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Langmuir 1993,9, 1582-1586
Monolayer Miscibility of a Hairy Rodlike Polyglutamate and n-Alkyl Amphiphiles K. Mathauer,?T. Vahlenkamp,t C. W. Frank,'$$and G. Wegnert Max-Planck-Institut fiir Polymerforschung, Ackermannweg 10, Postfach 3148,6500 Maim, Germany, and Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 Received December 22, 1992. In Final Form: March 11,1993 Surface pressure-area ( P A ) isotherms have been measured for mixtures of poly{(7-methyl-L-glutamatelco-(y-n-octadecyl-L-glutamats))(methyVoctadecyl=7/3)with a series of normal chain amphiphiles (CIS-, Cle-, and Ca-fatty acids and ethyl stearate). Excess areas and two-dimensional compreesibilities were determined from the F A isotherms. The deviations from ideality when these values are plotted against composition were interpreted on the basis of a two-phase model with the phases varying in composition depending on the amphiphile-amphiphile interaction. In the case of ethyl stearate a mixed phase is observed above a certain surface pressure, which corresponds to the phase transition of a pure ethyl stearate monolayer from the liquid condensed phase Lz to the superliquid phase LS. Complementary experimental evidence for the proposed model is given by fluorescence energy transfer experiments on transferred monofilms where the polymer component is substituted by a mixture of a carbazole and anthracene tagged polymer, respectively. FTIR studies on polymer/amphiphile mixtures in which the polymer has perdeuterated side chainsshowthat the polymer side chains become aligned more perpendicular to the substrate as a result of mixing with ethyl stearate.
Introduction Recently it has been shown that copolymers of yoctadecyl- and 7-methyl-L-glutamate1 and similar copolymersz are excellent materials for the formation of Langmuir-Blodgett films when spread on water from chloroform solution, in which the polymer exists in the a-helical conformation. The geometrical restrictions in these copolymers-30% octadecyl side chains are randomly distributed around the helix backbone-do not allow a completeclosepacking of the alkyl chainsin the monolayer, such as is observed with classical amphiphiles like fatty acids or amphiphilicpolymers.3 With regard to the phase behavior of monolayers, one can think of these polymers as a molecular composite of a rigid a-helical polypeptide and a disordered fluid or glassy phase of the side chains. The increased concentration of these disordered alkyl chains achieved by compressing the monolayer gives rise to steric repulsion and increase of osmotic pressure in the monolayer. Monolayers of a-helical polypeptides have been investigated intensivelyin the past. It has been shown that the a-helical conformation remains undisturbed at the airwater interface and that the helices are oriented parallel to the water ~urface.~Jj The shape of the isotherms and the collapse pressure are determined by the interaction with the water surfaceand by the side chain intera~tion.~J Two-dimensional mixtures of a-helical polypeptides and fatty acid type molecules have been investigated as model system for the lipid-protein interaction in biomembranes. In moet cases two-dimensional phase separationis observed Max-Planck-htitut Mr Polymerforschung. t Stanford University. (1) Duda, 0.;Schouten, A. J.; Amdt, T.; Lieaer, G.;Schmidt, G.F.; Bubeck, C.; Wegner, G. Thin Solid Films 1988,159,221. (2) Mathauer,K.; Schmidt,A.; Knoll, W.; Wegner,G.Macromolecules, in presa (3) Ul",A. An Introduction to Ultrathin Films: From LangmuirBlodgett to Self-Aaaembly, Academic Press: Boston, MA, 1991. (4) Malcolm, B. R. Polymer 1966, 7,595. (6) Loeb, 0. I.; Baier, R. E. J. Colloid Interface Sci. 1968,27, 38. (6) (a) Malcolm, B. R. J. Polym. Sci. C 1971,34,87. (b) Malcolm, B. R. J. Colloid Znterface Sci. 1986,15, 60. (7) Malcolm, B. R. Ado. Chem. Ser. 1975, No. 145,338.
