Langmuir 1993,9, 3115-3121
3115
Comparison of Different Spreading Techniques for Monolayers at the Air/Water Interface by External Infrared Reflection-Absorption Spectroscopy Arne Gericke, Johannes Simon-Kutscher, and Heinrich Huhnerfuss* Institute of Organic Chemistry, University of Hamburg, Martin-Luther-King-Platz6, 0-20146 Hamburg, Federal Republic of Germany Received March 10,1993. In Final Form: July 14,1993@ The single shot method (SSM) and the discontinuous compression method (DCM) are compared for the film-forming substances 1-hexadecanol, hexadecanoic acid methyl ester, and L-a-dipalmitoylphosphatidylcholine by external infrared reflection-absorption spectroscopy. In particular, the tilt angles of the molecules, the conformational order, and the headgroup structure are analyzed. It is shown that for 1-hexadecanolin the gaseous region different tilt angles are observed between the monolayers spread by the SSM and those compressed by the DCM. In the case of hexadecanoic acid methyl ester the molecules disseminated on the water surface by the SSM are oriented more vertically, the ester carbonyl group is mainly deprotonated, and the hydrophobic chain is conformationally more ordered in comparison with the monolayer compressed by the DCM. Upon relaxation the monolayer spread by SSM approaches the characteristics of the compressed monolayer,although after 80 min this state was not attained completely. L-a-Dipalmitoylphosphatidylcholinemonolayers spread by the SSM relaxate by achieving a higher conformational order, smaller tilt angles, and a higher deprotonation grade of the ester carbonyl groups in comparison with the initial state. A tentative explanation of the present results is given on the basis of a kinetic model recently published by Buontempo and Rice. Introduction Investigations of monomolecular films at the gas/water interface are usually performed by spreading the insoluble surface-active compound with the help of a so-called “spreading solvent”, and subsequently the monolayer is compressed by a movable barrier.l For the influence of the spreading solvent on the properties of monolayers a t the gas/water interface the reader is referred to ref 2. Regarding the spreading and compression techniques the following methods are used: in the case of the single shot method each surface concentration studied is obtained by a single addition to a fixed area of a pure water surface; in the case of the successive addition method the area/ molecule is decreased discontinuously by adding repeatedly well-defined portions of the spreading solution to the existing monolayer; the method of continuous compression, which includes simultaneous force and compression rate measurements, as a result a surface pressure/area isotherm is obtained; the discontinuous compression method, where the barrier is moved to definite positions and the force is observed until it becomes constant. Pallas and Pethica3 discussed the differences between the discontinuous compression method and the “single or one shot” method and compared these techniques with the method of successive additions? They measured IIlA isotherms for the transition region from the liquidexpanded (LE) phase to the liquid-condensed (LC) phase of pentadecanoic acid, and they observed that this region is more flat, i.e., without slope, in the case of the single shot and successive addition methods than applying the discontinuous compression technique. However, Hifeda
* To whom correspondence should be addressed. e Abstract
published in Advance ACS Abstracts, September 1,
1993. (1) Gaines, G. L., Jr. Insoluble monolayers at liquid-gas interfaces; Interscience: New York 1966. (2) Gericke, A,; Simon-Kutacher, J.; Hiihnerfuss, H. Langmuir 1993, 9, 2119. (3) Pallas, N. R.; Pethica, B. A. Langmuir 1986, 1 , 509. (4) Middleton, S. R.; Pethica, B. A. J. Chem. SOC., Faraday Symp. 1981, 16, 109.
