Langmuir 1992,8, 1243-1246
1243
Assembly of Fatty Acid Bilayers on Hydrophobic Substrates Using a Horizontal Deposition Procedure Samuel Lee, Jorma A. Virtanen, Sinikka A. Virtanen, and Reginald M. Penner* Institute for Surface and Interface Science (ISIS), Department of Chemistry, University of California, Irvine, California 9271 7 Received January 21, 1992. I n Final Form: March 19, 1992
A new procedure for assembling a stearate bilayer on a graphite or Ge(100) surface is described in this paper. Bilayer assembly occurs during the course of a single dipping step: A hydrophobic substrate is brought horizontally into contact with a Langmuir monolayer of either stearic acid or cadmium stearate and then slowly lifted from the surface. Transfer ratios of 2.0 (*0.2) are obtained by using this procedure indicating that the area equivalent of precisely one stearate bilayer is transferred to the substrate surface; bilayer deposition is confirmed by film thickness measurements obtained by ellipsometry. This horizontal depositionprocedure providesan alternativeto conventionalLangmuir-Blodgett depositionfor assembling bilayers (or an integral number of bilayers) on hydrophobic substrates. Introduction Langmuir-Blodgett (LB) deposition1t2is an extremely versatile method for transferring Langmuir monolayers of amphiphilic molecules onto hydrophilic substrates such as glass and q u a r t ~ etched ,~ 111-V and 11-VI semiconductors,4* mica,718 and oxidized metal surface^.^ LB deposition ie also an effective method for transferring monolayers to some hydrophobic substrates, such as hydrogen terminated or silanized Si(lll)lOJ1and silanized glass,12but chemically inert surfaces such as graphite and MoSl are likely to thwart deposition, yielding transfer (or deposition) ratios, 7,13 much smaller than 1. As a specific example from this laboratory, attempts to transfer monolayers and cadmium stearate or stearic acid to highly oriented pyrolytic graphite (HOPG) surfaces using the LB method have failed 7 values of 0.4-0.6 are obtained for deposition of each of the first two monolayers, and improvements in the efficiency of transfer are not obtained by manipulation of the subphase pH or the ionic strength, the dipper velocity, or the deposition pressure up to 50 dyn cm-l. A new procedure for assembling a stearate bilayer on a graphite or Ge(100) surface is described in this paper. In this procedure, the substrate surface is brought horizontally into contact with a floating Langmuir monolayer and the crystal is held in contact with the monolayer for -30 s and then slowly lifted from the surface at a velocity of 0.5 mm min-l. Transfer of a bilayer is evidenced by measurements of 7 and by ellipsometry measurements of the film thickness. Additional bilayers up to eight were deposited by repetition of this procedure following the
* Author to whom correspondence should be addressed.
(1) Blodgett, K. B. J. Am. Chem. SOC.1935,57, 1007. (2) Blodgett, K. B.; Langmuir, I. Phys. Rev. 1937,51, 964. (3) Kuhn, H.; Mobius, D.;Bucher, H. In Techniques of Chemistry; Weissberger, A., Rwiter, B. W., We.; Wiley: New York, 1972; Vol. I,
Part IIIB.
(4) Tredgold, R. H.; Winter, C. S. Thin Solid Films 1983,99, 81. (5) Kan, K. K.; Roberta, G. G.; Petty, M. C. Thin Solid Films 1983,
99, 291. (6) Clark,D.T.;Fok,T.;Roberts,G.G.;Sykes,R. W. ThinSolidFilms 1980,60, 261. (7) Marra, J. J. Colloid Interface Sci. 1985, 107, 446. (8)Marra, J.; Israelachvili, J. Biochemistry 1985,24, 4608. (B) Gainee, G. L., Jr. J. Colloid Interface Sci. 1960, 15, 321. (10) den Engelsen, D.J. Opt. SOC.Am. 1971,41,1460. (11) Fariss, G.; Lando, J.; Rickert, S. Thin Solid Films 1983,99, 305. (12) Honig, E. P.; Henget, J. H.; den Engelsen, D. J. Colloid Interface Sci. 1973, 46,92. (13) The transfer ratio, T = ALIAS,is the ratio of the decrease in area
occupied by the monolayer on the water surface, AL,and the coated area of the solid substrate, AS.
