Infrared Spectroscopic Comparison of Enantiomeric and Racemic N

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Langmuir 1994,10, 3782-3786

3782

Infrared Spectroscopic Comparison of Enantiomeric and Racemic N-Octadecanoylserine Methyl Ester Monolayers at the Air/Water Interface Arne Gericke and Heinrich Huhnerfuss* Institute of Organic Chemistry, University of Hamburg, Martin-Luther-King Platz 6, 20146 Hamburg, FRG Received March 30, 1994. I n Final Form: June 21, 1994@ The chiral discrimination of N-octadecanoylserine methyl ester monolayers at the airlwater interface is investigatedby external infrared reflection-absorption spectroscopy. For 297.15 K, evidenceis presented that the alkyl chain of the racemic surface film in the liquid-expanded phase and in the liquid-expanded/ liquid-condensed transition region is less ordered than the enantiomeric one. In the condensed phase these differences disappear largely. At 293.15 K, the differencesin alkyl chain order between the racemic and the enantiomeric films are reduced for large areas per molecule in comparison with the higher temperature, but they remain significant in the compressed state. In the case of the racemic film, the amide I band at 1626.0 cm-I is broadened in comparison with the enantiomeric film. The ester carbonyl group is slightlyless protonated in the case of the enantiomericfilm. The alcoholic C-0 stretching vibration 3 and is more close to a horizontal orientation than to a vertical direction. In the is found at ~ 1 0 8 cm-l case of the racemic film this band is broadened or split into two components at 1095 and 1079 cm-’.

Introduction The investigation of chiral effects in monomolecular surface films a t the airlwater interface is of considerable interest due to the influence of chiral molecules on many enzymatic processes, and therefore, in recent years a n increasing effort has been devoted to this task.’-8 The most intensive studies on chiral monolayers were carried out by Arnett and co-workers?-l2 who were able to show that chiral discrimination highly depends on factors like head group structure, surface pressure, temperature, and subphase composition. The investigation of the domain structure of chiral monolayers revealed that the shape of the domains is closely related to the stereochemistry of the surface and even in monolayers of achiral molecules chiral phases may be found15J6(due to the lack of head-tail symmetry in mon~layersl~). This so-called “chiral symmetry breaking”, i.e., the formation of chiral

* To whom all correspondence should be addressed. Abstract published inAdvanceACSAbstracts, August 15,1994. (1)Stewart, M.;Amett, E. M. In Topics in Stereochemistry;Eliel, E. L., Alinger, N. L., Eds.; Wiley: New York, 1982; Vol. 13. (2) Landau, E. M.; Grayer Wolf, S.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. J . Am. Chem. SOC.1989,111, 1436. Dupeyrat, M. Biochim. Biophys. Acta 1988, 938, (3) Boulossa, 0.; 395. (4) Dvolaitzky, M.; Guedeau-Boudeville,M. A. Langmuir 1989, 5, 1200. (5) Eckhardt, C. J.; Peachey, N. M.; Swanson, D. R.; Takacs, J. M.; Khan,M. A.; Gong, X.; Kim, J.-H.; Wang, J.; Uphaus, R. A. Nature 1993,362, 614. (6) Rietz, R.; Brezesinski, G.; Mohwald, H. Ber. Bunsen-Ges. Phys. Chem. 1993,97, 1394. (7) Qian, P.; Matsuda, M.; Miyashita, T. J . Am. Chem. SOC.1993, 115,5624. (8) Ahlers, W.; Miiller, W.;Ringsdorf,H.;Venzmer,J. Angew.Chem. 1990,102, 1310;Angew. Chem., Int. Ed. Engl. 1990,29, 1269. (9) Harvey, N.; Rose, P.; Porter, N. A.; Huff, J. B.; Amett, E.M. J. Am. Chem. SOC.1988,110,4395. (10) Amett, E. M.; Harvey, N.; Rose, P. L. Langmuir 1988,4,1049. (11) Harvey, N, G.; Amett, E. M. Langmuir 1989,5, 998. (12) Heath, J. G.; Amett, E. M. J . A m . Chem. SOC.1992,114,4500. (13)McConnell, H. M. Annu. Rev.Phys. Chem. 1991,42, 171. (14) Stine, K. J.; Uang, J. Y.-J.;Dingnam, S. D. Langmuir 1993,9, 2112. (15) Qiu, X.; Ruiz-Garcia,J.; Stine, K. J.;Knobler, C. M.; Selinger, J. V. Phys. Rev. Lett. 1991, 67, 703. (16) Ruiz-Garcia,J.; Qiu, X.; Tsao, M.-W.;Marshall, G.; Knobler, C. M.; Overbeck, G. A.; Mobius, D. J . Phys. Chem. 1993, 97, 6955. (17) Selinger, J. V.; Wang, 2.-G.; Bruinsma, R. F.; Knobler, C. M. Phys. Rev.Lett. 1993, 70, 1139. @

