Langmuir 1993,9, 2389-2397
2389
Perforated Monolayers: Fabrication of Calix[ Glarene-Based Composite Membranes That Function as Molecular Sieves’ Mark D. Conner, Vaclav Janout, Ivo Kudelka, Petr Dedek, Jiayi Zhu, and Steven L. Regen’ Department of Chemistry and Zettlemoyer Center for Surface Studies, Lehigh University, Bethlehem, Pennsylvania 18015 Received June 30,199P This paper reports the synthesis, monolayer behavior, and polymerization characteristicsof three new calix[6larenes, and the fabrication and permeation selectivity (permselectivity)of a series of composite membranesderivedfrom one such amphiphile. Monolayersof 5,11,17,23,29,35-hexakis(N-(2-(methyldithio)ethyl)carbamoyl)-37,38,39,40,41,42-helrakis(odyloxy)calix[6larene (I)exhibit a collapse pressure of 53 dyn/cm; those prepared from 5,11,17,23,29,35-herakis(mercaptom~yl)-37,38,39,40,41,42-her(nbutyloxy)calix[6larene(111,and 5 , 1 1 , 1 7 , 2 3 , ~ , 3 5 - h e ~ ~ y l - 3 7 , ~ , 3 9 , ~ , 4 1 , 4 2 - h e x a k i s ~ 3 , ~ o ~ - l - h e p t y l o ~ ) calix[6larene (111)collapse at 16 and 26 dyn/cm, respectively. At the collapse point, the areas that are occupied by 1-111 are 165,150,and 155 &/molecule, respectively. Polymerization of I via UV-induced disulfide disproportionationand I1 by air oxidation result in a modest reduction in surface pressure. In contrast, UV irradiation of 111results in the complete loss of surfacepressure and an apparent “buckling” of the fii. Composite membranes that have been fabricated from Langmuir-Blodgett multilayers of I, plus polymeric supports having large permanent pores (i.e., Celgard and Nuclepore membranes),show a significantreduction in permeabilitytoward He, Nz and SF6 but no enhancementin permeation selectivity (permselectivity). In striking contrast, analogous composites that have been constructed wing poly[1(trimethylsilyl)-l-propynelas supportmaterial,function as molecular sieves;i.e., He and Nz readilypermeate the composites, but SF6 does not. On the basis of intrinsic permeability coefficients that have been calculated for He and Nz as a function of LB film thickness, it is concluded that this permselectivity is due to the selective transport across homogeneous multilayers of I.
Introduction Membrane filtration methods represent the most energy-efficient and cost-effective means that are currently available for chemical separation, concentration, and purification.2s This fact, in and of itself, provides substantial incentive for the creation of new materials exhibiting high permeation selectivity (permselectivity) and high permeabilit~.~ At present, most of the synthetic membranes that are being developed for gas separations fall into one of three general categories: solution-diffusion, ultramicroporous, or Knudsen-diffusion membranese5 Solution-diffusion permeants
e
/e
1
0
4 1 I 0
solution-dltfusion
0
lli 0
ultramlcroporous
e 0
1 01 1 Knudsen Dltfusion
membranesare thin filmsof organicpolymer that separate gases on the basis of their inherent differencesin solubility
* Abstract published in Advance ACS Abstracts, August 15,1993.
(1)Supported by the Division of Basic Energy Sciences and the Department of Energy (DE-FG02-85ER-13403)and by Air Produds and Chemicals, Inc., Allentown, PA. (2) Haggin, J. Chem. Eng. News 1990, October 1, 22. (3) Lonsdale, H.K. J. Membr. Sci. 1982, 10, 81. (4) Kesting, R. E. Synthetic Polymeric Membranes: A Structural Perspectioe; John Wiley: New York, 1986. (6)The United States Department of Energy Industrial Energy Program: Research and Development in Separation Technology; DOE Publication DOE/NBM-8002773,1987.
