Langmuir 1994,10, 1005-1007
1006
Surface Pressure-Induced Layer Growth of a Monolayer at the Air-Water Interface J. Y.Fang and R.A. Uphaus’ Ames Laboratory, Iowa State University, Ames, Iowa 50011 Received September 2,1992. In Final Form: September 14,199P Spread monolayers containing a nematic liquid crystal and stearic acid were characterized at various mole fractionsby determination of surface pressurearea isotherms at the air-water interface. The surfacecomposition phase diagrams indicate that compression induces a new phase transition in the films, which changes from a mixed monolayer to a supermonomolecularsystem. X-ray diffraction and opticalabsorption spectra demonstrate that the supermolecular array consists of an island liquid crystal monolayer and a uniform stearic acid monolayer.
Introduction The phase transition is a fundamental physical phenomenon. Monolayers spread a t the air-water interface can provide an ideal model for phase transition effects and therefore have attracted considerable attention. Fatty acid molecules and lipid molecules often can show various monolayer phases a t the air-water interface, such as the gas, liquid-expanded, liquid-condensed, and solid phases. These monolayers have been used as model of twodimensional phase transitions and have been widely investigated by both experiment and theory.14 It is well known that some liquid crystal molecules which are insoluble in water can form stable Langmuir films a t the air-water interface. The intermolecular action and the interface effects in liquid crystal systems are different from those seen in fatty acid and lipid systems, and liquid crystal molecules can show differential phase behavior at the airwater interface. Recently, a new phase transition from two-dimensional monolayers to three-dimensional multilayers has been reported for some liquid crystal systems. For example, a ferroelectronic liquid crystal (HOBACPC) can be realized as a quantized layer growth from a monolayer to a bilayer, a bilayer to a trilayer, etc.,7 a smectic liquid crystal (8CB) monolayer can undergo a first-order transition to a trilayer? and a discotic liquid crystal shows a phase transition from a monolayer to a bilayereg The layer growth phase transition leads to a new surface phase at the air-water interface. .It provides a unique model system for study of phase transition problems. However, much less is known about data regarding the phase transition of liquid crystal-fatty acid systems at the airwater interface. The present study reports phase transition effects in a mixed system containing a nematic liquid crystal and a fatty acid. It is demonstrated that the phase transition of the mixed system occurs as a layer growth from a mixed monolayer to a supermonomolecular layer. AbstractpublishedinAdvance ACSAbstracts,October 15,1993. (1)k i n g , Th.;Shen, Y.R.; Kim, W. M.;Grubb, S. Phys. Rev. Lett. 1985,55, 2903. (2) Keller, D. J.: McConnell, H. M.;MOY,V. T. J. Chem. Phys. 1986, 90,2311. (3)Dutta, P.; Peny, J. B.; Lin, B.; Ketteraon, J. B.; hakash, M.; Georgopouloe, P.;Ehrlich, S. Phys. Rev. Lett. 1987,58, 2228. (4) Lm,B.;Shin, M. C.; Bohanon, T. M.;Ice, G. E.; Dutta, P. Phys. Rev. Lett. 1990, 65,191. (5)Schlossman, M.L.;Schwartz, D. K.; Perachan, P. S.; Kawamoto, E. H.; Kellogg, G. J.: Lee, S. Phys. Rev. Lett. 1991,66, 1599. (6) Qiu, X.; Garcia, J. R.; Stine, K. J.; Knoble, C. M.;Selinger, J. V. Phys. Rev. Lett. 1991, 67, 703. (7) Rapp, B.; Gruler, H. Phys. Rev. A. 1990,42, 2215. (8) Xue, J. 2.;Jung, C. S.; Kim, M.W. Phys. Rev. Lett. 1992,69,474. (9)Auweraer,M.V.D.; Catry, C.; Chi, L. F.; Karthaue, 0.; Knoll, W.; Ringsdorf, H.; Sawondny, M.;Urban, C. Thin Solid Films 1992,210/211, 39. @
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Figure 1. Molecular structures of the nematic liquid crystal (LC) and the stearic acid (SA).
Also discussed is the order of the layer growth phase transition, according to the Clausius-Clapeyron equation. Experimental Section Monolayer measurements were carried out in a Langmuir trough. Surface pressures were measured with a Wilhelmy plate to a precision of 0.2 mN/m. Distilled water was used as the subphase. The temperature of the subphase was controlled to an accuracy of f0.5 O C . The molecular structures of the nematic liquid crystal (LC) and the stearic acid (SA) used are shown in Figure 1. The LC and SA were dissolved in chloroform as the spreading solvent. Solutions were spread on a subphase containing 4 X 10-9M CdC12. After solvent evaporation,Langmuir f i i s were compressed at a slow speed. At constant surface pressures, the Langmuir filmswere depositedonto quartz plates by the vertical dipping LB method. Small-angle X-ray diffraction experiments were performed with a D/max-rB X-ray diffractometer. UV-vis absorption spectra were measured with a DUdB spectraphotometer. Results and Discussion Surface pressurearea isotherms of Langmuir films containing LC and SA at 25 OC are shown in Figure 2. The isotherms have abrupt slope changes a t about 30 mN/m, indicating that the Langmuir films undergo a phase transition. The two points a t 25 and 40 mN/m indicate two separate condensed phases. Figure 3 shows the mean molecular area vs composition at 25 and 40 mN/m. At low surface pressures, the mean molecular area deviates from linear, indicating that uniform mixed monolayers form at the air-water interface.10 A t high surface pressures, the mean molecular area is close to a constant of 20 A2,an unexpected result. Assuming that the SAmolecular area at the water surface in the mixed Langmuir monolayers is the same as that in the pure SA Langmuir monolayers, we can infer that the LC molecular area a t the water surface is nearly 0 A2 at the high surface (10)Gaines, G. L., Jr, Zmoluble monolayer at liquid-gas interfaces; Interscience: New York, 1966.
