Structure of Relaxed Monolayers of Arachidic Acid Studied by X-ray

D. Collected Abstracts of the 4th International Conference on Surface. X-ray and Neutron Scattering, Lake Geneva, WI, 1995; ANL: Argonne,. IL, 1995; p...
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Langmuir 1996, 12, 5148-5155

Structure of Relaxed Monolayers of Arachidic Acid Studied by X-ray Reflectometry and Atomic Force Microscopy E. A. Kondrashkina,*,† K. Hagedorn,‡ D. Vollhardt,‡ M. Schmidbauer,† and R. Ko¨hler† MPG-AG “Ro¨ ntgenbeugung”, Hausvogteiplatz 5-7, 10117 Berlin, Germany, and MPI fu¨ r Kolloid- und Grenzfla¨ chenforshung, Rudower Chausse 5, 12489 Berlin, Germany Received February 29, 1996. In Final Form: July 10, 1996X Following the constant pressure relaxation on different subphases and the deposition on solid substrates, the structure and morphology of arachidic acid monolayers were studied by X-ray reflectometry and atomic force microscopy. The growth of three-dimensional (3D) phase nuclei was observed from the first minutes of the relaxation process in a monolayer relaxed on a subphase of low pH e 3 with or without metal ions. With an increase in the relaxation time, the granule-like nuclei were transformed into platelike multilayer islands. The structure of 3D islands was commensurate with the B and C forms of bulk arachidic acid, with the B phase being prevalent after a long period of relaxation. During the storage of the samples in air, the granule-like islands were unstable and, after 2 months, transformed into the more stable platelike islands. The relaxation process was much slower in the case of binding the monolayer to cadmium or lead cations in a subphase of pH 5.3. After about 1 h of relaxation, only low folds arrayed with a hexagonal symmetry were observed on the monolayer of cadmium arachidate. On monolayers of lead arachidate, 1-2 bilayer islands emerged with hexagonal boundaries after a relaxation of some 3 h.

Introduction Langmuir-Blodgett (LB) films are considered to have many potential applications in the areas of optical and electronic devices, biosensors, and coatings and as a model of complicated biological systems.1 However, their wide technological and scientific use is restricted by the low stability and reproducibility of their preparation and properties as well as by their defect structure. The perfection of LB film structure is influenced by both the preceding state of the precursor monolayer on a water surface and the parameters of the LB transfer process. The three-dimensional (3D) phase formation in monolayers on a water surface during their compression to a pressure above the equilibrium surface pressure (ESP)2 has recently received attention from theoretical2-8 and experimental9-13 points of view. The fact that above the ESP the monolayers are in a supersaturated metastable state and undergo relaxation has been regarded as a consequence of nucleation and growth of a 3D phase in 2D monolayers. 2D-3D transformation kinetics can completely change the shape of the π-A isotherms. Such overgrown 3D structures disrupt the homogeneity and * To whom correspondence should be addressed. † MPG-AG “Ro ¨ ntgenbeugung”. ‡ MPI fu ¨ r Kolloid- und Grenzfla¨chenforschung. X Abstract published in Advance ACS Abstracts, October 1, 1996. (1) Swalen, J. D.; et al. Langmuir 1987, 3, 932. (2) Smith, R. D.; Berg, J. C. J. Colloid Interface Sci. 1984, 100, 465. (3) DeKeyser, P.; Joos, P. J. Phys. Chem. 1984, 88, 274. (4) Vollhardt, D. Adv. Colloid Interface Sci. 1993, 47, 1. (5) Vollhardt, D.; Retter, U. J. Phys. Chem. 1991, 95, 3723. (6) Vollhardt, D.; Ziller, M.; Retter, U. Langmuir 1993, 9, 3208. (7) Retter, U.; Vollhardt, D. Langmuir 1993, 9, 2478. (8) Retter, U.; Vollhardt, D. Langmuir 1992, 8, 1693. (9) Baglioni, P.; Gabrielli, G.; Guarini, G. G. T. J. Colloid Interface Sci. 1980, 78, 347. (10) Vollhardt, D.; Retter, U.; Siegel, S. Thin Solid Films 1991, 199, 189. (11) Angelova, A.; Ionov, R.; Reiche, J.; Brehmer, L. Thin Solid Films 1994, 242, 283. (12) Vollhardt, D.; Gutberlet, T. Colloids Surf. 1995, A102, 257. (13) Ariga, K.; Shin, J. S.; Kunitake, T. J. Colloid Interface Sci. 1995, 170, 440.

