J . Phys. Chem. 1989, 93, 4587-4593
interfacial tension dml is expected to vanish as u”, =
CO(T/T’ - 1)P
(4)
with a proportionality constant uo and a universal critical point exponent 1 for which scaling theory predicts a value 1 = 1.26. As the relative accuracy of the interfacial tension data decreases much less than the magnitude of the tensions in this near-critical region, the logarithmic form of eq 4 was used in the fitting pro-
4587
cedure. For the data in the temperature range 0.3 < T i T I < 2.4 the following best fit values were found: uo =
1 f 0.5 m N m-I;
p
= 1.15 f 0.15;
T , = 301.11 K
The best-fit value of T I agrees with the experimental value (300.95 K) within the experimental accuracy. Registry No. CsE,,19327-39-0; CI0E,, 23244-49-7; heptane, 14282-5; octane, 111-65-9; decane, 124-18-5.
Relation between Phase Diagram, Crystallization, and Optical Properties of Cyanine Dye/Stearic Acid Mixed Monolayers Claus Duschl,t,*Daniela Kemper,+,§Wolfgang Frey,# Paul Meller,§ Helmut Ringsdorf,§ and Wolfgang Knoll*,t Max- Planck-Institut f u r Polymerforschung, Ackermannweg 10, 0 - 6 5 0 0 Mainz, FRG, Institut fur Organische Chemie. Johannes-Gutenberg-Uniuersitat,J . - J.-Becher- Weg 18-20, 0-6500, Mainz, FRG, and Physik Department E 22, Technische Universitat Munchen, James- Franck-Strasse. 0-8046,Garching, FRG (Received: October 5, 1988; In Final Form: December IS, 1988)
The phase behavior of cyanine dye monolayers mixed with stearic acid as cosurfactant was investigated at various mole fractions, x, by recording pressure-area isotherms at the water-air interface. The resulting pressure-composition phase diagram shows a eutectic behavior with mixed crystal formation. In the miscibility gap ranging from x = 0.3 to x = 0.95 above the eutectic pressure re= 40 mN-m-l the two coexisting crystal modifications are characterized (among other techniques) by fluorescence microscopy and, after transfer to a suitable substrate, by electron diffraction. The dye-rich (x = 0.95) crystals show all the characteristic features of the “brick-stone” arrangement proposed for the molecular packing of the dye chromophores in J aggregates. The x = 0.3 phase boundary with a distinctly different crystal habit is stabilized by the areal match between the chromophore headgroups and the densely packed hydrocarbon chains. These thermodynamic and structural data are discussed in relation to the optical properties of the J-band aggregates.
Introduction Monomolecular layers of amphipathic organic molecules at the water-air interface have regained considerable interest1 in recent years for a number of reasons: (i) First of all, they are the basic structural unit for the buildup of functionalized multilayer assemblies, so-called Langmuir-Blodgett (LB) film^.^,^ It became clear that any technical application of LB layers4 (e.g., in electronic device^,^ as photoresists,6 in adhesion or lubrication,’ or for (nonlinear) optical devices*) requires a careful control of the geometric and electronic properties of the involved molecules already on the water surface. (ii) Their close structural relation with the lipid bilayer matrix of biological membranes renders them a valuable model system for studies aimed a t elucidating structure-function or order-function relationships in biomembrane~.~ (iii) Many basiclo and applied’’ questions in the field of colloidal physics and chemistry can be experimentally treated with surfactant monolayers. (iv) The quasi-two-dimensional nature of the monomolecular film makes it an attractive system for model studies of physical properties in low-dimensional systems (e& phase transition or magnetic ~ r d e r i n g ,to ’ ~mention only two). This expanding research activity also stimulated, of course, the development of new experimental techniques for the characterization of the structural and dynamical properties of monolayers. For many years mostly pressure-area diagrams (the two-dimensional analogue of van der Waals p-V diagrams) were used to derive information about molecular dimensions, packing properties, and phase transition^.^ A major breakthrough was achieved by *To whom correspondence should be addressed. Max-Planck-Institut fur Polymerforschung. *Present address: Institute of Biotechnology, Cambridge, U.K. Johannes-Gutenberg-Universitat Mainz. # Technische Universitat Miinchen.
