Langmuir 2001, 17, 6851-6854
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Anion and Cation Interactions with p-Dodecanoylcalix[4]arene Monolayers at the Air-Water Interface Patrick Shahgaldian and Anthony W. Coleman* Institut de Biologie et Chimie des Prote´ ines, CNRS UMR 5086, 7 passage du Vercors, F-69367 Lyon cedex 07, France Received April 17, 2001. In Final Form: July 12, 2001 The Langmuir compression isotherms of p-dodecanoylcalix[4]arene have been studied on subphases containing varying concentrations of group IA salts. The effect of anions on the stability of the monolayers shows that, for acetate and bicarbonate, there is a large increase in collapse pressure, when sodium is the countercation. In the case of cation effects, rubidium chloride shows a large stabilization of the monolayer.
Introduction The calix[n]arenes are a versatile class of macrocyclic host compounds.1 They are produced by the base-catalyzed reaction of formaldehyde with p-tert-butylphenol,2 and their “basket-shaped” structure is the basis of their widespread use in molecular recognition. A wide range of modified calix[n]arenes are now available for the selective complexation for small organic molecules and ions in solution.3 Also, recently, the use of calix[n]arenes or resorcinol-arenes as selective extractants for ions at the interfaces has undergone considerable expansion.4 At the solid-gas interface, films of various calix[n]arenes or resorcinol-arenes have been deposited by Langmuir-Blodgett,5 spin-coating,6 and vacuum thermal deposition7 techniques; such films have found numerous applications as molecular sensors. Ro¨sler et al.8 have reported on the development of a calix[n]arene-based sensor quartz crystal microbalance (QCM) for the detection of volatile organic compounds (VOCs) at the gas-solid interface. Thermal vapor deposited films have also been used in surface plasmon resonance (SPR) sensors, and Nabok et al.9 have published a study on the vapor phase detection of organic molecules using tetraphosphorylated resorcinol-arene LB films. Field effect transistors (FET) based on calix[n]arenes have been used for the detection of heavy metal ions.10 Langmuir-Blodgett films of calix* Corresponding author. E-mail:
[email protected]. (1) Vicens, J.; Bohmer, V. Calixarenes: A Versatile Class of Macrocyclic Compounds; Kluwer Academic Press: Boston, MA, 1991. (2) Gutsche, C. D.; Muthukrishnan, R. J. Org. Chem. 1978, 43, 4905. (3) Gutsche, C. D. Monographs in Supramolecular Chemistry, Calixarenes Revisited; Stoddart, J. F., Ed.; The Royal Society of Chemistry: Cambridge, U.K., 1998. (4) Chang, S.-K.; Cho, I. J. Chem. Soc., Perkin Trans. 1 1986, 211. Jin, T.; Mastaka, K.; Koyama, T.; Kobayashi, Y.; Hirata, H. Langmuir 1996, 12, 2684. Arena, G.; Contino, A.; Magri, A.; Sciotto, D.; Spoto, G.; Torrisi, A. Ind. Eng. Chem. Res. 2000, 39, 3605. (5) Davis, F.; O’Toole, L.; Short, R.; Stirling, J. M. Langmuir 1996, 12, 1892. Kim, J.-H.; Kim, Y.-G.; Lee, K.-H.; Kang, S.-W.; Koh, K.-N. Synth. Met. 2001, 117, 145 (6) Hassan, A. K.; Nabok, A. V., Ray, A. K.; Lucke, A. Smith, K.; Stirling, C. J. M.; Davis, F. Mater. Sci. Eng., C 1999, 8-9, 251. (7) Mlika, R.; Gamoudi, M.; Guillaud, G.; Charbonnier, M.; Romand, M.; Davenas, J.; Jaffrezic-Renault, N.; Lamartine, R.; Touhami, A. Mater. Sci. Eng., C 2000, 11, 129. (8) Ro¨sler, S.; Lucklum, R.; Borngra¨ber, R.; Hartmann, J.; Hauptmann, P. Sens. Actuators, B 1998, 48, 415. (9) Nabok, A. V.; Hassan, A. K.; Ray, A. K.; Omar, O.; Kalchenko, V. I. Sens. Actuators, B 1997, 45, 115.
