J. Phys. Chem. 1992, 96, 8982-8988
8982
they suggested other complexation products such as a dimer of the 1:l complex formed with or without a surfactant monomer. Another explanation has invoked a 3: 1 complex. We do not believe that such complexes are formed in this work. For example, if we plot the ratio (bound cyclodextrin)/(bound surfactant) against free cyclodextrin as shown in Figure 5 it is clear that the maximum value of this ratio is always less than 2 thus indicating that only 1:1 and 1:2 complexes are formed.
Acknowledgment. W.M.Z.W.Y. thanks the UK Commonwealth Scholarship Commission for a Commonwealth Fellowship. We also thank Unilever Ltd. for financial support and a maintenance grant (J.T.). Registry No. SDS-aCD (1:2), 143545-39-5;SDS-@CD (1:2), 143545-40-8;DTAB-aCD (1:2), 143547-76-6;DTAB-@CD ( l : l ) , 143509-50-6.
References and Notes (1) Bender, M. L.; Koyugana, M. Cyclodextrin Chemistry; SpringerVerlag: New York, 1987. (2) Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982;pp 194-201. (3)Griffiths, D. W.; Bender, M. L. Adu. Catal. 1973, 23, 209. (4)Cramer, E.;Saenger, W.; Spatz, H-Ch. J . Am. Chem. SOC.1967.89, 14. ( 5 ) Okubo, T.; Kitano, H.; Tse, N. J . Phys. Chem. 1976, 80, 2001. (6) Satake, I.; Ikenoue, T.; Takeshita, T.; Hayakawa, K.; Maeda, T. Bull. Chem. SOC.Jpn. 1985,58, 2146. (7)Satake, I.; Yoshida, S.; Hayakawa, K.; Maeda, T.; Kusomoto, Y. Bull. Chem. SOC.Jpn. 1986, 59,3991.
(8) Hersey, A.; Robinson, B. H.; Kelly, H. C. J . Chem. Soc., Faraday Trans. I 1986, 82, 1271. (9) Georges, J.; Desmettre, S. J . Colloid Interface Sei. 1987, 118, 192. (10)Palepu, R.; Reinsborough, V. C. Can. J . Chem. 1988, 66, 325. (1 1) Jobe, D. J.; Verrall, R. E.; Palepu, R.; Reinsborough, V. C. J . Phys. Chem. 1988, 92, 3582. (12)Palepu, R.; Richardson, J. E.; Reinsborough, V. C. Langmuir 1989, 5, 218 (13)Okubo, T.; Maeda, Y.; Kitano, H. J . Phys. Chem. 1989, 93,3721. (14)Saint Aman, E.;Serve, D. J . Colloid Interface Sei. 1990, 138, 365. (15) Park, J. W.; Song,H. J. J . Phys. Chem. 1989, 93,6454. (16)Smith, V. K.; Ndou, T. T.; de la Pena, M. A.; Warner, I. A. J . Inclusion Phenom. Mol. Recognit. Chem. 1991, 10, 471. (17)Lavandier, C. D.; Pelletier, M. P.; Reinsborough, V. C. Aust. J . Chem. 1991, 44,457. (18)Palepu, R.; Reinsborough, V. C. Can. J . Chem. 1989, 67, 1550. (19)Takisawa, N.; Hall, D. G.; Wyn-Jones, E.; Brown, P. J . Chem. Soc., Faraday Trans. I 1988.84, 3059. (20)Thomason, M. A.; Mwakibete, H.; Wyn-Jones, E. J . Chem. Soc., Faraday Trans. 1990,86, 15 1 1 . (21) Mwakibete, H.; Bloor, D. M.; Wyn-Jones, E. J . Inclusion Phenom. Mol. Recomit. Chem. 1991. 10. 497. (22) P a k e r , D. M.; Hall, D.’G.; Wyn-Jones, E. J . Chem. SOC.,Faraday Trans. I 1988, 84, 773. (23)Takisawa, N.;Brown, P.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J . Chem. Soc.. Faradav Trans. I 1989. 85. 2099. (24) Kelly, G.; Takkawa, N.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J . Chem. Soc., Faraday Trans. 1 1989,85, 4321. (25) Davidson, C.J. Ph.D. Thesis, University of Aberdeen, 1983. (26) Harata, K. Bull. Chem. SOC.Jpn. 1916, 49, 2066. (27)Harata, K. Bull. Chem. SOC.Jpn. 1976, 49, 1493. (28)MacPherson, Y. E.;Palepu, R.; Reinsborough, V. C. J . Inclusion Phenom. Mol. Recognit. Chem. 1990, 9, 137. (29)We thank the reviewer for this suggestion.
