Kinetic Appearance of First-Order Gas−Liquid ... - ACS Publications

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Langmuir 2006, 22, 1074-1078

Kinetic Appearance of First-Order Gas-Liquid Expanded and Liquid Expanded-Liquid Condensed Phase Transitions below the Triple Point Md. Mufazzal Hossain,† Toshiyuki Suzuki,‡ Ken-ichi Iimura,† and Teiji Kato*,† Venture Business Laboratory, Utsunomiya UniVersity, Yoto 7-1-2, Utsunomiya 321-8585, Japan, and Skin Care Products Research Laboratory, Kao Corporation, Tokyo, Japan ReceiVed August 17, 2005. In Final Form: NoVember 13, 2005 Phase diagram of Gibbs monolayers of mixtures containing n-hexadecyl phosphate (n-HDP) and L-arginine (L-arg) at a molar ratio of 1:2 has been constructed by measuring surface-pressure-time (π-t) isotherms with film balance and by observing monolayer morphology with Brewster angle microscopy (BAM). This phase diagram shows a triple point for gas (G), liquid expanded (LE), and liquid condensed (LC) phases at around 6.7 °C. Above this triple point, a first-order G-LE phase transition occurring at 0 surface pressure is followed by another first-order LE-LC phase transition taking place at a certain higher surface pressure that depends upon temperature. The BAM observation supports these results. Below the triple point, the π-t measurements show only one first-order phase transition that should be G-LC. All of these findings are in agreement with the general phase diagram of the spread monolayers. However, the BAM observation at a temperature below the triple point shows that the thermodynamically allowed G-LC phase transition is, in fact, a combination of the G-LE and LE-LC phase transitions. The latter two-phase transitions are separated by time and not by the surface pressure, indicating that the G-LC phase transition is kinetically separated into these two-phase transitions. The position of the LE phase below the triple point in the phase diagram is along the phase boundary between the G and LC phases.

Introduction Understanding of the physics of two-dimensional (2D) spread or Langmuir monolayers of water-insoluble amphiphiles has progressed considerably because of the development of some surface-sensitive techniques such as X-ray diffraction, neutron reflection, infrared reflection absorption spectroscopy, etc. Using these techniques, a variety of phases have been reported to exist in these monolayers. However, the generalized phase diagram of simple amphiphiles consists, at least, of four phases.1 These are gas (G), liquid-expanded (LE), liquid condensed (LC), and solid (S) phases. The phase transitions between two of the G, LE, and LC phases are first-order.2-4 All of the latter three phases coexist at a certain temperature, the so-called “triple point”. This triple point of an amphiphile only depends upon its nature and does not depend upon the surface pressure or any other variables. At a temperature above the triple point, a first-order G-LE phase transition occurs at a very low surface pressure. This transition is followed by another first-order LE-LC phase transition that takes place at a certain higher surface pressure depending upon the temperature. A first-order G-LC phase transition also occurs at a very low surface pressure and exists only below the triple point of the amphiphile.4 Although the phase diagrams of Gibbs adsorption layers of water-soluble surfactants are less rich with phases, the existence of the G, LE, and LC phases in these monolayers is now a wellknown phenomenon. A number of reports on the existence of * To whom correspondence should be addressed. Telephone: +81-28689-6170. Fax: +81-28-689-6179. E-mail: [email protected]. † Utsunomiya University. ‡ Kao Corporation. (1) Petty, M. C. Langmuir Blodgett Films, 1st ed.; Cambridge University Press: New York, 1996. (2) Pallas, N. R.; Pethica, B. A. J. Chem. Soc., Faraday Trans. 1 1987, 83, 585. (3) Winch, P. J.; Earnshaw, J. C. J. Phys.: Condens. Matter 1989, 1, 7187. (4) Moore, B. G.; Knobler, C. M. J. Chem. Soc., Faraday Trans. 2 1989, 82, 1753.