first deposition. A number of horizontal dipping or lifting protocols have been described p r e v i ~ u s l y ~ beginning *-~~ with Langmuir and Schaefer in 1938,18but in each case, a monolayer was deposited on each contact of the substrate to the monolayer surface or monolayer deposition was assumed. A horizontal deposition procedure yielding bilayers has not, to our knowledge, been previously reported, and we have not found it possible to deposit monolayers by the procedure described here. Experimental Methods Bilayer Preparation. Either highly oriented pyrolytic graphite (HOPG)or a germanium(100)single crystal were used as substrates. Prior to bilayer deposition, graphite crystalswere freshly cleaved and Ge(100) crystals were ultrasonicated sequentially in two volumes of chloroform/methanol (7030), chloroform, and methanol, and dried in N2 for 1 h. Langmuir monolayers of cadmium stearateor stearic acid were prepared from 10 mM solutions of stearic acid (Aldrich,>99%) in chloroform (Burdick & Jackson, GPC grade). Thirty-five microliters of this solution was injected onto the surface of an aqueous subphase to prepare each Langmuir monolayer. For cadmium stearate depositions, this subphase contained lo4 M CdCl2 (Aldrich,99.99+ %) at pH = 7.00-7.25prepared from Barnsted Nanopure water ( p > 18 Ma). For stearic acid depositions, a subphase of Nanopure water acidified to pH = 3.0 using HzSO, (Aldrich,99.999%) was employed. These subphases were contained in the all Teflon troughs of either of two commercialLang muir deposition s y s t e m ~ . l ~For - ~ ~both systems, monolayer pressures were measured using the Wilhelmy method with an oxidized platinum foil. Following injection of the stearic acid solution onto the subphase surface, chloroform was permitted to evaporate for 5 min before the first compression of the Langmuir film. Three pressure-molecular area (or P A ) isotherms were then recorded using compression rates of 6.0 A2 molecule-’ min-1 to 1dyn cm-l and 1.4 Azmolecule-’ min-l at higher pressures in order to anneal the monolayer and to obtain a measurement of the molecular area. The monolayer was then compressed to 35 dyn cm-l and, following 1-2 min at this pressure, the horizontal deposition procedure was commenced as described below. Horizontal DippingProcedure. A sequenceof photographs, shown in Figure 1, illustrate the horizontal dipping procedure. The substrate (graphite in Figure 1)was clamped to the dipper of a KSV Model 500019 deposition system in a horizontal orientation. The crystal was then lowered into contact with the (14) Schulman, J. H.; Waterhouse, R. B.; Spink, J. A. Kolloid Z . 1956, 146, 77. (15) Day, D.; Lando, J. B. Macromolecules 1980, 13, 1478. (16) Von Tscharner, V.; McConnell, H. M. Biophys. J. 1981,36,409. (17) Ta”, L. K.; McConnell, H. M. Biophys. J . 1985,47,105. (18) Langmuir, I.; Schaefer, V. J. J. Am. Chem. SOC.1938, 60, 1351. (19) KSV Instruments USA,Riverside, CT 06878.
0743-7463/92/2408-1243$03.00/00 1992 American Chemical Society
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1244 Langmuir, Vol. 8, No. 5, 1992
r.igure
1.
rnotograpns
01 the
horizontal deposition procedure, described in the text.