and Rayfields obtained identical results both with the singleshot and the continuous compression methods using samples of highest purity. The comparison of these two principal ways to obtain isotherms is of major importance, although the compressionmethod is the method most often used in studying monolayers at the air/water interface. But, on the other hand, for a number of investigations the single shot method or the successive addition method is applied due to instrumental limitations (see, e.g., refs 6-8). The objective of this work is the comparison of the single shot spreading and the discontinuous compressionmethod at a molecular level by external infrared reflectionabsorption spectroscopy, which includes the investigation of the molecular order, the tilt of the molecules, and the protonation/deprotonationratio of the headgroup. For this purpose substances are chosen exhibiting principally different n/Aisotherms: 1-hexadecanol showing a transition from liquid-condensed to solid, hexadecanoic acid methyl ester at room temperature exhibiting a liquidexpanded phase, and L-a-dipalmitoylphosphatidylcholine (DPPC) showing a liquid-expanded/liquid-condensed transition region. Experimental Part Materials. Hexane of analytical-reagent grade (Merck, Darmstadt, Germany) was distilled with the help of a 1.60-m bubble-cap column. Chloroform of analytical-reagent grade (Merck)was used as supplied (in order to prevent polymerization stabilized by ethanol). The purity of the solvents was checked by gas chromatography using a flame ionization detector. The water was deionized and purified by a SeralpurPro 9OC apparatus (Seral,Ransbach, Germany). The water quality was checked by fluid-fluid extraction with hexane followed by gas chromatographic analysis. 1-Hexadecanol of analytical-reagent grade (Merck) was recrystallized from pentane/ethanol (95:5 (v/v)), while hexadecanoic acid methyl ester 99.9% (Sigma, Germany) ( 5 ) Hifeda, Y. F.; Rayfield, G. W. Langmuir 1992,8, 197.
(6) Kawaguchi, M.; Tohyama, M.; Mutoh, Y.; Takahashi,A. Langmuir 1988,4, 407.
(7) McNally, E.; Zografi,G. J. Colloid Interface Sci. 1990,138, 61. (8) Sauer, B. B.; Chen, Y. L.; Zografi, G.; Yu, H. Langmuir 1988,4,111.
0743-7463/93/2409-3115$04.00/00 1993 American Chemical Society
Gen'cke et al.
3116 Langmuir, Vol. 9, No. 11,1993 and L-a-dipalmitoylphosphatidylcholine (DPPC) 99.W % (Fluka, Buchs, Switzerland) were used as received. The DPPC solutions, which were stored at 263 K, were used only once after allowing them to adapt to room temperature. The solutionsfor all surfaceactive compounds were prepared in the concentration range of 2.5 (f0.15)X 10-9 moVL. Methods. The surfacepressurelarea isotherms were recorded with the help of a Lauda FW-2 Langmuir trough (Lauda, Germany) that was temperature controlled in the limits of fO.1 K. The compression velocity was adjusted to 2 X 10-9 nm2/ (molecule min). Before the experiment was started, the barrier was moved to a small area, and the potentially available surface active pollutants were sucked off. Reexpansion and subsequent compression to small areas showed no change of the surface pressure. The external infrared reflection-absorption spectroscopy was performed by a Bruker IFS 66 (Karlsruhe, Germany) spedrometer equipped with a MCT detector and using a modified external reflection attachment of Specac (Orpington, Great Britain) for monolayersat the air/water interface that allowed thermostating the trough (f0.5 K). An incidence angle of 30° was applied. Furthermore, the Happ-Genzel apodiiation function with a resolution of 8 cm-l was used, and the spectra were taken by coadding 2000 scans. The reflection-absorption is defined as -log(R/Ro), where Ro and R are the reflectivities of the pure and the film-covered water surface, respectively. For details about the infrared reflection-absorption experiment at the aidwater interface the reader is referred to refs 9-11. The peak positions of the specific bands were determined by the "center of gravity" method.12 The wavenumbers of the stretching bands calculated according to this method are estimated to be accurate to about f0.1 cm-l. Before startingthe experiment the barrier was moved to a small area, and the potentially available surface-active pollutants were sucked off. 1-Hexadecanol and hexadecanoic acid methyl ester were spread from hexane, while DPPC was spread from chloroform. For the spreading procedure a 5-pL SGE syringe (Weiterstadt, Germany) was used. The areas/ molecule were determined with an accuracy of f0.002 nm2/ molecule. For the compression and the single shot methods the same volumes of the spreading solution were supplied to the surface. In the latter case the appropriate volume of the solution correspondingwith a specificcompression status of the monolayer (i.e., with a specific area/molecule) was inferred from the Langmuir curves of the respective compounds. Then, the barrier was moved to the desired area, and the spreading procedure was carried out.
L mNtm 1
un L: ma/m o h k 1
Figure 1. ILIA isotherm for a 1-hexadecanol monolayer at the &/water interface (continuous compreeeion; temperature 294
K).