Light-Induced Bubble-Stripe Transitions of Gaseous Domains in Porphyrin Langmuir Monolayers Mitsuru Yoneyama,* Akiteru Fujii, Shuichi Maeda, and Tetsuo Murayama Mitsubishi Kasei Corporation, Research Center, 1000 Kamoshida-cho, Midori- ku, Yokohama 227, Japan (Received: April 27, 1992; In Final Form: August 3, 1992)
We report the observation of bubble-to-stripe shape transitions of gaseous domains in Langmuir monolayers of 544-Ndodecylpyridiniumy1)-10,15,20-tri-ptolylporphyrinmixed with arachidic acid or 4-octyl-4’-(3-carboxytrimethyleneoxy)ambemne at the air-water interface. Monolayer textures are visualized by fluorescence microscopy with excitation of the porphyrin molecules by a focused beam. The same light is also utilized to expand the monolayer locally to induce shape transitions. Under continuous illumination, gaseous bubble domains emerge in the monolayer, which grow in size and then suddenly elongate into stripe structures at specified subphase pH. The critical bubble size characterizing the transition is found to increase with increasing pH, indicating the importance of electrostatic interactions in favoring stripe patterns.
Introduction Langmuir monolayers at the air-water interface afford a rich variety of model systems in which fundamental properties of two-dimensional pattern formation can be studied.’V2 The nature of patterns, i.e., lateral distributions of finite domains corresponding to solid, liquid, or gas, depends on the particular film-forming molecules and also on the thermodynamic-state variables. These nonuniformities can be easily visualized by fluorescence microscopy,) making it possible to correlate the growth, shapes, and sizes of domains directly with experimental conditions such as surface pressure, subphase temperature, and pH. Up-to-date various complex domain structures have been observed and characterized with fluorescence m i c r o s ~ o p y . ~Es-~~ pecially interesting is the observation of shape transitions between circular and noncircular domains. A host of theoretical works have focused on these shape transitions, ranging from simple transitions such as circular to elliptical domain^^^-^^ and square to rectangular domainsZoto more generalized transitions between regularly undulating shapes.2i*22Also, related theoretical descriptions have been developed that deal with phase transitions
between different infinite arrays of two-dimensional domain^.^^-^ All of these models build the physical mechanism for organization of domains on a combination of short-range attractions and long-range dipolar repulsions. Thus, they can be applied in principle to a wide variety of domains regardless of the nature of their phases. Most of the noncircular domains that have been experimentally observed have been concerned with s~lid-liquid~~~ or liquid-liquid13J6phases; it is only recently that the existence of elliptical or stripe structures in liquid-gas coexistence regions has come to our kn~wledge.~’-~~ Broadening the compass of monolayers that can show anisotropic gaseous domains should therefore facilitate a systematic characterization of domain shape transitions as well as a more profound understanding of the underlying physics and chemistry. In a previous paper,29we reported briefly the formation of gaseous stripe patterns in Langmuir monolayers composed of a surface-active pyridiniumylporphyrin and arachidic acid over a narrow range of subphase pH. There, the monolayer contained an excess amount of arachidic acid, leading to the three-phase
0022-365419212096-8982%03.00/00 1992 American Chemical Society
Bubble-Stripe Transitions of Gaseous Domains
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 8983
