Langmuir 1996, 12, 6399-6403
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Infrared Reflection Absorption Spectroscopic Study on Self-Assembling Processes of (Methacryloyloxy)alkyl Dihydrogen Phosphates on Evaporated Silver Films Hirotaro Ando, Minoru Nakahara, Masato Yamamoto, and Koichi Itoh* Department of Chemistry, School of Science and Engineering, Waseda University, Shinjuku-ku, Tokyo 169, Japan
Masako Suzuki Department of Molecular Science, College of Arts and Sciences, Showa University, Shinagawa-ku, Tokyo 145, Japan Received April 23, 1996X Infrared reflection absorption (IRA) spectroscopy was applied to study the adsorption process and structural change of (methacryloyloxy)alkyl dihydrogen phosphates, M10P and M2P (see Figure 1), during their self-assembling process at a silver substrate from a methyl methacrylate solution (5 × 10-5 mol/L). The saturation coverage of M10P was completed after about 50 min, while that of M2P was completed after about 25 min. The self-assembled monolayers of both M10P and M2P gave the νs(PO32-) band around 980 cm-1, indicating that the dissociation of the dihydrogen phosphate groups takes place on adsorption to the silver substrate. At an initial stage of the self-assembling process of M10P the IRA spectra gave only the νs(PO32-) band. After several minutes of incubation there arose the νas(PO32-) band near 1080 cm-1 and the intensity ratio of νas(PO32-)/νs(PO32-) increased with time, indicating that the axis of the PO32- group of the adsorbate changes from a perpendicular orientation to a tilted one. On the other hand, the IRA spectra of the self-assembled monolayer of M2P gave both the νs(PO32-) and νas(PO32-) bands, with the onset of adsorption and the intensity ratio of νas(PO32-)/νs(PO32-) virtually constant. Thus, in contrast to the case of M10P, the PO32- groups of M2P take an almost random orientation during the self-assembling process. The explicit differences in the kinetics of adsorption and structural change between M10P and M2P were explained in terms of a larger mobility of M2P at the substrate surface and/or an interaction between the alkyl moieties of M10P.
Introduction Since the classic paper of Timmons and Zisman1 reported that n-alkanoic acids with chain lengths of C14 and longer adsorb onto metal surfaces, forming closepacked monolayers, a number of studies have been performed to elucidate the adsorption and spontaneous organization processes of amphoteric molecules such as the n-alkanoic acids and alkanethiols on metal surfaces.2,3 These studies of so-called self-assembled monolayers have indicated that surface-binding interaction and intermolecular interactions play important roles in the formation of the ultimate structures of self-assembled monolayers.4 In addition to these factors, adsorption kinetics also plays an important role.5 In the present paper, by using infrared reflection absorption (IRA) spectroscopy, we studied the kinetics of adsorption and structural change of selfassembled monolayers of (methacryloyloxy)alkyl dihydrogen phosphates with C10- and C2-alkyl chains, i.e., (10methacryloyloxy)decyl dihydrogen phosphate, which is abbreviated M10P (see Figure 1), and (2-methacryloyloxy)ethyl dihydrogen phosphate (M2P; see Figure 1). Through these studies we have shown how the kinetic process interplays with the surface binding and intermolecular * Author to whom correspondence is addressed. Phone: +81-3-5286-3243. Fax: +81-3-5273-2606. E-mail: ki0092@ cfi.waseda.ac.jp. X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Timmons, C. O.; Zisman, W. A. J. Phys. Chem. 1965, 69, 984. (2) Ulman, A. An Introduction to Ultrathin Organic Films, From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (3) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvilli, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (4) Tao, Y.-T. J. Am. Chem. Soc. 1993, 115, 4350. (5) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45.