floating Langmuir'film, and the face of the crystal was immersed into the subphase by =0.2 mm (Figure la). Following 30 s in contactwith the Langmuirmonolayer,the crystal was lifted from the subphase surface at a velocity, v, of 0.5 mm min-l (Figure lb,c). Completion of the deposition cycle was signaled by the separation of the subphase meniscus from the face of the crystal (FigureId). Observationof the depositionprocessprovided clues to the mechanism of bilayer assembly; these are discussed in the results and discussion section below. During lifting, the monolayer pressure was maintained at 35 f 5 dyn cm-' by automated feedback control of the barrier position by the KSV 5000. A modificationof this method was employed for deposition on the Ge(100)substrate employed for Fourier transform IR (FTIR) measurements. Manipulationof the substrate was accomplished manually and bilayer deposition confirmed from measurements of 7. For these depositions, a Nima Technology trough20was employed to maintain constant pressure. Bilayer Characterization. Measurement of the bilayer thickness by ellipsometry was accomplished using a Gaertner Model L117 ellipsometer using an angle of incidence of 70" and X = 632.8 nm. Determinationof the refractiveindex of our HOPG crystalsfrom ellipsometrymeasurementsyielded a value of 3.041.95i at 631.8 nm and 50" incidence,21 and the refractive index of the LB bilayer was taken to be 1.51.22 IR spectra of stearic acid were recorded with a Nicolet 60SX FTIR spectrophotometerequipped with a liquid nitrogen cooled MCT detector. For each spectrum, 8192 scans were co-added for an eight-layer film (four bilayers) deposited onto the Get100) substrate. An angle of incidenceof 45" with respectto the parallel faces of the crystalwas employed to obtain 60 internal reflections. Spectral resolution was 4 cm-l. The spectrum of the clean Ge(20)Nima Technology, Ltd.,Coventry CV4 7EZ, England. (21)The refractive index of graphite exhibits a substantial dependence on the angle of incidence, 8, measured from the surface normal. 8 = 70" was employed for stearate film thickness measurements and refraction of the incident 632.8nm light at the film/air interface results in an angle of incidenceat the film/graphite interface of 4 0 " . Therefore, the refractive index of HOPG crystals employed in film thickness calculations(3.04-1.95)was determined from ellipsometrydata for freshly cleaved crystals at 8 = 50". (22)Pitt, C.W.; Walpita, L. M. Thin Solid F i l m 1980,68,101.
3.500 10'
3.650 io4
3.800 l~
3.950 lo4
4.100 io4
Trough Area, mm'
Figure 2. Monolayer pressure versus trough area isotherms delineating eight horizontal depositions. In this experiment, cadmium stearate is deposited from a subphase of 1V M CdClz at pH = 7.5 onto a freshly cleaved HOPG crystal (area = 215.3 mm2). The lift velocity employed for the deposition was 0.5 mm min-l and isotherms were obtained using a compression rate of 6.0 A2molecule-' min-l to 1dyn cm-l and 1.4 A2molecule-' mi+ at higher pressures. The deposition number and corresponding 7 value are as shown. (100) crystal was subtracted from all spectra and smoothing procedures were not employed. Water absorption was not subtracted and it is evident particularly in the range from 1600 to 1700 cm-l in both spectra.
Results and Discussion Figure 2 shows a typical series of pressure, a, versus trough area, A, isotherms delineating eight deposition cycles. In the experiment shown in Figure 2, cadmium stearate was transferred to a HOPG single crystal with an area of 215.3 mm2. For each isotherm, a rapid increase in pressure is observed near a = 1.0 dyn cm-l at an average molecular area (trough arednumber of cadmium stearate molecules) of 18.4 A2.With continued compression to the deposition pressure of 35 dyn cm-l, the molecular area
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Langmuir, Vol. 8, No. 5, 1992 1245
v-
0
2
4
6
8
Deposition Number
Figure 3. Ellipsometry measurements of the film thickness as a function of deposition number for up to eight horizontal depositions of cadmium stearate onto HOPG. Deposition conditions are identical to Figure 2.