domains, was also observed for racemic mixtures. Furthermore, this effect was recently theoretically described by Selinger et al.17 Acylamino acid esters are amphiphiles readily prepared18for the investigation ofchiral effects in monolayers. In particular, N-octadecanoylserine methyl esters turned out to show a high degree of chiral discrimination between the properties of its pure enantiomeric films and of the corresponding racemate.lg Stine et al.l4 compared the domain structures for the racemic and enantiomeric films and found distinct differences in the condensed phase domain structures. However, distinct insights into the correlation between this chiral discrimination effect and the alkyl chain orderhead group structure of the filmforming molecules are lacking. The present work aims a t filling this gap by investigating enantiomeric and racemic N-octadecanoylserine methyl ester monolayers at the air/ water interface for the temperatures 293 and 297 K by applying external infrared reflection-absorption spectroscopy, which is known to be a powerful tool for the determination of the alkyl chain order (translgaucheratio) with respect to the head group ~ t r u c t u r e . ~ OThese -~~ temperatures were chosen because earlier investigations by Stine et al.I4 and Harvey et al.19showed that a t the higher temperature (297 K) a chiral discrimination is encountered, while this is not the case a t 293 K. The reflection-absorption spectra of the surface films are compared for H2O and D2O subphases. In a subsequent paperz0racemic and enantiomeric long-chain a-hydroxy carboxylic acids are examined.

Experimental Section Materials. D- and L-serine were obtained from Sigma Chemicals (Munich, FRG),while n-octadecanoylchlorid was (18) Zeelen, F. J.; Havinga, E. Recl. Trau. Chim. Pays-Bas 1968,77, 267. (19) Harvey, N. G.; Mirajovsky,D.; Rose, P. L.; Verbiar, R.; Amett, E.M. J . A m . Chem. SOC.1989,111, 1115. (20)Neumann, V.; Gericke, A.; Hiihnerfuss, H. In preparation. (21) Gericke, A.;Simon-Kutscher,J.;H h e r f u s s , H. Langmuir 1993, 9. 2119. (22) Gericke,A.; Simon-Kutscher,J.;Hiihnerfuss,H. Langmuir 1993, 9, 3115. (23) Gericke, A.; Michailov, A. V.; Hiihnerfuss, H. Vib. Spectrosc. 1993,4, 335.

0743-746319412410-3782$04.5010 0 1994 American Chemical Society

IR Spectroscopic Comparison of Chiral Monolayers

Langmuir, Vol. 10, No. 10, 1994 3783

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Figure 1. Comparison of the WA isotherms for racemic and enantiomeric N-octadecanoylserine methyl ester monolayers at 297.15 K (rate of compression 0.05 nm2 molecule-' min-', spreading solvent CHCl3). Inset Comparison of the racemic surface film for the compression rates 0.05 (-) and 0.01 (- - -) nm2 molecule-' min-'.

obtained from Merck (Darmstadt, FRG). N-Octadecanoyl-Lserine methyl ester and the correspondingD-enantiomer were prepared according to the procedure described by Harvey et al.19 The melting temperatures of the bulk compounds in both cases were 362.15-362.65 K. The structures of the resulting compounds were checked by IH NMR and infrared spectroscopy. The spreading solvent chloroform of licrosolv grade (Merck, Darmstadt,FRG)was used as received. Harvey et al.19and Stine et al.14 used another spreading solvent, but it was shown by Gericke et aLZ1that the application of n-hexanelethanol (9:1) mixtures as spreadingsolvent may lead to remarkable losses of the film-forming substances into the subphase. The solution of the racemate was obtained by dissolving the same amounts of the D- and L-enantiomers in the spreading solvent. Measurements. The apparatusand the spreadingtechnique for the surface pressudarea (WA) measurements and the procedures for the IR spectroscopicmeasurementswere described e l s e ~ h e r e . ~The ~ ~angle ~ 3 of incidence of the IR beam was set to 30",and unpolarized radiation was used (i.e., only qualitative information about the tilt angle can be inferred from the data). The experimentalprocedure included discontinuous compression of the respective monolayer and a record time of approximately 4 min (2000 scans)for each state of compression. The accuracy of the surface pressure measurements was 0.1 mN/m.