the G-LE,5-7 LE-LC,8-13 or G-LC14,15 phase transitions in these monolayers have been readily available in the literature. Recently, we have reported that similar to the behavior of the spread monolayers, all of these phases coexist at a certain triple point of the amphiphile.16,17 Several attempts have been made to construct rather elaborated phase diagrams for Gibbs monolayers, which have implied that the surface phase behavior of both the Gibbs and Langmuir monolayers is similar.16,17 Despite much of the information regarding the physical properties of various phases in Langmuir monolayers obtained from different surface-sensitive diffraction and/or reflection techniques, microscopic techniques such as fluorescence microscopy (FM)18,19 and Brewster angle microscopy (BAM)20,21 have been playing outstanding roles. The visualization of the monolayer morphologies using FM or BAM provides us straightforward evidence of the equilibrium between the LE and LC phases in the transition region as well as insights into the shapes and textures of the domains. A large volume of data on the morphological changes during the LE-LC22-28 phase (5) Aratono, M.; Uryu, S.; Hayami, Y.; Motomura, K.; Matuura, R. J. Colloid Interface Sci. 1984, 98, 33. (6) Erichson, J. S.; Sundaram, S.; Stebe, K. J. Langmuir 2000, 16, 5072. (7) Subramanyam, R.; Maldarelli, C. J. Colloid Interface Sci. 2002, 253, 377. (8) Melzer, V.; Vollhardt, D. Phys. ReV. Lett. 1996, 76, 3770. (9) Vollhardt, D.; Melzer, V. J. Phys. Chem. B 1997, 101, 3370. (10) Hossain, M. M.; Yoshida, M.; Kato, T. Langmuir 2000, 16, 3345. (11) Hossain, M. M.; Kato, T. Langmuir 2000, 16, 10175. (12) Islam, M. N.; Kato, T. J. Phys. Chem. B 2003, 107, 965. (13) Pollard, M.; Pan, L. R.; Steiner, C.; Maldarelli, C. Langmuir 1998, 14, 7222. (14) He´non, S.; Meunier, J. J. Chem. Phys. 1993, 98, 9148. (15) Bois, A. G.; Panaiotov, I.; Baret, J. F. Chem. Phys. Lipids 1984, 34, 265. (16) Hossain, M. M.; Suzuki, T.; Kato, T. J. Colloid Interface Sci. 2005, 288, 342. (17) Hossain, M. M.; Suzuki, T.; Kato, T. Colloid Surf., A, in press. (18) von Tscharner, V.; McConnell, H. M. Biophys. J. 1981, 36, 36. (19) Losche, M.; Mo¨hwald, H. ReV. Sci. Instrum. 1984, 55, 1968. (20) He´non, S.; Meunier, J. ReV. Sci. Instrum. 1991, 62, 936. (21) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (22) Mo¨hwald, H. Annu. ReV. Phys. Chem. 1990, 41, 441. (23) Knobler, C. M.; Desai, R. C. Annu. ReV. Phys. Chem. 1992, 43, 207.

