Langmuir 2004, 20, 6297-6301
6297
Surface Phase Behavior in Gibbs Monolayers of Bis(ethylene glycol) Mono-n-tetradecyl Ether at the Air-Water Interface Md. Nazrul Islam and Teiji Kato* Satellite Venture Business Laboratory, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan Received January 22, 2004. In Final Form: April 19, 2004 We present the adsorption kinetics and surface morphology of the adsorbed monolayers of bis(ethylene glycol) mono-n-tetradecyl ether (C14E2) by film balance and Brewster angle microscopy. A cusp point followed by a plateau region in the pressure (π)-time (t) adsorption isotherm indicates a first-order phase transition in the coexistence region between a lower density liquid expanded (LE) phase and a higher density liquid condensed (LC) phase. A variety of condensed phase domains surrounded by the homogeneous LE phase are observed just after the appearance of the phase transition. The domains are of a spiral or striplike structure at lower temperatures. This characteristic shape of the domains is because of strong dipole-dipole repulsion between the molecules. At 18 °C, the domains are found to be quadrant structures. A slight increase in subphase temperature (around 1 °C) brings about a quadrant-to-circular shape transition in the domains. The circular domains return to quadrant structures as the subphase temperature is lowered. The domains completely disappear when the temperature is increased beyond 19 °C, suggesting that the critical temperature for the condensed domain formation is 19 °C. Above this temperature, the hypothetical surface pressure necessary for the phase transition exceeds the actual surface pressure attainable from a solution of concentration greater than or equal to the critical micelle concentration. An increase in molecular motion with increasing temperature results in a higher degree of chain flexibility. As a result, the molecules cannot accumulate in the condensed phase form when the subphase temperature is above 19 °C.
Introduction Monolayers of insoluble amphiphiles are not only interesting model membranes but also well-organized twodimensional (2-D) systems. These systems provide a unique opportunity to study the physical processes of molecular films on molecular interactions. From the fundamental point of view, research on phase behavior in 2-D systems is an interesting exercise since it provides new insights into the morphological features of adsorbed molecules, which are not common in three-dimensional systems. The phase diagram of these systems exhibits pressure-induced coexistence regions between 2-D condensed phases and a 2-D gaseous or liquid-expanded phase leading to a rich variety of morphological features at different temperatures. The development of sensitive microscopic techniques such as fluorescence microscopy1,2 and Brewster angle microscopy3,4 (BAM) allows in situ visualization of these phases at the air-water interface. With these techniques, an exciting variety of textures such as stripe,5-7 star,7-9 and boojum9,10 in monolayer phases * To whom correspondence should be addressed. Phone: 81-28-689-6170. Fax: 81-28-689-6179. E-mail: teiji@c c.utsunomiya-u.ac.jp. (1) von Tscharner, V.; McConnell, M. Biophys. J. 1981, 36, 409. (2) Lo¨sche, M.; Mo¨hwald, H. Rev. Sci. Instrum. 1984, 55, 1968. (3) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (4) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. (5) Rivie`re, S.; He´non, S.; Meunier, J. Phys. Rev. E. 1994, 49, 1375. (6) Ruiz-Garcia, J.; Qiu, X., Tsao, M.-W.; Marshall, G.; Knobler, C. M.; Overbeck, G. A.; Mo¨bius, D. J. Phys. Chem. 1993, 97, 6955. (7) Overbeck, G. A.; Ho¨nig, D.; Mo¨bius, D. Thin Solid Films 1994, 242, 213. (8) He´non, S.; Meunier, J. J. Chem. Phys. 1993, 98, 9148. (9) Fischer, T. M.; Bruinsma, R. F.; Knobler C. M. Phys. Rev. E 1994, 50, 413. (10) Schwartz, D. K.; Tsao, M.-W.; Knobler, C. M. J. Chem. Phys. 1994, 101, 8258.
