Langmuir 1989,5, 101-111
101
Formation of Multilayers by Self-Assembly Nolan Tillman, Abraham Ulman,* and Thomas L. Penner Corporate Research Laboratories, Eastman Kodak Company, Rochester, New York 14650 Received June 29,1988 Monolayer and multilayer films were formed by self-assembly of methyl 23-(trichlorosilyl)tricosanoate (1) from organic solution. In agreement with published results, this compound was found to form good
quality, close-packed monolayers on silicon surfaces. We have, however, found that films significantly thicker than the three monolayers previously obtained can be formed from continued chemisorption of monolayers of this compound followed by reduction of the surface ester with LiAlH4in THF to form an “alcohol surface”. The quality of monolayer formation in the multilayer films was monitored in detail by ellipsometry, contact angle, and FTIR measurements, and, although generally increasing disorder can be detected, films of up to 25 discrete monolayers can be successfully made. These results indicate that self-assembly is a viable alternative to the Langmuir-Blodgett transfer technique for the construction of relatively thick (0.1-hm scale), ordered, multilayer films. In recent years organized molecular systems have attracted growing attention. The techniques which are presently available for the construction of such systems include both Langmuir-Blodgett (LB)’ and self-assembly methods, by which ordered, monomolecular layers can be formed on hydrophilic surfaces. These systems are believed to have technological potential in both optical and molecular e1ectronics.l They allow the chemist to potentially design new organic materials at a molecular level by incorporating useful functional groups into such systems and controlling such variables as the spacing of these groups within and between monomolecular layers. Although the LB method has been studied intermittently for many years and has been found successful for the formation of an extremely wide variety of monolayer and multilayer films (including relatively thick films of even several hundred layers), the SA method offers important advantages for future applications in such areas as molecular electronics and optical applications. The use of derivatives of alkyltrichlorosilanes (e.g., octadecyltrichlorosilane, OTS) results in monomolecular layers which are durable, thermally stable! and resistant to degradation by a variety of strong reagents.%l6 The trichlorosilyl head group forms covalent bonds to the hydrophilic surface as well as cross-links to adjacent molecules via Si-0 bonds created upon hydrolysis with trace water. This method can be adapted to the formation of multilayers by designing monolayers containing terminal functional groups that can be treated with various reagents to unmask a fresh, hydrophilic surface upon which a succeeding monolayer can be adsorbed.’J’ (1) For recent reviews, see: (a) Roberts, G.G.Adu. Phys. 1985,34,475. (b) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.;Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987,3,932. (2) (a) Bigelow, W. C.; Pickett, D. L.; Zisman, W. A. J. Colloid Znterface Sci. 1946,1,513. (b) Bigelow, W. C.; Brockway, L. 0.J. Colloid Interface Sci. 1966, 11, 60. (c) Zisman, W. A. In Friction and Wear; Davies, R., Ed.;Elsevier: New York, 1959; p 110. (d) Allara, D. L.; Nuzzo, R. G.Langmuir 1985, 1, 45 and references therein. (e) Allara, D. L.; Nuzzo, R. G.Langmuir 1986,1, 52. (3) (a) Sagiv, J. J. Am. Chem. SOC.1980,102,92. (b) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984,100,465. (c) Gun, J.; Iscovici, R.; Sagiv, J. Zbid. 1984, 101, 201. (d) Gun, J.; Sagiv, J. Ibid. 1986, 112, 457. (e) Finklea, H. 0.;Robinson, L. R.; Blackbum, A.; Richter, B.; Allara, D.; Bright, T. Langmuir 1986, 2, 239. (4) (a) Nuzzo, R. G.; Fusca,F. A.; Allara, D. L. J.Am. Chem. SOC.1987, 109, 2358. (b)Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. F. D. Ibid. 1987,109, 3559. (5) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986,90, 3054. (6) (a) Maoz, R.; Sagiv, J. Thin. Solid Films 1985,132,135. (b) Maoz, R.; Sagiv, J. Langmuir 1987, 3, 1045.
Many practical devices will require the formation of high-quality, close-packed and highly ordered films with thicknesses of 0.5-2.0 pm. This means that the procedure for the formation of SA multilayers should be perfected so that films of a thickness of a hundred or more monolayers can be reproducibly constructed with minimal disorder. At present, however, most published reporb suggest that the quality of monolayers formed by self-assembly of trichlorosilane derivatives rapidly degrades as the thickness of the films increase^.'?^ For example, Pomerantz et a1.8 published results from an X-ray diffraction study of multilayers formed from methyl 23-(trichlorosily1)tridecanoate, (H3C02C(CH2)22SiC13), and it was found that a three-layer sample could be best fit to the data by assuming as a model a 55% two-layer and 45% three-layer sample. Furthermore, hexadecane contact angles for the unreduced, ester surface rapidly deteriorated, from 28’ for the initial monolayer on the silicon surface to 1 2 O for the third layer. We note that more successful results have since been obtained by the Sagiv group, employing modified procedures and amphiphiles, such as a p-tolyl ester analogous to L9 We are interested in the eventual fabrication of multilayers which contain useful functional groups, using the SA method. We decided to investigate the properties of 1 in order to become familiar with techniques of multilayer formation using the SA technique, identify potential problems in film fabrication, and assess the quality of multilayer films that might be formed. In light of the previously published results, we were pleasantly surprised to find that, in our hands, monolayers of appropriate thickness and reasonable contact angles could be adsorbed for considerably more than a few monolayers. We report here our investigation, presenting the results of a detailed study using ellipsometry, contact angle measurements, and FTIR spectroscopy for monolayers and multilayers based on 1.
Experimental Section Substrate Preparation. Substrates employed in this study were either p-doped, test grade, polished silicon wafers (Wacker (7) (a) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983,99,235. (b) Netzer, L.; Iscovici, R.; Sagiv, J. Zbid. 1983, 100, 67. (c) Netzer, L.; Sagiv, J. J.Am. Chem. SOC.1983, 105, 674. (8) Pomerantz, M.; Segmuller, A,; Netzer, L.; Sagiv, J. Thin Solid Films 1985, 132, 153. (9) Maoz, R.; Sagiv, J. In New Technological Applications of Phospholipid Bilayers, Thin Films, and Vesicles;J. A., Hayward, Ed.;Tenrife, Jan 6-9, 1986; to be published.
0743-7463/89/2405-OlO1$01.50/00 1989 American Chemical Society
102 Langmuir, Vol. 5, No. I, 1989 Chemitronic GMBH), or polished silicon ATR crystals (see below). Silicon wafers were cut into smaller pieces prior to cleaning. The substrates were cleaned with a detergent solution (ca. 5% Deconex 12PA) by using a soft camel hair brush, rinsing with large volumes of hot tap water followed by distilled, deionized water, and finally cleaning in a Harrick PDC-3XG plasma discharge cleaner at 1-Torr argon pressure, 30-W power, for 5-20 min. The removal of single monolayen from the ATR crystalsfor their reuse required longer plasma cleaning times (-20 rnin). We have not yet developed a completely adequate method for cleaning relatively thick multilayer films from the ATR crystals for their reuse. Adsorption of Monolayers of 1 onto Silicon Substrates (MeO2CCZ2Si/Si).Cleaned substrates were immersed for 2-3 min into solutions of 1 in a solvent mixture of 83/20 Isopar-G/CC& a t a nominal concentration of ca. 0.2% (4 x M). Isopar G (Exxon), a high boiling (157-172 "C) iso-parrafhic hydrocarbon solvent, was passed through an approximately equal volume of alumina prior to use. CC& (Aldrich, 99%+ or "Gold Label" grades) was used as supplied. The solvent mixture was prepared and allowed to stand over a few drops of distilled water per liter for 12 h prior to addition of 1 which was weighed directly into the mixed solvent and dissolved with gentle heating of the vial. The solutions of 1 formed a cloudiness upon standing and were filtered after standing for 10-20 min after mixing. The filtered solutions generally remained clear for 1-2 h longer but were filtered again as required. After immersion, the substrates were removed and rinsed with ligroin, methanol, and distilled water. The monolayer films were then cleaned with a dilute aqueous detergent solution (1% aqueous Deconex 12PA) by using a soft, camel hair brush. After detergent scrubbing the samples were rinsed with large volumes of hot tap water, followed by distilled, deionized water, and were dried in a stream of nitrogen. Generally, apparently complete monolayers (as evaluated by film thickness and measured by ellipsometry and contact angles) were formed by a single immenion for the indicated time; however, occasionally incomplete monolayers resulted, but it was found that reimmersion in the silanizing solution for several cycles usually resulted in the formation of complete monolayers. Occasionally monolayers with low hexadecane contact angles (particularily the receding hexadecane contact angle) were obtained, and these were discarded. Partial monolayers of 1 were obtained by following the same procedure, except that adsorption times of only 5-30 s were employed.
