4966
Langmuir 1996, 12, 4966-4968
Notes Langmuir Monolayers of p-(5′-m-Terphenylyl)benzoic Acid and Its 2′-Methyl Derivative at the Water/Air Interface
Scheme 1
Jan Czapkiewicz,* Patrycja Dynarowicz, and Piotr Milart Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland Received January 23, 1996. In Final Form: May 20, 1996
Introduction It is well-known that aliphatic carboxylic acids possessing at least 12 carbon atoms in the hydrocarbon chain may be spread with ease at the free surface of water.1 Since the hydrophobicity of a phenylene group is comparable to that of an aliphatic fragment composed of 4 ( 0.5 methylene groups,2 it may be expected that an aromatic carboxylic acid with the hydrophobic moiety consisting of four nonrigid phenyl groups should also exhibit filmforming properties. Such types of compounds do not appear to have been investigated so far. They might be treated as model aromatic analogues of alkanoic acids. We have recently demonstrated that sulfonates with an aromatic hydrophobic moiety based on the symmetrical triphenylbenzene ring system can be easily synthesized, and it has been shown that they exhibit strong selfassembly properties in aqueous solutions.3-5 Their mode of aggregation differs, however, from that which is characteristic of aliphatic amphiphiles. Thus, e.g., extensive dimerization of the sulfonates, preceding the operational critical micelle concentration (cmc), was demonstrated by the results of conductivity measurements.4 In the case of surface tension isotherms measured for the sodium triphenylbenzenesulfonate3 and for the derivative with a methyl group substituted at the central benzene ring,5 it was found that, at concentrations exceeding the cmc, the decrease of surface tension is not halted but tends to drop gradually with the increasing amphiphile’s concentration. This study has been carried out to examine the monolayer behavior of the carboxylic analogues of the aromatic sulfonates. We have prepared the model acids (4a, 4b; Scheme 1) by transforming 2,6-diphenyl-4-(4carboxyphenyl)pyrylium perchlorate (3) according to the general procedure described by Zimmermann and Fischer.6 4a was synthesized for the first time 30 years ago. A different two-step route was then applied: acetylation of 1,3,5-triphenylbenzene and oxidation of the acetyl derivative with sodium hypobromite.7 Experimental Section The perchlorate (3) was synthesized by applying the procedure (1) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (2) van Oss, N. M; Kok, P.; Bolsman, T. A. B. M. Tenside Surf. Det. 1992, 3, 175. (3) Czapkiewicz, J.; Milart, P.; Tutaj, B. J. Chem. Soc. Perkin Trans. 2 1993, 1655. (4) Czapkiewicz, J.; Rodriguez, J. R.; Tutaj, B. Colloids & Surfaces A 1995, 101, 147. (5) Czapkiewicz, J.; Milart, P. J. Chem. Soc., Perkin Trans. 2 1996, 187. (6) Zimmermann, T.; Fischer, G. W. J. Prakt. Chem. 1987, 329, 975. (7) Lewis, G. E. J. Org. Chem. 1965, 30, 2798; ibid. 1966, 31, 749.
S0743-7463(96)00079-0 CCC: $12.00
reported by Krygowski et al.,8 modified as follows: acetophenone (1) (14.5 g, 0.120 mol), 4-carboxybenzaldehyde (2) (10.0 g, 0.066 mol), and perchloric acid (70%) (20 mL) were stirred at 100 °C for 2 h. Then ethanol (50 mL) was added. An oily product separated out and solidified at ca. -10 °C. The material was filtered off and washed with cold ethanol and diethyl ether. For further purification the crude product was suspended in acetic acid (50 mL) and refluxed for 1 h. After this mixture was cooled to room temperature, the orange solid was separated, washed with diethyl ether, and then dried in air. Yield 7.8 g (25.8%); mp 312-314 °C (lit.8 mp 312-315 °C). 