S0743-7463(96)00398-8 CCC: $12.00
Figure 1. Structures of M10P (A) and M2P (B).
interactions to determine the overall features of the selfassembling process. Although a few papers have already been published with regard to the structures of selfassembled monolayers containing the phosphate group,6,7 little is known about the effect of the above-mentioned factors on the self-assembling process. M10P has received attention in dentistry because, when dental metals such as precious and nonprecious alloys are treated with it, the surfaces show stronger adhesion to a dental resin compared to untreated metal surfaces.8,9 The relationship between this practical aspect of M10P and its adsorption structure to the metal surfaces will be reported in a separate paper.10 Experimental Section Materials. (10-Methacryloyloxy)decyl Dihydrogen phosphate (M10P) and (2-methacryloyloxy)ethyl dihydrogen phosphate (6) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (7) Maoz, R.; Sagiv, J. Thin Solid Films 1985, 132, 135. (8) Omura, I.; Yamaguchi, J.; Nagase, Y.; Uemura, F. Japan Patent S58-21687, 1983. (9) Fujishima, A.; Fujishima, Y.; Ferracane, J. L. Dent. Mater. 1995, 11, 82. (10) Suzuki, M.; Fujishima, A.; Miyazaki, T.; Hisamitsu, H.; Ando, H.; Nakahara, M.; Itoh, K. J. Biomed. Mater. Res., in press.
© 1996 American Chemical Society
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Figure 2. pH dependence of the infrared transmission spectra of aqueous solutions of M10P and M2P: (A) M10P at pH 1; (B) M10P at pH 11; (C) M2P at pH 1; (D) M2P at pH 11. (M2P) were provided from Kuraray Co. Ltd. and Kyoeisha Chemical Co., respectively, and used as received. Methyl methacrylate (abbreviated to MMA) was purchased from Tokyo Kasei Co. Ltd. and used as a solvent without purification. Substrate Preparation. Silver was evaporated onto a glass slide (26 × 40 mm) from an aluminum Knudsen cell to form a silver substrate of a thickness of about 1000 Å. The evaporation was carried out at 1 × 10-6 Torr in a diffusion oil pumped system with a liquid nitrogen trap. Preparation of Spin-Coated Films. M10P was dissolved in MMA at concentrations in the range of 1 × 10-2-5 × 10-5 mol/L. A total of 20 µL of each solution was put on the substrate and spun dry at 3000 rpm for 20 s. Preparation of Self-Assembled Monolayers. M2P and M10P were dissolved in MMA at a concentration of 5 × 10-5 mol/L. The substrates were immersed into the solutions, which were kept at 15.0 ( 0.3 °C, and taken out after 30 s to 7 days. After being taken out from the incubation bath, the substrates were rinsed with MMA and the solvent was blown off by a purified dry nitrogen gas. The samples were kept in a desiccator at 20 ( 0.3 °C before IRA measurements. IRA Measurements. Infrared reflection absorption (IRA) spectra were recorded by using a Bio-Rad FTS-45A equipped with a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector. Each spectrum was run for 1024 scans at a resolution of 4 cm-1. A JEOL IR-RSC110 reflection attachment was used at the angle of incidence of 80°.
Results and Discussion pH Dependence of IR Spectra of M10P and M2P. Figure 2 illustrates the pH dependence of the IR spectra of M10P and M2P in aqueous solutions in the 1400-900 cm-1 region, where PO stretching vibrations are observed. At pH 1.0 (parts A and C of Figure 2) M10P and M2P are in the protonated state or in the ROPO3H2 form, where R denotes the (methacryloyloxy)alkyl groups (see Figure 1), and at pH 11 (parts B and D) they are in the dissociated state or in the ROPO32- form. Upon changing from pH 1 to 11, both samples show appreciable spectral changes ascribable to the dissociation, giving IR bands near 1085 and 980 cm-1 at pH 11, which are absent in the spectra at pH 1. Thomas et al.11 listed characteristic frequencies of the PO32- group of the alkylphosphonates, RPO32-, and (alkyloxy)phosphonates, ROPO32-. According to them, the PO32- asymmetric stretching band (νas(PO32-)) is observed in the 1055-1140 cm-1 region, and the PO32- symmetric stretching (νs(PO32-)) band is observed in the 970-1010 cm-1 region. Then, the 1083- and 1086-cm-1 bands, which are observed in parts B and D, respectively, of Figure 2 are assigned to the νas(PO32-) vibration, and the 977- and 981-cm-1 bands observed in parts B and D, respectively, (11) Thomas, L. C.; Chittenden, R. A. Spectrochim. Acta 1970, 26A, 781.
