2976
Langmuir 1992,8, 2976-2979
Planar Bilayers of Langmuir-Blodgett Films from 16-8 Diacetylene J. C. Hatfield, J. W. Taylor, and D. R. Bassett* Technical Center Union Carbide Chemicals and Plastics Company, Inc., 3200 Kanawha Turnpike, South Charleston, West Virginia 25303 Received October 24, 1991. In Final Form: February 21, 1992
A method to form large, planar bilayers of 16-8diacetylene at the aidwater interface and to transfer these bilayers toaluminized glase substrates using the LangmuhBlodgett technique has been demonstrated. Support for bilayer formation and deposition includes Langmuir trough data, ellipsometry of monolayers and bilayers following deposition onto aluminized glass slides, and FT-IR analyses of the deposited films. Langmuir trough data show that a stable bilayer is formed at the aidwater interface at a film pressure of 35 dyn/cm. The calculatedmolecular area after film formation suggestsa bilayer "sandwich"comprising two tightly packed monolayers. In addition, the ellipsometry of transferred bilayers shows them to be two monolayers in thickness. Finally, the quantitative transfer of up to four bilayers over aluminized slides was confirmed by FT-IR analysis; the spectra can be rationalized by head-to-head arrangement of the surface-active diacetylenic molecules in the bilayer assembly. Introduction The Langmuir-Blodgett (LB) technique by which multilayer structures are built one monolayer at a time is well established. The existence of stable, large-area, planar bilayers a t the aidwater interface, however, has received little study. This work documents a method for creating stable, large-area, planar bilayers of 16-8 diacetylene (10,12-nonacosadiyn-l-oic acid). Of equal, if not greater, importance is the discovery that such bilayers can be deposited onto solid substrates. This point is less important for phospholipids which have been shown to form bilayers a t the airlwater interface1 and which are primarily of biological interest. It is crucial, however, for the diacetylenes whose potential utility lies in their dramatic optical nonlinearity and in their ability to be fabricated into solid-state optical devices.2J It should be noted, too, that demonstrating bilayer formation in a class of surfaceactive molecules quite different from the phospholipids should stimulate further research to answer the question of what the molecular and processing requirements are to form stable bilayers at the airlwater interface. Some studies have been made of the formation and deposition of collapsed monolayer^.^,^ The current work differs from that of Kawabata et aL4 in that it is a neat system, not multicomponent. It differs from both Kawabata et aL4and Baker et aL5in that it is a different family of molecules and that the bilayer structure formed on the substrate after deposition is well supported by ellipsometric and FT-IR analyses.
Langmuir-Blodgett Technique. The bilayers and monolayers were manipulated on, and deposited from, a Langmuir trough manufactured by Brinkmann (preparative f i i balance, model P)in a class 10 clean environment. After the diacetylene fiiwerespreadfromchloroform solution(1mg/mL),thetrough lid waa left open to assure solvent dissipation. After 6 min, the lid was closed and the film compressed. To create stable, large-area bilayers at the airlwaterinterface, the trough temperature waa controlled at 26.0 O C . The f i waa compressed at 2.1 A2/min and then controlled by a feedback controlloop from the film pressure transducer to maintain a f i pressure of 36 dyn/cm. It is important to note that this pressure is much above the 20 dynlcm often used to create monolayers. The monolayers of this study, following compression at 2.1 Az/min, were 'annealed" for 1 h at 25.0 O C at a f i i pressure of 20 dynlcm. During the "annealing" period, the average molecular area decreased 0.3 Azto a value of from 26 to 27 Aa/molecde for monolayers formed at the water surface. The identical procedure for monolayers on an aqueous solution of CdClz (1 mM) produced a similar reduction in the average molecular area. For these f i i , the average area was about 22 AZ/molde just prior to deposition. Ellipsometry. Ellipsometry was performed with an AutoEL I11automated ellipsometermanufacturedby Rudolph Research. In this instrument,ellipticallypolarized6328-Awavelength light strikes the sample at an angle of 70° from the vertical. The accuracy of the readings was enhanced using the two-zone averaging technique. FT-IRAnalysis. The infrared spectra were obtained using an FTS-40 BioRad FT-IR. It was equipped with an FT-80 Spectra-Tech grazing angle accessory. The infrared light waa polarized through an aluminum wire grid polarizer. Spectrawere averaged for 266 scans at a resolution of 4 cm-'.
