Photoinduced Phase Separation and Miscibility in the Condensed

Stabilization of Langmuir monolayer of hydrophobic thiocholesterol molecules. Raj Kumar Gupta , K.A. Suresh. Colloids and Surfaces A: Physicochemical ...
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Langmuir 2004, 20, 8149-8154

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Photoinduced Phase Separation and Miscibility in the Condensed Phase of a Mixed Langmuir Monolayer P. Viswanath and K. A. Suresh* Raman Research Institute, Sadashivanagar, Bangalore 560 080, India Received March 30, 2004. In Final Form: June 7, 2004 We report our studies on the mixed Langmuir monolayer of mesogenic molecules, p-(ethoxy)-p-phenylazo phenyl hexanoate (EPPH) and octyl cyano biphenyl (8CB), employing the techniques of surface manometry and Brewster angle microscopy. Our studies show that the mixed monolayer exhibits higher collapse pressures for certain mole fractions of EPPH in 8CB as compared to individual monolayers. Also, a considerable reduction in the area per molecule is seen in the mixed monolayer, indicating a condensed phase. We have also studied the photostability of the mixed monolayer at different initial surface pressures. The mixed monolayer, under alternate cycles of UV and visible illumination, exhibits changes in surface pressures. This is due to the photoinduced transformation of EPPH isomers in the mixed monolayer. Our in-situ Brewster angle microscope studies for 0.5 mole fraction of EPPH in 8CB show a phase separation in the UV and a miscible phase in the visible, at low surface pressures (∼5 mN/m). At higher surface pressures (∼10 mN/m), under UV illumination, we find a phase separation which does not revert to a miscible phase under visible illumination.

1. Introduction Mixed Langmuir monolayer studies are useful in obtaining a stable monolayer of molecules that possess functional properties. Here, an amphiphilic molecule is mixed with a nonamphiphilic molecule possessing functional properties to obtain stable monolayers. Such stable monolayers can be transferred to substrates by the Langmuir-Blodgett (LB) technique for developing sensors.1 Studies on nonamphiphilic dichroic azo dye mixed with mesogenic amphiphilic molecules have been reported to form stable monolayers.2 Mixed monolayer studies on mesogenic molecules with donor-acceptor character leading to charge-transfer interactions have been reported to stabilize the system.3 These studies indicate that the shape of the molecules and their interactions play an important role in the stabilization of the mixed monolayer. Photoactive azo derivatives have been used as probes to study the structural changes in the azo-lipid mixtures at the air-water (A-W) interface.4 Recent studies on LB films show photocontrollable phase separation in mixtures of an azo polymer and a mesogenic molecule.5 They act as model systems to understand and control the orientation of molecules at the interfaces. In this paper, we report our miscibility and photostability studies on the mixed monolayer of mesogenic molecules. We have used a donor azo dye, p-(ethoxy)-pphenylazo phenyl hexanoate (EPPH), with an acceptor molecule, octyl cyano biphenyl (8CB). The mixed monolayer exhibited a stable condensed phase. Interestingly, under UV illumination, below the collapse pressure of 8CB, we find photoinduced phase separation. Under visible illumination, we find miscibility yielding a condensed phase. However, above the collapse pressure of 8CB, we find phase separation under UV illumination, which did * Corresponding author. E-mail: [email protected]. (1) Santos, J. P.; Zaniquelli, M. E. D.; Batalini, C.; De Giovani, W. F. J. Phys. Chem. B 2001, 105, 1780. (2) Martynski, T.; Biadasz, A.; Bauman, D. Liq. Cryst. 2002, 29, 281. (3) Ohlmann, A.; Rettig, W.; Diele, S.; Kuschel, F.; Weissflog, W. Thin Solid Films 1991, 199, 181. (4) Xu, X.; Iwamoto M. Jpn. J. Appl. Phys. 1997, 36, 7348. (5) Ubukata, T.; Ichimura, K.; Seki, T. J. Phys. Chem. B 2003, 107, 13831.

