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Phase Transition and Domain Morphology in Langmuir Monolayers of a Calix[4]arene Derivative Containing No Alkyl Chain Weijiang He,†,‡ Dieter Vollhardt,*,† Rainer Rudert,§ Longgen Zhu,‡ and Junbai Li| Max Planck Institute of Colloids and Interfaces, D-14424 Postdam/Golm, Germany; State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, P. R. China; Federal Institute for Materials Research and Testing, Unter den Eichen 87, D-12200 Berlin, Germany; and International Joint Lab, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Zhong Guan Cun, Beijing 100080, P. R. China Received August 13, 2002. In Final Form: October 16, 2002

The monolayer features of 5,11,17,23-tetra-tert-butyl-25,27-di(benzoylamidoethoxy)-26,28-dihydroxyl calix[4]arene (DBAC) at the air/water interface are experimentally studied. Using combined studies of surface pressure-area (π-A) isotherms, surface pressure relaxation, and monolayer morphology by Brewster angle microscopy (BAM), it is demonstrated that this calix[4]arene type, having no long alkyl chain, nevertheless, can form stable Langmuir monolayers. The π-A isotherms show a pronounced plateau region between ∼90 and ∼60 Å2/molecule after a linear increase of surface pressure. A first-order phase transition in the monolayer state takes place in the linear part of the pressure increase of the π-A isotherm for 5 °C e T e 25 °C clearly indicated at 5 °C by a small inflection region between 102 and 99 Å2/molecule and the simultaneous formation of condensed phase domains. The differences of the densities of the coexisting fluid and condensed phase are small. Temperature increase reduces the tendency to form the quasicrystalline condensed monolayer phase. The relaxation studies reveal that the DBAC monolayers are not in equilibrium under the conditions of compression/decompression at which the π-A isotherms were recorded. The corresponding BAM studies provide the exact area value for the onset of the two-dimensional phase transition. Under these conditions well-shaped monolayered dendritic structures are formed. The pronounced plateau region of the compression/decompression isotherm between ∼90 and 60 Å2/molecule represents the transition from a tightly packed monolayer to a bilayer/multilayer structure. Possible conformers of the DBAC molecule were calculated by structure optimization using force field Tripos 5.2. The dimensions calculated for the conformer with the low cross-sectional area are in reasonable agreement with the features of the monolayer obtained by the surface pressure and BAM studies.

Intrduction Over the past three decades, calixarene chemistry has been established as a molecular construction platform to synthesize a wide variety of cavity-containing and multifunctional molecules.1 The increasing interest in calixarenes is based on their broad potential in designing supramolecular host-guest structures and their close relation to current topics such as design and synthesis of enzyme mimics, selective molecular recognition of biorelevant species, but also metal cation complexation.2-5 Functionalized amphiphilic calixarenes have been prepared with the objective to study their monolayers, * Corresponding author. † Max Planck Institute of Colloids and Interfaces. ‡ Nanjing University. § Federal Institute for Materials Research and Testing. | Chinese Academy of Sciences. (1) (a) Bo¨hmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713. (b) Shinkai, S. Tetrahedron 1993, 49, 8933. (2) Asfari, Z., Bo¨hmer, V., Harrowfield, J., Vicens, J., Eds. Calixarenes 2001; Kluwer Academic Publishers: Dordrecht, Boston, London, 2001. (3) Hamuro, Y.; Calama, M. C.; Park, H. S.; Hamilton, A. D, Angew. Chem., Int. Ed. Engl. 1997, 36, 2680. (4) Gutsche, C. D. In Monographs in Supramolecular Chemistry, Calixarenes Revisited; Stoddart, J. F., Ed.; The Royal Society of Chemistry: Cambridge, U.K., 1998. (5) Arnaud-Neu, F.; McKervey, M. A.; Schwing-Weill, M. J. In Calixarene 2001, Asfari, Z.; Bo¨hmer, V.; Harrowfield, J.; Vicens, J., Eds.; Kluwer, 2001, p 642.