0.266
0.245 0
a \ a
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0.231
0 M I
0.223
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Results and Discussion In Figure 1,the II/A isotherm of 1-hexadecanol is shown as a reference, and in Figure 2 representative spectra for different areas/moleculeapplying the discontinuous compression method are displayed. The compression was started at an area of 0.43 nm2/molecule. The main bands of the spectrum are the methylene antisymmetric (around 2918 cm-9 and symmetric (around 2850 cm-') stretching vibrations. The strong upward band around 2300 cm-' is caused by the presence of COZ. The maximum of the baseline around 1670 cm-' is due to a strong change of the complex refractive index of water in that region. It is a typical feature of reflection-absorption spectra measured at the aidwater interface and cannot be avoided. Around 1468 cm-' the methylene bending band is observable. It has been demonstrated for long-chain hydrocarbon molecules that the frequencies of the antisymmetric and the symmetric methylene stretching vibrations are conformation-sensitivedue to perturbation of the stretching vibrations by Fermi-resonance interaction with the me~~
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Figure 2. Infrared reflection-absorption spectra in the range from 3100 to 1400 cm-* for a 1-hexadecanol monolayer at the &/water interface and different areas/molecule (discontinuous compmion; temperature 294 K). The intensity of the reflectiow absorption is indicated as a bar.
thylene bending vibration and that they can be correlated empiridywith the order (Le., with the trans-gaucheratio) of the hydrocarbon chains as follows:13-16 Lower wavenumbers are characteristic of highly ordered all-tram conformations while the number of gauche conformers (the "disorder" of the chains) increases with increasing wavenumbers and width of the band. Furthermore, the methylene bending band isknown to be extremelysensitive to interchain interactions. For a detailed description of the different arrangements associated with the wavenumbers of the bending band, the reader is referred to refs 16-18.
~
(9) Gericke, A.; Michailov, A. V.; Hherfuss, H. Vib.Spectrosc. 1993, 4, 335. (10)Dluhy, R. A. J. Phys.Chem. 1986,90, 1373. (11) Fina, L. J.; Tung,Y. S . Appl. Spectrosc. 1991, 45,986. (12) Cameron, D. G.; Kauppinen, J. K.; Douglas, J. M.;Mantech, H. Appl. Spectrosc. 1982, 36, 245.
(13) Snyder,R. G.;Hou, S. L.; Krimm,S . Spectrochim. Acta, Part A 1978,34,395. (14) Snyder, R. G.;Strauss,H. L.; Elliger, C. A. J. Phys.Chem. 1982, 86, 5145. (15) MacPhnil, R. A.; Strauss, H. L.; Snyder, R. G.;Elliger, C. A. J. Phys.Chem. 1984,88,334.
Comparison of Monolayer Spreading Techniques
Langmuir, Vol. 9, No. 11, 1993 3117
1470.0
[mN I ml ...................
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%
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Figure 3. Wavenumbers of the methylene bending vibration for a l-hexadecanol monolayer spread by the single shot method with respect to the relaxation time (temperature 294 K): -, 0.191 nm2/molecule; 0.268 nm2/molecule (fitted curves).
-
e,
During compression of the monolayer the wavenumber of the antisymmetric stretching vibration of the l-hexadecanol monolayer shifts only slightly, i.e., from 2918.2 (f0.2) cm-l (0.32 nm2/molecule) to 2917.5 (fO.l) cm-l (0.195 nm2/molecule). This implies that the chains of the molecules are well-ordered even in the LC/gaseous coexistenceregion (the state which is characterized by a surface pressure of =zero). The increase in the intensity between 0.266 nm2/moleculeand 0.234 nm2/moleculeindicates that the molecules arise from a tilted to a vertical position (the latter postulate was verified by polarized spectroscopyg). For an area of 0.266 nm2/molecule,the methylene bending band is situatedat 1465.0 (f0.2) cm-', while for 0.192 nm2/ molecule a wavenumber of 1466.5 (f0.2) cm-l is observed. At first glance, this result is surprising, because a wavenumber of 1466.5 cm-l corresponds to a liquid-like, disordered chain. However, an orthorhombic subcell would lead to a doublet at about 1472 and 1462 cm-l.'* Therefore, it is tentatively assumed that the band represents a state which is characterized by a mixture of orthorhombic (slightly tilted) and hexagonal (untilted) packed subcells,which results in a slightly broadened band between 1466 and 1467 cm-' (taking into account the low resolution used herein). If the single shot method is used for spreading a monolayer of l-hexadecanol at areas of 0.266,0.234, and 0.192 nm2/molecule,respectively, in all cases the wavenumber of the antisymmetric stretching vibration is the same as in the case of the discontinuous compressionwithin the experimental errors, while the larger intensities of the bands indicate that for the first two areas the molecules were slightly less tilted in comparison with the compression method. The wavenumbers and the intensities of the bands were absolutelystable during the experiment which took 80 min, i.e., no relaxation with respect to tilt and frequency can be concluded from these data. This result is inconsistent with the conclusion to be drawn from the shift of the methylene bending band which shows a relasation to lower wavenumbersfor the monolayer spread by the single shot method (see Figure 3; the observed values were fitted). This effect implies that a rearrangement in the monolayer occurs, which obviously is so small that it ~~
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(16) Cameron, D.G.;Umemura, J.; Wong, P. T. T.; Mantach, H. H. Colloids Surf. 1982,4, 131. (17) Kawai, T.;Umemura, J.; Takenaka, T.; Kodama, M.; Seki, S. J . Colloid Interface Sci. 1986,103, 56. (18) Weers, J. G.;.Scheuing, D. R. In Fourier transform infrared spectroscopy in collotd and interface science; Scheuing, D. R., Ed.;ACS Symposium Series No. 447; American Chemical Society: Washington, DC, 1991; p 87.