Reduction of Monolayers of MeOzCCz2Si/Si to HOC,Si/Si. Monolayers of 1 were immersed a t ambient temperature for 1.5-2.0 min in a solution of LiAIHl in T H F (1.0 M, from Aldrich, used as supplied), followed by 20% HCl(aq) and then large volumes of cold water and distilled, deionized water. The reduced monolayers were then used or characterized without any further cleaning or preparation. Ellipsometric Determination of Film Thickness. Film thickness was estimated by using a Gaertner L116B ellipsometer equipped with a 632.8-nm helium-neon laser. Optical constants for the bare substrates were predetermined for 5-10 spots on the cleaned, uncoated surfaces for each sample immediately prior to immersion in the silanizing solution. Measurements employed an estimated film refractive index, nf = 1.50. Independent measurement of the film refractive index with multilayer samples with the same instrument for relatively thick films (>15 monolayers) gave values of q = 1.49-1.51. Thicknesses were measured within 10 min after preparation or reduction of the monolayers. The variation (standard deviation) in measured thickness across the dimensions of an individual sample was on the order of f l A, although this increased in magnitude with increasing film thickness and was on the order of 1 8 8, for ca. 20-layer films. Contact Angle Measurements. Contact angles for various test liquids were determined by using a Rame-Hart NRL 100 goniometer. Advancing water contact angles were measured by growing small sessile drops on the monolayer surface (to1-3-mm diameter) from a syringe with a square-cut needle, withdrawing the needle, immediately measuring the angle on both sides of the drop, and recording the average value. This was found to be equivalent to applying the drop with thin capillary tubes. Careful attention to details was found to be necessary to obtain reproducible water contact angles on the hydrophilic reduced (alcohol) surfaws. since, for example, advancing contact angles measured
Tillman et al. by this technique (vs dropping the water droplet onto the surface) were not found to be equivalent, the latter giving a lower value. Receding contact angles were determined by withdrawing a portion of the water droplet with the syringe and averaging measurements for both sides of the freshly diminished drop. Water used for contact angles was distilled, deionized, and filtered through a Millipore-Q filtration system. Advancing contact angles for diiodomethane (Kodak, filtered through basic alumina) and n-hexadecane (Aldrich Gold Label grade, used as supplied) were measured in the same fashion by applying fresh drops from thin capillary tubes, while receding angles were measured for freshly diminished drops by withdrawing a portion of the drop with the capillary tube. Advancing contact angles were found to vary little (-&lo) across an individual substrate surface, except for water contact angles on the reduced monolayers, which showed less precision (ca. h3O). Measurements were made in the ambient atmosphere for convenience. No difference in advancing water contact angles was found if measured in a nitrogen atmosphere saturated with water vapor.
FTIR-ATR Spectroscopy of Monolayers and Multilayers. ATR (attenuated total reflection) IR spectra were obtained on 45O incidence, 50 X 10 X 3 mm silicon ATR crystals (internal reflection elements, from Harrick Scientific) by using an IBM Model IR44 spectrometer. Spectra were run in a dry atmosphere in a sample compartment purged for 5 h with nitrogen and were referenced to background spectra previously determined for the crystal under the same conditions. All spectra were run a t a resolution of 4 cm-' for 5000 scans. Polarized spectra were obtained with a germanium, single-diamond, Brewster's angle polarizer (Harrick Scientific), which could be rotated through 90' to produce either p-polarization (parallel to the incident plane) or s-polarization (perpendicular to the incident plane). Base lines were adjusted to zero absorbance electronically. Spectra for an individual monolayer in a multilayer sample were obtained by subtracting spectra of the multilayer sample (reduced surface) obtained prior to deposition of the layer. Formation of Multilayers from 1. Multilayer samples were fabricated by continuing the adsorption-reduction sequence as described above for monolayers of MeOzCCz2Si/Si and HOCH,,Si/Si. Ester surfaces were cleaned immediately after adsorption of 1 by using low-power ultrasonic cleaning. Samples were sonicated for 1-3 rnin in a 1% detergent solution (Deconex 12PA) in water with a Bransonic 220 ultrasonic cleaner, rinsed with large volumes of hot tap water, followed by distilled, deionized water, and blown dry in a stream of nitrogen. Monolayers of Octadecyltrichlorosilane (OTS). The procedure for adsorption of monolayers of OTS was strictly analogous to that described above for monolayers of 1. OTS (Aldrich) was vacuum distilled prior to use. Cleaned silicon substrates, or reduced multilayer samples, prepared as previously described, were immersed in 0.2% (w/v) solutions of OTS in 80/20 Isopar-G/CC14, removed, and rinsed with ligroin, methanol, and hot water, followed by distilled/deionized water. OTS monolayer samples on silicon substrates were then washed with a detergent solution by using a soft camel hair brush, rinsed with hot water and distilled water, and blown dry with nitrogen. OTS monolayen adsorbed as terminal monolayers on multilayer samples were cleaned by sonication in detergent solution, as described above for multilayer preparation, rinsed with hot water and distilled/deionized water, and blown dry with nitrogen. Synthesis of Methyl 23-(Trichlorosilyl)tricosanoate(1). To 16.0 g (0.044 mol) of methyl 22-tricosenoate (2) dissolved in 100 mL of refluxing glyme (1,2-dimethoxyethane, Aldrich Gold Label grade, distilled from calcium hydride immediately prior to use) under nitrogen, with 100 mg of a 10% (by weight) solution of chloroplatinic acid hexahydrate (Kodak, used as supplied) added as catalyst, was slowly added 11.8 g (0.087 mol) of trichlorosilane (Aldrich, used as supplied) in 40 mL of glyme. The time of addition was 2 h. Then the reaction was refluxed an additional 4 h and stirred overnight at room temperature. The solvent was removed by distillation a t room pressure, and the Torr, product was distilled under vacuum, bp 218-225 "C at to afford 9.8 g (0.0195 mol, 44%). Several grams of forerun and afterun were discarded. Although the sample of 2 employed contained ca. 21% isomerized olefin, no evidence of isomeric trichlorosilanes was found upon careful examination of the 'H
Formation of Multilayers by Self-Assembly NMR or 13CNMR spectra of the product. We also examined the product formed from methanolysis of 1 with pyridine/methanol and found no evidence of isomeric silanes by NMR analysis. The product had the following spectral properties: IR (CCW 2928 (w, CH2),2855(s,CH2),1742 ( 8 , C=O(OMe)), 1466 (w, CH2),1172 (w), 590 (8, SiC13),567 ( 8 , Sic&)cm-'; 'H NMR (CDC13= 6 7.24, 300 MHz) 6 1.20-1.50 (overlapping CH2multa, 38 H), 1.55-1.63 (overlappingmulta, 4 H), 2.29 (t,J = 7.6 Hz, 2 H), 3.65 (8, 3 H); I3C NMR (CDC13= 6 77.00, 75 MHz) 6 22.28, 24.34, 24.98, 29.03, 29.18, 29.29, 29.38, 29.48, 29.62, 29.72 (llc),31.84, 34.02, 51.38, 174.1 (the assignment of the 6 29.72 peak as 11 C is not certain, since peak intensities do not allow an unambiguous assignment of the positions of the 10 unresolved carbons). Anal. Calcd for C2,H1,O2SiC13: C, 57.41; H, 9.44. Found: C, 58.46; H, 9.65. Synthesis of Methyl 22-Tricosenoate (2). As a sample procedure, a total of 10.2 g (0.058 mol) of 22-tricosenoic acid (3; prepared according to literature procedureslO)was dissolved in 300 mL of methanol with 2.0 mL of concentrated sulfuric acid and refluxed for 2 h. The reaction mixture was then dissolved in chloroform and washed with water, 5 % potassium carbonate, and finally with water again. The chloroform solution was separated, dried with magnesium sulfate,and concentrated by rotary evaporation to leave 17.4 g of crude 2 as a slightly yellow waxy solid. The crude product was easily chromatographed on silica gel with 50/50 heptane/CH2C12as eluent to purify it from the small amount of free acid remaining; the yield was 15.8 g (0.043 mol, 74%). The product contained ca. 21% isomeric esters, originating from double-bond migration observed in the synthesis of 3. Although the procedure followed in the synthesis of 3 is reported to form the product without isomerizationof the terminal double bond, in practice it was found to be difficult to carry out the Huang-Minlon reduction of the keto-acid intermediate without some degree of double-bond isomerization. Spectral properties observed for 2 IR (CClJ 2928 (w,CH,), 2855 (8, CHZ), 1742 (8, C-0), 1642 (VW,CH=CH2), 1468 (w, CH2), 1171 (w), 912 (w, CHaCH2); 'H NMR (CDC1, = 6 7.24, 300 MHz) 6 1.20-1.40 (overlapping CH2 mults, 36 H), 1.56-1.63 (m, 2 H), 2.00-2.06 (m, 2 H), 2.29 (t,J = 7.5 Hz, 2 H), 3.66 (s,3 H), 4.90-5.03 (m, 2 H), 5.74-5.85 (m, 1H) (double-bond isomerism is indicated by the presence of an "impurity" peak at 6 5.40 (m, 0.4 H), and, correspondingly, the multiplet at 6 4.90-5.03 integrates to only 1.6 H, based upon the expected intensity), 13C NMR (CDC13,75 MHz) 6 24.93, 28.92, 29.12, 29.22, 29.41, 29.47, 29.57, 29.64 (llc), 33.76,34.07,51.26,113.99,139.14,174,14(peak intensities do not allow an unambiguous assignment for the peak positions of the 10 unresolved carbons;ale0 found were "impurity" peaks probably due to the presence of isomers of 1 at 6 12.63,26.81,32.65,123.47, and 124.41). Anal. Calcd for CuH4O2: C, 78.63; H, 12.65. Found C, 78.40; H, 12.72.
