Mixed Monolayers of Alkylated Azacrown Ethers and Palmitic Acid at

Kamil Wojciechowski*, Dmitry Grigoriev, Riccardo Ferdani, and George W. Gokel. Analytical and Biophysical ... Brewster Angle Microscopy. Keith J. Stin...
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Langmuir 2006, 22, 8409-8415

8409

Mixed Monolayers of Alkylated Azacrown Ethers and Palmitic Acid at the Air-Water Surface Kamil Wojciechowski,*,† Dmitry Grigoriev,‡ Riccardo Ferdani,§ and George W. Gokel§ Analytical and Biophysical EnVironmental Chemistry (CABE), Department of Analytical, Inorganic and Applied Chemistry, UniVersity of GeneVa, Sciences II, 30 quai Ernest Ansermet, CH-1211 Gene` Ve 4, Switzerland, Max Planck Institute for Colloid and Interfaces, Am Mu¨hlenberg 1, 14424 Potsdam/Golm, Germany, and Department of Molecular Biology & Pharmacology, Washington UniVersity School of Medicine, Campus Box 8103, 660 South Euclid AVenue, St. Louis, Missouri 63110 ReceiVed April 12, 2006. In Final Form: July 5, 2006 The Langmuir films of two alkylated azacrown ethers at the air-water surface were characterized using surface pressure-area isotherms, ellipsometry, Brewster angle microscopy, and constant-area surface pressure relaxation. The azacrown ether molecules aggregate in the monolayer, which significantly stabilizes the film against dissolution. Mixed azacrown ether-palmitic acid monolayers were also characterized; results suggest that at high compression the two molecules interact repulsively. The influence of Cu(II) ions present in the aqueous subphase on the single components and mixed monolayer characteristics was also studied.

Introduction Different arrangements of molecules at varied interfaces play a crucial role in their reactivity in heterogeneous systems. Crown ethers are among the most common macrocyclic complexing agents that are used in different extraction-based techniques. Since their discovery in the 1960s,1 the complexing and extracting properties toward many ions and neutral species have been extensively investigated.2,3 Most of the studies, however, deal with bulk properties of crown ethers, mostly due to their predominantly hydrophilic character, which prevents formation of stable monolayers. The Langmuir film formation of azacrown ethers continues to attract attention mostly due to the potential applications in chemical sensing,4-6 molecular electronics,7 and mimicking biological systems.8 Mo¨bius and Zaitsev extensively investigated the Langmuir monolayers of water-insoluble crown ether-based dyes.9,10 They found that complexation of metal ions by the crown ether monolayer has a profound effect on the monolayer properties and can be correlated with the metal-crown ether complex formation abilities. Tschierske et al. studied Langmuir monolayers of crown ether-appended rod-shaped amphiphiles and found that the surface pressure-area isotherms can be used to study molecular recognition of metal ions by these crown ethers.11 A variety of azacrown ethers have also been shown to form stable vesicles both in the presence and in the absence of †

University of Geneva. Max Planck Institute for Colloid and Interfaces. § Washington University School of Medicine. ‡

(1) Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 7017. (2) Gokel, G. W.; Leevy, W. M.; Weber, M. E. Chem. ReV. 2004, 104, 2723. (3) Inoue, Y.; Gokel, G. E. Cation binding by macrocycles; Marcel Dekker: New York, 1990. (4) Kele, P.; Orbulescu, J.; Calhoun, T. L.; Gawley, R. E.; Leblanc, R. M. Langmuir 2002, 18, 8523. (5) Liu, Y.; Gong, A.; Liu, M.; Xi, F. New J. Chem. 2001, 25, 970. (6) Moore, A. J.; Goldenberg, L. M.; Bryce, M. R.; Petty, M. C.; Moloney, J.; Howard, J. A. K.; Joyce, M. J.; Port, S. N. J. Org. Chem. 2000, 65, 8269. (7) Akutagawa, T.; Kakiuchi, K.; Hasegawa, T.; Nakamura, T.; Christensen, C. A.; Becher, J. Langmuir 2004, 20, 4187. (8) Lednev, I. K.; Petty, M. C. AdV. Mater. (Weinheim, Ger.) 1996, 8, 615. (9) Sergeeva, T. I.; Zaitsev, S. Y.; Tsarkova, M. S.; Gromov, S. P.; Vedernikov, A. I.; Kapichnikova, M. S.; Alfimov, M. V.; Druzhinina, T. S.; Mobius, D. J. Colloid Interface Sci. 2003, 265, 77. (10) Zaitsev, S. Y.; Sergeeva, T. I.; Baryshnikova, E. A.; Gromov, S. P.; Fedorova, O. A.; Alfimov, M. V.; Hacke, S.; Mobius, D. Colloids Surf., A 2002, 198-200, 473.