Experimental Section Synthesis of 16-8Diacetylene. The 16-8diacetylene of this study was synthesized by adapting the method of Tieke et al!J The product was purified in a silica gel column.
Results and Discussion Evidence of Bilayer Formation and Transfer. Figure 1 shows the compression process for the transformation of a monolayer to a bilayer. The axis labeled molecular area is a pseudoarea in that it assumes monolayer coverage which is true only a t time zero. At 4h the trough surface is covered by a bilayer. Between these extremes, the film is a collection of monolayer- and bilayer-covered regions. Throughout the collapse to a stable bilayer, the film pressure is maintained a t 35 dyn/cm. Figure 1 suggesta a gradual and continuous transformation from monolayer to bilayer coverage. The bilayer structure is quite stable in that it sustains an applied film pressure of 35 dynlcm without further
(1) Gerehfeld, N. L.Biophys. J . 1986,50 (3), 457. (2) Pitt, C.W.; Wdpitta, L. M. Thin Solid Film 1980,68, 101. (3) Vincett, P. S.; Roberta, G. G . Thin Solid F i l m 1980, 68, 135. (4) Kawabata, Y.;Sekiguchi, T.;Tanaka, M.; Nakamura, T.; Komizu, H.; Honda, K.; Manda, E.; Saito, M.; Sugi, M.; Iizima, S. J. Am. Chem. SOC.1985, 107, 5270. (5) Baker, S.;Petty, M. C.; Roberta,G . G.; Twigg, M. V. Thin Solid F i l m 1983, 99,53. (6) Tieke, B.; Wegner, G.; Naegele, D.; Ringsdorf, H. Angew. Chem., Znt. Ed. Engl. 1976, 15, 764. (7) Tieke, B.; Graf, H.-J.; Wegner, G.;Naegele, B.; Ringsdorf, H.; Banerjie, A.; Day, D.; Lando, J. B. Colloid Polym. Sci. 1977, 225, 521.
0743-7463/92/24082976$03.00/0 0 1992 American Chemical Society
Planar Bilayers of LB Films from 16-8 Diacetylene
Langmuir, Vol. 8, No. 12,1992 2911
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Figure 1. Compression/collapse of 16-8diacetylene film from monolayer to bilayer structure. Film pressure is controlled at 35 dyn/cm.
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Figure 3. Barrier movement during deposition of four succeeeive monolayers from water surface onto aluminized g h slide. Deposition pressure is 20 dyn/cm. Table I. Thickness Measurements (A) of Monolayers and
Bilayers.
~
monolayer bilayer 27.9 & 1 58.9 2.3 132 2 62.4 & 3.3 The monolayers were cast from a water solution of CdCl2 (1.0 X 10-8 M)over aluminized glass slides. The bilayers were cast from pure water over aluminized glass slides (see text for other details). layers 1 2
TIME (Seconds)
Figure 2. Barrier movement during deposition of four successive bilayers from water surface onto aluminized glass slide. D e p osition pressure is 35 dyn/cm. collapse. It is also significant that the final molecular area, calculated from the trough geometry and assuming no collapse of the monolayer, is 9 A2/molecule. For bilayer formation, however, this value is doubled to 18 A2/ molecule. This compares to about 26 A2/molecule for monolayers of 16-8diacetylene formed on a water surface and 22 A2/molecule for monolayers formed on a 1 mM CdClz aqueous solutionsurface. It suggeststhat each layer of the bilayer is more highly compressed than single monolayers formed at the aidliquid interface. Figure 2 shows the successive deposition of four bilayers onto an aluminized glass slide. Except for the fourth deposition, the barrier remained stationary during the downstroke and all deposition occurred on the upstroke, resulting in Z-type deposition. From the slope in Figure 2,the transfer efficiencywas from 105to 11596,indicating some collapse of the bilayer during deposition. In addition to 16-8diacetylene, we have demonstrated that films of 12-8diacetylene collapse to form stable bilayers on the Langmuir trough. For comparison, Figure 3 shows barrier movement during the successive deposition of four monolayers onto an aluminized glass slide. A small amount of deposition occurred and then reversed itself during the downstroke of the aluminized slide. Primary deposition, however, took place on the upstroke of the slide from the first to the fourth monolayers to give Z-type deposition as was the case for the bilayers. From the slope in Figure 3, the transfer efficiency was 98%. The thickness of thin films on reflective substrates can
