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Surface Induced Hydrogen-Bonded Macrocluster Formation of Methanol on Silica Surfaces Masashi Mizukami, Yasuhiro Nakagawa, and Kazue Kurihara* Institute of Multidisciplinary Research for Advanced Material, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Received May 7, 2005. In Final Form: August 9, 2005 Recently, we have succeeded in identifying the structure of the adsorption layer of ethanol on a silica surface in cyclohexane to be a hydrogen-bonded linear aggregate (polymer), which we call a surface molecular macrocluster. In this work, we studied the effect of the miscibility of liquids on the formation of the surface molecular macroclusters for confirming that this is a surface induced phenomenon. We investigated the interaction and the structure of methanol adsorbed on a silica surface in methanol-cyclohexane binary liquids by a combination of colloidal probe atomic force microscopy, adsorption excess isotherm measurement, and FTIR spectroscopy using the attenuated total reflection (ATR) mode, and compared the results with those of the ethanol-cyclohexane and 1-propanol-cyclohexane binary liquids. The former system is immiscible at methanol concentrations of ca. 8-90 mol %, and the latter two are miscible at any composition. At 0.03 mol % methanol, which is far from the critical concentration for the phase separation, the contact of the methanol macrocluster layers formed on the silica surface brought about the attraction from a distance of 42 ( 5 nm which was similar to that observed in ethanol-cyclohexane. At a methanol concentration of 9.0 mol %, above bulk phase separation, completely different force profiles were observed. These results demonstrated that the molecular macrocluster formation was different from the wetting induced by the bulk.
Introduction Adsorption at the solid-liquid interface is a key phenomenon in colloid and surface science and plays important roles in surface modification, catalysis, stabilization of a colloidal dispersion, wetting, and so forth.1 However, the study of liquid adsorption from binary liquids is still limited despite its basic and technological importance and has been mainly based on the adsorption isotherm measurement and its model analysis.2,3 The molecular level of understanding has been missing. Recently, we have succeeded in identifying the structure of ethanol adsorbed on a silica surface in cyclohexane by employing the combination of surface forces measurement, adsorption excess isotherm measurement, and infrared spectroscopy in the attenuated total reflection mode (ATRFTIR).4 It has been found that ethanol forms a linearly associated polymer extending ca. 15 nm from the surface through hydrogen-bonding between the surface silanol group and the hydroxy group of ethanol as well as between the hydroxy groups of the ethanol molecules. We use the term “surface molecular macrocluster” for this novel organized liquid structure. The adsorption layer consists of practically pure ethanol.4 A similar macrocluster formation was also found for 1-propanol on silica surfaces in cyclohexane.4 Such a thick liquid layer formation may * To whom correspondence should be addressed. Tel: 81-(0)22-217-5673. Fax: 81-(0)22-217-5674. E-mail: kurihara@ tagen.tohoku.ac.jp. (1) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; Wiley and Sons: New York, 1997. (2) Parfitt, G. D.; Rochester, C. H. Adsorption from Solution at the Solid/Liquid Interface; Academic Press: New York, 1983. (3) (a) Machula, G.; De´ka´ny, I.; Nagy, L. G. Colloids Surf. A: Physicochem. Eng. Asp. 1993, 71, 241. (b) Kira´ly, Z.; De´ka´ny, I.; Klumpp, E.; Lewandowski, H.; Narres, H. D.; Schwuger, M. J. Langmuir 1996, 12, 423. (c) De´ka´ny, I.; Nagy, L.; Tu´ri, L.; Kira´ly, Z.; Kotov, N. A.; Fendler, J. H. Langmuir 1996, 12, 3709. (d) De´ka´ny, I. Pure Appl. Chem. 1992, 64, 1499. (e) De´ka´ny, I. Pure Appl. Chem. 1993, 65, 901. (4) (a) Mizukami, M.; Moteki, M.; Kurihara, K. J. Am. Chem. Soc. 2002, 124, 12889. (b) Mizukami, M.; Kurihara, K. Chem. Lett. 1999, 28, 1005. (c) Mizukami, M.; Kurihara, K. Chem. Lett. 2000, 29, 256. (d) Mizukami, M.; Kurihara, K. Aust. J. Chem. 2003, 56, 1071.
