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Langmuir 2002, 18, 4530-4531
Characterization of the Liquid-Expanded to Liquid-Condensed Phase Transition of Monolayers by Means of Compressibility Zhi-Wu Yu,* Jing Jin, and Yang Cao Bioorganic Phosphorous Chemistry Laboratory, Department of Chemistry, Tsinghua University, Beijing 100084, China Received December 21, 2001. In Final Form: April 4, 2002
Introduction Amphipathic molecules at the air-water interface show a variety of physical states, namely, gas, liquid, and coherent solid monolayer films when subjected to compression. Certain amphiphiles, including phospholipids and fatty acids, display two kinds of liquid states of the films, that is, the liquid-expanded state (LE) and the liquidcondensed state (LC).1 The latter is sometimes called the solid-expanded phase.2 Transformation between the two phases is believed to be related to the reorientation of molecules at the interface and has thus received much attention by many research groups.3-7 Owing to the growing interest in monolayers by chemists, physicists, and biologists,3,8-10 proper characterization of these phases and transitions between them is of great importance. In this short note, a new method is proposed to characterize the phase transition from LE to LC.
Figure 1. Surface pressure-area per molecule isotherms of DPPC at four different temperatures.
Experimental Section 1,2-Dipalmitoylphosphatidylcholine (DPPC) (>99% pure) was purchased from Avanti Polar Lipids Inc. (Birmingham, AL), and R-tocopherol was from Sigma Chemical Co. (St. Louis, MO). Ethanol (>99% pure) and chloroform (>99% pure) were supplied by Beijing Chemical Co. (Beijing, China). All of the solutions were prepared by dissolving DPPC in chloroform and were stored at 4 °C. A KSV minitrough (KSV Instruments Ltd., Finland), equipped with a platinum Wilhemy plate, was used to acquire the surface pressure-molecular area (π-A) isotherms of monolayers. The precision in the measurement of surface pressure is (0.004 mN/m. The compression rate used in the study was 20 mm/min. No significant difference in the π-A isotherms was observed when higher compression rates up to 40 mm/min were applied.
Results and Discussion The surface pressure-area per molecule (π-A) isotherms of pure DPPC monolayers at a few temperatures are shown in Figure 1. The phase transition from the * Corresponding author. Tel: (+086) 10 6277 2767. Fax: (+086) 106277 1149. E-mail:
[email protected]. (1) Birdi, K. S. Self-Assembly Monolayer Structures of Lipids and Macromolecules at Interfaces; Kluwer Academic/Plenum Publishers: New York, 1999; Chapter 2. (2) Birdi, K. S. Lipid and Biopolymer Monolayers at Liquid Interfaces; Plenum Press: New York, 1989; Chapter 2. (3) Crane, J. M.; Putz, G.; Hall, S. B. Biophys. J. 1999, 77, 31343143. (4) Hwang, M. J.; Kim, K. Langmuir 1999, 15, 3563-3569. (5) Linden, M.; Rosenholm, J. B. Langmuir 2000, 16, 7331-7336. (6) Kawaguchi, M.; Yamamoto, M.; Nakamura, T.; Masatsugu, Y.; Kato, T.; Kato, T. Langmuir 2001, 17, 4677-4680. (7) Ruckenstein, E. J. Colloid Interface Sci. 1997, 196, 313-315. (8) Kampf, J. P.; Frank, C. W.; Malmstrom, E. E.; Kawker, C. J. Science 1999, 283, 1730-1733. (9) Ignes-Mullol, J.; Schwartz, D. K. Nature 2001, 410, 348-351. (10) Takamoto, D. Y.; Aydil, E.; Zasadzinski, J. A.; Ivanova, A. T.; Schwartz, D. K.; Yang, T. L.; Cremer, P. S. Science 2001, 293, 12921295.
Figure 2. Comparison between the A-π curve and β-π curve of a DPPC monolayer at 25.3 °C (A) and the deconvolution result of the β-π curve (B).
liquid-expanded (LE) to the liquid-condensed (LC) state, typified by the abrupt decrease in molecular area during compression, can be seen on these isotherms. The predominant difference between these isotherms is the initial point and range of the phase transition. The higher the temperature, the smaller the molecular area corresponding to the beginning point of the phase transition. This is in agreement with the literature results.11-13 To better characterize and elucidate the details of the transition from LE to LC, two-dimensional compressibility of the monolayers has been investigated. The following equation is used for the calculation of the compressibility coefficient.
β)-
1 ∂A A ∂π
( )
T
The results of DPPC as a function of surface pressure at 25.3 °C, together with the original π-A curve for (11) Albrecht, O.; Gruler, H.; Sackmann, E. J. Phys. 1978, 39, 301313. (12) Marra, J. J. Colloid Interface Sci. 1985, 107, 446-458. (13) Kubo, I.; Adachi, S.; Maeda, H.; Seki, A. Thin Solid Films 2001, 393, 80-85.
10.1021/la011840+ CCC: $22.00 © 2002 American Chemical Society Published on Web 05/03/2002
Notes
Figure 3. (A) Compressibility β versus surface pressure π at 16.5 (1), 20.6 (2), 25.3 (3), and 29.4 °C (4). (B) The temperature dependence of the starting surface pressure of the LE-LC phase transition.
comparison, are illustrated in Figure 2A. It can be seen from the figure that the LE-LC phase transition can be much better presented by the β-π curve. The “peak” point c corresponds to the state where the monolayer shows maximum compressibility. The phase transition range, from point a to point b, can be determined easily with the help of the β-π curve. In addition, the asymmetry of the peak indicates that the phase transition consists of at least two steps, which can be well displayed after deconvolution as shown in Figure 2B. This implies that two kinds of molecular reorientation, possibly in the lipid headgroup region, are involved in the phase transition. Similarly, coefficients of compressibility at other temperatures were calculated, and they are depicted in Figure 3. In Figure 3A, a sharp peak is seen for each of the isotherms during the phase transition. After baseline subtraction, the peak heights, which represent the maximum changes in β during phase transition at different temperatures, are almost the same, giving a value of ∆β ) 34 m/N. The starting surface pressures of the phase transition show a good linear relationship with temperature (Figure 3B). The slope is found to be 1.38 mN/m/K.
Langmuir, Vol. 18, No. 11, 2002 4531
Figure 4. Isotherm (A) and compressibility coefficient (B) of a monolayer containing DPPC and R-tocopherol with their molar ratio of 3 to 1 (25 °C).
Using the method, the LE-LC phase transition of monolayers containing DPPC with different amounts of R-tocopherol has also been investigated. As an example, Figure 4A shows the π-A isotherm of the binary mixture with a molar ratio of 3/1 (DPPC/R-tocopherol). A comparison between parts A and B of Figure 4 reveals that although no obvious LE-LC phase transition can be seen from the normal monolayer isotherm, the β-π curve shows clearly the phase transition, giving a transition range from 18 to 34 mN/m in surface pressure. To sum up, we have proposed in this short note a new method to characterize the liquid-expanded to liquidcondensed phase transition of molecular monolayers. With the help of the method, the widely known LE-LC phase transition of a pure DPPC monolayer was found to consist of at least two steps upon compression, implying that two kinds of molecular reorientation are involved in forming closely packed films. In addition, the method can increase significantly the detectability of the transition. Acknowledgment. Financial support from the National Natural Science Foundation of China (29973019, 20133030) and the key-faculty supporting scheme of the Ministry of Education of China is gratefully acknowledged. LA011840+