5′-Phenyl-1,1′:3′,1′′-terphenyl-4-carboxylic acid, in short PTCA (4a), was prepared by refluxing pyrylium salt (3) (6.8 g, 0.015 mol) and anhydrous sodium acetate (3.7 g, 0.045 mol) in acetic anhydride (30 mL) for 3 h with constant stirring. The orangeyellowish suspension changed to a brown solution in the course of the reaction. The mixture was cooled to room temperature. The colorless precipitate (sodium perchlorate and sodium acetate) was filtered off and washed with ethyl acetate. The solvents in the combined filtrate were removed under reduced pressure, and the oily residue was boiled for ca. 1 h with 50 mL of 20% aqueous NaOH solution until complete dissolution of the material. NaOH pellets were added if necessary, to ensure a strongly basic medium at the end of the process. The sodium salt of the acid precipitated upon cooling. It was recrystallized four times from water. The salt was converted into the acid by treating its hot aqueous solution with an excess of 18% hydrochloric acid. The colorless product was separated, washed with water, and dried in vacuum at 60 °C. It was then recrystallized from acetonitrile. Yield 2.5 g (47.5%), mp 259-260 °C (lit.7 mp 247-248 °C). Anal. Calc for C25H18O2: C, 85.68; H, 5.19. Found: C, 85.75; H, 5.31. IR (KBr) (cm-1) 3449, 3031, 2874, 2675, 2560 (OH + CH); 1689 (CdO), 1608, 1598, 1572, 1498 (Ar rings); 1432, 1408, 1296. 1H NMR (DMSO-d6) δ (ppm) 7.45 (t, 2H, Jortho ) 7.3 Hz, protons para of rings at 3′ and 5′); 7.54 (t, 4H, Jortho ) 7.3 Hz, protons meta of rings at 3′ and 5′); 7.91 (d, 4H, Jortho ) 7.3 Hz, protons ortho of rings at 3′ and 5′); 7.96 (t, 1H, Jmeta ) 1.6 Hz, proton 4′); 7.99 (d, (8) Krygowski, T. M.; Anulewicz, R.; Pniewska, B.; Milart, P. J. Phys. Org. Chem. 1991, 4, 121.
© 1996 American Chemical Society
Notes
Langmuir, Vol. 12, No. 20, 1996 4967
Figure 1. Surface pressure vs average area per molecule isotherms of PTCA at 20 °C recorded for different speeds of compression: (a) 1 cm2 min; (b) 10 cm2 min; (c) 50 cm2 min; and (d) 100 cm2/min. 2H, Jmeta ) 1.6 Hz, protons 2′ and 6′); 8.05 and 8.09 (2d, 4H, AA′BB′, JAB ) 8.5 Hz, protons of the ring with COOH group); 13.07 (1H, broad, COOH). 2′-Methyl-5′-phenyl-1,1′:3′,1′′-terphenyl-4-carboxylic acid, in short MPTCA (4b), was synthesized by refluxing 3 (6.8 g, 0.015 mol) and anhydrous sodium propionate (4.6 g, 0.045 mol) in propionic anhydride for 3 h with continuous stirring. The rest of the procedure was the same as that described for 4a. The sodium salt of 4b was recrystallized from a concentrated solution of NaCl and was then converted into the acid by treating its aqueous solution with an excess of 18% hydrochloric acid. The colorless product was separated, washed with water, and dried in vacuum at 60 °C. It was then recrystallized twice from acetic acid. Yield 1.5 g (27.5%), mp 303-304 °C. Anal. Calc for C26H20O2: C, 85.67; H, 5.54. Found: C, 85.71; H, 5.50. IR (KBr) (cm-1) 3439, 3021, 2853, 2665, 2550 (OH + CH); 1690 (CdO); 1605, 1580, 1492 (Ar rings); 1421, 1295. 1H NMR (DMSO-d6) δ (ppm) 2.05 (s, 3H, CH3); 7.37 (t, 2H, Jortho ) 7.3 Hz, protons para of ring at 3′ and 5′); 7.41-7.46 (m, 8H, other protons of ring at 3′ and 5′); 7.48 (s, 2H, protons 2′ and 6′); 7.80 and 7.95 (2d, 4H, AA′BB′, JAB ) 8.2 Hz, protons of the ring with COOH group); 13.02 (1H, broad, COOH). IR spectra were recorded on a Bruker IFS48 spectrometer as KBr pellets. 1H NMR spectra were taken at 500 MHz with a Bruker AMX500 spectrometer. Langmuir monolayers were prepared by spreading from a 100 µL Hamiltonian glass syringe an aliquot of the investigated acids, dissolved (0.5-1.0 g/L) in freshly distilled chloroform, on the surface of a 10-3 M HCl aqueous solution, held in a PTFE sliding barrier trough (NIMA Technology, U.K., Model 611), located on an antivibration table. The subphase temperature was controlled thermostatically to within 0.1 °C by circulating water. The experiments were performed at 20, 25, and 30 °C. After spreading, monolayers were left for 5 min for solvent to evaporate, after which compression was initiated. Surface pressure was measured with an accuracy of (0.1 mN/m. A Wilhelmy plate made from chromatography paper (Whatman Chr1) was applied as a pressure sensor. Routine measurements were performed at 25 °C, and at the speed of 50 cm2/min, a continuous mode of compression and expansion was used.
