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Structures and binding of LB films of calix-8-arenes
Suprtr,,tok~~rkrr.%.kr,c~c~ 4 ( 1997) 20 I 206 1 I997 Elscvicr Science Ltd Printed in Great Britain. All rights reserved 096X-5677/97/$17.00
PII:SO968-5677(97)00005-9 ELSEVIER
Structures and binding of LB films of calix-8-arenes Ziad Ali-Adib*,
Frank Davis&, Philip Hedge* and Charles J. M. Stirlingtl
*Department of Chemistry, University 9PL, UK t Department of Chemistry, University (Received 6 September 1996; revised
of Manchester,
Oxford
Road, Manchester
of Sheffield, Brook Hill, Sheffield 14 January 1997)
M 13
S3 7HF, UK
A series of three calix-8-arenes have been successfully deposited as Langmuir-Blodgett films up to 100 layers thick and studied by means of a quartz crystal microbalance, X-ray diffraction and infra-red spectroscopy. These layers have been shown to gain weight when exposed to dilute aqueous solutions of caesium bromide over periods of up to several hours. From our results we deduce that the ions are adsorbed verticall:y through the film rather than in a transverse manner. The kinetics of adsorption are similar to those found for other LB systems. 0 1997 Elsevier Science Ltd. All rights reserved.
INTRODUCTION Calixarenes and the related resorcarene compounds are a class of versatile macrocyclic compounds with a variety of potential uses1-3. Calix-8-arenes are easily synthesized by the base-catalysed condensation of a para-t-alkyl-phenol with formaldehyde (Scheme Z)4. They can easily be modified at either the lower rim by esterification or etherification5, or at the upper rim by removal of the t-alkyl group and replacement with many different functional groups6. The compounds are usually chemically and thermally stable. One potential use of calixarenes comes about as a result of their ability to bind metal ions selectively, a property widely studied in organic and aqueous solutions7’8. Interest has also been shown in the ability of calixarene monolayers on water to selectively bind alkali metal’ or lanthanide” ions. Monolayers of a calix-6-arene have also been deposited and studied by atomic force microscopy’ ‘. There have also been studies of LB film formation by various calix4-arenes and their use as selective gas-permeable membranes12. Previous work in these groups has been concerned with the ability of various substituted calix-8-arenes to form good-quality LB films, both for use in and as selective-ion-binding pyroelectric devicesi lilms14. The Sheffield group has previously shown that LB films of calix-&arenes, 1-3, when immersed in aqueous solutions of mixed alkali metal salts, selectively bind ions14. X-ray photoelectron spectroscopy showed that $To
whom correspondence
should be addressed
CH,=O, NaOH p-xylene *
R
BrCH2C02CH$H3 K&03,
KC
Acetone. /
R
R
R
R
R
R
X = -CH2C02CH2CH3
X = -CH,COCH,
R = -C(CH3kCHzWH3)3 Scheme 1
Synthesis
of the calix-8-arenes
the larger alkali metals such as caesium were bound in preference to such ions as potassium. However, no work up to then had been done on either the structure of the LB films or the detailed kinetics of adsorption into the multilayer. This further work on the nature of
SUPRAMOLECULAR
SCIENCE Volume 4 Numbers 3-4 1997
201
LB films of calix-Sarenes:
Z. Ah-Adib
the LB films and the extents adsorption is now presented.
et al.
and
kinetics
of
EXPERIMENTAL The synthesis, isotherms and dipping conditions of calix-8-arenes l-3 were described in our earlier paper14. LB depositions were made on a trough previously described in reference 15. All monolayer and multilayer experiments were carried out on a pure water (Millipore) subphase, pH = 5.0-5.5, at 20°C with no added salts. Quartz crystal microbalance (QCM) measurements were made with a Racal Dana 1991 frequency counter, using an uncoated crystal in air as a reference. The 1OMHz crystals (Elchema) were 12mm in diameter and 0.017 mm in thickness, with the gold-plated electrode area being 19.6mm2. The crystals were contained in an aluminium box which acted as a Faraday cage. X-ray diffraction studies were carried out by using a Philips PWl380 horizontal diffractometer fitted with a graphite crystal monochromator and using the Cu K, line (wavelength 0.1542 nm). Infra-red (IR) spectra were measured in reflection on a Perkin-Elmer 1725X spectrometer with a Harrick reflection accessory (angle of incidence SO”) and an MCT detector. Spectra shown are the sum of 100 scans. The calix-8-arenes were spread from 0.51.Omg ml-’ solutions in chloroform onto a pure water subphase. Substrates were clean, hydrophobic silicon wafers, glass slides coated with 5 nm Cr and then 1OOnm Au, or QCM crystals. Typically, 100 layers were deposited for X-ray studies and 11 or 100 layers for QCM studies. The mass of the QCM crystals was measured prior to and after LB film deposition. The
ion-binding properties of the calix-8-arenes were studied by immersing the QCM crystals into aqueous solutions of various ions for fixed periods of time. With our equipment it proved impossible to make in situ measurements of the QCM frequency whilst in solution, because stable readings could not be obtained. Therefore the crystals were removed from the solution at set times, rinsed with clean water and dried in a desiccator overnight. Quick rinses caused no mass variation. These crystals then gave stable frequency readings. Samples could be stored in dry air for several weeks without any change in frequency. For each deposition, adsorption and measurement at least two QCM crystals were used simultaneously and longer drying times (30min after each bilayer) were used to ensure good deposition.