LB films, and self-assembly films. Potential applications have been discussed in the field of medical diagnostic and miniaturized sensor technique.5,6 For obvious reasons calixarenes with long alkyl chains seem to be suited for such application, but it is interesting to note that also some calixarenes with short, or without any alkyl chains have been found to form monolayers at the air/water interface.7,8 Particularly, Langmuir monolayers of p-tert-butylcalixarene have been studied. For example, Baglioni et al. investigated not only the selective binding of NaCl, KCl, CsCl, and guanidinium thiocyanate to p-tert-butylcalix[6]arene monolayers9 but also the complexation of the corresponding calix[8]arene with C60 at the air/water interface.10 Kazantseva also obtained Langmuir films of p-tert-butylcalix[6-8]arene which had at the upper rim tert-butyl groups instead of long alkyl chains.11 Shinkai (6) Spichiger-Keller, U. E. Chemical Sensors for Medical and Biological Application; Wiley-VCH: New York, 1998. (7) Shinkai, S.; Mori, S.; Arimura, T.; Manabe, O. J. Chem. Soc., Chem. Commun. 1987, 238. (8) Nabok, A. V.; Richardson, T.; Davis, F.; Stirling, C. J. M. Langmuir 1997, 13, 3198. Davis, F.; O’Toole, L.; Short, R.; Stirling, C. J. Langmuir 1996, 12, 1892. (9) Dei, L.; Casnati, A.; LoNostro, P.; Baglioni, P. Langmuir 1995, 11, 1268. (10) Dei, L.; LoNostro, P.; Capuzzi, G.; Baglioni, P. Langmuir 1998, 14, 4143.

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Figure 1. Chemical structure of DBAC.

found that the lower rim ester-derived p-tert-butylcalix[4,5,6]arene can form monolayers where the ester groups should have a certain coordination atmosphere for alkali metal ions at the air/water interface.7 If the cavity of the molecules is enlarged from calix[4]arene to calix[6]arene, the lower rims of the three compounds can recognize Na+ and K+, respectively.7 He et al. found that binuclear Pd(II) complexes of lower rim amino acid derived p-tertbutylcalix[4]arenes can form robust monolayers at the air/water interface.12 Finally, Richardson and Stirling prepared monolayers of p-tert-octylcalix[8]arenes with lower rims substituted by carboxylic groups, ketone groups, and amino groups. Possible applications of these compounds in pyroelectric function, nanoparticles preparation, and selective ion binding have been studied.8,13 All these calixarenes are functionalized at the lower rim by hydroxyl groups or other polarized groups (connected with the hydroxyl groups), which should be the hydrophilic part of the amphiphilic calixarene molecule. The objective of the present paper is to study another calix[4]arene type where the hydrophilic lower rim is partly substituted by hydrophobic groups. An interesting candidate for such a study is 5,11,17,23-tetra-tert-butyl25,27-di(benzoylamidoethoxy)-26,28-dihydroxylcalix[4]arene (DBAC), wherein the lower rim is substituted by polar groups partially containing phenyl groups (Figure 1). Although this calix[4]arene does not have longer alkyl chains than those of the usual amphiphiles, it can form stable monolayers and condensed phase domains. Therefore, we focus primarily on the study of the characteristic features of the DBAC monolayers performed by coupling of surface pressure measurements with Brewster angle microscopy (BAM) at different temperatures. For a better understanding of the unexpected formation of stable monolayers, some possible conformers of the DBAC molecule are calculated and discussed in light of the monolayer results. The rotatable parts of the DBAC molecule are indicated by curved arrows in Figure 1. Experimental Section The tailored calix[4]arene DBAC (Figure 1) was synthesized by the acylation of the lower rim calix[4]arene diamine by benzoylic acid with DCC (N,N′-dicyclohexylcarbodiimide) as an accelerator. The detailed procedure will be published elsewhere. DBAC was obtained as colorless crystals. mp 263-265 °C. IR (KBr, cm-1): 3485 (CONH), 1665 (CONH). 1H NMR (δ ppm): 8.39 (m, 2H, NH), 8.26 (s, 2H, OH), 7.95 (d, J ) 7.1 Hz, 4H, ortho-H-Ph), 7.53 (m, 2H, para-H-Ph), 7.30 (m, 4H, meta-H-Ph), 7.05, 6.99 (2s, 8H, ArH), 4.20 (d, J ) 12.9 Hz, 4H, ArCH2Ar), 4.10 (11) Kazantseva, Z. I.; Lavrik, N. V.; Nabok, A. V.; Dimitriev, O. P.; Nesterenko, B. A.; Kalchenko, V. I.; Vysotsky, S. V.; Markovskiy, L. N.; Marchenko, A. A. Supramol. Sci. 1997, 4, 341. (12) He, W.; Liu, F.; Ye, Z.; Zhang, Y.; Guo, Z.; Zhu, L.; Zhai, X.; Li, J. Langmuir 2001, 17, 1143. (13) Richardson, T.; Greenwood, M. G.; Davis, F.; Stirling, C. J. M. Langmuir 1995, 11, 4623.

