Langmuir 1993,9, 1604-1607
1604
Notes structures of nonequilibrium states of the monolayers. While studies ofsuch metastable monolayersare of interest in their own right, the relationship between the structures of relaxed and unrelaxed monolayers remains to be established. At this time it is not clear that, in general, a relaxed monolayer is in an equilibrium state, so we shall Joseph T. Buontempo? Stuart A. Rice,"t S. Karaborni,: and J. I. Siepmannt continue to use the term relaxed to refer to a monolayer with time-independent properties. Department of Chemistry and The James Franck Institute, Infrared external reflection spectroscopy has often been The University of Chicago, Chicago, Illinois 60637, and used to determine the orientation of molecular adsorbates KoninklijelShell-Laboratorium,Amsterdam (Shell Research on metalsSl4 but it has not heretofore been applied to the BV), P.O.Box 3003, 1003 AA Amsterdam, The Netherlands study of molecular orientation in monolayers on liquids. This technique can also be used to study the internal Received January 15, 1993 conformationsof adsorbed molecules,e.g. the frequencies The macroscopic properties of monolayers of long chain of the antisymmetric and symmetric CHz stretching amphiphile moleculesassembled at the air-water interface motions of the alkane tail of an amphiphile molecule have been studied for nearly a century.lg However, it is provide information about the gauche "defects" in the only recently, by virtue of the application of grazing chain.1617 The coupling of the gauche defect concentration incidence X-ray diffraction (GIXD) to the study of these in the amphiphiles to the molecular packing in the systems,that information has become availableconcerning monolayer is of considerable interest but it has not, with the molecular packing in the several phases and mesotwo exceptions, been explored experimentallyor analyzed phases that such a monolayer s ~ p p o r t s .This ~ short report theoretically. The two exceptions are the seminal experdescribesthe use of a different technique,infrared external imental work of Dluhy, who has studied the internal reflection spectroscopy, to study molecular conformation molecular conformationsin monolayers of phospholopids and collective molecular tilt i n situ in monolayers of adsorbed at the air-water interface,lsm and the Monte heneicosanol (C21HaOH) at the air-water interface. SurCarlo simulations of a sei€-assembled monolayer by prisingly, the experimental data obtained provide strong Siepmann and McDonald,2l who show the existence of a qualitative evidence that the collective tilt ordering of correlation between the concentration of conformational heneicosanol molecules is different in relaxed and unredefects and the magnitude of the collectivemolecular tilt. laxed monolayers. This inference is supported by the We have constructed a sensitive infrared external results of molecular dynamics simulations, also described reflection spectrometer suitable for the in situ study of in this report. The experimental data and the results of monolayers of long chain amphiphiles at the air-water the simulations support the inference that the very slow interface.222s We have also developed an extension of the relaxation of the surface pressure which occurs when theory of infrared reflection spectroscopy of anisotropic compression of a high density monolayer is stopped is due films and a model which permits the determination of to the internal ordering of the long chain molecules coupled molecular orientation in such a film from measurements with molecular reorientation. of the external reflectivity of polarized light (without the It has been known for many years that it is very difficult need for transmission measurements).24 Our apparatus, to generate an equilibrium state of a Langmuir m~nolayer.~ which uses a novel nonimaging collector to enhance the For example, in 1939Nutting and Harkins observed that signal-to-noiseratio, is described else~here.~~1~3 We have the rate of compression used had an influence on the form chosen an angle of incidence close to the Brewster angle and positions of isotherms obtained for monolayers of long since there, for a dielectric medium, the absolute reflecchain alkanoic acids.s More recent investigations both tivity is low and the change in p-polarization reflectivity confirm this observation and show that a continuously compressed monolayer will noticably relax when the (9) Allera, D. L.; Nuzzo, R. G.Langmuir 198S,l, 52. compression is stopped.- Since almost all of the reported (10) Allera, D. L.; Swalen, J. D. J. Phys. Chem. 1982,86, 2700. GIXD studies of Langmuir monolayers have been carried J.Phys.Chem. (11) Umemura,J.;Kamata,T.;Kawai,T.;Takenaka,T. out without allowance for complete relaxation following 1990,94,62. (12) Kawai, T.; Umemura, J.; Tanaka, T. Langmuir 1990,6,672. compression, they are likely to be concerned with the Differences in the Structures of Relaxed and Unrelaxed Langmuir Monolayers of Heneicosanol: Dependence of Collective Molecular Tilt on Chain Conformation
t The University of Chicago.
t Koninklije/Shell-Laboratorium. (1) Adamson,A. W.PhysicalChemietryofSurfaces; Wiley: NewYork, 1990; Chapter 4. (2) Knobler, C. M. Adu. Chem. Phys. 1990, 77, 397. (3) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (4) See, for example, Wolf,S. G.;Deutsch, M.; Landau, E. M.; Lahav, M.; Leiserowitz, L.; Kjaer, K.; Ale-Nielesen, J. Science 1988,242,1286. Lin, B.; Shih, M. C.; Bohannon, T. M.; Ice, G.E.; Dutta, P. Phys. Rev. Lett. 1990,65,191. Barton, S.W.; Goudot, A.; Boulasea, 0.;Rondalez, F.; Lin, B.;Novak, F.;Acero, A.; Rice, S.A. J. Chem. Phys. 1992,M,1343. (6)Nutting, G.C.; Harkins, W. D. J . Am. Chem. SOC.1939,61,2040. (6) Bois, A. G.;Ivanova, M. G.;Panaiotov, I. 1. Langmuir 1987,3,216. (7) Bois, A. G.;Panaoitov, I. I.; Baret, J. F. Chem. Phys. Lipids 1984, 34,265. (8)Bois, A. G. J. Colloid Interface Sci. 1985, 105, 124.
