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Langmuir 2007, 23, 482-487
Self-Organization of a Wedge-Shaped Surfactant in Monolayers and Multilayers Nicholas Cain, Josh Van Bogaert, Douglas L. Gin, Scott R. Hammond, and Daniel K. Schwartz* Department of Chemical and Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed August 7, 2006. In Final Form: October 12, 2006 The self-organization behavior of a wedge-shaped surfactant, disodium-3,4,5-tris(dodecyloxy)phenylmethylphosphonate, was studied in Langmuir monolayers (at the air-water interface), Langmuir-Blodgett (LB) monolayers and multilayers, and films adsorbed spontaneously from isooctane solution onto a mica substrate (self-assembled films). This compound forms an inverted hexagonal lyotropic liquid crystal phase in the bulk and in thick adsorbed films. Surface pressure isotherm and Brewster angle microscope (BAM) studies of Langmuir monolayers revealed three phases: gas (G), liquid expanded (LE), and liquid condensed (LC). The surface pressure-temperature phase diagram was determined in detail; a triple point was found at ∼10 °C. Atomic force microscope (AFM) images of LB monolayers transferred from various regions of the phase diagram were consistent with the BAM images and indicated that the LE regions are ∼0.5 nm thinner than the LC regions. AFM images were also obtained of selfassembled films after various adsorption times. For short adsorption times, when monolayer self-assembly was incomplete, the film topography indicated the coexistence of two distinct monolayer phases. The height difference between these two phases was again 0.5 nm, suggesting a correspondence with the LE/LC coexistence observed in the Langmuir monolayers. For longer immersion times, adsorbed multilayers assembled into highly organized periodic arrays of inverse cylindrical micelles. Similar periodic structures, with the same repeat distance of 4.5 nm, were also observed in three-layer LB films. However, the regions of organized periodic structure were much smaller and more poorly correlated in the LB multilayers than in the films adsorbed from solution. Collectively, these observations indicate a high degree of similarity between the molecular organization in Langmuir layers/LB films and adsorbed selfassembled films. In both cases, monolayers progress through an LE phase, into LE/LC coexistence, and finally into LC phase as surface density increases. Following the deposition of an additional bilayer, the film reorganizes to form an array of inverted cylindrical micelles.
Introduction The control and understanding of the self-assembly behavior of amphiphilic molecules at surfaces have received considerable interest in recent years due to the fact that they spontaneously form highly organized structures with characteristic length scales of 2-5 nm. During the past decade, numerous surfactants have been shown to form organized micelle-like structures at the solidaqueous solution interface.1-8 Aggregate formation and the resulting geometries of the self-organized structures, which include spheres, hemispheres, cylinders, and hemicylinders, are dependent upon a variety of system conditions. In particular, surfactant shape and substrate chemistry play fundamental roles in the self-assembly of surface structures. As in bulk surfactant phases, the geometry of aggregates is generally correlated to the molecular shape; that is, roughly cylindrical molecules tend to form bilayers, while wedge-shaped molecules form aggregates with significant spontaneous curvature such as cylinders or spheres. * Corresponding author. Phone: (303) 735-0240. Fax: (303) 492-4341. E-mail:
[email protected]. (1) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (2) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 3207-3214. (3) Manne, S.; Schaffer, T. E.; Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky, G. D.; Aksay, I. A. Langmuir 1997, 13, 6382-6387. (4) Grant, L. M.; Ducker, W. A. J. Phys. Chem. B 1997, 101, 5337-5345. (5) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160-168. (6) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685-1692. (7) Velegol, S. B.; Fleming, B. D.; Biggs, S.; Wanless, E. J.; Tilton, R. D. Langmuir 2000, 16, 2548-2556. (8) Zou, B.; Qiu, D. L.; Hou, X. L.; Wu, L. X.; Zhang, X.; Chi, L. F.; Fuchs, H. Langmuir 2002, 18, 8006-8009.
Figure 1. Molecular structure of TDPMP.
