A Temperature-Dependent Two-Dimensional Condensation Transition

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A Temperature-Dependent Two-Dimensional Condensation Transition during Langmuir-Blodgett Deposition H. D. Sikes and D. K. Schwartz* Department of Chemistry, Tulane University, New Orleans, Louisiana 70118 Received April 3, 1997. In Final Form: June 11, 1997X Langmuir-Blodgett monolayers, deposited from a Langmuir monolayer (on the water surface) in a two-dimensional liquid (LE) state, were examined using atomic force microscopy (AFM), transmission infrared (IR) spectroscopy, and contact angle goniometry. Pentadecanoic and hexadecanoic acid films condensed into densely-packed islands on the mica substrate. IR spectra and contact angle data for these films suggested that molecules within islands are well-orderedssimilar to those within a monolayer deposited from a condensed monolayer phase (LC). Tetradecanoic acid monolayers, however, were uniformly (no condensation) covered by a monolayer of disordered molecules if deposited below 17 °C and displayed increasing numbers of dense, compact islands between 17 and 20 °C (resembling those in the longer chain acids). At slightly higher temperatures, lower porous islands were observed in addition to these dense, high islands. By about 22 °C, the surface was completely covered by the porous aggregates. We believe this represents a sequence of two transitions of the surface phase after transfer. We also present AFM images of 12-NBD octadecanoic acid monolayers deposited from the LE phase or the LE/LC coexistence region. Orientational alignment of the needle-shaped LC domains in the direction of growing islands lends support to the hypothesis of a surface tension gradient driven island growth mechanism.

Introduction In recent years there has been an increasing awareness that the process of Langmuir-Blodgett (LB) deposition can have a dramatic effect on molecular conformation and packing. Detailed studies of monolayer structure before (on the water surface) and after transfer have found, more often than not, that the details of the molecular packing were different.1-3 Typically these monolayers were compressed into a relatively close-packed state before transfer and changes in structure during transfer consisted of altered molecular lattice structure or correlations. Riegler and co-workers performed a series of experiments demonstrating that a two-component monolayer may be induced to phase separate during LB deposition.4-7 More recently, similar two-dimensional (2D) condensation during deposition has been observed even in single component monolayers.8-12 Such condensation can be alternately viewed as molecular aggregation on the solid surface and so has connections to the fields of adsorbed surfactant monolayers (e.g., in flotation processes) and self-assembled monolayer growth and structure, in which the possibility of 2D molecular aggregation is thought to be important.13-16 * To whom correspondence should be addressed: tel, 504/8623562; fax, 504/865-5596; e-mail, [email protected]. X Abstract published in Advance ACS Abstracts, July 15, 1997. (1) Tippmann-Krayer, P.; Mo¨hwald, H. Langmuir 1991, 7, 2298. (2) Steitz, R.; Mitchell, E. E.; Peterson, I. R. Thin Solid Films 1991, 205, 124. (3) Shih, M. C.; Peng, J. B.; Huang, K. G.; Dutta, P. Langmuir 1993, 9, 776. (4) Spratte, K.; Riegler, H. Makromol. Chem. Macromol. Symp. 1991, 46, 113. (5) Riegler, H.; Spratte, K. Thin Solid Films 1992, 210/211, 9. (6) Spratte, K.; Riegler, H. Langmuir 1994, 10, 3161. (7) Spratte, K.; Chi, L. F.; Riegler, H. Europhys. Lett. 1994, 25, 211. (8) Mikrut, J. M.; Dutta, P.; Ketterson, J. B.; MacDonald, R. C. Phys. Rev. B 1993, 48, 14479. (9) Rana, F. R.; Widyati, S.; Gregory, B. W.; Dluhy, R. A. Appl. Spectrosc. 1994, 48, 1196. (10) Chi, L. F.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Thin Solid Films 1994, 242, 151. (11) Fang, J.; Knobler, C. M. J. Phys. Chem. 1995, 99, 10425. (12) Sikes, H. D.; Woodward, J. T.; Schwartz, D. K. J. Phys. Chem. 1996, 100, 9093. (13) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354-3357.