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Figure 3. IRA spectra of the spin-coated films of M10P prepared from the MMA solutions with various concentrations: (A) 1 × 10-2 mol/L; (B) 5 × 10-3 mol/L; (C) 5 × 10-4 mol/L; (D) 1 × 10-4 mol/L; (E) 5 × 10-5 mol/L.
of Figure 2 are assigned to νs(PO32-). Presumably, the IR bands observed in the 1190-1170 cm-1 region in all of the spectra of Figure 2 are ascribable mainly to COC asymmetric stretching vibration (νas(COC)) of the methacryloyloxy group,12 and the band at 1020 cm-1 in Figure 2A is due to a coupled vibration of P-O stretching vibrations. Dissimilarity in the IR spectra in the region below 1170 cm-1 between M10P and M2P may be due to differences in the coupling of P-O and C-C stretching vibrations. IRA Spectra of a Spin-Coated Film of M10P on an Evaporated Silver. Figure 3, parts A-E, exhibits the IRA spectra of M10P spin-coated on an evaporated silver substrate from MMA solutions with various concentrations (1 × 10-2-5 × 10-5 mol/L). As shown in Figure 3A, the IRA spectrum of the spin-coated film prepared from a concentrated solution (1 × 10-2 mol/L) is similar to that of the neat sample (not shown in this paper), giving the 1720- and 1687-cm-1 bands, which are ascribable to CdO stretching bands (ν(CdO)) of the carbonyl groups in nonhydrogen-bonded and hydrogen-bonded states, respectively, and the 1173- and 1045-cm-1 bands, which are the counterparts of the 1170- and 1020-cm-1 bands observed for M10P in the aqueous solution at pH 1 (Figure 2A). The presence of the IRA bands at 1173 and 1045 cm-1 indicates that M10P in the spin-coated film exists in the protonated form. When the concentration of the solutions is reduced, the 1687-cm-1 band disappears and IRA bands appear around 1085 and 970 cm-1 (parts C-E of Figure 3), which correspond to the νas(PO32-) and νs(PO32-) bands observed at 1083 and 977 cm-1, respectively, in the IR spectrum of the aqueous solution at pH 11 (Figure 2B). These results indicate that the M10P molecules in the spin-coated films prepared from the diluted solutions form monolayers assuming a non-hydrogen-bonded state and that, upon direct interaction with the silver surface, the PO3H2 group undergoes dissociation. It should also be noted that the intensity ratios of νas(PO32-)/νs(PO32-) in parts D and E of Figure 3 are appreciably smaller than that in Figure 3C. The direction of the transition dipole of the νs(PO32-) band is parallel to the symmetry axis of the PO32- group, while that of the νas(PO32-) band is perpendicular to the axis. Then, referring to the surface selection rule for IRA spectroscopy,13 we can conclude from the above-mentioned results that the PO32- groups in the spin-coated film prepared from a relatively thick solution (Figure 3C) take on a tilted orientation, while the groups in the monolayers prepared from diluted solutions (parts D and E of Figure 3) adsorb to the silver surface with their axis almost (12) Manley, T. R.; Martin, C. G. Spectrochim. Acta 1976, 32A, 357. (13) Greenler, G. G. J. Chem. Phys. 1966, 44, 310.
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Figure 4. IRA spectral changes of the self-assembled monolayer of M10P as a function of time after preparation (see text). Numbers in the right-hand side of the spectra indicate time (h, hour; d, day): (A) IRA spectra in the 1800-1600 cm-1 region; (B) IRA spectra in the 1250-800 cm-1 region.