and diffusivity. Diffusion within these “solid” materials is generally thought to take place via a “jumping” mechanism whereby the permeant jumps from one transient gap to another along a concentrationgradient. These gaps are presumed to result from the thermal motion of the polymer chain segments. In contrast,ultramicroporous membranes maintain a tortuous, but continuous network of passages that extend from one side of the membrane to the other. Here, molecular separations are a consequence of the higher diffusional rates that are associated with the smaller-sized permeanta. Finally, Knudsendiffusionmembranes (havingeffective pore diametersthat are smaller than the molecule’s mean free path) separate gasesaccordingto Graham’slaw, where the rate of diffusion of each permeant is inversely proportional to the square root of ita molecular weight. We have become interested in creating fundamentally new classes of membranes for gas separations and have approachedthis problem from the view point of an organic chemist. In particular, we envisioned that membranes having well-defined and adjustable pore structures should be obtainable by molecular design. More specifically,we hypothesized that two-dimensionalassemblies of “porous molecules” should exhibit barrier properties that are governed by the effective diameter of each individual molecular In essence,we theorizedthat ‘perforated monolayers”should have the capability of functioning as molecular sieves. On the basis of this concept, we have begun to build prototype membranes by applying Langmuh-Blodgett (LB) methods of film construction to appropriately-designed“poroussurfactants”, e.g., polymerizable surfactant derivatives of calix[nIarer~es.~~~ (6) Markowitz,M. A.; Bielski, R.;Regen, S. L. J. Am. Chem. SOC. 1988, 110, 7546. (7) Markowitz, M. A.; Janout, V.; Castner, D. G.; Ftegen, S. L. J. Am. Chem. SOC.1989,111,8192.
0743-7463/93/2409-2389$04.00/0 0 1993 American Chemical Society
Conner et al.
2390 Langmuir, Vol. 9, No. 9, 1993
B
molecular pore
perforated monolayer
In this paper, we report the synthesis, monolayer behavior, and polymerization characteristics of three new calix[6larenes and the fabrication and permselectivity of a series of composites derived from one such amphiphile. In order to evaluate each composite for sieving activity, we have measured their barrier properties toward He, N2, and SF6. On the basis of the kinetic diameters of He, N2, and SF6 (2.6,3.6, and 6.6&respectively), and an estimated internal diameter of the calix[blarene framework of 4.8 A (CPK models), sieving action would be indicated if He and N2 readily crosses the composite, but SF6 does not." In addition, permselectivity ratios for He/N2 that are significantly greater than that which is predicted by Knudsen diffusion (Le., 2.6) would be indicative of molecular sieving.
'V
to exhibit surface activity due to the presence of the polar amide groups on the "upper" (wider) rim of the calix[6]arene. In addition, monolayers of I were expectad to crosslink upon exposure to ultraviolet light (264 nm) via disulfide disprop~rtionation.'~Calix[blarene I1 was also considered to be a worthy target. Here, the thiol moieties were intended to act as head groups and to provide a meam for cross-link formation via intermolecular disulfide formation.1C16 Examination of CPK molecular models of I1 indicates that intermolecular coupling should be strongly favored over intramolecular reaction and that the interstitial space (smaller holes between the calix[6]arene units) should be minimized. Finally, a styryl-type of calix[6]arene (111)was considered ashavingthe requisite amphiphilicity and polymerizability for our purpoee.17 Although we expected that I and I1 would maintain their two-dimensional structure upon polymerization, the situation for I11 was less clear.