arenes have also been used by Regen et al. for the fabrication of microporous membranes that function as molecular sieves.11 A number of studies of the formation of Langmuir monolayers at the air-water interface have been undertaken. Generally the presence of alkyl or acyl chains para to the phenolic hydroxyl groups induces a stabilization of the monolayers.12-22 Their interaction with cations and organic molecules has received attention. The first study of metal ion complexation by a calix[n]arene monolayer at the air-water interface was reported by Ishikawa et al.,12 and selectivities of p-tert-butylcalix[4]arene tetraester for the sodium ion and p-tert-butylcalix[6]arene tetraester for the potassium ion were demonstrated. More recent studies have shown the selective complexation of p-tert-butylcalix[6]arene hexamide with the guanidinium ion13 and p-tert-butylcalix[6]arene with the cesium ion.14 Capuzi et al. have recently reported selectivity for the cesium ion by 1,3-dioctyloxycalix[4]arenecrown-6-ether at the air-water interface and in micellar solutions.15 Other studies have shown the molecular recognition of organic compounds, including carbohydrates,16 aromatic ammonium salts17 and polyions.18 Esker et al.19 studied the stability of p-dioactadecylcalix[4]arene monolayers and the effect of subphase ions on (10) Cobben, P. L. H. M.; Egberink, R. J. M.; Bomer, J. G.; Bergved, P.; Verboom, W.; Reinhoudt, D. N. J. Am. Chem. Soc. 1992, 114, 10573. (11) Conner, M.; Janout, V.; Regen, S. L. J. Am. Chem. Soc. 1993, 115, 1178. (12) Ishikawa, Y.; Kunitake, T.; Matsuda, T.; Otsuka, T.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1989, 736. (13) Dei, L.; Casnati, A.; Lo Nostro, P.; Baglioni, P. Langmuir 1995, 11, 1268. (14) Lo Nostro, P.; Casnati, A.; Bossoletti, L.; Dei, L.; Baglioni, P. Colloids Surf., A 1996, 116, 203. (15) Capuzzi, G.; Fratini, E.; Dei, L.; LoNostro P.; Casnati, A.; Gilles, R.; Baglioni, P. Colloids Surf., A 2000, 167, 105. (16) Kurihara, K.; Ohto, K.; Tanaka, Y.; Aoyama, Y.; Kunitake, T. J. Am. Chem. Soc. 1991, 113, 444. (17) Tyson, J. C.; Moore, J. L.; Hughes, K. D.; Collard, D. M. Langmuir 1997, 13, 2068. (18) Zhang, L.-H.; Esker, A. R.; No, K.; Yu, H. Langmuir 1999, 15, 1726. (19) Esker, A. R.; Zhang, L.-H.; Olsen, C. E.; No, K.; Yu, H. Langmuir 1999, 15, 1716. (20) Shahgaldian, P.; Coleman, A. W.; Kalchenko, V. I. Tetrahedron Lett. 2001, 42, 577. (21) Mehri, G.; Munoz, M.; Coleman, A. W.; Barrat, G. Supramol. Chem. 1995, 5, 173. (22) Nakamoto, Y.; Kallinowski, G.; Bohmer, V.; Vogt, W. Langmuir 1989, 5, 1116.
10.1021/la010562b CCC: $20.00 © 2001 American Chemical Society Published on Web 10/06/2001
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Shahgaldian and Coleman
Figure 1. Schematic structure of p-dodecanoylcalix[4]arene (1).