Results and Discussion Monolayers of MeOzC(CHz)zzSiC18 (1) on Si and Reduction of the Terminal Ester Group. The formation of monolayers of l on silicon substrates (which we denote as Me02CCzzSi/Si)was straightforward, and our resulte are in essential agreement with literature We have used polished, p-doped silicon wafers and silicon ATR crystals, cleaned as described in the Experimental Section. The work reported on here was obtained under ordinary laboratory ambient conditions. Essentially complete monolayers were usually formed by immersion times of several minutes, as characterized by water and hexadecane contact angles on the ester surface, and by film thickness, as determined by ellipsometry.2d We found that there was a tendency for these initial monolayer films to show a slight excess film thickness greater than the 34-A thickness which may be estimated, using standard bond lengths, for a complete monolayer with perpendicular, fully-extended, all-trans alkyl chains. (10) (a) Barraud, A,; Roeilio, C.; Raudel-Teixier, A. J. Colloid Interface Sci. 1977,62,509. (b) Veale, G.; Girling, I. R.; Peterson, I. R. Thin Solid Films 1986,127, 293.
Langmuir, Vol. 5, No. 1, 1989 103 This excess thickness ranged from 1 to 15 A, with several angstroms being typical. Therefore we have employed washing the surface with a low-residue, alkaline, aqueous detergent solution ( 1 4 % ) and scrubbing with a soft, camel hair brush. This invariably resulted in a reduction in thickness of excessively thick films to values appropriate to true monolayers. IR evidence indicating that this treatment removes loosely bound impurity material was also found (see below). Various organic solvents, such as methanol, may, alternatively, be employed. Although such treatment may, at first glance, seem harsh, we have in fact discovered no evidence of measurable deterioration of monolayer quality, or adsorption of impurities onto the surface, on the basis of ellipsometry, contact angle, or IR measurements for monolayers of 1 on silicon substrates. However, as will be discussed below, this is not generally true for monolayers adsorbed onto multilayers samples. We also have not employed detergent treatment with the more hydrophilic hydroxyl surfaces formed upon reduction of the ester surfaces with LiAlH, (see below) since it was not found to be necessary, although several examples did not show any change in water contact angles after brief cleaning. We obtain, using these procedures, an average thickness of 34 f 2 A for 14 samples of Me02CCz2Si/Simonolayers, using an approximate value for the film refractive index of nf= 1.50. This value for nfseems well justified, since independent measurement of nf with the ellipsometer on relatively thick (650-700 A) multilayer films formed from 1 gave values of nf = 1.50 f 0.01. Advancing contact angles on these surfaces were 69 f 2' for water, 52 f 2' for diiodomethane, and 25 f 3' for n-hexadecane. Receding contact angles were 2-4' lower for water and 3-6' lower for hexadecane. The measured film thickness seems to be quite consistent with the close-packed monolayer structure of approximately vertical, fully extended alkyl chains, and our results for contact angle measurements are in good agreement with values previously published.s We note, however, that our ellipsometrically determined film thickness of -34 A is greater by 6 A than the value of 28 A found by using X-ray diffraction techniques.8 This may indicate that we have obtained more densely packed monolayers. When complete monolayers of 1 are immersed for 1.5-2.5 min in a solution of LiAlH, in THF (1.0 M), and then rinsed with 20% HC1 (aq), followed by large volumes of cold water and distilled/deionized water, a reduction in the advancing water contact angle to 30 f 3' and a decrease in the film thickness to 32 f 2 A is observed (13 samples). Receding water contact angles varied from 18 to 27'. Changes in other contact angles are smaller upon reduction of the monolayer than the decrease in the hydrophobic contact angle. The reduced monolayers showed diiodomethane contact angles of 37 f 1' and hexadecane contact angles of 18 f 4'. IR data (see below) indicate 290% disappearance of the ester carbonyl band a t 1743 cm-l. Further immersion in the reducing solution did not result in a significant additional change in the water contact angle, although one sample immersion for 2 X 2 min in the LiAlH, solution afforded an IR spectrum in which no carbonyl band was detectable. We attribute this, in agreement with previously published reports: to the reduction of the ester group to a hydroxyl group, forming a new "alcohol surface" monolayer, which we will denote as HOC23Si/Si. In light of the previous observation of a 50' advancing water contact angle for these monolayerss the observation of a 30' contact angle is puzzling. We believe this to be
104 Langmuir, Vol. 5, No. 1, 1989 due to differences in sample handling. Apparently the previous investigators routinely cleaned the monolayer surfaces, after reduction in ethereal THF and rinsing with concentrated HC1 and water, by Soxhlet extraction with fresh chloroform. When a sample monolayer initially showing an advancing contact angle of 31" and thickness of 34 A was immersed for 10 min in boiling, reagent grade chloroform and then dried in a stream of nitrogen, the contact angle rose to 60°, while the film thickness increased by 1A. This is a marked effect, which we do not observe with other, more hydrophobic surfaces such as OTS/Si, and suggests either that (1)trace amounts of hydrophobic impurities that drastically affect the contact angle are easily adsorbed onto the hydroxylated surface, (2) the surfaces swell with nonpolar solvents, or (3) the HOC23Si/Simonolayers restructure upon such treatment and minimize the free energy of the system by burying the hydroxyl groups as much as possible, given the restrictions imposed by covalent bonding to the surface, and exposing more CH2 groups to the surface. Such "surface reconstruction" has been postulated to occur for the surface of oxidized polyethylene, which consists largely of exposed carboxylic acid groups, with smaller amounts of ketones and possibly aldehyde groups." We have done some preliminary experiments to evaluate these possibilities. Four samples, which showed initial water contact angles and thicknesses after reduction with LiAlH4of 31 f 5" and 31 f 3 A were immersed in hot CC14 vapor.12 The resulting monolayers showed an increase in the water contact angle to 61 f 4" and a small increase in the film thickness to 35 f 4 A. Washing of one of these treated samples with detergent and rinsing with distilled water reduced the water contact angle and film thickness from 66" and 40 A to 57" and 32 A. This indicates that there may be some adsorption of material onto the surface, but this does not account for the full magnitude of the contact angle change. Small decreases in the water contact angle also resulted upon 30-h immersion of treated samples in cold water (before, 57", 32 A;after, 47", 34 A) or hot, 75 "C, distilled water (before 58", 34 A; after, 52", 37 A). Similarily, placing a treated sample overnight in a vacuum ( 1 Torr) also afforded little change (before, 62", 33 A; after, 58", 38 A). Two more control experiments were done. Two fresh HOC23Si/Simonolayers were prepared. One sample (with a water contact angle of 33" and a thickness of 32 A) showed no change after a 1.5-h soak in distilled, 75 "C hot water. The other sample was soaked for 30 h in room temperature CC14,and the result showed an increase in the water contact angle but not as dramatic a change as found for the samples subjected to the hot CC14vapor treatment (before, 32", 31 A; after: 42", 31 A). These results show that the results seen with the hot CC14treatment are not due to heating alone but that the change is driven by exposure to the nonpolar medium and proceeds much more slowly at ambient temperature than at the higher temperature of refluxing CC4. Thus, the exposure of HOC,,Si/Si monolayers to hot CC14 causes an increase in the water contact angle, a contribution to which is probably an irreversible restructuring of the monolayer to minimize the free energy of the N
(11) Holmes-Farley, S.R.;Whitesides, G. M. Langmuir 1987,3,62. (12) This experiment was done by placing the samples in a holder in a beaker with a small amount of refluxing CC14. Into the inside of the top of a beaker was built a water-circulatingcondenser coil. The samples treated with CCl, in this way were exposed only to hot CC14vapor or to condensing, fresh CCl,. The samples were removed from the vapor bath, rinsed with methanol and cold distilled/deionized water, and dried with a nitrogen stream.