0022-3654/89/2093-4587$01.50/0
introducing fluorescence microscopic technique^'^ that allowed for a direct observation of the lateral structure formation in monolayers, e g , during phase transitions. Many different systems have been studied since then, ranging from biomolecules to technical surfactants and polymeric systems. Most studies, however, were concentrating on one component systems and their phase-transition behavior. Far less attention has been paid, until now, to the phase behavior of mixed systems,I6 although it is well-known, e.g., from metal(1) See, e.g., the proceedings of the three International Conferences on Langmuir-Blcdgett Films: Thin Solid Films 1983, 99; Ibid. 1985, 133; Ibid. 1988, 159; Ibid. 1988, 160. (2) Blodgett, K. B.; Langmuir, I. Phys. Reu. 1937, 51, 980. (3) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (4) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross,E. A,; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabold, J . F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. ( 5 ) Roberts, G. G. Contemp. Phys. 1984, 25, 109. Sugi, M. J . Mol. Electron. 1985, I , 3. (6) Fariss, G.; Lando, J.; Rickert, S. Thin Solid Films 1983, 99, 305. Barraud, A. Ibid. 317. Broers, A. N.; Pomerantz, M. Ibid. 323. (7) Novotny, V.; Swalen, J. D.; Rabe, J. P. Langmuir 1989, 5, 418. (8) Swalen, J. D. J. Mol. Electron. 1986, 2, 155. Stegeman, G. I.; Seaton, C. T.; Zanoni, R. Thin Solid Films 1987, 152, 23 1 . (9) Sackmann, E. Ber. Bunsen-Ges. Phys. Chem. 1978,82, 891. (10) Hiemenz, P. C. Principles of Colloid and Surface Chemistry, 2nd ed.; Marcel Dekker: New York, 1986. (1 1 ) Rosen, M. J . StructurelPerformance Relationship in Surfactants, ACS Symposium Series 253; American Chemical Society: Washington, DC, 1984. (12) Losche, M.; Mohwald, H. Eur. Biophys. J . 1984, 11, 35. ( 1 3 ) Miller, A.; Knoll, W.; Mohwald, H. Phys. Reu. Lett. 1986, 56, 2633. (14) Pomerantz, M.; Dacol, F. H.; Segmiiller, A. Phys. Reu. Lett. 1978, 40, 246. ( 15) Tscharner, V.; McConnell, H. M. Biophys. J . 1981,36,409. Losche, M.; Sackmann, E.; Mohwald, H. Ber. Bunsen-Ges. Phys. Chem. 1983,87,848. Peters, R.; Beck, K . Proc. Natl. Acad. Sci. U.S.A. 1983, 80, 7183.
0 1989 American Chemical Society
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lurgy,'? that there is always an alloy that can be tailored to fit any material demands. Little is known about the phase behavior, phase transition, lateral structure, or two-dimensional pattern formation in binary surfactant mixtures. And even less is known of how these parameters influence and control the microscopic and macroscopic properties of an alloy monolayer, e.g., its (linear and nonlinear) optical properties. We address in the following these questions by combining information derived on the water surface from pressure-area (T-A) isotherms, fluorescence microscopy,I8 and spectroscopy, from absorption measurements and-after transfer of the monolayer to a solid support-from electron d i f f r a ~ t i o n . 'We ~ demonstrate that only this battery of methods allows for a sufficient characterization that enables one to correlate phase diagrams and crystallization behavior with the optical properties of a dye/surfactant mixture. The system studied in such detail as an example is a binary mixture of an amphipathic cyanine dye derivative and stearic acid as cosurfactant. Cyanine dyes in general are technically important as sensitizers in photography.20 Rather detailed investigations with cyanine dye monolayers have been performed to gain insight into the physical and chemical parameters that control the spectral sensitization of silver halides.20 For general optical studies these dyes are particularly interesting owing to their ability to form so-called Scheibe2*or J aggregates23with characteristic spectral features, e.g., a strong bathochromically shifted absorption peak. Kuhn and co-workers first demonstrated that long-tail derivatives of cyanine dyes can be spread and compressed as monomolecular layers at the water-air interface24 and still retain their ability to form, upon compression, two-dimensional J aggregate^.^^ We try in the following, for the first time, to correlate the thermodynamic parameters as well as kinetic factors that control this crystallization with the structural and optical properties of the grown J aggregates. We believe that only by such timeconsuming characterization will one eventually be able to optimize the performance of devices with integrated LB films.