not revert back to a miscible phase under visible illumination. 2. Experimental Section The materials p-(ethoxy)-p-phenylazo phenyl hexanoate (Eastman Kodak) and octyl cyano biphenyl (Aldrich) were obtained commercially. EPPH was recrystallized using ethanol as a solvent, and 8CB was used as procured. The EPPH molecule is rodlike in the trans state with a core length of about 9 Å, and it is bent in the cis state with a core dimension of 5.5 Å.6 In the cis state, the length of the molecule decreases, and the effective area increases. The 8CB molecule has a rodlike structure (see Supporting Information). The surface manometry experiments were carried out using a NIMA 611M trough. The subphase used was Millipore water (resistivity > 18.2 MΩ cm, pH ≈ 5.7), the temperature of which was maintained at 28 ( 1.0 °C. Stock solutions of 1.5 mM concentration of EPPH and 8CB in chloroform were used to prepare the mixtures. The solution was spread to form a monolayer using a microsyringe (Hamilton) and was equilibrated for about 10 min to allow the solvent to evaporate. The monolayer was compressed at the rate of 1.1 (Å2/molecule)/ min. The surface pressure (π)-area per molecule (A/M) experiments were carried out in a dark room for the mixed monolayer. We have used MiniBAM (NFT-Nanofilm Technologie) for our Brewster angle microscope (BAM) imaging. To study the stability of the mixed monolayer, we have measured the change in the area per molecule with time at constant surface pressure in a dark room. To probe the photostability of the mixed monolayer, we have used a stand-alone mercury arc lamp (100 W). Appropriate filters (Schott) were used to select the wavelengths, 360 nm to convert the EPPH molecules from trans to cis isomer and 440 nm to revert from cis to trans isomer. The lamp was placed at a distance of 15 cm from the monolayer to reduce heating effects. The intensities at 360 nm and 440 nm were nearly 0.5 mW/cm2 in the vicinity of the monolayer. In-situ BAM studies were carried out to study the photoinduced transformation of the mixed monolayer.

3. Results and Discussion The surface pressure (π)-area per molecule (A/M) isotherms for EPPH-8CB mixed monolayers carried out in a dark room are shown in Figure 1. The EPPH monolayer exhibited a surface pressure of less than (6) Irie, M. Adv. Polym. Sci. 1990, 94, 27.

10.1021/la0491899 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/06/2004

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0.1 mN/m above an A/M of 30 Å2. On compression, the surface pressure increased steeply at an A/M of 29 Å2. The monolayer collapsed at a surface pressure of 15.2 mN/m with a limiting area (A0) of 26 Å2. This is in agreement with the estimated cross-sectional area of the azobenzene chromophore. The π-A/M isotherm for 8CB is in agreement with earlier reports.7,8 For each composition studied, the π-A/M isotherm showed single collapse pressure (Figure 1), indicating a good miscibility between the EPPH and 8CB molecules. The collapse pressure is an important parameter in judging the miscibility and the stability of the mixed monolayer. Figure 2 shows the variation of collapse pressure for the EPPH-8CB mixed monolayer with a mole fraction (MF) of EPPH in 8CB. The value of the collapse pressure increases in the range 0.3-0.7 MF of EPPH in 8CB. Interestingly, the collapse pressures for the mixed monolayers were higher than that of the individual monolayers. The maximum collapse pressure occurred at about 0.7 MF of EPPH in 8CB and was around 25 mN/m. The π-A/M isotherm for the EPPH monolayer showed a spike, that is, a rapid fall in surface pressure after the collapse, whereas for 8CB, the surface pressure showed almost a plateau after the collapse (Figure 1). For the mixed monolayer, the nature of collapse showed a change from a plateau to a spike type with increasing composition of EPPH. Up to 0.7 MF of EPPH, the π-A/M isotherm exhibited a plateau. Such collapse behavior has been reported in systems with dissociated polar heads.9 Although the interactions in our system are quite different, we find a change from a plateau to a spike-type collapse.