0743-7463/94/2410-1005$04.50/00 1994 American Chemical Society
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1006 Langmuir, Vol. 10, No. 4, 1994
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Figure 2. Surface pressure-area curves of Langmuir films containing the LC and the SA at 26 O C : LCSA = 1:2 (-); LCSA 1:4 (- - -); LC:SA 1:6 (- * -1; LC:SA = 1:lO (A).
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Figure 3. Mean molecular area versus mole fractions. Surface pressures: 26 mN/m (A);40 mN/m (0).
Figure 5. X-ray diffraction pattern of the LB f i i transferred at 40 mN/m.
pressures. An obvious explanation for this result is that LC moleculesmay be extruded from the mixed monolayers, forming a bilayer. A possible model is shown in Figure 4. At a surface pressure of 25 mN/m, the alkyl chains and carbonyl group of the LC molecules are squeezed out of the water surfaceyand only the crown ether unit is in contact with the water. A mixed monolayer forms in this case. When this monolayer is compressed continuously, LC molecules may be slowly eliminated from the system, because LC molecules themselves can form monolayers at the air-water interface, and have strong hydrophobic interaction." The displaced LC molecules may form a monolayer below the condensed SA monolayer. Therefore, the mean apparent molecular area as measured is actually only the area of the SA molecule (20 A2). The model is in agreement with the experimentalreaulta. It is concluded that the phase transition in the Langmuir film is a layer growth from a mixed monolayer to a supermonomolecular system. In order to characterize further the structure of the two phases, the Langmuir films were transferred onto quartz plates with a mole fraction ratio of 1:6 at 25 and 40 mN/m. Under these conductions, the transfer ratios of both the
downward and upward processes were in the range from 0.8 to 1.1, indicating that the transferred LangmuhBlodgett (LB) films are the Y type. Figure 5 shows the X-ray diffraction pattern of an LB f i i transferred at 40 mN/m. Several diffraction peaks are visible. According to the Bragg equation, the long spacing D = 75 A. The length of the SA bilayer is about 50 A, while the length of the LC bilayer is 24 A (ref 11). The long spacing of 75 A is interpreted as a LC bilayer sandwiched between two SA monolayers. Because the LB films is the Y type, we can infer that the Langmuir f i i consista of a supermonomolecular layer containing a SA monolayer and a LC monolayer. In addition, some secondary maxima can be observed in Figure 5. The phenomenon is analogous to the diffraction of light waves from a grating with only a few slits. The secondary maxima have been reported for other alternating LB fiilms.l2 For the transferred LB film at 25 mN/m, X-ray diffraction shows a spacing D' of 48 A. The value correspondsto the length of the SA bilayer. These results demonstrate that the phase transition in the Langmuir films studied is layer growth. Figure 6 shows the UV-vis absorption spectra of the LB films. For the LB film transferred at 25 mN/m, the
(11) Fang,J.Y.;Lu,Z.H.; Pan,S.P.;Chen, 2.L.;Wei, Y.;Qin, J.; Xie, M.G. Solid State Commun. 1992,83, 1023.
(12)Fang,J. Y.;Xiao, S.J.; Lu, Z.H.;Wei, Y.;Sun,2.M.;Stroeve, P.Solid State Commun. 1991, 79,985.
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Langmuir, Vol. 10,No. 4, 1994 1007
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Figure 6. UV-vis absorption spectra of fiis transferred at 25 mN/m (A) and 40 N/m (B).
absorption peak is at 270 nm, a value close to the absorption peak of the LC in chloroform. This indicates that the LC molecules are in no aggregated state. For the LB film transferred at 40 mN/m, the absorption peak is at 285 nm. The red-shifted absorptionindicatesthat the LC molecules squeezed from the mixed monolayers are in an aggregated state. An island LC monolayer forms below the SA monolayer. Recently, the Clausius-Clapeyron equation has been used to discuss the order of the layer growth phase transition in Langmuir films.' In our experiments, the equilibrium pressure of the two phases as a function of temperature is shown in Figure 7. As can be seen, the equilibrium pressure decreases with increasing temperature. According to the Clausius-Clapeyron equation, SZ - SI = dp/dT (A2 - AI) [where A1 and AZare considered to be the areas of Langmuir films in the monolayer phase and the supermonomolecular layer phase, and SIand SZ are entropies of Langmuir films in the two phases], we can infer that the change of entropy during the layer growth is positive because AZ - A1 is always negative. This indicates that the phase transition of the layer growth is fiist order.
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Figure 7. Equilibrium pressure as a function of temperature.
Conclusions The surface pressure-induced pham transition of monolayers containing a nematic liquid crystal and stearic acid at the air-water interface has been investigated. It is concluded that the phase transition is a layer growth from a mixed monolayer to a supermonomolecular layer containing an island liquid crystal monolayer and a uniform stearic acid monolayer on the basis of surface pressurearea ieotherms and X-ray diffraction data. Our results also indicate the possibility to fabricate organic superlattices by transferring such supermonomolecularsystems onto solid supports for device production.
Acknowledgment. This work was funded in part by Ames Laboratory which is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7406-ENG-82, supported by the Office of Basic Energy Sciences'