have been reported in the literature. Usually monolayers formed by surfactants of relatively longer hydrocarbon chains possess high line tension and favor large circularshaped domains while dipolar repulsions between the headgroups tend to distort the domain. Generally, circular shapes are expected for isotropic systems since a circle minimizes the length of the solid/fluid interfacial line. When the line tension is weak compared to the dipolar repulsive interactions, domains become elongated to minimize the electrostatic repulsions.11-13 Vollhardt14 et al. studied the surface phase behavior of Langnuir monolayers of 1-monostearoyl-rac-glycerol and observed circular domains at lower temperatures which attain more and more elliptical shape as the temperature increases. They have attributed this phenomenon to a decrease in the line tension and viscosity of condensed domains with increasing temperature. Fainerman et al.15 studied the pH dependency of surface phase behavior of arachidic acid monolayers at the air-water interface. Their study reveals that the two-phase coexistence region of the π-A isotherm shifts to higher surface pressures with increasing the pH of the subphase. At the same time, irregularly shaped condensed domains are observed at pH around 13 that occurs due to an increase in the electrostatic repulsion between the charged headgroups. Thus, it appears that increases in pH value and increases in subphase tem(11) Andelman, D.; Brochard, F.; Joanny, J.-F. J. Chem. Phys. 1987, 86, 3673. (12) Keller, D. J.; McConnell, H. M.; Moy, V. T. J. Phys. Chem. 1986, 90, 2311. (13) Keller, D. J.; Korb, J. P.; McConnell, H. M. J. Phys. Chem. 1987, 91, 6417. (14) Vollhardt, D.; Gehlert, U.; Siegel, S. Colloids Surf., A 1993, 76, 187. (15) Fainerman, V. B.; Vollhardt, D.; Johann, R. Langmuir 2000, 16, 7731.
10.1021/la0400095 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/24/2004
6298
Langmuir, Vol. 20, No. 15, 2004
perature have a similar effect on the adsorption isotherm and the surface morphology of condensed domains. Recently, there has been an increasing interest in the 2-D phase behavior of water-soluble surfactants at the air-water interface.16-23 Even though the phase diagram of Gibbs monolayers is less rich with respect of molecular area compared to that of Langmuir monolayers, the coexistence of both gaseous-liquid expanded (G-LE) and LE-LC (liquid-condensed) phases in the Gibbs monolayer is now well established. In these systems, surfactant molecules dissolved in water being adsorbed at the airwater interface bring about a substantial increase in surface pressure with time. Studies on surface phase behavior of water-soluble amphiphiles of medium chain length reveal that depending on the nature and surface densities of the adsorbed molecules these systems can also form a variety of condensed-phase domains in the adsorbed layers showing a striking resemblance to those of Langmuir monolayers. However, due to slow and homogeneous growth process, the condensed phase domains formed in Gibbs monolayers are found to be more organized compared to those of Langmuir monolayers.17-19 In this work, we study the adsorption kinetics and surface morphology of condensed domains of bis(ethylene glycol) mono-n-tetradecyl ether (C14E2) at different temperatures using film balance and Brewster angle microscopy (BAM). Our BAM results reveal that the domains are of spiral structures at lower temperatures. Moy et al.24 observed the formation of spiral domains in phopholipid monolayers and demonstrated that spiral domains are not internally isotropic in nature, there is a long-range orientational order of the molecules which is associated with the tilt of the hydrocarbon chain with respect to the monolayer plane and that the tilt appears to be in the direction in which the monolayers are elongated. They elegantly addressed the situation in equilibrium and attributed the observed behavior of the domains to the delicate balance between the electrostatic repulsions among the headgroups of the molecules and the interfacial line tension along the boundaries. However, these studies have not included the nature of changes in domain shape with temperature. The present study has extended our understanding about the temperaturedependent changes in surface morphology of adsorbed monolayers. Although condensed domain formation in Gibbs monolayers is now a common phenomenon, the textural feature in these systems is limited to circular shape at lower temperatures and fractal or fingering patterns at higher temperatures.21,22 In the present study, we observed some quite distinct structures in the adsorbed monolayers of C14E2, which are highly sensitive to temperature change and, to the best of our knowledge, rather different from the results of previous observations. The spiral domains gradually become wide and short as temperature increases and finally attain quadrant and circular shapes at 18 and 19 °C, respectively. All these characteristic features are observed at the onset of the phase transition marked by a cusp in the adsorption isotherms. The goal of this paper is to present a complete (16) Melzer, V.; Vollhardt. D. Phys. Rev. Lett. 1996, 76, 3770. (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, 168, 231. (20) Hossain, M. M.; Yoshida, M.; Kato, T. Langmuir 2000, 16, 3345. (21) Hossain, M. M.; Suzuki, T.; Kato, T. Langmuir 2000, 16, 10175. (22) Islam, M. N.; Kato, T. J. Phys. Chem. 2003, 107, 965. (23) Islam, M. N.; Ren, Y.; Kato, T. Langmuir 2002, 18, 9422. (24) Moy, V. T.; Keller, D. J.; Gaub, H. E.; McConnell, H. M. J. Phys. Chem. 1986, 90, 3198.