coexistence, consisting of solid arachidic acid domains and liquid-gas regions of porphyrin:arachidicacid mixtures. The present paper focuses on the observation of critical shape transitions in two-phase regions of mixed monolayers, where gaseous domains are surrounded by liquid phase. The system examined is a binary mixture of 5-(4-N-dodecylpyridiniumyl)-l0,15,20-tri-p-tolylporphyrin and fatty acid. The monolayer is exposed to illumination & 20 of a focused beam, which allows fluorescence microscopic observations of the film textures and simultaneously a local expansion of the monolayer to induce shape transitions. We will show that 50 100 150 200 0 the evolution of stripe patterns from gaseous circular domains is Area per Porphyrin Molecule ($2) clearly observed and discuss its characteristic features such as dependence on subphase pH in terms of the theoretical model^.^^,^ Figure 1. r-A isotherms of a P - C , ~ P Y T T P : C(1:l) ~ ~ monolayer
$ t1
Experimental Section 5-(4-N-Dodecylpyridiniumyl)10,15,2O-tri-p-tolylporphyrin (pC,,PylTP) was synthesized as the bromide in our laboratory. Arachidic acid (C20) and 4-octyl-4'-(3-carboxytrimethylene0xy)azobenzene (8A3) were purchased from Fluka Chemie AG. and Dojin Kagaku Co., respectively, and used without further purification. Stock solutions of the porphyrin and the acids were prepared in spectroscopic grade chloroform with a concentration of 1.O X 1O-g M and then mixed. Monolayers were formed by spreading the mixed solutions onto a water surface of a Teflon-coated aluminum trough (25 X 10 an)mounted on the stage of a fluorescence microscope (Olympus BHMS). The water for the monolayer substrate was purified in a Milli-Q system, and its pH was adjusted by the addition of HCl and KHCOg. The trough was equipped with a motorized Teflon barrier, allowing the measurements of surface pressurearea (FA) isotherm of the monolayers. Surface pressures were measured with a filter paper Wilhelmy plate and are accurate to within 0.1 mN m-I. The water temperature was regulated by circulating thermostated water in the hollow aluminum trough. All of the monolayer studies here were made at 23 f 1 "C. Visible polarized absorption spectra of the monolayers were measured by a multichannel spectrophotometer equipped with optical fiber probes (MCPD-110, Otsuka Electronics). Transmission spectra were obtained: the polarized light traversing the monolayer was reflected by a mirror under the water surface and detected by a fiber probe following a second pass through the monolayer. The incident angle was 45O for both s-polarized light (electric vector of the light normal to the plane of incidence) and ppolarized light (electric vector of the light in the plane of incidence). Domain structures in the monolayers were observed by fluorescence microscopy. The porphyrin molecules were excited into the Soret band by light from a 100-W mercury lamp which was focused on the monolayer through an objective (Olympus MSPlanSO). The fluorescence image was separated from the exciting light with a dichroic mirror and viewed with a SIT television camera and was recorded on videotape. In the mixed monolayers in which the molar ratio of the acid was larger than that of the porphyrin, the excess amount of the acid tended to form crystalline domains29even in expanded states (-0 mN m-I). To avoid such inhomogeneity and to obtain well-mixed fluid monolayers, we employed equimolar mixtures (porphyrin:acid = 1:1) throughout the experiments. Results and Discussion Molecular Organization of Mixed Mooolayers. Figure 1 shows the r-A isotherms of p-C12PyTTPC20and p-CI2PyTTP:8A3 monolayers on pure water (pH 5.6) plotted as surface pressure against area of p-C12PyTTPmolecule. The isotherm of a pure P C , ~ P ~ T Tmonolayer P is also plotted for comparison. Essentially the same isotherms are obtained over the pH range that we have studied, Le., pH 3.7-6.6 for p-C12PyTTP:C20and pH 3.5-6.6 for p-C12PyTTP:8A3.By extrapolating the steeply rising part of the curve to zero pressure, the area occupied by a porphyrin molecule can be determined, namely, 110 A2 for purepCI2PyTTPand 147 A2 for p-C12PyTTP:C20and p-C12PyTTP:8A3. These values are smaller than the planar area of the porphyrin ring for which value
I\ I
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L
(-,
curve a), p-CI2PyTTP:8A3 (1:l) monolayer (-, curve b), and pure on pure water (pH 5.6, 23 "C). p-C,,PyTTP monolayer (.e.)
