pubs.acs.org/Langmuir © 2010 American Chemical Society
Stabilization of Alkylated Azacrown Ether by Fatty Acid at the Air-Water Interface Ali Zarbakhsh,*,† Mario Campana,† John. R. P. Webster,‡ and Kamil Wojciechowski*,§ †
School of Biological & Chemical Sciences, Queen Mary, University of London, Joseph Priestley Building, Mile End Road, London E1 4NS, United Kingdom, ‡ISIS Neutron Facility, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, OX11 0QX, United Kingdom, and §Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warsaw, Poland Received September 9, 2010. Revised Manuscript Received October 20, 2010 The adsorbed amount of partially deuterated dihexadecyl-diaza-18-crown-6 ether (d-ACE16) in the presence of different chain length fatty acids as a function of surface pressure was determined by neutron reflectometry technique. The highest adsorbed amount of the azacrown ether was observed for the mixture of ACE16 with hexadecanoic (palmitic) acid, pointing to the importance of chain length matching between the two species for optimum stabilization of the mixed monolayer. The contrast variation technique was used to estimate the contribution to the total adsorbed amount from stearic acid and ACE16. It was found that the mixed Langmuir monolayer is stable against dissolution up to a surface pressure of 20 mN m-1. Above this pressure, however, the spread and adsorbed amounts start to deviate, indicative of partial dissolution into the aqueous subphase. The consequences of this behavior for the transport of metal ions through the interfaces of permeation liquid membranes (PLMs) are discussed.
1. Introduction Macrocyclic amphiphiles have been shown to form Langmuir monolayers and have been used extensively in the past as models for biological membranes, as well as in chemical sensing1-3 and molecular electronics.4 Because of their hydrophilic nature, bare crown and azacrown ethers readily dissolve into the aqueous subphase. In order to obtain stable films, the macrocyclic ring should be appended with a water-insoluble moiety, e.g., longchain aliphatic or aromatic groups. For example, M€obius and Zaitsev extensively investigated the Langmuir monolayers of substituted benzodiaza-crown ether-based dyes in the context of selectivity of metal ions recognition,5,6 while Tschierske et al.7 studied Langmuir monolayers of a crown ether appended to rodshaped amphiphiles. Alkylated azacrown ethers (ACEs) in the presence of fatty acids have been widely used as carriers for the transport of metal ions in extraction-based techniques, for instance, in permeation liquid membranes (PLMs).8 The PLM has been used extensively for selective transport of Cu(II), Pb(II), Ni(II), and Cd(II) for environmental analysis purposes.8 Transport of ions through the PLM involves interfacial transfer at the aqueous-membrane *Author to whom correspondence should be addressed. E-mail: a.zarbakhsh@ qmul.ac.uk (A.Z.);
[email protected] (K.W.). (1) Kele, P.; Orbulescu, J.; Calhoun, T. L.; Gawley, R. E.; Leblanc, R. M. Langmuir 2002, 18, 8523. (2) Liu, Y.; Gong, A.; Liu, M.; Xi, F. New J. Chem. 2001, 25, 970. (3) 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. (4) Akutagawa, T.; Kakiuchi, K.; Hasegawa, T.; Nakamura, T.; Christensen, C. A.; Becher, J. Langmuir 2004, 20, 4187. (5) Zaitsev, S. Y.; Turshatov, A. A.; Mobius, D.; Gromov, S. P.; Alfimov, M. V. Colloids Surf. A 2010, 51-55, 354. (6) Sergeeva, T. I.; Gromov, S. P.; Zaitsev, S. Y.; Mobius, D. Colloids Surf. B 2009, 74, 410. (7) Plehnert, R.; Schroeter, J. A.; Tschierske, C. Langmuir 1998, 14, 5245. (8) Buffle, J.; Parthasarathy, N.; Djane, N. K.; Matthiasson, L. Permeation Liquid Membranes for Field Analysis and Speciation of Trace Compounds in Waters. IUPAC Series on Analytical and Physical Chemistry of Environmental Systems 2000, 6, 407.
