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Langmuir 2006, 22, 1586-1594
Influence of an Amide Group in Methyl Octadecanoates on the Monolayer Stability Katharina Dreger,† Li Zhang,‡ Hans-Joachim Galla,§ Harald Fuchs,‡ Lifeng Chi,‡ Ernst-Ulrich Wu¨rthwein,† and Hans Ju¨rgen Scha¨fer*,† Organisch-Chemisches Institut der UniVersita¨t Mu¨nster, Corrensstrasse 40, D-48149 Mu¨nster, Germany, Physikalisches Institut der UniVersita¨t Mu¨nster, Wilhelm-Klemm-Strasse 10, D-48149 Mu¨nster, Germany, and Institut fu¨r Biochemie der UniVersita¨t Mu¨nster, Wilhelm-Klemm-Strasse 2, D-48149 Mu¨nster, Germany ReceiVed August 22, 2005. In Final Form: December 12, 2005 The influence of a hydrogen bond donor and acceptor in the hydrophobic part of an amphiphile on the monolayer stability at the air/water interface is investigated. For that purpose, the amide group is integrated into the alkyl chain. Eight methyl octadecanoates have been synthesized with the amide group in two orientations and in different positions of the alkyl chain, namely, CH3O2C(CH2)mNHCO(CH2)nCH3 (n + m ) 14): 1 (m ) 1), 3 (m ) 2), 5 (m ) 3), 7 (m ) 14); and CH3O2C(CH2)mCONH(CH2)nCH3: 2 (m ) 1), 4 (m ) 2), 6 (m ) 3), 8 (m ) 14). The monolayers have been characterized by their π/A isotherms, their temperature dependence and Brewster angle microscopy (BAM). Amphiphile 1 with the amide group close to the ester group (m ) 1) behaves like an unsubstituted fatty acid ester, while 3, 5, and 7, with the amide group in an intermediate and terminal position, exhibit a two-phase region. The amphiphiles 2, 4, 6, and 8, with a reversed orientation of the amide group, all exhibit a two-phase region with higher plateau pressures and lower collapse pressures than those of 1, 3, 5, and 7. For 7 and 8, domains of the liquid condensed (LC) phase are visualized by BAM in the two-phase region. The liquid expanded (LE)/LC-phase transitions are all exothermic with enthalpies ∆H ranging from -31 to -12 kJ/mol. Comparison with other bipolar amphiphiles indicates that the LC phase is better stabilized by the hydroxy and dihydroxy groups than by the amide group. For model compounds of 1-4, optimized conformers in the LE and LC phases have been determined by density functional theory (DFT) calculations.
Introduction Amphiphiles are ordered at the air/water interface through noncovalent interactions. These comprise hydrogen bonds,1 steric repulsion, ionic,2 dipolar,3 hydrophobic and van der Waals forces.4 A possible way to obtain information on the kind, strength, and interplay of these interactions is the investigation of their monolayers using the Langmuir film balance technique5 often combined with Brewster angle microscopy (BAM).6 In amphiphiles, the alkyl chain generally contains no polar functional groups. Their installation can be an advantage, however, because they could act as reaction centers or as support for ion transport in membranes.7 The monolayer stability of longchain methyl carboxylates,8 however, is distinctly changed by introducing polar groups into the alkyl chain.9-12 In some of these papers, the investigation of isotherms has been combined * Corresponding author. Phone +49 8333231; fax +49 8336523; e-mail:
[email protected]. † Organisch-Chemisches Institut. ‡ Physikalisches Institut. § Institut fu ¨ r Biochemie. (1) (a) Kollman, P. A.; Allen, L. C. Chem. ReV. 1972, 72, 283-303. (b) Prins, L. J.; Reinhoudt, D. N.; Timmerman, P. Angew. Chem. 2001, 40, 2382-2426. (2) Shiloach, A.; Blankschtein, D. Langmuir 1998, 14, 7166-7182. (3) Thirumoorthy, K.; Nandi, N.; Vollhardt; D. J. Phys. Chem. B 2005, 109, 10820-10829. (4) (a) Sharp, K. A.; Nichols, A.; Fine, R. F.; Honig, B. Science 1991, 252, 106-109. (b) Widom, B.; Bhimalapuram, P.; Koga, K. Phys. Chem. Chem. Phys. 2003, 5, 3085-3093. (5) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience Publishers: New York, 1966; Chapter 3. (6) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590-4592. (7) (a) Menger, F. M.; Davis, D. S.; Persichetti, R. A.; Lee, L.-L. J. Am. Chem. Soc. 1990, 112, 2451-2452. (b) Renkes, T.; Scha¨fer, H. J.; Siemens, P. M.; Neumann, E. Angew. Chem. 2000, 34, 2512-2516. (8) Sackmann, H.; Doerffler, H. D. Z. Phys. Chem. (Leipzig) 1972, 251, 303313. (9) Menger, F. M.; Richardson, S. D.; Wood, M. G., Jr.; Sherrod, M. J. Langmuir 1989, 5, 833-838.