perpendicular to the surface. The IRA spectra in parts C and D of Figure 3 give the ν(CdO) band at 1727 cm-1, which is higher than the corresponding band (1720 cm-1) observed for the films prepared from the thick solutions (parts A and B of Figure 3); the results can be ascribed to a difference in intermolecular interaction between the bulk films and the monolayers. As can be seen from Figure 3E, the monolayer prepared from the most diluted solution gives the ν(CdO) band at 1711 cm-1, which is appreciably lower than the corresponding band observed for the other films and monolayers. Presumably, the M10P molecules in the former monolayer adsorb sparsely at the substrate surface, which allows the CdO groups to interact with the surface, causing the frequency lowering. IRA Spectral Changes as a Function of Time after Preparation of Self-Assembled Monolayers. Figure 4 illustrates the IRA spectral changes of a self-assembled monolayer of M10P as a function of time after being taken out from an incubation bath; the sample was prepared by immersing the silver substrate into the bath containing a MMA solution of M10P (5 × 10-5 mol/L) for 30 min. The spectrum measured after 1 h gives rise to the IRA bands at 1728, 1641, 1173, 1066, and 978 cm-1, which are assigned to the ν(CdO), ν(CdC), νas(COC), νas(PO32-), and νs(PO32-) vibrations, respectively. The presence of the νas(PO32-) and νs(PO32-) bands indicates that the adsorbates are in the dissociated form. The intensity ratio of νas(PO32-)/νs(PO32-) is much larger than the corresponding ratio observed for the spin-coated monolayers (parts D and E of Figure 3), indicating that the symmetry axis of the PO32- group of the adsorbate takes an appreciably tilted orientation with respect to the surface normal of the substrate. From Figure 4B it is clear that the intensities of the νas(PO32-) and νs(PO32-) bands show appreciable change with time. The intensities of these bands are plotted against time, together with those of the ν(CdO) and νas(COC) bands in Figure 5A. The result clarifies the following facts: (i) the intensity of the νs(PO32-) band increases with time, while that of the νas(PO32-) band decreases; (ii) the intensities of the ν(CdO) and νas(COC) bands are virtually constant in the time range up to several days. In addition to these facts the IRA spectral changes in the CH stretching vibration regions (not shown in this paper) indicate that (iii) the CH2 symmetric and asymmetric stretching (νs(CH2) and νas(CH2)) bands of the selfassembled monolayer giving the IRA spectra in Figure 4 are observed at 2852 and 2924 cm-1, respectively, and their intensities and frequencies are constant; the frequencies of the νs(CH2) and νas(CH2) bands suggest that
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Figure 5. Intensity changes of the IRA bands of the selfassembled monolayers of M10P (A) and M2P (B) as a function of time after preparation (see text): (0) ν(CdO) band; (1) νas(COC) band; (b) νs(PO32-) band; (4) νas(PO32-) band.
Figure 6. IRA spectral changes of the self-assembled monolayer of M2P as a function of time after preparation (see text). Numbers in the right-hand side of the spectra indicate time (h, hour): (A) IRA spectra in the 1800-1600 cm-1 region; (B) IRA spectra in the 1250-800 cm-1 region.
the alkyl moieties of the adsorbate are in an irregular state containing an appreciable amount of gauche conformations.14 Facts ii and iii prove that the (methacryloyloxy)decyl groups of M10P originally take a random orientation with the irregular alkyl moieties and keep its state during the time interval from 1 h to 7 days, and fact i proves that the PO32- group changes its symmetry axis from a tilted orientation to a less tilted one. Presumably, at a surface concentration of the sample the less tilted orientation of the PO32- group is thermodynamically more stable than the tilted orientation, and the orientation change can be interpreted as a conversion from a thermodynamically unstable (or a trapped) state to a more stable state (or an equilibrium state). Figure 5A indicates that the conversion is virtually completed after about 100 h. In order to investigate how the conversion process depends on the length of alkyl moieties, IRA spectral changes were measured also for a self-assembled monolayer of M2P prepared by immersing a silver substrate in a M2P solution in MMA (5 × 10-5 mol/L) for 30 min. The results are summarized in Figure 6. The monolayer gives the ν(CdO), ν(CdO), νas(COC), νas(PO32-), and νs(PO32-) bands at 1726, 1641, 1179, 1082, and 982 cm-1, respectively, at 1 h after being taken out from the incubation bath. The intensity ratio of νas(PO32-)/νs(PO32-) observed at this stage, however, is smaller than the corresponding ratio observed for the monolayer of M10P (cf. the spectrum measured at 1 h in Figure 4B), indicating that the axis (14) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316.