Experimental Section Pressure-area diagrams were recorded with two different home-built Langmuir troughs. Both were equipped with Wilhelmy plates for the pressure reading, and both had a fluorescence microscope that allowed for direct observation of the pressureinduced growth of single-crystalline domains with the help of a silicon-intensifiedtarget (SIT) camera (Hamamatsu). One trough had the microscope objective integrated in the bottom of the tankz6 the other had the whole microscope mounted above the monolayer.27 Both setups have their specific advantages as discussed. We never realized any difference in the microscopic pictures taken either from above or below the monolayer. Fluorescence microscopy photographs could be taken with the exciting or the emitted light being polarized, or both. All photographs presented here were obtained from the display screen and show an area corresponding to 120 pm X 170 pm on the water surface. I n addition, the intensity and the wavelength spectrum of the emitted light could be analyzed in an attached fluorescence (16) Cadenhead, D. A.; Miiller-Landau, F.; Kellner, B. M. J. In Ordering in Two Dimensions; Elsevier Biomedical Press: Amsterdam, 1980; p 73. Matuo, H.; Mitsui, T.; Motomura, K.; Matuura, R. Chem. Phys. Lipids 1981, 29, 5 5 . (17) Cahn. R. W.; Haasen, P. Physical Metallurgy, 3rd ed.; North-Holland: Amsterdam, 1983. ( I S ) Duschl, C.; Losche, M.; Miller, A,; Fischer, A,: Mohwald, H.; Knoll, W. Thin Solid Films 1985, 133, 65. (19) Duschl, C.; Frey, W.; Knoll, W. Thin Solid Films 1988, 160, 251. (20) Steiger, R.; Hediger, H.; Junod, P.; Mobius, D.; Kuhn, H. Phorogr. Sci. Eng. 1980, 24, 185. (21) Steiger, R.; Zbinden, F. J . Image Sci. 1988, 32, 64. (22) Scheibe, G. Angew. Chem. 1936, 49, 563. (23) Jelley, E. E. Nature 1936, 138, 1009. (24) Kuhn, H.; Mobius, D.; Bucher, H. In: Physical Methods of Chemisiry; Weissberger, A., Rossiter, B. W., Eds.; Wiley: New York, 1972; Vol. 1, Part 3B. (25) Czikkely, V.; Forsterling, H . D.; Kuhn, H . Chem. Phys. Lett. 1970, 6, I I . Biicher. H.; Kuhn, H . Ibid. 183. (26) Losche, M.; Mohwald, H . Rev. Sci. Instrum. 1984, 55, 1968. (27) Meller, P. Rec. Sci. Instrum. 1988, 59, 2225.
Duschl et al. a
Stra A
b
Tr
S 120, SteaAl,.,)
mN r-
i(
0 20
9 33 c35
04 2 050
c 90
(areoldye molecuielnw
Figure 1. (a) Structure formula of the employed cosurfactant, stearic acid and the cyanine dye S120, a derivative of the well-known pseudoisocyanine. Note that there is only one hydrocarbon tail attached to the chromophore. (b) Pressure-area ( 7 - A ) diagrams of binary mixtures of SI20 with stearic acid (SteaA) at various mole fractions, x , of S120. All isotherms are shifted relative to each other by 24 mN.m-'. The dashed lines indicate the extrapolation of the base line for each plot. Note that the abscissa is scaled to the area per dye molecule. The dashed arrows mark the onset of the crystallization with the formation of J aggregates. The full arrows indicate for intermediate mixtures the pressure of a second, eutectic transition. The dotted line IS a guide to the eye at an area per dye molecule of A = 0.55 nm2, which corresponds to the dense packing of the chromophore headgroups.
spectrometer. The absorption measurements were performed with a separate film balance built to fit into the sample chamber of a Varian 2300 absorption spectrometer.28 Transmission electron diffraction experiments of a single monolayer could be performed after transfer onto special hydrophobic substrates that were dipped horiz~ntally.~~ The electron beam illuminated an area of about 6-pm diameter on the sample, which allowed for the recording of Laue diffraction from single crystal^.'^ The employed cyanine dye (I-methyl- l'-octadecyl-2,2'-cyanine iodide, SI20 for short, structure formula given in Figure la) was synthesized by K. Wirthensohn (Physik Department E22, Technische Universitat Miinchen). Stearic acid (SteaA, Figure la) was from Fluka and used without further treatment. Monolayers were spread from chloroform solutions onto pure water (Millipore quality, pH 5 . 5 , T = 22 "C) for the dye-rich mixtures. For mixtures rich in SteaA, 10-4 M ethylenediamine tetraacetate (EDTA) was added to the subphase. This was necessary to remove traces of divalent ions that otherwise would have disturbed the crystallization process in the slightly negative monolayer. For dye-rich mixtures, where the monolayer is slightly positive, no EDTA could be added because it was found to interact directly with the monolayer. Compression under inert gas (Ar or N2)was (28) Heithier, H. Ph.D. Thesis, Universitat Ulm, 1981. (29) Fischer, A . Ph.D. Thesis, Technische Universitat Miichen, 1985.
The Journal of Physical Chemistry, Vol. 93, No. 11, 1989 4589
Cyanine Dye/Stearic Acid Mixed Monolayers S 120, S t e a A , ~ . ~ )
0
05
X
10
Figure 2. Pressure-composition phase diagram for the S120, SteaAl,
mixed monolayers based on the data derived from isotherms like the ones presented in Figure 1b. Open symbols mark the onset of the fluid-condensed coexistence range seen as a break in the isotherms (dashed arrows in Figure 1b); full symbols indicate the pressures of the second, eutectic transition (full arrows in Figure 1 b). The lines give the phase boundaries of the proposed eutectic phase diagram; dashed lines, coexistence between fluid phase and crystalline domains; solid lines, coexistence of two crystal modifications above the eutectic pressure, re.The dotted area marks the range of the miscibility gap in the solid state. The dashed area limits the range where a liquid-analogue phase at low pressures exists (mixtures very rich in SteaA ( x < 0.15) show a sublimation transition). done at very low speed (full a-A isotherm in about 3 h) to ensure unperturbed growth of single crystals.