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integrity of the precursor monolayer14 and lead to in-plane discontinuities and rearrangements of deposited LB multilayers.15 The 2D-3D transformation kinetics in monolayers below the collapse pressure was described by the nucleation-growth theory introduced by Smith and Berg2 and DeKeyser and Joos3 and recently developed by Vollhardt et al.4-7 The theoretical concept4-7 is based on the convolution of nucleation and growth of 3D centers and takes account of the overlapping of the growing centers in the successive stages of the transformation process. The concept has been supported experimentally by the constant pressure relaxation data of several amphiphilic monolayers.4,5,10,12 However, there are only a few direct morphological studies of monolayers during the relaxation process. With the introduction of Brewster-angle microscopy for the characterization of monolayers at the air-water interface, it has become possible to study the morphological transformations occurring during the relaxation of supersaturated monolayers. With the micron resolution of this method the growth of irregularly shaped, platelike 3D aggregates of several monolayers in height has been observed during the constant surface pressure relaxation.12,16 In recent studies,17,18 it has been demonstrated that atomic force microscopy (AFM) is an appropriate method to observe the microstructure of supersaturated monolayers, although in situ measurements cannot be made at the air-water interface. Growth and distribution of 3D islands formed during constant surface pressure relaxation of supersaturated arachidic acid monolayers have been quantitatively determined by grain-size analysis which (14) Morelis, R. M.; Girard-Egrot, A. P.; Coulet, P. R. Langmuir 1993, 9, 3101. (15) Angelova, A.; Penacorada, F.; Stiller, B.; Zetzsche, T.; Ionov, R.; Kamusewitz, H.; Brehmer, L. J. Phys. Chem. 1994, 98, 6790. (16) Siegel, S.; Ho¨nig, D.; Vollhardt, D.; Mo¨bius, D. J. Phys. Chem. 1992, 96, 8157. (17) Kato, T.; Matsumoto, N.; Kawano, M.; Suzuki, N.; Araki, T.; Iriyama, K. Thin Solid Films 1994, 242, 223. (18) Vollhardt, D.; Kato, T.; Kawano, M. J. Phys. Chem. 1996, 100, 4141.

© 1996 American Chemical Society

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provided the number, the average cross-sectional area, and the height of the islands.18 The subject of the present paper is to study both the structural and the morphological transformations in arachidic acid monolayers at the constant pressure relaxation and to explore the possibilities of a monolayer stabilization by adding bivalent metal ions to subphases with different pH values. It is known19 that the structure of monolayers, the LB deposition process, and the properties of LB films are influenced by the conditions in subsolution for fatty acid dissociation and its binding to metal cations. The influence of two selected examples of bivalent ions on the rate of 2D-3D transformations was reviewed in our study. The cadmium ions were chosen, since they were widely used in experiments with fatty acid mono- and multilayers. The lead ions were used as the most suitable for examination by X-ray technique due to the high intensity of their X-ray scattering. The subphase of pH 3 was tested as offering conditions without the dissociation of arachidic acid. The subphase of pH 5.3 was used for providing a preferable dissociation of arachidic acid and for binding the cations by the monolayer without hydrolysis. The molecular structure of the relaxed monolayers and the morphology of their surfaces were studied at different stages of the relaxation process by using X-ray reflectometry (including the analysis of X-ray small angle Bragg diffraction peaks) and AFM. The distribution of large 3D islands along the sample surface was studied by optical microscopy. The quality of monolayers transferred to silicon and to glass substrates was additionally estimated. Finally, the stability of relaxed monolayers stored on solid substrates in air was investigated as well. Materials and Methods Arachidic acid (99%, Merck) dissolved in n-heptane (p.a., Merck) was spread onto the surface of water of pH 3, or aqueous solutions of the salts CdCl2 or Pb(NO3)2 (p.a., 5 × 10-4 M, pH 5.3 or pH 3). Water from a Milli-Q system (Millipore) was used with the pH adjusted by HCl. A specially designed computer-interfaced film balance was used. The temperature of the aqueous surface was kept at (21 ( 0.5) °C during the experiment. After being spread, the monolayer was compressed to a surface pressure of 30 mN/m (see the π-A isotherm in Figure 1) with a rate of 0.05 nm2/ (molecule‚min), and then the surface pressure was maintained constant within an accuracy of (0.1 mN/m during the relaxation time. After the desired relaxation time, the monolayers were transferred to solid substrates using the scooping-up technique17 with a rate of 4 mm/min. This technique requires the substrate to have an inclination angle of 5° measured from the horizontal plane during rising up. The plates (10 × 20 × 0.5 mm3) cut from standard silicon wafers (Wacker, Burghausen), 100 mm in diameter, and optical microscope cover glasses (22 × 22 × 1 mm3) were used as solid substrates. The cleaning of silicon substrates by ultrasonification in ethanol, acetone, and hexane, followed by boiling in bidistilled water to remove contamination, did not destroy the natural silicon oxide layer on the surface. After cleaning, all the substrates were hydrophilic and, before use, were stored in distilled water. Between the measurements the samples of LB films were stored in air in closed boxes. The X-ray measurements were carried out on a triple-crystal diffractometer using a conventional X-ray tube (1.5 kW, Cu KR1 radiation), Ge(111) crystal monochromator and the same analyzer crystal (during adjustment) or an analyzing slit (during measurements) in front of the scintillation detector. The divergence of the incident beam was about 0.02°, and the angular resolution (19) Buhaenko, M. R.; Grundy, M. J.; Richardson, R. M.; Roser, S. J. Thin Solid Films 1988, 159, 253.