/
Callx[6]arene Framework
Results and Discussion Design of Second-GenerationPolymerizableCalix[glarenee. In previous work, we have attempted to fabricate permselective composites using dithiothreitolpolymerized monolayers of 1 plus Nucleopore membrane~.~*'J~ Because these fiis were highly cross-linked, we were limited to horizontal methods of transfer.I3 Despite repeated efforts, all of the resulting composites failed to show any permselectivity beyond that predicted by Knudsen diffusion. On the basis of these findings, we concluded that (i) the surfactant monolayer did not effectively span the large permanent pores of the support and/or (ii) the overlayer was "blown apart" when exposed tomodest pressure differentials,e.g., 0.1 atm. In any event, it was clear that the bulk of the permeant was passing through film defects and not through molecular pores. In order to create intact and robust surfactant overlayers, we have begun to focus our efforts on multilayers of polymerized calix[6larene. For this purpose, we sought &[61 arenes that would yield stable Y-type(headto head, tail to tail)multilayersand that could be polymerized after transfer from the gas-water interface. Three target molecules that were designed for this purpose were calix[Glarenee 1-111. The hexamidederivative (I)was expected (8)Gutache, C. D. Calixarenes; The Royal Society of Chemistry, Thomas Graham House, Science Park Cambridge, 1989. (9) Vicens, J.; Bohmer, V. Calixarenes: A Versatile Class of Macrocyclic Compoun&, Kluwer Academic Publishers: Boston, MA, 1991. (10)A preliminary account of this work has previously appeared: Conner,M.D.;Janout,V.;Regen,S.L. J.Am. Chem.Soc. 1993,115,1178. (11) Breck, D. W. Zeolite Molecular Sieves; John Wiley: New York, 1974. (12) Conner, M. D.; Janout, V.; Regen, S. L. Unpublished resulta.
(13) Ulman,A. An Introduction To Ultrathin Organic Films: From Langmuir-ElodgettTo Self-Assembly;Academic Press: New York, 1991.
CONHCHzCHzSSCH3
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II
111
Support Material. In this work, we have utilized three different types of support material for fabricating permselectivemembranes. Commerciallyavailable Nuclepore and Celgard membranes were chosen because of their large permanent pore structure. Specifically,we felt that if an LB film could effectivelycover such pores, diffusionacross the LB overlayer would have to be rate limiting. A third type of support was of special interest to us; these were cast films made from poly [l-(trimethylsily1)-1-propyne] (PTMSP). Unlike Nucleopore and Celgard membranes, PTMSP filmsbear a continuow polymeric surface (Figure l).l*J9 Because of their high internal free volume and a glassy state, PTMSP exhibits extraordinarily high permeability. In principle,the fact that largepermanentpores do not have to be traversed by the LB assembly should make it easier to construct defect-free surfactant overlayers. We also expected that a continuous surfacewould provide added mechanical strength to an LB overlayerby serving as a "backing", thereby reducing the chances that the permeant gas would be "blowing holes" through the film. Finally, we envisioned that a continuous support could function as a Ycaulking"agent, which reduces the (14) Organic Sulfur Chemistry: StructureAnd Mechanium;Oae, S., Doi, J. T., Eds.;CRC Press: Boca Raton, FL, 1991. (15) Ariiura,T.; Mataumoto, S.;Teshima, 0.; Nagasaki,T.; Shinkai, S. Tetrahedron Lett. 1991,32,5111. (16) Ahmad. J.: Astin. K. B. Colloids Surf. 1990.49. 281. (17) Conner; M.;Kudelka, I.; Ragen, S. L: h n g h l S 9 1 , 7,982. (18) Masuda, T.; Isobe, E.; Higashimura, T.;Takada, K. 3.Am. Chem. SOC.isa3.106.7473. (19) Polymers For Gas Separation;Toshima, N., Ed.;VCH Publiehing: New York, 1992.
Perforated Monolayers
Langmuir, Vol. 9, No. 9, 1993 2391
n-E - n & 0 ) !
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flow of permeant through film defects and thus helps to guide the flow through the molecular pores.20 The key issue with PTMSP, however, was whether or not the support would be sufficiently permeable such that diffusion through the LB overlayer would be rate limiting. Surfactant Synthesis. Alkylation of 37,38,39,40,41,42-hexahydroxycalix[6]arene21 with 1-bromooctane afforded the corresponding hexakis(n-octyloxy)ether (IV); sequential Friedel-Crafts acylation (CH&OCl), haloform (20)Henis, J. M. S.;Tripodi, M. K. Sep. Sei. Technol. 1980,15,1059. (21) Gutsche, C. D.; Lin, L. G. Tetrahedron 1986,42,1633.