Tillman et al. system and bury the surface hydroxyl groups. Loose adsorption of solvent onto the surface plays only a minor role. Rigorously speaking, it is still possible from the available data that CC14 becomes tenaciously intercalated in the monolayer and is not removed by any of the procedures reported here, despite the steric bulk of the CC14molecule. If there is, however, no solvent incorporation involved in this monolayer reorganization, then it is clear that there must be enough free volume distributed in the monolayer assembly to allow some unknown percentage of the hydroxyl-terminated alkyl chain tails to bend away from the air interface or for the alkyl chains to entangle. Furthermore, this is not a heat-driven process or simple melting, since soaking in hot water at a temperature comparable to refluxing CCll does not produce this change. Finally, under none of the conditions tested did we find this reconstruction to be completely reversible. The FTIR-ATR spectra of various monolayers were studied by adsorbing monolayers directly onto polished silicon ATR crystals. In Table I the data for Me02CCZSi/Siand HOC=Si/Si monolayers are compared with data for OTS monolayers on the same ATR crystals employed. Contact angle data were also collected for samples l a and lb, and for the OTS monolayers. Sample l a (MeOzCCPzSi/Si) showed contact angles of 69" for water and 24" for n-hexadecane. The monolayers of OTS showed both film thicknesses and contact angles reasonably in accord with previously published results for this wellcharacterized system: thickness, 23-28 A;3e,8913contact angles, 111"for water, 73-74" for diiodomethane, and 40" (sample 3) to 45" (sample 4) for n - h e ~ a d e c a n e . ~Al~*~~J~ though the hexadecane contact angle appears low for OTS sample 3, we feel that this is largely due to the particular batch of hexadecane employed at the time the spectrum for sample 3 was recorded. This is supported by the high water contact angle (111"for both samples 3 and 4) and the appropriate film thickness (26 A) observed. IR spectra for Me02CCz2Si/Siand HOC23Si/Si monolayers are presented in Figure 1and compared with the spectrum of bulk 1 in CC14solution. In this figure may be seen for the monolayers (Figure l b and IC)the symmetric and antisymmetric CH2 stretching modes, u, (CH,) and u,, (CH,) at 2851 and 2918 cm-l, respectively, and the CH, scissors bend, 6 (CH,), at 1468 cm-'. There is also an ester C=O stretch at 1742 cm-' in the spectrum of the ester monolayer (Figure lb), which nearly vanishes upon reduction with LiAlH4 (Figure IC)(discussed further below). The spectra presented in Figure l b and ICcorrespond to samples l a and l b in Table I. From a comparison of the data for samples l a and 3 (which were run on the same ATR crystal with the same sample handling conditions) it appears that the surface coverage of monolayers of MeOzCCz2Si/Siis, within the limit of experimental uncertainty, ca. 100% that of the reference OTS monolayer, based on the CH2 stretching intensities at ca. 2918 and 2851 cm-l, multiplying each intensity value by the peak widths at one half-height of 16 cm-' found for OTS and 15 cm-l found for Me02CCz2Si/Siand adjusting for the ratio of 22 CH2units in 1 to 17 in OTS. This is in good agreement with the results of Pomerantz et al., who determined that the coverage of Me02CCZSi/Si was ca. 90% relative to OTS.8 We note that all intensity data presented in Table I were recorded by using the same polarization (s-polarized incident radiation). The p,(CH2) frequencies of 2917-2918 cm-' observed for Me02CC2,Si/Siand OTS are consistent (13) Tillman, N.;Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. SOC.1988, 110,6136.
Langmuir, Vol. 5, No. 1, 1989 105
Formation of Multilayers by Self-Assembly
sampleb la
lb
2a
2b
2c
3 4
Table I. FTIR-ATR Data for MeOCCuSi/Si, HOCz3Si/Si, and OTS/Si Monolayersa substratec monolayerd condse thickness, A Y, cm-' intend 1 E A 30 f 4 2918 0.0426 2851 0.0280 1744 0.0084 1468 0.0040 R C 2918 0.0429 2851 0.0286 1744 0.0007 1468 0.0040 2 E B 37 f 3 2917 0.0534 2851 0.0349 1742 0.0090 1467 0.0058 A 34 f 2 2917 0.0458 2851 0.0300 1743 0.0083 1468 0.0045 R D 32 f 2 2917 0.0464 2851 0.0308 1744 0 1467 ~0.005 1 OTS/Si A 26 f 2 2919 0.0333 2851 0.0213 1 OTS/Si B 28 f 3 2918 0.0315 2851 0.2000 1468 ~0.004
dichroic ratio, 1.07 1.07 1.08
Rg
-
1.06 1.04
-
1.07 1.08 1.01 0.98 1.06 1.07 1.04 1.00 1.09 1.11 -
1.06 1.00 1.08 1.06
-
'Results of attenuated total reflection (ATR) spectra of monolayers adsorbed on silicon, 45O incidence, 50 X 10 X 3 mm ATR crystals. Each run number represents the same sample, with various treatments applied and indicated (in order) as a, b, etc. Substrates used were two different ATR crystals, numbered 1 or 2. dMonolayers are abbreviated as E = MeOzCCzzSi/Si,R = HOCz8Si/Si, OTS/Si = octadecyltrichlorosilane/Si. e Monolayers were prepared as described in text, and spectra were obtained under the following conditions: A, monolayers prepared and detergent washed with brushing of the surface; B, sonication for 2 min in detergent solution; C, MeOzCCBSi/Si monoalyers were reduced for 1.5 min, in LiAlH,/THF, as described in text; D, MeOzCCzzSi/Simonolayers were reduced for 2 X 2.0 min, in LiAlH,/THF, as described in text. /Intensity at peak maximum for selected wavelengths, Y, for s-polarized spectra. #Dichroic ratio R = ratio of s-polarized to p-polarized absorbance at specified wavelengths.