Results and Discussion The surface pressure/area isotherms for PTCA exhibit a characteristic broad plateau. Figure 1 illustrates the isotherms recorded at 20 °C by applying speeds of the barriers equal to 1, 10, 50, and 100 cm2/min. The surface pressure at the plateau (collapse region) slightly decreases (18.4-17.4 mN/m) whereas the second rise in pressure appears at lower areas per molecule as the rate of compression is slowed down. The range of temperatures studied was narrow (20, 25, and 30 °C); systematic trends, however, can be traced. Thus the area for the onset of pressure rise increases with the temperature rise (41-45 Å2/molecule) whereas the surface pressure at the collapse region decreases slightly (18.216.1 ( 0.2 mN/m). Upon expansion a drop of surface pressure, depending on the time of relaxation, is observed. Switching to compression restores the pressure im-
Figure 2. Compression-expansion cycles for PTCA at 25 °C.
Figure 3. Surface pressure vs average area per molecule isotherms of MPTCA at 25 °C recorded for (a) 50 cm2/min; (b) 15 cm2/min; and (c) 1 cm2/min.
mediately. Several compression-expansion cycles interrupted at various stages, as shown in Figure 2, form hysteresis loops, the compression fragments of which are identical. The area for the onset of pressure rise for PTCA measured at 25 °C occurs at about 44 Å2/molecule. This value compares well with that of 46 Å2/molecule estimated for the sodium polybenzenesulfonate analogue on the basis of surface tension data fitted to the Gibbs adsorption isotherm.3 For MPTCA, at high compression rates (50-150 cm2/ min) the pattern of the isotherms is comparable to those found for PTCA. When the speed of the barrier is slowed down to 15 cm2/min, the pressure at the collapse region relaxes. This phenomenon is augmented upon further speed reduction. Curve c in Figure 3 illustrates the isotherm recorded at the lowest compression rate attainable by the instrument (1 cm2/min). Such behavior suggests nucleation and growth of three-dimensional crystallites.9 Indeed, they were observed by the naked eye on the water surface after completing the compression at slow rates. Successive compression-expansion cycles performed for MPTCA produce loops shifted increasingly toward smaller areas. This effect was found to be the greater the longer the time left for aging of the decompressed layer. The isotherms measured for PTCA exhibit some similarities to those reported for 4′-alkyl-4-cyanoterphenyls10,11 and cyanobiphenyls.12-14 These liquid crystalline materi(9) Siegel, S.; Ho¨nig, D.; Volhardt, D.; Mo¨bius D. J. Phys. Chem. 1992, 96, 8157. (10) Daniel, M. F.; Lettington, O. C.; Small, S. M. Thin Solid Films 1983, 99, 61. (11) Dent, N.; Grundy, M. J.; Richardson, R. M.; Roser, S. J.; McKeown, N. B.; Cook, M. J. J. Chim. Phys. 1988, 85, 1003. (12) Xue, J.; Jung, C. S.; Kim, M. W. Phys. Rev. Lett. 1992, 69, 474.
4968 Langmuir, Vol. 12, No. 20, 1996
als exhibit a very broad plateau on the surface pressure vs average area per molecule isotherms. Furthermore, the area for the onset of pressure rise for alkylbiphenyl cyanides occurs in the region of 48-41 Å2/molecule as in the case of PTCA and MPTCA. Neutron reflectivity measurements,11 ellipsometry,12 and Brewster angle microscopy results13,14 indicate that the plateau represents a transition of a monolayer to an interdigitated multilayer state. The span of the plateau characteristic of PTCA suggests the formation of a bilayer. This is the first report (13) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1994, 10, 1251. (14) de Mul, M. N. G.; Mann, J. A. Langmuir 1995, 11, 3292.
Notes
on Langmuir film composed of an aromatic carboxylic acid which does not possess any alkyl chains deemed to be necessary for the formation of multilayer systems. The investigated acids, especially PTCA, seem to be promising model film-forming compounds due to their fluorescence in UV light. Acknowledgment. The authors (J.Cz. and P.M.) wish to thank the Committee on National Research (KBN, Poland) for the award of a grant (No. 2P303 030 06). LA960079E