near 200 A2 are measured.)' In the case of the pure porphyrin monolayer, the low molecular area is attributable to a large tilt of the porphyrin chromophore on the water surface with the ring face to face with each other. Such an arrangement was indeed found32 for monolayers of 5-(4-N-tetradecylpyridiniumyl)10,15,20-tri-p-tolylporphyrin(p-C14PyTTP)and may be typical of pyridiniumylporphyrin substituted with one long aliphatic chain. The measured molecular area is shifted to higher values by the addition of the acid components, but the values are independent of the nature of the acid (Cz0or 8A3). Since C20and 8A3 have different cross sectionsg3in pure monolayer form at the air-water interface (20 and 28 A2, respectively), the coincidence of the molecular areas suggests a monolayer structure in which molecular packing is determined mainly byp-C12PyTTP. Griiniger et aLg4 studied the molecular organization in mixed monolayers containing tetra-3-eicosylpyridiniumylporphyrin,arachidic acid, and methyl arachidate by r-A isotherm and reflection-absorption measurements. They found that the porphyrin ring was oriented parallel to the water surface and that the accessible space on top of the flat chromophore was filled with the lipid molecules. We believe such an arrangement to be realized in our mixed monolayers from the following additional experimental facts: (1) Polarized absorption measurements reveal larger absorption for s-polarization compared to ppolarization. Figure 2 shows the typical spectra measured at 160 A2 on pure water. The spectral features are independent of the subphase pH employed in our experiments. The ratio of the optical densities for s-polarization to p-polarization is about 1.55 at absorption maximum for both mixtures. Orrit et al.g5developed a procedure for quantitating the angular distribution of transition dipoles in monolayer and multilayer assemblies from polarized absorption spectra. They treated in particular the reflection from a monolayer on a water surface as well as the transmission through a monolayer on a glass substrate. By a detailed recalculationg2based on their method for the situation in the present paper, i.e., the transmission with a double pass through a monolayer at the air-water interface, the value of 1.55 is found to correspond to a nearly flat orientation of the porphyrin ring on the water surface. This orientation would be compatible with the measured molecular area (- 147 A,) if we assume a slight overlapping of the porphyrin plane with respect to each other as schematically illustrated in Figure 3. Thus,each porphyrin molecule provides as a free space an area of 127 A2, corresponding to the difference between the molecular area and the cross-sectional area required by the long alkyl substituent (-20 A2);this space is sufficient enough to accommodate a single C20 or 8A3 molecule. (2) It is found that photoisomerization of 8A3 causes no distinct change in the P A isotherm. For example, a pure 8A3 monolayer, when irradiated with UV light (360 nm, 1 mW cm-,) for 2 min, exhibits a significant expansion due to trans-cis isomerization of the azobenzene moieties, the limiting molecular area increasing from 28 to 35 A2. The isotherm of the p-CI2PyTTP:8A3 monolayer, however, is unaffected by the same extent of irradiation, although the Occurrence of a clear structural change from trans to cis isomer is confirmed by absorption measurements. This behavior constitutes another strong support for the existence of
8984 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992
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Figure 2. Polarized absorption spectra of mixed monolayers at a molecular area of 160 AZ: (a, top) p-C12PyTTP:C2p( 1 : l ) ; (b, bottom) p-C12PyTTP:8A3(1:l). The corresponding polarizations (s or p) are indicated for each spectrum.
P-c,
*m
C,, or 8A3
Figure 3. Schematic illustration of the molecular organization for mixed monolayers. The acid molecules (C20or 8A3) fill the space above the chromophore of P - C , ~ P ~ T T P .