0743-7463/93/2409-1582$04.00/0
in these mixtures.Sl0 Gabrielli et al. reported a region of miscibility between oleic acid and poly(ybenzy1-Lglutamate), but this result was only based on the variation of the collapse pressure with composition.11 It has been shown that the relative orientation of the hydrophobic parts of the two components is important for miscibility in monolayers.12J3 In order to probe the state of the hydrocarbon side chains of the copolyglutamates, monolayer miscibility with classical amphiphiles is investigated in the present study. A further reason for this investigation was the question of whether the film stability of the polymer films could be improved by additives of polar nature. From surface pressure/areaisothermsof pure and mixed monolayers, the excess areas of the binary systems can be derived. A negative excess area is an indication for less repulsiveinteractions in the mixture14or can be explained by an athermal space-filling mechanism16Jswhere cavities in the more expanded component are filled by the other component. In order to interpret the observed excess area in terms of a molcular interaction, one should have independent information about the binary monolayers. We therefore used polymers with carbazole and anthracene labels, respectively, in order to measure the interpolymer distance in the mixtures via fluorescence energy transfer measurements. In addition, a polymer with perdeuterated octa(8) (a)Albert,A.;Cordoba, J.;Otero-Aede, E.;Queralto,A. An. Qu~M., Ser. A 1983,79,704. (b)Albert, A.; Cordoba,J. Colloid Polym. Sci. 1984, 262, 811.
(9) Birdi, K. S.; Soereneen, K. E. Colloid Polym. Sci. 1979,267, Q42. (10) (a) Gabrielli,G.J. ColloidZnterfaceSci. 1876,53,148. (b) Gabrielli, G.;Baglioni, P.; Fabbrini, A. Colloids Surf, 1981, 3, 147. (11) Gabrielli, G.;DAubert, C. Colloid Polym. Sci. 1980,268, 56. (12) Ries, H. E., Jr.; Walter, D. C. J. Colloid Interface Sci. 1961,16, 361. (13) (a) Gabrielli, G.;Baglioni,P.; Maddii, A. J. Colloid Interface Sei. 1981, 79, 268. (b) Gabrielli, G.;Puggelli, M.; Gablioni, P. J. Colloid Interface Sci. 1982,86,485. (14) Costin, I. S.; Barnes, G.T. J. Colloid Interface Sci. 1975,51,106. (15) Dervichian, D. G. In Surface Phenomena in Chemistry and
Biology; Danielli, J. F., Pankhurst, K. G. A., Riddiford, A. C., Ede.; Pergamon: New York, 1958; p 70. (16) (a) Shah,D. 0.;Schulman, J. H. J. Lipid Res. 1967,8,215,227. (b) Shah,D. 0.;Schulman, J. H. Ado. Chem. Ser. 1968, No. 84,189.
0 1993 American Chemical Society
Langmuir, Vol. 9, No. 6,1993 1683
Monolayer Miscibility
[A2 ] Figure 1. Surface pressurearea isotherm at 18 O C and twodimensional compressional modulus C,-l of PG. Area per molecular unit
A-PG C-PG
R = A ;
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decyl side chains was used to monitor the side chain orientation of the polymer in the mixed monolayers via Fourier transform grazing incidence spectroscopy (after transfer to solid substrates).
Experimental Section Materials. The basic polymer poly[(y-methyl-L-glutamate)co-(y-n-octadecyl- glutamate)] (PG) contains 30 mol % octa-
J
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decyl side chains and has been synthesized by copolymerization of the corresponding N-carboxy anhydrides as described elsewhere." The average molecular weight of the polymer was determined by static light scattering according to the method described by Doty.ls The weight average molecular weight MW = 460 OOO lies in the range which is expected for the polymerization conditions.19 The terpolymers containing carbazole (C-PG) and anthracene (A-PG) labels have basically the same composition except that about 2% of the long side chains are replaced by the chromophore-bearing side chains (see Chart I). The synthesis was also carried out by copolymerization of the corresponding N-carboxy anhydrides." In d-PG2' all octadecyl side chains are replaced by perdeuterated side chains (94% deuterated). For
-,
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(19) Blout, E. R.; Karleon, R. H. J. Am. Chem. SOC.1966,78,941. (20) Mathauer, K.;Wegner, G. To be submitted for publication.