Figure 2. π-A isotherms of DBAC monolayers at different temperatures. The inset shows the inflection region observable in the isotherm at 5 °C. The marked points correspond to the BAM images of Figure 3. (m, 4H, OCH2), 3.47 (m, 4H, NCH2), 3.40 (d, J ) 12.9 Hz, 4H, ArCH2Ar), 1.24, 1.11 (2s, 36H, C(CH3)3). 13C NMR (δ ppm): 167.63 (CONH), 149.38, 148.38 (ArC-O), 148.03, 142.74 (ortho-C-Ar), 134.14 (4′-C-Ph), 132.75, 127.57 (para-C-Ar), 131.26, (1′-C-Ph), 128.19 (2′,6′-C-Ph), 127.33 (3′,5′-C-Ph), 125.97, 125.57 (metaC-Ar), 75.11 (OCH2), 39.54 (CH2NH), 34.09, 33.81 (C(CH3)3), 33.13, 32.16 (ArCH2Ar), 31.51, 30.99 (C(CH3)3). Anal. Calcd for C62H74N2O6: C, 78.95; H, 7.91; N, 2.97. Found: C, 79.24; H, 8.20; N, 3.28. ES-MS (LCQ system, Finnigan): 965.6 [M + Na]+, 943.5 [M + H]+. The spreading solvent was chloroform (p.a. grade) obtained from Baker, Holland. Ultrapure Milli-Q filtered (18.2 MΩ, pH 5.7) water was used as the subphase. Film Balance Measurements and BAM Observations. The surface pressure-area (π-A) isotherms of the DBAC monolayers were recorded with a computer-interfaced film balance. The surface pressure was measured with a reproducibility of (0.1 mN/m using the Wilhelmy method and a roughened glass plate. The film balance was sheltered in a cabinet to avoid excessive disturbances by convection and contamination by impurities. The monolayers were formed by stepwise spreading of 50 µL of 0.25 mM DBAC solution in chloroform. 20 min were given for the evaporation of the spreading solvent. All isotherms were recorded with a compression/decompression rate of 8.8 Å2/(min‚ molecule). The temperature of the subphase was controlled by a recyclic water system to (0.1 °C. Constant area experiments were carried out after stopping the compression at a certain position. Imaging of the DBAC monolayers was performed with a Brewster angle microscope (BAM1, NFT, Go¨ttingen, Germany) mounted on the film balance. The light source of the BAM was a He-Ne laser (10 mW). BAM uses the zero reflectance of p-polarized light at the Brewster angle at the air/water interface. A monolayer at the air/water interface leads to change in the reflectivity, thus allowing the direct visualization of the domain morphology with a special resolution of approximately 4 µm. Optical anisotropy in the monolayer is determined by the analyzer in the reflected beam path.

Result and Discussion π-A Isotherms of DBAC at Different Temperatures. The π-A isotherms of DBAC at different temperatures are presented in Figure 2. As can be seen, DBAC can form stable monolayers at the air/water interface between 5 and 25 °C, although the DBAC molecule has not the characteristic amphiphilic nature but rather two aromatic phenyl rings in the hydrophilic substituent at its lower rim. For all temperatures the surface pressure starts to increase nearly linear in the area region between 125 and 128 Å2/molecule up to a plateau, increasingly flat