Results Figure 1b summarizes typical pressure-area diagrams obtained for S120/SteaA mixtures of different mole fraction, x, of S120. Plotted is the surface pressure, a, versus the area per dye molecule for reasons that will become clear below. But first a few details are noteworthy. For x 5 0.1 5 all mixtures show for low pressures an expanded liquid-analogue phase. Upon increasing the pressure the breaks in the isotherms (dashed vertical arrows) mark the onset of the coexistence range of a first-order phase transition between liquid-expanded and solid-condensed state. The transition pressure, a I , shows a pronounced dependence on x: it first increases up to x = 0.55 and then decreases again until for x = 1 at a1 = 4 m N d the phase transition of the pure dye is observed. Between x = 0.60 and x = 0.70 a break in the isotherm is only barely detectable, and hence no reliable information on al is available. As for many other lipid systems here, too, the completion pressure of the phase-transition region is rather ill defined. The increase in force necessary to further compress the monolayer at the end of the coexistence "plateau" in the isotherm is caused, among other reasons, by the direct contact of the crystalline domains. Hence no direct information on the solidus line can be deduced from the a-A diagrams. As shown before,I* the condensed, rather incompressible crystalline state of pure SI20 is reached at an area of about 0.5-0.6 nm2 and is stable up to ca. 50 mNm-'. For mixtures containing between 30 and 90 mol % S120 a second transition is observed (except for the narrow range 0.60 < x < 0.70 where the collapse occurs before this transition would be reached). As can be seen from the near equidistant horizontal arrows in Figure 1, which mark this second transition for each isotherm, in that concentration range, the corresponding pressure is re N 40-42 m N d independent of the composition. However, the widths of the plateaus in the isotherms of this second transition (which are remarkably flat) depend very much on the mole fraction, x (see Figure 1). Increasing the SteaA content (1 - x) to about 65% does not change the area per dye molecule in the condensed state at r > 42 mNm-l, which means that the additional hydrocarbon chains of the SteaA do not require any additional area on the water surface (see dotted line in Figure 1). To show this clearly all isotherms have been plotted as a function of the dye area. Clearly,
Figure 3. Mean molecular area at r = 4 mNm-' for S120, SteaA,, mixed monolayers obtained from the r-A isotherms. The dashed straight line indicates the average area linearly interpolated between the value for pure S120 (A = 1.30nm2)and A = 0.24 nm2 taken for fluid-analogue SteaA. The maximum excess area (AA = 0.40 nm2) is found for x = 0.65, which is close to the eutectic mixture.
a disadvantage of these plots is that the area of the expanded state at, say, a = 4 mN-m-', has no immediate meaning for x # 1.0. Multiplication by x , however, gives the mean area per molecule. The different characteristic pressures of all isotherm are plotted in Figure 2 as a function of the mole fraction x, to derive a pressure-composition phase diagram. The shaded area for x 5 0.15 is introduced to avoid complications for the phase behavior of SteaA-rich mixtures associated with the sublimation transition of pure SteaA. As we will discuss below it is rather suggestive to interpret the characteristic features of the pressure-area curves on the basis of a eutectic phase behavior as tentatively drawn in Figure 2 by the dashed lines. Note that except in the more common (threedimensional) case of a temperature-composition phase diagram, here the characteristic features such as the miscibility gap in the crystalline state (dotted area) are upside down: Two coexistence regions with liquidus and solidus lines merge into a miscibility gap in the condensed state with two coexisting crystalline phases. The eutectic point, where three phases (two crystalline and one fluid) coexist, is reached at 40-42 mN-m-'. Extrapolation of the experimental points on the liquidus curves (open symbols) to the eutectic pressure (given by the full symbols) yields a eutectic composition of this still-fluid phase of x = 0.65. This compares well with the value obtained by plotting the mean molecular area at a = 4 mN-m-' as a function of x (see Figure 3): The excess area (defined in analogy to the excess volume in 3D mixtures as the difference to the averaged area given by the dashed line in Figure 3) has a maximum in the same range 0.6 C x C 0.7. This excess area is remarkably large and amounts to 0.40 nm2, which is almost 50%. Qualitatively, it is understandable that this mixture requires the highest lateral pressure to be forced to crystallize into domains. We will discuss this point later. The two-phase region in the crystalline state covers the composition range from about x1 = 0.30 to x2 = 0.95. The location of the upper phase boundary is certainly accurate to better than 5 mol % as the x = 0.90 mixture still shows clearly a little bit of a eutectic transition (see Figure 1). The lower phase boundary, however, is less accurate to locate because the low lateral diffusivity of the two components in the crystalline domains is the reason we cannot conduct the solidification process at x = constant but instead observe nonequilibrium crystallization. We will come back to this. The onset of the crystallization can also be followed by optical techniques. Figure 4 presents some spectroscopic results obtained with a pure S120 monolayer on the water surface. In part a absorption spectra are shown taken at different lateral pressures as indicated schematically on the isotherm shown in a perspective view. The intersections of the three dashed lines with the a-A curve mark the lateral pressures (or packing densities) at which
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The Journal of Physical Chemistry, Vol. 93. No. 11. 1989
a)
Wavelengthlnm
500
'
A
600
700
Wavcleqthlnm
Figure 4. (a) Absorption spectra taken directly from the water surface covcred with a monolayer of pure SI 20 at various lateral pressures corresponding to different degrees of crystalli7ation. This is indicated schematically by the dashed lines which intersect with the PA isotherm (shown perspectively and rotated by 90°) at different areas per molecule. In addition to the monomeric and (blue shifted) dimeric absorption band, the red-shifted J - aggregate peak appears after the monolayer was compressed beyond the break point in the isotherm. With increasing crystallization the J band grows considerably. (b) Fluorescence spectrum taken from a monolayer at the water-air interface after the pressureinduced formation of J aggregates. Virtually no fluorescence is observed (from monomeric and dimeric dye molecules) before the onset of the crystallization.