this purpose the starting material octadecyl-L-glutamate was synthesized by alkylation of the copper complex of glutamic acid with perdeuterated octadecyl bromide analogous to a method described in the literature. Arachidicacid, stearic acid, palmitic acid, and ethyl stearate (99% grade) were purchased from Aldrich and used as received. Langmuir-Blodgett Technique. All mixtures were spread from chloroform (Merck UVASOL) solutions (0.3 mg/mL) on a pure water subphase (MILLI-Q quality) contained in a commerical Lauda FW-1 film balance. Surface pressure/area measurements were recorded at 18 "C subphase temperature with a rate of compression of 3 min-1 molecule-'. The measured areas per molecule were reproducible within *0.5 Aa. Isotherms recorded at the highest possible compressionrate of 13.5 Asmin-1 molecule-l are slightly steeper and show an area per molecule which is about 4% higher at 20 mN/m for all investigated mixtures. Monolayer transfer was performed onto hydrophobic substrates with y-type deposition a t a lift speed of 2 cm/min. The quartz slides to be used as substrates for the fluorescence measurements on transferred monolayers were cleaned with CHCg in an ultrasonic bath for 15 min, then treated with a hot solution of NHlOH (25%)/H101(30%)/water(1:1:5) for 20 min and finally rinsed with water. Hydrophobization was achieved by treatment with a solution of hexamethyldisilazane in CHCb for 30 min at 50 "C.The gold substrates for the infraredrefledion measurementa were made by evaporation of first 50 A of Cr and then lo00 A of Au onto precleaned microscopic glass slides. The substrates were used for transfer ca. 12 h after preparation. Measurements. FT-infrared measurements on LB layers were carried out with a Nicolet 60 SX FTIR spectrometer at room temperature. Grazing incidence reflection spectra on gold substrates were measured as described previouslyB using the uncovered part of the substrate as reference. Fluorescence spectra of transferred monolayers were obtained at room temperature with a Spes Fluorolog 2 spectrophotometer (F 202) provided with a photomultiplier tube operated in the photon-counting mode to monitor the emission intensity. The band width for both excitation and emission was 4 nm. Emission spectra were not corrected.
Results A. Pressure-Area Isotherms. Figure 1 shows the surface pressure isotherm of PG together with the twodimensional compressional modulus C,-l (CB-l = A d?r/ d A ) . 2 4 In this and the following figures the molecular area and mole fraction of the polymer are based on an average monomer unit (215 g/mol). A plateau in the isotherm of a-helical polypeptides is generally regarded as a point where collapse of the monolayer or a transition to a bilayer (21) Schmidt, A.; Mathauer, K.;b i t e r , G.; Foster, M.; Sta", M.; Wegner, G.; Knoll,W. To be submitted for publication. (22) Van Heeawijk, W. A. R.; Eenink, M. J. D.; Fejien, J. Synthesis 1982,144. (23) Amdt, T.;Wegner, G. In Optical Technique8 to Characterize Polymer System; Baessler,H., Ed.;Elsevier Science: Amsterdam, 1989; p 41. (24)Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York 1963; p 265.
1584 Langmuir, Vol. 9,No. 6, 1993
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Figure 2. Area per molecular unit as a function of monolayer composition in mixed monolayers of PG with palmitic acid.
0
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Figure 4. Area per molecular unit as a function of monolayer composition in mixed monolayers of PG with ethyl stearate at 18 O C at the indicated surface pressures.
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Figure 3. Two-dimensional compressibility C, of mixtures of PGwithpalmitic acid at 18O C and the indicated surfacepreasures.
occur^.^ Possible side chain orientation effects are
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Figure 5. Two-dimensional compressibility C, of mixtures of PG with ethyl stearate at 18 “C and the indicated surface pressures.
