Langmuir 1997, 13, 4807-4809
4807
Silver(I) Ion Induced Monolayer Formation of 2-Substituted Benzimidazoles at the Air/Water Interface Minghua Liu,*,†,‡ Akira Kira,† and Hiroo Nakahara§ The Institute of Physical and Chemical Research (RIKEN), Wako-shi, Hirosawa 2-1, 351-01, Japan, and Faculty of Science, Saitama University, Urawa 338, Japan Received March 31, 1997. In Final Form: July 18, 1997X The monolayer formation of a series of 2-substituted benzimidazoles at the air/water interface was studied by the measurements of surface pressure-area isotherms. Concentrated silver(I) ion in the subphase can induce the monolayer formation of 2-alkylbenzimidazole derivatives (BzCn) with the alkyl chain equal to or greater than C5 although the derivatives cannot form monolayers on pure water surface except BzC17. Similar behavior of monolayer formation of 2-phenylbenzimidazole was also observed. The measurements of UV spectra and X-ray photoelectron spectroscopy analysis of the transferred films suggested that such monolayer formation was fulfilled through an in situ formation of a polymeric silver(I)benzimidazole complex at the air/water interface.
Introduction It is well-known that in order to form stable insoluble monolayers at the air/water interface the balance between the hydrophobic and hydrophilic groups within a molecule is very important. Usually amphiphiles with single straight alkyl chain ranging from C16 to C22 can form well-defined monolayers.1 Although some macrocyclic phthalocyanine and porphyrin derivatives containing numbers of short alkyl chains or even without long-alkyl chains can also form well-behaved monolayers at the air/ water interface,2,3 simple compounds with one short alkyl chain seem to be unsuccessful in forming monolayers at the air/water interface. For synthetic polymers, however, the requirement for the long alkyl chains is often unnecessary and those with short alkyl side chains can also form well-behaved monolayers.1 In this Letter, we report an unusual example of the monolayer formation of 2-substituted benzimidazole derivatives (Figure 4). The benzimidazole derivatives (C4) are spread on concentrated aqueous AgNO3 subphases, monolayers are formed. It is found that such monolayer formation is caused by an in situ formation of a polymeric silver(I)-benzimidazole complex. Experimental Section 2-Alkylbenzimidazoles (n ) 0-11, 13, 15, 17, abbreviated as BzCn) were prepared by the condensation of o-phenylenediamine and corresponding carboxylic acid (from formic acid to stearic acid) according to the literature method.4 2-Phenylbenzimidazole (BzPh) was from Tokyo Kasei and purified by recrystallization from ethanol. The monolayers were formed by spreading a chloroform solution (ca. 3 × 10-4 M) on aqueous subphases containing various concentrations of AgNO3 (99.8%, JUNSEI Chemicals). Deionized water from a Milli-pore Q system (18 MΩ cm) was used in all the cases. The surface pressure-area isotherms were measured with a Lauda film balance (FW-1) with †
Institute of Physical and Chemical Research. Present address: Graduate School of Bio-applications and Systems Engineering, Tokyo University of Agriculture and Technology, Tokyo, Japan. § Saitama University. X Abstract published in Advance ACS Abstracts, August 15, 1997. ‡
(1) Gaines, G. L., Jr. Insoluble monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (2) Ogawa, K.; Kinoshita, S.; Yonehara, H.; Nakahara, H.; Fukuda, K. J. Chem. Soc., Chem. Commun. 1989, 477. (3) Barwell, J.; Bolton, J. R. Photochem. Photobiol. 1984, 39, 735. (4) (a)Seka, R.; Mu¨ller, R. H. Monatsh. Chem. 1931, 57, 95. (b) Pool, W. O.; Harwood, H. J.; Ralston, A. W. J. Am. Chem. Soc. 1937, 59, 178.
S0743-7463(97)00325-9 CCC: $14.00
a compression speed of 10.5 cm2 min-1. For UV and X-ray photoelectron spectroscopy (XPS) measurements, the monolayers on 5 mM aqueous AgNO3 subphase were transferred at 20 mN m-1 onto quartz and ITO glass plates, respectively by a horizontal lifting method.5 The UV spectra were recorded by a Shimadzu UV-190 double-beam spectrophotometer, and the XPS spectrum was measured on an ULVAC PHI Model 550 spectrometer using Mg KR as the exciting source at a takeoff angle of 45°.
Results and Discussion When chloroform solutions of 2-alkylbenzimidazoles were spread onto a pure water (18 MΩ cm) surface, no monolayers were formed except BzC17.6 When the solutions were spread on a subphase containing an appropriate concentration of AgNO3 however, monolayers were formed for those benzimidazoles with the chain length equal to or greater than C5. Figure 1A shows, for example, the surface pressure-area (π-A) isotherms of 2-heptylbenzimidazole (BzC7) on subphases containing AgNO3 with various concentrations. On the subphase containing AgNO3 (10 µM), an onset of the surface pressure and an extrapolated molecular area are obtained as 0.14 and 0.10 nm2 molecule-1, respectively. This cannot be regarded as a true monolayer since the cross section of the alkyl chain is 0.2 nm2 molecule-1. The isotherm shifts to larger molecular areas when the concentration of AgNO3 in the subphase is increased. When the concentration of AgNO3 in the subphase is increased up to 5 mM, a monolayer with a limiting area of 0.25 nm2 molecule-1 is obtained. This value is reasonable in considering the cross section of alkyl chain and the molecular dimension of the benzimidazole,7 indicating the true monolayer formation of the BzC7 on concentrated AgNO3 subphase. No significant change was observed upon further increasing AgNO3 in the subphase. Similar monolayer behaviors were observed for 2-alkylbenzimidazoles with the alkyl length equal to or greater than C5 on concentrated AgNO3 subphase. For those derivatives with alkyl chain length longer than C8, monolayers were formed even on a subphase containing small amounts of AgNO3 (5 × 10-5 M). For those derivatives with the alkyl chain shorter than C5, no monolayers were formed even on concentrated AgNO3 (5 mM) subphase. It is further noted that (5) (a) Langmuir, I.; Schaefer, V. J. J. Am. Chem. Soc. 1938, 60, 1351. (b) Fukuda, K.; Nakahara, H.; Kato, T. J. Colloid Interface Sci. 1976, 54, 430. (6) Liu, M.; Kira, A.; Nakahara, H.; Fukuda, K. Thin Solid Films, 1997, 295, 250. (7) (a) Dik-Edixhoven, C. J.; Schenk, H.; Van der Meer, H. Crystallogr. Struct. Commun. 1973, 2, 23. (b) Escande, A.; Galigne´, J. L. Acta Crystallogry. 1974, B30, 1647.
