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Anomalous Phase Behavior in Langmuir Monolayers of Monomyristoyl-rac-Glycerol at the Air-Water Interface Md. Nazrul Islam and Teiji Kato* Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan Received June 13, 2005. In Final Form: October 4, 2005 The effect of temperature on the surface phase behavior in Langmuir monolayers of monomyristoylrac-glycerol (MMG) at the air-water interface has been studied by film balance and Brewster angle microscopy (BAM). It is observed that the domains of the MMG monolayers formed in the coexistence region between the liquid expanded (LE) and liquid condensed (LC) phases retain their circular shape over the studied temperature range, showing a sharp contrast to the temperature-dependent monolayer morphologies of amphiphilic systems where the shape of condensed domains changes either from compact circular to fingering or from irregular or spiral to compact patterns with increasing temperature. It is concluded that the system is capable of tuning the line tension of the interface by the effect of the increase in the hydrophobic character because of dehydration of the headgroup, which imparts to the molecules the properties of similar molecules but with less hydrophilic headgroups. As a result, the domains can retain their circular shape even up to the maximum possible temperature of the phase transition.
Introduction Over the past several years, there has been a growing interest in two-dimensional (2-D) phase behavior of amphiphilic systems at the air-water interface. This interest stems in part from their pressure-induced 2-D phase transition leading to the formation of a wide variety of structures at the air-water interface.1 From the applied aspects, Langmuir monolayers are used to study chemical and biochemical reactions in two dimensions2,3 and to fabricate Langmuir-Blodgett films. Although an enormous complexity arises in studying the phase behavior of two-dimensionally organized molecules at the water surface, Langmuir monolayers have been the fundamental method for characterizing their physical properties in a variety of phases.4-16 The identification of these phases and the boundaries between them has been characterized primarily from the measurement of surface pressure (π) as a function of molecular area (A). At low surface density, the molecules remain in a 2-D gaseous (G) state. An increase in surface density by lateral compression leads to the transformation of the G phase into an isotropic 2-D liquid expanded (LE) phase. On further compression, the * To whom all correspondence should be addressed. E-mail:
[email protected]. Phone: +81-28-689-6179. Fax: +8128-689-6170. (1) Mo¨hwald, H. Annu. Rev. Phys. Chem. 1990, 41, 441. (2) Magrioti, V.; Verger, R.; Violetta, C.-K. J. Med. Chem. 2004, 47, 288. (3) Debreczeny, M. P.; Svec, W. A; Wasielewski, M. R. Science 1996, 274, 584. (4) Suresh, K. A.; Nittmann, J.; Rondelez, F. Europhys. Lett. 1988, 6, 437. (5) Islam, M. N.; Kato, T. Langmuir 2005, 21, 2419. (6) Islam, M. N.; Kato, T. J. Phys. Chem. B 2003, 107, 965. (7) Johann, R.; Vollhardt, D.; Mo¨hwald, H. Colloid Polym. Sci. 2000, 278, 104. (8) Islam, M. N.; Kato, T. Langmuir 2004, 20, 10872. (9) Muller, P.; Gallet, F. J. Phys. Chem. 1991, 95, 3257. (10) Muller, P.; Gallet, F.Phys. Rev. Lett. 1991, 67, 1106. (11) Islam, M. N.; Kato, T. Langmuir 2004, 20, 6297. (12) Moy, V. T.; Keller, D. J.; Gaub, H. E.; McConnell, H. M. J. Phys. Chem. 1986, 90, 3198. (13) Gaub, H. E.; Moy, V. T.; McConnell, H. M. J. Phys. Chem. 1986, 90, 1721. (14) Weis, R. M.; McConnell, H. M. J. Phys. Chem. 1985, 89, 4453. (15) Heckl, W. M.; Lo¨sche, M.; Codenhead, D. A. Mo¨hwald, H. Eur. Biphys. J. 1986, 14, 11. (16) Islam, M. N.; Kato, T. J. Chem. Phys. 2004, 121, 10217.