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Figure 1. π-A isotherm measured during compression of a C20 monolayer on water of pH 3 at 21 °C. Inset: relaxation curves measured at a pressure of 30 mN/m and corresponding to the samples with different periods of relaxation chosen for X-ray and AFM measurements. of the analyzing slit was about 0.05°. During the experiment the specular reflection curves (θ/2θ scans) and the curves of diffuse scattering (θ/2θ scans with θ-offset) were recorded (where θ was the angle of incidence of the X-ray beam with respect to the sample surface and 2θ was the exit angle of the X-ray beam in terms of the direction of the incident beam). Some measurements of X-ray diffuse scattering were made with synchrotron radiation (λ ) 0.122 nm) at the D4.1 beamline of HASYLAB, DESY, Hamburg.20 A precise quantitative analysis of surface layers and their interfaces, carried out with an accuracy of tenths of a nanometer, is hardly practicable on the basis of the reflectivity measurements using a conventional X-ray tube.21,22 The small thickness of substrates leading to a possible curvature of samples affects the accuracy of the results.23 Nevertheless, even an approximate fitting of the experimental reflectivity curves provides some valuable information. A program of theoretical curve calculation was used, which is based on Parrat recurrent equations. The starting values of electron density necessary for calculations were taken from literature.24,25 The intensity of all the experimental curves was corrected according to the finite size of the samples24 and normalized in order to make the intensity equal to 1 in the region of total specular reflection. The diffuse scattering was subtracted21 from those specular reflection curves for which the fitting was carried out. The AFM images of the samples were taken in air with a NanoScope III (Digital Instruments, Inc.). In order to avoid surface damage of the samples, the noncontact tapping mode was used because of its small applied force (0.1-1 nN). In contrast to the contact mode, the tapping mode reduced drastically the shearing force, so that soft objects such as monolayers could not be easily deformed. Furthermore, etched silicon probes with curvature radii of 20 nm were used, which had been developed solely for the tapping mode. The probes were highly sharpened and cone-shaped tips, so that the effects of tip-sample convolution became considerably smaller and, ultimately, could be neglected. However, the improvement in resolution to be gained by using sharper tips on soft materials was limited because the deforma(20) Kondrashkina, E. A.; Schmidbauer, M.; Pfeiffer, J.-U.; Vollhardt, D.; Ko¨hler, R. Annual Report; HASYLAB, DESY: Hamburg, Germany, 1994; p 437. (21) Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Wasserman, S. R.; Whitesides, G. M.; Axe, J. D. Phys. Rev. 1990, B41, 1111. (22) Cowley, R. A.; Lucas, C. Colloq. Phys. 1989, C7, 145. (23) Bridou, F. J. Phys. III 1994, 4, 1513. (24) Pomerantz, M.; Segmu¨ller, A. Thin Solid Films 1980, 68, 33. (25) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Tippmann-Kraer, P.; Mo¨wald, H. Thin Solid Films 1988, 159, 17.