with previously published results for these mon01ayers~~J~ and are shifted from solution values by 10-11 cm-' from the 2928 cm-l we observe in CC14 for both OTS and 1. Studies of long-chain alkanethiols adsorbed on gold surwhich form similar, close-packed monolayer assemblies, have shown that peak frequencies shift to lower frequencies with increasing chain length, which may be attributed to increasing solid-phase character for the longer molecules (presumably resulting from increased close packing, driven by the increase in cohesive van der Waals interactions in the alkyl chains). The IR data in Table I also indicate the problem of adsorption of loosely held material onto the surface of monolayers of 1. Samples 2a and 2b represent a monolayer, spectra of which were obtained after first sonicating the monolayer-coated ATR crystal for 2 min in detergent solution and then washing the surface more vigorously by using detergent solution and a soft camel hair brush. After the detergentlbrushing treatment there is a reduction of 8% in the film thickness, from 37 to 34 A, and a decrease of 15% in the absorbance of the methylene stretching frequencies. We cannot a t this point specify unambiguously the nature of the material adsorbed, but it is likely to be a combination of solvent and partially polymerized 1, since the absorbance of the carbonyl ester band at 1742 cm-' has also diminished by 8%. Apparently this material is fairly tightly bound and was not removed by the short sonication. The use of polarized light allows one, in principle, to use ATR-IR to measure the average orientation of transition dipole moments for monolayers by measuring dichroic ratios of absorbances of orthogonally polarized incident beams,14although in practice it is difficult to get more than semiquantitative results for transition moments which are b
(14)Mirabella, F. M., Jr. Appl. Spectrosc. Rev. 1986,21, 45.
roughly parallel to the substrate surface, since the ratios are expected to be close to unity and to vary little with changes in orientation. Furthermore, disorder16 of adsorbed material on the monolayer will tend to change the observed dichroic ratios, with a value of R = 0.897 (defined below) predicted for a completely isotropic film on a silicon, 45O incidence ATR ~ r y s t a l . ~With ~ J ~this cautionary note, we present measurements of the dichroic ratio R, which we define as the ratio of absorbance intensities for s-polarized incident radiation (polarized perpendicular to the plane of incidence) to p-polarized incident radiation (polarized parallel to the incident plane). We have employed in a separate work this method to estimate the orientation of phenoxy groups incorporated into the alkyl chain of trichlorosilane monolayers and present details of our methods in that publi~ation.'~We indicate here only that the range of values of R for the CH2from 1.00 to 1.11 found in Table I correspond (for 45O incidence silicon ATR crystals) to tilts of the transition dipole moments of 65-90° from the axis perpendicular to the substrate surface, with The 1.09 being the value calculated for 90° orientatior1.3~J~ orientation of molecular features can be inferred provided that the orientation of the transition dipole moments with (15) The term 'disorder" we employ here in a general sense to refer to deviations from a close-packed, regular array of all-trans, fully extended alkyl chains of a monolayer assembly. Such a situation would result in a monolayer structure with exclusively ester or hydroxyl groups exposed to the solid-air interface. It is apparent that several forms of 'disorder" are possible, including entanglement and trans-gauche isomerism of the chains. It is apparent that deviations from a perfectly ordered close-packed monolayer structure will result in chruiges in the IR spectral features and in the surface wettability (since the intrinsic wettability of the ester and methylene groups, for example, is quite different and since wettability is believed to be the result of the structure of the outermost few Bngstroms of a surface"). Nevertheless,the information contained in currently available techniques such as we employ in this report does not allow distinctions to be made regarding the exact, detailed nature of any disorder.
Tillman et al.
106 Langmuir, Vol. 5, No. 1, 1989 H
IV U F
1,2928
I
1 "
51
$
?
\
;I
b
040-
I
1I 11
J
"c, /
COW, 1
Figure 2. Representation of the structure of the hydrocarbon chains in Me02CCzzSi/Simonolayers, assuming all-trans alkyl chains with a chain axis perpendicular to the substrate surface, showing the silane head group connected to the substrate and to adjacent silicon atoms via Si-0 bonds.
a000
2000
2500
3000
1503
I500
I200
Wavenumberr
is not surprising: the value of R for a film that is 90% perfectly ordered (with vertical alkyl chains) and 10% isotropic should be (0.90 X 1.09) (0.10 X 0.90) = 1.07; Le., the value of R should diminish by only 0.02 from that expected (R = 1.09) for perfectly ordered, vertical alkyl chains. Dichroic ratios for the ester carbonyl band at 1742 cm-I also indicate ordering of the monolayer. For Me02CCz2Si/Simonolayers with completely perpendicular, fully extended alkyl chains, the ester carbonyl should be approximately parallel to the surface, which is in fact observed, with dichroic ratios of 1.04 and 1.08 in samples 2b and l a (C=O stretch transition dipole moment tilt of 70-85' from the substrate normal). Taken together, these data are supportive of an ordered monolayer film with approximately vertical alkyl chains and surface-exposed, oriented methyl ester groups with C = O bonds parallel to the substrate surface, as diagrammed in Figure 2. The E t data also furnish information about the reaction of the Me02CCz2Si/Simonolayers with LiAlH4. Spectra for sample l b (also presented in Figure IC)were obtained after treatment of the monolayer in l a for 1.5 min in LiAIHl (1.0 M, at ambient temperature), followed by rinsing with cold, 20% HC1, large volumes of cold water, and distilled/deionized water. We observe 290% of the carbonyl band disappearing under these conditions, whereas in sample 2c, which was immersed for 2 X 2.0 min in the LiAIHl solution, no carbonyl band remained. We are not able to identify conclueivelyany IR spectral features due to the hydroxyl group in the reduced monolayer. For both of samples l b and 2c the intensity at 2918 cm-I suggests that 197% of the material is retained on the surface after reduction; however, a slight change in bandwidth could easily account for a 3% change in intensity. From the dichroic ratios we are not able to discern any major disordering of the monolayers, and the average value of 1.08 f 0.03 found for the values of R at 2918 and 2851 cm-' (for samples l b and 2c) corresponds to an alkyl chain axis tilt of -10 f 6O, Le., little change from the unreduced monolayer. In this subsection we have presented data which show that 1 adsorbs onto silicon surfaces to give ordered, close-packed monolayers which can be successfully treated with LiA1H4 to afford monolayers with hydroxylated surfaces with little loss of monolayer material or major disordering of the alkyl chains in the monolayer. This process can be continued indefinitely, in principle, to form
+
-
Il
[
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Si
2918
00401
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0
,
,
,
,
,
,
rn
,
,
1 , 3000
I
2R5l
, 2500
,
,
I
2000
IMO
1200
Wavenumberr
Figure 1. (a, Top) FTIR spectrum of 1 in CCll solution, (b, middle) attenuated total reflection (ATR) spectrum of the MeOzCCaSi/Simonolayer, and (c, bottom) FTIR-ATR spectrum of the monolayer in Figure l b reduced with LiAlH, to form a HOCC&i/Si monolayer. ATR spectra presented are s-polarized (polarizationperpendicular to incident plane), were recorded at dun-' resolution with 6000 acans on the same silicon ATR crystal, and are referenced to the same background spectrum. The spectra in b and c are tabulated as samples l a and Ib in Table I. respect to molecular coordinates is known. For the methylene stretching vibrations, v, (CHJ and va (CHJ, the transition dipole momenta lie perpendicular to the alkyl chain axis for a fully extended, all-trans alkyl chain.18 Considering samples l a and 2b, which apparently have the cleanest surfaces and should give the most reliable results, we obtain an average value of R for the two CH2stretching modes of 1.07 f 0.01, corresponding to an approximate chain axis tilt of 10 f 2 O , although this is probably an unreasonably low uncertainty. We note that the dichroic ratio is fairly insensitive to the details of sample handling, as the data for samples 2a (cleaned by sonication in detergent solution) and 2b (cleaned with detergent/brushing) show only a small decrease of 0.01 in the value of R. This
-
(16) (a) &bolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J . Chern. Phys. 1983,78,946 and references therein. (b) Reference 4b and references therein.
Langmuir, Vol. 5, No. 1, 1989 107
Formation of Multilayers by Self-Assembly
,
1000,
d
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80
/ I
I
,
I
1
600
-
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I
t
i
0
0 0
5
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15
20
25
5
30
Layer number
IO
15
25
20
M
toyer number
Figure 3. Film thickness, determined by ellipsometry, w layer number, measured on eight different multilayer samples.
