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Static and Dynamic Properties of Calixarene Monolayers at the Air/Water Interface. 2. Effects of Ionic Interactions with p-Dioctadecanoylcalix[4]arene Lan-Hui Zhang,† Alan R. Esker,‡ Kwanghyun No,§ and Hyuk Yu* Department of Chemistry, University of WisconsinsMadison, Madison, Wisconsin 53706 Received January 6, 1997. In Final Form: November 17, 1998 The static and dynamic properties of p-dioctadecanoylcalix[4]arene monolayers on the air/water interface are examined relative to the effects of inorganic and polymeric counterions (i.e., spermine, poly(ethylenimine), and poly(L-lysine)) in the subphase over a large range of pH. The methods adopted are the techniques of the Wilhelmy plate and surface light scattering (SLS). At moderate subphase pH values, 3.5-10, the polyions significantly expand the monolayer but do not appreciably change the monolayer surface viscoelasticity as detected by SLS. A scheme of complex formation and acid-base equilibrium is used to explain the surface pressure isotherms while strong lateral packing interactions of the hydrophobic chains and aromatic rings, which predominate over the electrostatic interactions of the phenoxide rings of calixarene, are invoked to explicate the viscoelastic properties. Simple univalent inorganic ions do not show any specific effects on either the static or dynamic properties of these monolayers.
Introduction Agonist or effector (ligand) binding to membrane-bound proteins (receptors) is a primary step in the signal transduction process across the cell membrane.1 However, how these ligands affect the lateral organization, dynamic responses, and the mobility of membrane components requires further study. To this issue, monolayer techniques can be considered as a convenient way to study membrane behavior since cell membranes can be modeled as a combination of two monolayers. However, this simplification introduces the problem of interfacial protein denaturation in monolayers.2 Consequently, a simple synthetic compound, which can serve as a model for a membranebound receptor in lipid monolayers is desired as a substitute. Calixarenes are a class of macrocyclic compounds with conelike three-dimensional molecular structures.3,4 They may possess ionophore activity and other manifestations of host-guest properties, similar in character to cell receptors but with greater monolayer stability. Hence, calixarenes may be suitable as model membrane-bound receptors in lipid monolayers for a wide variety of membrane mimetic studies. Adjusting the amphipathic nature of the calixarene molecule by chemical modification via functional groups can lead to the formation of stable monolayers at the air/water interface. In a previous study,5 a new calixarene compound, p-dioctadecanoylcalix[4]* To whom correspondence should be addressed. † Current address: National Institute of Standards and Technology, 100 Bureau Dr., Stop 8542, Gaithersburg, MD 20899-8542. ‡ Current address: Department of Chemistry, Lehigh University, Bethlehem, PA 18015. § Permanent address: Department of Chemistry, Sookmyung Women’s University, Seoul 140-742, Korea. (1) Mathews, C. K.; van Holde, K. E. Biochemistry; The Benjamin/ Cummings Publishing Company, Inc.: New York, 1990. (2) Macritchie, F. Adv. Protein Chem. 1978, 32, 283. (3) Gutsche, C. D. In Calixarenes: A Versatile Class of Macrocyclic Compounds; Vicens, J., Bo¨hmer, V., Eds.; Kluwer Academic Publishers: Boston, 1991. (4) Gutsche, C. D. In Calixarenes; Stoddart, J. F., Ed.; The Royal Society of Chemistry, Thomas Graham House, Science Park: Cambridge, 1989.
