k) Albrecht, O.; Laschewsky, A.; Ringsdorf, H. J. Membr. Sci. 1985, 22, 187. (l) Bruinsma, P.; Sturesson, C.; Spooner, G.; Coleman, L.; Koren, R.; Stroeve, P. Thin Solid Films 1992, 210/211, 440. (m) Chaing, C. Ph.D. Thesis, Case Western Reserve University, 1988. (n) Maximychev, A. V.; Matyukhin, V. D.; Stepina, N. D.; Yanusova, L. G. Thin Solid Films 1996, 284/285, 866. (o) Zhou, P.; Samuelson, L.; Alva, K. S.; Chen, C.; Blumstein, R.; Blumstein, A. Macromolecules 1997, 30, 1577. (p) Haubs, M.; Prass, W.; Hoechst, A.-G. PCT Int. Appl. W091/09669, 1991; PCT Int. Appl. W091/09670, 1991.
In recent studies, we have shown that composite membranes consisting of four LB monolayers of 1a on PTMSP exhibit the highest He/N2 selectivity within a related series of membranes.4f These studies have also shown that alkyl chain length can, in certain instances, significantly influence the permeation selectivity of an LB film. In particular, the relative permeation selectivities of LB films made from 1a-c were found to be 1a . 1b ≈ 1c. The primary aim of the work that is reported herein was to clarify two fundamental issues that were not addressed in our previous work: (i) Does hydrogen bonding (4) (a) Conner, M. D.; Janout, V.; Regen, S. L. J. Am. Chem. Soc. 1993, 115, 1178. (b) Conner, M. D.; Janout, V.; Kudelka, I.; Dedek, P.; Zhu, J.; Regen, S. L. Langmuir 1993, 9, 2389. (c) Dedek, P.; Webber, A. S.; Janout, V.; Hendel, R. A.; Regen, S. L. Langmuir 1994, 10, 3943. (d) Lee, W.; Hendel, R. A.; Dedek, P.; Janout, V.; Regen, S. L. J. Am. Chem. Soc. 1995, 117, 6793. (e) Lee, W.; Hendel, R. A.; Dedek, P.; Janout, V.; Regen, S. L. J. Am. Chem. Soc. 1995, 117, 10599. (f) Hendel, R. A.; Nomura, E.; Janout, V.; Regen, S. L. J. Am. Chem. Soc. 1997, 119, 6909.
10.1021/la9806496 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/03/1998
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(i.e., headgroup association) contribute to the cohesiveness and unusually high permeation selectivity associated with LB films derived from 1a? (ii) Why does an increase of four methylene units per alkyl chain, on going from 1c to 1b, result in a negligible increase in permeation selectivity, but a further increase of four methylene groups leads to substantially greater selectivity (i.e., 1b to 1a)? A secondary aim of this work was to clarify whether an analogous calix[4]arene, through which gaseous permeats could not pass, would lead to permeation-selective LB films. Specifically, could one take advantage of interstitial space and/or transient gaps between the assembled calix[4]arenes for the separation of gases? In this paper, we report the design and synthesis of two new calix[n]arenes plus monolayer and gas permeations measurements that were intended to provide insight into each of these questions. Experimental Section General Methods. Unless stated otherwise, all reagents and chemicals were obtained from commercial sources and used without further purification. 37,38,39,40,41,42-Hexakis(1hexadecyloxy)calix[6]arene and the “cone” conformer of 25,26,27,28-tetrakis(1-hexadecyloxy)calix[4]arene were prepared using established procedures;5 5,11,17,23,29,35-hexaamideoxime-37,38,39,40,41,42-hexakis-(1-hexadecyloxy)calix[6]arene (1a) was synthesized as previously described.4f All 1H NMR spectra were recorded on a Bruker 360 MHz instrument; chemical shifts are reported in ppm and were referenced to residual solvents. Highresolution mass spectra (fast-atom bombardment) were obtained at the Mass Spectrometry Facility of the University of California, Riverside. Elemental analyses were obtained from Midwest Microlab (Indianapolis, IN). Specific protocols that were used for surface pressure-area isotherm measurements, surface viscosity measurements, LB film fabrication, and gas permeation measurements have previously been reported.4f 5,11,17,23,29,35-Hexaacetyl-37,38,39,40,41,42-hexakis(1hexadecyloxy)calix[6]arene (2). The acetylated calix[6]arene, 2, was obtained as a colorless solid having mp 185-186 °C, and 1H NMR (C D Cl , 100 °C) δ 0.90 (t, 18 H), 1.30 (br s, 168 H), 2.32 2 2 4 (brs, 18 H), 3.77 (brs, 24 H), 7.60 (brs, 12 H). Anal. Calcd for C150H240O12: C, 80.59; H, 10.83. Found: C, 80.22; H, 10.63.6 5,11,17,23-Tetraamideoxime-25,26,27,28-tetrakis(1-hexadecyloxy)calix[4]arene (3). Synthetic procedures that were used to convert (i) the “cone” conformer of 25,26,27,28-tetrakis(1-hexadecyloxy)calix[4]arene into its corresponding tetrabromide form [90%, mp 73-74 °C; 1H NMR (CDCl3) δ 0.85 (t, 12 H), 1.24 (m, 104 H), 1.83 (m, 8 H), 3.55 (d, J ) 11.3 Hz, 4 H), 3.81 (t, 8 H), 4.31 (d, J ) 11.3 Hz), 4 H), 6.78 (s, 8 H)], (ii) the bromide into its corresponding nitrile form [56%, mp 89-90 °C; 1H NMR (CDCl3) δ 0.85 (t, 12 H), 1.23 (m, 104 H), 1.84 (m, 8 H), 3.22 (d, J ) 11.5 Hz, 4 H), 3.89 (t, 8 H), 4.40 (d, J ) 11.5 Hz, 4 H), 6.97 (s, 8 H)], and (iii) the cyanide into the corresponding amideoxime form (3) [71%, 194-195 °C; 1H NMR (CDCl3/CH3OH, 3/1, v/v) δ 0.87 (t, 12 H), 1.26 (m, 104 H), 1.66 (m, 8 H), 3.30 (d, 4 H), 3.40-4.50 (brm, 8 H), 3.80 (t, 8 H), 4.35 (d, 2 H), 6.10-7.05 (m, 8 H), 6.30-7.10 (brm, 4 H); HRMS-FAB calcd for C96H161N8O2 1554.2437, found 1554.2502] were analogous to those previously described for the synthesis of 1a.4f
Results Calixarene Design. In our original design of calix[6]arenes 1a-1c, it was assumed that intermolecular hydrogen bonding between the amideoxime groups would reinforce intermolecular hydrophobic interactions.4d To test this assumption, an acetyl analogue of 1a [i.e., 5,11,17,23,29,35-hexaacetyl-37,38,39,40,41,42-hexakis(1hexadecyloxy)calix[6]arene, 2] was chosen as a target for synthesis and investigation. In the case of 2, only (5) Markowitz, M. A.; Janout, V.; Castner, D. G.; Regen, S. L. J. Am. Chem. Soc. 1989, 111, 8192. (6) Conner, M. D. Ph.D. Thesis, Lehigh University, 1992.
