Mixed Monolayers Self-Assembled on Mica Surface - Langmuir (ACS

Muscovite mica was purchased from S&J Trading, U.S.A. .... into account the ratio between the actual and the geometric surface areas.24,26 However, th...
0 downloads 0 Views 367KB Size
Download PDF
4274

Langmuir 2001, 17, 4274-4281

Mixed Monolayers Self-Assembled on Mica Surface Vladimir Kitaev, Minseok Seo, Mark E. McGovern, Yen-jung Huang, and Eugenia Kumacheva* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6 Received November 2, 2000. In Final Form: March 27, 2001 Mixed self-assembled monolayers (MMLs) were prepared by exposing mica to (i) a solution of octyltrichlorosilane and 2,2,2-trifluoroethyl 11-(trichlorosilyl)-undecanoate in toluene and (ii) an aqueous solution of decamethonium bromide and dodecyltrimethylammonium bromide. X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle measurements were used to examine the composition, the thickness, the smoothness, and the stability of the self-assembled layers in different solvents. For both types of MMLs, we report the experimental conditions under which smooth mixed monolayers with a well-defined composition can be obtained. In silanization experiments, the ratio of silane species in the mixed layer was close to that in the solution, and the MMLs obtained from positively charged amphiphiles exhibited strong surface enrichment with dodecyltrimethylammonium bromide.

Introduction Over the past decade, studies of mixed monolayers (MMLs) have stimulated great interest in material science because of their potential applications in molecular recognition, electronics, chemical sensors, and nonlinear optics.1 Generally, in studies of MMLs the composition of a particular monolayer and the choice of a substrate were dictated by the potential application of the system. Modification of gold,2 silica,3 silicon,4 and other semiconductor surfaces5 with MMLs was investigated to a great extent. Studies of MMLs on the surface of mica remain scarce,6,7 although because of its smoothness, stability, and transparency mica is intensively used as a substrate in surface probe microscopy8 and in surface force balance experiments.9 Mica is a natural alumosilicate material with a hydrophilic negatively charged surface. Modification of the mica surface with a single component monolayer has been generally aimed at hydrophobization of the substrate and was carried out by Langmuir-Blodgett deposition or adsorption (self-assembly) of low-molecular-weight species from their solutions or vapors. The self-assembly-based modification of mica involved either covalent attachment of silanes to the surface of mica10 or physical adsorption * To whom correspondence should be addressed. E-mail: [email protected]. Tel/Fax: 416-978-3576. (1) Ulman, A. Thin Solid Films 1996, 273, 48. (2) (a) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (b) Nyquist, R. M.; Eberhardt, A. S.; Silks, L. A.; Li, Z.; Yang, X.; Swanson, B. I. Langmuir 2000, 16, 1793. (c) Beake, B. D.; Leggett, G. J. Phys. Chem. Chem. Phys. 1999, 14, 3345. (3) Lagutchev, A. S.; Song, K. J.; Huang, J. Y.; Yang, P. K.; Chuang, T. J. Chem. Phys. 1998, 226, 337. (4) (a) Banga, R.; Yarwood, J.; Morgan, A. M.; Evans, B.; Kells, J. Langmuir 1995, 11, 4393. (b) Hoffmann, H.; Mayer, U.; Krischanitz, A. Langmuir 1995, 11, 1304. (c) Takahara, A.; Kojio, K.; Ge, S. R.; Kajiyama, T. J. Vac. Sci. Technol., A 1996, 14, 1747. (d) Calistri-Yeh, M.; Kramer, E. J.; Sharma, R.; Zhao, W.; Rafailovich, M. H.; Sokolov, J.; Brock, J. D. Langmuir 1996, 12, 2747. (e) Yang, J. Y. M.; Frank, C. W. Org. Thin Films 1998, 695, 67. (5) Xu, F.; Zhu, J.; Mirkin, C. A. Langmuir 2000, 16, 2169. (6) Nakagawa, T.; Soga, M. Jpn. J. Appl. Phys. 1997, 36, 5226. (7) Fang, J.; Knobler, C. M. Langmuir 1996, 12, 1368. (8) Magonov, S.; Whangbo, M.-H. Surface Analysis with STM and AFM; VCH: Weinheim, 1996. (9) (a) Christenson, H. K.; Gruen, D. W. R.; Horn, R. G.; Israelachvili, J. N. J. Chem. Phys. 1987, 87, 1834. (b) Klein, J.; Kumacheva, E. J. Chem. Phys. 1998, 108, 6996.