with BAM and grazing incidence X-ray diffraction (GIXD) studies to provide deeper insight.11 The amide group is a widely used structural element both in nature and in the assembly of supramolecular structures.13 From crystal structures of different amides, it is known that sec-amides adopt an anti conformation of the aza-hydrogen and the carbonyl group.14 The presence of an amide group in the alkyl chain of a fatty acid in this manner could lead to a continual intermolecular hydrogen bond that would stabilize the monolayer of amphiphiles containing an amide group in the liquid condensed (LC) phase. There are some reports on amide groups integrated into the alkyl chain.15-18 N-Acyl-amino acids have been investigated with regard to a temperature-dependent chiral discrimination.15 For N-alkyl-β- and N-alkyl-γ-hydroxy-alkanoic acid amides, the phase (10) (a) Kellner, B. M. J.; Cadenhead, D. A. J. Colloid Interface Sci. 1978, 63, 452-460. (b) Asgarian, B.; Cadenhead, D. A. Langmuir 2000, 16, 677-681. (11) (a) Vollhardt, D.; Siegel, S.; Cadenhead, D. A. Langmuir 2004, 20, 76707677. (b) Vollhardt, D.; Siegel, S.; Cadenhead, D. A. J. Phys. Chem. B 2004, 108, 17448-17456. (c) Vollhardt, D.; Siegel, S.; Cadenhead, D. A. Colloids Surf., A 2005, 256, 9-15. (12) (a) Overs, M.; Fix, M.; Jacobi, S.; Chi, L.; Sieber, M.; Scha¨fer, H. J.; Fuchs, H.; Galla, H.-J. Langmuir 2000, 16, 1141-1148. (b) Fix, M.; Sieber, M.; Overs, M.; Scha¨fer, H. J.; Galla, H.-J. Phys. Chem. Chem. Phys. 2000, 2, 45154520. (c) Wang, L.; Jacobi, S.; Sun, J.; Overs, M.; Fuchs, H.; Scha¨fer, H. J.; Zhang, X.; Shen, J.; Chi, L. J. Colloids Interface Sci. 2005, 285, 814-820. (13) Palmore, G. T. R.; Macdonald, J. C. In The Amide Linkage: Structural Aspects in Chemistry, Biochemistry, and Material Science; Greenberg, A., Breneman, C. M., Liebman, J. F., Eds.; Wiley: New York, London, Sidney, 2000; pp 291-336. (14) (a) Leiserowitz, L.; Schmidt, G. M. J. Chem. Soc. A 1969, 2372-2382. (b) Leiserowitz, L.; Tuval, M. Acta Crystallogr. 1978, B34, 1230-1247. (15) Hoffmann, F.; Hu¨hnerfuss, H.; Stine, K. J. Langmuir 1998, 14, 45254534. (16) Melzer, V.; Vollhardt, D.; Brezesinski, G.; Mo¨hwald, H. Thin Solid Films 1998, 327-329, 857-860. (17) Melzer, V.; Weidemann, G.; Vollhardt, D.; Brezesinski, G.; Wagner, R.; Struth, B.; Mo¨hwald, H. Supramol. Sci. 1997, 4. 391-397. (18) Melzer, V.; Weidemann, G.; Wagner, R.; Vollhardt, D.; DeWolf, Ch.; Brezesinski, G.; Mo¨hwald, H. Chem. Eng. Technol. 1998, 21, 44-48.