Islam and Kato
Figure 1. π-t adsorption kinetics of a 2.0 × 10-5M aqueous solution of C14E2 at different temperatures: (I) 5 °C, (II) 10 °C, (III) 15 °C, and (IV) 18 °C. The arrows in the isotherms indicate the position of the appearance of phase transition.
account of the morphological features of the monolayers observed at different temperatures. In addition to the microscopic observation, a full analysis is made on the dependence of appearance of the phase transition on thermodynamic behavior of the adsorbed molecules. Experimental Section The amphiphile C14E2 (C14H29-O-CH2-CH2-O-CH2CH2-OH) was supplied by Nikko Chemicals Co., Ltd., Tokyo, Japan, with a purity of >99%, and was used without further purification. The experimental setup for the adsorption kinetics and surface morphology study was equipped with a home-built Langmuir trough of 2 mm depth above which a BAM is mounted. Surface pressure was measured by the Wilhelmy method using a small rectangular glass plate. Both sides of the plate were uniformly roughened to ensure its homogeneous wetting. The plate was soaked in 1% HF solution for about 1 min and was washed several times using ultrapure water to remove any impurities prior to each measurement. The BAM is composed of a 20 mW He-Ne laser, a Glan-Thompson polarizer, an analyzer, a zooming microscope of long working distance, a CCD camera of high sensitivity connected to a TV monitor, and a video recording system. Images recorded were treated with image processing software to maximize the contrast and to correct the distortion of the images caused by the oblique glancing of the microscope. Details of the instrumentation were reported elsewhere.25 The experiments were carried out pouring a definite amount of aqueous solution into the trough. The temperature of the surface was controlled precisely using a large number of integrated Peltier elements attached to the back of the base plate of the trough and was detected by a platinum wire temperature sensor of very small heat capacity, sealed in a piece of nickelplated copper. To attain equilibrium with the desired experimental temperature, the solution was allowed to stand for about 25 min before the start of the experiment. The molecules already adsorbed at the surface during this span of time were removed by sweeping the surface by the movable Teflon barriers at a constant sweep rate controlled by a program. Under these conditions, the surface concentration of the adsorbed molecules can be considered to be zero, i.e., π ) 0 at t ) 0. The increase in surface pressure was then followed with time, and simultaneously the change in surface morphology was observed by BAM.
Results and Discussion Adsorption Kinetics. Figure 1 shows the π-t adsorption kinetics of a 2.0 × 10-5 M aqueous solution of C14E2 at different temperatures. With time, the spontaneous (25) Kato, T.; Tatehana, A.; Suzuki, N.; Iimura, K.; Araki, T.; Iriyama, T. Jpn. J. Appl. Phys. 1995, 34, L911.
Monolayers of Insoluble Amphiphiles
adsorption of the amphiphile molecules from the bulk of the aqueous solution at the air-water interface results in a gradual increase in surface pressure. The initial increase in surface pressure before the appearance of the cusp point in the isotherm is related to the homogeneous distribution of the molecules in the fluidlike LE phase of the monolayer. When the surface pressure attains a definite value, the π-t curve shows a conspicuous cusp point in the adsorption isotherm. After that, the surface pressure changes only weakly with time showing a plateau region. Such a feature of the adsorption isotherm is indicative of a first-order phase transition from a lower density LE phase to a higher density LC phase.16-23 The appearance of the cusp point moves to early time as the rate of adsorption increases with increasing the subphase temperature. After the plateau region, the surface pressure again increases with increasing the adsorption time leading to global equilibrium. The curves at lower temperatures have not attained the equilibrium values within the specified time of the experiment. The adsorption process takes a prolonged period of time to attain the equilibrium value at lower temperatures. Surface Morphology of the Condensed Phases. Maldarelli et al.26 have measured π-A and γ-t adsorption isotherms of a series of poly(ethylene glycol) alkyl ethers including C14E2 and observed the surface