10.1021/la0522451 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/31/2005

First-Order G-LE and LE-LC Phase Transitions

transitions together with some reports on the G-LE29,30 phase transitions is also available. However, we are not aware of any study on the morphological changes during isothermal compression of the Langmuir monolayers in the G-LC phase transition region. The reason of neglecting the G-LC phase transition for such investigations is that, just after evaporation of the spread solvent, the monolayers are in compact forms, showing irregular morphological patterns. One way to investigate these systems is that, upon increasing temperature, the monolayers can be transformed into the completely G state. Then, a reproducible evolution of a circular LC phase (although not isothermal growth) can be achieved by slow cooling.31,32 However, all of these disadvantages can be overcome during the formation of the Gibbs monolayers, where the amphiphilic molecules are allowed to adsorb on a clean air-water interface. As a result, the isothermal morphological changes of the monolayers observed during the G-LC transition can be monitored by FM or BAM.14,16,17 Both He´non et al.14 and our group16,17 have reported that with time the surface concentration increases at the air-water interface and the LC phase starts forming in expense of the G phase. In this paper, we also endeavor to study the G-LC phase transition in Gibbs monolayers of a mixture containing n-hexadecyl phosphate (n-HDP) and L-arginine (L-arg) at a molar ratio of 1:2. Contrary to the previous papers, we present the first study that the G-LC phase transition is a kinetic combination of the G-LE and LE-LC phase transitions. Although not allowed by the thermodynamics of 2D systems, we show that these two latter phase transitions occur below the so-called triple point. It should be noted that the details of the phase behavior of the Gibbs monolayers of n-HDP have been published elsewhere.33-35 In some recent papers, we have shown that L-arg acts as an L-argininium cation in the aqueous solution of the mixtures containing n-HDP and L-arg and, depending upon the molar ratio, it forms mono- and di-substituted L-arg ionic complexes of n-HDP.35,36 At a molar ratio of 1:2 of n-HDP/L-arg, the formed complex is considered as a di-L-argininium complex of n-HDP [n-HDP-(L-arg)2] and behaves as a new amphiphile.36 We, therefore, choose this composition for further investigations at different temperatures. The kinetics of the formation of the monolayers is monitored by measuring surface pressure (π) with time (t). The existence of a first-order phase transition is concluded from the appearance of a plateau or a cusp point followed by a plateau in these π-t curves.5-17 This claim is further confirmed by observing two surface phases by the BAM. In comparison with the present understanding of the spread monolayers as well as observing domain morphology by BAM, the nature of the phases is confirmed. Experimental Procedures The amphiphile n-HDP was supplied by the Kao Corporation, Japan, with a purity g99%, and the material L-arg was purchased (24) Vollhardt, D. AdV. Colloid Interface Sci. 1996, 64, 143. (25) Miller, A.; Knoll, W.; Mo¨hwald, H. Phys. ReV. Lett. 1986, 56, 2633. (26) Weidmann, G.; Vollhardt, D. Langmuir 1997, 13, 1623. (27) Schwartz, D. K.; Tsao, M.-W.; Knobler, C. M. J. Chem. Phys. 1994, 101, 8258. (28) Riviere, S.; He´non, S.; Meunier, J. J. Phys. ReV. E 1994, 49, 1375. (29) Stine, K. J.; Rauseo, S. A.; Moore, B. G.; Wise, J. A.; Knobler, C. M. Phys. ReV. A 1990, 41, 6884. (30) Berge, B.; Simon, A. J.; Libchaber, A. Phys. ReV. A 1990, 41, 6893. (31) Poulis, J. A.; Boonman, A. A. H.; Gieles, P.; Massen, C. H. Langmuir 1987, 3, 725. (32) Gutberlet, T.; Vollhardt, D. J. Colloid Interface Sci. 1995, 173, 429. (33) Hossain, M. M.; Suzuki, T.; Kato, T. Langmuir 2000, 16, 9109. (34) Hossain, M. M.; Suzuki, T.; Kato, T. Colloid Surf., A 2002, 198-200, 53. (35) Hossain, M. M.; Suzuki, T.; Kato, T. J. Colloid Interface Sci. 2005, 292, 186. (36) Hossain, M. M.; Suzuki, T.; Kato, T. Colloid Surf., A, in press.