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Figure 2. X-ray specular reflection curves of C20 monolayers transferred to silicon substrates after relaxation on water for different periods. (a) Water of pH 3: (1) uncovered Si substrate; (2) unrelaxed monolayer; (3) monolayer relaxed for a period of 500 s; (4) 1000 s; (5) 1500 s; (6) 3000 s; (7) 4700 s. (b) Water of pH 5.3 with 5 × 10-4 M CdCl2: (1) 0 s (solid line); (2) 3000 s (dashed line). Water of pH 3.0 with 5 × 10-4 M Pb(NO3)2: (4) 1000 s; (5) 2000 s; (6) 5000 s. Water of pH 5.3 with 5 × 10-4 M Pb(NO3)2: (3) 0 s; (7) 11500 s; (8) 11 layers of lead arachidate. Successive curves are shifted by a factor of 20 along the intensity axis. The examples of theoretical fits are given by dotted lines. tion of the specimen by the probe was proportional to the pressure on the contact area. The optical microscope Olympus AX70 was used to take images of sample surfaces with a magnification of 200.

Results and Discussion A. X-ray Reflectometry of C20 Monolayers Deposited on Silicon Substrates after Constant Surface Pressure Relaxation on Water of pH 3. Figure 1 shows the typical π-A isotherm, which was recorded during compression of the C20 monolayer on pH 3 water at a temperature of 21 °C. The relaxation curves recorded during the relaxation processes at a constant pressure of 30 mN/m are presented in the inset of Figure 1. These curves correspond to the samples of monolayers at different stages of the relaxation process chosen for the preparation of LB films and following X-ray and AFM analyses. The relaxation curves point to the progressive-area loss by the monolayers, which bears evidence of 2D-3D transformations.2,18 The results of X-ray specular reflection measurements for C20 monolayers relaxed on water of pH 3 and transferred to silicon substrates are shown in Figure 2a along with the curves of the uncovered silicon substrate and the unrelaxed monolayer. The reflectivity data obtained from the uncovered silicon substrate (curve 1 in Figure 2a) correlate with the results published previously by other authors.21,22,26 An oscillation in this reflectivity curve suggests the presence of an additional layer, with an electron density nearly half that of silicon oxide. Such a layer is usually recorded when the silicon wafers with native silicon oxide are measured in air and some time after cleaning.21,22 Some authors have suggested that this layer was due to organic contaminant,21 whereas others have assumed that there was an absorbance of water by the silicon oxide surface.22 In measurements of justcleaned substrates and in a noble gas atmosphere21,27 the (26) Nitz, V. Ph.D. Thesis, Christian-Albrecht-University, Kiel, 1995, p 120. (27) Nakanishi, M.; Sakata, O.; Hashisume, H.; Foran, G.; Nakano, M. Photon Factory Aktivity Rep. 1993, 11, 316.

additional layer and the oscillation on the reflectometry curve were not found for incidence angles of more than 4°. This suggests the disappearance of an additional layer or a decrease in thickness of at least several angstroms. It can therefore be expected that in our case this additional layer had no considerable influence on the deposition of monolayers. Curve 2 in Figure 2a corresponds to the unrelaxed C20 monolayer. During the fitting of this curve it was difficult to identify precisely the head of the C20 molecule and the native silicon oxide because of the small difference in their electron densities.25 The fitted thickness of the C20 monolayer on the silicon substrate is 2.70 ( 0.05 nm, which correlates with the length of an untilted molecule.28 After relaxation on water of pH 3 the reflectivity curves of C20 monolayers (curves 3-7 in Figure 2a) reveal several peculiarities. With an increase in the relaxation time, the oscillations in intensity due to the monolayer on the substrate surface become less prominent without changing their positions. When starting with a relaxation time of 1000 s, the peaks that correspond to the Bragg diffraction on a 3D multilayer structure appear in the curves in addition to the reflection from the monolayer (the positions of the different-order Bragg peaks are marked in Figure 2a by Roman numerals). The simultaneous existence of the intensity oscillations from the monolayer and the Bragg peaks from the multilayer indicates that the 3D structure corresponds to islands rather than to a continuous film. The intensity of the Bragg peaks grows with an increase in the relaxation time, while the intensity oscillations decrease. This corresponds to a phase transition from a monolayer to 3D aggregates. The Kiessig oscillations, which are distinguishable on the curve corresponding to a relaxation time of 4700 s, show that the height of the islands has become more uniform. It is noticeable that the intensity of the even-order Bragg peaks in these curves is slightly suppressed. As shown by our simulations (see the dotted curve 7 in Figure 2a) (28) Robinson, I.; Jarvis, D. J.; Sambles, J. R. J. Phys. D: Appl. Phys. 1991, 24, 347.