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interface, analogous to the ion transfer at the interface between two immiscible phases. The contribution of the azacrown ether to the transport of metal ions has been studied using several experimental techniques, such as surface and interfacial tension9 and neutron10 and X-ray11 reflectivities. This contribution to the transport mechanism of metal ions has been understood and extensively discussed in the literature.12 However, the role of the added fatty acid in these transport processes is still not fully understood, especially in terms of its influence on the interfacial conformation of the azacrown ether and its stability. The latter is of paramount importance in the optimization and the efficient operation of PLM devices. It is believed that the fatty acid may enhance the retention of the azacrown ether at the interface, resulting in a more closely packed monolayer at the interface. The presence of a fatty acid in the close proximity of ACEs is also essential for the effectiveness of transport, especially for copper(II) ion transport. Recent mechanistic studies clearly suggest that a ternary copper(II)-ACE-fatty acid complex is formed at the interface prior to being transported through the PLM membrane.12 The same study also showed that fatty acid alone, even in the absence of azacrown ether is capable of coordinating Cu(II) in nonpolar environments, e.g., in the PLM membrane. The “paddle wheel” dimeric copper(II) alkoxylate thus formed, however, undergoes oligomerization leading to a significant worsening of the transport capabilities of the membrane. This process can be inhibited by the presence of the azacrown ether, which coordinates to the axial positions of the “paddle wheel” dimer. Given the experimental complexity, the mechanistic studies of interfacial processes in PLM must be performed in model systems, (9) Wojciechowski, K.; Grigoriev, D.; Ferdani, R.; Gokel, G. W. Langmuir 2006, 22, 8409. Wojciechowski, K.; Buffle, J; Miller, R. Colloid Surf. A 2005, 261, 49. (10) Zarbakhsh, A.; Webster, J. R. P.; Wojciechowski, K. Langmuir 2009, 19, 11569. (11) Wojciechowski, K.; Gutberlet, T.; Tikhonov, A.; Kashimoto, K.; Schlossman, M. Chem. Phys. Lett. 2010, 487, 62. (12) Wojciechowski, K.; Kucharek, M.; Buffle, J. J. Membr. Sci. 2008, 314, 152.
Published on Web 11/04/2010
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e.g., at water-air and oil-water interfaces. The mobility of ACE in the vicinity of the interface is achieved in PLM devices by some degree of dynamic exchange with bulk phases through partial dissolution of the ACE molecules.9 However, in previous Langmuir trough studies it has been shown that for bare ACE films some monolayer material is lost into the aqueous subphase, even for the long chain hexadecyl-substituted ACE (ACE16). As a result, a clear discrepancy between the actual spread and the adsorbed amount has been observed in neutron reflectivity (NR) experiments. In our previous paper,10 we reported a simultaneous surface-pressure isotherm measurement with an NR experiment for pure monolayers of partially deuterated ACE16 azacrown ether (d-ACE16), as a first step to resolve and understand the above discrepancy. NR provides a sensitive technique for resolving structures at interfaces on the order of a few angstroms perpendicular to an interface, taking advantage of the short wavelengths of the neutrons.13 NR, besides being nondestructive, provides a high degree of sensitivity when using isotopic substitutions to achieve large and tunable contrast14 between chemically discrete entities. The specular neutron reflectometry allows the determination of the neutron refractive index profile perpendicular to an interface. In a medium, the neutron refractive index, n, is commonly written as n ≈ 1 - (λ2/2π)Nb, where λ is the neutron wavelength, N is the number density of nuclei, and b is the average bound coherent scattering length of a nucleus. The multiple Nb is the scattering length density and is simply related to the volume fraction composition of a medium. In the present paper we have extended our previous NR study to mixtures of d-ACE16 and three different chain-length fatty acids (palmitic, stearic, and hexacosanoic) in order to more closely mimic the composition of an ion-transporting membrane used in actual PLM devices. In the present study we take advantage of neutron contrast variation, hence the necessity for the custom deuteration synthesis of the macrocyclic ligand used in this study. We report the systematic study of the effect of the chain length of the fatty acid on the stability of the mixed Langmuir monolayers of ACE16 with fatty acid at the air-water interface using NR. The detailed conformations of both the ACE16 and the stearic acid (SA) were determined taking advantage of the contrast variation technique offered by neutron scattering using mixtures of deuterated and protonated components. The effect of fatty acid on the adsorbed amount of ACE16 is also discussed.