of positively charged species, such as cationic surfactants11 or bipolar amphiphiles (bolaforms),12 onto the negatively charged surface of mica. Modification of mica through the reaction of the silanol groups of hydrolyzed organosilanes and surface hydroxyl groups of mica produces stable laterally polymerized layers. However, hydrolysis of silane molecules in the solution frequently leads to their partial polymerization and aggregation in the bulk,6,10b which brings a serious problem to surface modification, especially when the smoothness of the self-assembled layer is a priority. The silanization procedure is extremely sensitive to the variations in water content in the solution, to the temperature of the reaction, and to the chemical nature of the substrate. Generally, for a particular substrate, careful control of the amount of water and its location in the system, that is, in the bulk solution versus the surface, is helpful in producing reasonably smooth layers.10a,13a In contrast to silanization, surface modification driven by electrostatic interactions11,12 between negatively charged mica and species containing positively charged groups is a relatively simple process. The disadvantage of this approach is a relatively low stability of the layers caused by the mobility of molecules on the surface; however, this feature can be surmounted by lateral cross-linking of the surface-attached species.12 To the best of our knowledge, surface modification of mica with MMLs was reported by two groups, that is, by Nakagawa et al.6 and by Knobler et al.7 Both groups employed the trend of silanes to form fractal-like islands on the surface of mica. A mixed silane layer was formed in a two-step process: first, an incomplete layer of octadecyltrichlorosilane (OTS) was deposited onto the surface of mica by adsorption or the Langmuir-Blodgett (10) (a) Carson, G. A.; Granick, S. J. Mater. Res. 1990, 5, 1745. (b) Schwartz, D. K.; Israelachvili, J.; Steinberg, S.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354. (c) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Langmuir 1996, 12, 3626. (11) (a) Herder, P. C.; Claesson, P. M.; Herder, C. E. J. Colloid Interface Sci. 1987, 119, 155. (b) Chen, Y. L.; Chen, S.; Frank, C.; Israelachvili, J. J. Colloid. Interface Sci. 1992, 153, 244. (12) Mao, G.; Tsao, Y.; Tirrell, M.; Davis, T.; Hessel, V.; Ringsdorf, H. Langmuir 1995, 11, 942. (13) (a) McGovern, M. E.; Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607. (b) McGovern, M. E.; Thompson, M. Can. J. Chem. 1999, 77, 1678.

10.1021/la0015359 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/09/2001

Mixed Monolayers Self-Assembled on Mica Surface

Langmuir, Vol. 17, No. 14, 2001 4275

Table 1. Characteristics of Silane and Amphiphilic Molecules

Figure 1. Schematics of the formation of MMLs: (a) surfactant layers, x denotes positively charged trimethylammonium endgroups; (b) silane layers, b represents trichlorosilane endgroups, O designates siloxane groups coupled to the surface, and X denotes a functional ester group of F-Si (see Table 1).

method. The layer had pinholes with the size varying from 10 to 200 nm. In the second step, fluorinated silane molecules adsorbed to the surface which was not coated with OTS. The mixed layers had a patchy structure but were reasonably smooth. Here, our work was motivated by the results obtained in studies of self-assembly of synthetic rigid-rod polypeptides on the mica surface.14 It was found that polyglutamates strongly adsorb to the bare mica substrate in a lateral orientation, whereas on the hydrophobized mica surface polypeptide adsorption is very weak, and polymer layers can be easily washed out from the surface with a suitable solvent.14 It was assumed that by modifying the surface of mica with an essentially hydrophobic layer containing a small fraction of chemically reactive or charged groups, it would be possible to induce a homeotropic orientation of the polypeptide molecules in the adsorbed layer in a good solvent. More generally, we aimed at modification of mica with MMLs which would be suitable for immobilization of individual protein and DNA molecules. Such layers are of great current interest in the fields of genomic and proteomic microarray technologies. Ultimately, the surfaces bearing immobilized polymers had to be studied in the surface force balance experiments.9 Therefore, along with characterization of the composition of MMLs, our priority was to prepare smooth and stable layers. Modification of Mica Surface Using MMLs The modification of mica with MMLs was carried out (i) via chemical adsorption of mixed silanes from their solutions in toluene and (ii) through physical adsorption of mixed amphiphiles from their aqueous solutions. The chemical structures of the compounds used in the experiments are shown in Table 1, and the schematics of the expected organization of MMLs on the mica surface are shown in Figure 1. Both methods involved exposure of mica sheets to the solution containing a mixture of species to be deposited onto the surface. In the first approach (14) (a) Kitaev, V.; Schillen, K.; Kumacheva, E. J. Polym. Sci., Part B 1998, 36, 1567. (b) Sohn, D.; Kitaev, V.; Kumacheva, E. Langmuir 1999, 15, 1698.