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Figure 7. IRA spectral changes of the self-assembled monolayer of M10P as a function of incubation time (see text). Numbers in the right-hand side of the spectra indicate time (s, second; m, minute, h, hour): (A) IRA spectra in the 1800-1600 cm-1 region; (B) IRA spectra in the 1250-800 cm-1 region.
of the PO32- group of M2P assumes an orientation which is more perpendicular than that of M10P. The intensities of the νas(PO32-), νs(PO32-), ν(CdO), and νas(COC) bands are plotted against time in Figure 5B. The νs(PO32-) band increases its intensity, while that of the νas(PO32-) band decreases with time; the result indicates that the orientation change of the PO32- group from a tilted orientation to a less tilted one takes place, as in the case of M10P. From Figure 5B it is also clear that the process is completed after about 10 h, which is much faster than in the case of M10P. The molecular size of M2P is much smaller than that of M10P, and it is reasonable to consider that the mobility of M2P at the silver substrate is larger than that of M10P. This may cause the more rapid conversion to the perpendicular orientation (or the more stable state) observed for the PO32- groups of M2P compared to that for the groups of M10P. IRA Spectral Changes of the Self-Assembled Films of M10P and M2P as a Function of Incubation Time. Figure 7 summarizes the IRA spectral changes of the selfassembled monolayers of M10P against incubation time. As explained in the preceding section, the IRA spectral changes take place after taking out a self-assembled monolayer from the incubation bath. So, we measured the IRA spectra in Figure 7 after keeping each sample in a desiccator for 7 days after its preparation; at this time the conversion to an equilibrium state (or a thermodynamically stable state) is virtually completed. The IRA spectra measured after incubation of 30 s to 1 min give rise to the νs(PO32-) band at 975 cm-1, while the spectra give only a broad feature in the νas(PO32-) band region near 1080 cm-1; thus, at this stage (or at a relatively smaller surface concentration of the adsorbate) the axis of the PO32- group of M10P is almost perpendicular to the surface. Although the surface concentrations of the monolayers at 30 s to 1 min are similar to that of the spin-coated monolayer which gives the spectrum in Figure 3E, the former films give the ν(CdO) band at 1726 cm-1, which is appreciably higher than that observed for the latter monolayer; the result indicates that, in contrast to the case of the spin-coated monolayer, the self-assembling proceeds so that the CdO group does not interact directly with the substrate surface. The IRA bands in Figure 7 show appreciable intensity change with incubation time. Figure 8A plots the intensities of the νs(PO32-), νas(PO32-), νas(COC), and ν(CdO) bands against incubation time. From the plots the following facts are clarified: (i) at first, all the bands increase their intensity, indicating the increase in the
Figure 8. Intensity changes of the IRA bands of the selfassembled monolayers of M10P (A) and M2P (B) as a function of incubation time (see text): (0, 9) ν(CdO) band; (3,1) νas(COC) band; (O,b) νs(PO32-) band; (4,2) νas(PO32-) band.