2. Experimental Section Chemicals. The d-ACE16 N,N0 -di(hexadecyl-d33)-4,13-diaza18-crown-6 ether (C44H24D66N2O4) with molecular weight of 777.58 g mol-1 was synthesized by BDG Synthesis.15 Protonated fatty acids: h-palmitic acid (h-PA, C16H32O2), h-stearic acid (h-SA, C18H36O2) and h-hexacosanoic acid (h-HA, C26H52O2), as well as deuterated (98% D) stearic acid (d-SA) were purchased from Aldrich. All solutions and isotopic mixtures were prepared by mass. Null reflecting water (water with a neutron refractive index equal to that of air) contains approximately 8% (by volume) D2O mixed with ultrapure H2O (18 MΩ cm-1) processed in a Milli-Q water purification unit. Neutron Reflectometry. In order to understand the contribution of fatty acid to the stability of a spread monolayer of (13) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E.; Roser, S. J.; Mclure, I. A.; Hillman, A. R.; Richards, R. W.; Staples, E. J.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899. (14) Bacon, G. E.; Noakes, G. R. In Neutron Physics; Taylor & Francis: London, 1969. (15) BDG Synthesis, New Zealand, www.bdg.co.nz.
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azacrown ether (d-ACE16) at the air-water interface, we used a combination of a surface-pressure isotherm with NR measurements. In this experiment, both the d-ACE16 and the fatty acid solutions were prepared separately in distilled chloroform. Mixtures of d-ACE16 and fatty acid (1:1 (mol/mol)) were then prepared prior to the spreading. A standard NIMA Langmuir trough was used in these experiments, and the average area of the trough was varied in the range of 130-490 cm2 by two computercontrolled moveable barriers. Prior to use, the trough was cleaned in a standard way. The trough was then filled with the nullreflecting water, the water surface was swept, and aspirated before the deposition of the mixed azacrown ether and fatty acid film. After spreading, the solvent was allowed to evaporate for about 15 min. All measurements were conducted at T=(298 ( 0.5) K. The trough was situated on a vibration-isolation table covered with a Perspex lid with two windows to prevent any perturbation of the surface. A constant pressure was maintained during acquisition of each reflectivity profile by the automatic adjustment of the Langmuir trough area (which was in all cases larger than the neutron beam footprint). The height of the water surface was initially adjusted to within (1 μm of the same absolute position using a Micro-Epsilon optical displacement sensor prior to neutron alignments runs. This ensures that a constant scale factor is applied to the normalization of all raw reflectivity data. The SURF reflectometer13 at the pulsed neutron source at the STFC Central Facility at the Rutherford Appleton Laboratory in Oxfordshire, U.K., was used to measure the NR profiles. The reflectivity R(Q) was measured using the time-of-flight technique as a function of momentum transfer Q (Q=(4π sin θ)/λ, normal to the interface, where θ is the grazing angle of incidence of neutrons with wavelength λ). A polychromatic beam of neutrons with wavelengths in the range 0.5 < λ/A˚ < 6.5 was reflected at an incidence angle of θ = 1.5° and detected using a single 3He detector. The reflectivity profiles were fitted to a single slab model, thickness d, scattering length density Nb, and zero roughness. It has been shown the adsorbed amount Γ can be calculated using the following formula:16,17 Γ¼
ðNbÞlayer d • NA Σi bi
ð1Þ
where Σibi is the sum of the coherent scattering length for each species (fatty acid and the ACE16 molecules), d is the layer thickness, and NA is Avogadro’s number. For the characterization of the effect of the chain length of the fatty acid on the conformation of the azacrown ether, the mixtures of d-ACE16 with protonated fatty acids were used. For each mixture of fatty acid and d-ACE16, a series of reflectivity profiles was measured along the surface pressure isotherm. In the subsequent experiment, in order to fully characterize both the azacrown ether and the fatty acid conformation in the mixed monolayers on null-reflecting aqueous subphase, the following mixtures were used: d-ACE16 with h-SA and h-ACE16 with d-SA. In this case, the fitted Nb values can be written as a sum of the contributions from each of the four species representing the layer: ACE16, SA, air, and water. With the contribution of air and water (null-reflecting water) equal to zero (bair =bnrw =0), the total Nb of the layer can be written as Nblayer ¼ NACE16 bACE16 þ NSA bSA
ð2Þ
The value for the scattering length density of the layer (Nblayer) was obtained from a single layer fit to the data for both contrasts (Figures 2 and 4). These two values for the scattering length (16) Zarbakhsh, A.; Querol, A.; Bowers, J.; Yaseen, M.; Lu, J. R.; Webster, J. R. P. Langmuir 2005, 21, 11704. (17) Lu, J. R.; Thomas, R. K.; Penfold, J. Adv. Colloid Interface Sci. 2000, 84, 143.