RESULTS Mono- and multilayers of the calix-8-arenes l-3
A similar material, t-butylcalix-8-arene, has been previously shown to adopt a flexible pinched-loop conformation at the air/water interface16. This conformation then stands vertically as shown in Figure 1, causing a reduction in the surface area. Also, calix4-arenes have been studied and shown to adopt a perpendicular arrangement at the interface. The isotherms of the calix-8-arenes are shown in Figure 2 and, as discussed in our earlier paper14, appear to show a perpendicular arrangement of the macrocycles to the air/water interface for 1 and 2 and a parallel arrangement for 3 (Figure I, predicted areas from CPK models). The isotherms of 2 and 3 were reproducible, indicating no aggregation upon
Subphase Parallelorientation, about 3.0 nmz
Area/m01 Figure 1
202
Possible horizontal
and vertical orientations
SUPRAMOLECULAR
of calix-8-arenes
SCIENCE Volume
Perpendicularorientation ~rea/mol about 1.8run2 in Langmuir monolayers
4 Numbers 3-4 1997
LB films of calix-8-arenes:
Z. Ali-Adib et al.
60
50 S %
40
i $
3o
! v1
*O 10
0
I 0
-
Calix 1
................... Cdix 2 .._..r_... C& 3
2
1
3
4
Surface area per molecule (nm*)
Figure 2
Isotherms of the calix-Sarenes
compression and expansion of the film providing the collapse pressure was not reached. Calix 1 also gave reproducible isotherms up to the plateau region at about 15mNm-‘; above this pressure the film became very rigid and visible aggregates appeared on the water surface, which remained upon re-expansion. The films could all be deposited under the conditions previously usedI (Table I) to form LB films of the required thickness. Deposition was Y-type with regular deposition ratios of 0.9-1.0. Grazing-angle IR measurements were made on samples that had been deposited on gold. X-ray diffraction studies were undertaken on loo-layer LB films deposited on hydrophobic Si wafers. Calix 3 gave the best layer structure, with two Bragg peaks giving a bilayer spacing of 3.4nm (Figure :I). This indicates a degree of order much less than for classical amphiphiles such as stearic acid, and of the same degree as that found for multilayers of preformed polymers’5. Exposure overnight to CsBr solution (2.5x low3 M) caused an increase in intensity (cu. 300%) probably due to the increase in electron density within the films caused by incorporation of the heavy caesium and bromine atoms. Calix 1 and 2 gave somewhat poorer diffraction, each giving two smaller and much broader Bragg peaks indicating spacings of 3.55 and 4.60nm, respectively. Again, these peaks increased in intensity upon exposure to CsBr. Table 1 Langmuir-Blodgett
deposition conditions
Calix-8-arene
Deposition pressure (mN m-‘)
Deposition speed (mm min-‘)
Drying time (min)
Weight per layer (ng)
1 2 3
12 26 26
13 15 15
5 5 2
65 43 68
Adsorption of ions into LBfilms of l-3 Samples studied were loo-layer LB films of the calix-8-arenes on QCM crystals. Previously reported results indicated that Cs was the ion preferentially incorporated into calix-8-arene LB filmsi and its higher RMM (relative molecular mass) was thought to make the measurements easier and more accurate; therefore CsBr was used for the majority of the ionadsorption measurements presented here. As a control, a QCM crystal coated with 100 layers of calix 3 was soaked in pure water. Measurement of its frequency then showed no mass increase. This indicates that the change in QCM reading is not simply due to a reorganization of the film upon soaking in water. Table 2 and Figure 4 show the change of weight on exposure of the dipped crystals to aqueous 2.5x 10p3M CsBr. As can be seen the three calix-8arenes give quite similar results, showing an initial fast adsorption of ions, slowing down somewhat with time. The weight increase even after 5 h was much less than that expected for formation of a 1:1 calix-ion pair complex. The kinetics of adsorption of calix 3, the most widely studied of our materials, are shown separately and appear to show an initial fast weight increase followed by a slower linear increase. A tentative explanation is that the initial fast, first-order adsorption is into the bowls of the surface calix-8arene molecules and those exposed by any defects in the surface layer. The slower zero-order adsorption occurs after all of the easily available sites are occupied and the rate-controlling step then becomes that of diffusion of CsBr pairs from these sites deeper into the film, leaving new sites for rapid adsorption of fresh CsBr. As the films do not appear to adsorb ions fully, presumably because of permeation effects, we
SUPRAMOLECULAR
SCIENCE Volume 4 Numbers 3-4 1997
203
LB films of calix-Sarenes: Z. Ali-Adib et al.