thought to appear in the region where the compressional modulus is increasing with increasing pressure. We therefore analyzed the mixture isotherms only up to a surface pressure of 20 mN/m. As can be seen in Figure 1, the compressional modulus of a pure PG monolayer does not increase monotonically. Two distinct regions can be identified. The first steep rise between 0 and 2 mN/m might be due to reorientations of polymer rods or domains in the monolayer. Presumably the free space between individual domains is squeezed out in this initial part of the isotherms. The slower rise between 2 and 20 mN/m could be explained with an increasing side chain orientation. The area per molecular unit as a function of composition for the palmitic acid/PG mixtures is depicted in Figure 2. The excess area is positive for low surface pressures and becomes negative at higher surface pressures. With regard to the surface pressure/area isotherms and the excess area of mixing, it is reasonable to assume that the mixed monolayers of PG with fatty acids are twophase systemswith one liquid-condensedphase consisting mainly of the fatty acid and an expanded phase consisting mainly of the polymer side chains. Although there are not enough data to describe the palmitic acid/PG system completely,the plot of area vs compositionat higher surface pressures P 1 5 mN/m) seems to consist of two straight lines with the highest deviation from ideality at ca. 12mol % palmitic acid. This behavior is characteristic of a phase separated system with a 12% miscibility of the palmitic acid in the PG. Below 12% the partial molecular area of palmitic acid in the mixtures is almost zero. In Figure 3 the two-dimensionalcompressibilitiesare shown as a function of composition for the palmitic acid mixtures. Generally, a positive deviation from additivity is observed. Whereas a pure fatty acid monolayer shows almost a constant compressibility in the region between
2 mN/m and the transition to the LS phase,%PG shows a decrease of compressibility (seeincrease of compressional modulus C,-l in Figure 1) in the same surface pressure region. An increasingcompressibility of the mixtureswith surface pressure must therefore be related to a condensation effect in the mixtures at higher surface pressure. Figure 4 shows the molecular area as a function of composition for mixtures of PG and ethyl stearate. At lower surface pressures the behavior is very similar to the fatty acid mixtures, although no positive excess area is observed at 2 mN/m. A drastic increase of the negative excess area is observed above 12 mN/m and the plots of area vs composition consist basically of two straight lines with the minimum at 46 mol % ethyl stearate. The twodimensionalcompressibilitiesin Figure 5 showsthat at 20 mN/m, the compressibility of the mixtures above a concentrationof 46 mol % liesslightlybelow the additivity line and resembles the value of a monolayer with closepacked alkyl chains. It is reasonable to assume that in the case of the ethyl stearate, complete miscibility occurs with PG above 12 mN/m. The observed minimum of 12.6 A2per molecular unit for the mixtures with 46 mol % ethyl stearate is consistent with a model where all alkylchains in the mixed monolayer-an average of 0.3 octadecylchain per polymer repeat unit and the alkyl chains of ethyl stearateparticipate in a close packed structure which now determines the molecular area. Figure 6 shows the excess area for the mixtures as a function of surface pressure at a concentration of the fatty acid type molecules of 46 mol % . A characteristic feature is an almost linear decrease of the excess area to more negative values in all cases. Our results seem to indicate
(26) (a) Malcolm, B.R.J. Polym. Sci. C 1971,34,87. (b)Malcolm, B. R. J. Colloid Interface Sci. 1985, 16, 60.
(26) Hann, R.A. In Longtkr-EMgett Films; Roberts, G.,Ed.;Plenum Preee: New York, 1990.
Monolayer Miscibility
Langmuir, Vol. 9, No.6,1993 1585 1.8
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Figure 7. Ratio of the carbazole to anthracene emission (416 nm/349 nm) in fluorescence spectra of transferred mixed monolayers of C-PG and A-PG (k1.22)and various amounts of PG, ethyl stearate and cadmium arachidate as indicated. The fiis were transferred at a surfacepressure of 20 mN/m. Spectra are recorded with an excitation wavelength of 294 nm and a bandwidth of 4 nm. 0
5
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20
Surface pressure [mN/m] Figure 6. Excess area as a function of surface pressure for mixtures of palmitic (PA), stearic (SA), and arachidicacids (AA) and ethyl stearate (ES) with PG for a concentration of the amphiphiles of 46 mol 9%.