arene, was reported to form stable rigid monolayers over a large range of subphase pH. Additionally, it was shown that subphase acidity significantly affects monolayer packing density because of stepwise dissociation of the phenol groups in the calixarene molecule. Besides applications based on their ionophore properties, calixarenes have found uses as phase-transfer agents,6 accelerants in instant adhesives, hydrolysis catalysts,7,8 molecular switches,9 molecular capsules for substance delivery via “poly” calixarenes,10 and Langmuir-Blodgett films for membrane filtration.11 It has been reported that porous calixarene molecules might be used to fabricate the most energy-efficient and cost-effective membrane filters for chemical separation, concentration, and purification.12 However, the permselectivity of these perforated membranes often suffers from membrane defects, presumably due to the high rigidity of these monolayers.13 It has also been reported that polyions significantly affect the aggregation behavior of surfactants both in bulk systems and at interfaces, by inducing various assembly architectures. An example of a bulk system is single-chain cetyltrimethylammonium ions which form stable vesicles with polyacrylate counterions but form micellar aggregates with simple bromide counterions in aqueous media.14 At the air-water interface, polyion complexation of ionic amphiphiles with oppositely charged polyions is seen as one of the most fascinating techniques for the facilitated (5) Esker, A. R.; Zhang, L.; Olsen, C. E.; No, K.; Yu, H. Langmuir 1999, 15, 1716. (6) Araki, K.; Yanagi, A.; Shinkai, S. Tetrahedron 1993, 49, 6763. (7) Shinkai, S.; Shirahama, Y.; Tsubaki, T.; Manabe, O. J. Chem. Soc., Perkin Trans. I 1989, 1859. (8) Komiyama, M.; Isaka, K.; Shinkai, S. Chem. Lett. 1991, 937. (9) Murakami, H.; Shinkai, S. Tetrahedron Lett. 1993, 34, 4237. (10) Arimura, T.; Matsumoto, S.; Teshima, O.; Nagasaki, T.; Shinkai, S. Tetrahedron Lett. 1991, 32, 5111. (11) Conner, M. D.; Janout, V.; Kudelka, I.; Dedek, P.; Zhu, J.; Regen, S. L. Langmuir 1993, 9, 2389. (12) Lee, W.; Hendel, R. A.; Dedek, P.; Janout, V.; Regen, S. L. J. Am. Chem. Soc. 1995, 117, 6793. (13) Dedek, P.; Webber, A. S.; Janout, V.; Hendel, R. A.; Regen, S. L. Langmuir 1994, 10, 3943. (14) Wakita, M. A.; Edwards, K. A.; Regen, S. L. J. Am. Chem. Soc. 1988, 110, 5221.
10.1021/la9700174 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/30/1999
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deposition of stabilized monolayers.15,16 Stable monolayers formed from complexes of ionic surfactants with synthetic polyelectrolytes have been used to prepare LangmuirBlodgett films which mimic biopolymer systems where pH is used to reversibly open and close channel-like pathways in the multilayered assemblies.17-19 Polyelectrolytes were also used in the stepwise fabrication of multilayer films.20 In this work, attention is focused on the effects of a variety of cations in the subphase, including simple inorganic ions and polyions, on the static and dynamic behavior of p-dioctadecanoylcalix[4]arene monolayers. Experimental Section Materials. p-Dioctadecanoylcalix[4]arene was the same compound used in a previous study.5 Spermine tetrahydrochloride, H2N-(CH2)4-NH-(CH2)3-NH-(CH2)4-NH2‚4HCl (95∼98%, TLC) and poly(L-lysine hydrobromide) (PLS, Mw ∼ 56 000, Mw/ Mn ) 1.15) were purchased from the Sigma Chemical Co. Poly(ethylenimine) (PEI-18, Mw ∼ 1800) was obtained from the Dow Chemical Co. A fluorescently labeled lipid, 1-acyl-2-[12-[(7-nitro2-1,3-benzoxadiazol-4-yl)amino]dodecanoyl]phosphatidylethanolamine (NBD-PE) came from Avanti Polar Lipids, Inc. Sodium chloride, sodium tetraborate, sodium dihydrogenphosphate, potassium dihydrogenphosphate, cesium bromide, and hydrochloric acid were purchased from Aldrich with 99.999% purity. Sodium hydroxide was purchased from Aldrich with 