be reminiscent of the capillary condensation5 and prewetting transition6 which occur in binary liquids close to the phase separation. However, we think that this surface molecular macrocluster formation should be a phenomenon induced by surface and molecular interactions and not directly related to the miscibility because ethanolcyclohexane and 1-propanol-cyclohexane are miscible binary liquids at any composition. The aim of this study is to investigate the effect of the miscibility of liquids on the formation of surface molecular macroclusters. We have investigated the macrocluster formation of methanol on silica surfaces in methanolcyclohexane binary liquids which were immiscible at methanol concentrations of ca. 8-90 mol % and compared the results with those of ethanol-cyclohexane and 1-propanol-cyclohexane binary liquids and the interaction measured at a methanol concentration of 9.0 mol % which was in the bulk phase separation state. Experimental Section Materials. Reagent grade cyclohexane (Nacalai Tesque) and methanol were dried with sodium and magnesium, respectively, and distilled immediately prior to use in order to avoid the influence of water. Reagent grade hydrofluoric acid from Stella Chemifa, sulfuric acid from Nacalai Tesque and hydrogen peroxide from Santoku Chemical were used as received. Surface Forces Measurement. The interaction forces (F) between a glass sphere and a glass plate were measured as a function of the surface distance (D) in methanol-cyclohexane mixtures using atomic force microscope (AFM, Seiko Instruments, SPI3700-SPA300).7 Colloidal glass spheres (Polyscience) and glass plates (Matsunami, micro cover glass) were washed in a mixture of sulfuric acid and hydrogen peroxide (4:1, v/v) and thoroughly rinsed with pure water. A colloidal glass sphere (4-5 (5) Christenson, H. K. Chem. Phys. Lett. 1985, 118, 455-458. (6) At this transition, one component of a binary liquid is thought to fully wet the entire solid surface in contact with the liquid. (a) Beysens, D.; Este´ve, D. Phys. Rev. Lett. 1985, 54, 2123-2126. (b) Gurfein, V.; Beysens, D.; Perrot, F. Phys. Rev. A 1989, 40, 2543-2546. (7) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831.
10.1021/la0512190 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/09/2005
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µm in radius) was then attached to the end of a cantilever (Olympus, RC-800PS) with epoxy resin (Shell, Epikote1004). The spheres and the plates were treated with water vapor plasma (Samco, BP-1, 20 W, 13.56 MHz rf source in 0.6 Torr of argon and water, 50 mL/min flow rate) for 3 min just prior to each experiment in order to ensure the existence of silanol groups on the glass surfaces.8 The obtained forces were normalized by the radius (R) of the sphere using the Derjaguin approximation, F/R ) 2πGf.9 Here, Gf is the interaction free energy per unit area between two flat surfaces. Details of the forces measurement were previously described.4 Adsorption Excess Isotherm Measurement. The adsorption excess isotherm was measured using adsorbent glass spheres (typically 1 g), which were washed and treated with water vapor plasma in the same manner as for the forces measurement. The glass spheres (typically 1 g) dispersed in methanol-cyclohexane mixtures (10 mL) precipitated after they were equilibrated for about 24 h at 20 ( 0.5 °C. The adsorption excess amounts of methanol were obtained from the composition change of the supernatant after adsorption equilibrium (24 h at 20 ( 0.5 °C) which was determined using a differential refractometer (Otsuka Electronics, DRM-1021). The adsorption layer thickness (t) was estimated from the adsorption excess amount by assuming that only methanol is present in the adsorption layer and that the density of methanol in the adsorption layer is equal to that of liquid methanol (0.791 g/mL).10,11 The specific surface area of the adsorbent glass spheres (0.60 ( 0.04 m2/g) was calculated based on the size distribution of the glass spheres (ca. 1000 spheres) by assuming that the spheres were hard and nonporous. Infrared Spectroscopy in the Attenuated Total Reflection Mode. Infrared spectra were recorded on a Perkin-Elmer FTIR system 2000 using a TGS detector. All of the infrared spectra presented in this study were measured by using the background spectrum of pure cyclohexane. The ATR prism made of silicon crystal (Nihon PASTEC, 60 × 16 × 4 mm trapezoid, six times reflection with an incident angle of 45°) was used as a solid adsorbent surface. The silicon oxide layer, which exhibits similar properties to those of glass, formed on the silicon prism surface12 was used as an adsorbent surface. The silicon crystal surface was cleaned by immersing it in a mixture of sulfuric acid and hydrogen peroxide (4:1, v/v) and thoroughly rinsing with pure water. The crystal was then treated with water vapor plasma for 20 min immediately prior to each experiment. As a reference, the hydrogen-terminated silicon surface was also prepared by immersing the silicon prism in 0.5% hydrofluoric acid aqueous solution for 30 min. After this treatment, the contact angle of water on the hydrogen-terminated surface became 80 ( 1°, whereas that on the silicon oxide surface was 0°. In the ATR mode, the orientation of the adsorbed methanol on the silicon oxide surface was investigated by dichroic analysis of the OH stretching absorption obtained by using p- and s-polarized infrared light.13-16
Results and Discussions Figure 1a shows the surface forces profiles measured between the silica surfaces in the methanol-cyclohexane binary liquids. In pure cyclohexane, attractions appeared from a distance of 4 ( 2 nm which agreed well with the conventional van der Waals attraction calculated with the nonretarded Hamaker constants.9 Addition of methanol drastically changed the interaction force. For 0.03 (8) Okusa, H.; Kurihara, K.; Kunitake, T. Langmuir 1994, 10, 3577. (9) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991. (10) De´ka´ny, I.; Sza´nto´, F.; Nagy, L. G. Colloid Polym. Sci. 1978, 256, 150. (11) De´ka´ny, I.; Tu´ri, L.; Tomba´cz, E.; Fendler, J. H. Langmuir 1995, 11, 2285. (12) Sze, S. M. Semiconductor Devices: Physics and Technology; Wiley: New York, 1985; p 350. (13) Harrick, N. J. J. Opt. Soc. Am. 1965, 55, 851. (14) Harrick, N. J.; du Pre´, F. K. Appl. Opt. 1966, 5, 1739. (15) Fraser, R. D. B. J. Chem. Phys. 1953, 21, 1511. (16) Neivandt, D. J.; Gee, M. L.; Hair, M. L.; Tripp, C. P. J. Phys. Chem. B 1998, 102, 5107.
Figure 1. (a) Profiles of interaction forces between glass surfaces upon compression in binary liquids of methanolcyclohexane. Dashed and solid lines represent the van der Waals force calculated using the nonretarded Hamaker constants of 3 × 10-21 J for glass/cyclohexane/glass and 9 × 10-21 J for glass/ methanol/glass, respectively.9 (b) The schematic illustration of the contacting surfaces for the long-range attraction and the short-ranged repulsion.
Figure 2. Half the range of attraction and the adsorption layer thickness of methanol.