(Figure 1a), we employed the procedure developed by McGovern et al.13 for the modification of silicon surfaces. In this method, prior to silanization mica sheets were pre-exposed for a certain period of time to air with a controlled relative humidity. Then, adsorption of silanes was carried out from the mixed silane solution in a dry solvent. We assumed that this approach would enable us to suppress the formation of silane oligomers in the bulk solution. We used a mixture of octyltrichlorosilane (C-8) and fluorinated silane 2,2,2-trifluoroethyl 11-(trichlorosilyl)-undecanoate15 (F-Si), as is shown in Table 1 and Figure 1a. The reason for choosing F-Si was twofold. First, a protected ester group of F-Si should not interfere with the surface self-assembly of silane molecules. Second, once the molecules of F-Si are immobilized onto the surface, the ester linkage can be easily cleaved to form ionic carboxylic groups. Then, through a nucleophilic reaction with the carboxylic moiety, other functional groups can be coupled to the surface. C-8 was selected as an alkyl component with the minimum molecular length sufficient to form stable monolayers.16 In addition, the length of F-Si is about 3 methylene units longer than that of C-8, which makes F-Si/C-8 layers sterically favorable for nucleophilic SN2 reactions of the ester/carboxylic acid group.17 The concentration of F-Si in the layer was varied by changing the molar ratio F-Si/C-8, φF-Si, in the solution.18 In the preparation of mixed amphiphilic layers, aqueous solutions of decamethonium bromide (DMB) and dodecyltrimethylammonium bromide (DTAB) were used (Figure 1b). DTAB contains a single quaternary ammonium endgroup and forms smooth hydrophobic monolayers on negatively charged surfaces.11a DMB is a commercially available bipolar amphiphile terminated with quaternary ammonium groups at both ends of the hydrocarbon chain. The molecular length of DMB closely matches the (15) (a) Cheng, S. S.; Scherson, D. A.; Sukenik, C. N. Langmuir 1995, 11, 1190. (b) Fryxell, G. E.; Rieke, P. C.; Wood, L. L.; Engelhard, M. H.; Williford, R. E.; Graff, G. L.; Campbell, A. A.; Wiacek, R. J.; Lee, L.; Halverson, A. Langmuir 1996, 12, 5064. (16) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (17) Ulman, A. Private communication. (18) For immobilization of rigid-rod polypeptide molecules, such as poly(γ-benzyl-L-glutamate), in a homeotropic orientation on the surface, the required fraction of active functional groups in the layer is ca. 8 mol %. Thus, even with a relatively low yield of activation of ester groups in the mixed silane layer, the concentration of F-Si in the layer of ca. 15 mol % should be sufficient for polypeptide attachment.

4276

Langmuir, Vol. 17, No. 14, 2001

length of the DTAB molecule (Table 1). We assumed that self-assembly from mixed aqueous solutions containing the bolaform at a low molar ratio DMB/DTAB, φDMB, would result in incorporation of a small number of charged quaternary ammonium groups into the outer part of the mixed monolayers. Experimental Section Materials. 2,2,2-Trifluoroethyl 11-(trichlorosilyl)-undecanoate (F-Si) was synthesized according to Cheng et al.15a F-Si and octyltrichlorosilane (C-8) (97%, Sigma-Aldrich) were vacuum distilled within 4 weeks prior to use and stored refrigerated with the desiccant P2O5. Octadecyltrichlorosilane (OTS, Aldrich, 97%), decamethonium bromide (Flukabrand Ion-Pair Reagent, SigmaAldrich), and dodecyltrimethylammonium bromide (DTAB) (99%, Sigma-Aldrich) were used as received. Toluene (Spectrograde, Caledon Laboratories) was distilled over sodium immediately prior to silanization. Deionized water was purified with a Millipore Milli Q Plus system, distilled over KMnO4, and then passed through the Millipore Simplicity system. N,N-Dimethylformamide (DMF) (HPLC grade, Sigma-Aldrich), 1,4-dioxane (ACS grade, ACP Chemicals Inc.), and chloroform (Spectrograde, Caledon Laboratories) were used as received. Muscovite mica was purchased from S&J Trading, U.S.A. Sample Preparation. Prior to the silanization procedure, mica was cleaved into approximately 1 × 1 cm pieces and exposed for 14 ( 0.5 h to air at 53% relative humidity controlled by a saturated solution of Mg(NO3)2‚6H2O.13b The test tubes used for the preparation of the silane MMLs were first silanized with OTS to prevent adsorption of F-Si and C-8 onto the tube walls. Following silanization with OTS, the test tubes were baked at 130 °C for 40 ( 5 min and then swiftly transferred into the glovebox filled with dried air. A large tray of P2O5 was placed inside the glovebox. Silane solutions in freshly distilled dry toluene were prepared inside the glovebox using a separate OTSsilanized vessel and then poured into the test tubes containing mica plates. The total silane concentration in the solutions varied from 1.4 to 3.0 mM. The time of mica exposure to the silane solutions varied from 15 to 110 min, after which the samples were removed from the test tubes and washed with freshly distilled dry toluene. The silanized samples were dried in a laminar flow cabinet for 2-3 h prior to examination of the surfaces. Mixed surfactant monolayers were obtained by exposure of the freshly cleaved mica sheets to aqueous solutions of DTAB and DMB for the period of time varying from 30 to 300 min. The total concentration of surfactants in the solution was 0.01 M, and φDMB varied from 0 to 1.0. Immediately after surfactant adsorption, mica pieces were gently washed with distilled water. The samples were dried for about 3 h in a laminar flow cabinet. Sample Characterization. Contact Angle Measurements. Contact angles were measured at ambient temperature and humidity using an NRL contact angle goniometer (Rame´-Hart, Inc.). A droplet of water (2.0 ( 0.5 µL) was placed onto the surface of mica covered with the MMLs. Contact angles were determined immediately after placing the water droplet onto the surface and after 180 ( 15 s. For each sample, at least 12 measurements were carried out. Experimental results were reproducible within (2°. Atomic Force Microscopy. AFM experiments were conducted at ambient temperature and humidity using a Nanoscope III instrument (Digital Instruments, Santa Barbara, CA) and Si3N4 tips. Most of the images reported in this work were obtained in a contact mode with the cantilever spring constant 0.12 or 0.38 N/m. Scan rates varied from 1 to 4 Hz. To ensure that the layer is not damaged by the tip, control experiments were performed in a tapping mode using tips with the resonance frequency 300 kHz. All images presented in this work were obtained reproducibly over at least three spots on the sample surfaces. X-ray Photoelectron Spectroscopy. XPS spectra were obtained using a Leybold MAX200 XPS spectrometer (Leybold, Cologne, Germany) equipped with a monochromatic Al KR source (Ek ) 1486.6 eV) run at 15 kV and 20 mA, a hemispherical analyzer, and a multichannel detector. The operating pressure was ca. (4 ( 1) × 10-9 Torr.

