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Langmuir 1997, 13, 2414-2416
Mixed Monolayers of Lipid and Polymer Spread at the Air-Water Interface Masami Kawaguchi,* Midori Yamamoto, and Tadaya Kato Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1515 Kamihama, Tsu, Mie 514, Japan Received November 5, 1996. In Final Form: February 7, 1997
Introduction Binary mixed monolayers of polymer and fatty acids have been studied since the report of Ries and Walker in 1961.1 Gabrielli and co-workers have extensively investigated the interfacial properties of various combinations of polymers of fatty acids in monolayer blends.2-4 They discussed the miscibility by taking into account differences in the orientations, such as horizontal and vertical orientations, between the two compounds at the interface. There are several methods to determine the miscibility in the mixed monolayers:5,6 conventional data analysis, such as compressibility and collapse pressure trends with blend composition, isobaric data of mean surface area against blend compositions, and calculation of the excess free energy of mixing are discussed, and these data can be obtained from the surface pressure measurements. On the other hand, investigations on changes in the morphology of the mixed monolayers by fluorescence or by Brewster angle microscopies7 are also useful to understand the miscibility behavior. A combination of surface pressure measurements and fluorescence microscopy should provide the interfacial properties of the mixed monolayers independently and enable a good check of the consistency. In this note, using both methods the miscibility of the binary blends for poly(vinyl acetate) (PVAc) and pentadecanoic acid (PDA) is investigated as a function of PVAc surface concentration.
Figure 1. Plots of surface pressure as a function of surface concentration of PVAc for pure PVAc and various PVAc-PDA mixed monolayers: 9, PVAc; 2, PDA surface area of 60 Å2/ molecule; O, PDA surface area of 50 Å2/molecule; 0, PDA surface area of 40 Å2/molecule; 4, PDA surface area of 30 Å2/molecule.
Experimental Section Materials. PDA (Tokyo Kasei Co.) was purified several times by crystallization of its n-hexane solution. N-(7-Nitro-2,1,3-benzoxadiazol-4-yl)-L-R-dipalymitoyl phosphatidylethanolamine (NBD-PE) (Molecular Probes, Inc., Oregon) was used as a florescence probe without further purification, and its concentration was ∼1.5 mol % during the entire fluorescence microscopy measurement. PVAc was synthesized by radical polymerization of freshly distilled vinyl acetate with azobis(isobutyronitrile) as an initiator in benzene at 60 °C. The resulting PVAc was poured into a large amount of methanol, and the precipitated PVAc was dried in a vacuum. PVAc was dissolved in acetone and fractionated into 10 fractions using n-hexane at 25 °C as a precipitant. We chose one fraction PVAc, whose molecular weight was determined to be 300 × 103 from the intrinsic viscosity measurement in benzene at 25 °C.8 The spreading solvent used to prepare all monolayers at the air-water interface was spectrograde chloroform, and we used it without further purification. (1) Ries, H. E., Jr.; Walker, D. C. J. Colloid Sci. 1961, 16, 361. (2) Gabrielli, G.; Maddii, A. J. Colloid Interface Sci. 1978, 64, 19. (3) Puggelli, M.; Gabrielli, G. Colloid Polym. Sci. 1985, 263, 879. (4) Puggelli, M.; Gabrielli, G. Colloid Polym. Sci. 1987, 265, 432. (5) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; John Wiely & Sons: New York, 1966. (6) Birdi, K. S. Lipid and Biopolymer Monolayers at Liquid Interfaces; Plenum: New York, 1989. (7) Petty, M. C. Langmuir-Blodgett Films An Introduction; Cambridge University Press: Cambridge, 1996. (8) Kawaguchi, M.; Nagata, K. Langmuir 1991, 7, 1478.
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Figure 2. Plots of the mean surface areas as a function of PVAc content for fixed surface pressures of 4 (0), 8 (9), and 12 (O) mN/m. Surface Pressure Measurements. The surface pressure measurement apparatus was the same as described in detail before.8 A Teflon trough was filled with aqueous 0.01 N HCl solution with the temperature controlled at 25 ( 0.2 °C. Monolayer of PVAc was spread on surface of the aqueous HCl solution in the trough with an area of 290 cm2 by applying the method of “successive additions” of the chloroform solution with a Hamilton microsyringe to adjust the surface concentration. For the preparation of the mixed monolayers of lipid and polymer, PDA monolayer was first spread at fixed surface areas of 30, 40, 50, and 60 Å2/molecule from its chloroform solution, the PVAc solution was added successively, and changes in the surface pressure at the respective surface areas of PDA were monitored as a function of surface concentration of PVAc. The solvent was allowed to evaporate for at least 30 min before the experiment was started. Unless the surface pressure did not remain constant over 10 min, we regarded it as an equilibrium surface pressure. The equilibrium value was not changed if longer time periods than 30 min were passed. The experimental errors in the surface pressure measurements were less than 0.1 mN/m.