surface concentration, (ii) the intensity increases of the νas(COC) and ν(CdO) bands are almost completed after about 50 min, which means that saturation coverage takes place around this time, (iii) the intensity of the νs(PO32-) band begins to decrease after about 10 min, while the νas(PO32-) band still keeps its intensity increasing, and the intensity reversal of the νs(PO32-) and νas(PO32-) bands occurs at about 10 min. These facts suggest that the M10P molecules at first adsorb at the substrate, keeping the axis of the PO32- group almost perpendicular to the surface, and that, as the surface concentration increases further, the axis starts to tilt from the perpendicular orientation. Figure 8A indicates that this orientation change is almost completed after about 50 min. It is also clear from Figure 7 that the IRA spectra give a complicated feature in the 1090-1050 cm-1 region, where the νas(PO32-) bands are observed; actually, a band near 1051 cm-1 dominates at 3 min, a band near 1068 cm-1 dominates at 30 min, and after several hours a band near 1083 cm-1 appears. Presumably, the PO32- groups in the tilted orientation are in some different interaction states with the substrate surface, causing the complicated feature in the νas(PO32-) region. Figure 9 summarizes the IRA spectral changes of the self-assembled monolayers of M2P against incubation time. The IRA spectra were measured after keeping each sample film in a vacuum desiccator for 7 days after its preparation. The IRA spectra measured at an initial stage of incubation (the incubation time of 30 s to 1 min) give rise to a broad band centered at 1059 cm-1 due to νas(PO32-) in addition to the IRA band at 975 cm-1 assigned to νs(PO32-). The broadness of the νas(PO32-) band suggests a multiinteraction state of the PO32- group with the silver substrate, and we can exclude the possibility that all the M2P molecules take an orientation at a given tilt angle. Thus, in contrast to the case of M10P, the PO32- group of the M2P molecule does not take a specific orientation but an almost random orientation from the initial stage of adsorption. Although the intensity ratio of νas(PO32-)/νs(PO32-) seems to slightly increase with time, the intensity reversal of the νas(PO32-) and νs(PO32-) bands observed for M10P cannot be detected. This can be seen from Figure 8B, where the intensities of the νs(PO32-), νas(PO32-), νas-
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on incubation time especially near the time of saturation coverage; the reason for this result has not been clarified. The discrepancy of the data may be ascribable to this critical dependence on the incubation time.) Conclusion
Figure 9. IRA spectral changes of the self-assembled monolayer of M2P as a function of incubation time (see text). Numbers in the right-hand side of the spectra indicate time (s, second; m, minute, h, hour): (A) IRA spectra in the 1800-1600 cm-1 region; (B) IRA spectra in the 1250-800 cm-1 region.
(COC), and ν(CdO) bands in Figure 9 are plotted against incubation time. The absence of the explicit orientation change from the perpendicular to tilted state in the case of M2P suggests that the alkyl moiety of M10P plays an important role in the orientation change. As the surface concentration of M10P increases, an interaction between the adsorbates (presumably between the alkyl groups) may stabilize the tilted orientation of the PO32- group. Figure 8B indicates that the intensity of all the bands shows a rapid increase at the beginning of adsorption and arrives at a maximum level after about 25 min. The time of saturation coverage is appreciably shorter than that of M10P (ca. 50 min). This can be explained by considering that the mobility of M2P on the silver substrate is larger than that of M10P. (The intensity ratio of νas(PO32-)/νs(PO32-) in the spectrum measured at 30 min in Figure 9B is slightly larger than the corresponding ratio in the spectrum measured at 9 h in Figure 6B, although the former spectrum was measured for the sample, which was prepared by incubating the substrate for 30 min and keeping it for 7 days. The intensity ratio critically depends
The conclusions are summarized as follows. (1) On adsorption to the silver substrate M2P and M10P dissociate the dihydrogen phosphate moiety, forming the PO32- group. (2) Saturation coverage of M10P takes a longer time (ca. 50 min) than that of M2P (ca. 25 min), which may be ascribable to the larger mobility of M2P compared to M10P. (3) At a certain level of surface concentration (e.g., after about 30 min of incubation) the PO32- groups of both M10P and M2P take at first a tilted orientation and change it to a less tilted one. The conversion to the less tilted orientation (or an equilibrium orientation) of M10P takes a longer time (ca. 100 h) than M2P (ca. 10 h), which may also be ascribable to the larger mobility of M2P compared to M10P. (4) At an initial stage of the self-assembling process (30 s to ca. 10 min) of M10P the axis of the PO32- group in an equilibrium state assumes mainly an orientation perpendicular to the substrate surface, and after ca. 10 min there occurs an orientation change from the perpendicular orientation to a tilted one. (5) In contrast to the case of M10P the PO32- group of M2P does not show such an explicit conversion from the perpendicular orientation to the tilted one; i.e., the axis of M2P takes on an almost random orientation during the self-assembling process. (6) The absence of the explicit orientation change of the PO32- group in the case of M2P suggests that an interaction between the alkyl groups of M10P plays a crucial role in stabilizing the tilted orientation of the PO32- group. Acknowledgment. The authors thank Prof. H. Hisamitsu, Prof. T. Miyazaki, and Dr. Fujishima of Showa University School of Density for their helpful discussion. LA9603986