10.1021/la0522799 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/19/2006
Influence of Amide Groups on Monolayer Stability
transitions and morphological structures of monolayers have been studied.16-18 In this contribution, the behavior of methyl octadecanoates with a polar amide group integrated in two orientations and several positions into the main chain is investigated at the air/water interface. The results should provide information on the balance of intermolecular hydrogen bonds and repulsive forces in the LC phase for the amide group compared to other bipolar amphiphiles. Preliminary results have been reported in a short communication.19 Experimental Section General Synthetic Methods. Solvents and reagents were purified, if necessary, using literature methods. The reagents were used as supplied by Aldrich, Fluka, and Merck, if not specified otherwise. Thin-layer chromatography (TLC) was performed on aluminum plates (5 × 7.5 cm) coated with Merck Silica Gel 60F254. For silica gel column chromatography, Merck flash Silica Gel 60 was used. Elementary analyses were performed on a Varian EL III apparatus by the Department of Microanalytical Services of the Institute of Organic Chemistry, University of Mu¨nster. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker IFS 28 spectrometer. Mass spectra (GC/MS) were obtained from a Finnigan MAT 8230 instrument, linked with a Varian GC 3400 using data system SS 300. 1H and 13C NMR spectra were recorded on Bruker ARX 300 and Bruker AMX 400 spectrometers. Melting points were determined on a Kofler heating plate microscope and are presented uncorrected. Film Balance Measurements. Surface pressure-area isotherms were taken on a Langmuir film balance type 622 (Nima, Coventry) with a total trough area of 1300 cm2. The surface pressure was measured with a Wilhelmy plate system at a compression rate of 80 cm2/min and, if not otherwise stated, at 293 K. The isotherms were reproduced at least three times with the deviations given in the tables and figures. Brewster Angle Microscopy. BAM was performed on a Langmuir film balance type 601BAM (Nima, Coventry) with a total trough area of 750 cm2 and a compression rate of 60 cm2/min. The pictures were taken with a BAM2+ (NFT, Go¨ttingen). The light source was a Nd:YAG laser. The reflected beam was recorded with a CCD camera and videotaped. Materials. Methyl 2-(Pentadecanoylamino)ethanoate (1). Oxalyl chloride (0.25 mL, 3.0 mmol) was slowly added to pentadecanoic acid (0.360 g, 1.5 mmol) in dry CH2Cl2 (10 mL) and cooled to 273 K under argon. After stirring for 30 min at 273 K and for 20 h at room temperature, the solvent and the remaining oxalyl chloride was removed under vacuum. The formed acid chloride was slowly added to a solution of methyl glycinate hydrochloride and triethylamine (1.0 mL, 7.2 mmol) in dry CH2Cl2 (10 mL). The solution was stirred for 30 min at 0 °C, at 273 K, and at room temperature. After addition of CH2Cl2 (20 mL) and water (20 mL) and acidification of the solution with hydrochloric acid, the phases were separated, and the aqueous phase was extracted with CH2Cl2 (2 × 20 mL). The combined organic phases were washed with saturated NaHCO3 and dried (MgSO4). After the solvent was evaporated, the residue was purified by flash chromatography (cyclohexane/ethyl acetate, 3:2) to obtain 1 (0.344 g, 1.1 mmol, 73%) as a white solid. mp: 341-344 K. 1H NMR (CDCl3, 300 MHz) (δ, ppm): 0.89 (t, 3J ) 6.7 Hz, 3H, CH2CH3), 1.22-1.36 (m, 22H, 11CH2), 1.65 (m, 2H, NHCOCH2CH2), 2.24 (t, 3J ) 7.6 Hz, 2H, NHCOCH2), 3.76 (s, 3H, CO2CH3), 4.05 (d, 3J ) 5.0 Hz, 2H, H3CO2CCH2NH), 6.07 (m br, 1H, NH). MS (GC/MS) [m/z (%)]: 313 (