(@ CH2=CHSnBu3 Pd(PPh3)4
111
Figure 1. Scanning electron micrograph of (A, top) Celgard 2500, (B, middle) Nuclepore (300 A diameter pores), and (C, bottom) cast film made from PTMSP; bar represents lo00 A.
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oxidation, acid chloride formation, and condensation with 2-methyldithiaethylamineafforded calixarene I (Scheme I). The yield of I, starting from IV, was 13%. Calix[6]arene I1 was synthesized via chloromethylation of 37,38,39,40,41,42-hexa-n-butoxycalix[61arene (VII),followed by thioester formation and reduction (Scheme11). Finally, alkylation of 37,38,39,40,41,42-hexahydroxycalix[ 61arene with 3,6-dioxa-l-heptylbromide followed by bromination and vinylation afforded the requisite styryl monomer IV (Scheme 111). Monolayer Properties. The monolayer behavior that was observed for each of the polymerizable surfactants, 1-111, was similar to those of analogous 0-alkylated, paramercurated calixC61arenes (Figure 2). One noticeable difference, however, was that I appeared to be somewhat more c~mpressible.~ The collapse pressures for I, 11, and I11 were 53,16, and 26 dyn/cm. At the collapse point, the
Conner et al.
2392 Langmuir, Vol. 9,No. 9, 1993
7
A n a (Angslmmr * m ” u l e )
Figure 2. Surface pressure-area isotherms over water at 25 ‘c: (-)
I; (---)
11; (...)111.
0 0
10
20
30
40
50
60
Time (minutes)
Figure 4. Surface pressure of an air-oxidized (6 h, pH 6.6) monolayer of I1 ( 0 )and an analogow monolayer (0)after 26 min of air exposure at pH 8.6, in the presence of a 4mm slit opening (canal viscometer) as a function of time.
Tim (mlnutee)
Figure 3. (A, top) Plot of surface pressure of a monolayer of I as a function of time of exposure to ultraviolet light (254 nm). (B,bottom) Surface pressure of aW-treated (0)and nontreated ( 0 )monolayer of I in the presence of a 4-mm slit opening (canal viscometer) as a function of time.
area that was occupied by calix[6]arenes 1-111 was 165 f 10 A2, 150 f 10 A2, and 155 f 10 A2 per molecule, respectively. These areas are in excellent agreement with a minimum area that is predicted from CPK models (154 A2/molecule),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;Le., the pore axis lies perpendicular to the water surface. The higher-compressibility of I is a likely consequence of a “surfacing”of some of the hydrophobic disulfide groups, alongside the hydrophobic calix[6larene core. These groups, however, appear to be effectively “tucked under” the calix[6larene core as the monolayer is compressed. Polymerization Behavior at the Gas-Water Interface. Brief UV treatment (254nm) of monolayers of I, which were maintained at asurface areaof 155A2/molecule, resulted in a decrease in surface pressure from 19 to 9.0
dyn/cm after 5 min (Figure 3A). Further UV treatment for 20 min resulted in a decrease in surface pressure of ca. 0.7dyn/cm (not shown). Exposureof this W-treated film to a 4-mm slit opening of a canal viscometer resulted in a negligible decrease in surface pressure after a 2-h period (Figure 3B). In striking contrast, the surface pressure of a nonirradiated monolayer of I fell to 0 dyn/cm after 5 min. These results imply that W irradiation leads to the formation of a cross-linkedassembly? Similarreeulta were obtained with 11. Thus, exposure of compressed monolayers of I1 to air for 20 min (basic subphase, pH 11) resulted in a modest reduction in surface pressure (9.78.7 dyn/cm) and a dramatic increase in surface viscosity (Figure4). Analogous results were obtained when a pure water subphase (pH5.5)was used,and the time of exposure to air was extended to 6.0 h. In contrast to I and 11, monolayers of I11 did not maintain surface coverage upon W irradiation. Thus, W treatment (254 nm)of I11 at the gas-water