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intermolecular hydrophobic interactions are available for producing a cohesive monolayer. In principle, therefore, if films derived from 1a were to exhibit greater cohesiveness and integrity than those based on 2, this would constitute compelling evidence in support of our assumption. A calix[4]arene analogue of 1a (i.e., 5,11,17,23-tetraamideoxime-25,26,27,28-tetrakis(1-hexadecyloxy) calix[4]arene, 3) was also considered as an attractive target for probing the question of diffusion through versus between the surfactants. In contrast to 1a, which has an estimated internal diameter of 4.8 Å (CPK models), the internal diameter of 3 is ca. 2.0 Å (“lower rim”, alkoxy side). Thus, He and N2 (having kinetic diameters of 2.6 and 3.64 Å, respectively) would be small enough in size to pass through 1a but not 3.4a A comparison of the monolayer and LB film properties of 3 with those of 1a was also expected to provide insight into the influence that the number of n-alkyl chains and amideoxime groups have on intermolecular association and film cohesiveness. Since the outer diameter of 3 is estimated to be ca. 11.6 Å (CPK models, “upper rim”, amideoxime side), and since a previous estimate for the diameter of pores within cast film of poly[1-(trimethylsilyl)-1-propyne] (i.e., our support material used in this and previous studies) is 10 Å, one would expect the LB film to remain assembled on the surface of the support.7
Calixarene Synthesis. With these ideas in mind, 37,38,39,40,41,42-hexakis(1-hexadecyloxy)calix[6]arene was subjected to Friedel-Crafts acylation, affording a 56% isolated yield of 2.6 By using procedures that were similar to those that were previously described for the synthesis of 1a, bromination of the “cone” conformer of 25,26,27,28-tetrakis(1-hexadecyloxy)calix[4]arene with N-bromosuccinimide (NBS), followed by cyanation and addition of hydroxylamine afforded 3 (Scheme 1).4f Monolayer Properties. Surface pressure-area isotherms that were recorded for 1a and 2 over a pure water subphase at 25 °C are shown in Figure 1. Both surfactants form stable monolayers, having significant hysteresis. After two consecutive compression-expansion cycles, however, such hysteresis is eliminated. Extrapolation from the condensed region of the isotherms to zero surface (7) Yampol’ski, Y. P.; Shantorovich, V. P.; Cherrynyakovski, F. P.; Kornilov, A. I.; Plate, N. A. J. Appl. Polym. Sci. 1993, 47, 85.
Permeation Selectivity of L-B Films
Langmuir, Vol. 14, No. 22, 1998 6547 Scheme 1
Figure 1. Surface pressure-area isotherms for 1a, 2, and 3 over a pure water subphase at the indicated temperatures. Monolayers were compressed at a rate of 25 cm2/min; maximum surface pressures that were reached for these hysteresis experiments are indicated in each isotherm. Solid and dashed lines represent the first and second compression-expansion cycles, respectively; isotherms that were obtained from subsequent compression-expansion cycles were unchanged. The collapse pressures for 1a, 2, and 3 are in excess of 50 dyn/cm (not shown).
pressure (after two compression-expansion cycles) yield a limiting area of 162 Å2/molecule for both 1a and 2, respectively. Similar isotherms that were recorded at elevated temperatures showed little hysteresis in the first compression-expansion cycle, and an increase in limiting areas (Figure 1). The increase in the limiting area that is observed on going from 25 to 45 °C for 1a (and 25 to 50 °C for 2) suggests that a monolayer phase change has occurred, i.e., that a transition from a solid-analogous to liquid-analogous state has been made, where each molecule occupies greater area. In sharp contrast, 3 (which also formed a stable monolayer) showed no hysteresis at 25 °C and a limiting area of 107 Å2/molecule. These areas for 1a, 2, and 3 are in good agreement with minimum areas that are estimated from CPK models, if it is assumed that each calixarene lies in a tightly packed array such that all of the polar moieties are in contact with the water surface; i.e., the pore axis lies perpendicular to the water surface. To judge the cohesiveness of each of these calix[n]arenes in the monolayer state, we examined their relative surface viscosities at the air-water interface by use of canal viscometry.4f Data that are reported in Figure 2A show that films derived from 1a and 2 are highly viscous at 25 °C, as indicated by a pressure drop of