a free space around each 8A3 molecule, allowing its efficient photoisomerization in mixed monolayer systems. Bubble-Stripe Transitions in Mixed Monolayers. From the molecular organization deduced from the previous section (Figure 3), we can anticipate that nearly perfect miscibility would be achieved in p-CizPyTTP:CzOand p-ClzPyTTP:8A3 monolayers in liquid phase. This is indeed confirmed by fluorescence microscopic observation: a uniform bright field is obtained for both mixtures persistently at high surface densities where the pressure deviates from zero. As the density is decreased by film expansion to an area per p-C12PyTTPmolecule >160 A2;small dark spots appear randomly in the bright background. These regions are apparently low-density two-dimensional gaseous bubbles in the surrounding higher density liquid. Moreover, if the subphase pH is adjusted within a specific range, the bubbles grow with further expansion to a certain critical size beyond which they turn quite abruptly into stripe structures. This shape transition can be monitored in experiments in which the monolayer is expanded by the movable barrier several hours after the monolayer has been formed. We have found, however,
Yoneyama et al. that it is also possible and much easier to track successive shape changes of individual domains by means of a local expansion technique, which utilizes thermal expansion of the monolayer caused by continuous illumination with the exciting light; the illuminated portion of the monolayer self-expands at the cost of the light energy absorbed by p-ClzPylTP. Figure 4 shows the typical fluorescence images obtained in this way for pCi2PyTTPC20monolayers at various pH. The arrow in each micrograph indicates a gaseous bubble at its critical size of the transition. In Figure 5, the measured diameter (D) of the critical bubble is plotted as a function of pH to clarify the pH dependence. It is evident from these results that the critical size increases monotonously as the pH increases. The shape transitions are clearly observed in a narrow pH range (pH 3.7-5.5); at pH 5.5 the transitions do not occur because the bubbles can not grow beyond their critical size due to repulsion from neighbors. The marked pH dependence indicates the importance of electrostatic interactions between the film-forming molecules in producing the bubble-stripe changes as will be seen in detail later. Similar pH-dependent transitions are found for p-Ci2PyTTP:8A3 monolayers as shown in the microscope images in Figure 6 and the D vs pH plot in Figure 5, the principal difference from the Cz0 mixture being an increase in D at the same pH. Throughout this series of observations, the microscope stage was slowly moved in order to illustrate the evolution from bubble to stripe structures in a single picture. Thus, for the micrographs in Figure 4 for example, the monolayer moves upward at a constant speed, traversing the illuminated spot inside the octagonal area in about 5 s. This leads to a larger light-induced reduction in the surface density in the upper part of the monolayer, resulting in the seemingly inhomogeneous texture with the coexistence of various bubbles and stripes. We have also performed measurements of illumination on a fixed portion of the monolayer. As an example of the domain growth obtained in such measurements, the progress of microscope images is depicted for a p C,2PyTTPC20monolayer at pH 4.68 in Figure 7. A fairly homogeneous texture is observed, while the criticle size remains unchanged compared to the inhomogeneous case at the same pH (see Figure 4c); hence the critical size estimated from Figure 4 or Figure 6 may well represent a characteristic quantity of the system, unaffected by the inhomogeneous appearance. This circumstance presumably arises from relatively uniform surface density achieved around the bubble domains on a length scale comparable to their size. As discussed later, we are not in a position to make quantitative analyses of our experimental data at the present stage; however, it is possible to interpret the observed shape transitions qualitatively in terms of a general treatment of domain shape changes by McConnell and c o - w o r k e r ~ . ' ~They - ~ ~ have focused on shape changes of solid lipid domains from circular to noncircular structures on monolayer compression and developed a theory describing such instability based on a competition between line tension and electrostatic dipoler repulsions. Their calculations have shown that, when in-dipole components of the film-forming molecules can be neglected, an isolated circular domain undergoes a transition to an ellipse at the diameter where X is the isotropic line tension and p is the difference in molecular dipole density between the solid and liquid phases. This formula signifies the sensitivity of the critical size to factors affecting p such as subphase pH and may be therefore suited to explain the sort of transition observed here. As seen in Figure 4 and Figure 6, the circular domains are not very concentrated and well approximated as isolated from each other. In our case, however, we should note that it is the gaseous domains instead of solid domains that undergo shape changes and that the changes are caused by film expansion instead of film compression; on expanding the monolayer, a certain number of gaseous circular domains appears either by nucleation or by spinodal decomposition.
Bubblestripe Transitions of Gaseous Domains
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 89%5
c.