the three corresponding absorption spectra were taken. At the lowest pressure, just above the break point in the isotherm, the first indication of J aggregates with their characteristic absorption peak at X = 580 nm is found in addition to the absorption of dye monomers ( A = 531 nm) and dimers (A = 496 nm). This observation parallels results by Mobius and Griiniger obtained with a reflection technique.30 Upon increasing the lateral pressure the J-band absorption grows considerably at the expense of the monomeric and dimeric contributions. The integrated optical density, however, remains constant when corrected for the increasing lateral density of the dye and is hence independent of the aggregational state of the chromophore. Obviously, the oscillator strength of the single molecule is constant. The appearance of the first J-band absorption is accompanied by a strong fluorescence of the dye monolayer at X = 582 nm (Figure 4b). Both spectral features are strictly correlated. As we have shown before,'* the fluorescence intensity-again when corrected for dye density-remains nearly constant for the pure SI20 monolayer but shows a 6-fold increase for the x = 0.5 mixture above the eutectic pressure. This points to the different quantum yields of radiative deexcitation for crystal modifications of SI 2O/SteaA mixtures grown under different conditions (mole fractions, lateral pressures, etc). As discussed before, it is this high fluorescence quantum yield of the dye crystals that allows for a microscopic observation of the J aggregates with high contrast relative to the surrounding, still-fluid mixtures of cosurfactants with monomeric and dimeric dye molecules. A selection of typical pictures is presented in Figure 5. We concentrate on the composition range 0.3 < x < 1 .0 (see Figure 2). Figure 5a shows the starlike aggregates grown at A (30) Miibius. D.: Griiniger. H. In Charge and Field Eflects in Biosystems; Allen, M. J.. Usherwood, P. N. R., Eds.; Abacus Press. Tunbridgc Wclls. C.K.. 1984; p 265.
takcn from S 120, SteaAl-, Fiplure 5. f.luorcsccncc microscopic . pictures . mixed monolaycrs at thc watct-air intcrfacc at various composition, x. and pressures. A. Polarized excitation; all photographs show an area of approximately I30 pm X 170 pm on the water surface. (a) x = 0.29, A = 20 "em-'. Note the hexagonal superstructure of the starlike aggregates; (b) x = 1.0. A = 20 mN0m-I; (c) x = 0.5. A = 22 "em-'; (d) x = 0.87, A = 15 "em-'; (e) x = 0.5, A = A, = 42 "am-'; (f) x = 0.87, A = A,.
= 20 mN0m-I in the S 1 200,29SteaAo.71 mixture, which is the last system (coming from pure SteaA) that does not yet show the eutectic tran~ition.~'The excitation light of the microscope for this and the following picture.. was polarizd. Some characteristic features may be summarized: ( i ) the single crystallites appear to be composed of centrosymmetrically arranged needles or segments; (ii) the aggregates are all about equal in size with a mean diameter of 30 pm; (iii) they repel each other and form a hexagonal superlattice on the water surface. At sufficiently high lateral pressure almost densely packed crystalline monolayers can be prepared with highly indented stars (see Figure 6) which still show the hexagonal superstructureas seen by rotating the polarizer by nearly 120' (Figure 6). The (nearly) pure SI20 aggregates show quite a different morphology (Figure Sb): Large elliptically shaped crystals with dimensions > I O 0 pm can be grown. Their size distribution is rather irregular, and no distance correlation exists between the single crystallites. Their fluorescence is highly polarized and completely homogeneous over the whole single crystal. There is no obvious repulsive interaction bctween single domains; merging aggregates exhibit straight grain boundaries (see, e.g., Figure 2e in ref 18). The same behavior is also found if 5% SteaA is added to the spread dye solution (not shown). The fluorescence microscopic pictures obtained with intermediate S 1 2O/StcaA mixtures show a characteristically differcnt behavior. Figure 5c demonstrates the formation of centrosymmetrical aggregates if a monolayer of 5070 SI20 (x = 0.5) is compressed to A > 18 mN0m-I. The dark bars in the single crystals rotate as the polarizer is rotated. The density of the domain increases until at renew crystals with different properties appear (see Figure Sc): (i) although growing in a twinlike configuration, they seem to bc more uniform in thcir crystallographic behavior; (ii) their fluorescence is homogeneously polarizcd and orientation dcpcndent-rotating the polarizer turns off completely the ( 3 I ) Duschl. C.; Frcy. W.; Hclm. C.; hls-Niclscn, J.; W. Thin Solid Films 1988. 159. 286.