Kitaev et al. For each sample, a survey spectrum was acquired to determine the elements present on the surface. The binding energy scales for all spectra were referenced to the C(1s) peak at 285.0 eV. The peak areas were converted to atomic concentrations using the sensitivity factors empirically derived by Leybold:19 C(1s) 0.32, O(1s) 0.75, N(1s) 0.51, F(1s) 1.00, Al(2p) 0.23, Si (2p) 0.36, and K(2p) 1.39. In the angle-dependent XPS experiments, the takeoff angle, θ, was 90°, 60°, 45°, 30°, 20°, and 10° relative to the surface. The survey spectra and the data for quantitative analysis were obtained at a pass energy of 150 eV and the spot size of 1.5 × 0.7 mm. Freshly cleaved mica samples were used to obtain the reference average intensity of Al(2p) and K(2p) signals. To evaluate the thickness of the silane layers adsorbed to mica, the attenuation of K(2p) or Al(2p) signals from the bare substrate, that is, I(K,mica) or I(Al,mica), was compared with the signal obtained for the same substrate covered with the silane layer, that is, I(K,layer) and I(Al,layer), respectively.20 The thickness of the layer was determined by collecting data on I(K,mica)(θ) and I(K,layer)(θ) for the different takeoff angles. For example, for the K(2p) signal the adsorption ratio I(K,layer)(θ)/I(K,mica)(θ) was

I(K,layer)(θ) I(K,mica)(θ)

) e-(d/λ sin θ)

(1)

where λ is the electron mean free path of K(2p) electrons through the adsorbed layer, θ is the angle between the beam and the substrate, and d is the thickness of the layer.21,22 For a flat and uniform layer, a plot of ln[I(K,layer)(θ)/I(K,mica)(θ)] versus 1/sin(θ) gives a straight line with a slope a ) -d/λ. Here, the slope was used to determine the relative thickness of the layer in units of the electron mean free path λ.23

Results and Discussion Mixed Silane Layers. Contact Angle Measurements and AFM Studies. Prior to adsorption of mixed silane monolayers, the silanization procedure was examined for pure octyltrichlorosilane by varying the time of exposure of mica sheets to solutions of C-8 with various silane concentrations. It was found in the AFM experiments that for 2.9 mM C-8 solutions short-time adsorption of ca. 15 min gave reasonably smooth silane layers with the roughness of ca. 1-2 nm. Adsorption time exceeding 15 min resulted in the formation of surface aggregates with the height of about 10-15 nm. On the other hand, exposure of mica to more dilute solutions of C-8 led to either poor surface coverage (short exposure times) or aggregation on the surface (long-time adsorption). Washing of the silanized surfaces with chloroform in order to remove aggregates and the excess of weakly adsorbed C-8 did not improve the quality of the layers. On the basis of these results, studies of mixed layers of C-8 and F-Si were carried out using toluene solutions with the total concentration of silanes of 2.9 ( 0.1 mM. Mixed silane layers were prepared from the solutions with φF-Si ) 0.14 ( 0.01 and φF-Si ) 0.24 ( 0.01. Figure 2 shows the AFM images of the mixed silane layers prepared following different adsorption times for φF-Si ) 0.24 ( 0.01. In Figure 2a, the layer prepared after 15 min silanization is relatively smooth, with the roughness not exceeding 1.5-2 nm. The layer obtained after 30 min exposure to the silane solution features aggregates with the height of ca. 10 nm (Figure 2b). The largest roughness was observed (19) Berresheim, K.; Mattern-Klossen, M.; Wilmers, M. Fresenius’ J. Anal. Chem. 1991, 341, 121. (20) The signal of the C(1s) peak was not reliable because of the unavoidable carbon contaminations. (21) Andrade, J. D. Surface and Interfacial Aspects of Biomedical Polymers; Plenum Press: New York, 1985. (22) Solletti, J. M.; Botreau, M.; Sommer, F.; Brunat, W. L.; Kasas, S.; Tran Minh Duc; Celio, M. R. Langmuir 1996, 12, 5379. (23) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1670.

