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Langmuir 1996, 12, 2298-2302
Phases and Phase Transitions in Langmuir Monolayers by Second-Harmonic Generation Tongguang Zhang, Zhiming Feng, G. K. Wong, and J. B. Ketterson* Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208 Received February 20, 1996X We have studied phase transition phenomena in Langmuir monolayers of a molecule incorporating NPP dye by optical second-harmonic generation. From these measurements we obtain information on the molecular orientation and the molecular conformation in the film.
I. Introduction A Langmuir monolayer, viewed as a two-dimensional molecular system, can exist in several different phases.1 Clasically, the phases and phase transitions of Langmuir films were inferred from the Π-A isotherm, the plot of surface pressure, Π, versus surface area per molecule, A. The identification of the phases of a monolayer and the boundaries between them was based primarily on the behavior of the Π-A isotherm.2 Recently, many new methods, for example, fluorescence microscopy,3 synchrotron radiation diffraction,4 and optical second- and sumfrequency generation,5,6 were employed to study the phases and phase transitions in Langmuir films. Among these methods, second-harmonic generation (SHG) studies can provide valuable information about molecular orientation in various two-dimensional phases, which supplies new insight into the microscopic features of the rich world of two-dimensional phase transitions. In this paper we will study a Langmuir film which shows an interesting phase coexistence behavior in its Π-A isotherm. By the SHG method, we can obtain information on the molecular conformation within the film during film formation and, especially, near the phase transition points. II. Experiment The molecule used in the experiment is an NPP dye containing an amphiphilic molecule. Figure 1 shows the molecular formula. The molecule has a hydrophilic OH group at one end and a weakly hydrophilic NO2 group on the other end, with the carbon chain between serving as the hydrophobic group. The UV-vis absorption spectrum of this molecule was measured as shown in Figure 2. The absorption peak was found at 410 nm. The Langmuir film was spread in a type FW-1 Lauda trough. A chloroform solution of NPP molecules was spread on a pure (pH ) 7) deionized water subphase to form a monolayer. A movable barrier was used to compress the film. The surface pressure was measured by a Langmuir balance. The surface area, surface pressure, and polarized SHG intensity were all recorded. A mode-locked, Q-switched YLF laser operating at a wavelength of 1054 nm was used in the experiment. Figure 3 shows the experimental setup. The laser beam was focused onto the sample at an incident angle of 60°. A polarization rotator was X
Abstract published in Advance ACS Abstracts, April 15, 1996.
(1) Adamson, A. W. Physical Chemistry of Surfaces, 4th ed.; Wiley: New York, 1982; Chapter 4. (2) Harkins, W. D. The Physical Chemistry of Surface Films; Reinhold: New York, 1952; Chapter 2. (3) (a) Losche, M.; Mohwald, H. Eur. Biophys. J. 1984, 11, 35. (b) LeGrange, J. D. Phys. Rev. Lett. 1991, 66, 37. (4) Lin, B.; Shih, M. C.; Bohanon, T. M.; Ice, G. E.; Dutta, P. Phys. Rev. Lett. 1990, 65, 191. (5) Raising, Th.; Shen, Y. R.; Kim, M. W.; Grubb, S. Phys. Rev. Lett. 1985, 55, 2093. (6) Hunag, J. Y.; Superfine, R.; Shen, Y. R. Phys. Rev. A 1990, 42, 3660.
S0743-7463(96)00157-6 CCC: $12.00
Figure 1. Molecular formula of the NPP dye containing molecule.
Figure 2. UV-vis absorption spectrum of the NPP dye molecule. It shows the absorption peak is at 410 nm. used to change the polarization of the input beam. The beam reflected from the sample passed through a glass filter (which removed the fundamental beam) and a polarization analyzer. The polarized, second-harmonic generated light passed through an interference filter and was then detected by a photomultiplier tube (PMT). More experimental details are given in ref 7. (7) Zhang, T. G.; Zhang, C. H.; Wong, G. K. J. Opt. Soc. Am. B 1990, 7, 902.