that below 11 mN/m two phases exist in all monolayers, an almost pure fatty acid (ester) phase and a polymer phase with a certain amount of dissolvedfatty acid (ester). At about 11 mN/m the transition in the ethyl stearate mixtures is expressed as a sharp drop of the excess area. The slopes in Figure 6 can be interpreted as an excess compressibility of the two-phase systems caused by the dissolution of the fatty acid type amphiphile in the PG phase. A rough comparison of all four compounds can be made based on the heats of fusion of the bulk materials because in the mixed phase, the fatty acid molecules are expected to be in a disordered hydrocarbon environment. The slopes in Figure 6 are indeed proportional to the melting enthalpies of the used n-alkylamphiphilesin their crystalline form which is stable at the melting point (Cform of fatty acid and a-form of ethyl stearate2'). Another striking feature in Figure 6 is that the positive excess area observed for the fatty acid mixtures at low surface pressure is increasingwith decreasingchain length of the fatty acid. This can be explained on the basis of an unfavorable interactionbetween the disorderedpolymer side chainsand the alignedfatty acid molecules. The lower interactions in shorter fatty acids can allow more polymer to be dissolved in the fatty acid phase and, thus, leads to a higher positive excess area. B. Energy Transfer. Nonradiative fluorescence energy transfers provides a way to determine intermolecular distances in the nanometer range. In order to test the conclusion drawn from the surface pressure/area isotherms, we replaced the polymer in the mixtures by a 1:l mixture of anthracene and carbazole tagged copolyglutamates, respectively, which otherwise have the same structure as PG. Chart I shows the structure of these polymers. This pair of chromophores has a characteristic Foerster distance for nonradiative energy transfer of 29 (27) Markley, K. S.,Ed. Fatty Acids, Part 4; IntersciencePubliihers: New York, 1967; p 2588. (28) Farster, T. Ann. Phys. 1948.2.55: . . . 2.Naturforsch. 1949, A4.321: Diecuss. Faraday SOC.19S9,27,7.
A.2e In Figure 7 the ratio of the carbazole to anthracene fluorescence intensity (at 349 and 415 nm) is shown for transferred monolayers where the second component is either untagged PG, ethyl stearate, or cadmium arachidate. The monolayers were transferred at 20 mN/m onto hydrophobic substrates during the downstroke, and the substrates were removed from the subphase behind the moving barrier through the clean air-water interface. In the case of mixed monolayers with cadmium arachidate, the interpolymer distance does not change at all over the whole range of composition. In the mixtures with more A than 50% ethyl stearate a clear increase of the I ~ I ratio and, thus, the interpolymer distance can be seen. This is in agreementwith the conclusionsdrawn from the isotherm studies. Up to about 40 mol % ethyl stearate free cavities of the polymer are filled by the amphiphile and there is no change of the interpolymer distance. With higher amphiphile concentrations the polymer chains start to be separated. C. FTIR Spectroscopy. Figure 8 shows a comparison of grazing incidence FTIR spectra of multilayers on gold of 10layers of pure d-PG (PG with perdeuteratsd octadecyl side chains) and 10 layers of mixed monolayers of 25% ethyl stearate and 75% d-PG. Both monolayers were transferred at a surface pressure of 13 mN/m. Typical polymer bands (NH stretching at 3300 cm-l, amide I at 1660cm-9 and amide I1at 1552cm-l), which do not overlap with any band of ethyl stearate, show the same intensity in both spectra. On the other hand, the mixture spectrum showsincreased band intensities in regionswhere the ethyl stearate shows characteristicbands (CH stretching around 2900 cm-l, region between 1500 and lo00 cm-1). Both observations support the conclusions drawn from the isotherms that the ethyl stearate is incorporated in free volume of the polymer without changing the average polymer distance. A remarkable decrease in intensity is observed in the C-D stretching vibrations at 2098 and 2196 cm-1 for the mixed system compared to pure the pure d-PG multilayer. This is a clear indication that the polymer side chains are becoming oriented more perpendicular to the substrate than in the pure polymer multilayer. The C=O stretching band at 1737 cm-l shows almost the same intensity in both spectra. Due to orientation effects, this band is expected to be small in a (29) Berlamn,I.Energy TronsferParametersofAromatic Compounds; Academic Press: New York, 1973.
1586 Langmuir, Vol. 9, No. 6, 1993 0.01
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Mathauer et al. I
1
Wavenumber [cm'1
Figure 8. FTIR grazing incidence reflection spectra of 10 monolayerson gold of PG-d (dottedline)and a mixtureof 75 mol % PG-dand 25 mol % ethyl stearate. The transferwas performed at a surface pressure of 13 mN/m and a subphase temperature of 18 O C .
ethyl stearate monolayep and reduced for the polyglutamate in the mixture; thus, the same intensity in the mixture than in the pure polymer monolayer is possible.