The excess area per molecule obtained for the mixed monolayer of EPPH and 8CB indicates attractive interactions between the components (see Supporting Information). The stability of the mixed monolayer was analyzed by calculating the excess Gibbs free energy (∆GEXC), which was obtained by integrating the excess area, AEXC, over surface pressure.10 The ∆GEXC was negative throughout the composition and was minimum at 0.5 MF of EPPH in 8CB (see Supporting Information). Similar ∆GEXC values have been reported in the context of complex formation.10 The Brewster angle microscope images obtained in a dark room for the EPPH monolayer are shown in Figure 3. At large A/M, corresponding to the zero surface pressure region, irregularly shaped 2D crystalline domains (gray) which were less mobile coexisted with the gas phase (voids). The 2D crystalline domains exhibited grainy textures (Figure 4). On compression, we could observe some small three-dimensional (3D) crystallites nucleating even in the steeper region of the isotherm and prior to collapse. This indicated that the EPPH monolayer was crystalline in nature and was not very stable at the A-W interface. In the collapsed state, well-developed 3D crystallites (bright) coexisted with the crystalline monolayer (Figure 4). The microscope images of 8CB monolayer at the A-W interface were in accordance with the earlier studies.7,8,11 We have carried out BAM studies in detail for different compositions of EPPH in 8CB. For the compositions 0.3, 0.5, and 0.7 MF of EPPH, the images corresponding to the steeper region of the isotherm showed a homogeneous texture, indicating good miscibility (see Supporting Information). Also, the images did not show the nucleation of 3D crystallites in this region as seen for the EPPH monolayer, indicating the stability of the mixed monolayer. On the basis of the surface manometry and BAM studies, we suggest that there may be a weak charge-transfer interaction between the donor EPPH and acceptor 8CB molecules in the formation of the condensed complex at the A-W interface. In this context, it should be mentioned that there has been a report on the mixed Langmuir monolayer of mesogenic molecules in which one component was the electron acceptor and the other component was the electron donor.3 In these studies, the individual monolayers which were unstable formed a stable mixed monolayer exhibiting a large reduction in the A/M. Such charge-transfer interactions, in the case of bulk liquid crystalline mixtures, play an important role in enhancing the stability, increasing the range, and in the induction of a new mesophase.12 We have carried out UV-visible spectroscopic studies to detect the complex formation in the bulk EPPH-8CB mixture dissolved in chloroform (concentration 0.07 mM). The spectra showed no shift, in the absorption peak, that would arise due to charge transfer. However, further investigation is needed on the mixed system at the A-W interface and on LB films. Studies on the mixed monolayer of nonamphiphilic dichroic azo dyes with strongly polar 8CB show that the packing of the molecules and the stability strongly depend on the concentration of the azo dyes.2 Our studies on the EPPH-8CB mixed monolayer indicated that the presence of alkyl chains in addition to donor group in EPPH molecules stabilizes the system. The stability of the monolayers in a dark room was studied by monitoring the A/M with time, at a constant

(7) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1994, 10, 1251. (8) Mul, N. G. M. D.; Mann, J. A., Jr. Langmuir 1994, 10, 2311. (9) Angelova, A.; Vollhardt, D.; Ionov, R. J. Phys. Chem. 1996, 100, 10710.

(10) Goodrich, F. C. Proc. Int. Congr. Surf. Act., 2nd 1957, 1, 85. (11) Suresh, K. A.; Bhattacharyya, A. Langmuir 1997, 13, 1377. (12) Praefcke, K.; Singer, D. In Handbook of Liquid Crystals: Low Molecular Weight of Liquid Crystals II; Demus, D., Goodby, J., Gray, G. W., Spiess, H. W., Vill, V., Eds.; Wiley-VCH: New York, 1998.

Figure 1. Surface pressure (π)-area per molecule (A/M) isotherms for the EPPH-8CB mixed monolayer at different mole fractions (MF) of EPPH at a temperature of 28 °C carried out in a dark room.

Figure 2. Variation of the collapse pressure (πc) with the mole fraction (MF) of EPPH in 8CB.

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Figure 3. BAM images for EPPH at different A/M. (a) Irregularly shaped 2D crystalline domains (gray) with gas phase (voids). (b) Collapsed state. Here, bright 3D crystallites coexist with the crystalline monolayer (gray) in the background. The scale of each image is 4.8 × 3.2 mm2.