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Figure 3. Comparison of specular reflection and diffuse scattering data for relaxed C20 monolayers transferred to the different substrates. (a) (1) relaxation time of 1500 s, silicon substrate; (2) 1900 s, glass; (3, 3d) 3000 s, silicon; (4, 4d) 2500 s, glass; successive pairs of curves are shifted by a factor of 20 along the intensity axis; diffuse scattering curves (θ - offset ) 0.25°) are marked by the letter “d”. (b) Isointensity contour map (log scale) for the sample with a relaxation time of 2500 s and a glass substrate, measured with synchrotron radiation.

and as outlined in the literature,24 the reason for such suppression is supposed to be the periodic bilayer structure of islands that have two molecules within the unit cell. The heads of the molecules form a maximum of electron density in the center of the unit cell. A small gap between the tails of the molecules provides a minimum of electron density at the edges of the unit cell. Another important feature of the curves observed at the early stages of monolayer relaxation is the splitting of the Bragg peaks of each order into two peaks (see two peaks of the third order on curve 5 in Figure 2a marked by the letters B and C; the additional meaning of this marking will be explained below). The distance between these two peaks increases with the order of diffraction, which proves that they correspond to two different multilayer phases. The periods of these phases calculated from the positions of the third-order Bragg peaks are 4.87 ( 0.03 and 4.41 ( 0.03 nm. This corresponds to the structures of the B and C phases of bulk arachidic acid.28,29 Assuming that the length of the two untilted molecules is equal to 5.36 nm28 (our results for the unrelaxed monolayer had shown that the length of an untilted molecule is 2.70 nm), it is possible to estimate the tilt of the molecules in the layers of these multilayer phases. It is 25° and 35° for the B and C phases, respectively. At the early stages of relaxation the amount of the C phase is greater than that of the B phase. With an increase in relaxation time, the B phase becomes predominant, but even at the late stages of relaxation, phase C is still visible as an enlarged intensity of Kiessig oscillation at the right side of the Bragg peaks (compare experimental and theoretical curves 7 in Figure 2a; calculations correspond to a multilayer structure consisting of one monolayer and eight bilayers of B phase). The narrowing of the Bragg peak width in successive stages during the relaxation process is due to an increase in the number of multilayer periods, i.e. the height of the 3D islands. The number of periods estimated from the half width of the Bragg peaks is approximately 10 bilayers for 4700 s of relaxation and 5 bilayers and less for 3000 s and shorter times of relaxation. B. X-ray Reflectometry of C20 Monolayers Relaxed on a Subsolution of Cadmium and Lead Cations. (29) Kitaigorodskii, A. I. Organic Chemical Crystallography; Publishing Consultants Bureau: New York, 1961.

Figure 4. Stability of C20 monolayers relaxed for periods of 500 s (1), 700 s (2), 800 s (3), and 4700 s (4). Dashed and solid curves were measured in the first and the third month after sample preparation, respectively. The pairs of curves 1, 2, and 3 are shifted successively by a factor of 20 along the intensity axis; dashed and solid curves 4 are shifted by a factor of 400 and 2000 with respect to curves 3.

The reflectivity curves for the samples of C20 monolayers relaxed on aqueous subsolutions of bivalent counterions are shown in Figure 2b. The cadmium ions in the subphase of pH 5.3 stabilize the monolayer, so that there is no difference between the reflectivity curve taken after maintaining the monolayer at a constant pressure of 30 mN/m for a period of 3000 s and the monolayer curve at the relaxation time of zero (Figure 2b, dashed line curve 2 and solid line curve 1, respectively). The lead ions in water of pH 5.3 also considerably slow down the relaxation process of the C20 monolayer. The curve of the monolayer relaxed for such a long period as 11500 s (curve 7 in Figure 2b) indicates only very broad Bragg peaks at those positions that correspond to Bragg peak positions in the curve of the continuous multilayer film of lead arachidate, shown for comparison in Figure 2b, curve 8. The period of the 3D phase structure

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Figure 5. AFM images of C20 monolayers relaxed on water and transferred to silicon substrates. Water of pH 3: (a) relaxation time of 1500 s; (b) 3000 s; (c) 4700 s. Water of pH 3 with 5 × 10-4 M Pb(NO3)2: (d) 1000 s; (e and f) 2000 s.