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In previous work we showed that simple fatty acids, when deposited from the 2D liquid (LE) phase, often condensed into close-packed islands during transfer.12 Such condensation occurred on mica substrates and on silicon oxide substrates at high pH, but not on silicon oxide substrates at low pH. This led us to believe that surface charge was an important factor in the condensation. In addition, the morphology of the 2D aggregates was interesting: small islands were compact while large islands were dendritic. This implied an instability in the 2D growth process of the aggregatesswe suggested a process based on Marangoni (surface tension gradient-driven) transport. In this paper we extend our previous work in an attempt to understand the condensation transition better. In particular, we report the behavior of the transition as a function of deposition temperature. In one system, the condensation is found to stop completely below a critical temperature. Also, we present observations of film morphology that we believe clearly demonstrate the presence of surface tension driven interfacial flow during the condensation process. Experimental Details Tetradecanoic acid (TDA), pentadecanoic acid (PDA), hexadecanoic acid (HDA, Aldrich, 99%)), or 12-NBD octadecanoic acid (NBDOA, Molecular Probes, 99%) was spread from chloroform solution onto a pure water surface (Millipore Milli-Q UV+) contained in a NIMA LB trough held at constant temperature (0.2 °C. The films were transferred to mica substrates by vertical dipping while the monolayer was held at constant surface pressure (π) in the LE phase or in the LE/LC coexistence region. Mica substrates were freshly cleaved immediately before use. Transfer ratios were typically 1.0 ( 0.1. Imaging was performed using a Nanoscope III atomic force microscope (AFM) under ambient conditions using a 15 µm × 15 µm or a 150 µm × 150 µm scan head and a silicon nitride tip on an integral cantilever with spring constant 0.12 N/m in contact mode (PDA and HDA) or a silicon tip on a “diving board” cantilever in “tapping mode” (TDA and NBDOA) Images were obtained from at least five (14) Allara, D. L.; Parikh, A. N.; Judge, E. Chem. Phys. 1994, 100, 1761-1764. (15) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Langmuir 1996, 12, 3626. (16) Woodward, J. T.; Schwartz, D. K. J. Am. Chem. Soc. 1996, 118, 7861.

© 1997 American Chemical Society

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Figure 1. AFM images of pentadecanoic acid monolayers deposited from the LE phase (at π ) 2 mN/m) at (a) 22, (b) 28, or (c) 34 °C, demonstrating the gradual decrease in size of both dendritic and compact aggregates with increasing temperature. The total number of islands increases, keeping the total surface coverage approximately constant. macroscopically-separated areas on each sample; at least two independent samples of each type were prepared and imaged. Representative images are presented below. For measurements of the so-called “static” contact angle, we followed a procedure described by Bain et al.17 in which a 1 mL drop of hexadecane was formed at the end of a needle and brought into contact with the surface. The needle was removed and the contact angle measured. Results were reproducible to within (2°. Infrared absorption data were obtained using a Mattson Cygnus 100 spectrometer in transmission mode with a 6.4 mm pinhole defining the incident beam. Because of the low signal from the monolayer samples, data were obtained for 3200 scans (about 1.5 h) for both the film and the bare substrate in order to minimize statistical noise. In order to obtain background spectra that could be subtracted reliably, we found it necessary to use the same substrate held in the same position in the spectrometer. Accordingly, we fashioned a sample holder that (17) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; et al. J. Am. Chem. Soc. 1989, 111, 321-335.

could be reproducibly positioned in the spectrometer to within (0.1 mm (much less than the beam size). We first measured the spectrum of the monolayer sample. We then removed the entire holder and placed it in a UV-oxygen cleaner (Boekel Industries) to destroy the monolayer (both sides of the sample were cleaned). The holder was then repositioned within the spectrometer and the background spectrum was measured. Because of the transmission geometry, periodic oscillations were typically observed in the spectra due to interference effects. Depending on the substrate thickness, these oscillations were more or less severe, and with the appropriate choice of substrate the actual peaks could be easily distinguished. IR measurements were repeated on at least two independent samples of each type. The lineshape of each IR spectrum, in the region 2800-3000 cm-1, was fit to a function consisting of two independent Lorentzian peaks with a linear background. The nonlinear least-squares fitting was performed using the SigmaPlot software package.