salts.12 Diazacrown ethers can readily be substituted at the macroring nitrogen atoms. Attachment of alkyl chains creates an amphiphile that is more or less lipophilic depending upon the alkyl chain length. Such alkylated diazacrown ethers can combine metal ion complexing properties with the ability to preferentially partition into either a polar or a nonpolar phase or to adsorb at their interface. N,N′-Didecyl-4,13-diaza-18-crown-6 ether (ACE10) in a mixture with lauric (LAH) or palmitic (PAH) acid was extensively used as a carrier to transport metal ions through permeation liquid membranes (PLMs). Such membranes are selective to Cu(II), Pb(II), and Cd(II) and were applied to study the chemical speciation of these ions in the environment.13-15 The PLM device consists of two aqueous compartments (source and strip) separated by an organic phase. The organic phase contains the carrier, which selectively transports given chemical species and might be embedded in micropores of an inert polymeric support to reduce the necessary amount of the carrier and thus the cost of the device. Since both ACE-10 and fatty acids are surface active,16 it is possible that the transport of metal ions through the water-membrane interface takes place via an interfacial path, as was suggested, e.g., for extraction of Ni(II) with long alkyl chain 8-quinolinols.17 To investigate the possible role of both azacrown ether and fatty acid in Cu(II) ion recognition at the water-air interface, we investigated the Langmuir films of both compounds as well as their mixture. For this purpose surface pressure-area isotherms combined with ellipsometry and Brewster angle microscopy were used in this study. Experimental Section ACE-10 was obtained from Merck (Kryptofix 22DD). Palmitic acid (puriss. p.a.) was purchased from Fluka. N,N′-Dihexadecyl4,13-diaza-18-crown-6 (ACE-16) was prepared as described in ref 18. (11) Plehnert, R.; Schroeter, J. A.; Tschierske, C. Langmuir 1998, 14, 5245. (12) De Wall, S. L.; Wang, K.; Berger, D. R.; Watanabe, S.; Hernandez, J. C.; Gokel, G. W. J. Org. Chem. 1997, 62, 6784. (13) Buffle, J.; Parthasarathy, N.; Djane, N. K.; Matthiasson, L. IUPAC Ser. Anal. Phys. Chem. EnViron. Syst. 2000, 6, 407. (14) Guyon, F.; Parthasarathy, N.; Buffle, J. Anal. Chem. 2000, 72, 1328. (15) Zhang, Z.; Buffle, J.; van Leeuwen, H.; Wojciechowski, K. Anal. Chem., in press. (16) Wojciechowski, K.; Buffle, J.; Miller, R. Colloids Surf., A 2005, 261, 49. (17) Shioya, T.; Nishizawa, S.; Teramae, N. Langmuir 1998, 14, 4552. (18) Gatto, V. J.; Arnold, K. A.; Viscariello, A. M.; Miller, S. R.; Morgan, C. R.; Gokel, G. W. J. Org. Chem. 1986, 51, 5373.