morphology using fluorescence microscopy at 22.5 °C. Their study reveals that C14E2 cannot form condensed phase domains in the monolayer, which is consistent with both π-A and γ-t isotherms of the amphiphile having no cusp point. We observed that the C14E2 monolayers form condensed domains at lower temperatures showing an exciting variety of structures. Figure 2 shows the BAM images of C14E2 monolayers observed after the appearance of the cusp point in the π-t adsorption isotherms at different temperatures. The bright 2-D condensed-phase domains surrounded by a less dense continuous phase are found to be spiral at lower temperatures (images A-C). Once the initial seed domains have formed, the width of the spiral domains does not increase appreciably, rather they become longer and more spiral with increasing the adsorption time. Moy et al.24,27 reported that monolayers of phospholipid having a chiral center show spiral condensed-phase domains using fluorescence microscopy. Their study reveals that long-range orientational order of the molecules is responsible for the spiral structure that occurs due to anisotropy in line tension between two sides of a domain giving a unidirectional molecular tilt. In the present work it is shown that the domains formed in the case of C14E2 up to 15 °C are quite similar to those observed for phospholipid monolayers although no sign of optical anisotropy is observed in the domains. Thus it appears that apart from unidirectional molecular tilt responsible for optical anisotropy, condensed domains can also attain spiral structures being favored by the electrostatic repulsions between the headgroups of the molecules. It has been found that monolayers of dimyristoylphosphatidic acid do not normally form elongated domains, but if the pH of the subphase is raised to about 11, elongated domains form, which are presumably due to the ionization of the headgroup of the amphiphile.28 In a previous paper,22 we have reported that the condensed domains of ethylene glycol mono n-tetradecyl ether, which (26) Pollard, M. L.; Rennan, P.; Steiner, C.; Maldarelli, C. Langmuir 1998, 14, 7222. (27) Moy, V. T.; Keller, D. J.; McConnell, H. M. J. Phys. Chem. 1988, 92, 5233. (28) Heckl, W. M.; Lo¨sche, M. Cadenhead, D. A.; Mo¨hwald, H. Eur. Biophys. J. 1986, 14, 11.
Langmuir, Vol. 20, No. 15, 2004 6299
Figure 2. BAM images of the adsorbed monolayers of C14E2 observed after the appearance of phase transition at different temperatures: (A) 5 °C, (B) 10 °C, (C) 15 °C, and (D-F) 18 °C. The bar in image A indicates 100 µm.
bears only one ethylene oxide (EO) unit in its headgroup, are circular at lower temperatures while they are of a fingering pattern at higher temperatures. Line tension of the interface is believed to be the governing factor for the domain shapes over the studied temperature range. The introduction of one more EO unit in the C14E2 head has appreciably increased its dipole moment. As a result, molecules adsorbed at the water surface experience a greater extent of repulsive interactions between the headgroups showing striplike structures which become longer and spiral with increasing the adsorption time. This result is in line with the previous reports,11,12 which conclude that when the area of the domain is large, the electrostatic repulsions favor the formation of long elongated structures to minimize the repulsive interactions between the dipoles. At 18 °C, the domains attain quadrant structure with two straight LE/LC boundaries (Figure 2, images D-F). The uniform brightness all over the domains suggests that all the molecules in the domains are perpendicularly oriented with respect to the surface normal. Fischer et al.29 observed the formation of some less stable two-wall triangular domains, which undergo square or rectangular structures on slow cooling. Similar structure formation has been observed in the case of L- and racemic Nstearoylserine methyl ester monolayers.30 All these struc(29) Fischer, B.; Tsao, M.-W.; Ruiz-Garcia, J.; Fischer, T. M.; Schwartz, D. K.; Knobler, C. M. J. Phys. Chem. 1994, 98, 7430.
6300
Langmuir, Vol. 20, No. 15, 2004
Figure 3. Changes in domain shape in the temperature range 15-19 °C: (A) at 16 °C; (B) at 17 °C; (C-E) the course of shape transition with change in temperature from 18 to 19 °C; (F) fully circular domains at 19 °C return to quadrant structure at 18 °C. The bar in image A indicates 100 µm.
