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Langmuir 1998, 14, 2582-2584
Polymer Adsorption Induced Pattern Formation in Lipid Monolayers 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 24, 1997. In Final Form: February 24, 1998
Introduction Studies of the interaction of water-soluble polymers in the aqueous subphase with Langmuir films of insoluble lipids are important to understand the foam and emulsion stability and the relation between monolayer structure and the properties of deposited Langmuir-Blodgett (LB) films.1-7 Some lipid monolayers spread at the airwater interface show phase transitions as a function of their spread amount.8,9 Pattern morphology of the spread lipid monolayers due to their phase transitions10 should be changed by penetration of polymer into the monolayers when polymer penetration (or adsorption at the air-water interface) acts as a sort of quench. To investigate such a pattern formation, a combination of different experimental techniques should provide the effect of polymers in the subphase on phase transformations in lipid monolayers independently and enable a good check of the consistency. Among several combinations, both the surface pressure and light microscopy measurements are the most useful, and these techniques have been widely employed for the individual and the mixed monolayer systems. Dynamic behavior of polymer penetration into the monolayers should be easily monitored by the respective methods. Particularly, a fluorescence or Brewster angle microscope has been widely utilized in studies of the equilibrium phase behavior and the kinetics of phase transition and pattern formation in monolayers at the air-water interface.10 Thus, the microscopic technique should be suitable to observe changes in the morphology of monolayers due to the phase transition induced by the penetration of polymer. Recently, we have reported the miscibility and pattern changes of the binary blends of poly(vinyl acetate) (PVAc) and pentadecanoic acid (PDA) at the air-water interface, where PVAc and PDA are immiscible, as a function of PVAc surface concentration using fluorescence microscopy and surface pressure measurements.11 Both methods (1) de Gennes, P. G. J. Phys. Chem. 1990, 94, 8407. (2) Naumann, C.; Dietrich, C.; Lu, J. R.; Thomas, R. K.; Rennie, A. R.; Penfold, J.; Bayerl, T. M. Langmuir 1994, 10, 1919. (3) Ariga, K.; Shin, J. S.; Kunitake, T. J. Colloid Interface Sci. 1995, 170, 440. (4) Chatellier, X.; Andelman, D. Europhys. Lett. 1995, 32, 567. (5) Baekmark, T. R.; Elender, G.; Lasic, D. D.; Sackmann, E. Langmuir 1995, 11, 3975. (6) Armstrong, N. J.; Chari, K.; Penner, T. L. J. Colloid Interface Sci. 1996, 183, 617. (7) Rosilio, V.; Boissonnade, M.-M.; Zhang, J.; Jiang, L.; Baszkin, A. Langmuir 1997, 13, 4669. (8) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966. (9) Birdi, K. S, Lipid and Biopolymer Monolayers at Liquid Interfaces; Plenum: New York, 1989. (10) Petty, M. C. Langmuir-Blodgett Films: An Introduction; Cambridge University Press: Cambridge, U.K., 1996.
Figure 1. Plots of changes in the surface pressure, ∆π, as a function of time, t, elapsed after the injection of an aqueous poly(NIPAM) solution for various surface areas, A, of PDA monolayers: b, A ) 0; O, A ) 60 Å2/molecule; 0, A ) 35 Å2/ molecule; ∆, A ) 25 Å2/molecule.
were beneficial to the understanding of the phase transition of PDA induced by the addition of polymer. In this paper, we investigated the incorporation of poly(N-isopropylacrylamide) (poly(NIPAM)) into PDA monolayers spread at constant areas by injection of aliquots of an aqueous poly(NIAPM) solution. Poly(NIAPM) chains adsorb at the air-water interface,12-14 leading to squeezing of the spread PDA monolayers and causing the phase transition of PDA. Adsorption of poly(NIPAM) from the aqueous subphase at the air-water interface, where PDA was pre-spread, was measured in terms of an increment of an initial surface pressure of PDA monolayer and changes in fluorescence images of phase domains as a function of adsorption time. Experimental Section Materials. PDA (Tokyo Kasei Co.) was purified several times by crystallization of its n-hexane solution.11 N-(7-Nitro-2,1,3-benzoxadiazol-4-yl)-L-R-dipalymitoyl phosphatidylethanolamine (NBD-PE) (Molecular Probes, Inc., Eugene, Oregon) was used as a fluorescence probe without further purification, and its concentration was present around 1.5 mol % in all fluorescence microscopy measurements. We used a fractionated poly(NIPAM) with Mw ) 5070 × 103.14 The spreading solvent used to prepare PDA monolayers at the air-water interface was spectrograde chloroform, and we used it without further purification. Surface Pressure Measurements. The surface pressure was measured by a Wilhelmy system (NL-PS80-MTC; Nippon Laser & Electronics Lab., Nagoya, Japan). A filter paper of 5 mm width was used as a Wilhelmy plate attached to a displacement transducer. A Teflon-coated trough with an area of 15.8 × 9.0 × 1.0 cm3 was separated into two components by a Teflon circular ring with a diameter of 6.2 cm, and it was filled with an aqueous 0.01 N HCl solution with the temperature controlled at 25 ( 0.2 °C. Monolayers of PDA were first spread at fixed areas of 25, 35, and 60 Å2/molecule on the surface of the aqueous HCl solution on the inside of the circular ring by applying the chloroform solution with a Hamilton microsyringe. The solvent was allowed to evaporate for at least 30 min, and unless the surface pressure did not remain constant, we regarded it as an equilibrium surface pressure. After attaining the equilibrium surface pressure of PDA, 0.3 mL of an aqueous poly(NIPAM) solution with a concentration of 4.2 g/L was deposited randomly on the water surface on the outside of the ring barrier using a Hamilton microsyringe and then it was gently mixed to disturb (11) Kawaguchi, M.; Yamamoto, M.; Kato, T. Langmuir 1997, 13, 2414. (12) Kawaguchi, M.; Saito, W.; Kato, T. Macromolecules 1994, 27, 5882. (13) Kawaguchi, M.; Hirose, Y.; Kato, T. Langmuir 1996, 12, 3523. (14) Saito, W.; Kawaguchi, M.; Kato, T.; Imae, T. Langmuir 1996, 12, 5947.