0.0
1
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0.20
0.25
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Figure 4. II/A isotherm for a hexadecanoic acid methyl ester monolayer at the &/water interface (continuous compression, temperature 294 K).
is not reflected by a shift of the conformationally less sensitive wavenumber of the methylene stretching vibration. As a reference, Figure 4 shows the lI/A isotherm of hexadecanoic acid methyl ester, while in parts A and B of Figure 5 the spectra are given for this film-forming substance spread by the single shot method at an area of 0.272 and 0.225 nm2/molecule,respectively. In addition to the bands present in the spectrum of l-hexadecanol, the spectrum of hexadecanoic acid methyl ester shows the carbonyl stretching vibration, which very sensitively responds to relaxation processes. The less tilted molecule normally corresponds with a band of the unprotonated carbonyl group around 1739 cm-l, while the band of the protonated carbonyl group is situated around 1720 cm-l. A t 0.272 nm2/moleculethe compressed monolayer exhibits a slightly broadened band at 1722 cm-l which indicates that only a very small portion of the molecules is deprotonated at the carbonyl group. In contrast to this result, the spectrum of the monolayer spread by the single shot method shortly after the spreading exhibits a strong band at 1738 cm-l, with a small shoulder at 4 7 2 0 cm-l. Then the monolayer obviously relaxes and approaches the state of the compressed monolayer. The decrease in the intensity of the antisymmetric CH2-stretching vibration indicates that the molecules become more and more tilted during the relaxation process (Figure6). In principal, the same behavior is observed for 0.225 nm2/molecule(Figure 5B), with the only difference that also in the case of the compressed monolayer a remarkable portion of the molecules is deprotonated. (Basically, according to its orientation and deprotonation status a band of a deprotonated carbonyl group is more intensive than the band of a protonated one. Therefore, it can be safely assumed that the majority of the molecules are still protonated.) For the single shot method again a strong band of a deprotonated carbonyl group is observed, which upon relaxation of the monolayer changes to a more protonated one, while the intensity of the antisymmetric methylene stretching vibration decreases (Figure 61, and the wavenumber of the bending band shifts slightly to lower wavenumbers (not shown). In Figure 7 the relaxation of the antisymmetric stretching vibration with respect to the wavenumber is shown. While during the first 40 min the band positions for areas of 0.193 nm2/molecule and for 0.272 nm2/moleculestrongly shift to higher wavenumbers, the band shifts for 0.225 nm2/molecule are considerably smaller. A possible explanation for this behavior is that
Gericke et al.
3118 Langmuir, Vol. 9, No. 11,1993 compressed monolayer
1
80 min.
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Figure 6. Reflection-absorption of the antisymmetricmethylene
stretching vibration for hexadecanoic acid methyl ester monolayers spread by the single shot method with respect to the relaxation time (temperature, 294 K): -, 0.193 nma/molecule; ---,0.225 nm2/molecule; 0.272 nm2/molecule(fittedcurves).
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compressed monolayer 80 min.
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10 min.
stretching vibration for hexadecanoic acid methyl ester monolayers spread by the single shot method with respect to the relaxation time (temperature, 294 K): -, 0.193 nm2/molecule; ---,0.225 nm2/molecule; 0.272 nmz/molecule(fittedcwes).