monolayer undergoes a transition between the LE and the liquid condensed (LC) phases, showing a cusp followed by a plateau region in the π-A isotherm. By the use of microscopic techniques, such as fluorescence microscopy (FM)17,18 and Brewster angle microscopy (BAM),19,20 one can readily visualize an exciting variety of structures such as circular,4-6 faceted,7,8 needlelike,9,10 quadrant,11 and spiral structures in the monolayers of both chiral12-15 and in nonchiral11,16 amphiphiles at the air-water interface, which are not common in three-dimensional systems. Generally, it is observed that condensed-phase domains are circular at lower temperatures, which undergo a fractal or fingering pattern at higher temperatures because of a decrease in line tension of the LE-LC interface.4-7 Quite recently, we have reported that condensed-phase domains can also attain an increasingly compact shape with increasing temperature,5,8,11,16 which is quite opposite to the usual temperature-dependent phase behavior of amphiphilic systems at the air-water interface. The experimental findings have been explained by considering the fact that the dehydration of the headgroup increases the hydrophobicity and imparts to the molecules the properties of similar molecules with less hydrophilic headgroups. As a result, the line tension of the interface increases showing more and more compact domains with increasing temperature. Therefore, one might expect that condensed-phase domains should retain their shape if the system can adopt the state from the effects of the increase in the hydrophobic character and the decrease in the dipolar repulsive interaction between the molecules caused by the dehydration of the headgroup that minimizes the free energy of the system. With a view to this phenomenon, in the present work, we have studied the phase behavior of monomyristoyl glycerol (MMG) monolayers over the temperature range between 5 and 32 °C and observed that the condensed-phase domains can preserve their circular shape over the whole temperature range which, to the best of our knowledge, is a new phenomenon in monolayer morphologies of amphiphilic systems at the (17) Lo¨sche, M.; Mo¨hwald, H.; Rev. Sci. Instrum. 1984, 55, 1968. (18) Von Tscharner, V., McConnell, H. M. Biophys. J. 1981, 36, 409. (19) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. (20) Ho¨nig, D.; Mo¨bius, D. J. Chem. Phys. 1991, 95, 4590.
10.1021/la051563x CCC: $30.25 © 2005 American Chemical Society Published on Web 10/25/2005
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Figure 1. π-A isotherms of MMG monolayers at different temperatures.
air-water interface. Furthermore, thermodynamic behavior of condensed-domain formation has also been taken into account considering the dependence of critical surface pressure necessary for the phase transition and the maximum surface pressure attainable before the collapse of the monolayers on temperature. Experimental Section Materials. The amphiphile MMG (CH3(CH2)12CO-O-CH2CHOH-CH2OH) was purchased from Sigma and was used without further purification. The solution of the amphiphile was prepared in hexane. The subphase was ultrapure water of resistivity 18 MΩ‚cm (Elgastat UHQ-PS). Experimental Procedure. A home-built Langmuir trough of 2 mm depth, equipped with a Brewster angle microscope, was used to measure the π-A isotherms and in situ surface morphology of the monolayers. The surface pressure was measured by the Wilhelmy method using a small rectangular glass plate. The temperature of the surface was controlled using a large amount of integrated Peltier elements attached to the base plate of the trough and was detected by a platinum-wire temperature sensor. The compression was started 20 min after spreading the amphiphile solution to allow complete evaporation of the spreading solvent and to attain equilibrium with the desired experimental temperature. Details of the BAM setup21 and the experimental procedure22 have been reported elsewhere.
Results and Discussion π-A Isotherms. Figure 1 shows the pressure-area (πA) isotherms of MMG monolayers at different temperatures. It is clear in the figure that the isotherms show a well-defined cusp point followed by a plateau region after which the surface pressure increases steeply with a small decrease in the molecular area. Such a feature is indicative of a first-order phase transition between a lower density LE phase and a higher density LC phase at the air-water interface. The initial increase in surface pressure before the appearance of the cusp point is indicative of homogeneous distribution of the molecules at the water surface. The critical surface pressure necessary for the phase transition increases with increasing temperature as the molecules need to be braced against a higher degree of thermal motion and chain flexibility with increasing temperature. Surface Morphology. Figure 2 shows the in situ surface morphology of the condensed-phase domains observed just after the appearance of the cusp points in the π-A isotherms over the temperature range between 5 and 30 °C. It is clear in the figure that, at all temperatures, the domains are circular with internal texture. This type of texture occurs due to regions of different molecular orientation within a domain. Upon further compression of the monolayers, the condensed domains form by the sacrifice of the LE phase. Under such cir-
Figure 2. BAM images of MMG monolayers at different temperatures: (A) 5 °C, (B) 10 °C, (C) 15 °C, (D) 20 °C, (E) 25 °C, and (F) 30 °C.
cumstances, the condensed domains come close to each other and start to deform toward the accessible LE phase.11 Usually, it is observed that condensed domains are circular at lower temperatures, which attain fractal or fingering pattern because of the decrease in the line tension of the LE-LC interface and the viscosity of the condensed phase with increasing temperature.4,24 On the other hand, if the dehydration of the headgroup imparts to the molecule an increase in hydrophobic character,24,25 then the domains can attain more and more compact shapes with increasing temperature.8,10,15 This occurs when the headgroup is relatively large as observed in the case of surfactants containing two or more ethylene oxide (EO) units8,10,15 or a glycerol26 unit in the headgroup. This conclusion is further supported by the neutron reflectivity experiments on water-soluble amphiphiles.27 The experimental data reveal that the alkyl chains show a closer molecular packing as the temperature increases which has been ascribed to an increase in the dehydration of the headgroup (21) Kato, T.; Tatehana, A.; Suzuki, N.; Iimura, K.; Araki, T.; Iriyama, K. Jpn. J. Appl. Phys. 1995, 34, L911. (22) Iimura, K.; Yamauchi, Y.; Tsuchiya, Y.; Kato, T.; Suzuki, M. Langmuir 2001, 17, 4602. (23) Vollhardt, D.; Gehlert, U.; Siegel, S. Colloids Surf. A 1993, 76, 187. (24) Islam, M. N.; Kato, T. Langmuir 2003, 19, 7201. (25) Wongwailikhit, K.; Ohta, A.; Seno, K.; Nomura, A.; Shinozuka, T.; Takiue, T.; Aratono, M. J. Phys. Chem. B 2001, 105, 11462. (26) Islam, M. N.; Kato, T. J. Colloid Interface Sci., in press. (27) Lu, J. R.; Li, Z. X.; Thomas, R. K.; Staples, E. J.; Thompson, L.; Tucker, I.; Penfold, J. J. Phys. Chem. 1994, 98, 6559.