99.99% purity. Sodium chloride, sodium tetraborate, and cesium bromide were further purified by baking them at 550 °C for 5 h to eliminate possible organic impurities. The other materials were used without further purification. The subphases were prepared with distilled water which was further purified by a Millipore Q2 system (Millipore). Surface Pressure Measurements. Experiments were carried out in a Teflon trough (28.5 × 11.0 × 1.25 cm3) housed in a Plexiglas box. To maintain high relative humidities (>80%) which keep the sandblasted platinum plate (2.63 × 1.11 × 0.01 cm3) well-wetted, and to reduce the effect of carbon dioxide in the air on the subphase pH, humid nitrogen gas was gently flowed through the box. The trough, moving barrier, and platinum plate were cleaned before each experiment with an H2SO4/Nochromix mixture (Godax Laboratories, Inc.) and were rinsed with Millipore-Q filtered distilled water. In all the experiments, calixarene and NBD-PE were mixed in a 100:1 molar ratio and dissolved in HPLC grade chloroform (Aldrich) to make spreading solutions with total concentrations of ∼0.1 g‚L-1. About 200 µL of spreading solution was spread on the subphase with a Hamilton microsyringe to prepare the monolayers. At least 30 min were allowed for the solvent to evaporate before measurements were taken. The subphase solutions were prepared to keep the ionic strengths around 0.04 M. The pH values of the subphase solutions were measured before and after each experiment to ensure the pH remained constant. The surface tensions of the bare and monolayer-covered surfaces were measured by the Wilhelmy plate technique with a Cahn 2000 model electrobalance. Surface tensions were taken, and surface light-scattering measurements (SLS, see below) were begun once a stable surface tension value was obtained (typically, ∆σs < 0.04 mN‚m-1 over a 5 min period). The surface concentration was controlled by either stepwise compressions of the Teflon barrier or successive additions of spreading solution. The temperature of the subphase was held constant at 23.0 ( 0.1 °C by circulating thermostatted water (Lauda RM6) through a glass coil placed in the bottom of the trough. Surface Light Scattering. All SLS experiments were performed simultaneously with the static surface tension measurements. The SLS experimental setup has been described in (15) Shimomura, M.; Kunitake, T. Thin Solid Films 1985, 132, 243. (16) Higashi, N.; Kunitake, T. Chem. Lett. 1986, 105. (17) Higashi, N.; Sunada, M.; Niwa, M. Langmuir 1995, 11, 1864. (18) Higashi, N.; Shimoguchi, M.; Niwa, M. Langmuir 1992, 8, 1509. (19) Niwa, M.; Mukai, A.; Higashi, N. Macromolecules 1991, 24, 3314. (20) Mao, G.; Tsao, Y.; Tirrell, M.; Davis, H. T.; Hessel, V.; Ringsdorf, H. Langmuir 1993, 9, 3461.
Figure 1. The pH dependence of the molecular area of p-dioctadecanoylcalix[4]arene/NBD-PE (100:1, molar ratio) monolayers on subphase solutions with and without (vertical bars) a ligand (2 mM PEI, open squares; 0.1 mM PLS, filled diamonds) at three surface pressures (23.0 °C, ionic strength ) 0.04 M). The open circles indicated by the solid and dashed arrow show the results with K+ and Cs+ in subphase, respectively. The curves are only provided to highlight the overall trends. detail elsewhere,21 with a design based on Ha˚rd et al.22 The laser wavelength, λ, the incident angle, θ, and the distance from the interface to the detector, R, were 632.8 nm, 64.4°, and 3.64 m, respectively. The laser beam profile was measured to ensure that it has a Gaussian shape. The full width at half-maximum intensity of the Gaussian laser beam profile ∆ui and the wave vector k were obtained by calibrating against water as the standard. In this work, the fourth-, fifth-, and sixth-order diffraction spots were used to define the scattering angles. The corresponding wave vectors were k ) 266, 328, and 389 cm-1, respectively.