© 1996 American Chemical Society
Phases and Phase Transitions in Langmuir Monolayers
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Figure 3. Experimental setup for SHG measurement. In the diagram A is a polarization rotator, B is a focusing lens, C is a color glass filter, D is a polarization selector, E is a color glass filter, and F is an interference filter.
Figure 5. p-Polarized SHG intensity as a function of the polarization angle from the film at two different surface densities.
Figure 4. Π-A isotherm of the monolayer at the different temperatures. By measuring the polarized SHG intensity, we can determine the molecular orientation of the film. In the system we studied, since the molecular absorption peak is far away from the wavelength we worked at, the frequency dispersion of the susceptibilities can be ignored. The number of independent susceptibility components involved in SHG from a Langmuir film depends on its symmetry.8 For an unoriented film there are only two independent susceptibility components, as also described by eq 2 of ref 8. The molecular orientation in this work was calculated on the basis of the expression developed in ref 7 in which two susceptibility components were considered.
III. Results and Discussion Figure 4 shows the Π-A isotherm of the monolayer of the NPP dye containing molecules at various temperatures. All of these isotherms have long flat plateau regions at intermediate pressures. Because the compressed Langmuir film can be expanded and recompressed to show the same features, the plateau region is presumed to arise from the monolayer structure. The height of these plateaus depends on temperature. The existence of the plateaus indicates that there is a phase coexistence region between the two phases. It is important to note that the isotherm at the lowest temperature is almost directly above that for the highest temperature. This is an unusual feature, since isotherms at lower temperatures typically lie lower. This kind of anomalous isotherm has been (8) Kajikawa, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys. 1991, 30, 1050.
observed in Langmuir films involving other molecules exhibiting phase coexistence behavior.9 In ref 9, a breakdown of hydrogen bonding between monolayer molecules was proposed to interpret the phase transition and the negative temperature dependence. The similarities between those isotherms and ours suggest that the explanation may be similar. SHG measurements can provide more direct information on the molecular conformations in each phase (we will have a clearer picture of this phenomenon after our data analysis). The data to be discussed here were obtained on a film at 24.8 °C. Data collected at other temperatures had similar features. Figure 5 shows the p-polarized SHG intensity as a function of the polarization angle of the input beam fitted using the equation given in ref 7
Ip(2ω) ) (Ap cos2 R + Bp sin2 R)2I2(ω)
(1)
at two different molecular surface densities. The direction of the electric field vector lay in the incident plane (corresponding to p-polarization). The polarization angle, R, of the scattered beam is defined as the angle between the incident plane and the direction of the polarization. Ap and Bp are functions of the susceptibilities, dielectric constant, and incident angle. From the constants Ap and Bp obtained by fitting this expression, χzyy/χzzz and the molecular orientation angle, ψ, can be calculated. The molecular orientation as a function of the area per molecule is shown in Figure 6. Figure 7 shows the p-polarized SHG field generated by an s-polarized pump beam, denoted by Esp, as a function of the area per molecule. The decreasing SHG intensity indicates that the nonlinear optical efficiency is decreasing, which we assume is due to the formation of a more centrosymmetric structure within the film. We define Na as the net number of nonlinearly active molecules; i.e., N is the excess number of molecules oriented in a given direction. The remainder of the molecules are presumed to average to a centrosymmetric (non-SHG) structure. Figure 8 shows the normalized value of Na as a function of the area per molecule. Na is calculated from the data shown in Figures 4 and 5 by (9) Glazer, J.; Alexander, A. E. Trans. Faraday Soc. 1951, 47, 401.
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Figure 6. Molecular orientation of the film as a function of surface density.
Figure 8. Number of active molecules as a function of the surface density.