Results and Discussion In Figure 1, the WA isotherms for the enantiomeric and racemic N-octadecanoylserine methyl ester (OSME) surface films are shown (297.15 K). The curves, which are largely in accordance with the isotherms presented by Stine et al.14and Harvey et al.,19 reveal remarkable differences between the racemic and the enantiomeric films, while the curves of the two enantiomeric films are identical (not shown). However, at the beginning of the liquid-expandedfliquid-condensed(LE/LC) region ( ~ 0 . 5 6 nm2/molecule) of the racemic film, a small maximum in the WA isotherm is discernible. This maximum disappears if the rate of the compression is reduced from 0.05 nm2molecule-' min-'(2 h for the whole isotherm) to 0.01 nm2molecule-I min-l. Stine et al.14also used the slowest available compression rate (0.04 nm2 molecule-' min-') for the racemic film a t this temperature, while in all other cases a 4.5 times higher compression rate was chosen (but they did not comment on these compression rates). They investigated the relaxation behavior of the domain structures of the racemic film (rate of compression 0.15 nm2molecule-l min-') a t 0.50 nm2/molecule (Le. close to the beginning of the plateau region) for a period of 7 min. Initially, a fractal growth of LC domains was observed, and after e 3 0 s, clockwise and counterclockwise hooks were formed. A slower compression rate (0.04 nm2

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Figure 3. Comparison of the antisymmetric methylene stretching vibration for racemic and enantiomeric N-octadecanoylserine methyl ester monolayers at the aidwater interface for different areas per molecule (spreading solvent CHCl3). The inset is an enlarged part for small areas per enantiomeric film, 293 molecule: (W) racemic film, 293 K, (0) K (0)racemic film, 297 K ( 0 )enantiomeric film, 297 K.

molecule-l min-') immediately led to a domain structure, which was found for the higher compression rate aRer a n expansion and a second compression of the surface film only. The maximum in the WA isotherm observed for the higher compession rate used herein (Figure 1)presumably can be attributed to less-ordered domain structures, which upon relaxation approach a n optimum arrangement of the film-forming molecules, implying a decrease in surface pressure (Stine et al.14showed that the surface pressure decreases during this conversion). At compression rates 10.04nm2 molecule-l min-', the compression rate is slow enough to give the film the chance to relax during the compression procedure. This example shows that for the separation of racemic monolayers in enantiomeric domains the relaxation time, and therefore the rate of compression, is a n important parameter. At 293.15 K the WA isotherms for the enantiomeric and racemic monolayers are indicative of a condensed film ~ t r u c t u r e ,although '~ the isotherm for the racemic film is still slightly shifted to higher areas per molecule (not shown). In Figure 2 the structures of N-octadecanoyl-L-serine methyl ester and the corresponding D-enantiomer are displayed, while in Figure 3 the wavenumbers of the antisymmetric methylene stretching vibration (v,(CH)2) are compared for the enantiomeric and the racemic surface films a t 293 and 297 K for different areas per molecule. It is generally accepted that the wavenumbers of the symmetric and antisymmetric methylene stretching vibrations of long-chain hydrocarbon molecules are conformation-sensitive and that they can be correlated empiricallywith the order (i.e., with the trandgauche ratio) of the hydrocarbon chain as follow^:^^-^^ Lower wave(24) Snyder, R. G.; Hou, s. L.; Krimm, s.Spectrochim. Acta Part A 1978,34, 395. (25) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J . Phys. Chem. 1982, 86, 5145. (26)MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984,88, 334.