I
3.0
0”
I
I
2.0
1.0
I
3.0
from 100 layers of calix 3 on Si, before and after treatment
attempted to use thinner films of calixes on QCM crystals. However, we found that when bare QCM crystals were exposed to CsBr solution, they increased dramatically in weight. This increase was much faster than noted for crystals coated with thick LB films. It is thought that LB films ‘protect’ the QCM crystals
Table 2 Adsorption Calix 1
of CsBP by LB films of calixarenes Calix 2
1-3h
Calix 3
Time (min)
Wt increase (ng)
Time (min)
Wt increase (ng)
Time (min)
Wt increase (n)
30 120 300
41 149 220
10 30 60 180
19 25 66 124
10 20 30 60 150 330
39 63 70 92 126 187
“From aqueous ‘100 LB layers
204
2.5 x 10m3M CsBr
SUPRAMOLECULAR
2.0
1.1
After CsBr
Before CsBr Figure 3 X-ray diffraction
(.y
with CsBr
from CsBr. Similar effects have been previously noted for other compounds such as H2Se on bare gold”. It may not be possible to compare results for bare QCM and coated crystals in that adsorption of CsBr onto a bare crystal may arise by penetration of the ions into the actual gold electrode, thereby altering its properties. However, for coated crystals this effect will be negligible because, as shown by many authors, the frequency/weight increase relationship of LB films remains linear up to a large number of layers, and no adsorption into the actual electrode takes place. When thin (11 -layer) films of calixarenes 1 and 2 were dipped they also showed rapid adsorption, but 1l-layer films of calix 3 gave more reproducible adsorption and the results are given in Table 3. The increase is proportionately much larger than that of the loo-layer films, indicating that, in the thicker films, adsorption is only taking place in the outer layers. This result also indicates that adsorption on the time scale used is limited to the outer layers of the films. Were
SCIENCE Volume 4 Numbers 3-4 1997
LB films of calix-Sarenes:
Z. Ali-Adib et al.
0
220200160B E.
160140-
CaIix 1 0 Calix2
Cl
W Calix3
0
60
160
240
320
400
time (min) Figure 4
Weight increase of the 1004ayer LBs of the calix-8-arenes upon exposure to CsBr for varying lengths of time, curve fitting done for
cahx 3
Table 3
Adsorption of CsBr” by an I I-layer LB film of calixarene
3h Time (min) Wt increase (ng)
30 100
60 133
90 183
180 700‘
“From aqueous 2.5x 10m3 M CsBr hIncrease of 76 ng per layer “Probably a result of adsorption of ions into QCM crystal
adsorption of CsBr isotropic within the multilayer, then the thicker films would display much higher mass increases than the 1l-layer films. Previous studies on the adsorption of metal ions and species such as HzS and H$e by LB films of fatty acids indicated that the diffusion was parallel to the surface in the films, i.e. along the interface between the acid groups of alternate layers’7-‘9. This is reasonable because diffusion perpendicular to the substrate would be greatly hampered by the well-packed alkyl side chains of the fatty acids. Vertical diffusi’on was, however, thought to be a strong possibility in these calixarene LB films because of their porous nature, as seen by their permeability to certain gases12. Further evidence on diffusion in the layers was obtained by depositing identical loo-layer samples of calix 3 on two QCM crystals. For one of the crystals, that part of the crystal surrounding the central gold electrode was cleaned. The difference in readings between the two crystals before and after exposure to CsBr was small. Because only the mass of the film actually on the g,old surface is measured2’ this suggests that lateral diffusion does not generally occur, as we would expect a much faster apparent initial adsorption for a cleaned crystal due to ions not having to diffuse as far from outside the boundary of the electrode. In the vertical diffusion mode, whether this is through or between the calixarene bowls is not clear. Besides QCM and X-ray measurements, we also attempted to use IR-visible ions such as sodium acetate, azide and nitrate. Although NaCl has been
previously shown14 to be adsorbed by LB films of 1, 2 and 3, no adsorption of any of the ions could be detected by IR, even when more concentrated solutions (0.1 M) and longer exposure times, up to 72 h, were used. The spectra of the films remained constant, showing the LB film was not removed from the substrate.