Figure 4. Variation of the surface pressure (π) with time (t) on alternate illumination with UV (360 nm) and visible (440 nm) radiations for the EPPH monolayer.

surface pressure. This will reveal the depletion of the molecules due to dissolution in the subphase13 or nucleation and growth of 3D structures at the A-W interface.14 For the case of the EPPH monolayer, the A/M decreases rapidly at constant surface pressure. The rate of decrease in A/M was much higher at higher surface pressures (see Supporting Information). This shows the unstable nature of the EPPH monolayer, which can be attributed to the nucleation and growth of 3D crystallites. However, for the EPPH-8CB mixed monolayer, there was no appreciable decrease in A/M with time (see Supporting Information), indicating the stability. We have probed the photostability of the EPPH monolayer at a given A/M, by alternately illuminating it with UV and visible radiations. When illuminated with UV radiation (360 nm), the EPPH molecule, which possesses a photoactive azo moiety, transforms from the trans to the cis isomer. On illumination with visible radiation (440 nm), it reverts to the trans isomer. These transformations led to a change in the surface pressure. The variation in the surface pressure for the EPPH monolayer on alternate illumination with UV and visible radiation with time is shown in Figure 4. In the first cycle, the EPPH monolayer when illuminated with UV radiation leads to a sharp increase in the surface pressure from a value of about 4.5 to 11 mN/m. However, when illuminated with visible radiation, the surface pressure decreases to about 2 mN/m. We find that, for four cycles of illumination, the peak surface pressure decreases from a value of about 11 to about 6.3 mN/m. Correspondingly, the dip also decreases from a value of about 4.5 to 1.4 mN/m. (13) Viseu, M. I.; Silva, A. M. G.; Costa, S. M. B. Langmuir 2001, 17, 1529. (14) Smith, R. D.; Berg, J. C. J. Colloid Interface Sci. 1980, 74, 273.

Figure 5. Variation of the surface pressure (π) with time (t) for 0.5 MF of EPPH in 8CB on alternate UV and visible illumination.

We find that the photoinduced transformation also occurred for the EPPH-8CB mixed monolayer in the condensed phase. The decreasing trend in the peak and the dip values in surface pressures for 0.7 MF of EPPH in 8CB during alternate illumination of UV and visible radiations were similar to those of the individual EPPH monolayer (see Supporting Information). For 0.5 MF of EPPH in 8CB, the variation of the surface pressure with time on alternate illumination with UV and visible radiations for three cycles is shown in Figure 5. Here, we find that, for the initial surface pressures of 4.8 and 10 mN/m, the peak values did not decrease considerably as in the case of EPPH monolayer. However, the decrease in the dip value for repeated cycles of illumination depends on the initial surface pressure. For the initial surface pressure of 4.8 mN/m, the dip value was almost close to that of the initial value. However, for the initial surface pressure of 10 mN/m, the dip value decreased to about 7 mN/m after the first cycle and remained unchanged for further cycles of illumination. We have carried out in-situ BAM studies for the EPPH and for the mixed monolayers of EPPH and 8CB with alternate illumination of UV and visible radiations. Under UV illumination, the texture of the EPPH monolayer (Figure 3a) transformed to a collapsed state. Here, the coexistence of bright domains, gray domains, and dark regions was seen (Figure 6). The bright domains correspond to 3D crystallites of EPPH. The gray domains are similar to those domains obtained in a dark room (Figure 3a). They correspond to the crystalline monolayer of EPPH molecules in the trans state. The dark region corresponds to the fluidlike monolayer of EPPH molecules in the cis state. However, under visible illumination, a fraction of EPPH isomer in the cis state transformed back to the trans state, while the bright regions (3D crystallites of EPPH) remain unchanged. Subsequent cycles of al-

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Figure 6. BAM image of the EPPH monolayer on UV illumination. Here, the initial surface pressure was 4.5 mN/m. Also shown is the coexistence of bright domains, gray domains, and dark regions in the background. The scale of the image is 6.4 × 4.8 mm2.