mol % methanol, the long-range attraction, which extended to 42 ( 5 nm, appeared and turned into repulsion at distances shorter than 2.3 ( 1.2 nm. The pull-off force also increased to 179 ( 10 mN/m. The shape of this long range interaction force profile was similar to those observed in ethanol-cyclohexane as well as in 1-propanol-cyclohexane, indicating that the long range attraction was similarly caused by the contact of opposed adsorption layers, and the short-range repulsion was ascribed to a steric force caused by the overlap of structured layers (Figure 1b).4 To again confirm the origin of the long-range attraction, the adsorption layer thickness was estimated from the adsorption excess amount by assuming a pure methanol layer formation (Figure 2). The methanol adsorption layer thickness of ca. 15 nm was close to half of the attraction range, 20 ( 2 nm, at methanol concentrations of 0.030.12 mol %. This supported that the adsorption layers on glass surfaces consisted of almost pure methanol and the long-range attraction was caused by the contact of these methanol adsorption layers. With increasing the methanol concentration, the attraction started to decrease at 0.16 mol % methanol, the
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Table 1. Long Range Attraction and Adsorption Data for Various Alcohol-Cyclohexane Binary Liquids concentration at which the attraction appeared concentration at which the attraction disappeared maximum attraction range (concentration) maximum pull-off force (concentration) apparent adsorption layer thickness (concentration) concentration at which the cluster starts to form in cyclohexane
attraction range was 31 ( 3 nm and became almost identical to that observed in pure cyclohexane at 0.8 mol % methanol, whereas the methanol adsorption layer thickness remained nearly the same. Similar concentration dependencies of the attraction range and the adsorption layer thickness were observed in other two binary liquids.4 A plausible explanation for the decrease in the long-ranged attraction sensitive to the alcohol concentration is the exchange of alcohol molecules between clusters on the surface and those in the bulk solution,4 which has been confirmed by NMR spectroscopy in our laboratory.17 One may note that the concentrations where the long-range attraction starts to decrease are close to the concentrations where hydrogenbonded alcohol clusters appear in the bulk. To investigate the interactions involved in the adsorption of methanol molecules, ATR-FTIR spectra were measured on the silica surface in methanol-cyclohexane binary liquids. Cyclohexane was used to measure the background spectrum. The general spectral characteristics of hydrogen-bonded alcohols in a nonpolar solvent for the fundamental OH stretching region have been well established (Figure 3a).18 The ATR spectrum obtained at a 0.1 mol % methanol concentration was similar to those for ethanol-cyclohexane (Figure 3b). It exhibited a broad absorption band at 3000-3600 cm-1, which was assigned to the doubly hydrogen-bonded OH group (polymer OH). The negative band at 3680 cm-1 (free silanol) indicated that the hydrogen bonding between the silanol group and methanol was essential for the adsorption of methanol. This mechanism was also confirmed using the hydrogen terminated silicon surfaces exhibiting no adsorption. These observations were identical to those for ethanol and 1-propanol. The completely free silanol groups in a vacuum shows the peak at 3750-3740 cm-1 and this peak shift to ca. 3680 cm-1 in nonpolar liquids such as cyclohexane due to the weak interaction with cyclohexane. Here, the term “free silanol” means the non-hydrogen bonded silanol group. A mean orientation angle of the OH groups in the methanol macrocluster was estimated to be 46 ( 5° from the dichroic ratio at a 0.1 mol % methanol concentration. This was also similar to the value of 40 ( 4° obtained in ethanol-cyclohexane at 0.1 mol % ethanol.4 A plausible structure of the methanol macrocluster formed on the silica surface is shown in Figure 3c. Here, the properties of surface macroclusters of methanol, ethanol, and 1-propanol are compared to see whether the miscibility affect the phenomenon. In Table 1, the data for the long-range attraction, the pull-off forces, and the apparent adsorption layer thicknesses obtained for various alcohol-cyclohexane binary liquids are sum(17) Endo, S.; Nakagawa, Y.; Mizukami, M.; Kurihara, K. Trans. MRS-J. in press. (18) (a) Bellamy, L. J. Advances in Infrared Group Frequencies; Methuen & Co. Ltd.: London, 1968. (b) Graener, H.; Ye, T. Q.; Laubereau, A. J. Chem. Phys. 1989, 90, 3413.