Figure 4. Fluorescence microscope pictures of pCI2PyTTPC2,,(1:l) monolayers taken under continuous illumination at various pH: (a) pH 4.35; (b) pH 4.50; (c) pH 4.68; (d) pH 5.05. The monolayer was expanded to an average molecular area of about 165 A2 before starting illumination. The microscope stage was slowly moved in a horizontal direction to scan the monolayer surface during the observation. The arrow in each picture indicates a gaseous domain at the onset of bubble-to-stripe transition (critical bubble). Each bar is 50 pm.
Figure 5. Experimental plot of the diameter D of critical bubbles versus subphase pH for p-C12PyTTPC20(1:l) monolayers ( 0 ) and pC12Py'ITP8A3(1:l) monolayers (0).The uncertainty on D is around *0.2 pm.
On further expansion, these bubbles progressively grow in size until they reach the transition diameter described by eq 1, at which the sharp transition to stripe structures take place. The pH dependence of the critical size can be ascribed to a pHdependent electrostatic nature of the film-forming molecules, especially Cm and 8A3. In fact, the dipole moments for fatty acid monolayers normal to the air-water interface are known36*37 to increase almost linearly with decreasing pH from surface potential measurements. Thus, if the pH dependence of p is assumed to come mostly from that of C20or 8A3 in the mixed monolayers, increasing pH leads to decreasing p and eventually to increasing critical size in accord with the observed features in Figure 5. The
different pH dependence between p-C12PyTTPC20and pC12PyTTP8A3is thought to be the result of differences in p and/or h under the same experimental conditions; we cannot decide which factor plays a more important role at the present stage. Evolutionof Stripe Patterns. The thcbry by McConnell and ~ o - w o r k e r spredicts ' ~ ~ ~ ~ that after the transition point the domain width initially shows a sharp decrease and then gradually a p proaches an asymptotic value while the length increases as the area becomes large with film expansion. This behavior is indeed found in our experiments as noted in Figure 4 and Figure 6. At the beginning of the elongation when the fraction of the gaseous phase is not large, the gaseous domains extend straight, but as their tips get closer to the boundaries of other domains the domain-domain interactions begin to impede their free extension; the domain tips then search for other directions to continue their growth, leading to serpentlike structures. At this stage we obtain two-dimensional finite arrays of twisted stripes, and their characteristic features such as stripe width and lattice period could be more precisely analyzed by theoretical calculations including long-range electrostatic repulsions between different domains.2s26 When there is no more space left for domain elongation, the gaseous stripes grow in area by thickening on further expansion of the monolayer. The series of micrographs in Figure 8 illustrates an example of the successive deformation of the stripe patterns. The pictures are of the same field of the p-C12PyTTPC20 monolayer at pH 4.50 taken under continuous illumination with the exciting light. As the thickening proceeds, the gaseous regions develop into polygonal cells that are separated from each other by continuous liquid strips, leading to a foamlike structure3** (Figure 8d). Once the foam is formed, the growth of the cells slows under illumination presumably because the fraction of the
Yoneyama et al.
0986 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 I
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Figure 6. Fluorescence microscope pictures of p-C,2PyTTP:8A3 (1:l) monolayers taken under continuous illumination: (a) at pH 3.75; (b) at pH 4.12. The monolayer was expanded to an average molecular area of about 160 A2before starting illumination. Other experimental conditions and notations are the same as in Figure 4.
