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Infrared Spectroscopy Analysis of the Structure of Multilayer Langmuir-Blodgett Films: Effect of Deposition Velocity and pH M. Elena Diaz, B. Johnson, K. Chittur, and R. L. Cerro* Department of Chemical and Materials Engineering, University of Alabama in Huntsville, Huntsville, Alabama 35899 Received June 9, 2004. In Final Form: October 29, 2004 Multiple Langmuir-Blodgett (LB) films of arachidic acid were deposited on germanium (Ge) substrates from subphase solutions of 10-4 M CdCl2 at different pH values and at different deposition speeds. Attenuated total reflectance infrared (ATR-IR) spectroscopy was used to obtain information on the molecular order and structure of these multilayer LB films. At pHs higher than the pKa of the fatty acid/cation system, transfers took place only during the downstroke, indicating X-type deposition. At pH ) pKa and large deposition speeds, deposition partially failed during the downstroke, resulting in Z-type depositions. Analysis of the infrared spectra indicates that multiple LB films deposited only during the upstroke (Z-type) or during downstrokes (X-type) have a centrosymmetric structure typical of films deposited during the upstroke and downstroke, except for a slight decrease in molecular order and tilt angle as the pH increases (X-type). The centrosymmetric structure indicates that rearrangement of layers takes place between cycles. Experimental evidence of such rearrangement occurring in a fatty acid/divalent cation salt subphase is shown here, and rearrangement alternatives are discussed.
1. Introduction The deposition of multilayer Langmuir-Blodgett (LB) films is classified into three basic modes1,2 (Figure 1). X-type depositions take place only during immersion (downstroke), and Z-type depositions take place only during removal (upstroke). On the other hand, Y-type depositions take place during immersion (downstroke) and during removal (upstroke). The type of deposition, however, does not guarantee the structure of multiple LB films, and a distinction should be made between the type of deposition and the resulting overall structure. The most common structure, also denoted as the basic unit,3 is the centrosymmetric, head-to-head and tail-to tail configuration4 (Figure 2 (1)). If there is no rearrangement after deposition, X-type depositions should produce asymmetric, head-to-tail structures where the hydrophobic tails are parallel and in the same direction as the normal to the solid surface, and Z-type depositions should produce asymmetric structures where the hydrophobic tails are parallel and in the opposite direction as the normal to the solid surface (Figure 2 (2)). Although there is a wealth of applications for LB films showing the basic structural unit, the overall molecular polarity required for nonlinear second-order optical effects is particularly well achieved by asymmetric organic materials. Hence, there would be a great deal of applications for X- and Z-type depositions, if they could render asymmetric LB films.3 Asymmetric films have also found applications in the fields of pyroand ferroelectrics.1,5 * Corresponding author. Phone: (256) 824-7313. Fax: (256) 8246839. E-mail:
[email protected]. (1) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, 1990; p 317. (2) Peterson, I. R. J. Phys. D: Appl. Phys. 1990, 23, 379. (3) Schwartz, D. K. Surf. Sci. Rep. 1997, 27, 241. (4) Takamoto, D. Y.; Aydil, E.; Zasadzinski, J.; Ivanova, A.; Schwartz, D. K.; Yang, T.; Cremer, P. Science 2001, 293, 1292. (5) Bune, A. V.; Zhu, C. X.; Ducharme, S.; Blinov, L. M.; Fridkin, V. M.; Palto, S. P.; Petukhova, N. G.; Yudin, S. G. J. Appl. Phys. 1999, 85 (11), 7869.