tures formed in the above cases are transient ones in nature. Our BAM results reveal that the quadrant-shaped domains are quite stable regardless of domain size and time length of the experiment. Images D-F of Figure 2 show the condensed domains formed at 18 °C. It is clearly seen that the domains preserve their shape until the surface becomes fully covered by the densely packed condensed domains. With increase of the adsorption time, the domains grow in size by the sacrifice of the fluidlike LE phase. At the latter stage, the domains start to touch each other. Nevertheless, the domains do not fuse, rather they get deformed and arrange themselves in a well-defined way to minimize the spaces between them (image F). Only a little change in the temperature of the surface brings about a dramatic change in the surface morphology of condensed domains in the temperature range between 15 and 19 °C. Starting with an experiment at 15 °C, we have increased the temperature slowly (about 0.2 °C/min) to observe the change in surface morphology. Figure 3 shows the changes in domain shape with changes in the temperature. The pattern shown in Figure 2 at 15 °C (image C) becomes more wide and less spiral at 16 °C (Figure 3, image A). With further increase in temperature by 1 °C, the domains initially become long striplike structures (image B), which may attain somewhat bent (30) Stine K. J.; Uang, J. Y.-J.; Dingman, S. D. Langmuir 1993, 9, 2112.
Islam and Kato
structures if the domains are too long (image not shown). Thus, it appears that the domains become less prone to twists and bends at higher temperatures. Condensed phase domains with straight LE/LC or LC/LC boundaries within a domain of different tilt orientation have been reported in the literature.31,32 In the present study, we observed both straight and curved LE/LC boundaries in the quadrant shape domains at 18 °C. The appearance of this type of structure is not clearly understood. However, the BAM observation suggests that the quadrant structures mark a boundary line between the predominance of the dipolar repulsion and the line tension. Below 18 °C, the formation of spiral or striplike structures suggests the predominance of repulsive interactions between the dipoles carried by the molecules. When the temperature of the subphase increased by 1 °C, at a rate of about 0.2 °C/min, the domains undergo a quadrant-to-circular shape transition. Images C-E of Figure 3 show the course of changes in domain shape with increasing temperature. If the temperature is lowered gradually at the same rate, the domains return to quadrant structures again (image F). This suggests that the shape transition with changes in temperature is reversible in nature, and the domains observed at 18 °C are equilibrium structures not transient ones. However, a rapid heating/cooling process turns the domains into small fragments. Previous experimental results of both Langmuir33 and Gibbs22,23 monolayers reveal that at lower temperatures domains have compact shape while at higher temperatures due to a decrease in line tension they are found to be fractal or fingering pattern. In the present study, we observed the formation of circular domains at higher temperatures suggesting that line tension becomes more favorable for domain formation as the temperature increases. To explain the observed behavior we need to take into account the effect of temperature on dipole-dipole repulsions and hydrophobicity of the surfactant simultaneously. Details of the observed behavior have been described in a previous report.34 Briefly, dehydration around the EO unit increases the hydrophobicity35,36 and imparts to the molecules a longer chainlike character. Thus, being enhanced by the dehydration, the line tension of the interface increases with increasing temperature since the cohesive tendency between the molecules thereby line tension increases with increasing the length of the hydrophobic alkyl chain. On the other hand, the dipolar repulsion between the molecules decreases with increasing temperature since random thermal motion of the molecules opposes the tendency of their dipoles to line up in an electric field. Thus, the line tension approaches the dipolar repulsion as the temperature increases and, finally, dominates over the dipolar repulsions when the temperature is 19 °C. As a result, domains gradually become wide and short with increasing temperature and finally attain circular shape at 19 °C. When the temperature is increased beyond 19 °C, the domains completely disappear. We have carried out experiments with a solution of concentration around 2.75 × 10-5 M (data not shown) while the critical micelle concentration (cmc) value of the amphiphile is around 0.35 × 10-5 M at this temperature,35 but no condensed (31) Johann, R.; Vollhardt, D.; Mo¨hwald, H. Colloid Polym. Sci. 2000, 278, 104. (32) Weidemann, G..; Brezesinski, D.; Vollhardt, D.; DeWolf, C.; Mo¨hwald, H. Langmuir 1999, 15, 2901. (33) Suresh, K. A.; Nittmann, J. A.; Rondelez, F. Europhys. Lett. 1988, 6, 437. (34) Islam, M. N.; Kato, T. Submitted for publication in J. Chem. Phys. (35) Islam, M. N.; Kato, T. Langmuir 2003, 19, 7201. (36) Wongwailikhit, K.; Ohta, A.; Seno, K.; Nomura, A.; Shinozuka, T.; Takiue, T.; Aratono, M. J. Phys. Chem. 2001, 105, 11462.