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Cyanine Dye/Stearic Acid Mixed Monolayers
The Journal of Physical Chemistry, Vol. 93. No. I I, 1989 4591
Y
SI
Figure 7. Electron microscopic picture of a 1 : I SI2O:SteaA mixed monolayer transferred to the Elul grid at x = 25 "em-'. The mean aggregate size is about 30 pm. The contrast between aggregates and surrounding (amorphous) material is achieved by the charge decoration technique.29
Figure 6. Fluorescence microscopic picture taken from a mixed monolayer at x = 7,. The polarizer in the S1200.2~teaAo~7, excitation was rotated each time by nearly 120° from (a) to (c).
fluorescence light from single bright domains whereas dark ones become brightly fluorescing; (iii) the maximum fluorescence intensity from the new crystals is much brighter. The reversed appearance of the two types of crystallites is found if a mixture with x > 0.65 is compressed. An example with x = 0.87 is given in Figure 5d.f. First, at A' > 5 m3.m-l homogeneous crystals are found nearly identical with those observed with pure S120 (Figure 5d). These can be grown to ellipses of several tens of micrometers in size until at A, new crystals appear, this time reminiscent of the centrosymmetricaldomains found for the 1 : l mixture of A' < A < A, (cf. Figure 5c.f): again the dark bars in the small circular crystals rotate if the polarizer in the excitation beam is turned. Both crystal modifications can be "melted" in the reverse sequence of their appearance if the monolayers are expanded again. I n this sense, both types of crystals are equilibrium modifications depending only on the composition of the mixture and on the lateral pressure. To further characterize the structural properties of the two crystal forms on the molecular level, transmission electron diffraction experiments were performed with monolayers transferred onto suitable substrates.3' The successful transfer could be controlled either by light (fluorescence or polari7ation) microscopy or by electron microscopy. An example of the latter obtained with an x = 0.5 mixture transferred at A 30 "em-' to an EM grid is shown in Figure 7. The charge decoration contrast32allows one to identify the single aggregates grown on the water surface
-
.
~~~~
(32)Fischer, A.; Sackmann, E. Nature 198s. 313, 299.
and transferred with almost no detectable deterioration. It is therefore reasonable to assume that the structural information obtained by elcctron diffraction from a transferred monolayer also mirrors the situation on the water surface. The two most important diffraction patterns are reproduced in Figure 8. The upper hexagon of diffraction spots (Figure 8a) is obtained from a mixed SteaA/S120 monolayer with x = 0.3. This mixture was chosen because it corresponds to the SteaA-rich phase boundary (see Figure 2). This way one can ensure that structural information of only the starlike crystal modification is derived. The depicted unperturbed hexagonal spot pattern is found throughout the single domain contrary to the crystals grown at higher pressure in a 1:l mixture, where toward the outer edges of the crystals the hexagons get more and more di~torted.~'The lattice plane distance derived from the diffraction spots amounts to d = 0.42 nm and is therefore identical with the spacing found for a pure fatty acid m0nolaye9~in its fully condensed state. Only the slight blurr in the single diffraction spots indicates some loss of lateral order. A completely different diffraction pattern is obtained for a pure S120 monolayer transferred at A = 40 mN0m-I (Figure 8b). The spot pattern (which is not changed if hexadecane is equimolarly added to support the mechanical stability of the J aggregates) could be fully indexed'* on the basis of the brick-stone model,2s assuming two bricks (=chromophores) per unit cell. No evidence for contributions from the hydrocarbon tails is found. The pattern position remains constant when the electron beam is scanned along a single crystalline domain but eventually jumps to a different position when a neighboring crystal with a different orientation is reached. The single diffraction spots are rather sharp, indicative of a high positional correlation even though a true line-shape analysis could not be performed. The dimensions of a single brick thus derived were 0.36 nm X 1.54 nm. which compared well with the area per dye molecule obtained from the A-A isotherms on the water surface. Discussion
The combination of thermodynamic data derived from A-A curves with the fluorescence microscopic control of the growing
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Duschl et al.