Gericke and Hiihnerfuss

3784 Langmuir, Vol. 10,No. 10,1994 numbers are characteristic of highly ordered all-trans conformations while the number of gauche conformers (the "disorder" of the chains) increases with increasing wavenumbers and width of the band. It is obvious that the quite strong chiral discrimination as observed for the higher temperature with the help of WA measurements is also reflected by the position of the va(CH2)vibration; i.e., the racemic OSME monolayer in the LE and LELC transition regions exhibits higher wavenumbers for v,(CH2) and thus a lower chain order than the enantiomeric film. However, in the LC region the differences are not significant any longer. For the lower temperature (293 K), basically the same characteristics were found; i.e., the larger wavenumbers of the va(CH2)vibration indicate a lower chain order within the racemic monolayer. The wavenumber differences observed for the racemic and the enantiomeric films are significant but much smaller for large areas per molecule than for the higher temperature and remain significant even for small areas per molecule (see inset in Figure 3). The increasing chain order in the LELC transition region of the racemic surface film (297 K) is a result of the so-called "chiral symmetry breaking" (i.e., the separation in enantiomeric domains, as observed by Stine et al.14) and the better arrangement which can be achieved in enantiomeric domains (in the case of a homochiral behaviol.2'). The small but significant differences between the racemic and the enantiomeric monolayers in the chain order a t the lower temperature (293 K) are in line with the observation of Stine et al.14 that the LC domains of the enantiomeric film exhibit sharper edges Gee.,higher order) than the LC domains of the racemic one. In addition, Stine et al.14found in both cases a small amount ( ~ 5 %of) LE phase (although the temperature is below the triple-point temperature; this nonequilibrium effect was also observed for fatty acid monolayers d d is expected to decrease with time28ag).Therefore, it cannot be excluded that the reduced chain order in the racemic film can also be a result of a higher content of LE phase. However, as it will be shown in a subsequent paper20 concerning a-hydroxy fatty acids in the presence of Pb2+cations, a more condensed enantiomeric monolayer (asindicated by the WA isotherm) will not necessarily lead to a higher chain order (in that case the less condensed racemic monolayer showed less gauche conformers). As the chirality center of N-octadecanoylserine methyl ester is part of the head group region (Figure 2), it is of superior interest to investigate IR spectroscopically the vibrations of the head group components, i.e., especially the vibrations ofthe ester carbonyl group and ofthe amide I and amide I1 bands. Stine et al.14 found distinct differences between the IR spectra of the racemic and enantiomeric bulk compounds (KBr) due to inter- and intramolecular hydrogen-bondformation. At the aidwater interface these hydrogen bonds have to compete with the hydrogen-bond formation between the monolayer and the adjacent water subphase, and as a consequence, in comparison with the bulk compounds, other structures may become dominant. In Figure 4a (293 K) and 4b (297 K) the IR reflectionabsorption spectra ofN-octadecanoyl-L-serinemethyl ester monolayers at the aidwater interface are shown for different representative areas per molecule. The low intensity of the reflection-absorption bands of the methylene stretchingvibrationindicates that the film-forming (27) Andelman, D. J. Am. Chem. SOC.1989,111,6536. (28) Moore, B. G.; Knobler, C. M.; Akamatsu, S.; Rondelez, F. J. Phys. Chem. 1990,94,4588. (29) Qiu, X.; Ruiz-Garcia, J.;Knobler, C. M. Muter. Res. Symp. Ser. 1992,237,263.

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molecules, as already proposed by Stine et al.,14are tilted. The band of the ester carbonyl group in the compressed state is mainly located a t 1739-1735 cm-', while a shoulder is found a t ~ 1 7 2 cm-'. 0 For the higher temperature (297 K) the latter band is slightly more intensive than a t 293 K. The amide I band is in both cases observed