calculated from the position of the third-order Bragg peak in curve 7 in Figure 2b is about 5.7 nm, which corresponds to a bilayer. The number of bilayers in the 3D phase islands, estimated from the half width of the same thirdorder Bragg peak, is not more than 2. The addition of lead ions in a subsolution of pH 3 does not slow down the relaxation process of C20 monolayers. The Bragg peak intensity of curves 4-6 in Figure 2b is only slightly lower than that in curves 4-7 in Figure 2a. The Bragg peak positions in curves 4-6 and the position of oscillation from the monolayer in curve 4 in Figure 2b are the same as the positions of the peaks in curves 4-7 and the oscillation position in curve 3 in Figure 2a. This means that the lead ions do not bind to the arachidic acid

monolayer at low pH. They do not leave the subphase either during the LB transfer process19 or during 2D-3D transformations, and thus they do not essentially influence the relaxation process. C. X-ray Measurements of Relaxed C20 Monolayers Deposited on Glass Substrates. The typical X-ray specular and off-specular reflection curves from monolayers of arachidic acid transferred after relaxation to glass substrates are compared in Figure 3 with those for silicon. The difference between the curves of the samples on silicon and those of the samples on glass substrates is that the decline of specularly reflected intensity proceeds much more quickly in the case of glass (compare curves 2 and 4 with curves 1 and 3 in Figure 3a) and that the

Relaxed Monolayers of Arachidic Acid

background level of our measurements with a conventional X-ray tube is much more rapidly reached. This is the reason why the intensities of the high-order Bragg peaks have become indistinguishable on curves 2 and 4, although they have been observed on those curves measured with synchrotron radiation (see Figure 3b and the synchrotron data not shown here30 ). The reason for the instant decline of the specularly reflected intensity is that the surface roughness of the samples is relatively high.31 The estimated root mean square (rms) roughness is ∼1.5 nm for glass and ∼0.5 nm for silicon substrates. These estimates were obtained by comparing curves 3 and 4 in Figure 3a with the theoretical simulations of the tail decline of the curves for substrates with different surface roughnesses. Thus, the microscope cover glass substrates are at least three times rougher than industrial silicon wafers. The roughness of the substrates is transferred to the monolayer and to interfaces inside the 3D multilayer islands, as is obvious from the gathering of diffusely scattered intensity into so-called “bananas”32 (see the peaks at the angles corresponding to the first-order Bragg peaks in the off-specular reflection curves 3d and 4d in Figure 3a and the strips parallel to the θ-offset axis on the intensity map in Figure 3b). The vertical correlation length of the roughness estimated from the width of the “banana”32 (the size along the θ axis) in curve 4d of Figure 3a is about 10 bilayers and approximately coincides with the height of the islands. The lateral correlation length of the roughness defined by the length of the “banana” (the size along the θ-offset axis) is about 0.7 µm and corresponds to the average lateral size of the 3D islands. D. Stability of Relaxed Monolayers. All X-ray measurements discussed above were carried out within the first month after preparation of the samples, and no changes in sample structure were noticed when X-ray measurements were repeated during this period. However, the reflectivity curves from several samples of arachidic acid monolayers with 500-800 s relaxation time, which were repeated in the third month after sample preparation, showed significant changes in structure (compare dashed and solid line curves 1-3 in Figure 4; dashed curve 1 corresponds to curve 3 in Figure 2a). The multilayer islands became much higher and their density increased, as was indicated by an increase in the intensity of Bragg peaks and a decrease in the intensity oscillations resulting from the monolayer itself. Nevertheless, the reflectivity curve of the sample of arachidic acid with 4700 s relaxation time measured in two and even four months did not show any changes (compare the dashed and solid line curves 4 in Figure 4; dashed curve 4 corresponds to solid curve 7 in Figure 2a). Thus it is possible to conclude that, at the early stages of relaxation, monolayers are less stable than those formed later. Additional information about the samples obtained by means of AFM allows us to discuss the other details of this effect below. E. Atomic Force Microscopy of Relaxed C20 Monolayers. Typical AFM images of relaxed monolayers are shown in Figure 5. They correspond to C20 monolayers relaxed on water of pH 3, for which the X-ray curves shown in Figure 2a (curves 3-7) and in Figure 2b (curves 4-6) were obtained. Two kinds of 3D islands can be found on (30) Kondrashkina, E. A.; Schmidbauer, M.; Ko¨hler, R.; Vollhardt, D. Collected Abstracts of the 4th International Conference on Surface X-ray and Neutron Scattering, Lake Geneva, WI, 1995; ANL: Argonne, IL, 1995; p 121. (31) Nevot, L.; Croce, P. Rev. Phys. Appl. 1980, 15, 761. (32) Holy, V.; Baumbach, T. Phys. Rev. 1994, B49, 10669.