Block copolymers exhibit self-organizing behavior that is fundamentally similar to that observed with surfactants, albeit with somewhat larger length scales. Nanostructured block copolymer films have found applications as templates for nanomaterials and lithography.9-11 Thin films of dendrimers also show promise in this area.12-14 These polymer films are amenable to conventional fabrication methods such as spin coating, and typically display characteristic lengths scales in the 10-50 nm range. Analogous nanostructured surfactant films have characteristic length scales of 2-5 nm, and could potentially provide a complementary approach to polymer films as templates in this smaller size range. However, there are different challenges (9) Thurn-Albrecht, T.; Schotter, J.; Kastle, C. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126-2129. (10) Tsui, O. K. C.; Russell, T. P.; Hawker, C. J. Macromolecules 2001, 34, 5535-5539. (11) Xu, T.; Stevens, J.; Villa, J. A.; Goldbach, J. T.; Guarim, K. W.; Black, C. T.; Hawker, C. J.; Russell, T. R. AdV. Funct. Mater. 2003, 13, 698-702. (12) Percec, V.; Cho, W. D.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 2001, 123, 1302-1315. (13) Zhang, L.; Huo, F. W.; Wang, Z. Q.; Wu, L. X.; Zhang, X.; Hoppener, S.; Chi, L. F.; Fuchs, H.; Zhao, J. W.; Niu, L.; Dong, S. J. Langmuir 2000, 16, 3813-3817. (14) Jung, H. T.; Kim, S. O.; Ko, Y. K.; Yoon, D. K.; Hudson, S. D.; Percec, V.; Holerca, M. N.; Cho, W. D.; Mosier, P. E. Macromolecules 2002, 35, 37173721.
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Figure 2. A surface pressure-area isotherm of TDPMP obtained at 20 °C showing representative behavior consistent with several two-dimensional phases including a gas phase (G), a liquid expanded phase (LE), and a liquid condensed phase (LC), and the transitions between them.
Figure 4. Isotherms for the TDPMP monolayer obtained at various temperatures as indicated in the legend. Note the increasing surface pressure of the plateau regions as temperature increases. At the lowest temperatures, no clear plateau is visible. With the exception of the 28.8 °C isotherm, the horizontal data were adjusted for emphasis of trends and ease of viewing.
Figure 3. Representative BAM images taken from a compression isotherm at 22.2 °C. All images are the same size scale. Uneven illumination is caused by the image area being larger than the laser spot, and the circular distortions are optical interference. (a) G-LE coexistence (the dark region is the gas phase). (b) LE phase. (c) LE-LC coexistence (the small white droplets are LC regions). (d) LC phase.
involved in depositing organized nanostructured films of small molecules that are stable under ambient conditions where they can be used as templates for further processing. For example, it is not generally possible to create well-organized conformal films of small molecules via spin-coating or other casting processes. More typically, organized monolayers or multilayers are achieved through self-assembly (spontaneous adsorption on a solid surface) or Langmuir-Blodgett (LB) deposition (layerby-layer transfer from the liquid/vapor interface to a solid substrate). Also, surface micelles of conventional surfactants at the substrate-aqueous solution interface lack technological appeal due to the fact that these structures are generally stable only in contact with solution.8 While some exceptions exist, such as films of amphiphiles whose stability is enhanced through interactions of mesogenic groups,15,16 we feel that the assembly of inverted-micellar systems from nonaqueous solution provides a more general route to the preparation of nanostructured smallmolecule films that are stable under ambient conditions. In these systems, the hydrophobic moieties of the surfactants are exposed to the surrounding solvent, with the hydrophilic portion of the molecules shielded in the interior of the supermolecular assemblies. We showed in earlier work that this configuration (15) Zou, B.; Wang, L. Y.; Wu, T.; Zhao, X. Y.; Wu, L. X.; Zhang, X.; Gao, S.; Gleiche, M.; Chi, L. F.; Fuchs, H. Langmuir 2001, 17, 3682-3688. (16) Wang, M. F.; Qiu, D. L.; Zou, B.; Wu, T.; Zhang, X. Chem.-Eur. J. 2003, 9, 1876-1880.
remains stable and intact upon removal from solution,17 and that nanostructured self-assembled films of reverse-micelle-forming surfactants can be successfully deposited from nonaqueous solution provided there is a sufficiently strong interaction between the molecular headgroup and the solid substrate upon which the film is deposited. In this Article, we systematically explore the process of selfassembly of a model inverted-micelle-forming surfactant in thin films, by comparing the structure of the Langmuir monolayers, LB films, and adsorbed self-assembled films they form as a function of surface concentration and film thickness. This comparison provides fundamental insight into the dynamical process of self-assembly in a constrained quasi-two-dimensional geometry. Experimental Section Materials. The synthesis and purification of disodium-3,4,5-tris(dodecyloxy)phenylmethylphosphonate (TDPMP, see Figure 1) has been described previously.18 Isooctane (99.9%; Fisher Scientific, Fair Lawn, NJ) was used as the solvent for self-assembly. Tetrahydrofuran (99.9%) and pentane (99.9%), also purchased from Fisher Scientific, were used as solvents for Langmuir monolayer deposition. Methanol (99.9% purity), sulfuric acid, and hydrogen peroxide were also purchased from Fisher Scientific. Water was purified with a Milli-Q UV+ purification system (Millipore, Bedford, MA). Muscovite mica (purchased from Ted Pella, Inc., Redding, CA) was cut into 12.7 mm diameter disks and then cleaved just before use. For Langmuir monolayer deposition, TDPMP was dissolved in tetrahydrofuran/pentane (3:7 by volume) at a typical concentration of 0.4 mg TDPMP/mL solvent. After the TDPMP was added to the solvent mixture and shaken for several minutes, the solution was sonicated in a Fisher Scientific FS30 for 45 min and heated at 50 °C for 45 min to completely dissolve the TDPMP. For self-assembly experiments, sonication of TDPMP/isooctane solutions for 1 h was required for complete dissolution. All immersions for self-assembly were performed at room temperature for specified times ranging from a few seconds to 2 h. (17) Nelson, M.; Cain, N.; Taylor, C. E.; Ocko, B. M.; Gin, D. L.; Hammond, S. R.; Schwartz, D. K. Langmuir 2005, 21, 9799-9802. (18) Hammond, S. R.; Zhou, W. J.; Gin, D. L.; Avlyanov, J. K. Liq. Cryst. 2002, 29, 1151-1159.