-
e,
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m
m
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Figure 5. (A, top) Infrared reflection-absorption spectra of a
hexadecanoic acid methyl ester monolayer at the air/water interface spread by the single shot method at an area of 0.272 nmz/molecule in the range from 3100 to 1400 cm-1 for different relaxation times. As referencethe spectrumfor the corresponding area applying the discontinuous compression method is given. The intensity of the reflection-absorption is indicated by a bar (temperature, 294 K). (B,bottom) Infrared reflection-absorption spectra of a hexadecanoic acid methyl ester monolayer at the &/water interface spread by the single shot method at an area of 0.225 nm2/molecule in the range from 3100 to 1400 cm-' for different relaxation times. As reference the spectrum for the corresponding area applying the discontinuous compression method is given. The intensity of the reflection-absorption is indicated by a bar (temperature, 294 K).
at higher surface pressures (ie., at 0.193 nm2/molecule) the relaxation process is accelerated, and for 0.272 nm2/ molecule a larger area is available for the relaxation. The latter hypothesis is supported by the resulta shown in Figure 6, because the relaxation velocity as reflected by the change of the tilt of the molecules (intensity of band) is the fastest for an arealmolecule of 0.272nm2/molecule. Application of the single shot method at an area of 0.225 nm21moleculeobviouslyleadsto monolayer characteristics between these two different situations. It can be concluded from parts A and B of Figure 5 that after 80 min neither the tilt angle nor the protonation/deprotonation grade of the compressed monolayer is achieved (the same is valid
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Figure 8. II/A isotherm for a L-a-dipalmitoylphosphatidylcholine monolayerat the &/water interface (continuouscompression; temperature, 295 K).
for 0.193 nm2/molecule,not shown), while the wavenumbers for the antisymmetricmethylene stretching vibration are equal within the experimental error. If DPPC monolayers are spread by means of the single shot method, a different situation is observed (asreference in Figure 8 the IIlA isotherm of DPPC is given), because the molecules relax to a less tilted packing (Figure 9). Furthermore, the carbonyl band upon relaxation approaches a more deprotonated status, although this process is not so pronounced for 0.488 nm2/molecule,because after 10min the band is already situated at 1727cm-' and during
Comparison of Monolayer Spreading Techniques
Langmuir, Vol. 9, No. 11, 1993 3119
m
compressed monolayer
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30 min. 30min.
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Figure 9. (A, top) Infrared reflection-absorption spectra of a L-a-dipalmitoylphoephatidylcholine monolayer at the &/water interface spread by the single shot method at an area of 0.610 nm2/moleculein the range from 3000 to 1400 cm-l for different relaxationtimes. As reference the spectrumfor the corresponding area applying the discontinuous compression method is given. The intensity of the reflection-absorption is indicated by a bar (temperature,295 K). (B,bottom) Infrared reflection-absorption spectra of a L-a-dipalmitoylphoephatidylcholinemonolayer at the &/water interface spread by the single shot method at an area of 0.488 nm2/moleculein the range from 3000to 1400 cm-l for differentrelaxation times. As reference the spectrumfor the corresponding area applying the discontinuous compression method is given. The intensity of the reflection-absorption is indicated by a bar (temperature, 295 K).
Figure 10. (A, top) Infrared reflection-absorption spectra of a L-a-dipalmitoylphoephatidylcholinemonolayer at the &/water interface spread by the single shot method at an area of 0.610 nmYmolecule in the range from 1300 to 900 cm-l for different relaxationtimes. As reference the spectrumfor the Corresponding area applying the discontinuous compression method is given. The intensity of the reflection-absorption is indicated by a bar (temperature,295 K). (B,bottom) Infraredreflection-absorption spectra of a L-a-dipalmitoylphosphatidylcholinemonolayer at the &/water interface spread by the single shot method at an area of 0.488 nm2/moleculein the range from 1300 to 900 cm-1 for differentrelaxationtimes. As reference the spectrum for the corresponding area applying the discontinuom compreesion method is given. The intensity of the reflection-absorption is indicated by a bar (temperature, 295 K).
