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Unusual Transition in a Two-Dimensional Condensed Phase to a Mosaic Texture Md. Mufazzal Hossain, Toshiyuki Suzuki,† and Teiji Kato* Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan, and Skin Care Products Research Laboratory, Kao Corporation, Tokyo, Japan Received May 31, 2000. In Final Form: September 18, 2000 Both first- and second-order phase transitions have been studied in adsorbed monolayers of n-hexadecyl phosphate at the air-water interface by Brewster angle microscopy (BAM). An initial plateau region in the surface pressure-time (π-t) curve is caused by a first-order phase transition. This claim is supported by the coexistence of two surface phases which are observed by BAM. A second-order phase transition is indicated by a gradual change in the surface morphology, from a uniformly bright isotropic to an anisotropic mosaic textured phase, which is accompanied by a continuous change in surface pressure. This unusual transition from an isotropic to an anisotropic phase can be understood considering the intermediate nature of the former phase between liquid-expanded and liquid-condensed states.
Introduction Phase behavior of insoluble amphiphiles in two-dimensions (2-D) has been studied extensively by measuring π-A isotherms together with some other microscopic and/ or spectroscopic results. The π-A isotherms of Langmuir monolayers suggest the existence of both first- and secondorder phase transitions during compression.1 It is now widely accepted that a horizontal line in the isotherms indicates a first-order transition whereas a small kink reveals a transition of higher-order between two phases of different compressibility. Microscopic observation and X-ray diffraction studies of different phases in the monolayers confirm this prediction from the π-A isotherms.2-7 These techniques have revealed that even a small change in the isotherm can correspond to a phase transition. Our particular interest is on Brewster angle microscopy (BAM),8,9 which can visualize the coexistence of two phases of different densities, providing direct evidence for a firstorder transition. In addition, the change in the film texture at transitions between tilted phases or between tilted and untilted phases can be observed using BAM. Rivie`re et al.6 reported that any sharp change in contrast during BAM observation is also indicative of a first-order transition. In contrast, a second-order transition can be ascertained microscopically by observing a gradual change in the image contrast which is correlated by a small kink or a continuous rise in surface pressure in the isotherms.4-7 During such a transition, monolayers usually change from a tilted to less tilted (or untilted) phase by the compression. Although a variety of mesophases have been observed to date, the generalized phases in Langmuir monolayers are gas (G), liquid-expanded (LE), liquid-condensed (LC), * To whom correspondence should be addressed at Utsunomiya University. Phone: +81-28-689-6170. Fax.: +81-28-689-6179. E-mail:
[email protected]. † Skin Care Products Research Laboratory. (1) Harkins, W. D.; Copeland, L. E. J. Chem. Phys. 1942, 10, 272. (2) Moore, B. G.; Knobler, C. M.; Akamatsu, S.; Rondelez, F. J. Phys. Chem. 1990, 94, 4588. (3) Knobler, C. M. Adv. Chem. Phys. 1990, 77, 397. (4) Overbeck, G. A.; Ho¨nig, D.; Mo¨bius, D. Langmuir 1993, 9, 555. (5) Ramos, S.; Castillo, R. J. Chem. Phys. 1999, 110, 7021. (6) Rivie`re, S.; He´non, S.; Meunier, J.; Schwartz, D. K.; Tsao, M.-W.; Knobler, C. M. J. Chem. Phys. 1994, 101, 10045. (7) Riviere-Cantin, S.; He´non, S.; Meunier, J. Phys. Rev. E 1996, 54, 1683. (8) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. (9) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590.
and solid (S) phases. Harkins et al.1,10 suggested that an intermediate phase (I) exists between the LE and LC phases and that the transitions LE-I and I-LC are second-order. The principle characteristic of this I phase is extremely high compressibility. However, there was no direct experimental evidence of the presence of such an I phase in the monolayers. The phase behavior of water-soluble surfactants in Gibbs adsorption layers has not received much attention until recently. Introduction of BAM, which has the advantage of allowing in situ studies during surface pressure measurement at the water surface, has opened the door to investigate the phase transition and consequent condensed domain formation in these monolayers.11-13 Presently, reports on first-order transitions from G to LC as well as from LE to LC in these monolayers are available in the literature.11-20 Several attempts have been made to compare the properties of Gibbs monolayers with those of spread monolayers.17-20 Vollhardt et al.17,18 have found similarities in both thermodynamic behavior and major morphological features of the domains of these monolayers. We have reported that the LC domains behave similarly toward the experimental variables regardless of the types of monolayers.20 In this Letter we show that both firstand second-order phase transitions occur in Gibbs monolayers of n-hexadecyl phosphate (n-HDP). The most interesting feature of our study is the unusual transition, which is second-order in nature, from an isotropic to an anisotropic phase in condensed monolayers with an increase in the surface concentration. To the best of our knowledge, we observe such an unusual phase transition in the monolayer systems for the first time. (10) Harkins, W. D. The Physical Chemistry of Surface Films; Reinhold: New York, 1952; Chapter 2. (11) He´non, S.; Meunier, J. J. Chem. Phys. 1993, 98, 9148. (12) Melzer, V.; Vollhardt, D. Phys. Rev. Lett. 1996, 76, 3770. (13) Hossain, M. M.; Yoshida M.; Kato, T. Langmuir 2000, 16, 3345. (14) Berge, B.; Faucheux, L.; Schwab, K.; Libchaber, A. Nature 1991, 350, 322. (15) Casson, B. D.; Braun, R.; Bain, C. D. Faraday Discuss. 1996, 104, 209. (16) Pollard, M. L.; Pan, R.; Steiner, C.; Maldarelli, C. Langmuir 1998, 14, 7222. (17) Vollhardt, D.; Melzer, V. J. Phys. Chem. B 1997, 101, 3370. (18) Melzer, V.; Vollhardt, D.; Brezesinski, G.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 591. (19) Hossain, M. M.; Yoshida, M.; Iimura, K.; Suzuki, N.; Kato, T. Colloids Surf., A 2000, 171, 105. (20) Hossain, M. M.; Kato, T. Langmuir, in press.