Langmuir Monolayers of a Calix[4]arene Derivative

with the increase of temperature. The surface pressure inflects at the beginning of the plateau at around 95 Å2. Similar results were obtained for other calix[4]arenes14 and calix[4]arene derivatives.15 It is important to note that the π-A isotherm at 5 °C shows a kink appearing already at around 25 mN/m. At further compression after a small maximum (see inset of Figure 2) it follows a slight decrease of the surface pressure before the surface pressure increases steeply again. This behavior indicates not only a first-order phase transition to a more condensed phase but also a slight supersaturation of the more fluid phase. This phase transition has been corroborated by observation with BAM (vide infra). A small region of 3 Å2 (between 102 Å2 and 99 Å2) with only a small area change (see inset of Figure 2) suggests that the difference between the densities of the two coexisting phases is small. At temperatures T g 10 °C the kink disappears and the surface pressure increases linearly, independent of the temperatures up to the kink point at around 35 mN/m in the same area region. At T g 20 °C a real flat plateau without an inclination up to areas A e 60 Å2/molecule is found. After that, further compression leads again to a linear surface pressure increase. The area/molecule at the end of the plateau is obviously smaller than the calculated cross-sectional area of the DBAC molecule between 128 and 156 Å2 (vide infra) and suggests the formation of multilayer aggregates. BAM Studies at Different Temperatures. Recent studies of the morphological features of Langmuir monolayers using BAM have led to a more detailed understanding of the nature of the two-dimensional condensed phase structures.16,17 It has been demonstrated how compression or decompression of monolayers can affect the shape and inner structure of the two-dimensional condensed phase domains.18,19 Despite the powerful potential of BAM for the characterization of the monolayer morphology, so far this technique has not yet been used to study the phase behavior and other features of the condensed monolayer phases of calixarenes. Therefore, it is of special interest to perform BAM studies of the DBAC monolayers, as the chemical structure of this special calixarene deviates considerably from that of a usual amphiphile. The DBAC molecule does not contain long alkyl chains (as do usual amphiphiles), and the hydrophilic part at the lower rim is partly substituted by hydrophobic phenyl groups. At first, the morphological features of the DBAC monolayers are considered at different temperatures and monolayer compressions. Interesting information is already provided by a series of BAM images (Figure 3) obtained for designated points a-d of the π-A isotherm at 5 °C (Figure 2). First, small domains appear already at the end of the linear pressure increase before the small pressure maximum b (see inset of Figure 2) at an area of ∼103 Å2, corresponding to a surface pressure of 24.4 mN/m (Figure 3a). At compression to the small maximum (inset of Figure 2) at 102 Å2 (25.5 mN/m) the number of domains increases (Figure 3b). Then, a large number of small condensed phase domains irregularly shaped are formed at further compression only to the slightly changed area of ∼99 Å2 at the small minimum, nearly completely (14) Shahgaldian, P.; Coleman, A. W. Langmuir 2001, 17, 6851. (15) Dermody, D. L.; Lee, Y.; Kim, T.; Crooks, R. M. Langmuir 1999, 15, 8435. (16) Vollhardt, D. Adv. Colloid Interface Sci. 1996, 64, 143. (17) Vollhardt, D. Adv. Colloid Interface Sci. 1999, 79, 19. (18) Gehlert, U.; Vollhardt, D. Langmuir 1997, 13, 277. (19) Weidemann, G.; Vollhardt, D. Biophys. J. 1996, 70, 2758.

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Figure 3. BAM images of DBAC monolayer obtained at compression to the correspondingly marked points in Figure 2 at 5 °C.