DISCUSSION The kinetics of adsorption of CsBr into the loo-layer LB films shows an initial rapid adsorption of ions that slows down with time. For calix 3 the adsorption/time plot shows what could be a two-stage process. The first is a first-order process for which the weight increase is due to binding of CsBr into easily available calixarene sites such as those of the top layer and those calixarenes exposed by defects. Once these have been tilled, further adsorption appears to be a zeroorder process in which ions diffuse from these sites into other less exposed calixarenes and more CsBr is then adsorbed by the vacated surface calixarenes. Comparison of the weight increase on deposition with that after adsorption of CsBr for calixarene 3 indicated an average distribution of 0.28 ion pairs per calixarene. This indicates that even after 5 h adsorption is not complete. This agrees with earlier work on multilayers which indicates that adsorption in LB films takes place ~lowly’~, e.g. over 6h for total conversion of nine layers of stearic acid into cupric stearate”. Exposures longer than 5 h were not attempted as the electrode contacts showed a tendency to corrode and affect the final readings. The proportionately much greater adsorption of ions by thinner LB films is probably an artefact caused by adsorption of CsBr into the QCM crystal. X-ray results show that films 1 and 2 are disordered and probably contain defects, allowing ions to penetrate. Films of calix
SUPRAMOLECULAR
SCIENCE Volume 4 Numbers 3-4 1997
205
LB films of calix-8-arenes:
Z. Ali-Adib
et al.
3 are probably
more ordered, as indicated by X-ray results, and contain fewer defects. Assuming that the weight increase after 90min is totally due to complexation within the LB of calix 3 gives a figure of 2.25 CsBr pairs per calix-8arene. Calix-Carenes are known to form 1:l complexes with Cs in bulk2’ and calix-8-arenes form 1:2 complexes with lanthanide ions**. This apparent greater adsorption rate for thin films indicates that adsorption is limited to the outer layers of the thicker loo-layer films. The stability of the IR spectra of the calixes on exposure to aqueous nitrate, azide or acetate confirms that the films are unaffected by water and that bulkier anions such as nitrate cannot penetrate the matrix as readily as spherical halide ions.
REFERENCES I. 2. 3. 4. 5. 6.
I. 8.
9.
CONCLUSIONS
10.
Calix-8-arenes are easily deposited as LB films. Thick multilayers of these systems slowly and reproducibly adsorbed CsBr from dilute aqueous solutions. The diffusion is thought to be perpendicular to the substrate and takes place mainly in the outer layers. Bulky anions such as acetate are shown by IR not to be adsorbed, whereas the QCM results presented here and previous XPS resultsI show that halide ions are. The wide possible variations in calixarene structure and the rapid initial responses of the films indicate the possibility of using these systems as selective detectors for a wide range of ions.
11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
ACKNOWLEDGEMENTS We thank the University of Sheffield and EPSRC for funding.
206
SUPRAMOLECULAR
SCIENCE Volume 4 Numbers
22.
Gutsche, C. D., Culixarenes. Monographs in Supramolecular Chemistry, Royal Society of Chemistry, 1989. Vicens, J. and Bohmer, V. (eds), Calixarenes, A Versatile Class ofMacrocyclicCompounds. Kluwer, Dordrect, 1991. Bohmer, V., Angew. Chem. Int. Edn, 199535, 713. Cornforth. J. W., D’Arcy Hart, P., Nicholls, G. A., Rees, R. J. W. and Stock, J. A., Br. J. Pharmacol.. 1955, 10, 13. Arduini, A., Pochini, A., Reverberi, S. and Ungaro, R., J. Chem. Sot., Chem. Commun., 1984,981. Vanloon, J. D., Arduini, A., Coppi, L., Verboom, W., Pochini, A., Ungaro, R., Harkema, S. and Reinhoudt, D. N., J. Org. Chem., 1990,55, 5639. Izatt, S. R., Hawkins, R. T., Christensen, J. J. and Izatt, R. M., J. Am. Chem. Sot. 1985, 107.63. Arnaud-Neu, F., Collins, E. M., Deasy, M., Ferguson, G., Harris, S. J., Kaitner, B., Lough, A. J., McKervey, M. A., Marques, E., Ruhl, B. L., Schwing-Weill, M. J. and Seward, E. M.. J. Am. Chem. Sot., 1989.111.8681. Dei, L., Casnati, A., Lonostro, P. and Baglioni, P., Langmuir, 1995, 11, 1268. Ludwig, R., Matsumoto, H., Takeshita, M., Ueda, K. and Shinkai, S., Supramol. Chem., 1995,4, 319. Namba, M., Sugawara, M., Buhlmann, P. and Umeza, W. A., Langmuir, 1995, 11, 635. Dedek, P., Webber, A. S., Janout, V., Hendel, R. A. and Regen, S. L., Langmuir. 