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Figure 1. π-t adsorption isotherms for the mixtures containing n-HDP and L-arg at a molar ratio of 1:2 at different temperatures. Concentrations of n-HDP and L-arg were chosen arbitrarily to 4.0 × 10-5 and 8.0 × 10-5 M, respectively. from Sigma (Germany) with a certified purity of g98.5% and used here without further purification. The purity of n-HDP was checked by 1H NMR (Varian Unity INOVA) and digital tensiometer (Kru¨ss K10). Ultrapure water of resistivity 18 MΩ cm (Elgastat, UHQ-PS) was used throughout the present study. All of the experiments were performed in a home-built shallowtype Langmuir trough above which a BAM was mounted. The surface pressure was measured by the Wilhelmy method using a small rectangular glass plate. This plate was cleaned by immersion into 1% HF acid followed by washing with ultrapure water prior to each experiment. The temperature of the trough was controlled by using a large number of integrated Peltier elements attached to the base plate of the trough and was detected by a platinum wire temperature sensor. The BAM is composed of a 20 mW He-Ne laser, a GlanThompson polarizer, an analyzer, a zooming microscope, a CCD camera of high sensitivity connected to a TV monitor, and a videorecording system. Special software was employed to adjust the contrast and to revise the image distortion caused by the Brewster angle. Details of the trough were published elsewhere.37 The aqueous solutions of the mixtures containing n-HDP (4.0 × 10-5 M) and L-arg (8.0 × 10-5 M) at a molar ratio of 1:2 were prepared by weighing an appropriate amount of the components into a 1000 mL volumetric flask. The experiments were carried out by putting a definite amount of aqueous solutions of the mixtures into the trough. The solution was allowed to stand for about 25 min. The molecules already adsorbed at the surface during this span of time were removed by sweeping the solution surface by the movable Teflon barriers. The surface pressure was then followed with time, and simultaneously, the change in surface morphology was observed by BAM. Details of the experimental procedure were reported elsewhere.11,16

Results and Discussion Figure 1 shows the π-t adsorption curves of the mixtures containing n-HDP and L-arg at a molar ratio of 1:2 at different temperatures. The concentration of n-HDP in the mixtures was chosen arbitrarily to 4.0 × 10-5 M, considering the fact that the existence of phase transitions becomes evident from the π-t measurements of the solutions having the same concentration and composition over a wide range of temperatures. The ultimate concentration of L-arg necessary to maintain the above composition is 8.0 × 10-5 M. Under these conditions, a new amphiphile, n-HDP-(L-arg)2, is formed.35,36 Each of the π-t curves at g8 °C shows an initial plateau at about 0 surface pressure. After a certain period of induction time, the surface pressure rises with time. At a certain critical surface pressure (πc), which depends upon the temperature, this π-t curve shows a cusp point followed by another plateau. Just after the latter plateau, the surface pressure rises again slowly and finally reaches the equilibrium value. On the other hand, each of the π-t curves at e5 °C also shows the (37) Kato, T.; Tatehana, A.; Suzuki, N.; Iimura, K.; Araki, T.; Iriyama, K. Jpn. J. Appl. Phys. 1995, 34, L911.

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initial plateau at about 0 surface pressure. The surface pressure then rises continuously to the equilibrium value. Although, several interpretations for these plateaus in the π-t curves were suggested previously,38-40 there is now a consensus that these plateaus are caused by first-order phase transitions in the Gibbs monolayers.6-17 Thus, the π-t measurements of these monolayers clearly indicate that two-step first-order phase transitions take place at g8 °C. Because the equilibrium surface pressure is higher at lower temperatures and no other cusp point, kink, or plateau except the initial plateau exists in the π-t curves before reaching the equilibrium at e5 °C, one can conclude that only one first-order phase transition occurs at these temperatures. The values of πc necessary for the phase transitions at different temperatures were measured from the Figure 1. These values are plotted against temperature, which constitute the phase diagram (Figure 2). The phase diagram of this adsorption layers is very much similar to the generalized diagram of the spread monolayers.1,22-24 In comparison with the phase diagram of the spread monolayers, we can assign the phases observed under different conditions in the Gibbs monolayers. The assigned phases are shown in the Figure 2. However, in the following paragraph, we shall provide evidence by BAM that the properties of the assigned phases are exactly consistent with those of the respective phases. It is clear from the figure that, at higher temperatures, the initial phase transition that occurs at 0 surface pressure is a G-LE phase transition, while the second one at a certain higher surface pressure is an LE-LC phase transition. On the other hand, the only one phase transition that occurs at 0 surface pressure and at lower temperatures is a G-LC phase transition. The πc for the G-LC or G-LE phase transition is independent of the temperature, whereas that of the LE-LC phase transition increases linearly with an increasing temperature. These characteristics of the phase transitions are also in line with the present understanding of the spread monolayers.1-4 The πc-temperature (T) curve of the LE-LC transition intersects that of the G-LC and/or G-LE phase transitions at 6.7 °C. This is, indeed, the triple point of the monolayers formed by a mixture containing the n-HDP and L-arg at a molar ratio of 1:2. To make sure that our classification of phases is not done arbitrarily, simultaneous BAM images observed during the π-t measurement at 15 °C are presented in Figure 3 (a corresponding π-t curve is shown in Figure 1). However, the BAM images at different temperatures above the triple point are also similar.35 During the initial plateau at 0 surface pressure of the π-t curve,