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Figure 6. Two-dimensional phase diagram for TDPMP Langmuir monolayers on water with schematic structural diagrams. The diagram was constructed using BAM video and isotherm data.
Results and Discussion Figure 5. BAM images of low-temperature phase phenomena in TDPMP Langmuir monolayers. (a) G-LC coexistence at 7.9 °C. (b) All three phases in coexistence near the triple point at 8.9 °C. Langmuir Monolayers. Langmuir monolayers of TDPMP were prepared by depositing drops of solution on the surface of pure water (Millipore Milli-Q UV+) contained in a custom-built Teflon Langmuir trough. The temperature in the subphase was controlled to within (0.5 K using a combination of a recirculating water bath (Neslab) and thermoelectric Peltier elements. A Teflon-encapsulated thermocouple probe served to monitor the temperature of the water subphase. The surface area of the trough was adjusted by means of a motor-driven barrier, allowing compression or expansion of the monolayer. The surface pressure was monitored using a filter paper Wilhelmy plate and an R&K electrobalance. Imaging of the monolayer was performed by means of a custom-built Brewster angle microscope (BAM)19,20 as described previously.21 Langmuir-Blodgett Film Preparation. Langmuir-Blodgett films were prepared in a Nima 611 trough. Freshly cleaved mica substrates were immersed just below the water surface prior to monolayer deposition. The monolayer was compressed to the desired surface pressure (at a rate of 20 cm2/min), and one or more layers was transferred to the mica by successive dipping (at a rate of 2 cm/min) while the Langmuir film was held at the desired surface pressure and temperature. AFM Imaging. Atomic force microscope (AFM) imaging was performed with a Digital Instruments (now Veeco, Santa Barbara, CA) Nanascope III MMAFM instrument, equipped with an E scanner. All images were obtained through tapping mode using etched silicon probes (NanoDevices, Inc., Santa Barbara, CA) with fundamental resonance frequencies (fo) of 300 kHz. These cantilevers were 125 µm in length and have a spring constant (k) of 40 N/m, width of 45 µm, and thickness of 4 µm. Phase and height (topographical) images were captured simultaneously at scan rates 1.20-2.35 Hz. Images were acquired from at least three macroscopically separated areas of each sample, and the reported surface coverage and height differences are the result of the general trends found in these areas. The images were minimally flattened. The domain heights and sizes were determined from cross-sectional analysis. The two-dimensional (2D) spectra were obtained by a Fourier transform and used to determine the spacing of the observed columnar structures. (19) He´non, S.; Meunier, J. ReV. Sci. Instrum. 1991, 62, 936. (20) Ho¨nig, D.; Mobius, D. J. Phys. Chem. 1991, 95, 4590. (21) Ignes-Mullol, J.; Schwartz, D. K. Langmuir 2001, 17, 3017-3029. (22) Sauer, B. B.; McLean, R. S.; Thomas, R. R. Langmuir 1998, 14, 30453051. (23) Taylor, C. E.; Schwartz, D. K. Langmuir 2003, 19, 2665-2672.
BAM and π-A Isotherm Studies. Using data from the surface pressure-area isotherms correlated to BAM video from monolayer compression at different temperatures, the locations of monolayer phase transitions were determined. The surface pressures of the visible phase coexistence in the BAM video coincided well with the pressure “plateaus” in π-A isotherms. Figure 2 shows a representative π-A isotherm, obtained at 20 °C, and Figure 3 shows representative BAM images from each of the four distinct regions in the isotherm. At areas greater than about 100 Å2/molecule, the surface pressure is negligible (