10.1021/la0609928 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/25/2006

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Figure 1. Structure of two azacrown ethers: ACE-10 (R ) C10H21) and ACE-16 (R ) C16H33). The monolayer components were dissolved as received in toluene (puriss. p.a. ACS reagent for UV spectroscopy) or hexane (Uvasol), purchased from Fluka and Merck, respectively. Fresh Millipore water (18.2 × 106 Ω‚cm-1) of pH 5.5 was used as a subphase. For experiments with Cu(II) in the subphase, a 1 mM solution of Cu(NO3)2‚3H2O (p.a. from Merck) was used (pH 5.0). The surface pressure-area (Π-A) isotherms were recorded in a rectangular homemade PTFE Langmuir trough with a total area of 150 cm2 equipped with two moving barriers allowing for symmetric and asymmetric compressions. Compression was performed in an asymmetric way at two barrier velocities of 8.48 × 10-3 and 8.3 × 10-4 nm2/(s‚molecule) for pure ACE-10, ACE-16, or PAH monolayers and at 6.2 × 10-4 nm2/(s‚molecule) for mixed ACE-16-PAH monolayers. The subphase temperature in the trough was kept at 23 ( 0.2 °C. The monolayers were spread from a 0.2 mM solution of the corresponding substance in toluene such that the initial surface coverage corresponded to a monolayer state of weak interactions between molecules (usually liquid-expanded, LE). Spreading was done in a conventional way using a Hamilton microsyringe. Recording of the Π-A isotherms was started about 10 min after spreading to guarantee mechanical and temperature preequilibration of the monolayer. Each measurement was repeated at least three times to reach a satisfactory reproducibility of data. The Langmuir trough was equipped with a Brewster angle microscope (BAM-1) from NFT, Germany. Both the BAM unit and Langmuir trough were placed in a covered Plexiglas box to avoid impurities from the surrounding air and to keep the air humidity near the monolayer at a constant level. Additionally, the whole laboratory was thermostated at 23 °C. To register the surface pressure changes in systems with the spread monolayers during ellipsometric measurements, a circular PTFE vessel with a total area of 63 cm2 and a microbalance with a Wilhelmy plate were embedded in the commercial ellipsometric setup “Multiskop” from Optrel GbR, Germany. The angle of incidence of the laser beam was 50°. Measurements of dynamic surface tension and ellipsometric measurements were started simultaneously a short time (usually about 10 min) after spreading, which was necessary for the mechanical equilibration of the surface layer and for solvent evaporation.

Results and Discussion ACE-10 and ACE-16 Monolayers. In the first part of the study, the monolayers of ACE-10 (Figure 1) were investigated since this azacrown ether is normally used in our PLM systems and is commercially available. The films thus formed are, however, not stable; the surface pressure decreases after the spreading solvent evaporates. Upon compression, the monolayers collapse at unrealistically small areas per molecule (10-20 Å2/molecule). In addition, a strong hysteresis is observed in the consecutive compression-expansion cycles, which could not be avoided even by presaturating the subphase with ACE-10 for 1 h prior to spreading. Indeed, the aqueous solubility of ACE-10 ((2.7 ( 0.1) × 10-5 M at 25 °C)19 is sufficient to dissolve the whole monolayer material in the volume of the trough (about 80 cm3). Therefore, we conclude that the hysteresis of the isotherms is mostly caused by monolayer dissolution. This hypothesis was further strengthened by the results of dynamic surface tension (DST) and dynamic ellipsometry (see the Supporting Information). (19) Parthasarathy, N.; Buffle, J. Anal. Chim. Acta 1991, 254, 1.

Figure 2. Dynamic surface pressure of an ACE-16 monolayer spread on water (pH 5.5) at two different areas per molecule: 105 Å2 (9) and 60 Å2 (O).

To study more quantitatively the effects in alkylated azacrown ethers, the monolayer stability was improved by replacing ACE10 with an analogue having longer aliphatic chains (-C16H31): ACE-16 (Figure 1). Owing to the presence of six additional methylene groups on each arm of the molecule, the stability of the monolayer spread on water increased significantly. When the ACE-16 monolayer was compressed to Π < 30 mN/m, the isotherms were fully reversible and almost no hysteresis was observed (see Figure S3a, Supporting Information). Brewster angle microscopy revealed no micrometer scale aggregates during the compression-expansion cycles between 0 and 30 mN/m. A small shift observed in the consecutive isotherms can be assigned to loss of small amounts of material from the ACE-16 monolayer in this state. To exclude any convective effects due to compression and decompression, the DST under quiescent conditions was measured as before. The ACE-16 monolayer was spread in a Petri dish to give an area per molecule of 105 Å2/ molecule, corresponding to Π ) 16 mN/m (Figure 2). This pressure corresponds to half the maximum pressure up to which the monolayer was continuously compressed in the cyclic experiments described above. At the same time the ellipsometric angles ∆ and Ψ were monitored by ellipsometry. The results are shown in Figure 3, and the best-fit parameters are collected in Table 1. Unfortunately, the changes of both ∆ and Ψ were not sufficiently large to allow for reliable calculation of the ellipsometric thickness. Despite this, the weak time dependence of ∆ suggests that the ACE-16 monolayer undergoes only minor changes in time in comparison to that of ACE-10. When the ACE-16 monolayer was compressed further above 30 mN/m, a phase transition was observed, marked by an almost flat region at Π ) 35 mN/m (Figure 6). The onset of the transition (70 Å2/molecule) corresponds well to a situation in which the macrocycle lies flat and the alkyl chains stand upright toward air (see below). The ellipsometric angle ∆ (Table 1) measured at 60 Å2/molecule, i.e., above 30 mN/m (see Figure 3), changes to a smaller extent than at 105 Å2/molecule (Π < 30 mN/m). Both the time constant of the DST and equilibrium surface pressure are higher at greater compression (60 Å2/molecule). Although gradual dissolution is also observed for ACE-16 compressed to 60 Å2/molecule (Figure 2), the monolayer stability is increased in this state with respect to the expanded one (Π < 30 mN/m). Upon compression of the ACE-16 monolayer below 40 Å2/ molecule, the pressure increased again and unrealistically small