© 1997 American Chemical Society
4808 Langmuir, Vol. 13, No. 18, 1997
Letters
Figure 2. XPS spectrum of a 40-layer LB film of BzC7 on ITO glass.
Figure 1. Surface pressure-area isotherms of BzC7 and BzPh on aqueous subphase containing various concentrations of AgNO3: (A) BzC7, (a) 1 × 10-5 M, (b) 1 × 10-4 M, (c) 5 × 10-4 M, (d) 5 × 10-3 M; (B) BzPh, (a) 1 × 10-4 M, (b) 1 × 10-3 M, (c) 5 × 10-3 M.
2-phenylbenzimidazole (BzPh) can also form a stable monolayer on concentrated AgNO3 solution, as evidenced by the π-A isotherms shown in Figure 1B. The monolayers were relatively stable. The area losses of the monolayer of BzC7 on AgNO3 (5 mM) subphase were found to be 0.9, 1.2, and 1.3%/min at constant 20, 30, and 40 mN m-1, respectively, and those of BzPh were 0.9, 1.6, and 1.8%/min, respectively. All the isotherms of these monolayers are similarly independent of the alkyl chain lengths. The monolayers show no phase transition, and the surface pressure grows rapidly after they are compressed to around 0.3 nm2 molecule-1. The monolayers show no clear collapse pressure, and the surface pressure increases gradually after collapse. These features are characteristic of the monolayers of synthetic polymers.1 It has been reported that imidazole and benzimidazole can react with many transition metal ions such as Ag(I), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), and so on to form metal complexes, and the metal complexes are essentially polymeric in nature.8,9 The above results from the π-A measurements suggest that a polymeric Ag(I) complex between the benzimidazole and Ag(I) in the subphase is formed at the air/water interface. Such a polymeric Ag(I) complex causes the formation of the monolayers. It seems that the complex formation at the interface depends on (8) Sundberg, R. J.; Martin, R. B. Chem. Rev. 1974, 74, 471. (9) (a) Jarvis, J. A.; Well, F. A. Acta Crystallogr. 1960, 13, 1027. (b) Brown, G. P.; Aftergut, S. J. Polym Sci., Part A 1964, 2, 1839. (c) Bauman, J. E.; Wang, T. C. Inorg Chem. 1964, 3, 36. (d) Goodgame, M.; Cotton, P. A. J. Am. Chem. Soc. 1962, 84, 1543. (e) Cordes, M. M.; Walter, J. T. Spectrochim. Acta 1968, 24A, 1421. (f) Xue, G.; Zhang, J.; Shi, G.; Wu, Y. J. Chem. Soc., Perkin Trans. 2 1989, 33.
the Ag(I) ion concentration in the subphase and the alkyl chain length of the benzimidazoles. For the benzimidazoles with short alkyl chains (C5-C8), a higher concentration of Ag(I) is needed to complete the complex formation. For those derivatives with long alkyl chains (>C8), the complex formation can be fulfilled on a subphase at relative lower concentration. For those derivatives with shorter alkyl chains (C4) spread on dilute AgNO3 (0.1 mM), although a true monolayer (with the molar area larger than 0.2 nm2 molecule-1) was not obtained, the “monolayer” could be scraped onto quartz plates. The spectra of these films showed the same spectra as those from the monolayers on concentrated AgNO3 solution. This fact indicates that the small area in “monolayer” on dilute AgNO3 subphase is due to partial dissolution of derivative into the water or incomplete complex formation. Sigwart et al. have discussed the structure of the polymeric 1:1 copper(I)-imidazole complex.13 By application of their structure to our case, the monolayer can
Acknowledgment. This work was supported by the Basic Science Research Program, Fund of Science and Technology Agency of Japan and in part by Grant-in-Aid for Scientific Research (No.07854033) from the Ministry of Education, Science, Culture and Sports of Japan. Authors are grateful to Miss Y. Nagahama of Saitama University for the XPS measurement. LA9703252 (13) Sigwart, C.; Kroneck, P.; Hemmerich, P. Helv. Chim. Acta 1970, 53, 177. (14) Nagel, J.; Oertel, U. Polymer 1995, 36, 381. (15) Werkman, P. J.; Schouten, A. J. Thin Solid Films 1996, 284/ 285, 24. (16) Storrier, G. D.; Colbran, S. B. J. Chem. Soc., Dalton Trans. 1996, 2185. (17) Hnapp, R.; Schott, A.; Mehahn, M. Macromolecules 1996, 29, 478. (18) Manners, I. Angew. Chem., Int. Ed. Engl. 1996, 35, 1603. (19) Wang, B.; Wasielewski, R. J. Am. Chem. Soc. 1997, 119, 12.