*
be determined by ellipsometry.8 The refractive index of unpolymerized 16-8diacetylene was determined by the immersion method to be 1.738. Assuming the transferred monolayers and bilayers to be isotropic and homogeneous with a refractive index of 1.738, the thickness values in Table I were derived. (In these measurements, the plane defined by the incident and reflected beams was perpendicular to the dipping direction of the aluminized slide.) Although the films are, in fact, anisotropic, the assumption of isotropy does not materially change the calculated thicknesses. More importantly, the thickness ratios of one to two monolayers, two monolayers to one bilayer, and one to two bilayers is even less affected. Of central importance is the observation from Table I that one bilayer is close to two monolayers thick and that two bilayers are close to four monolayers thick. Further comments on orientation within the monolayers and bilayers are found in the next section. The FT-IRspectra of the bilayers and monolayers were obtained using polarized grazing angle (PGA) reflectance. Figure 4 shows the absorbance of the symmetrical CH2 stretch at 2850 cm-l plotted against the number of bilayers deposited onto an aluminized glass slide from a water surface, the number of monolayers deposited onto an aluminized glass slide from a water surface, and the number of monolayers deposited from an aqueous CdC12 solution. From a linear fit of the data, the methylene absorbance at 2850 cm-l is 0.001 28 per bilayer, a value roughly twice that of the methylene absorbance at 2860 cm-l obtained from an equivalent number of monolayers deposited from either water or CdClz solution. Thus, absorptionsat 2850cm-l support the quantitative transfer of bilayers formed at the aidwater interface. Orientation. The PGA reflectance method produces infrared light with its electric vector parallel to the z axis (Figure 5). Molecular segments having oscillating dipole moments parallel to this vector will absorb a fraction of the incident light while those whose moments are per(8) Spaniar, R.F.Znd. Res., Sept. 1975, 73.
2978 Langmuir, Vol. 8, No. 12, 1992
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Figure 6. Oscillating dipole momenta of the methylene group in a hydrocarbon chain. pendicular to it will not. As shown in Figure 5, the oscillatingmoment for the asymmetricCHz stretch at 2919 cm-I is perpendicular to the plane of the hydrocarbon chain, whereas the moment for the symmetric CHz stretch at 2850 cm-l is parallel to the H-C-H plane and bisects the H-C-H angle.9 The PGA reflectance spectrum of two bilayers of 16-8 diacetylene deposited from water is shown in Figure 6. For reference, the spectra of four monolayers deposited from a water substrate and four monolayers deposited from an aqueous CdClz solution are included. Also shown is a transmission spectrum of 16-8 diacetylene cast over a sodium chloride disk. For a chain perpendicular to the substrate, both 2919- and 2850-cm-l bands should be weak and of similar intensity; neither would have a component of its oscillating moment parallel to the electric vector of the incident radiation. As shown in Figure 6, the peak height ratio (2919/2850 cm-l) is 2 or greater for both monolayer and bilayer structures. These results suggest that hydrocarbon chains which compose the monolayer and bilayer structures are tilted from the z axis. For the f i s t deposited monolayer, a peak height ratio (2919/2850 cm-l) of 2 is obtained, and the first bilayer gives a peak (9) A k a , D. L., Swalen, J. D.J. Phys. Chem. 1982,86, 2700.