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Table 1. Best-Fit Parameters for Monoexponential Decay (y0 + A exp(-t/τ)) of the DST and of the Ellipsometric Angle ∆ for Monolayers of ACE-16 Spread on Water (pH 5.5) at Two Different Areas Per Molecule area per molecule (Å2)

A (mN/m)

DST τ (s)

y0 (mN/m)

A (deg)

∆ τ (s)

y0 (deg)

60 105

29. ( 0.1 9.2 ( 0.1

5000 ( 25 1700 ( 5

4.1 ( 0.1 7.3 ( 0.1

0.903 ( 0.012 0.355 ( 0.004

5766 ( 150 2348 ( 76

181.386 ( 0.013 181.556 ( 0.003

values of the area per molecule were observed (Figure 6). The minimum area per molecule estimated by various methods for a monolayer of ACE-16 is 50-157 Å2.20 The lowest possible area per single ACE-16 or ACE-10 molecule would correspond to the sum of two alkyl chains, i.e., ca. 40 Å2/molecule, if the size of the crown ether cavity could be neglected. At a toluenewater interface a similar area per molecule was obtained from the maximum adsorption (Γ∞). The interfacial tension isotherm of ACE-10 was fitted to the reorientation model, assuming two different adsorption states of the molecule. The low-coverage state would correspond to a 2D gaseous state of the Langmuir monolayer, with an area per molecule of 322 Å2. In a fully compressed state, one molecule of the azacrown ether would fill only 42.7 Å2.16 Such dense packing would require at least partial overlap between the molecules in the monolayer. We conclude that the phase transition for ACE-16 starting at Π ) 32 mN/m in Figure 6 is related to aggregation of the azacrown ether molecules in the monolayer.12 The BAM pictures corresponding to different points of the isotherms are shown in Figure S3 (Supporting Information). Once the pressure exceeds 32 mN/m, a new phase with a different refractive index appears in BAM. Most probably the aggregates on the BAM pictures are 2D crystals of a new phase. No distinct Newton rings around them, which are typical for the 3D objects, were observed. Unfortunately, due

to the small thickness of the monolayer, no conclusions can be drawn on the changes of the thickness of the monolayer during the transition. Taking into account the ellipsometric and BAM results, dissolution alone cannot explain the hysteresis observed in Figure 6. Probably the monolayer in this new state at Π > 32 mN/m is very inert and upon expansion is not completely dissociated on the time scale of the experiment. This is confirmed by the fact that the aggregates are seen microscopically (BAM) even below Π ) 32 mN/m upon expansion of the monolayer (see, e.g., point F in Figure S3). No clear 3D collapse is observed in the first compression cycle, even when an area per molecule as low as 20 Å2 is reached. Film rupture can be observed only in the consecutive compression of the film (not shown). Unfortunately, due to a strong hysteresis caused by partial dissolution and irreversibility of the transition observed at Π ) 32 mN/m, the corresponding area per molecule could not be determined. The relaxation experiments were performed by a quick compression to a given surface pressure, after which the trough barrier was stopped and the surface pressure was monitored in parallel with BAM video recording. During relaxation of the monolayer quickly compressed to the phase transition pressure (32 mN/m), the surface pressure remains almost constant for the first 2 min (Figure 4). Only after that, the first objects are apparent

Figure 4. Constant-area relaxation curve and corresponding BAM pictures for an ACE-16 monolayer spread on water (pH 5.5) and quickly compressed to Π ) 32 mN/m. The basal length of each BAM picture corresponds to 750 µm.