these monolayers. For the monovalent cations Na+ and K+, no apparent effects on film stability were observed. In this publication, we present a systematic study of the absorption and interaction with anions at the airwater interface, with p-dodecanoylcalix[4]arene, 1, and sodium as the countercation. The study was undertaken at variable salt concentration, and the effects on both the collapse pressure and the apparent molecular area of the film are presented. The work is extended to give comparative data concerning effects of group IA, NH4+, and H+ cations with Cl- as the counteranion. It is shown that both anions and cations have significant effects on the layer stability. With regard to anion interaction, acetate and bicarbonate lead to increased stability, in terms of higher collapse pressure (πc) values, while tetrafluoroborate slightly destabilizes the monolayer. It will, also, be shown that rubidium gives rise to the largest monolayer stabilization. Experimental Section p-Dodecanoylcalix[4]arene, 1, was synthesized as previously described.20 All chemicals were purchased from Acros Organics and used without further purification. Subphases were prepared with deionized water purified with a Millipore MiliQ water system to obtain a resistivity of at least 18 MΩ cm. Spreading solutions were prepared by dissolving a known quantity (5-6 mg) of p-dodecanoylcalix[4]arene in 5 mL of chloroform (HPLC grade) and stored at -15 °C to prevent solvent evaporation. Solutions were changed at least every 3 days. Measurements were carried out in a Teflon trough of 400 mL. Solutions of p-dodecanoylcalix[4]arene were deposited as a volume of 30 µL with a micropipet (Gilson) at the air-water interface. A period of 30 min was allowed for the solvent evaporation and equilibration. Isotherms were carried out on a Langmuir type balance (Nima 610). Compressions were performed continuously at a rate of 20 cm2 min-1 from 510 cm2 to 50 cm2. Each sample was run at least three times to ensure reproducibility of results (deviation of area and pressure were less than 3%). To confirm the absence of surfactant contaminants in the salts, solutions at 1 M concentration were allowed to stand in the Langmuir trough overnight, and then the system was compressed; in no case was a change in surface pressure observed.
Results and Discussion The molecular structure of 1 is given in Figure 1. The compression isotherm for 1 on a pure water surface is presented in Figure 2, and the apparent molecular area at the collapse pressure of 15.2 mN/m is 106 Å2. These results are in close agreement with the area, 100 Å2, obtained by extrapolation for a calix[4]arene carrying four stearoyl functions at the upper rim and bearing one ester function at the lower rim.21 The stability of the monolayer is in accord with the work of Esker et al.,19 on dioctadecanoylcalix[4]arene, where a collapse pressure of 8 mN/m was observed, and in contrast to the work of Nakamoto et al.,22 where no stable layer was observed for a p-tetraalkyl-substituted calix[4]arene on a pure water subphase. The hydrophilic-hydrophobic balance in the
Figure 2. Langmuir isotherm of 1 on pure water. Table 1. Isotherm Data for 1 in the Presence of Group IA Cationsa salt LiCl NaCl KCl RbCl CsCl NH4Cl HCl
collapse collapse pressure (mN m-1) area (Å2) Alim (Å2) A0 (Å2) A1 (Å2) 27.0 26.3 26.8 32.4 26.5 23.4 14.8
112 105 103 110 109 122 105
148 141 130 147 148 160 123
170 170 140 156 156 196 156
152 152 133 150 150 174 145
a A lim is the extrapolated molecular area, A0 is the apparent molecular area at π ) 0, and A1 is the apparent molecular area at π ) 1 mN/m.
case of 1 is maintained by a larger number of shorter chains as compared to the molecules studied by Esker et al. and only a reduction in chain length compared to the molecule studied by Nakamoto.22 The aspect of the isotherm and in particular the rapid “takeoff” of the surface pressure shows that the monolayer is relatively rigid. The determination of the compressibility modulii (formula 1) Cs-1 ) 115 mN/m shows that the monolayer is in a liquid condensed phase.23
Cs-1 ) -A(δπ/δA)
(1)
In Table 1 are summarized the isotherm data for 1 on pure water and in the presence in the subphase, at a concentraton of 0.2 M of the monovalent cations Li+, Na+, K+, Rb+, Cs+, and NH4+; the data for varying concentrations of HCl in the subphase are given for comparison. In all cases the counterion is Cl-. In Figure 3 are presented the variation of the collapse pressures (πc) for the monovalent cations, as a function of salt concentration. No effective variation in the apparent molecular area of 1 at the collapse pressure is observed, even in the presence of 0.2 M concentration of monovalent cations in the subphase. No significant variations in compressibility modulii are observed (data not shown). This contrasts with the previously reported work of Ishikawa et al.,12 where with other amphiphilic calix[n]arenes variations in the molecular area were reported in the presence of salts in the subphase. The authors reported that for the monolayers of p-tert-butylcalix[4]arene tetraester, there was considerable expansion of the layer on an NaCl subphase as compared to subphases containing KCl or LiCl. In contrast, the monolayer of p-tert-butylcalix(23) Gaines, J. L., Jr. Insoluble Monolayers at Liquid-Gas Interface; Interscience Publishers: New York, 1966.