1994, 10, 3943. Richardson, T., Greenwood, M. B., Davis, F. and Stirling, C. J. M., Langmuir, 1995, 11, 4623. Davis, F., O’Toole, L., Short, R. and Stirling, C. J. M., Langmuir, 1996, 12, 1892. Tredgold, R. H., Rep. Progr. Phys.. 1987, SO, 1609. Lonestro, P., Casnati, A., Bossoletti, L., Dei, L. and Baglioni, P.. Colloids Surf. A. 1996. 116. 203. Urquhart, R. S., Furlong, D. N., Gengenbach, T., Geddes, N. J. and Griest, F., Langmuir, 1995, 11, 1127. Yuan, C. W., Wu, C. R., Bai, J. J., Yang, W. Y. and Wei, Y., Langmuir, 1995, 11, 5. Chen, H. J., Chai, X. D., Wei, Q., Jiang, Y. H. and Li, T. J., Thin Solid Films, 1989, 178, 535. Ebara, Y. and Okahata, Y., Langmuir, 1993, 19, 574. Harrowfield, J. M., Ogden, M. I., Richmond, W. R. and White, A. H., J. Chem. Sot., Chem. Commun., 1991, 1159. Furphy, B. M., Harrowfield, J. M., Kepert, D. L., Skelton, B. W., White, A. H. and Wilner, F. R., Inorg. Chem., 1987, 26, 4231.
3-4 1997
PII:SO968-5677(97)00005-9 ELSEVIER
Structures and binding of LB films of calix-8-arenes Ziad Ali-Adib*,
Frank Davis&, Philip Hedge* and Charles J. M. Stirlingtl
*Department of Chemistry, University 9PL, UK t Department of Chemistry, University (Received 6 September 1996; revised
of Manchester,
Oxford
Road, Manchester
of Sheffield, Brook Hill, Sheffield 14 January 1997)
M 13
S3 7HF, UK
A series of three calix-8-arenes have been successfully deposited as Langmuir-Blodgett films up to 100 layers thick and studied by means of a quartz crystal microbalance, X-ray diffraction and infra-red spectroscopy. These layers have been shown to gain weight when exposed to dilute aqueous solutions of caesium bromide over periods of up to several hours. From our results we deduce that the ions are adsorbed verticall:y through the film rather than in a transverse manner. The kinetics of adsorption are similar to those found for other LB systems. 0 1997 Elsevier Science Ltd. All rights reserved.
INTRODUCTION Calixarenes and the related resorcarene compounds are a class of versatile macrocyclic compounds with a variety of potential uses1-3. Calix-8-arenes are easily synthesized by the base-catalysed condensation of a para-t-alkyl-phenol with formaldehyde (Scheme Z)4. They can easily be modified at either the lower rim by esterification or etherification5, or at the upper rim by removal of the t-alkyl group and replacement with many different functional groups6. The compounds are usually chemically and thermally stable. One potential use of calixarenes comes about as a result of their ability to bind metal ions selectively, a property widely studied in organic and aqueous solutions7’8. Interest has also been shown in the ability of calixarene monolayers on water to selectively bind alkali metal’ or lanthanide” ions. Monolayers of a calix-6-arene have also been deposited and studied by atomic force microscopy’ ‘. There have also been studies of LB film formation by various calix4-arenes and their use as selective gas-permeable membranes12. Previous work in these groups has been concerned with the ability of various substituted calix-8-arenes to form good-quality LB films, both for use in and as selective-ion-binding pyroelectric devicesi lilms14. The Sheffield group has previously shown that LB films of calix-&arenes, 1-3, when immersed in aqueous solutions of mixed alkali metal salts, selectively bind ions14. X-ray photoelectron spectroscopy showed that $To
whom correspondence
should be addressed
CH,=O, NaOH p-xylene *
R
BrCH2C02CH$H3 K&03,
KC
Acetone. /
R
R
R
R
R
R
X = -CH2C02CH2CH3
X = -CH,COCH,
R = -C(CH3kCHzWH3)3 Scheme 1
Synthesis
of the calix-8-arenes
the larger alkali metals such as caesium were bound in preference to such ions as potassium. However, no work up to then had been done on either the structure of the LB films or the detailed kinetics of adsorption into the multilayer. This further work on the nature of
SUPRAMOLECULAR
SCIENCE Volume 4 Numbers 3-4 1997
201
LB films of calix-Sarenes:
Z. Ah-Adib
the LB films and the extents adsorption is now presented.