falls nearly linearly to -17.9 A2. These isotherms are similar to those previously reported for cadmium stearate Langmuirfilmsat25°C.a Followingeachdeposition (and before the first deposition), three P A isotherms were obtained with excellent reproducibility. In Figure 2, two of these three isotherms are shown for clarity. Successive pairs of P A isotherms are displaced to smaller values along the trough area axis as a result of transfer of cadmium stearate molecules from the Langmuir film to the graphite substrate surface. 7 for each deposition is the ratio between the average shift in area at a pressure, ?r = 35 dyn cm-1, and the eubstrate area. For each deposition, these shifts reveal the area equivalent removed from the Langmuir monolayer both as a consequence of dissolution into the subphaseand transfer to the graphite substrate. However, dissolution of the Langmuir film wae found to be negligible for the subphase compositions employed in these experimenta on the time scale of hours. The average 7 obtained from these data for the first seven dips is 2.06 f 0.10. These data demonstrate that the area equivalent of one biluyer of cadmium stearate is transferred to the graphite crystal for each contact and lift cycle. Horizontal transfer achieves results of similar quality for the assembly of stearic acid bilayers onto graphite and the assembly of either cadmium stearate or stearic acid bilayers onto Ge(100) substrates. Deposition becomes erratic in all cases after the transfer of seven to nine bilayers and 7 values thereafter fluctuate between 1.5 and 3.0. This is apparent in Figure 2 by the 7 value of 1.47 obtained for the eighth deposition. In Figure 3, the film thickness measured by ellipsometry is plotted versus the deposition number for the series of eight transfers of cadmium stearate to HOPG. With each successive deposition, the total thickness of the deposited film increases by incrementa of 50.8 A. The thickness of a condensed phase cadmium stearate monolayer is 25.2 A,24thus the 50.8-A increment is consistent with deposition of a cadmium stearate bilayer. Thus, Figure 3 shows that bilayer-by-bilayer deposition occurs for the first eight depositions. FTIR spectra of eight-layer stearic acid and cadmium Stearate films, shown in parta A and B of Figure 4, respectively, c o n f i ithe chemical identity of the deposited film and yield information on its structure. In Figure 4A, carbonyl stretching is observed at 1700 cm-l, and a broad adsorption between 2500 and 3500 cm-l is typical for a hydrogen bonded carboxylate-hydroxyl stretching vibra(23) See for example: Hann, R. A. In Longmuir-Blodgett Films; Roberta, G. G., Ed.;Plenum Prese: New York, 1990; Chapter 2. (24) Matauda, A.;Sugi, M.; Fukui, T.; Iiima, S.; Miyahara, M.; Oteubo, Y. J. Appl. Phys. 1977,48, 771.
6M)
900
1200
1500
1800
2100
2400
2700
3ooo
3300
Wavenumber, cm"
Figure 4. FTIR spectra of eight layers (i.e., four bilayers) of stearic acid (A) and cadmium stearate (B)on a Ge(100)crystal.
t i ~ n .In~ addition, ~ the absorption at 942 cm-l in this spectrum is indicative of out-of-plane OH bending of a dimeric carboxyl group, which is diagnostic of Y-type deposition of stearic acid.26 In contrast, carbonyl stretching is absent in the spectrum of Figure 4B demonstrating that virtually no stearic acid is deposited from the Langmuir monolayer on the cadmium-containing subphase. Instead, a very strong absorption, characteristic of an ionized carboxylate group, is observed at 1543.5 cm-l.26 These FTIR data further reveal that both stearic acid and cadmium stearate multilayer8 are substantially crystalline. This is indicated by the CH2-wagging band ' addition, progression in the region 1180-1340 ~ m - l . ~In in Figure 4B the CH2-bending vibration is split into two peaks of equal height at 1473 and 1463 cm-l due to crystal field splitting.28 It is now established by that the exact position of the CH-stretchingabsorption provides a further measure of the degree of crystallinity.29 In previous work with phospholipid^,^^*^^ for a highly crystalline state the asymmetric CH2 stretch is near 2916 cm-l, while in a disordered state it is close to 2224 cm-l. On this basis, the spectrum of Figure 4A for stearic acid exhibits greater crystallinity (2916 cm-l) than that of Figure 4B for cadmium stearate (2918 cm-l). Finally, in the various crystalline forms of stearic acid, alkyl chain packing is essentially invariant but the conformation about the car(25) Silverstein, R. M.; Bawler, G. C.; Morrill, T. C. Spectroscopic Identification of Organic Compounds; Wiley: New York, 1981;Chapter IV. (26) Kimura, F.;Umemura, J.; Takenaka, T. Langmuir 1986, 2, 96. (27) Snyder, R.C.J.Mol.Spectrosc. 1960,4, 411. (28) Snyder, R. C. J. Mol.Spectrosc. 1964, 7,116. (29) Naselli, C.; Rabolt, J. F.; Swalen, J. D.J. Chem. Phys. 1988,82, 2136. (30) Cameron, D.G.; Casal, H. L.; Mantsch, H. H.Biochemistry 1980, 19, 3665. (31) Lotta,T. 1; Laakkonen, L. J.; Virtanen, J. A.; Kinnunen, P. K. J. Chem. Phys. Lipids 1988,46, 1.