p-Dodecanoylcalix[4]arene Monolayers
Langmuir, Vol. 17, No. 22, 2001 6853 Table 2. Isotherm Data for 1 in the Presence of Group-Varying Anionsa
salt
collapse pressure (mN m-1)
collapse area (Å2)
Alim (Å2)
A0 (Å2)
A1 (Å2)
NaCl NaNO3 NaH2PO4 Na2SO4 NaCH3CO2 NaBr NaI NaHCO3 NaBF4
26.3 24.2 19.6 28.4 35.2 25.3 25.9 38 14.2
105 113 123 105 103 110 112 118 108
141 142 160 148 140 132 171 158 132
170 158 205 174 166 150 192 190 142
152 147 190 170 158 140 184 170 130
a A lim is the extrapolated molecular area, A0 is the apparent molecular area at π ) 0, and A1 is the apparent molecular area at π ) 1 mN/m.
Figure 3. Evolution of collapse pressure with concentration of LiCl (1), NaCl (2), KCl (b), RbCl (0), CsCl (O), NH4Cl (9), and HCl (]).
[6]arene hexaester shows an expansion on a KCl subphase and the monolayer of p-tert-butylcalix[8]arene shows effectively no change in molecular area on subphases containing LiCl, NaCl, or KCl. Similar changes were reported by Dei et al.13 for monolayers of p-tert-butylcalix[6]arene. In the work of Zhang et al.18 the effects of NaCl, NaH2PO4, KH2PO4, and CsBr were studied. The variation in the collapse pressure on the stability of the monolayer shows a number of interesting points. First in the presence of HCl at concentrations up to 0.1 M and for NaOH subphase concentrations of below 10-6 M, i.e. at pH values between 1 and 8, the collapse pressure is effectively invariant. It was previously reported by Nakamoto et al.22 that the monolayer formed by poctadecylcalix[4]arene was not stable under neutral or acidic conditions. Here we observe that 1 forms a stable monolayer under strongly acidic conditions. This result is in agreement with the results of Dei et al.,13 where it was shown that no deprotonation was observed for phenolic groups of p-tert-butylcalix[6]arene for pH values between 7 and 1. Also, the variation in πc follows an apparent Langmuir type adsorption isotherm suggesting that strong interactions occur between the salts and the monolayer of 1 at the interface.24 Similar strong adsorption, again following Langmuir adsorption behavior, has previously been reported for the absorption of cations by calix[4]arene at a water-chloroform interface by Marecˇek et al.25 The variation in πc is lowest for NH4Cl, with a ∆πc value of 8.2 mN/m. For LiCl, NaCl, KCl, and CsCl the adsorption curves are experimentally equivalent, leveling out at ∆π ) 11.5 ( 0.3 mN/m for all concentrations above 0.1 M in salt. In the case of RbCl, a value of ∆πc ) 12.2 mN/m is observed for salt concentrations of 5 × 10-2 and 0.1 M; above this concentration ∆πc again increases reaching 17.2 mN/m for 0.2 M. For 1, there apparently exists no apparent selectivity between the chloride salts of Li+, Na+, K+, and Cs+. However the monolayer is most strongly stabilized in the presence of Rb+ in the subphase. It is known that calix[4]arene is an excellent ligand in the solid state for Na+.26 A large volume of work exists on the construction of modified calix[4]arenes for the complexation and extrac(24) Hunter, R. J. Foundations of Colloid Science; Oxford University Press: New York, 1989; Vol. 2. (25) Marecek, V.; Lhotsky´, A.; Ja¨nchenov’a, H. J. Electroanal. Chem. 2000, 483, 174. (26) Bott, S. G.; Coleman, A. W.; Atwood, J. L. J. Am Chem. Soc. 1986, 108, 1709.