S0743-7463(97)01282-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/11/1998
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Figure 2. Fluorescence microscopic images of PDA monolayers with surface areas of 60 (I), 35 (II), and 25 (III) Å2/molecule for various adsorption times, t, after the injection of the aqueous poly(NIPAM) solution and π. For I: a, t ) - 14′ 21′′, π ) 0 mN/m; b, t ) 54′00′′, π ) 0.63 mN/m; c, t ) 54′57′′, π ) 0.85 mN/m; d, t ) 55′08′′, π ) 0.95 mN/m. For II: a, t ) - 10′00′′, π ) 2.5 mN/m; b, t ) 45′48′′, π ) 11.1 mN/m; c, t ) 45′54′′, π ) 11.5 mN/m; d, t ) 46′12′′, π ) 12.1 mN/m. For III: a, t ) - 7′20′′, π ) 8.5 mN/m; b, t ) 29′48′′, π ) 12.3 mN/m; c, t ) 33′27′′, π ) 13.1 mN/m; d, t ) 37′36′′, π ) 14.0 mN/m. the spread PDA monolayer, leading to a final poly(NIPAM) concentration of ca. 8.8 × 10-3 g/L in the subphase. Thus, changes in the surface pressure were monitored as a function of the time elapsed after the deposition and mixing of poly(NIPAM). The experimental errors in the surface pressure measurements were less than 0.1 mN/m, and the errors in the time when changes in surface tension were first observed due to the polymer adsorption were less than 3 min. Therefore, the reproducibility of the kinetic measurements is good. Fluorescence Microscopy. The same trough mentioned above was mounted on the stage of an Olympus BH2-UMA epifluorescence microscope attached to a DAS-512 ICCD camera (Imajista Co., Tokyo, Japan).11 The surface pressure measurements and fluorescence microscope observation were carried out simultaneously. Images of the monolayers were observed through Olympus NeoSplan 40 (40×) ultralong-working-distance objective lenses. 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. The images were analyzed by a Himawari 60 digital image analyzer (Library Co., Tokyo, Japan).
Results and Discussion Surface Pressure Measurements. Changes in the surface pressures, ∆π ) π(t) - π0, caused by poly(NIPAM) adsorption for the PDA monolayers with fixed PDA surface areas of 0, 60, 35, and 25 Å2/molecule are plotted in Figure 1 as a function of elapsed time, t, after injection of a poly(NIPAM) solution, where π(t) is the surface pressure at a given time after the deposition of an aqueous poly(NIPAM) solution and π0 is the surface pressure of the pure PDA just before the deposition. The values of π0 are 0, 2.5, and 8.5 mN/m for PDA surface areas of 60, 35, and 25 Å2/molecule, respectively. Hereafter, we will call the plot of ∆π against t the adsorption kinetic curve. The higher the spread amount of PDA is, the earlier the change in the surface pressure is observed. The values of ∆π gradually approach the respective plateau values, and
the plateau surface pressure increases with an increase in the spread amount of PDA. On the other hand, the adsorption kinetic curves of 60 and 35 Å2/molecule have a kink point around 12 and 9 mN/m of ∆π, respectively, which is attributed to the liquid expanded (LE)/liquid condensed (LC) transition region of PDA. Above the kink point the rate of change in the surface pressure seems to be almost the same as that of poly(NIPAM). Fluorescence Microscopy. After spreading the 1.5 mol % NBD-PE chloroform solution without PDA on the water surface, poly(NIPAM) was deposited and changes in the surface tension and in the fluorescence microscopic images were monitored. The circular gas domains due to the presence of NBD-PE were observed here and there at early adsorption time of poly(NIPAM). However, they gradually faded with an increase in the adsorption time due to covering them by the adsorbed poly(NIPAM) layer. At the plateau surface tension of the poly(NIPAM) solution the fluorescence image was less bright than that of the pure water. This indicates that the fluorescence probe is not participated within the poly(NIPAM) monolayer. Thus, it is preferentially soluble in the LE phase of PDA for the blend films of PDA and poly(NIPAM) since the fluorescence probe of NBD used here is selectively soluble in LE regions.15 Figure 2 shows typical changes in fluorescence microscopic images of PDA-poly(NIPAM) mixtures at various PDA surface areas of 60, 35, and 25 Å2/molecule as a function of adsorption time, respectively. The pure PDA surface area of 60 Å2/molecule corresponds to the coexistence of gas (G)-liquid expanded (LE) phases, and the pure PDA surface areas of 35 and 25 Å2/molecule correspond to the LE phase only and the coexistence of LE(15) Moore, B. G.; Knobler, C. M.; Akamatsu, S.; Rondelez, F. J. Phys. Chem. 1990, 94, 4588.