Figure 7. Esp as a function of surface density.
using the equation
Na ∼
Aχzyy 1 sin2 ψ cos ψ 2
∼
AEsp 1 sin2 ψ cos ψ 2
(2)
where A is the area per molecule. Figure 8 implies that the number of nonlinearly active molecules Na decreases with increasing surface pressure. We now propose a molecular model which is consistent with the above results as well as other calculated results. When the area per molecule A > 175 Å2, both the surface density and surface pressure are low. The low SHG signal suggests the molecules lie flat on the surface under these conditions. The proposed molecular conformation is shown in Figure 9a. When 175 Å2 > A > 120 Å2, the surface pressure and the SHG signal increase as the surface density increases. The molecular tilt angle becomes smaller, implying the molecules tend to “stand up” under compression. The fact that Na is almost constant indicates that few molecules form antiparallel centrosymmetric structures. The proposed molecular conformation is shown in Figure 9b. This
Figure 9. Proposed molecular conformation of the film.
kind of configuration, in which both ends remain in contact with the water, will be referred to as an inverted U. (We could also use the intersection symbol, ∩, of set theory.) This conformation would arise from the existence of two hydrophilic groups in the molecule. Molecules with two hydrophilic groups were studied in ref 10, where the inverted-U conformation was proposed. Our molecular orientation measurements provide a direct measure of the average molecular shape. The fact that the signal is stable as a function of time in this region indicates that (10) Kellner, B. M. J.; Cadenhead, D. A. J. Colloid Interface Sci. 1978, 63, 452.
Phases and Phase Transitions in Langmuir Monolayers
the film is spatially uniform; i.e., no large domains (islands) drift in and out of the area illuminated by the laser. Such a drift would cause the measured signal to fluctuate in time. When 120 Å2 > A > 80 Å2 the SHG signal decreases as the molecular surface density increases. The decreasing of Na indicates that the molecules start to form a centrosymmetric structure, which we model as antiparallel pairs. The formation of these pairs is likely due to the dipole-dipole interaction between the dye groups in the molecules. A continuous decrease of the average molecular orientation indicates that the inverted-U configuration is still relatively compressible. Figure 9c shows the molecular conformation at this stage. From this proposed conformation we can calculate Na as a function of the area. Other studies have found that a high dye density causes aggregates to form, shifting the absorption spectrum; this in turn reduces the molecular hyperpolarizabilities.11 In our system, since the absorption peak of the molecule is far away from our SHG wavelength, a shift in the absorption due to aggregation is unlikely to have a large effect on β (at our wavelength). As a simple model, let Nu be the number and Au be the area of the “standing” molecules and Nd be the number and Ad be area of the inverted-U molecules. Assuming molecular area and number conservation, we have the equations
N ) Nd + Nu, A )
Nu Nd Au + Ad N N
(3)
where N is the total number of molecules and A is the measured area per molecule. At this stage we assume the surface is fully covered by molecules. Assuming the standing molecules are antiparallel pairs, only the inverted-U molecules contribute to SHG; i.e.,
Na ) Nd The solution of these equations is
Na A - Au ) N Ad - Au
(4)
From the Π-A isotherm we determine that Au ) 25 Å2. If we assume Ad decreases linearly from 120 to 110 Å2 in this region (due to the compressibility of the inverted-U molecules), then Na as a function of A is given by the solid line in Figure 8. The solid line agrees well with our experimental data. For the case where all molecules are aligned with the NO2 group out of the water, the SHG will also be canceled. However in this case we have the cancellation condition
Na ) Nd - Nu The solution then becomes
Na 2A - Au + Ad ) N Ad - Au
(5)
The slope of this theoretical Na-A plot will be twice that of the previous result and is shown as the dashed line in Figure 8, which does not agree with the experimental data. We can also estimate the energy of a standing molecule with its OH group out of the water. To estimate this energy we use the principle of independent surface action, given (11) Carpenter, M. A.; Willand, C. S.; Penner, T. L.; Williams, D. J.; Mukamel, S. J. Phys. Chem. 1992, 96, 2801.