Discussion Monolayersof long chain fatty acids3132and their ethyl esters33,34display a distinctive polymorphism depending on surface pressure and temperature. In the surface pressure range of 0-30 mN/m two distinct phases exist at room temperature for stearicacid, palmitic acid,and ethyl stearate: a compressible low pressure phase Lz,in which the alkyl chains are tilted with respect to the surface normal,%and a superliquid phase LS,% where the molecules are vertically oriented but can undergo both rotational and translational motion. For the fatty acids the phase transition occurs around 23 mN/m, whereas the ethyl stearate monolayer shows the transition at 11mN/ m. The occurrence of a transition to a one-phase system in the mixtures of PG and ethyl stearate at this surface pressure showsthe existence of a two-phasesituation below this point. A model for a investigated mixtures is given in Figwe 9. A condensation effect in mixed monolayers at higher surfacepressures has to our knowledge not been observed so far. In comparable mixtures of poly(octadecy1methacrylate) (PODMA),poly(octadecy1acrylate) (PODA),or poly(viny1stearate)with octadecanol,Fukuda et al.37and Barnes et al.38 observed the opposite behavior. For these (30) Puterman, M.; Fort,T., Jr.; Lando, J. B. J. Colloid Interface Sci. 1974,47, 705. (31) (a) Staellbrg-Stenbagen,S.; Stenhagen, E.Nature 194,156,239. (b) Stenhagen, E.In Determination of Organic Structures by Physical
Methods; Braude, E. A., Nachod, E.C., Eds.; Academic Preas: New York, 1955. (32) Bibo, A. M.; Peterson, I. R.Adu. Mater. 1990,2,309. (33) Lundquist, M. Chem. Scr. 1971,1,197. (34) B i b , A. M.; Knobler, C. M.; Peterson, I. R.J. Phys. Chem. 1991, 95,5591. (35) Kenn, R M.; Boehm, C.; Bibo, A. M.; Peterson, I. R.; Mtihwald, H.; Ah-Nieleen, J.; Kjaer, K. J. Phys. Chem. 1991,95,2092. (36) Harkina, W. D.; Copeland, L. E.J. Chem. Phys. 1942,10, 272.
Water
Figure 9. Model for mixed film of PG with n - e l amphiphiles: (a) ethyl stearate below the phase transition at 11 Mn/m and fatty acids. (b) ethyl stearate above the transition at 11 Mn/m. View along the a-helixaxis of the polymer backbone. The circles representacross-sectionof the a-helixincludingthew-estergroup.
polymers the excess area was negative below 10 mN/m and became positive at higher surface pressure. The major difference between those systems and the present case lies in the fact that in the hairy rod system the polymer side chains are forced into a random state by the attachment to the a-helical polymer backbone, whereas in the amphiphilic flexible polymers the side chains can already align at low surface pressures. For amphiphilic polymers a major driving force for the film formation is the alignment of alkyl side chains. In the case of monolayers of hairy rodlike polymers, energy has to be applied to the monolayer in the form of compression to orient the alkyl side chains of the polymer. Due to the dilution of these side chains, this orientation cannot lead to considerable van der Waals interactions unless the free volume can be filled by a miscible low molecular alkyl component as observed with ethyl stearate.
Conclusion The use of amphiphilic mixtures has permitted some fundamentalunderstandingof the function of side chains in a monolayer of a hairy rodlike polymer. We have shown that the disordered side chains in this class of monolayers can be forced to align similar to that of amphiphilic polymers when the free volume is filled with miscible n-alkylamphiphileleadingto a low compressiblecomposite monolayer.
Acknowledgment. This work w& supported by the German minister of research and technology (BMFT) as part of the project on "Ultrathin Layers of Polymers as New Materials for Optics, Electronics and the Life Science". K.M. and T.V. acknowledgesupport through a Kekulbscholarship, granted by the Verband der Chemischen Industrie. C.W.F. acknowledgespartial support by the Polymers Program of the National Science Foundation. (37) Fukuda, %,Kato,T.;Machida, S.;Shimizu,Y.J. ColloidInterface Sci. 1979,68, 82. (38)Drummond, C. J.; Elliott,P.; Furlong, D. N.; k e a , G. T. J. Colloid Interface Sci. 1992, 151, 189.