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Figure 6. Optical microscopy photographs of C20 monolayers relaxed for periods of 3000 s (a) and 4700 s (b), respectively.

the surface of the samples. These islands look like granules and plates. The granules presented in the images of Figure 5 are not spiky because the vertical scale of the images is expanded by two orders of magnitude (the lateral scale in Figure 5 is 10 µm, and the vertical scale is 100 nm). Thus the real dimensions of the granules are several tens of nanometers to a few hundred nanometers in lateral size and several nanometers to several tens of nanometers in height. Consequently the 3D granules are rather flat. The large platelike islands reach a lateral size of several microns. Their height grows with an increase in the relaxation time and averages 4, 6, and 12 bilayers for 1500, 3000, and 4700 s, respectively (Figure 5a-c). These values correlate with the estimates made from X-ray measurements (4, 5, and 10 bilayers, respectively). The same growth tendency of the platelike islands can be observed by optical microscopy. The photographs in Figure 6a and b correspond to samples with relaxation times of 3000 and 4700 s, and the growth of the platelike island height appears as an increase in the darkness of the images. There were virtually no optically visible islands available on the surface of unrelaxed or relaxed monolayers for 500 s. The tendency in growth for granule-like islands is not so evident from the AFM data available as it is for platelike islands. As was shown in previous experiments18 under equal conditions, there were granules existing at the early stages of relaxation, and with an increase in the relaxation time, their density continued to increase until platelike islands appeared. However, our AFM measurements have shown that granule-like islands together with the platelike islands were found on all samples, although the density of plates was very low at the early stages of relaxation. The reason for such a discrepancy may be attributed to the difference in the initial state of the monolayer. A slow constant surface pressure relaxation, when only granules and no platelike islands could be observed at the early stages of relaxation,18 pointed to an obviously higher metastability of initial monolayers due to a smaller amount of defects. As a probable reason for the appearance of defects in the initial monolayer, the process of spreading can be mentioned. The local increase in surface pressure during the spreading of additional drops of solvent can lead to an irreversible collapse and to the formation of platelike islands. In spite of the presence of solitary platelike islands, we may conclude that the granules are predominant at the early stages of relaxation.

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Figure 7. AFM 3D images and their projections of relaxed monolayers of arachidates: (a and c) water of pH 5.3 with 5 × 10-4 M CdCl2, 3000 s; (b and d) water of pH 5.3 with 5 × 10-4 M Pb(NO3)2, 11500 s.

It is worth mentioning that our AFM measurements were carried out after the X-ray study and 1-2 months after preparation of the LB films so that the instability of relaxed monolayers, which was shown by our X-ray data, could have already affected the AFM results. Then the instability of monolayers relaxed for a shorter period of 500-800 s and the stability of the sample with a 4700 s time of relaxation could lead to the following assumption. The granule-like islands, which predominate at the early stages of relaxation, are less stable and can transform into more stable platelike islands either by increasing the time of constant pressure relaxation or even by a longterm storage on solid substrates. Evidence of coalescence of granule-like islands into platelike ones can also be found on the AFM images (see, e.g., Figure 5c-e). The possibility for granules to coalesce into platelike islands may suggest that granules have also a multilayer structure. Since granules are the less stable 3D aggregates, it might be assumed that they have a C structure. However, it was difficult to prove this assumption when measuring the exact height of granules by way of AFM or to separate their influence on the X-ray reflectivity curves. The analysis of the AFM images of platelike islands has shown that they consisted of domains of different heights which corresponded to an integer number of bilayers in line with the B form or C-form of bulk arachidic acid (compare, e.g., the steps on the surface of the plate island in Figure 5d). Moreover, the adjacent domains