Figure 9. Molecular models showing the near match between the areal requirements of the cyanine chromophore-had and the hydrocarbon tails if 2.5 SteaA molcculcs are added and closely packed with thc dye's one chain. Only schematically ignoring packing restraints in the plane of the monolayer. Figure 8. Transmission clcctron diffraction pattern obtained from the S 1 2 0 ~ t e a A l - ,mixed monolayers transferred at A = 40 mNm-' to suitable substrates. (a) x = 0.3. The lattice plane spacing obtained amounts to d = 0.42 nm. which corresponds to a close packing of the alkyl chains. (b) x = 1 .O. The difference diffraction spots can be fully indexed on the basis of the "brick-stone"arrangement of the chromophores.
aggregates gave rather clear evidence that the phase behavior of S 120 mixed with SteaA can be best described on the basis of a eutectic phase diagram with mixed crystal formation. To our knowledge this is the first example of such a "textbook" eutectic diagram for a quasi-two-dimensional monomolecular system. The experimental situation in this case was rather advantageous because the intrinsic fluorescence of the growing J aggregates not only allowed us to discriminate between crystalline material and still-fluid regions but, even more conclusive. showed the coexistence of two types of crystals-the best evidence for a miscibility gap in the crystalline state. Contrary to most other binary systems we thus also did not need to add any fluorescence label molecules to generate an optical contrast between different phases. All our phase information. therefore, was disturbed only (if at all) by some residual chemical impurities. Given for the two molecules in our study the very different steric requirements for a dense packing on the water surface (see Figure l a ) it is intuitively understandable that only a limited solubility of one in the other is found. While, however, the tight crystal packing of pure SI20 in the brick-stone arrangement squeezes out almost all additional fatty acid chains (hence x2= 0.95), there is a substantial solubility of SI 20 in a pure SteaA matrix: Solid solutions up to xI = 0.3-0.33 corresponding to about 2-2.5 SteaA chains per S I20 chain can be formed. As we suggest in Figure 9 by a very simplifying cross section through a monolayer, the architecture of these mixed crystals is given by an array of S120 chromophores facing the water, with 2-2.5 SteaA molecules sitting on top of the chinolin rings next to the dye's own chains in a hexagonal close packing giving rise to the observed electron diffraction pattern. This phase boundary, therefore. corresponds to a match point between the areal requirements of the chromophore headgroup and the alkyl chains. The two lattices in this sense are commensurate at that molar ratio. Further addition of SI20 does not lead to a "dilution" of the chain packing but
instead to the formation of a new, separated phase. It is interesting to note that a specific interaction bctwecn the carboxyl group of the SteaA and the cationic chromophore of the dye must play an important role because a 1:l mixture of S120 and stearoyl alcohol shows phase separation already in the fluid state,'* whereas octadecane can be added to SI 20 equimolarly without noticeable effect on the A-A diagrams or on the J aggregates (except for a slight mechanical stabiliiation during the transfcr of the monolayer to a solid support). As for other (three-dimensional) liquid-solid transformations, kinetic effects can lead to nonequilibrium situations. We know that the grown J aggregates are very rigid with possibly very low lateral diffusion of their constituents. For a curved solidus line like the one given in Figure 2 between x = 0.1 5 and x = 0.3 this would mean that a crystallite grown after nucleation at A == 20 mN0m-I with a mole fraction of about x = 0.20 would have to change its composition to x = 0.25 when compressed to A = 3 0 mN0m-I. We have, in fact, evidence from fluorescence microscopy (see, e.g., Figure 7c in ref 17) and electron diffraction3' that details of the molecular packing in the center of the SteaA-rich starlike aggregates are slightly different than in the other region grown at a later stage at somewhat changed conditions. Although this is quite indircct evidence, we nevertheless assume therefore that the solidus line is somewhat curved in this range. This nonequilibrium solidification leads to a local enrichment of S120 in the fluid state (too much SteaA is bound in the inner parts of a growing crystal) with the consequence that wc do not cross the phase diagram vertically along a line of constant composition, or isopleth, but instead steadily drift to higher x values. Unfortunatcly, we cannot fully quantify this cffcct. Therefore, we cannot exactly locate the position of the star-aggregate phase boundary xI because we lose the strict correlation bctwccn the starting molar ratio, x, and the appearance of the first indication of a second eutectic transition in the A-A curves. This effect also makes it imposible to precisely locate the solidus line experimentally. In principle, we could have obtained this by analyzing the fluorescence microscopic pictures with an image analysis system and derive at a givcn lateral pressure the dcgrce of crystallization for diffcrcnt mixturcs. Since we cannot apply strictly the lever rule any more, we cannot transform this infor-