methanol
ethanol
1-propanol
0.03 mol % 0.8 mol % 42 ( 2 nm (0.03 mol %) 212 ( 2 mN/m (0.05-0.16 mol %) 14 ( 2 nm (0.1-0.8 mol %) ca. 0.5 mol %
0.1 mol % 1.4 mol % 35 ( 3 nm (0.1 mol %) 134 ( 4 mN/m (0.1-0.4 mol %) 13-16 nm (0.1-2.2 mol %) ca. 0.5 mol %
0.1 mol % 5.0 mol % 69 ( 9 nm (0.1 mol %) 110 ( 7 mN/m (0.1-0.3 mol %) 15 ( 3 nm (0.5-2.8 mol %) ca. 0.5 mol %
Figure 3. (a) Structures of monomer alcohol and of hydrogenbonded alcohol in a linear cluster (polymer), and the notation of various OH-stretching modes, νR, νβ, νγ, and νδ, which correspond to the absorption peaks at different wavenumbers. (b) ATR-FTIR spectra measured on the silicon oxide surface in methanol-cyclohexane binary liquids at methanol concentrations of 0.0, 0.1, 0.3, 0.5, 1.0, and 2.0 mol %. An ATR-FTIR spectrum measured on the hydrogen-terminated silicon surface at 0.3 mol % methanol is shown by a dotted line. (c) Schematic illustration of methanol macrocluster formed on the silica surface.
marized. If the adsorption of alcohol was caused by a prewetting transition or capillary condensation, (1) the adsorption should occur at a concentration close to the phase separation and (2) the thickness should increase with increasing the concentration toward the phase separation.5,6 However, the thick adsorption layer appeared at a much lower concentration for methanol (0.03 mol %) than its phase separation concentration (ca. 8 mol %). The maximum attraction range and the apparent adsorption layer thickness showed no significant difference among the three alcohols and showed no increase with increasing methanol concentration further. The long range attraction appeared at lower concentrations for methanol (0.03 mol %) than ethanol and 1-propanol (0.1 mol %); however, it also disappeared at a lower concentration in the case of methanol compared to the other two. Therefore, the observed thick adsorption layer formation is not directly related to the miscibility but should be a phe-
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nomenon induced by the surface which interacts with alcohol molecules. To confirm this directly, the interaction forces between glass surfaces were measured at 9.0 mol % methanol which was higher than the concentration of bulk phase separation (ca. 8 mol %). At this condition, the methanol and cyclohexane cause the phase separation, and the pure methanol layer should be formed on the surface. The surface forces measured at 9.0 mol % methanol were completely different from those measured at methanol concentrations of 0.03-0.16 mol %. The attraction upon approaching appeared from a distance of ca. 900 nm, which was much longer than the range of attraction caused by the contact of methanol macroclusters (42 ( 2 nm at 0.03 mol % methanol). On the other hand, the adhesive force measured at 9.0 mol % methanol was ca. 7.4 mN/m which was much weaker than those caused upon separation of methanol macrocluster layers (212 ( 2 mN/m at 0.050.16 mol % methanol). These significant differences clearly demonstrated that the molecular macrocluster formation was different from wetting induced by bulk phase separation. Conclusion In conclusion, we have demonstrated that the methanol formed molecular macrocluster on silica surfaces in cyclohexane which was similar to the ethanol and 1-pro-
panol molecular macroclusters. The attraction caused by the contact of the methanol macrocluster layers and the adsorption layer thickness showed no significant difference compared to those measured for ethanol and 1-propanol. The forces measured at 9.0 mol % methanol (above bulk phase separation) were completely different from those observed at concentrations of 0.03-0.16 mol % methanol. We concluded that the surface molecular macrocluster is not related to the bulk phase separation and the miscibility of binary liquids and is the phenomenon induced by the surface. This study has also demonstrated that the perfect wetting of solid surfaces with one liquid component in binary liquids is possible by chemical interaction (hydrogen bonding) independent of their miscibility. This should extend the scope of the wetting study, which has considered that the phase separation is necessary to achieve perfect wetting of a surface in binary liquids. Acknowledgment. This work was supported by the CREST program of Japan Science and Technology Agency (JST) and by Grant-in-Aids for Scientific Research and 21st Century COE Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. LA0512190