Figure 4. Water (circles)and n-hexadecane (squares) advancing contact angles on Me02CCz2Si/Simonolayers vs layer number, measured on five different multilayer samples.
multilayer structures. In the next subsection we will present our results for multilayer films which we have fabricated. Multilayers from 1. The formation of multilayer films from 1could be affected by a continued application of the procedures outlined above for the chemisorption of monolayers of 1 on silicon. However, it was found that the treatment of the ester surfaces of multilayer samples with a detergent solution and a soft brush resulted in deterioration of the multilayer samples. Adsorption of terminal monolayers of OTS onto samples which had been so treated resulted in OTS layers which streaked when retracted from methanol, by which OTS monolayers are normally completely unwetted. A high-quality OTS monolayer on a silicon wafer normally emerges completely dry from methanol, but these samples emerged without even, complete peeling of the methanol. Brushing of the surface with a suitable solvent, such as methanol, instead of the detergent salution, still showed the same streaking, even though ellipsometry measurements did not indicate any depletion of monolayer material. The application of water drops to the surface after reduction with LiAlH4 for the measurement of contact angles yielded oval shaped drops, indicating anisotropy of the wetting properties of the surface. The origin of this disordering is not clear. Therefore, we have employed only sonication of the multilayer samples in detergent solution, using a low-power sonication apparatus. Although this does not appear to give a completely clean surface, we have discovered no evidence of multilayer destruction as a result of this treatment. In all other respects, the procedure for adsorption of monolayers onto multilayer samples was completely analogous to the procedure outlined for monolayer formation on silicon substrates. In this manner multilayer samples were easily fabricated on silicon wafers in which a linear relationship between the film thickness and the layer number is observed (Figure 3). The linear regression line through the data points (measured after treatment with LiA1H4 for eight independent multilayer samples and a total of 59 data points) shows a slope of 35 A/layer, hence it appears that about 11% more material is adsorbed than the 32 &layer which would be expected for the results for HOC,,Si/Si monolayers. This is likely due largely to the more effective cleaning which could be employed for the monolayers on silicon substrates than is possible for the multilayer samples. Thus it is probable that the process of multilayer formation we employ incorporates a certain amount of loosely held, “impurity” material, either by adsorption on
Table 11. Contact Angles and Thicknesses of OTS Monolayers Adsorbed on Multilayers from 1 on Silicon Wafers layers of 1 0 1 5 10 15 25
OTS layer thickness, Ab 25 k 2 26 & 2 26 k 3 28 2 34 5 NAC
**
contact angle,’ deg (*2O) H20 CHJ2 n-ClsH, 111 74 45 111 74 45 110 74 44 109 73 43 110 71 44 106 70 43
OAdvancing contact angles of sessile drops. bThicknessof OTS layer by subtraction of thickness of multilayer of 1, measured by ellipsometry. Not available since the ellipsometer employed failed to give accurate numbers for more than -20 monolayers.
top of the monolayer surfaces or inclusion in the monolayer bulk, although 35 A is certainly a very reasonable thickness value. There is a tendency to decreasing precision of thickness measurements with increasing layer number: the uncertainty (expressed as standard deviation) on the readings of film thickness at various positions across the face of the sample increased from ca. fl A for only a few monolayers to ca. f8 A for very thick films of ca. 20 monolayers. We also monitored contact angles for both the ester and the reduced, alcohol surfaces during the process of multilayer fabrication. We were very surprised to find no apparent deterioration of either the hexadecane or water contact angles on the ester surface with increasing layers (Figure 4). In fact, the tendency was for contact angles to increase after the initial monolayer, and after three or four monolayers the contact angles appeared to stabilize at about 69 f 2O for water and 30 f 2O for hexadecane, as compared with 68 f 2 O and 27 2O for the initial monolayers employed in the multilayer work. As a check on these results, we have also capped the monolayer samples by adsorbing terminal monolayers of OTS and measuring the contact angles (Table 11). Although some loss of hydrophobicity and oleophobicity can be detected after 5-10 monolayers, it is not until more than 15 monolayers (we measured 25 monolayers plus OTS) that a deviation of more than our estimated experimental error (f2O) is observed. These terminal OTS monolayers also appear to show a generally increasing film thickness with increasing number of layers in the films (for example, 34 A for the 15-layer-plus OTS sample). We do not observe a correlation between OTS layer thickness and the contact angles.
*
Tillman et al.
108 Langmuir, Vol. 5, No. 1, 1989
0301
,2917
2851
I
A
Wovenumberr
0
10
5
15
20
25
I
30
,2917
toyer number
Figure 5. Advancing water contact angles on HOCzzSi/Si monolayers w layer number. Measurements were taken with four different multilayer samples, with the values for each plotted by using different point types. Table 111. FTIR-ATR Data for a Multilayer Sample on a Silicon ATR Crystal' thickness, dichroic laversb surface' 8, Y, cm-' intensitvd ratio, Re 1.01 1-7 R 240 6 2917 0.319
*
8
E
R
29
*6
40 f 6
2851 1741 1721 1468 2917 2851 1742 1470 2917 2851 1741 1466
0.208 0.0060 0.0037 0.032 0.0454 0.0298 0.0090 0.0048 0.0476 0.0315 0.0013 0.0063
1.01
1.03 1.00 1.02 0.96
1
0Wok
2851
f
1.03 1.06
f
'Spectra were recorded as described in note a, Table I, and are of a multilayer sample prepared on the initial monolayer presented in Table I as sample 1. bThe notation 1-7 means the spectrum of the seven-layer sample; layer eight is the spectrum of the eighth layer obtained by subtraction of the seven-layer spectra from the eight-layer spectra. The samples were either those with an ester surface (E) or reduced (R)to the alcohol surface. ds-polarized intensity, absorbance units. a Ratio of the s-polarized to p-polarized intensities. f Not available due to relatively low signal-to-noise ratio and consequent uncertainty of intensities. Although the water and hexadecane contact angles appear to stabilize at a constant value on the ester surfaces, the same cannot be said for the water contact angle on the reduced, hydrophilic alcohol surfaces (Figure 5). There is a significant general increase in the advancing water contact angle (we note also that receding contact angles also tend to increase), so that, by about 20 monolayers, the contact angle for water on the reduced 20th layer has increased to -49" from the initial value of -30". Based upon a study of the water contact angles of imcomplete monolayers of HOC&i/Si (see below), we believe that this reflects a general tendency to increasing disorder in the monolayers with increasing layer number. This water contact angle appears, in fact, to be the most sensitive and obvious indicator of disorder in the monolayer structure. FTIR-ATR data for a multilayer sample, analogous to the data in Table I for monolayers, are collected in Table 111, and spectra are presented in Figure 7 for a seven-layer sample (reduced) and, for the eighth layer, both ester and reduced (alcohol). We note that the seven-layer sample (in Table I1 and Figure 7a) was a continuation of the identical sample which also appears in Figure l b and IC
l
,
,
,
,
4000
I
3xa
.
.
,
,
,
,
,
,
,
3000
,
,
2500
,
,
2000
I500
1200
Wovenumberr
Figure 6. FTIR-ATR spectra for (a, top) a seven-layer sample, terminated by an HOCzsSi monolayer (Le., reduced); (b, middle) the MeOzCCzzSi eight layer adsorbed on the sample in a; (c, bottom) the HOCzsSi eighth layer after treatment of the sample in b with LiA1H4. Spectra shown are s-polarized (perpendicular to incident plane), were obtained on the same ATR crystal, and are referenced to a common background. Spectra were obtained a t 4-cm-' resolution and were run for 5000 scans.
0
40
U 0 %OD
j
1 01 0
10
,
20
30
40
Monolayer thickness
Figure 7. Advancing water (circles) and n-hexadecane (sqquares) contact angles vs monolayer thickness (determined by ellipsometry) for MeOzCCzzSi/Simonolayers and partial monolayers.