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ternate UV and visible illuminations led to the appearance of more 3D crystallites. We can interpret the gradual decrease in the peak and dip values of the surface pressure with time for the EPPH monolayer during alternate UV and visible illuminations (Figure 4) on the basis of our BAM observations. The decrease in the peak and dip values of surface pressure is likely due to the nucleation and growth of 3D crystallites, which depletes the EPPH molecules at the A-W interface. This process continues with successive cycles of illumination, resulting in the decreasing trend. The in-situ BAM images for 0.5 MF of EPPH in 8CB for an initial surface pressure of 4.8 mN/m showed phase separation during UV illumination and miscibility during visible illumination (Figure 7). Figure 7a shows, under visible radiation, the homogeneous texture of the condensed EPPH-8CB mixed monolayer. This texture is similar to that obtained in a dark room (see Supporting Information). Under UV illumination, voids were seen to develop from the homogeneous texture (Figure 7b). Under visible illumination, these voids disappeared and trans-

Figure 7. BAM images for 0.5 MF of EPPH in 8CB for an initial surface pressure of 4.8 mN/m during alternate UV and visible (V) illuminations. (a) Initial homogeneous texture of the condensed mixed monolayer. (b) Appearance of voids under UV illumination. (c) Phase separation (foam texture) under UV illumination, which evolves to small circular domains under visible illumination (d). (e) Phase separation (foam texture and bright circular domains) under UV illumination, which transforms under visible illumination to coalescing domains with holes (f). The scale of each image is 4.8 × 3.2 mm2.

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Figure 8. BAM images for 0.5 MF of EPPH in 8CB for an initial surface pressure of 10 mN/m during alternate UV and visible (V) illuminations. (a) Small circular domains (separated phases) under UV illumination. (b) Faint domains (separated phase) with a homogeneous phase in the background under visible illumination. (c) Foam texture. (d) Under UV illumination, bright circular domains (multilayers) are seen to coexist with elongated domains. (e) Domains with holes seen under visible illumination. The scale of each image is 4.8 × 3.2 mm2.

formed back to the homogeneous texture (Figure 7a). Illumination again with UV radiation yielded a fluidlike foam texture (Figure 7c). At this stage, under visible illumination, these foam textures transformed to small circular domains (Figure 7d), which evolved back to the homogeneous texture (Figure 7a). Further UV illumination yielded foam textures and bright circular domains (Figure 7e). This clearly indicated the phase separation of coexisting monolayer and multilayers of 8CB (Figure 7e). Under visible illumination, multilayer domains developed holes (Figure 7f), which again revert back to the homogeneous texture, indicating miscibility. During UV illumination, the EPPH isomer in the trans state (rodlike) gets converted to the cis state (bent), which is incompatible with the shape of the 8CB molecule (rodlike). Thus, the 8CB molecules with a low collapse pressure squeeze out from the EPPH-8CB mixed monolayer, leading to phase separation. During visible illumination, EPPH isomer in the cis state reverts to the trans state.

This is compatible with the shape of the 8CB molecules and forms miscible mixed monolayer. We have also carried out in-situ BAM studies for 0.5 MF of EPPH in 8CB at a higher initial surface pressure of 10 mN/m (Figure 8). Here too, there was a phase separation during UV illumination. However, the separated phases did not completely revert back to a homogeneous miscible phase under visible illumination. The initial homogeneous texture of the mixed monolayer in the condensed state transformed to small circular domains (separated phase) under UV illumination (Figure 8a). Under visible illumination, there was a perturbation in the texture, and faint domains were seen with the homogeneous texture in the background (Figure 8b). These faint domains evolved to a foam texture (Figure 8c). Under UV illumination, the foam texture transformed to bright circular domains, which coalesced to elongated domains (Figure 8d). These were much brighter than that observed for a low initial surface pressure (4.8 mN/m), suggesting