et al.
and
kinetics
of
EXPERIMENTAL The synthesis, isotherms and dipping conditions of calix-8-arenes l-3 were described in our earlier paper14. LB depositions were made on a trough previously described in reference 15. All monolayer and multilayer experiments were carried out on a pure water (Millipore) subphase, pH = 5.0-5.5, at 20°C with no added salts. Quartz crystal microbalance (QCM) measurements were made with a Racal Dana 1991 frequency counter, using an uncoated crystal in air as a reference. The 1OMHz crystals (Elchema) were 12mm in diameter and 0.017 mm in thickness, with the gold-plated electrode area being 19.6mm2. The crystals were contained in an aluminium box which acted as a Faraday cage. X-ray diffraction studies were carried out by using a Philips PWl380 horizontal diffractometer fitted with a graphite crystal monochromator and using the Cu K, line (wavelength 0.1542 nm). Infra-red (IR) spectra were measured in reflection on a Perkin-Elmer 1725X spectrometer with a Harrick reflection accessory (angle of incidence SO”) and an MCT detector. Spectra shown are the sum of 100 scans. The calix-8-arenes were spread from 0.51.Omg ml-’ solutions in chloroform onto a pure water subphase. Substrates were clean, hydrophobic silicon wafers, glass slides coated with 5 nm Cr and then 1OOnm Au, or QCM crystals. Typically, 100 layers were deposited for X-ray studies and 11 or 100 layers for QCM studies. The mass of the QCM crystals was measured prior to and after LB film deposition. The
ion-binding properties of the calix-8-arenes were studied by immersing the QCM crystals into aqueous solutions of various ions for fixed periods of time. With our equipment it proved impossible to make in situ measurements of the QCM frequency whilst in solution, because stable readings could not be obtained. Therefore the crystals were removed from the solution at set times, rinsed with clean water and dried in a desiccator overnight. Quick rinses caused no mass variation. These crystals then gave stable frequency readings. Samples could be stored in dry air for several weeks without any change in frequency. For each deposition, adsorption and measurement at least two QCM crystals were used simultaneously and longer drying times (30min after each bilayer) were used to ensure good deposition.
RESULTS Mono- and multilayers of the calix-8-arenes l-3
A similar material, t-butylcalix-8-arene, has been previously shown to adopt a flexible pinched-loop conformation at the air/water interface16. This conformation then stands vertically as shown in Figure 1, causing a reduction in the surface area. Also, calix4-arenes have been studied and shown to adopt a perpendicular arrangement at the interface. The isotherms of the calix-8-arenes are shown in Figure 2 and, as discussed in our earlier paper14, appear to show a perpendicular arrangement of the macrocycles to the air/water interface for 1 and 2 and a parallel arrangement for 3 (Figure I, predicted areas from CPK models). The isotherms of 2 and 3 were reproducible, indicating no aggregation upon
Subphase Parallelorientation, about 3.0 nmz
Area/m01 Figure 1
202
Possible horizontal
and vertical orientations
SUPRAMOLECULAR
of calix-8-arenes
SCIENCE Volume
Perpendicularorientation ~rea/mol about 1.8run2 in Langmuir monolayers
4 Numbers 3-4 1997
LB films of calix-8-arenes:
Z. Ali-Adib et al.
60
50 S %
40
i $
3o
! v1
*O 10
0
I 0
-
Calix 1
................... Cdix 2 .._..r_... C& 3
2
1
3
4
Surface area per molecule (nm*)
Figure 2
Isotherms of the calix-Sarenes
compression and expansion of the film providing the collapse pressure was not reached. Calix 1 also gave reproducible isotherms up to the plateau region at about 15mNm-‘; above this pressure the film became very rigid and visible aggregates appeared on the water surface, which remained upon re-expansion. The films could all be deposited under the conditions previously usedI (Table I) to form LB films of the required thickness. Deposition was Y-type with regular deposition ratios of 0.9-1.0. Grazing-angle IR measurements were made on samples that had been deposited on gold. X-ray diffraction studies were undertaken on loo-layer LB films deposited on hydrophobic Si wafers. Calix 3 gave the best layer structure, with two Bragg peaks giving a bilayer spacing of 3.4nm (Figure :I). This indicates a degree of order much less than for classical amphiphiles such as stearic acid, and of the same degree as that found for multilayers of preformed polymers’5. Exposure overnight to CsBr solution (2.5x low3 M) caused an increase in intensity (cu. 300%) probably due to the increase in electron density within the films caused by incorporation of the heavy caesium and bromine atoms. Calix 1 and 2 gave somewhat poorer diffraction, each giving two smaller and much broader Bragg peaks indicating spacings of 3.55 and 4.60nm, respectively. Again, these peaks increased in intensity upon exposure to CsBr. Table 1 Langmuir-Blodgett
deposition conditions
Calix-8-arene
Deposition pressure (mN m-‘)
Deposition speed (mm min-‘)
Drying time (min)
Weight per layer (ng)
1 2 3
12 26 26
13 15 15
5 5 2
65 43 68
Adsorption of ions into LBfilms of l-3 Samples studied were loo-layer LB films of the calix-8-arenes on QCM crystals. Previously reported results indicated that Cs was the ion preferentially incorporated into calix-8-arene LB filmsi and its higher RMM (relative molecular mass) was thought to make the measurements easier and more accurate; therefore CsBr was used for the majority of the ionadsorption measurements presented here. As a control, a QCM crystal coated with 100 layers of calix 3 was soaked in pure water. Measurement of its frequency then showed no mass increase. This indicates that the change in QCM reading is not simply due to a reorganization of the film upon soaking in water. Table 2 and Figure 4 show the change of weight on exposure of the dipped crystals to aqueous 2.5x 10p3M CsBr. As can be seen the three calix-8arenes give quite similar results, showing an initial fast adsorption of ions, slowing down somewhat with time. The weight increase even after 5 h was much less than that expected for formation of a 1:1 calix-ion pair complex. The kinetics of adsorption of calix 3, the most widely studied of our materials, are shown separately and appear to show an initial fast weight increase followed by a slower linear increase. A tentative explanation is that the initial fast, first-order adsorption is into the bowls of the surface calix-8arene molecules and those exposed by any defects in the surface layer. The slower zero-order adsorption occurs after all of the easily available sites are occupied and the rate-controlling step then becomes that of diffusion of CsBr pairs from these sites deeper into the film, leaving new sites for rapid adsorption of fresh CsBr. As the films do not appear to adsorb ions fully, presumably because of permeation effects, we
SUPRAMOLECULAR
SCIENCE Volume 4 Numbers 3-4 1997
203
LB films of calix-Sarenes: Z. Ali-Adib et al.
I
3.0
0”
I
I
2.0
1.0
I
3.0
from 100 layers of calix 3 on Si, before and after treatment
attempted to use thinner films of calixes on QCM crystals. However, we found that when bare QCM crystals were exposed to CsBr solution, they increased dramatically in weight. This increase was much faster than noted for crystals coated with thick LB films. It is thought that LB films ‘protect’ the QCM crystals
Table 2 Adsorption Calix 1
of CsBP by LB films of calixarenes Calix 2
1-3h
Calix 3
Time (min)
Wt increase (ng)
Time (min)
Wt increase (ng)
Time (min)
Wt increase (n)
30 120 300
41 149 220
10 30 60 180
19 25 66 124
10 20 30 60 150 330
39 63 70 92 126 187
“From aqueous ‘100 LB layers
204
2.5 x 10m3M CsBr
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2.0
1.1
After CsBr
Before CsBr Figure 3 X-ray diffraction
(.y
with CsBr
from CsBr. Similar effects have been previously noted for other compounds such as H2Se on bare gold”. It may not be possible to compare results for bare QCM and coated crystals in that adsorption of CsBr onto a bare crystal may arise by penetration of the ions into the actual gold electrode, thereby altering its properties. However, for coated crystals this effect will be negligible because, as shown by many authors, the frequency/weight increase relationship of LB films remains linear up to a large number of layers, and no adsorption into the actual electrode takes place. When thin (11 -layer) films of calixarenes 1 and 2 were dipped they also showed rapid adsorption, but 1l-layer films of calix 3 gave more reproducible adsorption and the results are given in Table 3. The increase is proportionately much larger than that of the loo-layer films, indicating that, in the thicker films, adsorption is only taking place in the outer layers. This result also indicates that adsorption on the time scale used is limited to the outer layers of the films. Were
SCIENCE Volume 4 Numbers 3-4 1997
LB films of calix-Sarenes:
Z. Ali-Adib et al.
0
220200160B E.