Formation of Multilayers by Self-Assembly and as sample 1 in Table I. Spectral features for the multilayer resemble those for the monolayer sample. The vg (CH,), v, (CH,), and 6 (CH,) bands appear at the expected frequencies of 2917,2851, and 1468 cm-l, respectively, with somewhat "excessive" intensities for each (for example, the absorbance at 2917 cm-l of 0.319 is -6% greater than the 7 > 0.0429 = 0.300 expected on the basis of the initial HOC2&3/Si monolayer intensity; a similar calculation for the 2851-cm-' band gives +4%). We note that this is probably due, as we believe to be the case for the monolayer thickness measurements in the multilayer samples, to the difference in experimental protocol between the monolayer/Si sample and the multilayer samples; the former sample was brushed with detergent solution, which could not be employed for the multilayer sample. Relative intensities and bandwidths are also similar (peak widths at half-height were 16 cm-' at 2918 cm-' for the monolayer and 15 cm-l at 2917 cm-l for the multilayer). Dichroic ratios are somewhat lower in the multilayer sample, indicating greater tilt of the alkyl chains. The value of 1.01 found for both of the methylene stretching vibrations corresponds formally to an alkyl chain axis tilt of -22' from the normal or -12O greater than for the monolayer sample. The numbers obtained are still significantly different from the -0.90 which would be measured for a completely disordered, isotropic system, indicating that considerable ordering is still present. Also conspicuous are the small impurity peaks at 1741 and 1721 cm-'. Probably these correspond to residual ester carbonyl bands resulting from incomplete reduction of the MeOC(0) group, especially the peak at 1741 cm-'; perhaps the 172-cm-' band is also an ester carbonyl band which is localized in a different chemical environment. These impurity bands are too intense to be solely the result of incomplete reduction of the seventh layer and must be present throughout the entire bulk of the sample. When this seven-layer sample was treated with 1 in the usual fashion, an eight-layer sample was obtained, yielding a thickness of 39 f 6 A for the eighth layer (the uncertainty is estimated from the deviation in measured values across the surface of the substrate) and advancing contact angles of 69' for water and 28' for n-hexadecane (vs 69' and 24' for the initial monolayer). The absorbances of the methylene stretching vibrations are, like the multilayer film, 11% greater than would be expected on the basis of the Me02CCz2Si/Sispectrum; the ester carbonyl band is also about 7% larger than would be expected from the initial monolayer. Treatment with LiAlH, in THF for 1.5 min followed by rinsing with 20% HCl and then large volumes of cold water and distilled/deionized water afforded a reduced monolayer with almost exactly 100% of the expected intensity for the methylene stretching vibrations (based on the unreduced eighth layer), allowing for the 23/22 ratio of methylene units in the alcohol/ester monolayers. The ester carbonyl band in the IR is not as completely reduced as was observed for the initial monolayer; the residual peak at 1741 cm-' is almost twice that found for the HOC23Si/Si monolayer. We note here that this residual carbonyl band has a barely discernable shoulder at 1725 cm-'. The water contact angle resulting was 44' vs 35' for the HOC2,Si/Si monolayer. Dichroic ratios for both the unreduced and reduced eighth layer are indicative of ordering in the alkyl chains (chain axis tilt of -15' (R = 1.05) to 22' ( R = l . O l ) , although, like the seven-layer sample, they are not as high as found for the initial monolayer. We wish to point out that these data, although suggestive, must be viewed with some caution, since thy spectra of the eighth layer were obtained by subtraction
-
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Langmuir, Vol. 5, No. 1, 1989 109
-
of two spectra differing by only 13% in absorbance intensities. Thus, any relative errors for the eighth layer will necessarily be larger than for a single monolayer on silicon. Probably the intensity data, especially for the methylene stretching vibrations, which are relatively intense, are accurate; however, less certainty can be associated with data which rely on very small differences, especially R. The ellipsometry data, absorbance intensities, and dichroic ratios for the multilayer samples all suggest that the multilayer samples are clearly composed of distinct monolayers and not layers of bulk film. The IR data indicate that there may be more tilting or disordering of the alkyl chains in the seven-layer sample than for the monolayer samples. These data do not unambiguously answer the important questions of whether disorder tends to increase with increasing film thickness. The film thickness shows a steady, even increase with increasing layer number (although the variability of the measurements tends to increase), and the advancing contact angles on the ester surface tend to stabilize at constant values. However, the contact angles for water on the reduced, alcohol surfaces show a distinct and entirely significant increasing trend with the addition of more layers. The IR sample showed a 35-45' change when the first and eighth layers were compared, while multilayer samples on silicon wafers showed a similar, but slower, tendency to increase, with 45' being the typical value for -15 monolayers (Figure 5). Surprisingly, no such drastic change was observed for either the water or n-hexadecane contact angles for the unreduced layers. Which of these parameters, the contact angles on the ester surface or the water contact angles on the alcohol surface, conveys the most information about the quality of the monolayers which are deposited? In order to begin to answer this question, it is necessary to be able to introduce, in some kind of controlled fashion, disorder into the monolayers and to measure the resulting contact angles. We found that we could form partial monolayers (measured by ellipsometry) on silicon wafers by briefly immersing the wafers into the silanizing solution (5-30 s, as compared with the 2-4 min usually employed) and rinsing and washing the monolayer surfaces in the usual fashion. Although partial monolayer formation represents only one kind of the several kinds of disorder which might be expected to be possible in monolayer assemblies, it is probably representative. The alkyl chains would be expected to bend and intertwine in order to fill the introduced voids. It is also quantifiable, since partial coverage should lead to reduced film thicknesses, which can be detected with a reasonable level of confidence by the ellipsometer. The results, plotted as contact angle vs film thickness, are presented in Figures 7 and 8. From these data it appears that the less sensitive indicators of disorder are the advancing water and hexadecane contact angles on the ester surface, while the most sensitive contact angle data are to be found for the water contact angle on the reduced, alcohol surface. Above about 25-A thickness there is little change in the contact angles on the ester surface; for the alcohol surface, on the other hand, the water contact angle changes from -40' at 25-A thickness to -30' at the close-packed thickness of 34-35 A. This represents a clear trend, although there is considerable scattering of the data in Figure 8, and justifies the assertion made above that there is a general tendency toward increasing disorder in the multilayer samples with increasing number of layers. The idea that increasing disorder would increase the water contact angle is intuitively plausible, since disorder would effectively expose
-.
110 Langmuir, Vol. 5, No. 1, 1989 7
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Tillman et al. 1
0
0 0 0
0
80
201-
O
I 1
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0
I
IO
-
_
_
-
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A
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.
20
Monolayer thickness
-
-
30
40
A
Figure 8. Advancing water contact anglea VB monolayer thickness (determined by ellipsometry) for HOC&i/Si monolayers and
partial monolayers.
methylene units to the surface, which are intrinsically hydrophobic to water drops. What these data do not, by themselves, make clear is what percentage of the -30' contact angle actually observed is due to residual disorder at the monolayer/air interface, what the minimum water contact angle obtainable with trichlorosilyl systems is, and to what degree the quality of monolayer adsorption onto alcohol surfaces (and hence multilayer film fabrication) can be correlated with the hydrophilic contact angle. Current research on alkane thiols has suggested that self-assembled monolayers of o-hydroxy-substituted thiols on noble metals form highly ordered, close-packed systems, with the monolayer-air interface composed largely of the terminal g r o ~ p . ~ Advancing J~ contact angles of such monolayers are highly sensitive to the terminal functional groups substituted on the alkane thiol chains." Monolayers of 11-hydroxy-1-undecanethiolafforded contact angles of ==O" on gold17J8and -20" on silver'* substrates. Thiol monolayers appear to be among the most highly ordered monolayer assemblies presently obtainable, and if these are taken as model systems, a comparison with the data presented in this report suggests that there is a contribution of CH2 groups to the wettability of the HOC2&X/Sisurface, since the water contact angle (-30') is higher than would be expected for a pure hydroxylated surface. The origin of this disordering is still a matter for speculation, but several possibilities include (1)imperfect packing of the ester group, (2) disorder introduced by the chemical reaction of the ester group with LiAlH,, and (3) disorder introduced by the polysiloxane network formed by the trichlorosilyl head group, which might disorder the surface indirectly by disordering the alkyl chains. Experimental investigation into these questions will continue in our labs to try to answer these questions. We have not systematically investigated various other wettability parameters, such as receding contact angles. One which also appears to be useful is the receding hexadecane contact angle on the ester surface, which appears from preliminary results to show considerable sensitivity to monolayer ordering, decreasing to -10' for the 20th layer on one multilayer sample from an initial value of 20' for the first layer on Si, while the advancing contact angle remained unchanged. Although these results are encouraging for the eventual fabrication of relatively thick films by self-assembly, some (17) Bain, C. D.; Whitesides, G. M. J.Am. Chem. SOC.1988,110,3665.