Figure 3. Time changes of the ellipsometric angles ∆ and Ψ for a pure water surface (O) and an ACE-16 monolayer spread on water at two different areas per molecule: 105 Å2 (0) and 60 Å2 (4). The solid lines correspond to the best fit to the monoexponential decay (see the text).

in the BAM camera and the pressure starts to decay. Interestingly, no such lag time was observed for monolayers spread on a Petri dish for dynamic surface tension measurement (see Figures 2 and S1, Supporting Information). In the latter experiment, the monolayer was in the condensed state from the beginning, so the lag time was not observed due to the 10 min equilibration time after spreading. In contrast to that, in the trough experiment, the equilibration of the monolayer was done in the expanded state, after which the monolayer was quickly compressed and measurement started. This short delay between the moment of compression and measurement allowed the lag time to be (20) De Wall, S. L.; Barbour, L. J.; Gokel, G. W. J. Phys. Org. Chem. 2001, 14, 383.

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Table 2. Best-Fit Parameters for Biexponential Decays (y0 + A1 exp(-t/τ1) + A2 exp(-t/τ2)) or Monoexponential Decay (A1 ) 0) of the Constant-Area Relaxations for ACE-16 Monolayers and ACE-16-PAH Mixed Monolayers Spread on Water (pH 5.5) after a Quick Compression to Π0 ACE-16 Π0 (mN/m)

y0 (mN/m)

A1 (mN/m)

τ1 (s)

A2 (mN/m)

τ2 (s)

50 32 22

39.89 ( 0.01 17.35 ( 0.05 9.20 ( 0.01

67.96 ( 0.51 13.32 ( 0.03

77 ( 1 146 ( 2

5.60 ( 0.01 7.92 ( 0.04 12.50 ( 0.01

683 ( 1 4200 ( 40 1845 ( 1

ACE-16-PAH Π0 (mN/m) 37 23

y0 (mN/m) 22.99 ( 0.02

Π > 23 mN/m A (mN/m) 23.40 ( 0.01

τ (s)

y0 (mN/m)

681 ( 2

11.42 ( 0.01 6.39 ( 0.01

observed. Initially the aggregates grow dendritically in all directions, but with time some branches start to dominate as shown in Figure 4. The rate of domain growth could be estimated from the pictures to be equal to ca. 1 µm/s. A similar relaxation experiment after compression to 22 mN/m, i.e., below the phase transition, resulted in a different pattern (Figure 5, bottom line).

Figure 5. Constant-area relaxation curve and corresponding BAM pictures for an ACE-16 monolayer spread on water (pH 5.5) and quickly compressed to Π ) 50 or 22 mN/m. The basal length of each BAM picture corresponds to 750 µm.

Figure 6. Surface pressure-area isotherms of (a) PAH, (b) an ACE-16-PAH mixture (1:1), and (c) ACE-16 on water (pH 5.5). The directions of the arrows correspond to the compression and expansion cycles.

The pressure decreased monoexponentially without any time lag, in agreement with the results shown in Figures 2 and 3 and in agreement with a dissolution mechanism. During relaxation, no aggregates were observed by using BAM. The monolayer compressed to the highest possible pressure (50 mN/m) was also less soluble than the low-pressure phase.