3000.0 2000.0 WAVENUMBERS, RES c4.0
Figure 6. FT-IRgrazing angle spectra of 16-8 diacetylene. height ratio (291912850 cm-') of 7.8. As shown in Table I, these observations are supported by ellipsometric data. The calculated length of the fully extended 16-8 diacetylene hydrocarbon chain is 36.8 A. From this value, and an observed thickness of 27.9 A for a monolayer cast from a CdClz-water solution, the closepacked molecules are tilted 41' from the z axis. Assuming the fmt monolayer remains constant during the second deposition, the second layer tilts only 20° from the z axis. From Table I, a single bilayer is inclined 37O from the z axis. This assumes that the bilayer is composed of two identical monolayers, each made of fully extended molecules having the same orientation. Also from Table I, assuming the first bilayer is unchanged by the deposition of a second layer, the second bilayer tilts only 7O from the z axis. The above results from FT-IR and ellipsometry show that both the first deposited monolayer and the first deposited bilayer are tilted. The ellipsometricdata further suggest that for both monolayers and bilayers, the second layers tilt less than the first. Layer-to-LayerStructure. It is hypothesized that hydrogen bonds between opposing carboxylicacid groups produce a head-to-head arrangement in the transferred bilayers. This ties up protons associated with these groups and preventstheir migration to basic s i t e on the aluminum oxide surface. This hypothesis is supported by the absorption at 1725 cm-l in the bilayer spectrum of Figure 6, and a series of absorptions (not shown) between 1348 and 1196 cm-l. Such absorptions between 1348 cm-' and 1196 cm-' are consistent with those of carboxylic aciddo in which formation of the carboxylateanion does not occur. In contrast, proton migration, together with head-totail organization between layers, is inferred for monolayers transferred from the water surface. The spectrum of four monolayers transferred from water (Figure 6) shows an absorption at 1587 cm-l. This implies dissociation of carboxylic acid moieties in the transferred monolayers to form anionic carboxylate species. It appears from the literature that the chemical structure of the oxide layer consists of a bulk aluminum oxide with a highly hydroxylated surface.11J2 This hydroxylated structure is thought (10) Kimura, F.; Umemura, J.; Takenaka, T. Longmuir 1986,2, 97. (11) Evans, H. E.; Boweer, W. M.; Weinberg, W. H. Appl. Surf. Sci. 1980,5,258. (12) Boweer, W. M.; Weinberg, W. H. Surf. Sci. 1977, 64, 377.
Planar Bilayers of LB Films from 16-8 Diacetylene
to result from the thermal oxidation of the aluminum surface in the presence of small traces of water. Allara and Nuzzo13have suggested that carboxylates form on an aluminum oxide surface by the reaction of the acid with the surface hydroxyls to release water and/or with the Al-0 lattice to form surface hydroxyls. The presence of charged carboxylates argues against head-to-head arrangement in monolayer assemblies. The structure in monolayers transferred from the water surface, therefore, is taken to be head-to-tail, with the heads pointing to the aluminum oxide surface. It is expected that the basic surface sites eventually saturate with protons and at some number of deposited monolayersthe carboxylic acid group should reappear. In fact, the spectrum of four monolayers confirms this by showing a peak carboxylic acid absorption at 1710 cm-l (Figure 6). The case for monolayers transferred from the air/CdCln solution interface is different still. Such monolayers exist as the cadmium carboxylate salt and are transferred in this form as evidenced by the carboxylate anion absorption at 1543 cm-' seen in Figure 6.14 (13)Allara, D.L.;Nuzzo, R. G. Langmuir 1986, 1, 45. (14) Rabolt, J. F.;Burns,F. C.; Schlotter,N.F.;Swalen,J. D. J. Chem. Phys. 1983, 78, 946.
Langmuir, Vol. 8, No. 12, 1992 2979
Conclusions Langmuir trough data suggest the formation and transfer of stable, tightly-packed bilayers of 16-8 diaceb ylene. Ellipsometry shows the thickness of one and two bilayers transferred to an aluminized slide to be, respectively, two and four monolayers thick. In other words deposited films of bilayers are twice the thickness of deposited monolayers. FT-IR analysis supporta the deposition of diacetylenicbilayers onto aluminum-coated slides. Langmuir-Blodgett-assembled films of surface-active diacetylenea are of interest because of their nonlinear optical properties as well as their electrical conductivity. Applications such as optical switching elements and gas sensorsderive from such material properties. The impact of the more highly organized bilayers suggested by this research may further enhance these basic material characteristics. Important diffraction studies on the bilayer structure remain to be done. At the very least, the discovery of stable bilayers in this unique chemical family allows their deposition at twice the previous rate. Acknowledgment. The fine effortsof James F. Winter for the Langmuir trough work and Richard A. Hammock for the synthesis are gratefully acknowledged. Registry No. 16-8Diacetylene, 66990-34-9.