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with increasing temperature. However, the present study reveals that the domains preserve their circular shape over the studied temperature range even though the amphiphile contains a glycerol unit along with a carbonyl group in the headgroup that seems to be contradictory to the observations of the previous studies. This observation suggests that the line tension of the interface predominates over the dipolar repulsion between the headgroups even at higher temperatures. Now the question arises as to how the line tension, which usually decreases with increasing temperature, preserves its predominance to retain compact circular domains over the whole temperature range. To understand this unusual phase behavior, the following thermally controlled factors should simultaneously be taken note of: first, the dehydration of the headgroup and its effect on the hydrophobicity of the molecules and the line tension of the LE-LC interface, and second, the change in the dielectric constant of the subphase water and its effect on the repulsive interaction between the headgroups of the molecules. Since, the dielectric constant of water is higher at lower temperatures, the dipolar repulsion between the headgroups lying beneath the water surface will not be pronounced enough to dominate over the line tension of the interface. As a result, line tension being a dominating factor allows the molecules to form compact circular domains at lower temperatures. As the temperature increases, the headgroups will experience a stronger repulsion because of the decrease in the dielectric constant of the subphase water with increasing temperature. At the same time, due to the dehydration-induced shrinkage of the headgroup size and random thermal motion of the molecules, the repulsive interactions between the molecules will tend to decrease with increasing temperature. Probably, the amount of the increase in the dipolar repulsion between the headgroups caused by the decrease in the dielectric constant of the subphase water is offset by the combined decrease in the repulsive interactions caused by the shrinkage of the headgroup size and the random thermal motion of the molecules. Additionally, being favored by the dehydration of the headgroup, the hydrophobicity of the molecules increases, resulting in an increase in the favorable van der Waals interaction between the molecules. This provides the molecules the capacity to withstand the increase in thermal motion and chain flexibility, and compensate for the decrease in the line tension of the interface, showing circular domains at higher temperatures. As shown in Figure 3, the critical surface pressure necessary for the phase transition increases linearly with increasing temperature (curve I). On the other hand, the maximum attainable pressure before the collapse of the monolayer decreases with increasing temperature (curve II). The two curves meet at a point, which is the maximum temperature for the appearance of phase transition. For MMG monolayers, this temperature is found to be 32 °C. Our BAM results also reveal that the condensed-phase domains surrounded by the LE phase exist up to 32 °C above which two-phase coexistence state does not occur irrespective of the amount of spreading solvent and molecular area. Above this
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Figure 3. Effect of temperature on the critical surface pressure necessary for the phase transition (I) and maximum attainable pressure before the collapse of the monolayers (II) at different temperatures.
temperature, for example at 33 °C, the system should attain a surface pressure of 43 mN/m for the appearance of phase transition (shown by the arrows). However, at this temperature, the system can attain a surface pressure of around 40 mN/m before the collapse of the monolayer (data are not shown, however, the collapse occurs when maximum surface pressure is around 40 mN/m at the molecular area of 0.195 nm2), which lags behind the hypothetical surface pressure necessary for the phase transition by 3 mN/m. This result suggests that the critical temperature for the phase transition is 32°C. Above this temperature line tension vanishes as the molecular cohesion becomes too weak to allow the molecules to form high-density states at the air-water interface. As a result, phase transition and thereby condensed domain formation is not possible when the subphase temperature is g33°C. Conclusions In the present work, we have studied the surface phase behavior of monomyristoyl glycerol (MMG) monolayers from π-A measurements and in situ BAM observations over a wide range of temperatures. Bright two-dimensional condensed-phase domains with internal texture are observed just after the appearance of the phase transition marked by a cusp point followed by a pronounced plateau region in the π-A isotherms. Contrary to the usual behavior, where domain shapes change either from circular to fingering or from irregular to compact patterns with increasing temperatures, the domains of MMG monolayers do not undergo changes in shape with increasing temperature. It is concluded that the increase in the dipolar repulsion caused by the decrease in the dielectric constant of the subphase water is offset by the decrease in the hydration-induced repulsive interaction between the headgroups and the random thermal motion of the molecules with increasing temperature. Additionally, taking advantage of the increase in the dehydrationinduced hydrophobicity, the molecules can compensate the decrease in the line tension of the LE-LC interface with increasing temperature, showing circular domains over the studied temperature range. LA051563X