interface resulted in the complete logs of surface pressure after 13 min (Figure SA). Similar experiments that were carried out, in which the film was maintained at a constant surface pressure (18 dyn/cm), resulted in substantial film contraction; e.g., an initial surface area of 155 A2/molecule was reduced to 85 A2/moleculeafter 5 min of irradiation (Figure5B). Further W treatment reduced the occupied area to ca. 45 A2/molecule. Apparently, polymerization leads to a ‘buckling” of the film and to its eventualremoval from the air-water interface. Because of this twodimensional instability, I11 was not investigated further. Langmuir-Blodgett Multilayers. Calix[Glarene I was found to have excellent LB transfer characteristics. In particular, multilayers of I that were built on a silicon wafer gave transfer ratios of 1.06 f 0.13 (water to air) and 0.94 f 0.09 (air to water), using a surface preesure of 24 dyn/cm and a transfer speed of 0.8 cm/min.22 Film thickness measurements that were made by ellipsometry (Figure6) indicated an average monolayer thickness of 18 f 2 A, which is in excellent agreement with the thickness that is estimated from CPK models. Irradiation of a 21layered LB film for 7 min (254nm) led to an apparent decrease in overall film thickness of ca. 20% as judged by (22) The transfer ratio ia defined a~the decrease in monolayer area at the gas-water interface divided by the geometrical surface area of the substrate that paeees, vertically, through the interface.
Langmuir, Vol. 9, No. 9, 1993 2393
Perforated Monolayers
+
I
To Flowmeter
Gas In
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'Backing screen (when used)
Membrane
O-rings
5
0
10
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15
Time (minutes)
201
0
'
.
.
.
.
I
. .
'
I
20
10
.
. . ..
.
I 30
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Figure 5. (A, top) Influence of UV irradiation (254 nm) on the surface pressure of a monolayer of I11 that was maintained a t 155 A2/molecule. (B, bottom) Influence of UV irradiation (254 nm) on the area that is occupied per calixarene,while maintaining a surface pressure of 18.0 dyn/cm; a significant change in area was noted only during the time that the UV light was on.
A Number of Layers
Figure 6. Ellipsometric film thickness as a function of the number of monolayers of I that have been transferred to a silicon wafer at 24 dyn/cm. All thickness values represent the average of at least five measurements made on different sections of the silicon wafer.
ellipsometry. In contrast to a non-UV irradiated film, which was completelyremoved by washing with chloroform and methanol, only ca. 24 % of a UV-treated film could be removed by similar washing (estimated via ellipsometry). Further washings did not remove any additional quantity of film. The insolubilityof this UV-treated film is a likely consequence of cross-linking. Although I1 appeared to form reasonable Y-type multilayers, having transfer ratios of ca. 1.2 (water to air) and 0.8 (air to water), visual inspection of the substrate (glass
Table I. Flux of He, N2,and SFe across Membrane Compositesa mono- preslayers sure P/Z(cm3/(cm2-s*cmHg)) membrane ofIb (atm) He N2 sF6 0 0.03 2800 X lo-" 1100 X lo-" 540 X lo-" Celgard 6 0.01 680X lo-" 580X lo-" 320X lo-' Celgard/I 6 0.01 2100Xlo-" 9OOX lo-" 46OXlo-" Celgard/IC 14 0.01 41OXlo-" 390X lo-" 200Xlo-" Celgard/I 14 0.01 1500Xlo-" 650X lo-" 3 4 0 x 1 0 " Celgard/IC 26 0.03 3 1 x 1 0 " 2 4 X 10-4 1 4 X lo-" Celgard/I 26 0.03 100X lo-" 8 8 X lo-' 4 8 X l o - " Celgard/Ic 0 0.03 630X lo-" 250X lo-" 130X10-' Nuclepore 0.2 9.6 X lo-" 4.0 X lo-" 1.9 X lo-" Nuclepore/I 25 0.2 19OX lo-" 7 6 X 10-4 39x104 Nuclepore/IC 25 0 0.7 530X 10-6 540X 10-6 310Xlo-g PTMSP 12 0.7 96 X 10-6 5.8 X 10-6