10.1021/la0007527 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000
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Figure 1. (a, top) π-t adsorption kinetics of a 2.5 × 10-5 M aqueous solution of n-HDP at 20 °C. The points A, B, C, and so forth in the figure indicate the position of BAM observation shown in part b. (b, bottom) BAM images of a 2.5 × 10-5 M aqueous solution of n-HDP at 20 °C with time, showing both first-order (images A-C) and second-order (images C-F) phase transitions. Image size: 300 × 200 µm2.
Experimental Section The material n-HDP was obtained from Kao corporation, Japan, with a purity above 99% and used here without further purification. The purity was checked by 1H NMR (Varian Unity INOVA). Ultrapure water of resistivity 18 MΩ‚cm (ElgastatUHQPS) was used throughout the present research. All the experiments were performed in a home-built shallow type (2 mm depth) Langmuir trough above which a Brewster angle microscope was mounted. The surface pressure was measured by the Wilhelmy method using a small glass plate. A 20 mW He-Ne laser was used as a light source for BAM observation. The experiments were carried out by putting an aqueous solution of n-HDP into the trough at 20 °C. After 25 min, which is sufficient for acquiring
the experimental temperature, the solution surface was swept rapidly with computer-controlled movable Teflon barriers. The surface pressure was monitored and the surface was observed by BAM simultaneously. Details of the instrumentation21 and the experimental procedure19 were published elsewhere.
Results and Discussion Figure 1a presents the surface pressure-time (π-t) adsorption kinetics of a 2.5 × 10-5 M aqueous solution of n-HDP. The points A, B, and so forth in the figure indicate (21) Kato, T.; Tatehana, A.; Suzuki, N.; Iimura, K.; Araki, T.; Iriyama, K. Jpn. J. Appl. Phys. 1995, 34, L911.
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Figure 2. BAM images of a 3.0 × 10-5 M aqueous solution of n-HDP at 20 °C with time, showing a transition from isotropic to tilted phase. The time interval between images A to F is about 1.5 h. Image size: 400 × 250 µm2.
the time of BAM observation shown in Figure 1b. Although a gradual increase in π with time is expected due to continuous adsorption of the amphiphilic molecules, a plateau region is observed in the π-t curve at about zero surface pressure. This zero pressure is also confirmed by digital tensiometer (Kru¨ss K10), which shows an initial surface tension of 72.8 mN/m (standard surface tension of water at 20 °C). After some time the surface pressure increases rapidly to its equilibrium value. This type of surface tension or surface pressure relaxation for some other amphiphiles was previously explained by the existence of the first-order phase transition.11,22,23 On the other hand, just after sweeping the surface with barriers, small size bright domains begin to grow (image A). Because of the rapid rate of adsorption, the domain does not increase in size, but rather it increases in number (image B). The surface is fully covered with fused domains (image (22) Lin, S.-Y.; McKeigue, K.; Maldarelli, C. Langmuir 1991, 7, 1055. (23) Sundaram, S.; Ferri, J. K.; Vollhardt, D.; Stebe, K. J. Langmuir 1998, 14, 1208.