Mixed Monolayers Self-Assembled on Mica Surface

Langmuir, Vol. 17, No. 14, 2001 4277 Table 2. Results of Contact Angle Measurements on Silane MMLs total time of exposure silane exposure time to φF-Si concn in to water silane [mol %] soln [mM] vapors [h] soln [min]

Figure 2. AFM images of the mica surface exposed to solutions of C-8 and F-Si in dry toluene at total silane concentration of 2.9 ( 0.1 mM. φF-Si ) 0.24 ( 0.01. Exposure time: (a) 15 min, (b) 30 min, and (c) 120 min.

for the layer obtained after 120 min adsorption, shown in Figure 2c, in which the height of irregular aggregates reached ca. 70 nm. The surface roughness of all layers did not notably change after the surfaces were washed with chloroform. Comparison of the AFM images shown in Figure 2 with the results of contact angle measurements presented in Table 2 showed correlation between the higher values of contact angles and the surface roughness. Larger contact angles were measured for the rough and thick silane layers obtained after long-time adsorption. For example, the silane layer shown in Figure 2c had the highest contact angle of 96°, whereas a smooth layer shown in Figure 2a had a moderate contact angle of 74°. A similar trend was noticed earlier for the single-component C-8 layers. The

0.25 0.25 0.23 0.23 0.15 0.15 0.15 0.15

2.9 2.9 3.0 3.0 3.0 3.0 3.0 3.0

0 20 ( 1 14 ( 1 14 ( 1 14 ( 1 14 ( 1 14 ( 1 14 ( 1

120 120 15 30 15 30 45 15

0.15

3.0

14 ( 1

15

0.14

2.8

14 ( 1

15

exposure time to solvents

contact angle [deg]

none none none none none none none DMF, 45 h at room temp DMF, 67 h at 80 °C none

91 96 74 78 78 79 79 76 76 79

effect of surface roughness on spreading of liquid droplets on homogeneous and especially heterogeneous surfaces is a complicated issue which is beyond the scope of this paper. The relation between the surface roughness and contact angle on the same solid surface is given by the Wensel equation which takes into account the ratio between the actual and the geometric surface areas.24,26 However, the Wenzel relation frequently conflicts with experimental observations. In our work, further measurements are necessary, which would include the examination of advancing and receding contact angles on the mica surface modified with MMLs of the same thickness. Here, it should be stressed that the results of contact angle measurements should be treated with great caution when the preparation of smooth monolayers is the objective. In such experiments, a combination of contact angle measurements with AFM, XPS, and ellipsometry would be very helpful. For the mixed F-Si/C-8 layers obtained under the same conditions, the smoothness of the layer improved when the molar fraction of F-Si in the solution decreased. When φF-Si decreased from 0.24 ( 0.01 to 0.14 ( 0.01, the surface roughness reduced from 2-2.5 nm (Figure 2a) to ca. 1.2 nm (Figure 3a). Further characterization of the silane MMLs was performed using the latter system.18 First, the stability of the layer in DMF was examined, because this solvent can be used as a medium for the reaction of nucleophilic substitution for activation of carboxylic groups of F-Si. In the first series of experiments, the mica sheets modified with the F-Si/C-8 layer were exposed to DMF for 45 h at 25 °C. The smoothness of the silane layer did not notably change after such treatment. In the second series of experiments imitating conditions of a nucleophilic reaction, the modified mica samples were heated in DMF for 67 h at 80 °C. Following such exposure, the smoothness of the layers and the values of contact angles underwent an insignificant change, as is shown in Figure 3b and Table 2, respectively. A small decrease in contact angles from 78° to 76° could occur because of solvation of the silane layers with DMF and an incomplete removal of DMF upon drying of the samples in ambient conditions for 2 h (the boiling temperature of DMF is 153 °C). XPS Experiments. Two mixed F-Si/C-8 layers obtained from the mixed solutions at φF-Si ) 0.14 ( 0.01 and 0.24 ( 0.01 were examined in XPS experiments. The first sample referred to as F-Si-14 was obtained following 15 (24) (a) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (b) Neumann, A. W.; Good, R. J. J. Colloid Interface Sci. 1972, 38, 341. (25) The value of λ ) 3.75 ( 0.25 nm was found for pure hydrocarbons. We assumed that the same λ can be used for C-8 molecules composed of a relatively long hydrocarbon tail and a single functional silane group. (26) Ulman, A. In Characterization of Organic Films; ButterworthHeinemann: Boston, 1995; pp 21-31.