liquid phase that absorbs light becomes too small to bring efficient light-induced expansion of the monolayer. The domain growth is then begins to be governed by two elementary processes (TI and T241)typically seen in two-dimensional foam evolution. Reliability of tbe Light-Induced Expansion Metbod. We have demonstrated that the light-induced local expansion provides a convenient way of following the evolution of individual gaseous domains. This method is based on the LeChatelier-Brown principle: the light energy absorbed by pC12PyTTPis partly converted into thermal energy, which is in turn consumed by self-expansion of the monolayer. There is an inherent disadvantage in this method, however, of being unable to control precisely the locally achieved values of the experimental parameters such as final temperature and surface density in the monolayer, making certain kinds of quantitative results difficult to obtain. In this section, we discuss the effects of such impreciseness. We have confirmed that the subphase temperature remains constant at 23 f 1 "C just under the focused portion of the monolayer throughout the microscopic observations, but we have not been able to measure exact temperature of the monolayer itself, which may be influenced to some extent by irradiation. Temperature perturbation is likely to affect thermodynamic properties of monolayer systems so that it is necessary to examine its effect on the domain shapes. We have attempted to estimate the effect by observing highly expanded monolayers at the onset of shape transitions under illumination with less intense light. In this way it is possible to trigger the shape changes immediately after the monolayer is irradiated, minimizing light-induced temperature changes. As an example, the fluorescence image of a pC,2PyTTp:C20monolayer at pH 5.05 is shown in Figure 9. The monolayer was expanded before irradiation to an average mo-
\ % -
Figure 7. Progress of fluorescence microscope images for a p C,2PylTPCm(1:l) monolayer at pH 4.68. The images are of the same field under continuous illumination. The arrow indicates a critical bubble. Each bar is 50 pm.
lecular area of about 210 A2, and the light intensity was reduced to one-tenth of that for obtaining Figure 4d. It is evident from a comparison between Figure 4d and Figure 9 that the size of the critical bubble and the nature of its evolution are not noticeably influenced by the light intensity. Similar features are always observed over the pH range studied and also for pC12PyTIP8A3 mixtures, indicating that temperature perturbation, if any, has little effect on the shape transitions under the experimental conditions employed here. These results also eliminate the possibility of other types of light-induced side effects such as photochemical effects which are found to produce domain elongation in liquid monolayers by Rice and McC0nne1l.I~ The surface density in the observed portion of the monolayer varies continuously under illumination. This is not a serious problem as long as the instability of isolated domains is concerned because the domain size parameters do not depend explicitly on surface density, as predicted by the theoretical treatmenti9*20 (see eq 1). On the other hand, one needs to know experimental values relating to surface density in order to discuss thermodynamic properties of bubble or stripe arrays involving interdomain repulsions. In our case, the area fractions of the liquid and gaseous phases may be directly obtained by measuring the fractions of the bright and dark areas in the micrographs respectively, using image analysis. By combining such an analysis with careful PA isotherm measurements, one could make definitive comparisons of the experimental results with theoretical calculation^.^^-^^ As discussed so far, the light-induced expansion method can be safely applied to studies of domain shape transitions, the results being unaffected by the accompanying uncertainties in some experimental parameters. Here, we should stress that the shape changes have been generated by expansion of the monolayer, not by the action of the light. One might then expect that the
Bubble-Stripe Transitions of Gaseous Domains
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 0987 "1IJ c
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Figure 8. Evolution of a foam structure from a gaseous stripe phase for a pC,2PyTTPCm (1:l) monolayer at pH 4.50. The images are of the same field under continuous illumination. It took about 6 s to change from (a) to (d). The bar in (d) is 50 pm and indicates the scale of all pictures.
Figure 9. Fluorescence microscope picture of a p-C,2PyTTP:Cm(1:l) monolayer at pH 5.05. The monolayer was expanded to an average molecular area of about 210 A2before starting illumination. The light intensity was reduced by a ND filter to one-tenth that for taking Figure 4. This particular picture was taken immediately after illumination. The arrow indicates a critical bubble, and the bar is 50 pm.