Cyanine Dye/Stearic Acid Mixed Monolayers mation into values for the composition of the crystalline phase, even though we measure rather accurately the liquidus line from a-A isotherms. Another detail of the phase’behavior of these mixtures that we attribute, at least partly, to the curved phase boundaries is the apparent differences in the habit of crystals grown at different conditions: The centrosymmetric aggregates grown in mixtures with x = 0.3 are clearly more starlike with indentations (Figure 5a) compared to the almost circular crystals grown for a 1:l mixture below ire(Figure 5c) or for the dye-rich systems above a, (Figure 5f) although all three belong to the same phase boundary. Similarly, clear differences between the (almost) pure S 120 crystals grown from mixtures with x > 0.7 below ac (Figure 5b,d) and those obtained at a > a, for, e.g., a 1:l mixture (Figure 5e), can be seen. It is, on the other hand, well conceivable that these differences must be associated with the fact that these molecules have in addition to the outer also inner degrees of freedom. In other words, it is very likely that configurational differences of the molecules at different lateral pressures modify the interaction potentials and hence details of the packing properties of mixed crystals. In passing, we want to comment on the quite obvious superstructure formation of the star aggregates (see Figure 5a,b). As postulated for other monolayers we attribute this, too, to an electrostatic interactionI2 of the single crystals due to charge or dipole density differences relative to the surrounding fluid mixture. This can cause an additional contribution to the phase-transition pressure in charged lipid monolayer^.^^ For our mixture, however, it does not seem to have any major influence because within experimental uncertainty we find the eutectic pressure independent of the composition, x, although x controls (through the level rule) the relative (interaction) distance between the crystallites at a,. A last comment to the presented phase behavior concerns the eutectic mixtures itself. We were unable to observe a true eutectic solidification with a simultaneous growth of the two types of crystals from the pure melt for two reasons: First of all, for mixtures close to the extrapolated eutectic mixture, x,, we always found in the fluorescence microscopy either the one or the other crystal type already for pressures below re.We believe that this is caused by a local variation of the mixture composition, x. Different solubilities of the dye and SteaA in the spreading solvent could cause some segregation during the spreading process already. Given the lateral dimensions on the water surface, it might take too long to equilibrate this by a mere diffusion process. In compressing we then cross locally either one of the two liquidus curves. The second reason is that these fluid mixtures seemed to be particularly unstable at higher pressures, so that we observed the collapse of the monolayer already at pressures below re. (33) Helm, C. A,; Laxhuber, L.; Losche, M.; Mohwald, H. Colloid Polym. Sci 1986, 264, 46.
The Journal of Physical Chemistry, Vol. 93, No. I I , 1989 4593 Finally, we want to comment briefly on the optical properties of the grown aggregates that were the original motivation for the whole study: Their strong narrow absorption band makes these J aggregates an attractive candidate for spectroscopic studies, e.g., for the investigation of dye monolayers interacting with the plasmonic states of a metal substrate.34 We have seen that the two types of crystals represent two stable phase boundaries of the pressure-composition-phase diagram of this mixture. Only the knowledge of the complete phase behavior allows us now to prepare ”pure” states, which could considerably simplify the optical behavior. Putting it another way, a satisfying interpretation of optical studies with aggregates grown, e.g., in a 1:l S12O:SteaA mixture will not be possible without knowing that the transferred film consists of two different aggregates with different optical properties. In addition, not only are these equilibrium considerations important, but as we have seen also minor kinetic or other nonequilibrium factors that influence the steric and electronic structure of the final two-dimensional crystal need to be controlled. We do not know yet in detail all the optical and spectroscopic differences of the two crystal forms. Clearly, the fluorescence intensity and polarization properties are very different. Details of the absorption spectra like the width of the J-band peaks seem to differ slightly. Unfortunately, we cannot correlate this with observable differences in the chromophore packing because only for the (near) pure S120 crystal can the arrangement of the dye headgroups be elucidated from electron diffraction. For the star aggregates we could not find any diffraction spots that could be assigned to the brick-stone or any other chromophore arrangement. We cannot decide currently whether this means that a chromophore lattice needs a much better positional correlation to show up in electron diffraction than to show up optically as a J-band structure. It has be=en estimated that less than 10 correlated dipoles can be identified spectroscopically already as aggregate^.^^ Such a small number is certainly unable to produce a detectable electron diffraction pattern. We also do not know yet what determines the distribution ( and how this looks like) of different J aggregates that are observed in photothermal s p e c t r o ~ c o p ythrough ~~ their different quantum yield for radiationless deexcitation.
Acknowledgment. We are most grateful to Birgit Wallner, who performed some of the experiments with great care. Helpful discussions with A. Fischer, H. Kuhn, A. Miller, D. Mobius, H. Mohwald, and R. Steiger are acknowledged. This work was financially supported by the Deutsche Forschungsgemeinschaft (Kn 249/1-2). Registry No. SlzO, 116786-51-7; stearic acid, 57-1 1-4. (34) Knoll, W.; Philpott, M. R.; Swalen, J. D.; Girlando,A. J . Chem. Phys. 1981, 75, 4795. (35) Daltrozzo, E.; Scheibe, G.; Gschwind, K.; Haimerl, F. Phofogr. Sci. Eng. 1974, 18, 441. (36) Knoll, W.; Coufal, H. Thin Solid Films 1988, 160, 333.