further relaxation the band shifts to 1735 cm-l. For the area 0.488nm2/moleculethe intensity of the antisymmetric methylene stretching vibration is only slightly enhanced, and it remains at the initial position which is equal to the wavenumber observed for the compressed monolayer (2917.7f 0.2 cm-l). At an area of 0.610 nm2/molecule a wavenumber for the antisymmetric stretching vibration of 2919.0 (f0.3) cm-' is observed for the compressed monolayer, while for the single shot method a frequency of 2921.7 (f0.2) cm-' is encountered during the first 40 min, and then a sudden decrease to the final value of 2920.7 (f0.2)cm-l is taking place (the values for the compressed monolayer within the indicated experimental errors are
the same as reported by Dluhy and Cornelll9). In Figure 10 the region between 900 and 1300 cm-' is shown for the DPPC spectrum. At -1222 and -1090 cm-' the antisymmetric and symmetric POz- vibrations are observable, and the bands a t d 8 0 and at -976 cm-l can be attributed to the antisymmetric ester C-0 stretching vibration and the C-N stretching vibration of the quarternary nitrogen in the choline headgroup, respectively (for the band (19)Dluhy, R. A.; Cornell, D. G. In Fourier transform infrared spectroscopy in colloid and interface science; Scheuing, D. R., Ed.;ACS Sympoeium Series No. 447; American Chemical Society: Weahington, DC, 1991; p 192.
3120 Langmuir, Vol. 9, No. 11, 1993 assignments see ref 20). Basically, the frequency shift of the bands during the relaxation process allows insight into hydratioddehydration processes of the phosphate group. However, in this case the shift of the band cannot be analyzed in detail, because the “center of gravity algorithms” for the determination of the exact band position are less accurate for low intensity broad bands and yield a strong scatter of the data. In spite of this obstacle it can be qualitatively inferred from the change of the intensities that the orientation of the respective groups changes during the relaxation process. This in turn reveals that rearrangements of the headgroup region occur. The spectra obtained for the compressed monolayer of DPPC and for the single shot method after 80 min are still different, as it can be seen from Figures 9 and 10. The present results clearly show that the spreading technique is of vital importance in determining the relaxation process of monolayers a t the aidwater interface. At fiist glance, the dependence of the respective relaxation procedure as expressed by the tilt angle and the number of gauche defects on the initial area available for the spreading of the three film-formingsubstances investigated in this work appears to be partly inconsistent and in part even counterintuitive (e.g., SSM). However, the recent findings by Buontempo and Rice21and by Collazo et al.22 seem to offer a possibility to integrate the present data in a general scheme: Buontempo and Rice21evoked a kinetic model in which the following occur: The collective tilting of the film molecules can respond to a change in area more quickly than can a reduction in the number of gauche conformers. In general, the middle part of the alkyl chain is the most ordered regime of the molecule. If the headgroup is smaller than any of the other groups along the amphiphilic chain, the headgroup region of the chain tends to be less ordered than the tail end region, because the headgroup has more space to move and can thereby easier support gauche conformationsof the chain. As the surface pressure increases, i.e., the area/molecule decreases, the hydrocarbon chain becomes more ordered, and the molecules may retain a collective tilt. If the headgroup is larger than any of the other groups along the amphiphilic chain, the tail end region of the chain tends to be less ordered due to its increased conformational freedom and may contain more gauche defects. Although these conclusions drawn by Buontempo and Rice21have to be verified by a larger data set, they allow a tentative explanation of the present results: The sterical demand of the DPPC headgroup is quite high. According to Collazo et al. one would expect an increased number ofgauche defects at the tailend, because the headgroup will not arrange perfectly directly after the “single shot” application. Upon relaxation, however, the headgroups may reorientate to arrangements with less sterical demand, which in turn leads to a reduction of the gauche conformers and of the tilt. The initial spreading to larger areas/molecule applying the DCM also leads to a larger number ofgauche conformers, but they are typical of films in the LE/gaseous coexistence region and are “squeezed out” upon compression, because the discontinuous compression is slow enough for conformational motions. For 1-hexadecanol the situation is different; i.e., it contains a headgroup that is smaller than any of the other (20) Hunt, R. D.; Mitchell, M. L.; Dluhy, R. A. J. Mol. S t r u t . 1989, 214, 93.
(21)Buontempo, J. T.; Rice, S. A. J. Chem. Phys. 1993,98,5835. (22) Collazo, N.;Shin, S.; Rice, S. A. J. Chem. Phys. 1992, 96,4735.