Results and Discussion Temperature Dependence. Figure 1 shows typical AFM images of PDA monolayers transferred from the LE

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Figure 2. Transmission IR spectra (CH stretching region) of hexadecanoic acid monolayers deposited from the LC phase and the LE phase. The solid lines are fits to Lorentzian peak shapes. The peak positions and widths of the two spectra are identical, suggesting that the molecules within islands (deposited from the LE phase) have a similar conformation as those deposited from the LC phase. The dashed lines are drawn at 2851 and 2919 cm-1, respectively, as guides to the eye.

phase onto mica substrates at π ) 2 mN/m over a range of temperatures. An inhomogeneous monolayer is observed with high islands surrounded by lower areas. The same phenomenon is observed with hexadecanoic acid. The height of the islands, 2 nm, is consistent with the fully-extended length of the molecules, suggesting that the islands consist of relatively close-packed well-ordered molecules and that they are surrounded by bare substrate. Contact angle and IR data support this picture. We have compared the hexadecane contact angle and IR spectra of monolayers deposited from the LE phase with those deposited from the densely-packed LC phase. The hexadecane contact angle of PDA and HDA monolayers deposited from the LC phase is 35 ( 3°, consistent with a fairly, but not perfectly, ordered methyl surface. (The contact angle on well-ordered self-assembled monolayer surfaces may be as high as 46°.) By comparison, the contact angle measured for HDA monolayers deposited from the LE phase was 23 ( 3°. We can use the Cassie equation18 as a simple way to model the expected contact angle of a heterogeneous surface consisting of patches of monolayer. This gives an expression for the contact angle

cos θ ) cos θmica + χ(cos θmonolayer - cos θmica) where χ is the fractional monolayer coverage. Using θmonolayer ) 35°, θmica ) 0°, and χ ) 0.5 gives an expected contact angle of 24.5° for the heterogeneous surface, consistent with our measurements. Figure 2 compares the CH stretching region of the IR spectra of HDA monolayers deposited from the LE and LC phases. Two peaks are visible, corresponding to the symmetric methylene stretch, νs(CH2), at about 2851 cm-1, and the antisymmetric methylene stretch, νa(CH2), at about 2920 cm-1. The positions and widths of the peaks are the same for both films. The positions of these peaks are typical (although at the high end of the acceptable range) for all trans well-ordered alkyl chains.19 This is again consistent with our picture that the molecules within the islands (in PDA and HDA) have a structure and conformation (18) Cassie, A. B. D. Discuss. Faraday Soc. 1952, 75, 5041. (19) Cameron, D. G.; Dluhy, R. A. In Spectroscopy in the Biomedical Sciences; Gendreau, R. M., Ed.; CRC Press: Boca Raton, FL, 1986; p 53.