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by Langmuir,12,13 in which one assumes that each part of a molecule possesses a local surface energy. Energies of 25 × 10-2 eV/molecule for an OH group, denoted as E(OH/ air), and 6.1 × 10-2 eV/molecule for the hydrocarbon group, denoted as Eh, have been used for alcohol molecules.13 For an NO2 group the surface energy E(NO2/air) is 18.7 × 10-2 eV/molecule from its hydrophilicity. The dipolar energy is estimated as in ref 12. The molecular energy for an inverted-U molecule is
E ) Eh + Ed + Ee where Eh is the hydrocarbon surface energy, Ed is the dipole energy, and Ee is the elastic energy due to the hydrocarbon chain bending. On compression the molecules are forced to stand up, which releases the elastic energy. The standing molecular energy for the unpaired case is
Eu ) Eh + E(NO2/air) + Ed(unpaired) ) 24.8 × 10-2 eV + Ed(unpaired) and that for the antiparallel paired case is
Ep ) Eh + 1/2(E(NO2/air) + E(OH/air) + Ed(paired)) ) 27.9 × 10-2 eV + Ed(paired) For molecules with small dipole moments, the weak hydrophilic end will leave the water surface, as proposed in ref 13. In our case of the NPP dye molecular film, the large dipole moment in the NPP group will play an important role in the film structure. For example, for a molecular dipole moment of 10 D, an intermolecular distance r ) 11.8 Å, and a tilt angle of ψ ) 45°, the dipole energy of the unpaired molecules is estimated to be from 5.3 × 10-2 to 7.5 × 10-2 eV/molecule, depending on the distribution of the in-plane component of the dipole moment.14 The dipole energy for those molecular pairs is expected to be much lower than that for the unpaired molecules. If we take Ed(unpaired) ) 6.25 × 10-2 eV and Ed(paired) ) 0, the energy difference per molecule is
δE ) Ep - Eu ) -3.1 × 10-2 eV The Boltzmann factor, exp(-δE/kBT), is 3.4, which favors the paired conformation (kBT ) 2.5 × 10-2 eV at T ) 295 K). On compression the unpaired dipole energy becomes even larger (due to the reduction in the intermolecular distance and in the molecular orientation angle). The molecules will then favor pairing. At r ) 10.5 Å and ψ ) 40° the dipole energy ranges between 11.2 × 10-2 and 13.8 × 10-2 eV/molecule. The Boltzmann factor becomes 43, which strongly favors the antiparallel paired configuration. When the area is between 80 and 30 Å2, the surface pressure levels out. The constant molecular orientation indicates that inverted-U molecules can no longer be compressed. The continuous decrease in Na indicates that more molecules form antiparallel pairs. On the basis of our observation we suggest that the inverted-U molecules convert to antiparallel molecular pairs. At the end of this (12) Langmuir, I. Colloid Symposium Monograph; The Chemical Catalog Company: New York, 1925; p 48. (13) MacRitchie, F. Chemistry at Interface; Academic Press: San Diego, CA, 1990; p 37. (14) Philips, M. C.; Cadenhead, D. A.; Good, R. J.; King, H. F. J. Colloid Sci. 1971, 37, 437.
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region essentially all the molecules exist as antiparallel pairs. The molecular conformation is given in Figure 9d. The calculated Na from eq 5 for Ad ) 110 Å2 and Au ) 25 Å2 is plotted in Figure 8. The phase transition is associated with some NO2 groups being “squeezed out of the water surface”, which is associated with a breakdown of hydrogen bonding between the NO2 group and the water surface. When the area is between 30 and 25 Å2, the pressure rises very rapidly and the SHG signal becomes very small. In this phase all molecules are antiparallel coupled and close packed, as shown in Figure 9e. IV. Conclusions The SHG measurements provide us with unique molecular orientation information on the film, which can be
Zhang et al.
used to interpret the various phase transitions. For the NPP monolayer in the liquid phase, it is suggested that the molecules are in an inverted-U conformation; in the solid phase the molecules are argued to be in the antiparallel paired conformation. The phase transition between the two phases is associated with the NO2 groups of some of the molecules being “squeezed out” from the water surface. Our molecular conformation model is consistent with both the molecular area conservation law and various known molecular interaction energies. Acknowledgment. This work was supported by the NSF under Grant DMR8706456 and the Department of Energy under Grant DE-FG02-84ER45125. LA960157Y