C) within 12 min. The initially observed two-phase coexistent state clearly supports the concept of the firstorder transition as described above. The fusion of the domains indicates their fluid-like behavior. After completion of the first-order transition, the surface pressure increases abruptly from zero to the final equilibrium value and the morphology of the uniform monolayers undergoes a gradual change to form a mosaic texture (images D-F), although no indication is found either of a plateau or of a kink in the π-t curve. These results, at least, demonstrate that the latter transition is not of first-order. In addition, we do not observe coexistence of two phases at the latter stage. Rather, we see the beginning of the formation of anisotropic monolayers represented by bright and dark regions (images E, F). Overbeck et al.4 observed a continuously decreasing contrast in BAM images at the region of a small kink in a π-A isotherm of a spread monolayer. They also classified the transition as a secondorder phase transition. In some other literature,6,7 a gradual change in the anisotropy of the monolayers that
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is approached by increasing the surface pressure has been taken as evidence for the second-order phase transition. Thus, our observation of a gradual increase in the anisotropy accompanied by an increase in the surface pressure indicates a second-order phase transition. Moreover, absence of nucleation of a new phase during the transition is also consistent with a second-order phase transition.4 We certainly need to prove whether the final phase is untilted or tilted because it is reported that the monolayer can show mosaic texture with a weak contrast even when the molecules are not tilted.6,24 This texture is apparently due to the anisotropy of the unit cell. The change in morphology of the condensed monolayers with adsorption becomes clearer when we use a more concentrated solution of 3.0 × 10-5 M n-HDP (Figure 2). Just after sweeping the surface with the barriers, the surface pressure begins to increase immediately and we observe uniformly bright monolayers containing fused condensed domains of fluid-like nature (image A). At the beginning stage, a two-phase coexistent state is not observed with this higher concentration solution because of the very rapid rate of adsorption. With time, the surface pressure increases rapidly toward equilibrium. The surface of the monolayer shows inhomogeneity within a few minutes (image B). This inhomogeneous monolayer later shows a mosaic texture containing molecules with either different tilt directions or different unit cell orientations (image C). However, after a long time (1.5 h) we observe perfectly anisotropic condensed monolayers with segments of different brightnesses (images E, F). The strong contrast among the segments of the monolayers clearly suggests that the final anisotropic form is a tilted LC phase. To characterize the initially observed bright monolayer phase more clearly, we performed the experiment at a low concentration of 2.0 × 10-5 M to reduce the rate of adsorption so that the domain size is increased (Figure 3). Because of the low bulk concentration, the surface pressure remains unchanged for a longer time and the second-order transition to mosaic texture does not occur. However, we observe uniformly bright domains which undergo fusion when they touch each other (images A, B). Although the BAM images suggest that the bright phase should be untilted in nature, the existence of such a phase is very unlikely under these experimental conditions. The ordering in this phase should be at least lower than that of the LC phase. The question arises, what could explain the observed phases and their transitions, particularly from the isotropic to anisotropic phase? The present understanding of the phase diagram of 2-D monolayers allows only the existence of G-LE, G-LC, and LE-LC transitions to form the LC phase, and all of them are first-order.2 Definitely, the initially observed dark phase (Figure 1b, image A) is G type because, just after sweeping of the surface, the 2-D concentration of the amphiphilic molecules is very low. Since the mosaic textured phase is LC type, the only remaining name for the other phase is LE. But the bright monolayer phase observed during the former transition should not be LE because the second-order LE-LC transition as observed during the experiment is not allowed. The BAM images are also consistent with the conclusion that this phase should rather be of a condensed type. Evidently, the latter transition is induced by the increased surface concentration because, with a lower concentration solution, this transition is not observed even after long time and/or, with increased surface concentra(24) Overbeck, G. A.; Ho¨nig, D.; Wolthaus, L.; Gnad, M.; Mo¨bius, D. Thin Solid Films 1994, 242, 26.
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Figure 3. BAM images of a 2.0 × 10-5 M aqueous solution of n-HDP at 20 °C. Images were taken at 21 min (A) and 35 min (B). Image size: 400 × 250 µm2.
tion, contrast among the segments of the tilted phase becomes clear within several minutes (Figure 2, images C-F). Thus, any interpretation by a relaxation of the isotropic phase can be ruled out. A remedy of the problem is to consider the intermediate nature (I) of the initially observed bright monolayer phase between LE and LC, as suggested by Harkins et al.1,10 The observation of the second-order transition of I-LC is also consistent with their prediction. The density as well as the properties of this phase should also be intermediate. We still do not have any explanation about the mechanism by which this unusual transition takes place. The nature of the initial bright monolayer phase could not be determined exactly because we can characterize the monolayer phases in the macroscopic level, which reveals an average of the property of the molecules inside the domains. More experimental work, particularly at the molecular level, is, therefore, necessary to understand the exact nature of this phase and the mechanism by which such an unusual transition takes place. In conclusion, we wish to emphasize here that we observe for the first time a new phase in 2-D systems that is associated with an unusual transition from the isotropic to anisotropic phase with increasing surface concentration. However, the exact interpretation for them is still elusive. Research is in progress with polarization modulation FTIR spectroscopy and grazing incidence X-ray diffraction to characterize this new phase. Acknowledgment. We are thankful to Prof. R. A. Dluhy of the University of Georgia for the English grammatical corrections of the manuscript. Part of this research is supported by SVBL of Utsunomiya University. LA0007527