Π < 23 mN/m A (mN/m) 46.92 ( 0.03 16.53 ( 0.01

τ (s) 1303 ( 1 1355 ( 1

The relaxation curve (Figure 5, upper curve) showed an even longer lag time than in Figure 4. The morphology of the aggregates seen in BAM is different from that seen before and probably represents a collapsed monolayer. The domain growth rate estimated from BAM (ca. 14 µm/s) is more than 1 order of magnitude higher than that in the relaxation curve from Figure 4. During relaxation experiments the phase transition starting at Π ) 32 mN/m was almost completed after 2-3 min, and the slow decay afterward probably results from the dissolution of the monolayer material. The rate of this dissolution is significantly smaller than that of the first process. In the absence of any reasonable theoretical model describing the phenomenon, the decay of Π after the lag time in Figure 4 was fitted using a biexponential decay model (Table 2). A monoexponential decay could describe the experimental data only if the slow process, showing up after the first quick decay, was neglected. The time constants from the biexponential fitting are τ1 ) 150 s and τ2 ) 4 × 103 s. The second relaxation time is in good agreement with the relaxation time (τ ) 2 × 103 s) of the monolayer compressed to 22 mN/m, i.e., below the phase transition point. The similar time scale of both relaxation processes suggests that they may have the same origin, i.e., monolayer dissolution. The first relaxation time after quick compression to Π ) 50 mN/m (τ1) is governed by other phenomena, probably related to the monolayer collapse, and the time scale of the relaxation is thus different from the other two. After compression to Π ) 32 mN/m, the first fast process (τ1) in the relaxation curve following the lag time (Figure 4) is selfregulated. When the surface pressure drops after the lag time, aggregate growth stops and only slow dissolution of the coexisting less condensed phase contributes to the relaxation curve (τ2). On the time scale of the experiment, the phase transition is, however, irreversible. After the pressure drop, the molecules remain mostly in the condensed 2D crystalline state (insoluble). This irreversibility, together with dissolution of the coexisting less condensed phase, is responsible for the observed compression-expansion hysteresis in the ACE-16 monolayer (Figure 6). Thus, when the monolayers are spread without proper care, local oversaturation occurs. This leads to the local phase transitions and aggregates observed by BAM even before the compression starts. Mixed Monolayers of PAH and ACE-16. Under the conditions used for the PLM experiments, ACE-10 was mixed (1:1 mol/mol ratio) with LAH (C11H23COOH) or PAH (C15H31COOH). We have shown in a previous paper16 that, at least at the toluene-water interface, the studied azacrown ethers are significantly more surface active than lauric acid. When both LAH and ACE-10 are present, the former may coadsorb with ACE-10. The coadsorption may result from salt bridge formation

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Figure 7. Constant-area relaxation curve and corresponding BAM pictures for mixed ACE-16-PAH monolayers spread on water (pH 5.5) and quickly compressed to Π ) 37 mN/m (upper curve) or 23 mN/m (bottom curve). The basal length of each BAM picture corresponds to 750 µm.

that occurs when the acid transfers a proton to a macrocyclic amine nitrogen atom. Proton transfer could be preceded or augmented by hydrogen bonding between the amine and acid.21 At the water-air interface, however, due to lack of an organic solvent phase, the relative surface activity of azacrown ether and fatty acid might be quite different. Therefore, we decided to study the mixed Langmuir films of the azacrown ether and fatty acid. To minimize water solubility of the monolayers, ACE-16 and palmitic acid were used instead of ACE-10 and lauric acid, as used in the PLM. The mixed monolayers show significantly less hysteresis of the compression-expansion isotherms, even at pressures exceeding those required for the phase transition in a pure ACE-16 monolayer (Figure 6). Also BAM pictures show reversible changes in the morphology of the monolayer on the time scale of the experiment. Only after very high compression (55 mN/m) is a strong hysteresis observed in the mixed monolayer, when eventually both ACE-16 and PAH domains collapse (not shown). To quantify the interaction between ACE-16 and PAH molecules in mixed monolayers, the excess area per molecule was calculated according to the following formula:22

ωexc ) ωmix - (XACE-16ωACE-16 + XPAHωPAH)

(1)

where ω is the area per molecule, X is the mole fraction of PAH and ACE-16, and the subscript “mix” corresponds to the mixed ACE-16-PAH monolayer. The nonmonotonic behavior of the excess area per molecule as a function of surface pressure for the mixed monolayer of ACE-16-PAH (Figure S4, Supporting Information) suggests that the interaction between the two molecules in the Langmuir monolayer at the air-water interface is more complicated than in Gibbs monolayers at the liquid-liquid interface.21 At the water-air interface, PAH and ACE-16 are almost immiscible at low surface pressure (32 mN/m). For the same reason a pressure jump, characteristic for oversaturated monolayers, is seen in the mixed monolayer isotherm just before the plateau region at 40 Å2/ molecule (Figure 6). Interestingly, such repulsion was not observed at the toluene-water interface before. This is not surprising, however, in view of the dynamic nature of the liquidliquid interface, where adsorption is reversible and spontaneous. In the case of Gibbs monolayers, the surface pressure is regulated by the adsorption process and not externally imposed by compression. As soon as the interaction between the two molecules becomes repulsive, desorption of one of the components could take place to minimize this effect. After the plateau region, at the beginning of the second steep part of the compression branch, squeezing out of the less condensed ACE-16 phase takes place. The pressure increase is less steep than in the pure PAH monolayer (Figure 6). Finally, the monolayer freed from ACE-16 collapses at Π ) 52 mN/m, similar to that in the pure PAH monolayer. Relaxation curves at constant area of the mixed monolayer compressed to Π ) 38 mN/m showed two distinct relaxation processes, in addition to the lag time. The best-fit parameters using two different monoexponential decays for the curves from Figure 7 are collected in Table 2. Due to the pressure drop related to the first relaxation process, another phase transition is reached at Π ) 26 mN/m. Interestingly, this transition is not seen in the compression isotherm, most probably because the rate of compression and relative changes of molecular area during compression are much higher than those corresponding to the relatively smooth conditions in the relaxation experiments. Distinct changes were also observed by using BAM (Figure 7). The small aggregates disappeared after Π reached a value of 26 mN/m to give rise to a multitude of pointlike aggregates at lower values of surface pressure. When the monolayer was compressed to Π ) 23 mN/m, i.e., below the onset of the second observed transition (separate experiment), the same relaxation time was observed: 1.3 × 103 s (Figure 7, bottom curve, and Table 2). BAM pictures revealed the same morphology observed during the second phase of the previous relaxation. The appearance of the distinct structure observed by BAM suggests that the nature of the process is not simple dissolution of the monolayer. As suggested above, the immiscibility of the two components at low surface pressures could be responsible for this phenomenon. If so, the low-pressure aggregates would be composed of a demixed PAH phase. Influence of the Presence of Cu(II). The presence of Cu(II) in the aqueous subphase at pH 5.0 (1 mM) influences the compression isotherms of both ACE-16 and PAH, as well as of their 1:1 mixture (Figure 8). As observed previously for behenic acid (C21H43COOH),23 the presence of Cu(II) ions leads to expansion of the palmitic acid monolayer. Dutta et al. on the basis of X-ray diffraction studies of Langmuir monolayers of