4278

Langmuir, Vol. 17, No. 14, 2001

Kitaev et al.

Figure 3. AFM images of the mixed silane layers obtained following 15 min exposure of mica samples to a mixture of C-8 and F-Si in toluene with the total silane concentration of 2.9 ( 0.1 mM and φF-Si ) 0.14 ( 0.01: (a) prior to incubation in DMF and (b) after incubation in DMF at 80 °C for 67 h.

min adsorption time and had a structure similar to that shown in Figure 3a. The second sample designated as F-Si-24 was obtained after a 2 h silanization procedure. This layer featured a very rough surface, as is shown in Figure 2c. A pure C-8 layer with the roughness of ca. 1-2 nm was used as a reference sample. Figure 4 shows the angular dependence of the ratios of intensities for the Al(2p) signal (Figure 4a) and for the K(2p) signal (Figure 4b) measured for the bare mica and the mica surface bearing silane layers. Because the slope, a, of the graph is proportional to the thickness of the layer, it can be concluded that the average thicknesses of the F-Si-14 layer and the C-8 layer are close and that both of them differ significantly from that of the F-Si-24 layer.

For Al(2p) photoelectrons with Ek ) 1412.5 eV, λ ) 3.75 ( 0.25 nm was used,23,25 and the thickness of the C-8 layer dC-8 ) 1.1 ( 0.1 nm was found using eq 1. This value of dC-8 was in good agreement with the theoretical length of the C-8 molecule of ca. 1.25 nm given in Table 1. To find the thickness of mixed F-Si/C-8 layers, the same value of λ ) 3.75 ( 0.25 nm was used taking into consideration a relatively low fraction of heavier fluorine and oxygen atoms in the layer. The estimated thickness of the F-Si-14 layer was 1.7 ( 0.1 nm; that is, it was intermediate between the theoretical length of C-8 and F-Si molecules (1.25 and 2.1 nm, respectively, see Table 1). It should be noted, however, that the silane molecules can be tilted with respect to the surface, and thus the

Mixed Monolayers Self-Assembled on Mica Surface

Langmuir, Vol. 17, No. 14, 2001 4279

Table 3. Composition of Mixed Silane Layers Obtained in XPS Measurements experimental C/F ratio at different takeoff anglesa

φF-Si [mol %]

theoretical C/F ratio

90°

60°

45°

30°

20°

average C/F ratio

φF-Si estimated for MML

0.14 ( 0.01

19.8

11.6

19.6

20.3

22.3

30.8

20.9 ( 3.4

0.16 ( 0.03

a

C/F ratio was normalized by sin(θ), where θ is the takeoff angle.

average thickness of the MML can be smaller than that expected from the theoretical value.26 The mixed F-Si-24 layer had an estimated thickness of 3.0 ( 0.2 nm. The evaluation of the layer thickness using the K(2p) signal was more complicated, because the first layer of potassium ions is partially removed from the surface of mica during mica cleavage and silane self-assembly. Thus, either the XPS data had to be adjusted11a or the exact value of λ had to be determined using supplementary methods. Because in the current work we were unable to account for the loss of potassium atoms, only the relationship between the thicknesses of the silane layers expressed in the units of electron mean free path was found. The relation between the estimated thicknesses of the silane layers was the same as that obtained using the Al(2p) signal: the F-Si-24 layer had the largest thickness, and the thickness of the F-Si-14 layer was only slightly larger than that of the C-8 layer. The composition of the mixed silane layers was estimated using the following considerations. In the smooth MMLs, the fluorine atoms are expected to be localized in the upper part (about 1 Å thick) of the layer. Thus, the F(1s) signal shows no angle dependence, whereas the C(1s) signal increases in proportion to 1/sin(θ) because of an increase in the effective probing length. Therefore, the intensities of the C(1s) and F(1s) peaks were normalized by sin(θ). In Table 3, the values of the C/F ratio measured at different takeoff angles for the F-Si-14 layer correspond to the average molar fraction of F-Si of 0.16 ( 0.03. Appreciable scatter in the C/F ratios for different takeoff angles was presumably caused by the low intensity of the fluorine signal. On the basis of these results, it can be concluded that the molar ratio of C-8 and F-Si in the mixed silane layer is almost the same as in the mixed bulk solution used for mica modification and that no preferential adsorption of either component occurs during self-assembly. The composition of the F-Si-24 layer was not determined, because based on the AFM results its thickness exceeded the probing length of XPS measurements of about 3λ ≈ 10 nm. In addition, for the rough disordered layers of F-Si25 the normalization of the C/F ratio based on the assumption of localization of fluorine atoms at the very top of the MML may not be applicable. Mixed Surfactant Layers. Contact Angle Measurements and AFM Studies. First, the experimental conditions for the preparation of smooth DTAB layers were examined, because this surfactant was supposed to be the major component of the MMLs. The concentration of DTAB, cDTAB, in solutions varied from 0.3 to 10 mM, whereas the time of adsorption changed from 30 to ca. 20 h. To avoid adsorption of micelles on mica, cDTAB was maintained below its critical concentration of micellization (cmc), that is, below 0.0154 M at 20 °C.27 By use of cDTAB ) 0.3 mM, hydrophobic surfaces with a relatively high contact angle of 84° were obtained only after long-time adsorption of ca. 20 h. For higher DTAB concentrations of 1 and 10 mM, significantly shorter adsorption times were sufficient to obtain the layers with