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Figure 1. Reflectivity data for the mixed Langmuir monolayer of h-palmitic and d-ACE16 (1:1 mol/mol) for surface pressures in the range 11 to 40 mN m-1 on null reflecting water measured at the incidence angle θ = 1.5°. The solid lines correspond to a single layer model with film layer thickness 22 ( 1 A˚. The scattering length density Nb increases with surface pressure from 3.26 to 3.78 10-6 A˚-2. The data are shifted up by multiples of 10 (10, 100, etc.) for the purpose of clarity. Figure inset highlights the changes in the reflectivity profiles as a function of surface pressure.
Figure 2. Reflectivity data for the mixed Langmuir monolayer of h-stearic acid and d-ACE16 (1:1 mol/mol) for surface pressures in the range of 11-40 mN m-1 on null reflecting water measured at the incidence angle θ = 1.5°. The solid lines correspond to a single layer model with film layer thickness 21 ( 1 A˚. The scattering length density Nb increases with surface pressure from 3.27 to 3.68 10-6 A˚-2. The data are shifted up by multiples of 10 (10, 100, etc.) for the purpose of clarity.
Table 1. Coherent Neutron Scattering Length for ACE16 and SA
deuterated protonated
ACE16 bACE16/fm molec-1
SA bSA/fm molec-1
671.300 -2.122
363.966 -3.356
densities are then used to calculate the number densities, NACE16 and NSA by solving eq 2 for the two contrasts. The values for the coherent scattering lengths (bACE16 and bSA) for both protonated and deuterated ACE16 and SAs are given in Table 1.
3. Results The reflectivity profiles for mixtures of d-ACE16 with h-palmitic and h-stearic for surface pressures in the range 11 to 40 mN m-1 are shown in Figures 1 and 2. The respective mixture with the longest alkyl-chain derivative, h-hexacosanoic acid (Figure 3) could only be compressed to the surface pressure Π=15 mN m-1 using the current setup. The analysis of NR data involves determining the Nb(z) from the measured reflectivity spectrum R(Q) by the optical matrix method18,19 based on a layer model (each layer with a characteristic thickness, composition and interlayer roughness). It was found that a simple one-layer fit was adequate to describe the reflectivity profiles. The fits to the data are shown as solid lines in Figures 1-3. The fitted layer thickness for the ACE16 in the presence of palmitic acid was 22 ( 1 A˚, which corresponds well to the length of a fully stretched hexadecyl chain (∼21.7 A˚) of the ACE16. No significant changes in the layer thickness were observed for ACE16 in the presence of SA (21 ( 1 A˚). However, for the mixture of ACE16 and hexacosanoic acid, the fitted layer thickness dropped to 19 ( 1 A˚. (18) Born, M.; Wolf, E. In Principles of Optics; Peragamon: Oxford, 1980. (19) Heavens, O. S. In Optical Properties of Thin Films; Butterworth: London, 1955.
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Figure 3. Reflectivity data for the mixed Langmuir monolayer of h-hexacosanoic acid and d-ACE16 (1:1 mol/mol) for surface pressures in the range of 11-15 mN m-1 on null reflecting water measured at the incidence angle θ = 1.5°. The solid lines correspond to a single layer model with film layer thickness 19 ( 1 A˚. The scattering length density Nb increases with surface pressure from 3.19 to 3.43 10-6 A˚-2. The data are shifted up by multiples of 10 (10, 100, etc.) for the purpose of clarity.
The scattering length density (Nb) for this layer was also lower than the values for the other two fatty acids (Figures 1-3). This is because of some degree of dissolution of the ACE16 by this longer chain fatty acid, caused by the larger mismatch of the alkyl chain of the two species involved (see below). The respective fitted scattering length densities of these layers obtained from the best-fits were then used to estimate the total adsorbed amount of the azacrown ether in the mixed layers as a function of surface pressure using eq 1. The corresponding adsorbed amounts of d-ACE16 in the presence of different fatty acids are shown in Figure 5. Clearly, the highest adsorbed amount of ACE16 is observed for the mixture with hexadecanoic (palmitic) acid, pointing to the pivotal role of chain length matching for the optimum stabilization of the mixed monolayer. In a second experiment, the individual contributions of both the SA and ACE16 at the interface were determined using a series Langmuir 2010, 26(23), 18194–18198
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Figure 4. Reflectivity data for the mixed Langmuir monolayer of d-stearic and h-ACE16 (1:1 mol/mol) for surface pressures in the range 11 to 40 mN m-1 on null reflecting water measured at the incidence angle θ = 1.5°. The solid lines correspond to a single layer model with film layer thickness 24 ( 1 A˚. The scattering length density Nb increases with surface pressure from 1.91 to 2.8210-6 A˚-2.