Figure 4. Evolution of collapse pressure with concentration of NaCl (b), NaNO3 (9), NaCH3CO2 (2), NaI (1), NaH2PO4 (O), Na2SO4 (0), NaHCO3 (4), NaBF4 (3), and NaBr (]).
tion of monovalent cations.4 It has been shown that the p-tert-butylcalix[6]arene is highly selective for the complexation of the Cs+.27 However the present work is, to our knowledge, the first in which a specific interaction with Rb+ has been observed. The apparent stronger interaction between 1 and RbCl may arise from two possibilities. First that complexation of the cation occurs in the intermolecular interstices between the molecules of 1 forming the film. The second possibility would occur by initial complexation of the Clanion at the interface, with Rb+ forming the second ion at the electrical double interface.28 The second part of this study concerns the effects of the counteranion on the monolayer stability. Generally for simple calix[n]arenes, the possibility of anion complexation has been largely ignored in the literature. To show the influence of anion on the stability of monolayer of 1 we have studied a large range of anions, Cl-, Br-, I-, NO3-, SO42-, H2PO4-, CH3CO2-, BF4-, and HCO3-, while retaining the cation, Na+, constant (Table 2). The variation of πc with anion concentration is presented as Figure 4. We can clearly see that the stability of monolayer varies with the nature of counteranion. For the noncomplexing anion BF4-, no or even a slight decrease in πc is observed for all salt concentrations. The stabilization of the monolayer is weak with H2PO4-; ∆πc ) 4.4 mN/m. The stabilization of the layer is more important with NO3- and SO42-, ∆πc ) (27) Talanov, V. S.; Talanova, G. G.; Bartsch, R. A. Tetrahedron Lett. 2000, 41, 8221. (28) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1991.
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9, 13.2 mN/m, and for Cl-, Br-, and I-, ∆πc ) 11.1, 10.1, and 10.7 mN/m, respectively. For CH3CO2- and HCO3the increase of ∆πc is much larger: 20 and 22.8 mN/m, respectively. This shows that the layer is largely stabilized by the presence in the subphase of certain anions, in particular with acetate and carbonate. While the behavior of the ionic interaction of 1 with salts is complex, the process involved seems to fit a Langmuir adsorption model of a single species, throughout the concentration range studied. This can be compared to the interaction of tert-butyldimethylsilyl-06-β-cyclodextrin with CaCl2 in the subphase where multistep complexation of first Ca2+, then replacement by Cl-, and finally the proposed complexation of the CaCl+ ion pair has been observed.29 The exact mechanism of 1-salt interactions remains unclear. However the effects of anion and cation presence in the electric double layer will strongly influence the hydratation of the monolayer; this has been reported for the monolayers of amphiphilic cyclodextrins where variations in the static surface potential arise from differing amphiphile-water interactions.30 (29) Eddaoudi, M.; Bazkin, A.; Parrot-Lopez, H.; Coleman, A. W. Langmuir 1995, 11, 13.
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The above results would appear to correlate with anionic interaction by 1 at the air-water interface being the most significant process involved in the interaction of 1 with salts of monovalent ions. Conclusion The study of the interfacial interaction of the amphiphilic calix[4]arene, 1, with monovalent salts shows a number of interesting points. First, the interaction between anions and 1 is the determinant factor in changes in the stabilization of the monolayer, with noncomplexing anions having no effect and HCO3- and CH3CO2- have the strongest effects on the layer stability. Second, there is an apparent selective larger stabilization of the monolayer by the rubidium cation. Acknowledgment. P.S. acknowledges the financial support of Central PBV. LA010562B (30) Tchoreloff, P.; Baszkin, A.; Boissonade, M.-M.; Zhang, P.; Coleman, A. W. Supramol. Chem. 1994, 4, 169.