contact angles of up to 80°. For example, the contact angle for the DTAB layer obtained after 300 min adsorption from the 10 mM solution was 75 ( 3°. This value is close to 73° reported by Eriksson et al.28 for the saturated monolayers of a similar cationic surfactant, CTAB. However, the DTAB layers were not stable in organic solvents, such as DMF and dioxane: the contact angles measured after 30 min exposure of the surfactant layers to these solvents decreased from 75 ( 3° to 7° and 55°, respectively. In contrast, in nonpolar solvents, such as toluene and cyclohexane, the DTAB layers had reasonably good stability, retaining the contact angle value of about 70° after 30 min of exposure. Upon examination with AFM, the layers of DTAB on mica featured a very smooth surface, which within the resolution of the instrument was almost indistinguishable from the bare mica surface. Self-assembly of mixed DMB/DTAB layers was carried out from the solutions with the total concentration of amphiphiles of 10 mM. The time of self-assembly was 300 min. The variation in the values of contact angles for the mixed surfactant layers is shown in Figure 5. The contact

(27) Inglese, A.; De Lisi, R.; Milioto, S. J. Phys. Chem. 1996, 100, 2260.

(28) Eriksson, L. G. T.; Claesson, P. M.; Eriksson, J. C.; Yaminsky, V. V. J. Colloid Interface Sci. 1996, 181, 476.

Figure 4. Variation of (a) ln(I(Al,layer)/I(Al,mica)) vs 1/sin(θ) and (b) ln(I(K,layer)/I(K,mica)) vs 1/sin(θ) obtained for the pure C-8 layer (]), mixed silane layer F-Si-15 (0), and F-Si-25 (4). The slope of the graphs, a, gives the thickness of silane layers expressed in units of the mean free path, λ: (a) aC-8 ) -0.298, aF-Si-14 ) -0.469, and aF-Si-25 ) -0.808; (b) aC-8 ) -0.439, aF-Si-14 ) -0.559, and aF-Si-25 ) -2.11.

4280

Langmuir, Vol. 17, No. 14, 2001

Kitaev et al.

Figure 5. Contact angles measured for the mica samples after 300 min exposure to mixed solutions of DTAB and DMB of different compositions at total surfactant concentration of 10 mM without washing (]) and following washing (O).

angles measured for the pure DMB layers formed on mica had a relatively low value of ca. 40 ( 5°, which can be anticipated from the polar nature of DMB molecules. In addition, a possibility of adsorption of DMB in a flat configuration by anchoring to the surface with two quaternary ammonium groups cannot be ruled out. When DTAB was introduced in the solution, the value of contact angle for the mixed layer substantially increased. For the layers prepared at φDMB ) 0.15 and φDMB ) 0.25, the values of contact angles were 67° and 73°, respectively. Figure 5 shows the variation of contact angles on the mica surface covered with freshly prepared MMLs and the MMLs washed with water. The values of contact angles on the washed samples are noticeably higher. This observation indicates that washing removes the excess of surfactant initially trapped in the layer via dipole-dipole interactions with the adsorbed molecules.11 The MMLs prepared from surfactants were stable in nonpolar solvents, such as cyclohexane and in toluene. For φDMB ) 0.15 and φ ) 0.25, that is, in the most important range of DMB concentrations,18 the MMLs were very smooth and did not differ from the pure DTAB layers. XPS Studies of Mixed DMB/DTAB Layers. XPS experiments were conducted on the MMLs produced by exposing mica sheets to mixed solutions of DMB and DTAB with the total surfactant concentration of 10 mM. Because the Al(2p) signal of the mixed surfactant layers was very scattered, only the K(2p) signal was used in the analysis of the XPS data. In Figure 6, the ratios of intensities of K(2p) signals coming from the modified mica and from the bare mica substrate are plotted as a function of the takeoff angle. The results obtained for the pure DTAB and DMB layers (Figure 6a) and for the mixed DTAB/ DMB layers for φDMB values of 0.5, 0.25, and 0.15 (Figure 6b) show reasonably low scatter, which allowed us to compare the slopes of the graphs. On the basis of the variation of a, it can be concluded that the thicknesses of the DTAB layer and of the mixed surfactant layers are very similar. By use of λ ) 2.11 nm and λ ) 2.19 nm for DTAB and DMB, respectively,29,30 the thickness of the DTAB layer was estimated to be 1.65 nm; the average thicknesses of the mixed layers formed from (29) Tanuma, S.; Powell, S. J.; Penn, D. R. Surf. Sci. 1987, 192, L849. (30) The values of λ for K(2p) were estimated using our data obtained for K(2p) and Al(2p) for C-8 and mixed silane layers. These values of λ were lower than λ ) 3.4-3.6 nm commonly accepted23 because of partial loss of potassium atoms during formation of MMLs.11a However, the value of λ ) 2.1-2.2 nm agrees well with the theoretical estimate of λ ) 2.15 nm for K(2p) (ref 29).