covering the surface (Figure 3c). Finally, in the region of the renewed increase of surface pressure (point d in Figure 2) the condensed phase domains appear more irregular and more coalesced with each other. The stronger brightness indicates the formation of multilayered areas has started (Figure 3d). The BAM studies at 5 °C corroborate the conclusion that can be drawn from the π-A isotherm. The twodimensional phase transition to the condensed monolayer phase takes place in the inflection region of the linear pressure increase with the small maximum and minimum. The appearance of the small maximum, and consequently of the minumum, instead of a small plateau region, indicates metastability of the monolayer in this region as a consequence of supersaturation before onset of the phase transition. This is supported by the fact that the first domains are developed already before the maximum is reached.20 The formation of a large number of small domains is obviously the consequence of a high twodimensional nucleation rate. The quick increase of the domain number up to the state where finally the twodimensional condensed phase domains are tightly packed in this small area range between ∼103 and ∼99 Å2 is in agreement with the conclusion obtained from the isotherms, that the densities of the two coexisting phases differ only slightly. The shape and size of the condensed phase domains formed during compression of DBAC monolayers at higher temperatures (e15 °C) are similar to those at 5 °C. Although the domain size increases somewhat with temperature, it remains smaller than 40 µm at 15 °C. Nevertheless, at this size it is seen that the domains have a rhombic shape and domains of different reflectivity are formed. With the increase of temperature, the domains appear at higher surface pressure than that in the case of 5 °C. As the small inflection region with a small maximum and minimum in the region of the linear pressure increase is missing in the isotherms at T > 5 °C, BAM images obtained at 10 and 15 °C cannot be interpreted in the same way as that at T ) 5 °C. (20) Vollhardt, D.; Fainerman, V. B. J. Phys. Chem. B 2002, 106, 345.

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Figure 4. π-A isotherms of the first compression/decompression cycle for the DBAC monolayer at 20 °C. The corresponding BAM images for the marked points are given in Figure 5. Figure 6. BAM images of the domain growth during the constant area relaxation of the DBAC monolayer at 104 Å2. The first domains appear after ∼50 s (a) and covered approximately half of the surface after ∼1 h. T ) 20 °C.

Figure 5. BAM images of the DBAC monolayer at the marked points in Figure 4: (a-c) at compression; (d-f) at decompression; T ) 20 °C.

Therefore, more detailed studies of the DBAC monolayers have been performed at T ) 20 °C. To obtain more information on the well-developed plateau in the π-A compression isotherm at π ∼ 35 mN/m, the situation at decompression has also been studied and the corresponding compression/decompression loop at 20 °C was recorded (Figure 4). The decompression curve indicates a reverse transition process, but the extended plateau region appears only at around 30 mN/m. BAM images were taken at the designated points of the compression/decompression loop to characterize the changes in the morphological features during this process (Figure 5). The first domains appear at 94 Å2, just before the onset of the plateau (Figure 5a). Further compression to lower area values within the plateau (Figure 5b) and also into the region of the renewed pressure increase (Figure 5c) does not lead to a remarkable increase in the domain number. The size of the domains is ∼60 µm, and their different reflectivities (bright and black domains) indicate different azimuthal orientation of the molecules in the different domains. In the decompression plateau (∼29 mN/m), the BAM studies show likewise only a few domains but with different morphology in comparison to those obtained at compression. They are obviously larger and develop a nonuniform brightness with progressive decompression. This behavior cannot be completely understood. It is important to note that dynamic conditions prevail in the present case of a compression/decompression cycle. As the position of the plateau region in the compression/ decompression isotherms is situated between around 100 and 60 Å2, this plateau cannot represent a first-order phase

transition to a condensed monomolecular phase but rather the transition from a monolayered to a bilayered state. The area value of ∼100 Å2 at the beginning of the plateau region corresponds roughly to the presence of a tightly packed monomolecular layer. In this state the BAM images provide no indication of a long-range oriented condensed phase. The tightly packed monolayer should be in a nonoriented fluidlike state. Under these dynamic conditions, also the bilayered state is formed mainly without long-range ordering. The few visible domains may be single multilayered domains which after extension disintegrate slowly in the lower plateau of the decompression isotherm, as concluded from the increasing nonuniform brightness of the domains and the concomitant decrease in the brightness (Figure 5d-f). At further increasing temperature, the tendency to form domains during the compression of the DBAC monolayers decreases. Whereas at 25 °C very few small domains can be seen at the end of the plateau (which can be understood to be similar to those formed at 20 °C), no domains can be observed at 28 °C. The formation of ordered textures in the DBAC monolayers is obviously inhibited under dynamic conditions. Relaxation Studies at Different Constant Areas at T ) 20 °C. It was the objective of the relaxation experiments to characterize the dynamic DBAC monolayers during approaching the equilibrium state at different areas per molecule. The experiments have been carried out by stopping the compression at the selected molecular areas of 105.6 Å2 (22.3 mN/m) and 104.0 Å2 (25.6 mN/m) with both area positions greater than the plateau-onset, of 95.6 Å2 (33.5 mN/m) just before the plateau and of 83. 5 Å2 (34.4 mN/m) within the plateau. At the position of 105.6 Å2 corresponding to an initial surface pressure of 22.3 mN/m, the relaxation studies did not show any indication that condensed phase textures have been developed in the DBAC monolayer, even after 40 min. That means, also under equilibrium conditions the DBAC monolayer is really in a two-dimensional fluidlike state where condensed phase domains cannot exist. This is consistent with the fact that the domains disappear at ∼24 mN/m during expansion. The situation changes drastically if the molecular area decreases only slightly. At 104 Å2, the first well-shaped domains can be observed after approximately 50 s (Figure 6a). Afterward, more domains develop and start to grow (Figure 6b and c). At the beginning, the shape is needlelike with a small arm on each side in the center of the main axis. More sidearms evolve to characteristic dendritic