Monolayers of Insoluble Amphiphiles
Langmuir, Vol. 20, No. 15, 2004 6301
condensed domain formation is possible only when the line tension of the interface dominates over the kinetic energy of the adsorbed molecules. With increasing temperature, the cohesive energy between the molecules decreases while the kinetic energy of the molecules increases. The kinetic energy starts to exert its influence as the temperature increases and finally dominates over the cohesive energy when the temperature exceeds 19 °C. Furthermore, a higher degree of kinetic energy imparts a poorer packing of the molecules in the adsorbed monolayer and weakens the favorable van der Waals interactions between the alkyl chains. As a result, the molecules cannot conveniently accumulate in the form of condensed domains when the temperature is above 19 °C. Conclusions
Figure 4. Dependence of critical surface pressure (πc) necessary for the phase transition (I), and equilibrium surface pressure attainable at gcmc (π(cmc)) (II) on temperature.
domains are observed. This indicates that the critical temperature for the formation of condensed domain is 19 °C. The question of particular interest is why the condensed domains are not observed even though the bulk concentration is sufficiently high. To find out the reason behind this, we need to take into account the effect of temperature on the critical surface pressure necessary for the phase transition, and the equilibrium surface pressure attainable at gcmc simultaneously. These two factors approach each other in the course of temperature change depending on a number of factors22 and become equal at a definite temperature, which is the maximum temperature for the formation of condensed-phase domains. This can be clarified as follows: Figure 4 shows that the critical surface pressure necessary for the phase transition increases with increasing temperature, while the equilibrium surface pressure attainable at gcmc decreases slowly with increasing temperature. A decrease in the equilibrium surface pressure (obtained by subtracting the surface tension of the solution of concentration gcmc from the standard surface tension of pure water) is indicative of a decrease in the surface concentration of the adsorbed molecules at the interface, which in turn increases randomness in the adsorbed molecules. Again, Brewster angle microscopic study reveals that condensed domains in the adsorbed monolayers exist up to 19 °C. Above this temperature, for example at 20 °C, the monolayer cannot show any indicative feature of phase transition even with a solution of concentration 2.75 × 10-5 M, which is much higher than the cmc value of the amphiphile at this temperature. The critical surface pressure data plotted in Figure 4 (curve I) suggest that at 20 °C the monolayer should attain a surface pressure of 48 mN/m for the occurrence of the phase transition. On the other hand, the equilibrium surface pressure attainable at gcmc is around 46.4 mN/m (curve II) at this temperature, which is less than the hypothetical surface pressure necessary for the phase transition by 1.6 mN/m. This result suggests that a phase transition is not possible in the C14E2 monolayer above 19 °C regardless of the bulk concentration. Again,
The adsorbed monolayers of C14E2 undergo a firstorder phase transition in the two-phase coexistence region showing a cusp point in the π-t isotherms. The critical surface pressure necessary for the phase transition increases while the equilibrium surface pressure attainable at gcmc decreases with increasing temperature. The two curves coincide at 19 °C, which is the maximum temperature for the appearance of the phase transition. In this study we observed a variety of condensed-phase domains in the monolayers over the studied temperature range, which are quite different from the previous observations. At lower temperatures, the domains are found to be spiral, and become more and more spiral as the domains become longer with increasing the adsorption time. This characteristic feature of the domain is because of the dipole-dipole repulsive interaction between the molecules. At 18 °C, the domains are found to be quadrant structure with two straight boundaries. When the subphase temperature is increased to 19 °C, the domains undergo a quadrant-to-circular shape transition. This suggests that the line tension is more favorable for domain formation as the temperature increases. An increase in temperature results in dehydration around the EO chain which increases the hydrophobicity and imparts the molecules a longer chainlike character. As a result, the line tension gradually increases and finally dominates over the repulsive interactions between the molecules showing circular domains at higher temperatures. On lowering of the temperature to 18 °C, the domains get back their quadrant structure which is suggestive of reversible nature of shape transition in the heating/cooling process. The maximum temperature for the appearance of the phase transition is 19 °C. Above this temperature, the actual surface pressure attainable by the adsorbed molecules cannot reach the hypothetical surface pressure necessary for the phase transition. An increase in temperature results in a higher degree of molecular motion giving rise to a poorer packing in the adsorbed monolayer. As a result, the system cannot acquire the surface pressure necessary for the phase transition when the temperature is above 19 °C. Acknowledgment. The authors appreciate much the financial support from the Satellite Venture Business Laboratory of Utsunomiya University. LA0400095