Figure 6. The adsorbed amount for (a) stearic acid and (b) ACE16 obtained from two H/D contrasts for mixtures of protonated/deuterated SA and ACE16. The solid lines show the estimated adsorbed amounts of the individual components from the area of the trough as a function of surface pressure assuming no dissolution. The inset in panel b shows the adsorbed amount for the ACE16 in the presence of SA (filled circles), compared to the case when ACE16 is the only component of the spread monolayer (open squares, taken from ref 10).
Figure 5. The adsorbed amount of ACE16 in the mixed layer of d-ACE16 with different chain-length fatty acids (palmitic acid (O), stearic acid (b), and hexacosanoic acid (2)) as a function of surface pressure. The adsorbed amount10 of d-ACE16 alone (0) as a function of surface pressure is shown for comparison.
of reflectivity profiles measured for a number of H/D contrasts. For this purpose, the following solutions were used: d-ACE16 with h-SA (Figure 2) and h-ACE16 with d-SA (Figure 4) spread on the null-reflecting aqueous subphase. The adsorbed amounts of both components (SA and ACE16) were then estimated using eq 2. The results are shown in Figure 6a,b, respectively. The solid lines show the adsorbed amounts of the individual components estimated from the area of the trough as a function of surface pressure. For these estimations, we have assumed that no dissolution of any component takes place, thus the deviation of experimental NR data (points) from these lines is a measure of the extent of losses of monolayer material. The inset in Figure 6b clearly indicates an increase of the adsorbed amount of the ACE16 in the presence of SA (filled circles), compared to the case when ACE16 is the only component of the spread monolayer (open squares).
4. Discussion The adsorbed amount of SA as a function of surface pressure in the presence of ACE16 was estimated from the reflectivity Langmuir 2010, 26(23), 18194–18198
profiles. Using two contrasts, the obtained reflectivity profiles enabled us to estimate the contribution to the total adsorbed amount from SA and ACE16 separately (Figure 6a,b, respectively). Figure 6a clearly shows that the fatty acid in the mixed Langmuir monolayer is stable against dissolution up to a surface pressure of 20 mN m-1. Above this pressure, however, the calculated and estimated values for the adsorbed amount of SA start to deviate, indicative of partial dissolution of the fatty acid into the aqueous subphase. Such discrepancy has already been observed in the literature for fatty acids.20,21 In the present system, this process is further complicated by the presence of ACE16, because of the mutual interactions between the two molecules. It has been shown previously that ACE and fatty acid share a proton of the carboxylic acid and are hydrogen bonded both in bulk and at the interface.22 In addition to the headgroup interaction, the strong effect of fatty acid chain length on the retention of d-ACE16 of the mixed ACE-fatty acid monolayers suggests a significant contribution of van der Waals interactions between the alkyl chains of ACE and fatty acid (see below). Figure 6b shows the corresponding contribution of the adsorbed amount of ACE16 in the presence of SA. The solid line shows the estimated surface concentration of ACE using the amount spread and the area of the trough under assumption of no dissolution of ACE16 taking place. In contrast to the corresponding surface concentration of SA (Figure 6a), clearly a deviation from the no-dissolution line is (20) Heikkila, R. E.; Deamer, D. W.; Cornwell, D. G. J. Lipid Res. 1970, 11, 195–200. (21) Qi, S.; Roser, S.; Deutsch, D.; Barker, S.; Craig, D. J. Pharm. Sci. 2008, 97 (5), 1864–1877. (22) Wojciechowski, K.; Buffle, J. Biosens. Bioelectron. 2004, 20, 1051.