(18)Ulman, A.;Littman, J.; Tillman, N., to be published.
problems remain. One of these is the problem of impurity adsorption. Ellipsometry and IR have shown that there is a tendency for excess material to adhere to the monolayer surface (a similar conclusion has been reached by other workers3deven for the more hydrophobic and oleophobic surfaces of OTS monolayers). Ellipsometry and IR together suggest that the nature of this impurity is a combination of solvent and ester-containing material, presumably polysiloxanes derived from 1 adsorbed during the silanation step. Examination of the surfaces by optical microscopy at 500-1OOOX has disclosed that particulate matter of 1-10-mm dimensions was present on two multilayer samples of 5- and 10-monolayer thickness. Detergent washing either with sonication or brushing failed to completely remove the particles, although regions free from these could be detected after the latter treatment. We found no distinctive particles on two unwashed monolayer samples, although an excess thickness of 15 A on one of them was detected, which could be removed by detergent and brushing. It appears, therefore, that there may be adsorbed impurities present as both a thin film and as gel slugs, dust, or other particulate matter on the surface. Other gross features, such as cracks in the film, could not be unambiguously detected under the microscope. As the films grow thicker (>20 layers), they acquire a blue color due to interference effects, and the surface can be visually evaluated. Except for small regions, usually at the wafer edges, where the process of multilayer formation has evidently been disrupted, the wafers appear to be evenly coated. Much remains to be learned regarding the extent and nature of film defects in these multilayer samples and what improvements in experimental procedures can mitigate these problems.
Conclusion We have presented a detailed evaluation of the process of multilayer film formation by self-assembly, and the results are encouraging for the prospects for eventual construction of relatively thick films. In agreement with the results of Pomerantz et a1.8 we have shown that 1 forms close-packed, ordered monolayer structures. These assemblies contain surface-exposed, oriented ester groups, in which the C=O bond is approximately parallel to the substrate surface. These monolayers can be successfully treated with LiAlH, in THF to form a hydroxyl-terminated monolayer with 100% of the packing of the original monolayer and no major disordering of the alkyl chains detectable by FTIR-ATR. The resulting advancing water contact angle (-30' for the best-quality monolayers) is highly sensitive to disordering effects. The HOC,,Si/Si monolayers are not perfectly ordered, and this value of 30' probably includes an unknown degree of CH2 contribution to the surface properties. Multilayers as thick as 25 monolayers can be formed from 1, although we find that there is gradually increasing disorder, based on water contact angle data for the reduced alcohol surfaces. These multilayer films clearly consist of sequentially deposited, discrete monomolecular layers. It is probable, we believe, that further improvements in substrate quality, experimentalconditions, and amphiphile design will result in even more encouraging results. In this context, we note that the Sagiv group has presented results indicating that p-tolyl esters analogous to 1 form multilayers at least nine layers thick.g Thus it appears that
-
(19)Lee, H.; Kepley, L. J.; Hong,H.-G.; Mallouk, T. E. J. Am. Chem. SOC.1988,110,618.
Langmuir 1989,5,111-113 further improvements in amphiphile design can be expected to significantly increase the number of monolayers which can be successfully adsorbed. We note also that new reactions useful for the self-assembly of multilavers can be expected to be discovered, of which the recerk report of a seven-layer film linked by zirconium-phosphonate bonds is a recent and creative example.19 A still unclear issue is to what degree multilayer formation will remain compatible with incorporation of diverse functionalities into-the structure of the film, and we are continuing to explore this problem. From the results presented here it
111
appears, however, that self-assembly represents a viable alternative to the Langmuir-Blodgett technique for the construction of ordered, multilayer films of thicknesses amroaching -- 0.1 um.
Acknowledgment. We acknowledge the contributions of vita DePalma, for obtaining optical microscopy results, and of Ravi Sharma, for stimulating discussions, both of EivemifiedTecholot$es Research Group, Eastman Kodak Lo-
Registry No. 1,103946-41-4; L m ,16853-85-3; Si, 7440-21-3.
Association between Amphiphilic Cyclodextrins and Cholesterol in Mixed Insoluble Monolayers at the Air-Water Interface Svetla Taneva,? Katsuhiko Ariga, and Yoshio Okahata* Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan
Waichiro Tagaki Department of Applied Chemistry, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558, Japan Received July 19, 1988 Two-component insoluble monolayers of amphiphilic cyclodextrins (a-, 0-, r-CleCDs) and cholesterol (CH) were studied at the air-water interface. Surface pressure vs area measurements were carried out at various compositions of the mixtures. Negative deviations of the mean molecular area from the additivity rule are observed in the three binary monolayers (a-C16CD/CH,P-C16CD/CH,and r-C16CD/CH). Both calculations and monolayer data showed that cholesterol can be included into only the r-CleCD cavity. monolayers, cholesterol seems to be accommodated in the alkyl In the cases of mixed a-or @-Cl6CD/CH chain region of amphiphilic cyclodextrins.
Introduction Cyclodextrins (CDs) are unique molecules capable of forming inclusion complexes with molecules of an appropriate size in their apolar cavities. Because CDs are able to encapsulate guest molecules and thus modify their physicochemical properties, the practical application of CDs in the pharmaceutical and food industries has gained in importance.'I2 Recently, the interaction between cyclodextrins and biological membrane components such as steroids and phospholipids has been studied in connection with the hemolysis of human erythrocytes. For example, a high concentration of CDs has been reported to hemolyze human erythrocytes in the order 0- > a- > y-CD,s*4and this effect has been attributed to the removal of membrane components by CDs. Inconsistent data, however, have been reported by other workers concerningthe interaction of CDs with steroids in aqueous solutions: formations of a stable inclusion complex between a-CD and cholesterol with a molar ratio of almost 1:16 and no interactions of a-CD with steroid molecules.6 p- and y-CDs have been proposed to include a part of steroid molecules in their cavities.w A qualitative monolayer study has showed that 'On leave from the Department of Physical Chemistry, Faculty of Chemistry, University of Sofia, Anton Ivanov 1,1126 Sofia, Bul; garia.
the surface pressure of a cholesterol monolayer decreases when CDs are dissolved in the subphase in the order 8> y- > a!-CDmg In the present work, the monolayer method has been employed to investigate the association between cyclodextrins and cholesterol by using mixed monolayer system of amphiphilic cyclodextrins (a-, b-, and r-C,,CDs) and cholesterol (CH). Formation of stable insoluble monolayers by amphiphilic @-cyclodextrinshaving seven long alkyl chains'O and deposition of Langmuir-Blodgett films (1) Saenger, W. Angew. Chem., Int. Ed. Engl. 1980,19, 344. ( 2 ) Szejtly, J. Cyclodextn'na and Their Inclusion Complexes;Akademia Kiado: Budapest, 1982; Chapter 5. (3) hie, T.;Otagiri, M.; Sunada, M.; Uekama, K.; Ohtani, Y.; Yamada, Y.; Sugiyama, Y. J. Pharmacobio-Dyn. 1982,5, 741, (4) Szejtli, J.; Czerhati, T.; Szogyi, M. Carbohydr. Polym. 1986,6,35. ( 5 ) Hammami, M.; Maume, G.; Maume, B. Cell. Biol. Toxicol. 1986, 2, 41. (6) Kempfle, M.; Mueller, R.; Palluk, R. Freaeniwr. 2.Anal. Chem. 1984,317, 700. (7) Kempfle, M.; Mueller, R.; Palluk, R.; Winkler, H. Biochim. Biophys. Acta 1987,923,83. (8) Lach, J.; Pauli, W. J . Pharm. Sci. 1966,55, 32. (9) Miyajima, K.; Saitq H.; Nakagaki, M. J. Chem. SOC.Jpn. 1987, 306.
(10)Kawabata, Y.; Matsumoto, M.; Tanaka, M.; Takahaehi, H.; Irinatau, Y.; Tamura, s.; Tagaki, W.; Nakahara, H.; Fukuda, K. Chem. Lett. 1986, 1933.
0743-7463/89/2405-0111$01,50/00 1989 American Chemical Society