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analogous to those in a homogeneous monolayer deposited from the condensed LC phase. In PDA films prepared at 22 °C, a few large dendritic aggregates are observed along with smaller, compact islands. As the deposition temperature is increased, the number of large aggregates decreases until, at 34 °C, the aggregates are relatively uniform in size. The total number of islands, therefore, increases with temperature. However, the fraction of the surface covered by islands remains at approximately 50%, consistent with condensation of molecules from the dilute LE phase on the water surface (area/molecule ) 40 Å2) to an approximately closepacked state (area/molecule ) 20 Å2). In previous work,12 we proposed that as the water layer between the molecules and substrate thins, isolated nuclei of a two-dimensional condensed phase form and attach to the substrate. These aggregates grow by adsorption of additional molecules from the remaining LE phase until this reservoir is used up or else the process is quenched for another reason such as drainage of the water layer. The dendritic morphology of the larger aggregates demonstrates that the growth process of the islands is unstable, possibly driven by surface tension gradients. Within this picture, the increasing number of islands with temperature is consistent with an increase in the nucleation rate. When the nucleation rate is small (at low T) we expect to find fewer and larger aggregates, but when there are many nuclei (at high T) aggregates do not grow very large before all of the molecules in the LE phase are exhausted. The decrease in nucleation rate with decreasing temperature suggests that we are nearing a transition temperature, at low temperatures, beyond which islands would not nucleate at all. Unfortunately, the phase diagram of PDA is such that the LE phase does not persist below about 21 °C. In TDA, however, the triple point is at lower T and we were, in fact, able to find a temperature below which islands do not form during deposition from the LE phase. Figure 3 shows typical AFM images of TDA monolayers deposited from the LE phase, at π ) 2 mN/m, at deposition temperatures ranging from 16.7 to 22.6 °C. Below about 17 °C (Figure 3a), the monolayer surface is homogeneous. However, transfer ratios, as well as the IR and contact angle data shown below, demonstrate that a monolayer is indeed present. Between deposition temperatures of 17 and about 20 °C, increasing numbers of dense islands are observed (Figure 3b), reminiscent of those in PDA or HDA. Figure 4 shows the number of observed dense islands as a function of temperature in this range, extracted from observations of 15 separate films. For the sake of comparison, the line drawn through the data represents the expected functional dependence of the nucleation rate on temperature for a simple first-order phase transition in two-dimensions

[

N ∝ exp

-a T(T - T0)

]

where T0 is the transition temperature and a was used as a fitting parameter. Since the theory of homogeneous nucleation provides an expression for a in terms of the line tension and the difference in free energy between the phases,20 we had originally hoped to extract approximate values of the line tension from these data. However, given the mediocre agreement with the functional form, the large uncertainties of and interdependencies between the fitting parameters, and the difficulties involved in determining the shape of the phase diagram, we were forced to abandon this hope. Therefore, we certainly do not claim any (20) Muller, P.; Gallet, F. Phys. Rev. Lett. 1991, 67, 1106.

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Figure 3. AFM images of tetradecanoic acid monolayers deposited (at π ) 2 mN/m) at various temperatures: (a) 16.7 °C, the surface is featureless; (b) 18.2 °C, a few dense islands are observed; (c) 19.9 °C, dense island regions coexist with porous regions of lesser height; (d) 22.6 °C, porous aggregates only.

profound agreement with this functional form, simply a qualitative consistency. The temperature behavior of this “condensation” is intriguing and somewhat counterintuitive. If the island formation truly represents an equilibrium phase transition, then we have observed a transition to a solid phase from a liquid phase by increasing the temperature. This would imply that the “island” phase has an anomalously high entropy. At temperatures near the transition, when only a few islands are observed, the surface coverage is clearly much less than 0.5. Thus the islands do not account for all of the molecules in the monolayer; many must remain in the regions between the islands. This may explain why good images were not obtained using contact mode AFM on these films; it was necessary to use tapping mode. At deposition temperatures of about 20 °C, a second type of island morphology is observed to coexist with the dense islands. These aggregates are porous and are not as high as the dense islands (see Figure 3c). As the

deposition temperature is increased further, the porous aggregates eventually replace the dense islands completely (Figure 3d). We interpret this sequence of images as evidence that a 2D condensation transition occurs during LB transfer in TDA at all temperatures above 17 °C. However, the structure of the condensed phase that forms below 20 °C is different from that of the phase that forms above 20 °C. The IR absorption in TDA monolayers (see Figure 5) was dramatically weaker (by almost an order of magnitude) than in analogous HDA monolayers. Therefore, it was difficult to extract the same degree of quantitative information. We can make several conclusions with confidence, however. The presence of IR peaks from the monolayers deposited at 16.7 °C (which appeared homogeneous with the AFM) is conclusive evidence that a monolayer is indeed present. The fact that these peaks are so weak and very broad may indicate that the molecules are disordered and/or are not oriented normal