IR Spectroscopic Comparison of Chiral Monolayers

Langmuir, Vol. 10, No. 10, 1994 3785

a t 1626 cm-l, while in the wavenumber range ofthe amide

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I1 vibration several bands occur, with the most intensive ones a t e1562 and 1542 cm-'. The methylene scissoring vibration is represented by a relative broad band at e1473 cm-'. The reflection-absorption band at e1083 cm-' is presumably due to the C-0 stretching vibration of the alcoholic group. In comparison with saturated alcoh o l ~ ,this ~ ~band , ~ ~is quite strong, which indicates that the alcoholic C-0 bond is oriented nearly parallel to the water surface. The interpretation of the amide I band is complicated by the baseline maximum which is normally observed between e1750 and e1400 cm-' (Figure 4a and 4b). This maximum is related to the change ofthe complex refractive index of the HzO substrate in that region and cannot be avoided. This baseline maximum becomes narrower in the presence of a n intensive band, as, e.g., observed for carboxylate monolayer^,^^ and the position of the band can be reliably investigated in the limits of the optical resolution used herein, while the intensity and the shape of the band are influenced more strongly by the optical characteristics of the substrate and the angle of incidence. However, in order to verify the position of the amide I band, we repeated the same experiments on a DzO substrate, where the baseline maximumis shifted to lower wavenumbers and the amide I band is located a t the high wavenumber side of the maximum. As a disadvantage of the shift of the baseline maximum, the amide I1 band region and the methylene scissoring band cannot be investigated. Furthermore, it cannot be excluded that the presence of HDO molecules and a different effective purging of the atmosphere may have influenced the baseline characteristics. However, apart from these basic obstacles it can be concluded that even on DzO substrates the amide I band is quite narrow. The center of gravity of the band shifts from 1626.0 cm-l (HzO) to 1628.4 cm-l (DzO),which may be in part attributed to the shift of the baseline, but it is also in accordance with the theory of the reflection-absorption of monolayers a t the airlwater interfaceeZ3 In conclusion, the experiment on a DzO substrate suggests that the disturbance of the amide I band on HzO substrates is not so strong that the investigation of this band on a HzO substrate becomes misleading. Clearly, it is possible to investigate the amide I band of monolayers even with HzO substrates, but it is recommended to check the position of the band with DzO substrates. In the spectral region of the C=O stretching vibrations the relative intensities of the two components a t e1739 and e1720 cm-l invert; i.e., in the case of a HzO substrate the first one is more intensive, while for D20 substrates the second component (e1720 cm-l) is slightly stronger. This can be a result either of slightly different pH (pD) values or ofthe change of the optical substrate properties. This result shows that caution should be applied when intensities of the carbonyl band are interpreted (see also rep3), because small differences between these two components can also be a result of the effects described above; i.e., they must not necessarily reflect different amounts of unprotonated (e1739 cm-') and monoprotonated carbonyl groups ( ~ 1 7 2 0~ m - l ) A . ~prevalence ~ of one species can only be safely assumed if one band is clearly preferred. In Figure 6a-c the IR reflection-absorption spectra of racemic and enantiomeric N-octadecanoylserine methyl (30) Gericke,A.;Hiihnerfuss, H. In 9th International Conference on Fourier Transform Spectroscopy; Proc. SPIE-Int. SOC.Opt. Eng., Bellingham, 1993; Vol. 2089, p 570. (31) Gericke, A.;Hiihnerfuss, H. In preparation. (32) Gericke, A.; Hiihnerfuss, H. Thin Solid Films 1994, 245, 74. (33) Gericke, A,; Huhnerfuss, H. J. Phys. Chem. 1993, 97, 12899.

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Figure 6. (a, Top) External infrared reflection-absorption spectra for a racemic and enantiomericN-octadecanoylserine methyl ester monolayer in the range 2000-1000 cm-I at 293 K (0.289 nm2/molecule). (b, Middle) External infrared reflection-absorption spectra for a racemic and enantiomeric N octadecanoylserinemethyl ester monolayer in the range 20001000cm-l at 293 K (0.355nmVmolecule). (c,Bottom)External infrared reflection-absorption spectra for a racemic and enantiomericN-octadecanoylserinemethyl ester monolayer in the range 2000-1000 cm-l at 293 K (0.435 nm2/molecule). esters are compared for different areas per molecule (293 K) in the wavenumber range 2000-1000 cm-l. For all states of compression the amide I band of the racemic monolayer a t 1626 cm-' is broadened in comparison with the enantiomeric one (for 0.355 nm21moleculea splitting ofthe band is observable). In particular, in the compressed state (Figure 6c) the ester C=O group of the racemic film appears to be slightly more protonated than the respective group of the enantiomeric film. The interpretation of the

3786 Langmuir, Vol. 10, No. 10, 1994

Gericke and Huhnerfuss hm'Imolccule1

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Figure 7. Externalinfrared reflection-absorption spectra for a racemic and enantiomericN-octadecanoylserinemethyl ester monolayer in the range 2000-1000 cm-l at 297 K (0.355 nm2/ molecule).