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Figure 3. Double-logarithmic plots of R as a function of t t0 ) ∆t, where t0 is the time at which the LC domain was first observed and R is the average radius of the LC domains at a given time t. Each point represents an average of several LC domains. The line indicates least-squares fits to the data.
liquid condensed (LC) phases, respectively.15,16 It can be expected that the dark regions at 60 Å2/molecule and those at 25 Å2/molecule correspond to the G and LC phases, respectively. At 35 Å2/molecule a uniformly bright film corresponds to the formation of a single phase of LE. Adsorption of poly(NIPAM) at the air-water interface, where the PDA monolayer was pre-spread, induces changes in the fluorescence images. At the PDA surface area of 60 Å2/molecule, beyond the elapsed time of 50 min, the resulting G domains are deformed with their rugged periphery, the amplitude of the meandering periphery is larger, and then they coalesce. The image as shown in Figure 2-I-d is similar in shape to that observed for the buckling instability developed in an ethyl heptadecanoate monolayer by heating.17 Such morphology changes occurred within 1 min as shown in Figure 2-I. Furthermore, we notice that the shape changes occur with very little change in surface pressure. It seems more likely that the instability comes from a change in line tension between the G and LE phases caused by the adsorption of poly(NIPAM). Though the G domains are drastically changed in shape with an increase in adsorption time, the areas of the individual domains almost remain during changes in shape. With further increasing adsorption time, a homogeneous and less bright image, in which no dark domains are observed, was observed, and it is similar to that of poly(NIPAM) alone. At the PDA surface area of (16) Pallas, N. R.; Pethica, B. A. Langmuir 1985, 1, 509. (17) Stine, K. J.; Knobler, C. M. Phys. Rev. Lett. 1990, 65, 1004.
Notes
35 Å2/molecule, small circular LC phase domains in the LE phase appear beyond the ∆π value of around 10 mN/m due to aggregation of PDA, and the size of the LC domains increases without coalescence with an increase in adsorption time as shown in Figure 2-II. Dynamic growth of the LC domains is analyzed by plotting the average domain radius R as a function of t - t0 ) ∆t, where t0 is the time at which the LC domain was first observed and the R values were calculated from the domain area S by R ) (S/π)1/2 at a given time t. A typical logarithmic plot of R against ∆t is shown in Figure 3, where it is seen that a power law with an exponent of 0.51 gives a good representation of the data. The resulting exponent is much larger than the exponent of ca. 0.3 for the growth of the circular gas bubbles of PDA.18 On the other hand, a similar exponent was obtained for domain growth in two-dimensional binary immiscible fluids by computer simulation, in which the number and ratio of immiscible particles is fixed during the simulation.19 Although the concentration is not fixed by the polymer adsorption in the present work, the agreement between both exponents seems to stem from little change in surface tension during the growth of LC domains. In addition, such a good agreement is consistent with the fact that the LC and LE domains behave as immiscible fluids at the air-water interface. At the PDA surface area of 25 Å2/molecule, circular LC domains coarsened without coalescence, “salami” structures in the LC domains appeared and became larger, and then they coalesced with increasing adsorption time as shown in Figure 2-III. Formation and sequential growth of the salami structures means that poly(NIPAM) chains first penetrate into the LE phase and subsequently migrate into the LC domains. The light domains inside LC domains correspond to LE domains, which are stabilized at smaller diameters by the presence of polymer, namely, the line tension effect. Furthermore, the polymers do not mix with PDA in the molecular level. The salami structures are frequently observed in immiscible polymer blends in bulk.20 LA971282S (18) Berge, B.; Simon, A. J.; Libchaber, A. Phys. Rev. A 1990, 41, 6893. (19) Coveney, P. V.; Novik, K. E. Phys. Rev. E 1996, 54, 5134. (20) Echter, A. Angew. Makromol. Chem. 1977, 58/59, 175.