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Figure 4. Number of islands, per 100 µm2, observed on tetradecanoic acid monolayers prepared at various temperatures. The solid line represents the best fit to the functional form expected for the nucleation rate of a first-order condensation transition as discussed in the text.

Figure 5. Transmission IR spectra (CH stretching region) of a tetradecanoic acid monolayer deposited from the LC phase and monolayers deposited from the LE phase at 16.7, 18.2, or 22.6 °C.

to the surface on average. The peaks obtained on films prepared at 18.2 °C are quite broad compared to those from a film deposited from the LC phase. In fact, the peak at 2850 cm-1 appears split. This is consistent with the implication from AFM images that only a fraction of the molecules in these films were condensed into islands; others remained between islands, presumably with different molecular conformations. Aside from this broadening, however, the spectrum at 18.2 °C is similar to the spectrum of a film deposited from the LC phase. This is consistent with the picture that the islands that form between 17 and 20 °C in TDA are analogous to those seen in PDA and HDA, in which the molecules are well-ordered and densely packed. The weaker peaks in the spectrum from a film at 22.6 °C suggest that the molecules in the porous aggregates are less well-ordered than those in the dense aggregates formed at lower temperatures. The 10° contact angle of hexadecane on TDA monolayers prepared at 16.7 °C also indicates that a monolayer is present but that the molecules are poorly ordered. The contact angle was 19° for monolayers deposited at 18.2 °C

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and 22° for films deposited at 22.6 °C. These are generally consistent with the contact angles obtained from PDA and HDA monolayers deposited from the LE phase as previously discussed. Surface Tension Gradients. In previous work we proposed a possible mechanism to explain the dendritic shapes observed in large aggregates. The mechanism was analogous to growth instabilities during crystallization (e.g., Mullins and Sekerka instability);21 however, the driving force was suggested to be Marangoni (surfacetension driven) flow rather than diffusion. When the initial nucleus is formed there must be a depletion of surfactant molecules in the liquid phase surrounding the nucleus (because the molecules in the solid phase take up only half the area). Molecules are transported toward the nucleus because of the resulting surface tension gradient. However, as molecules reach the island they are converted to a solid phase, and a steady-state surface tension gradient is maintained. This gradient is enhanced near a protrusion that occurs as a fluctuation in the shape of a growing domain, and therefore, the compact shape is inherently unstable. A consequence of such a growth mechanism is that a steady interfacial flow must be established toward a growing island. We were able to measure the effect of such flow by observing the alignment of pre-existing anisotropic crystallites during the deposition process. Monolayers of the fluorescent molecule, NBDOA, undergo a 2D liquid to solid phase transition near room temperature.20,22 LB monolayers of this substance deposited from the 2D liquid phase onto mica substrates exhibit a similar morphology as the previously mentioned fatty acidsssmall compact islands and large dendritic islands are observed by AFM (see Figure 6). It is well-known, from fluorescence microscopy observations, that domains of the solid phase on the water surface (in the 2D liquid/solid coexistence region) are highly anisotropic crystallites (needles).20,22 Thus it is noteworthy that the islands grown during deposition from the liquid phase have no needle-like character despite the anisotropic molecular interactions. This is consistent with fast growth driven by surface flow. Figure 7 shows a typical AFM image of an LB film deposited from a Langmuir monolayer in the liquid/solid coexistence region. Several types of structures are seen: small compact and larger dendritic aggregates (as were seen in films deposited from the liquid phase) and needles such as are present on the water surface. We note that the needles are generally clustered around, and pointed toward, the porous, dendritic islands. This is consistent with the hydrodynamic behavior expected for rod-shaped particles under flow; they will move in the direction of flow and be oriented by the flow as well.23,24 We believe that these images are evidence of long-range interfacial flow fields in the direction of the growing aggregates consistent with our hypothesized surface tension gradient driven island growth mechanism. Conclusions Langmuir-Blodgett monolayers of several fatty acids were examined using atomic force microscopy, transmission infrared (IR) spectroscopy, and contact angle goniometry. AFM images suggested that molecules within pentadecanoic and hexadecanoic acid films, deposited from Langmuir monolayers in an expanded liquid phase, condensed into densely-packed islands on the solid (21) Mullins, W. W.; Sekerka, R. F. J. Appl. Phys. 1963, 34, 323. (22) Muller, P.; Gallet, F. J. Phys. Chem. 1991, 95, 3257. (23) Jeffery, G. B. Proc. R. Soc. London, Ser. A 1922, 102, 161. (24) Okagawa, A.; Cox, R. G.; Mason, S. G. J. Colloid Interface Sci. 1973, 45, 303.