Results and Discussion Effects of Univalent Inorganic Ions. Previously,5 it was reported that sodium ions in the subphase affect neither the static nor dynamic properties of p-dioctadecanoylcalix[4]arene monolayers. The effects of other univalent inorganic ions have been studied as well. Figure 1 shows the surface area occupied by each p-dioctadecanoylcalix[4]arene molecule in a monolayer as a function of subphase pH at a given surface pressure. It can be seen that the molecular areas of monolayers with potassium or cesium ions in the subphase follow the same tendencies as monolayers with sodium ions in the subphase, which implies that these species of univalent ions do not induce any significant changes in p-dioctadecanoylcalix[4]arene surface activity. These results are different from those observed by Ishikawa et al. and Dei et al. for monolayers of other calixarene molecules.23,24 For the case of Ishikawa et al.’s work, the difference in results may be due to structural differences as the ion-binding moieties are ester (21) Sano, M.; Kawaguchi, M.; Chen, Y.-L.; Skarlupka, R. J.; Chang, T.; Zografi, G.; Yu, H. Rev. Sci. Instrum. 1986, 57, 1158. (22) Hard, S.; Hamnerius, Y.; Nilsson, O. J. Appl. Phys. 1976, 47, 2433. (23) Ishikawa, T.; Kunitake, T.; Matsuda, T.; Otsuka, T.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1989, 736. (24) Dei, L.; Casnati, A.; Lo Nostro, P.; Baglioni, P. Langmuir 1995, 11, 1268.
Effect of Subphase Ions on Calixarene Monolayers
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Figure 2. Surface pressure isotherms of p-dioctadecanoylcalix[4]arene monolayers with (filled symbols) and without (open symbols) 1 mM spermine at two pH values (5.5, circles; 8.4, triangles; 23.0 °C, I ) 0.04 M). The dashed vertical line at 0.78 nm2‚molecule-1 corresponds to the value predicted by CPK models for calix[4]arenes in a conelike conformation.
groups versus the phenol groups here. This is probably not the case for the work of Dei et al. as their calixarene, p-tert-butylcalix[6]arene, also has phenol groups for ion binding. It should also be noted that these two groups disagree on whether p-tert-butylcalix[6]arene forms stable monolayers. Considering this, it is much more likely that the discrepancies on univalent ion effects are due to the high salt concentrations of the other studies (1 M). Under high salt concentrations, changes in the isotherm may be due to changes in the solvent quality of the interface rather than specific binding interactions. An additional problem at high salt concentrations is that even high-purity ionic compounds can have substantial quantities of trace impurities which are either surface-active or ligands for the monolayer in question. Effects of Polyions. The greater charge of polyions results in stronger electrostatic interactions with oppositely charged species. It is possible for polyions to attract several counterions simultaneously and act as crosslinking agents. The studies by Markowitz et al.25 and Conner et al.11,26 show that both ionic and covalent crosslinking of calixarene molecules produce more condensed monolayers with significantly larger surface shear viscosities. To highlight the binding interaction between monolayer molecules and linking agents in the subphase, spermine as a linking agent on p-dioctadecanoylcalix[4]arene monolayers was investigated at multiple subphase pH values, since the calixarene monolayer is sensitive to subphase pH. Figure 2 shows surface pressure isotherms of these monolayers with and without spermine in the subphase at pH ) 8.4. The isotherm with spermine is slightly expanded, which might be taken as an indication that the spermine is incorporated into the calixarene monolayer. It was observed earlier in this laboratory that the binding of spermine to stearic acid monolayers produces a more condensed monolayer.27 These apparently contradictory results can be understood in terms of differences in molecular size and charge magnitude. Around pH 6, both monoprotic stearic acid and spermine molecules are completely ionized. Since the cross-sectional area of each stearic acid molecule is small, it is possible (25) Markowitz, M. A.; Janout, V.; Castner, D. G.; Regen, S. L. J. Am. Chem. Soc. 1989, 111, 8192. (26) Conner, M. D.; Janout, V.; Regen, S. L. J. Am. Chem. Soc. 1993, 115, 1178. (27) Yazdanian, M. Ph.D. Thesis, University of WisconsinsMadison, Madison, WI, 1990.