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Figure 8. Surface pressure-area isotherms of (a) PAH, (b) an ACE-16-PAH mixture (1:1), and (c) ACE-16 in the presence of Cu(II) ions in the subphase (pH 5.0). The directions of the arrows correspond to the compression and expansion cycles.

heneicosanoic acid (C20H41COOH) on a copper(II)-containing subphase proposed that tilting of the chains takes place upon Cu(II) complexation.24 The pictures from BAM show that, even at Π ) 0, a high-contrast needlelike structure exists (Figure S5, Supporting Information). The linear structure of these aggregates might arise from polymerization of copper(II) palmitate, as was observed in the X-ray structure of bulk crystals of long-chain alkanoates of copper(II).25,26 In such structures, individual dimeric copper(II) alkanoate units are connected through Cu(II)-oxygen coordination between two neighboring dimers. The magnetic studies of collapsed copper(II) behenate are in agreement with the polymeric structure.23 Further compression of the PAH monolayer in the presence of Cu(II) ions in the subphase leads to formation of a homogeneous layer (Figure S5). The collapse mechanism is also different in the presence of Cu(II) ions: instead of an abrupt pressure drop, only the slope of the isotherm decrease is observed. In the case of ACE-16, the phase transition takes place at higher surface pressure than in the absence of Cu(II) (41 mN/m vs 32 mN/m). In the presence of Cu(II), the transition is marked by a pressure jump, similar to that observed in mixed ACE16-PAH monolayers. Since the rate of compression is quite slow, as in the mixed monolayer (8.3 × 10-4 nm2/(s‚molecule)), the oversaturation is probably a result of monolayer stabilization by complexed copper(II) ions. The increase of the plateau pressure is generally taken as an indication of an increase in monolayer hydrophilicity.11 The comparison of the plateau pressure of Figures 6 and 8 confirms that hydrophilic Cu(II) ions from the subphase are incorporated into the ACE-16 monolayer. Electrostatic repulsion between Cu(II) ions bound to the monolayer could also contribute to the increase of the phase transition surface pressure. Constant-area relaxation curves for ACE-16 in the presence of Cu(II) are similar to those obtained in its absence. However, when the monolayer is compressed to Π ) 42 mN/m, i.e., above the collapse pressure, the relaxation follows the dissolution mechanism, with surface pressure decaying exponentially to Π ) 1.5 mN/m within 30 min (not shown). This is probably caused by dissolution of the collapsed monolayer of the Cu(II)-azacrown ether complex formed in situ. (23) Bettarini, S.; Bonosi, F.; Gabrielli, G.; Martini, G. Langmuir 1991, 7, 1082. (24) Lin, B.; Bohanon, T. M.; Shih, M. C.; Dutta, P. Langmuir 1990, 6, 1665. (25) Del Sesto, R. E.; Deakin, L.; Miller, J. S. Synth. Met. 2001, 122, 543. (26) Lomer, T. R.; Perera, K. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1974, B30, 2912.