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Figure 7. AFM image of a 12-NBDOA monolayer deposited from the liquid/solid coexistence region. Porous, dendritic islands are observed similar to the one in Figure 6b. In addition, long, needle-shaped 2D crystallites (present on the water surface) are observed clustered around, and pointing toward the dendritic islands.

Figure 6. AFM images of 12-NBD octadecanoic acid monolayers deposited from the two-dimensional liquid phase. (a) Small compact and (b) large dendritic islands are observed in qualitative similarity to monolayers of PDA and HDA deposited from the LE phase.

substrate so long as the substrate was prepared to have a significant negative surface charge (e.g., mica, or silicon oxide at high pH). IR spectra and contact angle data for pentadecanoic and hexadecanoic acid films suggested that molecules within islands were well-orderedssimilar to those within a monolayer deposited from a condensed monolayer phase (LC). Tetradecanoic acid LB monolayers, however, were uniform (no islands) if deposited below 17 °C, and displayed increasing numbers of dense, compact islands between 17 and 20 °C (resembling those in the longer chain acids). At slightly higher temperatures, lower porous islands were observed in addition to these dense, high islands. At even higher temperatures, the surface was covered by the porous aggregates only. IR and contact

angle data for tetradecanoic acid monolayers were consistent with disordered molecules below 17 °C. The broad IR peaks in the CH stretch region at 18.2 °C were consistent with coexistence of two or more types of coexisting molecular conformations. Taken together the evidence was consistent with the following picture: below 17 °C, tetradecanoic acid monolayers are transferred intact from the disordered LE phase on the water surfacesno condensation occurs, but the molecules are relatively disordered as in the LE phase on the water surface. Between 17 and 20 °C increasing numbers of solid-like islands nucleate during transfer. At higher temperatures, molecules still condense during transfer; however, they form a less ordered surface phase. Monolayers of NBDOA also formed dense molecular aggregates when deposited from the 2D liquid phase. These aggregates were compact when small and dendritic when large, exactly as was observed with the n-alkanoic acids; there was no indication of faceting or anisotropy in these islands. Monolayers deposited from the liquid/solid coexistence region displayed the distinctive porous, dendritic islands formed during deposition along with the needle-shaped solid domains pre-existing on the water surface. These needles were clustered around and pointing toward the porous islands consistent with the orientation of rodlike rigid particles by surface flow. This is consistent with the proposed surface tension gradient driven mechanism for island growth. Acknowledgment. This work was supported by the National Science Foundation (Grant No. CHE-9614200), the Camille and Henry Dreyfus New Faculty Award Program, the donors of the Petroleum Research Fund, and the Center for Photoinduced Processes (funded by the National Science Foundation and the Louisiana Board of Regents). LA970346W