Figure 3. Schematic illustration of the interactions between monolayer components and binding agents in the subphase.
for four stearic acid molecules to bind to the four charge sites of an individual spermine molecule as illustrated in Figure 3a. The end-to-end distance of four chargeneutralized stearic acid molecules is 15.0 Å, calculated from the experimental surface area occupied by each molecule, A, assuming maximal packing density on the surface. This result is a bit smaller than the maximum end-to-end distance of a spermine molecule (15.5 Å). This is presumably due to charge neutralization resulting in reduced electrostatic repulsions. As a result, the stearic acid monolayer is condensed by spermine via cross-linking from the subphase. In the case of p-dioctadecanoylcalix[4]arene on a pH 8.4 subphase, both the calixarene and spermine molecules are partially dissociated with every spermine molecule carrying roughly 2 units of charge (pKa’s of bulk spermine are 10.94, 10.12, 9.04, and 7.97).28 Since the molecular area of calixarene is much larger than that of stearic acid and a calixarene molecule can carry multiple negative charges, the binding pattern illustrated in Figure 3b may be applicable. For this situation, each calixarene molecule may bind to the center of two or more neighboring charged sites with the packing density depending on the distance between two neighboring charge centers. According to such a binding pattern, spermine inserting from the subphase results in a more expanded calixarene monolayer. For the case of the pH ) 5.5 subphase (Figure 2), the expansion effect of spermine on calixarene monolayers can be illustrated by a binding pattern similar to the one shown in Figure 3c. Under these conditions, the experimental distance between neighboring calixarene molecules happens to be the distance between two charge centers in the spermine molecule if each calixarene molecule binds to two positive charges of a spermine molecule. This result requires further explanation considering the calixarene molecule used here is not dissociated at pH ) 5.5.5 (28) Templeton, D. M.; Sarkar, B. Can. J. Chem. 1985, 63, 3122.
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To understand the binding mechanism, it is first useful to consider the acid-base equilibria operative between molecules in the monolayer and species in the bulk: m
m
m n n
n
n m
m
n
n
where H4L and B donate calixarene and spermine molecules (or PEI and PLS, see below), respectively. Without spermine in the subphase, only equilibrium (1) exists and undissociated H4L is the predominant species in the monolayer at pH ) 5.5. With spermine in the bulk, equilibria (3) and (4) also exist. The formation of the complex (BHn)m(H4-mL)n in the monolayer may be energetically favorable, which induces an expanded monolayer according to the aforementioned binding pattern. Because of complex formation, the pH range over which complexes form is larger than the corresponding pH range for singlecomponent calixarene dissociation. Essentially, this electrostatic interaction between calixarene and spermine induces the adsorption of spermine onto the monolayer. Obviously, if the subphase pH value is low enough, equilibrium (1) shifts to left and spermine will have no effect. It should be pointed out that an alternative explanation of spermine’s effect is expansion due to the penetration of spermine molecules into the monolayer. However, as seen in Figure 2, this incorporation of spermine does not appear to affect the collapse concentration, which still corresponds to a closely packed cylindrical conformation with all aromatic rings essentially perpendicular to the interface. The observed limiting area is in excellent agreement with the value predicted from Corey-PaulingKoetun (CPK) models (0.78 nm2‚molecule-1). We may infer from this that adsorbed spermine ions do not insert into the monolayer, at least at the surface concentration where collapse occurs. This is reasonable, considering spermine is extremely hydrophilic and shows no surface activity in the absence of calixarene, and also consistent with the conclusion that electrostatic interactions are the primary factor in the adsorption mechanism. Figures 4 and 5 show results for other subphase polyions, polyethylenimine and poly(L-lysine), which support the above adsorption mechanism and monolayer binding pattern (filled symbols). For the convenience of comparison, relevant results in the absence of binding agents are included (open symbols). The subphase pH dependence of molecular area for these monolayers with PEI or PLS in the subphase is also shown in Figure 1 for three surface pressures (1, 2, and 5 mN‚m-1). Once again, these results are consistent with the formation of polyion complexes which occur at pH values lower than expected purely based on calixarene dissociation. These results show that a small amount of PEI in the subphase significantly expands the monolayer when the subphase pH is between 3.5 and 10, but has smaller-tonegligible effects outside this range. This can be explained by equilibria (1)-(4). With increasing subphase pH, equilibrium (1) shifts to the right and equilibrium (2) shifts to the left. Consequently, the intermediate range of subphase pH at 3.5-10 strongly favors complex formation and significant binding effects are observed. When the subphase pH is lower than 3.5 or higher than 10,
Figure 4. Surface pressure isotherms of p-dioctadecanoylcalix[4]arene monolayers on subphases with and without 2 mM poly(ethylenimine) at various pH values (23.0 °C, I ) 0.04 M).