© 1997 American Chemical Society
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Langmuir, Vol. 13, No. 8, 1997 2415
Figure 3. Fluorescence microscopic images of pure PDA and various PVAc-PDA mixed monolayers. Fluorescence Microscopy. A homemade fluorescence microscope was used. A Teflon trough with a diameter of 15 cm was used, and it was filled with aqueous 0.01 N HCl solution. The temperature of the solution in the trough was controlled within 25.0 ( 0.1 °C by circulating thermostated water. The trough was fitted with the stage of an Olympus BH2-UMA epifluorescence microscope attached to a DAS-512 ICCD camera (Imajista Co. Tokyo). Monolayers were spread on the surface of the aqueous HCl solution by applying the same method as described in the part of the surface pressure measurements. Images of monolayers were observed through Olympus NeoSplan 40 (40×) or Olympus NeoSplan 20 (20×) ultralong working distance objective lenses. Unless noted, the microscopic images were followed by the Olympus NeoSplan 40 lens. The condenser was equipped with one set of optical filters allowing observation of fluorescence after excitation at 465 nm. A mercury lamp (USH-102D, Ushio, Tokyo, Japan) was used as the excitation light source.
Results and Discussion Surface Pressure Measurements. Figure 1 shows surface pressures of pure PVAc and PVAc-PDA mixed monolayers as a function of PVAc surface concentration. The resulting collapse surface pressure of pure PVAc monolayer is lower than that spread at a pure water surface,8 indicating less expanded state of PVAc chains on the surface of aqueous 0.01 N HCl solution. At a fixed concentration of PVAc the surface pressure increases with an increase in the mixed ratio of PDA, the plateau surface pressure is observed above a PVAc concentration of 2 mg/ m2, irrespective of PDA content, and it increases with an increase in the amount of PDA. For the determination of the miscibility of PVAc and PDA, we estimated the mean areas at fixed surface pressures as a function of molar ratio from Figure 1, and the resulting plots of the mean
surface areas versus the molar ratio at various surface pressures of 4, 8, and 12 mN/m are shown in Figure 2. The mean surface areas are almost positively deviated from the additive line, indicating that PVAc and PDA are immiscible in a two-dimensional state. This is in agreement with the fact that the mixed monolayers of PVAc and various fatty acids with long chains are not miscible,1 where PVAc is horizontally oriented and the fatty acids are vertically oriented at the interface, respectively, and such a steric orientation prevents the miscibility. Fluorescence Microscopy. Figure 3 shows typical changes in fluorescence microscopic images of PVAc-PDA mixtures at various PDA surface areas of 60, 50, 40, and 30 Å2/molecule as a function of surface concentration of PVAc. The pure PDA surface areas of 60 and 50 Å2/molecule correspond to coexistence of gas (G)-liquid expanded (LE) phases,9 and the pure PDA surface areas of 40 and 30 Å2/molecule correspond to the LE phase only and coexistence of LE-liquid condensed (LC) phases,9 respectively. Since the fluorescence probe of NBD used here is able to be selectively soluble in LE regions, it can be expected that the dark regions at the 60 and 50 Å2/ molecule and those at the 30 Å2/molecule correspond to the G and the LC phases, respectively. At 40 Å2/molecule a uniformly bright film corresponds to the formation of a single phase of LC.10 In addition, at around 20 Å2/molecule of pure PDA, coarsening of the LC phase domains is observed, and above the surface area the bright regions almost disappear in the fluorescence images (not shown here). (9) Pallas, N. R.; Pethica. B. A. Langmuir 1985, 1, 509. (10) Moore, B. G.; Knobler, C. M.; Akamatsu, S.; Rondelez, F. J. Phys. Chem. 1990, 94, 4588.
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Notes
Addition of PVAc to the PDA induces changes in the fluorescence images. For the surface areas of 60 and 50 Å2/molecule, the LC phase circular domains appear in the fluorescence images at a PVAc surface concentration of 0.358 mg/m2 through the disappearance of bubbles of the two-dimensional gas followed by the uniformly bright single LE phase regions. Further addition of PVAc causes an increase in the size and/or the number of LC phase circular domains at 60 Å2/molecule, while at 50 Å2/molecule coarsening of the LC region is observed, leading to the small and bright circular domains at the highest concentration of PVAc. The changes in patterns at 60 and 50 Å2/molecule of PDA by the addition of PVAc are qualitatively in agreement with those accompanied with the G-LE and the LE-LC transitions for the pure PDA monolayer. This indicates that the addition of PVAc leads to a compression of PDA because of the immiscibility of PDA and PVAc. For the immiscible pairs of porphyrinphospholipid11 and lipoteichoic acid-phospholipid,12 vari-
ous phase-separated domains were observed in their fluorescence micrographs, and the results were similar to ours. For the PDA surface areas of 40 and 30 Å2/molecule, the LC phase domains are more numerous and larger with an increase in PVAc surface concentration. Above the PVAc surface concentration of 0.715 mg/m2, where the surface pressures of the PDA-PVAc mixtures are almost in the respective plateau regions, coarsening of the LC phase domains occurs and the bright regions of the LE phase almost disappear. On the other hand, PVAc monolayer containing the NBD-PE did not show any dark regions in the fluorescence microscopic images up to the PVAc surface concentration of 4 mg/m2. This indicates that the fluorescence probe was randomly distributed within the PVAc monolayer due to the formation of an expanded type monolayer of PVAc.
(11) Mo¨hwald, H.; Miller, A.; Stich, W.; Knoll, W.; Ruaudel-Teixier, A.; Lehmann, T.; Fuhrhop, J.-H. Thin Solid Films 1986, 141, 261.
(12) Gutberlet, T.; Milde, K.; Bradaczek, H.; Haas, H.; Mo¨hwald, H. Chem. Phys. Lipids 1994, 69, 151.
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