Figure 6. Variation of ln(I(K,layer)/I(K,mica)) vs 1/sin(θ) obtained for the layers formed by amphiphilic molecules. (a): DTAB (]), DMB (0); (b): mixture of DTAB and DMB at φDMB ) 0.15 ( 0.01 (]), φDMB ) 0.25 ( 0.01 (0), and φDMB ) 0.50 ( 0.01 (4). The total surfactant concentration in the solution is 10 mM. The slope of the graphs, a, gives the layer thickness expressed in units of the mean free path, λ: (a) aDTAB ) -0.781 and aDMB ) -0.658; (b) aφ(DMB))0.15 ) -0.740, aφ(DMB))0.25 ) -0.712, and aφ(DMB))0.5 ) -0.805. Table 4. Thickness of Mixed Amphiphilic Layers Obtained in XPS Measurements φDMB [mol %]

estimated layer thickness [nm]

15 25 50

1.53 1.58 1.74

solutions at φDMB values of 0.15, 0.25, and 0.5 were 1.53, 1.58, and 1.74 nm, respectively, as shown in Table 4. These thicknesses of the mixed amphiphilic layers compare reasonably well with the estimated lengths of the DTAB and DMB molecules of 17.7 and 17.9 Å, respectively (Table 1). The compositions of the mixed surfactant layers were estimated from the ratios of normalized intensities of C(1s) and N(1s) peaks. The experimental C/N ratio calculated for four takeoff angles was normalized by 1/sin(θ) in a manner similar to that for the C/F ratio for the silane layers. The variation in the C/N ratio was plotted against the composition of the surfactant solutions, as shown in Figure 7. Despite a significant scatter of experimental data resulting from the low intensity of the N(1s) signal, an interesting trend can be seen in Figure 7. When φDMB < 0.25, the value of the C/N ratio does not significantly change and is close to the theoretical C/N ratio for pure DTAB. This observation suggests that (a) incorporation of the DMB molecules into the layer is suppressed and (b) for φDMB < 0.25, the MMLs are strongly enriched with DTAB. This conclusion is supported by the results of contact angle measurements, in which the value of the

Mixed Monolayers Self-Assembled on Mica Surface

Langmuir, Vol. 17, No. 14, 2001 4281

significant amount of DMB into the MML its molar fraction in the solution has to be increased, as is shown in Figure 7. Conclusions

Figure 7. Variation in C/N ratio measured for the mica samples vs the composition of the surfactant solution. The time of adsorption is 60 min; the total surfactant concentration in the solution is 10 mM. The measurements were carried out at takeoff angles of 90° (]), 60° (0), 45° (4), and 20° (O). The estimated error in the C/N ratio is ca. (5%. A theoretical C/N ratio is shown by a solid line.

contact angle for φDMB < 0.25 is very close to that measured for the DTAB layer. The suppression of DMB adsorption in the MML can be caused by the energetically unfavorable incorporation of polar DMB molecules in a hydrophobic layer formed by DTAB. Therefore, to incorporate a

A procedure was established for modification of mica with smooth silane MMLs containing octyltrichlorosilane (C-8) and fluorinated silane 2,2,2-trifluoroethyl 11(trichlorosilyl)-undecanoate. The molar fraction of fluorinated silane in the optimized silane MMLs was 0.16 ( 0.03, which was close to the molar ratio of the species in the bulk toluene solution. Silane MMLs on mica demonstrated good stability in organic solvents and can be of great utility for further studies. In the second approach, smooth MMLs on mica were obtained from solutions containing positively charged amphiphiles decamethonium bromide and dodecyltrimethylammonium bromide. These MMLs were enriched with DTAB and exhibited a poor stability in polar organic solvents. Acknowledgment. This work was supported by the National Science and Engineering Research Council of Canada. The authors are grateful to Professor A. Ulman and Professor M. Thompson for fruitful discussions. LA0015359