Figure 5. Surface pressure isotherms of p-dioctadecanoylcalix[4]arene monolayers on subphases with and without 0.1 mM poly(L-lysine) (PLS) at various pH values (23.0 °C, I ) 0.04 M).
equilibrium (1) or (2) lie to the left, which results in no significant binding between calixarene in the monolayer and PEI in the subphase (the bulk dissociation constant pKa of PEI is 8.56).29 Therefore, no effect is detected. The surface pressure isotherms for monolayers outside the pH range of 3.5-10 are essentially identical to those without polyions in the subphase. The similarity between the molecular area at pH ≈ 8.4 and for pH values in the 10-12 range is probably just coincidental as some PEI is most likely incorporated into the film at intermediate pH values. Although the effect of poly(L-lysine) (PLS) is generally the same as that of polyethylenimine, binding resulting in more expanded monolayers, PLS shows some differences. The first as seen in Figure 1 is that the binding interactions seem to be occurring at lower pH values. This may be interpreted as the interaction between poly(Llysine) and calixarene is stronger than that between polyethylenimine and calixarene. Second, at the surface (29) Kobayashi, S.; Hiroishi, K.; Tokunoh, M.; Saegusa, T. Macromolecules 1987, 20, 1496.
Effect of Subphase Ions on Calixarene Monolayers
pressures shown in Figure 1, the packing density of the calixarene monolayer is almost the same over the pH range of 5.5-8.4 but increases steadily at higher pH. This is in contrast to the continuous change as a function of pH seen for PEI in Figure 1. At this point, it is helpful to consider the pKa difference between PEI and PLS to understand this second difference. Once p-dioctadecanoylcalix[4]arene has gone through the first interfacial dissociation step, increasing pH has no effect on its ability to bind PEI and PLS until the second dissociation step is approached. However, this is not the only acid-base equilibrium. For the case of PEI, the reported pKa value of 8.5629 lies between those for p-dioctadecanoylcalix[4]arene (6.4 and 9.2).5 Thus, the gradually decreasing positive charge on PEI in this region leads to diminished binding interactions and enhanced electrostatic repulsions between calixarene molecules such that the film continues to expand. The overall result is a nearly continuous expansion from an uncharged calixarene film at low pH to a dissociated calixarene film at high pH. This can be contrasted with the PLS case. According to Applequist et al.,30 poly(L-lysine) with a high degree of polymerization exists as a charged random coil for pH < 8.5. Considering the bulk intrinsic dissociation constant of poly(L-lysine) (pKa ) 10.44),31 the charge density of poly(L-lysine) can be regarded as essentially constant for pH values from 5.5 to 8.4, unlike the case of PEI above. Therefore, no significant changes in film structure are anticipated. This is supported by the plateau observed in Figure 2, indicating the packing density is essentially constant. For pH values greater than 8.4, this is no longer true. The reason for this is PLS undergoes a helix-coil transistion in the pH range of 8.5-10.5 as the bulk intrinsic dissociation constant is approached.30 The reduced charge density of the partially helical PLS, along with additional dissociation of p-dioctadecanoylcalix[4]arene, is consistent with the remarkable expansion of the films in this region. Finally, it is worth emphasizing that both poly(ethylenimine) and poly(L-lysine) expand the p-dioctadecanoylcalix[4]arene monolayer as spermine does. Here, the effect of polyion binding is similar to that of poly(ethylenimine) binding to fatty acid monolayers. It was reported that poly(ethylenimine) expanded a stearic acid monolayer at moderate subphase pH32 and expanded an arachidic acid monolayer at a high subphase pH (∼10).12 As explained