two surface phases are found to coexist. Bright and circular domains are formed in a dark background (image a). Because the amphiphilic molecules are allowed to adsorb on a clean airwater interface, which is made by sweeping of the solution surface by the barriers, the surface concentration at the beginning stage is very low. Thus, the dark phase observed during the initial plateau is a G phase. The bright domains are deformed by the movement of the monolayers and undergo fusion while touching each other. Indeed, the monolayers were always in rapid movement, and the images were taken at the time of the sudden stop of the flow. These characteristics are very common in the LE phases during the G-LE phase transitions.29,30 With time, the fraction of the surface covered by the LE phase increases, and finally, at the end of the initial plateau of the π-t curve, the surface is fully covered by this bright monolayer phase (image b). At this stage, the surface pressure rises continuously and the surface of the monolayers remains homogeneously bright before showing a cusp point at about 7.8 mN/m of the surface pressure in the π-t curve. Just after this cusp point, the brighter domains having a fractal shape emerge at the expense of the LE phase (image c). The surface is fully covered again by this brighter phase, and the surface pressure rises again in the π-t curve. The fractal domains in monolayers are possible only in a phase where the dipole moments of the headgroups are well-ordered, resulting in strong dipole-dipole interactions.35 These interactions overcome the line tension of that phase and allow for the formation of fractal domains.11,41-43 Such a well-ordered phase in 2D monolayers can be classified as an LC phase. Thus, the classification of the G, LE, and LC phases and the existence of the first-order G-LE and LE-LC phase transitions at 15 °C are not made arbitrarily. Moreover, this classification is consistent with our previous papers.16,17,35 The most important and surprising part of this paper involves the BAM observation at a temperature lower than the triple point. Figure 4A shows a π-t curve of a mixture containing n-HDP and L-arg at a molar ratio of 1:2 at 2 °C, and Figure 4B presents the simultaneous BAM images. The positions at which images were taken are marked in Figure 4A. We chose this temperature for presenting BAM images because this temperature is well below the triple point. A further lowering of the temperature causes solidification of the aqueous solution. However, the BAM images at other temperatures below 6.7 °C are almost similar. The π-t curve shows a plateau, which predicts that thermodynamically a first-order phase transition exists under these conditions. In contrast, the BAM images show evidently the existence of two first-order phase transitions at 2 °C. The figure presents the growth of circular and bright domains in a dark background at the beginning stage of the plateau (images a and b). We have already indicated in the previous paragraph that the dark phase must be a G phase. Because the properties of the bright phase are similar to those of the LE phase during the G-LE phase transition at higher temperatures, this bright phase is also an LE phase. The homogeneous LE phase continues to exist for a little time (image c), but the surface pressure in the π-t curve still remains constant at a 0 value. After some time, brighter domains having a fractal shape, which have been found at the second plateau of the π-t curve at higher temperatures, emerge at the expense of the LE phase (images d and e). At the end of the plateau, the surface is fully covered by this brighter phase (image f). At this stage, the surface pressure starts rising continuously to the equilibrium value. This latter transition is

(38) van der Vegt, W.; Norde, W.; van der Mei, H. C.; Busscher, H. J. J. Colloid Interface Sci. 1996, 179, 57. (39) Sengupta, T.; Razumovsky, L.; Damodaran, S. Langmuir 1999, 15, 6991. (40) Ybert, C.; di Meglio, J.-M. Langmuir 1998, 14, 471.