transition would be most readily produced by a mechanical expansion from the liquid phase to the liquid-gas coexistence region. Indeed, we have confirmed the Occurrence of a clear shape transition via movement of the barrier with keeping the exciting light at the weakest possible power to minimize light-induced expansion. This approach would be extremely useful if the density of the monolayer could be varied uniformly over the entire surface so that the observed phase behavior could be directly related to average surface densities. It is found, however, that the monolayer tends to show a heterogeneous texture in the liquid-gas region;
in one part of the trough one may see all gas, in another part, nearly all liquid. Moore et a1.42 have reported the observation of similar inhomogeneity in pentadecanoic acid monolayers, which forced them to scan the entire surface of the trough to get representative features at a fixed average density. We therefore believe it more realistic to use the light-induced expansion method in studying the evolution of gaseous domains. Several workers20*2'*26 have argued that domain elongation is a consequence of nonequilibrium domain growth, one that may be most relevant to actual experiments. We have also observed a deviation from global equilibrium in our monolayers; once the gaseous domains are formed, further monolayer expansion leads to the growth of the domains rather than the formation of new gaseous domains, indicating that the domains can be in a metastable state with respect to their shape but they are not in equilibrium with respect to their number. The total number of gaseous domains is determined by nucleation at the early times of monolayer expansion. This is experimentally hard to control but clearly depends on the mode of expansion as can be seen by comparing the pattern shown in Figure 4c with the pattern shown in Figure 7b. When the monolayer is expanded uniformly, the domains appear randomly distributed in the field, and they are so concentrated that their elongation stops soon due to repulsions from neighbors (Figure 7b). Typically, the stripes cannot lengthen beyond 50 pm. In contrast, the domains are smaller in number and much more extended when the expansion is not uniform (Figure 4c). We sometimes observe stripes of length >200 pm. In such cases, the tips of the stripes initially formed are thought to provide nucleation sites for less expanded regions, allowing efficient expansion of the liquid phase via elongation of the seed stripes without much help of the formation of new domains.
Conclusions We have found reproducible shape transitions from bubbles to
8988 The Journal of Physical Chemistry, Vol. 96, No. 22, I992
s t r i p in mixed monolayers composed of porphyrin and fatty acid on film expansion under continuous illumination. The critical bubble size for the transition is quite sensitive to the subphase pH, a demonstration of the importance of electrostatic interactions in stabilizing elongated domains. The shape transition prescnted here is not a unique feature that is restricted to the particular monolayer systems examined. For example, our preliminary investigationsusing stearylamine instead of the acid components show that a sharp bubblcstripe transition does take place when the subphase pH is increased to about 10. This suggests that shape changes are widely observable in any types of charged monolayers by tuning electrostatic forces via subphase pH. Such universality supports strongly the appeal of a description of domain shape instabilities in terms of the fairly simple picture of competing interactions.'e26 In the present paper, we have not carried out quantitative explanations of our shape changes. There remain two problems to be solved before making a thorough analysis of the experimental results. First, the dipole moment normal to the interface must be experimentally obtained as a function of pH from surface potential measurements; this will help to correlate the critical bubble size directly with the dipole moment by using eq 1. Second, proper estimation must be given to the effects of in-plane dipoles. It has been suggested by Heckl and M6hwald5 that a sudden appearance of in-plane components of the polarization can give rise to the shape transition in their dimyristoylphosphatidicacid monolayers. If such a mechanism is applied to the sharp transition presented hen, the transition should be accompanied by an abrupt, oooprative change in molecular tilt order, which scans improbable for gaseous domains. Actually, we have encountered no positive evidence of anisotropic orientational order of the porphyrin chromophores in the liquid phase; they are found to have no preferential orientation from absorption measurements and microscopic measurements using polarized illumination. For drawing decisive conclusions, however, it should be clarified whether or not the aliphatic chains of the film-forming molecules have ferroelectric long-range tilt order. Bubblestripe shape transitions can in principle also be brought about by changes in the ratio X/p2 in eq I. Furthermore, a sudden change in this ratio may achieve favorable situations for other class of shapes with higher harmonic to develop. Thus, it is of experimental interest to search for a means to vary the dipole density difference I.L or the line tension X in a controlled manner. McConnel122has proposed the use of short, focused light pulses to induce a photochemical transformation that changes the value X/p2 for a single circular domain. Another attractive approach is through the incorporation of photochromic materials as film-forming molecules that undergo reversible structural changes. In such system, one can directly alter the charge density or polarization of the monolayer by illumination and thus produce a reversible, light-controlled shape transition. It is therefore anticipated that our monolayer containing the azobenzene derivative 8A3 will exhibit shape changes by irradiation with the light that causes trans-cis isomerization. This is one focus of our work currently underway and will be the subject in a forthcoming paper.
Yoneyama et al.
Acknowledgment. This work was performed under the management of FED (the R&D Association for Future Electron Devices) as a part of the RCD of Basic Technology for Future Industries supported by N E D 0 (New Energy and Industrial Technology Development Organization).
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