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Figure 7. The adsorbed amounts for d-ACE16 in the presence of fatty acids calculated from the Nima trough versus that determined by the fits to the NR spectra (palmitic acid (O),stearic acid (b), hexacosanoic acid (2), and d-ACE16 alone (0), taken from ref 10). The solid line shows the calculated ideal profile for the d-ACE16 adsorbed amount with no discernible dissolution of the monolayer taking place.
observed, even at very low surface pressures. Taken together, the data from Figure 6 point to a much higher depletion of ACE16 than that of SA from the mixed monolayer. Nevertheless, it should be stressed that the extent of ACE16 dissolution from the spread Langmuir monolayer is significantly reduced in the presence of SA. (Figure 6b inset). The effect of fatty acid chain length on the retention of ACE16 at the interface can be deduced from the NR results shown in Figures 1-3. The adsorbed amounts for the d-ACE in presence of the three fatty acids (palmitic, stearic, and hexacosanoic acids) were obtained from the fits of the reflectivity profiles and are shown in Figure 5. The highest adsorbed amount for the ACE was observed in the presence of the palmitic acid, closely followed by stearic acid and hexacosanoic acid. This result points to the importance of matching the lengths of alkyl chains of ACE16 to those of the fatty acid. Similar observations have already been reported for several types of molecules forming mixed monolayers.23 Both palmitic acid and stearic acid are very efficient in retaining the ACE16 in the mixed Langmuir monolayer up to the adsorbed amount of about 2 10-6 mol m-2 (actual value, derived from the NR data), corresponding to the surface pressure of about 20 mN m-1. This is indicated by relatively minor deviations of the NR experimental points from the “no-dissolution” line in Figure 7. The line shows the expected linear relation between the actual adsorbed amount derived from the NR data and the one derived (23) Wang, Z. N.; Li, G. Z.; Mu, J. H.; Zhang, W. X. J. Surfactants Deterg. 2002, 5(4), 391.
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from the deposited amount (i.e., assuming that the amount of monolayer material deposited does not change due to dissolution in the subphase). We have shown already in our previous paper,10 that bare d-ACE16 monolayers show marked deviations from the “non-dissolution” line. The corresponding data is shown for comparison also in Figure 7. Up to the adsorbed amount of 2 10-6 mol m-2, the attraction between ACE16 and fatty acid is sufficient to maintain the integrity of the monolayer. However, above this value both the fatty acid and d-ACE16 start desorbing, and the monolayer becomes more and more depleted. The slopes of the curves for mixtures of d-ACE16 with palmitic acid and stearic acid become comparable to that of bare d-ACE16 (Figure 7), suggesting that above the threshold surface pressure of 20 mN m-1 the fatty acid is not able to maintain the azacrown ether any more. The deviation of the experimental points from the “no-dissolution” line for SA in the mixed monolayer in Figure 6a suggests that the fatty acid may dissolve together with the ACE16, e.g., as a hydrogen-bonded complex. The interaction between the two molecules is thus beneficial for retaining the azacrown ether at the interface, but only at low and intermediate surface pressure regions. Above it, the tendency of both azacrown ether and fatty acid to dissolve into the aqueous subphase drastically increases, hence the availability of the two carrier molecules to take part in chemical reactions (e.g., ion complexation). These observations throw new light on the role of mutual interactions between the two carrier molecules (fatty acid and azacrown ether) in the mechanism of ion transport through the PLM systems.
5. Conclusions Spread monolayers of mixtures of d-ACE16 with different chain-length fatty acids (palmitic, stearic, and hexacosanoic) were analyzed using NR at the air-water interface. By combining the surface pressure-area isotherm with NR measurements, the effect of the presence of fatty acids on the extent of the d-ACE16 monolayer dissolution at the air-water interface was quantified. Although all the fatty acids were capable of enhancing the retention of ACE16 at the interface, the optimum condition was achieved when the chain length of the fatty acid was matched to that of the ACE16 (i.e., palmitic acid). These results provide a clear experimental justification for the current empirical composition of the carrier in the PLM membrane for metal ion transport, where the chain length of the fatty acid (dodecanoic acid) is closely matched to that of the azacrown ether (ACE10).8 In addition, the surface pressure-dependent stability of the mixed monolayer may help to understand the complex role of the two carrier components in the PLM. Acknowledgment. This work was financially supported by the Polish Ministry of Science and Higher Education (Grant No. N N204 236934). The authors wish to thank the STFC for allocation of beam time at ISIS and for provision of consumables and subsistence.
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