Reports that asymmetric head-to-tail films can be successfully prepared have been published,6,7 but there is also experimental evidence that regardless of the deposition type the basic centrosymmetric structure prevails,8,9 due to molecular rearrangement taking place either during or after deposition. The reasons for rearrangement may be traced back to more favorable energetic arrangements and are outside the scope of this paper. There is ample evidence of molecular rearrangement from X-type depositions inside the water subphase.4,10-12 There are some instances of molecular rearrangement of Z-type films that may take place outside the water subphase13,14 for perfluoro fatty acids, polyethylenimine, and a specific amphiphilic pyridine. Experimental strategies conducive to different types of deposition have been extensively studied.3,15,16 Some experimental results indicate that the X-type films are favored by increasing the pH16-18 or the subphase concentration of metal cation.17 The accumulated immersion time, associated with dipping speeds and time spent by deposited films under the water, has been shown to have (6) Allen, S.; Hann, R. A.; Gupta, S. K.; Gordon, P. F.; Bothwell, B. D.; Ledoux, I.; Vidakovic, P.; Zyss, J.; Robin, P.; Chastaing, E.; Dubois, J.-C. Proc. Soc. Photo-Opt. Instrum. Eng. 1987, 682, 97. (7) Kowel, S. R.; Hayden, L. M.; Selfridge, R. H. Proc. Soc. Photo-Opt. Instrum. Eng. 1987, 682, 103. (8) Fankuchen, I.; Bikerman, J. J.; Schulman, J. H. Phys. Rev. 1938, 53, 909. (9) Prakash, M.; Peng, J. B.; Ketterson, J. B.; Dutta, P. Chem. Phys. Lett. 1986, 128, 354. (10) Ehlert, R. C. J. Colloid Sci. 1965, 20, 387. (11) Angelova, A.; Ionov, R.; Reiche, J.; Brehmer, J. Thin Solid Films 1994, 242, 283 (12) Choi, J.; Cho, K.; Lee, W. H.; Lee, H. S. Thin Solid Films 1998, 327-329, 273. (13) Werkman, P. J.; Wilms, H.; Wieringa, R. H.; Schouten, A. J. Thin Solid Films 1998, 325, 238. (14) Lee, M. H.; Ha, T. H.; Kim, K. Langmuir 2002, 18, 2117. (15) Binks, B. P. Adv Colloid Interface Sci. 1991, 34, 343. (16) Gaines, G. L., Jr. Insoluble monolayers at liquid-gas interface; Interscience Publishers: New York, 1966. (17) Peng, J. B.; Ketterson, J. B.; Dutta, P. Langmuir 1988, 4, 1198. (18) Diaz Martin, M. E.; Cerro, R. L. J. Colloid Interface Sci., submitted for publication, 2004.
10.1021/la048572a CCC: $30.25 © 2005 American Chemical Society Published on Web 12/13/2004
Analysis of the Structure of Multilayer LB Films
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Figure 2. Possible configurations of the LB multilayers: (1) centrosymmetric structure with head-to-head and tail-to-tail conformation and orthorhombic subcell packing of the hydrocarbon chains; (2) asymmetric structure with head-to-tail conformation and hexagonal subcell packing of the hydrocarbon chains.
Figure 1. Schematic representation of the three multilayer deposition types observed during LB film depositions: (A) Y-type; (B) X-type; (C) Z-type.
an effect on transitions from Y-type to X-type depositions.17-19 At slow dipping speeds (or longer times spend under the water), some fatty acid/divalent cation systems rapidly lose the ability to deposit during the upstroke and X-type depositions take place. Dipping speeds leading to Y-type multilayers are limited by the rate of drainage of the water between the solid and the monolayer being deposited. If this limit is reached or overcome, water entrainment occurs and the deposition of the subsequent layer on the way down is hampered by the presence of a thin layer of liquid. Values of pH and dipping speeds at which transitions take place are specific to the cation used in the subphase. The objective of this paper is to study the effect of subphase pH and dipping speeds on the structure and molecular properties of multilayer LB films of arachidic acid over a CdCl2 subphase. The structure and order of cadmium arachidate multilayer LB films deposited on germanium (Ge) substrates were studied using attenuated total reflectance infrared (ATR-IR) spectroscopy, a nondestructive analytical technique providing very specific information. Transfer ratios (TRs) were used to determine the deposition type and the amount of the monolayers (19) Diaz Martin, M. E.; Montes, F. J.; Cerro, R. L. J. Colloid Interface Sci., submitted for publication, 2004.