Langmuir Monolayers of a Calix[4]arene Derivative

Figure 7. BAM images of the domain growth during the constant area relaxation of the DBAC monolayer at 95.6 Å2. The first domains appear after a few seconds (a). In the succeeding stages of growth the domains gradually occupy the whole surface, a second layer seems to overgrow a first layer, and finally (after ∼30 min), the surface is completely covered (c-e). T ) 20 °C.

structures during the domain growth. Second-order branches from these sidearms or even lower order branches develop during the domain growth (Figure 6d and e). In these branched domains, the axial growth of the domains is much quicker than that in the lateral direction. The first-order branches form an angle of ∼54° with the long axis (Figure 6a-c). Finally, after 1 h the domains can reach a length of ∼1000 µm, but also after 3 h no more than half of the available surface is covered by them. The domains show the same reflectivity over the whole shape, so an inner texture does not exist. On the other hand, again two types of domains (black and white) which reflect completely differently (Figure 6b) are observable, suggesting two different orientations of the molecules. The comparison of the BAM results, which can be obtained after compression stop in the linear part of the pressure increase of the π-A isotherm at the slightly different constant areas of 105.6 and 104 Å2/molecule (see Figure 2), allows interesting conclusions. Whereas at the higher area value of 105.6 Å2/molecule a phase transition from a fluid phase to a condensed phase with crystalline features cannot take place, it can be convincingly observed at 104 Å2/molecule. That means, at T ) 20 °C this phase transition occurs at ∼105 Å2/molecule in the steep part of the pressure increase of the isotherm without indicating this in the shape of the isotherm. Considering the results obtained at the low temperature of 5 °C, it can concluded that at this area the phase transition occurs from a fluid phase to a condensed phase having nearly the same density in the monolayer state. This conclusion is also corroborated by any indication for a phase transition in the shape of the isotherm being missing and by the fact that, near the point where the phase transition first starts, approximately half of the available surface is covered by the new condensed phase when approaching the equilibrium state. Consider now the situation at the constant area of 95.6 Å2, just before the plateau region. The growth of lancetlike condensed domains starts already after a few seconds (Figure 7a and b). The number and size of the domains increase rapidly. After ∼12 min, domains develop which have dimensions of ∼800 µm in the long axis and ∼100 µm in the short axis. The domains show homogeneous

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Figure 8. BAM images of the domain growth during the constant area relaxation of the DBAC monolayer at 83.5 Å2. The first domains appear already before reaching the fixed position. Finally, they cover the whole surface and overgrow each other irregularly within 1000 s (d). T ) 20 °C.

reflectivity at the beginning. However, in the succeeding stages of growth the domains gradually occupy the whole surface, a second layer seems to overgrow a first layer, and finally (after ∼30 min), the surface is completely covered (Figure 7c-e). In the plateau region at 83.5 Å2, the first domains appear already during the compression of the monolayer (Figure 8a). New domains which grow rapidly are formed after stopping the compression (Figure 8b and c). The single domains reflect homogeneously but differ from each other. With increasing size the domains touch each other, coalesce easily (Figure 8c), and finally, not only cover the whole surface but also overgrow each other irregularly within 1000 s (Figure 8d). The BAM images reveal irregular areas of different reflectivity (Figure 8d). All the BAM images taken at constant areas of