in the case of spermine, these results are due to the fact that the large size of p-dioctadecanoylcalix[4]arene relative to stearic acid is the dominant factor at low pH. The diminished charge density of PEI/PLS will play a more important role at high pH. Surface Viscoelasticity of Calixarene Monolayers. Figure 6 shows the frequency shift (fs) and the corrected full width at half-maximum intensity (∆fs,c) of the power spectrum from SLS at a scattering wave vector of k ) 328 cm-1 for p-dioctadecanoylcalix[4]arene monolayers on buffer (pH ) 8.4, I ) 0.04 M) with and without binding agents. The relevant limits (pure liquid dynamics, the maximum damping coefficient for a perfectly elastic surface film, and infinite lateral modulus dynamics) obtained from the Lucassen-Reynders-Lucassen disper(30) Applequist, J.; Doty, P. In Polyamino Acids, Polypeptides, and Proteins; Proceedings of the International Symposium; Stahmann, M. A., Ed.; University of Wisconsin Press: Madison, WI, 1962; p 161. (31) Katchalsky, A.; Shavit, N.; Eisenberg, H. J. Polym. Sci. 1954, 13, 69. (32) Chi, L. F.; Johnston, R. R.; Ringsdorf, H. Langmuir 1991, 7, 2323.
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Figure 6. fs-Π and ∆fs,c-Π for p-dioctadecanoylcalix[4]arene/ NBD-PE (100:1, molar ratio) monolayers on buffer (pH ) 8.4, I ) 0.04 M) with (triangles) 1 mM spermine, (crosses) 2 mM PEI, and (circles) 0.1 mM PLS and without (squares) binding agents at 23.0 °C and k ) 328 cm-1. The curves marked by the roman numerals correspond to I ) pure liquid limit, II ) maximum damping coefficient for a perfectly elastic surface film, and III ) infinite lateral modulus dynamics (see text).
sion equation33 are also included on the graphs. There exists no significant difference in the dynamics in the presence and absence of binding agents in the subphase. All monolayers behave as films with infinite lateral moduli for Π > 1 mN‚m-1. Even though the linking agents expanded the isotherms of p-dioctadecanoylcalix[4]arene, the dynamics were not affected by spermine, PEI, and PLS. These results are consistent with both the pH and NaCl results presented previously.5 Another way of examining these data is to consider a plot of the complex frequency for different surface pressures at an arbitrary reference state. This approach was first used by Ha˚rd and Neuman34 and has more recently been applied to a variety of homopolymers and copolymers.35 For this approach, the Lucassen-ReyndersLucassen dispersion equation33 is solved for the dynamic dilational elasticity, d, and the corresponding surface viscosity, κ, at multiple wave vectors under the assumption that the static and dynamic surface tensions are equivalent (σs ) σd) and the transverse viscosity is zero (µ ) 0). These assumptions should be valid considering the excellent agreement between the experimental data, and the calculated limits of the dispersion equation (calculated with µ ) 0) shown in Figure 6. Next, an arbitrary reference state is chosen (k ) 324 cm-1, σs ) 71.97 mN‚m-1, µ ) 0 mN‚s‚m-1, F ) 997 kg‚m-3, and η ) 0.894 mPa‚s) and equivalent values of fs,eq and ∆fs,c,eq are calculated from the experimentally determined values of d and κ. These equivalent values can then be averaged for multiple wave vectors and plotted on a single plot of ∆fs,c,eq versus fs,eq as shown in Figure 7. On this plot, isobars of fixed elasticity are represented by the solid curves, while isobars of fixed viscosity are represented by dashed curves. The plot clearly shows the three different limits of viscoelastic behavior, indicated in Figure 6. From the trajectory, we can surmise (33) Lucassen-Reynders, E. H.; Lucassen, J. Adv. Colloid Interface Sci. 1969, 2, 347. (34) Ha˚rd, S.; Neuman, R. D. J. Colloid Interface Sci. 1987, 120, 15. (35) Esker, A. R. Ph.D. Thesis, University of WisconsinsMadison, Madison, WI, 1996.