160140-
CaIix 1 0 Calix2
Cl
W Calix3
0
60
160
240
320
400
time (min) Figure 4
Weight increase of the 1004ayer LBs of the calix-8-arenes upon exposure to CsBr for varying lengths of time, curve fitting done for
cahx 3
Table 3
Adsorption of CsBr” by an I I-layer LB film of calixarene
3h Time (min) Wt increase (ng)
30 100
60 133
90 183
180 700‘
“From aqueous 2.5x 10m3 M CsBr hIncrease of 76 ng per layer “Probably a result of adsorption of ions into QCM crystal
adsorption of CsBr isotropic within the multilayer, then the thicker films would display much higher mass increases than the 1l-layer films. Previous studies on the adsorption of metal ions and species such as HzS and H$e by LB films of fatty acids indicated that the diffusion was parallel to the surface in the films, i.e. along the interface between the acid groups of alternate layers’7-‘9. This is reasonable because diffusion perpendicular to the substrate would be greatly hampered by the well-packed alkyl side chains of the fatty acids. Vertical diffusi’on was, however, thought to be a strong possibility in these calixarene LB films because of their porous nature, as seen by their permeability to certain gases12. Further evidence on diffusion in the layers was obtained by depositing identical loo-layer samples of calix 3 on two QCM crystals. For one of the crystals, that part of the crystal surrounding the central gold electrode was cleaned. The difference in readings between the two crystals before and after exposure to CsBr was small. Because only the mass of the film actually on the g,old surface is measured2’ this suggests that lateral diffusion does not generally occur, as we would expect a much faster apparent initial adsorption for a cleaned crystal due to ions not having to diffuse as far from outside the boundary of the electrode. In the vertical diffusion mode, whether this is through or between the calixarene bowls is not clear. Besides QCM and X-ray measurements, we also attempted to use IR-visible ions such as sodium acetate, azide and nitrate. Although NaCl has been
previously shown14 to be adsorbed by LB films of 1, 2 and 3, no adsorption of any of the ions could be detected by IR, even when more concentrated solutions (0.1 M) and longer exposure times, up to 72 h, were used. The spectra of the films remained constant, showing the LB film was not removed from the substrate.
DISCUSSION The kinetics of adsorption of CsBr into the loo-layer LB films shows an initial rapid adsorption of ions that slows down with time. For calix 3 the adsorption/time plot shows what could be a two-stage process. The first is a first-order process for which the weight increase is due to binding of CsBr into easily available calixarene sites such as those of the top layer and those calixarenes exposed by defects. Once these have been tilled, further adsorption appears to be a zeroorder process in which ions diffuse from these sites into other less exposed calixarenes and more CsBr is then adsorbed by the vacated surface calixarenes. Comparison of the weight increase on deposition with that after adsorption of CsBr for calixarene 3 indicated an average distribution of 0.28 ion pairs per calixarene. This indicates that even after 5 h adsorption is not complete. This agrees with earlier work on multilayers which indicates that adsorption in LB films takes place ~lowly’~, e.g. over 6h for total conversion of nine layers of stearic acid into cupric stearate”. Exposures longer than 5 h were not attempted as the electrode contacts showed a tendency to corrode and affect the final readings. The proportionately much greater adsorption of ions by thinner LB films is probably an artefact caused by adsorption of CsBr into the QCM crystal. X-ray results show that films 1 and 2 are disordered and probably contain defects, allowing ions to penetrate. Films of calix
SUPRAMOLECULAR
SCIENCE Volume 4 Numbers 3-4 1997
205
LB films of calix-8-arenes:
Z. Ali-Adib
et al.
3 are probably
more ordered, as indicated by X-ray results, and contain fewer defects. Assuming that the weight increase after 90min is totally due to complexation within the LB of calix 3 gives a figure of 2.25 CsBr pairs per calix-8arene. Calix-Carenes are known to form 1:l complexes with Cs in bulk2’ and calix-8-arenes form 1:2 complexes with lanthanide ions**. This apparent greater adsorption rate for thin films indicates that adsorption is limited to the outer layers of the thicker loo-layer films. The stability of the IR spectra of the calixes on exposure to aqueous nitrate, azide or acetate confirms that the films are unaffected by water and that bulkier anions such as nitrate cannot penetrate the matrix as readily as spherical halide ions.
REFERENCES I. 2. 3. 4. 5. 6.
I. 8.
9.
CONCLUSIONS
10.
Calix-8-arenes are easily deposited as LB films. Thick multilayers of these systems slowly and reproducibly adsorbed CsBr from dilute aqueous solutions. The diffusion is thought to be perpendicular to the substrate and takes place mainly in the outer layers. Bulky anions such as acetate are shown by IR not to be adsorbed, whereas the QCM results presented here and previous XPS resultsI show that halide ions are. The wide possible variations in calixarene structure and the rapid initial responses of the films indicate the possibility of using these systems as selective detectors for a wide range of ions.
11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
ACKNOWLEDGEMENTS We thank the University of Sheffield and EPSRC for funding.
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