amide I1 band region is complicated by the occurrence of several bands, and it cannot be excluded that the less intensive amide I1 band is disturbed by not completely compensated water vapor bands. Therefore, we refrain from interpreting this band. Furthermore, in the case of the racemic monolayer, the C-0 stretching vibration of the alcoholic group is broadened (Figure 6b) or split into two components a t 1095 and 1079 cm-l (Figure 6a,c). At 297 K (Figure 7) the amide I band of the racemic monolayer seems to be less broadened than a t the lower temperature (Figure 6b), but it is less intensive, and a stronger influence of the optical properties of the HzO substrate cannot be excluded. In contrast to a HzO substrate, the amide I band of the racemic film remains slightly broadened on a DzO substrate in comparison with the respective band for the enantiomeric film (not shown). This holds for both 293 and 297 K. It would be premature to derive the head group structure from the present results, because the data set and the IR spectroscopic experience about such monolayers are too small a t this stage. However, some preliminary conclusions can be drawn: A more condensed film structure is expected to favor intra- and intermolecular hydrogen-bond formation of the film-forming molecules at the expense of hydrogen-bond formation between the monolayer and the water subphase. On the other hand, the collapse point at 0.27-0.28 nm2/molecule indicates a less-condensed film structure, and as a result, water molecules should be able to penetrate the surface film. The principal ability of the water molecules to penetrate into the monolayer can be shown by a reduction in the pH value, which leads to a n increasing protonation of the ester carbonyl group and thus to a shift of the vibration frequency of the ester carbonyl group to lower wavenumbers (Figure 8). However, the amide I band remains a t ~ 1 6 2 6cm-l. For pH 4 and in the compressed state, the antisymmetric methylene stretching vibration shifts about 0.3 cm-l to higher wavenumbers in comparison with pH 6. More detailed information about the head group structure can possibly be inferred from the orientation of the alcoholic group. Principally, the alcoholic group may form a n intramolecular hydrogen bond with the ester carbonyl group and with the amide C-0 group, respectively. In the first case, this would result in a six-membered ring, where the alcoholic C-0 bond would be more close to a n orientation perpendicular to the water surface (depending on the overall orientation of the molecule), while in the second case a seven-membered ring is formed, implying an orientation of the alcoholic C-0 bond more parallel to

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the water surface. The intensity of the reflectionabsorption due to the alcoholic C-0 vibration and the partly deprotonated character of the ester carbonyl group appears to be more consistent with the formation of a hydrogen bond between the amide C-0 group and the alcoholic group. If we tentatively accept this hypothesis of a n intramolecular hydrogen-bond formation, it can be assumed that the hydrogen-bond formation is disturbed in the case of the racemic surface film. For a more detailed structural analysis, especially with respect to whether or not an intra- or intermolecular hydrogen bond is formed, additional experiments have to be carried out.

Conclusions For 297 K, strong differences in the alkyl chain order were observed between the racemic and enantiomeric N-octadecanoylserine methyl ester monolayers for large areas per molecule, which are considerably reduced in the compressed state. At 293 K, these differences were much smaller for large areas per molecule, but they remained significant for small areas per molecule. For the enantiomeric surface film the amide I band a t 1626 cm-l is much narrower than the racemic one. The amide I1 region is comparable for both cases. The band which represents the C-0 stretching vibration of the alcoholic group is broadened for the racemic monolayer. The intensity of the band is in both cases indicative of a more or less horizontal orientation of the C-0 bond. On a DzO substrate the position of the amide I band shifts approximately 2 cm-l to higher wavenumbers in comparison with a HzO substrate, and the intensities of the bands representing the ester carbonyl group (unprotonated or protonated species) are slightly changed for the DzO substrate. However, in spite of these minor differences, the present data confirm that the amide I band of monolayers a t HzO substrates can be reliably investigated. External infrared reflection-absorption spectroscopy has been shown to be a powerful tool for the investigation of chiral recognition in monomolecular surface films a t the aidwater interface.

Acknowledgment. The authors thank A. Jobman for the preparation of the surface-active compounds and the Langmuir-trough measurements of the surface pressure/ area isotherms.