deposited onto the solid substrate. The TR is defined as the ratio of the decrease in the area occupied by the monolayer at constant surface pressure to the area of the substrate that is immersed in water. Experimental values of pH were selected to achieve three different ionization conditions of the monolayers. At pH 4.5, little or no ionization occurs. At pKa ) pH ) 5.5, half of the fatty acid carboxylic heads are ionized, and at pH 6.8, complete ionization is assumed. Dipping speeds were chosen as a compromise between the time films spend under the water versus the production of wet LB films due to water entrainment. The slowest dipping speed was 3 mm/min, and the largest dipping speed was 65 mm/min, the minimum and maximum speeds allowed by the experimental setup. Two intermediate dipping speeds of 19 and 33 mm/min were also used during experiments. Additional experimental evidence of Z-type and X-type depositions is shown for arachidic acid/cadmium chloride systems. Multiple deposited films, however, show the basic centrosymmetric structure expected from a Y-type deposition, indicating molecular rearrangement during or after film transfer. 2. Experimental Setup The solution in the water subphase was prepared with 10-4 M cadmium chloride (ACS reagent grade) and deionized water collected using a Millipore Direct-Q5 water purifier. The pH of the solution was increased to the desired value by adding sodium hydroxide (ACS reagent grade) or decreased using hydrochloric acid (ACS reagent grade) both provided by Acros. This solution was then placed in a 612D Nima Technology LB trough. The experimental setup is contained inside a Sterilgard III Advance Class II Laminar Flow cabinet. The surface pressure and dipping speeds are controlled by a Nima microprocessor interface. Trapezoidal Ge ATR crystals (45°) with dimensions of 105 mm × 10 mm × 2 mm were used as deposition substrates. The crystals were cleaned by scrubbing the crystals with Liqui-nox rinsable detergent, rinsing thoroughly with deionized water, scrubbing
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Table 1. Effect of Substrate Speed and pH on the Type of Multilayer Deposition substrate velocity (mm/min) pH 4.5 5.5 6.8
3 Y X
19 Y X
33
65
Y X
Y Z-Y X
with HPLC grade 99.9% 2-propanol, and again rinsing with deionized water. Arachidic acid (99%) purchased from Aldrich Chemical Co. was used without any further purification. The spreading solution (1 mg/mL) was prepared using chloroform (HPCL grade) as solvent provided by Fisher Scientific Inc. This solution was spread evenly on the surface of the dipping solution. The subphase and the monolayer were maintained at 25 °C using a Lauda Ecoline RE204 water circulator and heater. The spreading solution was then compressed at a velocity of 40 cm2/min until the surface pressure reached 25 mN/m. This pressure was maintained for ∼45-50 min prior to deposition, ensuring that the monolayer was in stable equilibrium and that the molecules were arranged vertically except for the experiments performed at pH 4.5 at which collapse of the monolayer was observed after ∼15 min of compression at 25 mN/m. In this case, monolayers were deposited immediately after the first compression and deposition was performed only at the highest speed of 65 mm/min to ensure the deposition under stable conditions. After deposition, germanium substrates were removed from the dipping mechanism and allowed to air-dry completely. All experiments were done at least three times to ensure reproducibility. Multiple deposited films were analyzed using a Digilab FTS-60A infrared spectrometer equipped with a Harrick attenuated total reflectance attachment and a wire grid polarizer (IGP-225, Molectron Detector, Inc.). Each spectrum was collected by coadding 512 scans at a nominal resolution of 4 cm-1. Dichroic ratios were calculated from absorbance spectra obtained using the parallel-polarized light with those obtained using perpendicular-polarized light.
3. Experimental Results A summary of experimental observations of deposition type from solutions at different pH values and at different dipping speeds is shown in Table 1. At pH e pKa for the arachidic acid/cadmium chloride system, depositions were predominantly of the Y type; that is, deposition takes place during immersion and during removal. At the highest dipping speed, 65 mm/min, water entrainment often took place during the first few strokes. When water entrainment is present, if the wet films are not allowed to dry between dipping cycles, TRs of