(41) Keller, D. J.; McConnell, H. M.; Moy, V. T. J. Phys. Chem. 1986, 90, 2311. (42) Andelman, D.; Brochard, F.; Joanny, J. F. J. Chem. Phys. 1987, 86, 3673. (43) Siegel, S.; Vollhardt, D. Thin Solid Films 1996, 284/285, 424.

Figure 2. Phase diagram for Gibbs monolayers of the mixture containing n-HDP and L-arg at a molar ratio of 1:2. The arrow at 6.7 °C indicates the triple point of the system.

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Figure 3. Typical growth of monolayer phases at 15 °C, which is above the triple point. Images a, b, and c were taken at around 60, 150, and 500 s, respectively, after the start of the experiment. The corresponding π-t curve is shown in Figure 1. The bar in image a indicates 100 µm.

Figure 4. (A) π-t isotherm at 2 °C of a mixture containing n-HDP (4.0 × 10-5 M) and L-arg (8.0 × 10-5 M) at a molar ratio of 1:2. The points a-f indicate the positions of the BAM images that are shown in B. (B) Growth of surface phases observed during the π-t measurement at 2 °C. The positions at which images were taken are marked in A. Domains in images a and b are deformed by movement of the monolayers. The bar in image a indicates 100 µm.

consistent with the LE-LC phase transition as described above. Thus, although thermodynamically a G-LC phase transition is allowed to exist at 2 °C, the BAM images evidently show that there exist two first-order phase transitions, which are almost similar to the G-LE and LE-LC phase transitions at higher temperatures. The length of the plateau at 2 and/or 5 °C is almost equal to that of the combined G-LE and LE-LC phase transitions at higher temperatures. This also supports the above prediction. These two-phase transitions at lower temperatures are separated

by time and not by the surface pressure. This means that the thermodynamically allowed G-LC phase transition is separated kinetically into a G-LE and an LE-LC phase transition at a temperature lower than the so-called triple point. The question arises, where could be the position of the LE phase observed below the triple point in the phase diagram? The πc-T curve of the G-LE and that of the LE-LC phase transitions intersect each other at the triple point. This means that both the transitions merge at and below the triple point. Thus, at e6.7 °C,

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both the LE and the LC phases are expected to exist at 0 surface pressure. For the spread monolayers, all of the molecules remain in the vicinity just after evaporation of the solvent at the airwater interface and directly form the high-density LC phase. In contrast, for the Gibbs monolayers, the 2D surface concentration increases with the adsorption time. As a result, just after sweeping of the solution surface by the barriers, the 2D concentration of the molecules is too low to form a high-density LC phase but sufficient to form a low-density LE phase. When the surface concentration becomes sufficiently high for the formation of the LC phase (time-dependent phenomenon in our system), this phase starts forming at the expense of the LE phase. Thus, the position of the LE phase below the triple point in the phase diagram is along the phase boundary of the G-LC phase transition, and this phase appears only kinetically.

Conclusions In the present paper, we have shown that a first-order G-LE phase transition, which is followed by a first-order LE-LC phase transition, is found at a temperature higher than the triple point, 6.7 °C. Both π-t measurements and BAM observation confirm

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the order of phase transitions and the nature of phases. At e6.7 °C, the π-t curves show a first-order phase transition that should be G-LC. However, the BAM observation at lower temperatures shows that there exist two-phase transitions, which are almost similar to the G-LE and LE-LC transition at higher temperatures. These two-phase transitions are separated by time and not by the surface pressure, indicating that the thermodynamically allowed G-LC phase transition at lower temperatures is kinetically separated into G-LE and LE-LC phase transitions. Although not allowed thermodynamically, we have shown evidently for the first time that both the G-LE and LE-LC phase transitions occur below the so-called triple point. Acknowledgment. The financial support from the venture business laboratory (VBL) of Utsunomiya University is highly appreciated. One of the authors (K. I.) acknowledges the financial support from the Grant-in-Aid for young scientists (B) (15750112) 2003-2004 and (A) (17685012) 2005 from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. LA0522451