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The surface pressure-area isotherm of mixed ACE-16-PAH monolayers does not change drastically in the presence of Cu(II) ions, the major difference being an increased hysteresis of the isotherm. The morphology of the mixed monolayer on the Cu(II)-containig subphase as revealed by BAM is also similar to that in the absence of Cu(II). With the exception of the pure ACE-16 monolayer, the presence of Cu(II) leads to an increase of hysteresis in the consecutive compression-expansion cycles. This is in agreement with an increase of hydrophilicity of the monolayers upon complexation of hydrophilic copper(II) ions. Both PAH and ACE-16 incorporated into the monolayer at the air-water interface are thus capable of complexing copper(II) ions. In the presence of the latter, in all cases distinct changes of the monolayer properties can be observed. The exact mode of interaction cannot, however, be elucidated from the current results. The fact that both fatty acid and azacrown ether are capable of binding copper(II) ions from the aqueous phase at a nonpolar-polar interface is a strong indication that they may act cooperatively also in the PLM. We have shown previously16 that when both azacrown ether and fatty acid are present in the toluene phase, they coadsorb at the toluene-water interface. Taking into account that the solvent of the membrane phase of our PLM consists of a mixture of toluene and structurally similar hexylbenzene, it is very likely that such coadsorption occurs also at the PLM membrane-water interface. As a consequence, both carrier molecules may participate synergistically in the metal ion binding at the polar-nonpolar interface like that of the PLM.

Conclusions The N,N′-dialkylated azacrown ethers are capable of forming quite stable monolayers on a water subphase. Using dynamic surface tension and ellipsometric measurements, it was shown that the didecyl-substituted derivative ACE-10 dissolved significantly in the subphase during compression. Decreased aqueous solubility of the dihexadecyl-substituted derivative ACE-16 allowed us to study the Langmuir films of dialkylated azacrown ethers. Lengthening the alkyl chains resulted in an increase of the characteristic times of ellipsometric angle ∆ and surface pressure decays by an order of magnitude. Upon compression above Π ) 32 mN/m, the ACE-16 molecules in the monolayer started to aggregate to form a 2D crystalline phase. In this state, the monolayer of ACE-16 showed higher stability against dissolution and the phase transition was irreversible on the time scale of our experiments. In the mixed ACE-16-palmitic acid monolayers, the two molecules interacted repulsively or did not mix, depending on the state of compression. The mixed monolayers collapsed in two stages: first ACE-16 was squeezed out from the monolayer, and only then did the palmitic acid domains collapse. The presence of Cu(II) ions in the subphase influenced both ACE-16 and PAH monolayers, as well as their equimolar mixture, suggesting that at the surface both molecules can complex copper(II) ions. This observation, together with the previous observations showing that, at the toluene-aqueous interface the fatty acid and the azacrown ether coadsorb, suggests that both molecules may participate in Cu(II) transfer through the interface of the PLM. The best performance of the PLM, which is observed when both ACE-10 and LAH are present, may thus originate from their synergistic action at the interface, in addition to their yet unclear roles in the bulk of the membrane, which are subject to further studies in our group. The major drawback of Langmuir films as models for more complicated systems such as the PLM is that the nature of the

ACE-PAH Mixed Monolayers at the Air-Water Surface

monolayer is different from that in the liquid-liquid interface, the latter being formed by spontaneous adsorption. The solubility of the fatty acid and the azacrown ether in the organic phase makes desorption possible if the interaction between the two molecules at the liquid-liquid interface become repulsive. As a result, phenomena such as repulsion between ACE-16 and PAH can be observed at the air-water interface, but not at the liquid-liquid one. Even though the water-air interface is not the best model for the much more complex interface between the PLM membrane and water, it was shown that some useful information on the

Langmuir, Vol. 22, No. 20, 2006 8415

mechanism of Cu(II) transfer through such an interface can be obtained using this relatively simple method. Acknowledgment. This work was supported by a project of the European Space Agency (FASES MAP AO-99-052) and by a grant from the NIH to G.W.G. (GM 36262). Supporting Information Available: Best-fit parameters of the dynamic surface tension and ellipsometric data for ACE-10 (Table S1) and figures showing the dynamic surface tension and ellipsometric data for ACE-10 and ACE-16 monolayers (Figures S1-S5). This material is available free of charge via the Internet at http://pubs.acs.org. LA0609928