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erties of p-dioctadecanoylcalix[4]arene monolayers, despite strong effects on the static surface pressure isotherms. Conclusions
Figure 7. Equivalent complex frequency plot for p-dioctadecanoylcalix[4]arene/NBD-PE (100:1, molar ratio) monolayers on buffer (pH ) 8.4, I ) 0.04 M) with (triangles) 1 mM spermine, (crosses) 2 mM PEI, and (circles) 0.1 mM PLS and without (squares) binding agents at 23.0 °C. The open circles show the results of poly(vinyl acetate). Reference conditions: k ) 324 cm-1, σs ) 71.97 mN‚m-1, µ ) 0 mN‚s‚m-1, F ) 997 kg‚m-3, and η ) 0.894 mPa‚s. The roman numerals correspond to I ) pure liquid limit, II ) maximum damping coefficient for a perfectly elastic surface film, and III ) infinite lateral modulus dynamics (see text). The points at limit III correspond to Π > 1 mN‚m-1, while the other points correspond to Π < 1 mN‚m-1 (increasing from right to left).
readily that viscous forces play an important role in capillary wave propagation on p-dioctadecanoylcalix[4]arene monolayer-covered surfaces. As expected from the raw data in Figure 6, this plot also shows that the dynamics are consistent with an incompressible surface film possessing nearly infinite lateral modulus dynamics (d > 100 mN‚m-1, κ × 105 > 100 mN‚s‚m-1) for Π > 1 mN‚m-1 within experimental errors. These results are consistent with a previous study,5 as well as the static elasticities reported for a different calixarene molecule,24 and demonstrate that the linking agents examined here have no significant SLS-detectable effect on the viscoelastic prop-
On the basis of the above experimental results, it is clear that p-dioctadecanoylcalix[4]arene monolayers do not show any special interaction with univalent inorganic ions in the subphase solution. Additionally, polyions in the subphase appear to adsorb onto p-dioctadecanoylcalix[4]arene monolayers by forming complexes through electrostatic interactions between oppositely charged species. This complex formation may be energetically favored and should take place at some subphase pH values beyond the normal dissociation regions of p-dioctadecanoylcalix[4]arene and the polyions. Whether the binding from the subphase results in an expansion or condensation effect depends on the molecular size and the charge distribution of the linking agent in the subphase. Ion binding from the subphase has no effect on the highest packing density of p-dioctadecanoylcalix[4]arene monolayers (surface concentrations near the point of film collapse), which is determined by the cross-sectional area of the cylindrical calixarene basket with all of the aromatic rings more or less perpendicular to the interface. Finally, polyion binding from the subphase does not result in significant SLS detectable changes in surface viscoelasticity, which is still predominantly determined by strong lateral packing interactions of the hydrophobic chains and aromatic rings, and not the electrostatic interactions of the hydrophilic headgroups. Acknowledgment. This research is supported in part by an NSF grant (DMR 92-03289), Eastman Kodak, and the Helfaer Endowment Fund. A.R.E. was supported in part by a National Research Service Award 5 T32 GM08349 from the National Institute of General Medical Sciences and the Henkel Corporation 1994-1995 Research Fellowship in Colloid and Surface Chemistry. K.N. is grateful to the Organic Chemistry Research Center (OCRC) for their support. LA9700174