Gericke et al. Table I. Comparison of Molecular Order, Tilt Angle, and the Protonation/DeprotonationRatio for 1-Hexadecanol, Hexadecanoic Acid Methyl Ester,and DPPC Spread by Means of the Discontinuous Compression Method (DCM) and the Single Shot Method (SSM), Respectively surface active compound
molecular order
tilt angle
rotonationl Beprotonation ratio
1-hexadecanol m.o.mM = m.o.aM t.a.ncM > t.a.=M hexadecanoic m.o.mM < m.o.aM t.a.ncM > t.a.aM p/dncM > P/&M acid methyl ester DPPC m.o.mM > m.o.sM t.a.mM < t.a.wM PIdncM < P/&M
groups along the amphiphilic chain. Furthermore, longchain alcohols are known to form LC as well as higher ordered domains even upon spreading to large areas/ molecule. Since the SSM was carried out near or in the LC or solid region, the available area is quite restricted, and the molecules are forced in an almost vertical position with a high conformational order. Upon relaxation the available space per molecule increases due to a more homogeneous distribution and a stronger tilt. This larger space in turn leads to an increase in conformational disorder (presumably in the head group region), and the molecule relaxes to a more tilted state. In the case of hexadecanoic acid methyl ester the headgroup is principally able to form two conformers with different space requirements. the 2-conformer, in which the alcoholic methyl group continues the all-trans configuration of the long acid alkyl chain, or, alternatively, the E-conformerwith a higher sterical demand. The first conformation leads to an unprotonated carbonyl group and less tilt, while in the second case the majority of the carbonyl groups are protonated, and the molecule is more tilted.23 The SSM gives rise to strong hydrophobic interactions, less tilt, less gauche conformers and an unprotonated carbonyl group, because the initial space is so restricted that the alcoholic methyl group is forced into a 2-conformation. It is assumed that upon relaxation isomerization takes place, i.e., the E-conformer with considerably higher sterical demand is preferred which in turn leads to an increase in tilt and in gauche defects. In particular, this latter result supports the postulate of Collazo et al. that both tilt and conformational order are closely related to the sterical demand of the headgroup in relation to the alkyl chain. Finally, it is worth noting that orientating measurements have shown that the results presented above for the SSM are generally comparable to the results obtained by the successive addition method.
Conclusions External infrared reflection-absorption spectroscopy measurements showed that for 1-hexadecanol, hexadecanoic acid methyl ester, and DPPC monolayers strong differences between the discontinuous Compression method and the single shot method are observed for the tilt angle, conformational order, and headgroup structure (see Table I). In the case of 1-hexadecanol,which exhibits a condensed II/A isotherm, the two spreading techniques mainly give rise to different tilt angles for areas >0.20 nm2/molecule, while the wavenumber of the antisymmetric methylene stretching vibration is the same. For a 1-hexadecanol monolayer a relaxation can only be inferred from the shift of the wavenumber of the methylene bending band. (23)Gericke, A,; Hiihnerfuss, H. Manuscript in preparation.
Comparison of Monolayer Spreading Techniques In the presence of a hexadecanoic acid methyl ester monolayer, a strong relaxation process is revealed by a change of the tilt angle of the molecules, by a wavenumber shift of the antisymmetric methylene stretching vibration toward higher wavenumbers, which implies a less ordered chain, and by the protonation/deprotonationratio of the carbonyl band. Upon relaxationthe monolayer approaches the characteristicsof the compressed monolayer,although after 80 min this state was not achieved completely. The relaxation characteristics of DPPC monolayers spread by the single shot method differ considerably from hexadecanoic acid methyl ester films: The tilt angle of the chains and the frequency of the antisymmetric stretchingvibration decrease, i.e., the conformationalorder increases during an experimental period of 80 min, and the carbonyl band shifta to the more deprotonated species. In summary, the single shot method and the discontinuous compression technique lead to different states of
Langmuir, Vol. 9, No. 11,1993 3121 the monolayer at a molecular level. However, in all cases investigated herein, the monolayers spread by the single shot method relax by approaching the state of the compressed monolayer which implies that this state is thermodynamically more stable. A tentative explanation of the present results is given on the basis of a kinetic model recently published by Buontempo and Rice.21 Our results seem to support the postulate of Buontempo and Rice that both tilt and conformational order are closely related to the sterical demand of the headgroup in relation to the alkyl chain. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft, FRG (Schwerpunktprogramm Methoden der Fernerkundungvon Atmaphire und Hydrosphiire, project OMMI HU 583/1-l), and by the Fraunhofer Gesellschaft FRG (Contract T/RF36/ L0013/L1309 SAXON-FPN).