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the predominant existence of multilayers of 8CB. Under visible illumination, the textures transformed to domains of varying intensities, indicating multilayers of different thickness. This texture persisted even for a longer duration of illumination with visible radiation, suggesting its inability to return to a homogeneous miscible phase. The difference in the miscibility behavior for 0.5 MF of EPPH in 8CB during visible illumination at low and high initial surface pressures can be interpreted as follows. At low initial surface pressure (comparable or less than the collapse pressure of 8CB), the monolayer phase of 8CB readily mixed with the monolayer phase of the trans isomer of EPPH (rodlike) due to the attractive interactions between them, resulting in considerable reduction in the A/M. This gives rise to voids which facilitated the multilayers of 8CB to dissolve (Figure 7f) and yield a miscible phase. This is also indicated in the variation of the surface pressure with time (Figure 5), where the dip values recover nearly to the initial value (4.8 mN/m). At a higher initial surface pressure (much higher than the collapse pressure of 8CB), multilayers of 8CB were predominant as compared to the monolayer of 8CB. In this case, although the multilayers dissolved to some extent, giving rise to holes, the remaining multilayers persisted, leading to immiscibility (Figure 8e). This is also seen in the variation of the surface pressure with time (Figure 5), where the dip value drops to about 7 mN/m and does not recover to the initial value (10 mN/m). There have been some reports on the photoisomerization studies of azobenzene derivatives of carboxylic acid at the A-W interface. For an azobenzene derivative, where the molecules were densely packed in the monolayer, Freimanis et al. have reported the absence of photoinduced transformation of isomers.15 For another system, where the molecules were not that densely packed in the monolayer, Tabe and Yokoyama report a photoinduced transformation.16 In the same system, Yim and Fuller find a photoinduced transformation of isomers in the monolayer at lower surface pressures, while it was hindered at higher surface pressures.17 We find in the EPPH monolayer a photoinduced transformation during the illumination of UV and visible radiations. These studies show that the packing of the molecules in the monolayer plays an important role in the photoinduced transformations at the A-W interface. Xu and Iwamoto have reported the influence of photoillumination in the mixed Langmuir monolayer of the azobenzene derivative with saturated and unsaturated phospholipids.4 This technique has been used as a probe to detect the packing differences between the alkyl chains of lipids due to photoisomerization of the azobenzene derivative. Recently, Ubukata et al. have also reported on the photocontrollable phase separation in LB films of an azo polymer mixed with a mesogenic molecule.5 They found

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changes in the morphology during alternate illumination of UV and visible radiations in high humid conditions, whereas no changes in the morphology were found in dry conditions. This showed that the presence of water molecules facilitates the photoinduced transformations as seen in the EPPH-8CB system at the A-W interface. 4. Conclusions We have carried out surface manometry and Brewster angle microscopy studies on the EPPH-8CB mixed system. We find for the mixed monolayer a large reduction in the area per molecule and an increase in the collapse pressure. We infer that the mixed monolayer is more stable when compared to individual monolayers and forms a condensed phase. The stability of the mixed monolayer in the condensed phase was checked by monitoring the change in the surface pressure with time during alternate illumination of UV and visible radiations. Our experiments indicated that the photoisomerization process occurred in the mixed system. The change in textures during photoillumination has been studied using BAM at different initial surface pressures. Reversible phase separation and miscibility were seen to occur at a low initial surface pressure (comparable or less than the collapse pressure of 8CB). The phase separation is due to the shape incompatibility of EPPH in the cis state (bent) and 8CB (rodlike). The miscibility is due to the shape compatibility between the monolayer of EPPH molecules in the trans state (rodlike) and 8CB (rodlike). In addition, we suggest that the weak charge-transfer interactions enhance the miscibility, resulting in the formation of a stable condensed phase. However, for initial surface pressures, higher than the collapse pressure of 8CB, we find that the phase separation was not reversible during photoillumination due to the predominantly present multilayers of 8CB. Acknowledgment. We thank G. S. Ranganath for useful discussions. Supporting Information Available: Chemical structures of EPPH isomers and 8CB. Variation of A/M for the mixed monolayer and for the ideal case with MF of EPPH at different surface pressures. Variation of excess Gibbs free energy with MF of EPPH at different surface pressures. BAM image for 0.5 MF of EPPH in 8CB. Variation of the logarithm of normalized area with time for two different surface pressures for the EPPH monolayer. Variation of the logarithm of normalized area with time for 0.5 MF of EPPH for two different surface pressures. Variation of surface pressure with time for 0.7 MF of EPPH on alternate illumination with UV and visible radiations. This material is available free of charge via the Internet at http://pubs.acs.org. LA0491899 (15) Freimanis, J.; Markava, E.; Matisova, G.; Gerca, L.; Muzikante, I.; Rutkis, M.; Silinsh, E. Langmuir 1994, 10, 3311. (16) Tabe, Y.; Yokoyama, H. Langmuir 1995, 11, 4609. (17) Yim, K. S.; Fuller, G. G. Phys. Rev. E 2003, 67, 041601.