actually possessed either the same B structure (then the steps appeared due to the different numbers of bilayers in the domain height) or the different B and C structures (then the steps on the surface appeared due to the different bilayer lengths of different structures that accumulated with the growth of the domain height). It was difficult to estimate the width of the domain boundary, but as can be seen in Figure 5d (boundaries on the side wall of the plate island), it is about 0.1 µm. The presence of multilayer fragments possessing B and C structures was also observed in continuous LB multilayer films of long chain fatty acids28,33 by means of X-ray reflectometry. The size of coexisting fragments was estimated to be about 1 µm. F. Atomic Force Microscopy of C20 Monolayers Relaxed on Subsolutions of Cadmium and Lead Ions. In Figure 7 the AFM images of relaxed monolayers of cadmium and lead arachidate are shown. They corroborate the X-ray data and indicate that the relaxation process is incomparably slower for the monolayers of fatty acids bound to metal ions. The X-ray reflectometry curves do not indicate any changes in the monolayer of cadmium arachidate after 3000 s of relaxation. Nevertheless, in this case the AFM images show that there were low folds of heights slightly greater than one molecule (Figure 7a and c). The directions of these folds possess the hexagonal symmetry of the molecular structure of the original monolayer. Direct evidence of the hexagonal symmetry (33) Leuthe, A.; Riegler, H. J. Phys. D: Appl. Phys. 1992, 25, 1786.

Relaxed Monolayers of Arachidic Acid

of island boundaries that correspond to the molecular structure was recently adduced.34 Here AFM studies were performed with a molecular level resolution for the cadmium arachidate LB film, where 3D islands appeared after aging the film under an aqueous surface. The AFM images of a lead arachidate monolayer, which was relaxed for a long time period of 11 500 s, show granule-like and platelike islands of a height of not more than 1 -2 bilayers (Figure 7b and d). The boundaries of the platelike islands also reveal the hexagonal symmetry of the molecular structure of the islands. There is a small amount of particular granule-like islands on both cadmium and lead arachidate monolayers (Figure 7). The height of these granules is more than 2 bilayers. They are probably formed by molecules of pure arachidic acid, which are more mobile than salt molecules. This fact indicates that not all monolayer molecules in a subsolution of pH 5.3 are bound to cadmium or lead ions,19 but we observed only a small amount of such molecules. Conclusions The X-ray and AFM data obtained have shown that at constant surface pressure relaxation the structure and surface morphology of arachidic acid monolayers were largely affected by the presence of bivalent ions as well as by the pH of the subsolution. At a low pH e 3 the monolayer molecules are not dissociated and the addition of cadmium or lead ions to the subphase does not result in binding of the monolayer to metal cations. As a consequence, the maintenance of such a monolayer under a constant surface pressure above the ESP leads to similar results of relaxation on the subphase with or without bivalent ions. The 3D phase nuclei that resemble granules and plates begin to grow in a monolayer from the first minutes of the relaxation process. The granule-like islands are prevalent at the early stages of relaxation. Then they are transformed into more stable platelike islands either by increasing the (34) Schwartz, D. K.; Viswanathan, R.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 10444.

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relaxation time up to 1000 s or during storing of the relaxed monolayer as an LB film on a solid substrate for longer than one month. At the early stages of relaxation, the structure of the 3D islands corresponded to the B and C phases of bulk arachidic acid. With an increase in the relaxation time, the structure assumed predominantly the B form. There were no differences between relaxed monolayers deposited on hydrophilic silicon and glass substrates other than the three times larger surface roughness of the glass substrates, which was transferred to the monolayer. The use of a subsolution of pH 5.3 provided the binding of the major part of cadmium and lead cations by the arachidic acid monolayer, which made the relaxation process incomparably more slow. X-ray reflectometry did not indicate any changes in the monolayer of cadmium arachidate after relaxation for 3000 s. AFM images, which are more sensitive to the surface morphology, show low folds with a height of about one length of a molecule arranged in a hexagonal symmetry, thus repeating the molecular structure of the original monolayer. A long relaxation for 11 500 s of the lead arachidate monolayer gave rise to 1-2 bilayer granules and plates with boundaries possessing a hexagonal symmetry. The combination of the two powerful methods, i.e. X-ray reflectometry and atomic force microscopy, is very useful for the study of 2D-3D transformations in monolayers, providing both structural and morphological information, local and averaged along the sample surface. Acknowledgment. The authors are grateful to colleagues from MPG-AG “Ro¨ntgenbeugung”, to S. Stepanov for providing the program for simulating the reflectivity curves, and to H. Raidt, W. Mo¨hling, and V. Kaganer for very helpful discussions. The authors appreciate the valuable discussions with T. Kato of Utsunomiya University (Japan). The assistance of I. Berndt and U. Modrow in preparation of the LB films is greatly appreciated as well. LA960188T