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boxylate group is substantially different in these various forms. Accordingly, a-methylene and carboxylate absorptions provide information on the crystal structure.32 For C-form stearic acid, a-methylene bending occurs at 1300cm-l and out-of-planeOH-bendingat 940 cm-l, which are the same as we have observed. As noted above, observation of horizontal deposition provides clues to the mechanism of deposition. As shown in parts b and c of Figure 1, during lifting of the crystal from the surface of the trough, the acute contact angle of the subphase with the crystal surface demonstrates that an initially hydrophobic graphite surface is now hydrophilic. This observation suggests that the first stearic acid monolayer is present on the surface of the graphite crystal at this point and that the orientation of the first monolayer is such that carboxylate heads are exposed at the monolayer-air interface. With continued lifting, the meniscus moves from the perimeter of the crystal to the center (Figure IC)before separating (Figure Id), leaving a small residual drop of subphase (shown). Following deposition, the surface of the bilayer is hydrophobic with a contact angle for water of ==90°. Consequently, the orientation of the second monolayer is such that alkyl chains are exposed at the bilayer surface and the overall deposition is Y-type, as indicated by the FTIR data of Figure 4. A mechanism consistent with these observations is shown schematically in Figure 5. Transfer ratio and ellipsometrydata both indicate that the orientation of stearate molecules in both monolayers transferred is nearly perpendicular to the substrate surface. The first stearate monolayer is deposited with alkyl chains perpendicular to, and in contact with, the graphite crystal. The exposed acid moieties of this first monolayer provide a hydrophilic surface which allows wetting of the monolayer surface during lifting and receding of the meniscus. In analogy to vertical deposition, this wetting behavior is likely to be a requirement for deposition of a second stearate monolayer in which stearate molecules are oriented with alkane chains exposed at the bilayer-air interface. Likely, the formation of strong (=7kcal mol-l) interlayer carboxylatecarboxylate H-bonds increasesthe adhesion of stearic acid monolayers in the bilayer. However, the presence of this interlayer H-bonding is not a prerequisite for bilayer formation since the FTIR data above clearly indicate that multilayer8of pure cadmium stearate can be deposited by the horizontal method.
Summary We describe a simple procedure enabling the preparation of bilayers from Langmuir monolayers of stearic acid or cadmium stearate on hydrophobic substrates. Bilayer (32) Holland, R. F.;Nielsen, J. R.J. Mol. Spectrosc. 1962, 9, 436.
n
1.
I
2*
vaphite -p cadmium stearate
4
n
aaueous 0.1mM Cd2+
5.
n
Figure 5. Schematic diagram illustrating the probable mechanism of bilayer assembly effected by the horizontal dipping procedure described in this paper.
deposition is evidenced unequivocally by measurement of the transfer ratio and ellipsometry measurements of the film thickness. The precision obtained for assembly of the first seven bilayers equals the best results we have been able to obtain using the conventional LB method (i.e. vertical deposition) for bilayer transfer to Si(ll1) substrates of equal size. Of course, the precision of 7 improves for both vertical and horizontal depositions with increasingsubstrate size as the aredperimeter ratio of the substrate increases and edge effects become negligible. The horizontal deposition procedure described in this paper provides an alternative to the conventional LB method for assembling a bilayer (or an integral number of bilayers) on hydrophobic surfaces.
Acknowledgment. Financial support from the University Exploratory Research Program of the Procter & Gamble Corporation and the Committee on Research of UC Irvine is gratefully acknowledged. In addition, S.A.V. thanks Emil Aaltonen’s Foundation for financial support. We express our appreciation to Mr. Art Moore of Union Carbide for generous donations of highly oriented pyrolytic graphite, to Professor John Hemminger of UC Irvine for providing accessto the ellipsometeremployed for these experiments, and to Professor Veronica Burrows of the Arizona State University for providingassistance with the FTIR experiments described here.