0143-7463/93/2409-1604$04.Ool0
(13)Rabolt, J. F.; Burns, F. C.; Schlotter,N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78,946. (14) Song, Y. P.; Petty, M. C.; Yarwood, J.; Feast, W. J.; Tsibouklis, J.; Mukherjee, S.hngmuir 1992,8, 267. (15) Snyder, R. G.;S t r a w , H. L.; Elliger, C. A. J. Phys. Chem. 1982,
86. - - ,5145.
(16) MacPhail, R. A.; Straws, H. L.; Snyder, R. G.;Elliger, C. A. J . Phys. Chem. 1984,88,334. (17) Casal, H. L.; Cameron,D. G.;Mantach, H. H. Can.J . Chem. 1983, 61, 1736. (18) Dluhy, R. A.; Reilly, K. E.;Hunt, R. D.; Mitchell, M. L.; Mautone, A. J.; Mendelsohn, R. Biophys. J . 1989,66,1173. (19) Hunt, R. D.; Mitchell, M. L.; Dluhy, R. A. J. Mol. Struct. 1989, 214, 93. (20) Mitchell, M. L.; Dluhy, R. A. J. Am. Chem. SOC.1988,110,712. (21) Siepmann, J. I.; McDonald, I. R. Mol. Phys. 1992,75,265; in press. (22) Buontempo, J. T.; Rice, S. A. J. Chem. Phys., in press. (23) Buontempo, J. T.; Rice, 5. A. Appl. Spectrosc. 1992,46, 726. (24) Buontempo, J. T.; Rice, S.A. J. Chem. Phys., in press.
0 1993 American Chemical Society
Notes
Langmuir, Vol. 9, No. 6, 1993 1605
14 13 12 11 10 9 8
7 6 5 4
3
2 1 0 19
20
21
22
23
24
25
26
K2/molecule
Figure 1. Compression isotherm for a heneicosanol monolayer at 20.03 f 0.02 "C. The dashed line represents the presumed phase boundary. The insert shows the corresponding compression (0)and expansion ( 0 )isotherms at the same temperature. Each point along these isothermsrepresentsa fully relaxed state of the monolayer. (parallel to the plane of incidence) with change in molecular orientation is substantial. For any particular infrared resonance, the ratio of the normalized p-polarized reflectivity to the normalized s-polarized reflectivity (perpendicular to the plane of incidence) yields information about the orientation of the molecular transition moment relative to the normal to the surface. To obtain sufficient light throughput for a useful signal-to-noise ratio (with the optics available to us), the aperture in front of the globar source had to be maintained large enough that the incident beam could not be collimated to better than 2" (divergence or convergence). Unfortunately, near the Brewster angle the influence of this small deviation from perfect collimation on the inversion of the experimental data leads to considerable uncertainty in the absolute value of the collective tilt," with the consequence that only qualitative information concerning the collective tilt of the molecules in the monolayer can be obtained from the current data. However, since the angle of incidence and the incident beam collimation are fixed experimental parameters, systematic changes in the collective tilt as a function of surface coverage are not obscured by the uncertainty in the calculation of the absolute magnitude of the collective tilt. We have invested considerable effort in generating relaxed states of monolayers of pure heneicosonal on water.22v25With apparatus and methods described elsewhere we have achieved stability of the surface pressure, a t constant temperature (h0.02 "C), of h0.05 dyn/cm for as long as 10 h (after which measurement of the surface pressure was stopped). It was often the case, particularly in the high density region of the monolayer phase diagram, that relaxation to the stationary state surface pressure following a compression of 0.1 A22/moleculerequired 2 h. The isotherms we have obtained typically show a reproducible small hysteresis, implying that the monolayer is in some long lived metastable state, e.g. a "glass" state, rather than in an equilibrium state; in a few instances the isotherms obtained are completely reversible. Figure 1 shows the compression isotherm for heneicosanol a t 20.03 f 0.02 "C; the insert shows both the expansion and compression isotherms for the same temperature. The upper end of the range of surface pressure shown is the (25) Buontampo, J. T.; Novak, F. Reo. Sei. Instrum., in press.
limit above which the monolayer appears to collapse; although a relaxed steady state pressure could be achieved when the area per molecule was less than that a t this limiting point, that pressure was independent of the area per molecule. The isotherm displayed was obtained by application of successive small compression-relaxation cycles;several days were required to traverse this isotherm. For surface pressures greater than shown, there is a discontinuity in slope (a kink) above which the isotherm rises very steeply. GIXD studies of this system,2ein which the isotherm shown is traversed in a few hours, and in which the surface pressure is held constant at a particular value by continuous compression to compensate for relaxation, unambiguously show that in the very low pressure region (tothe right of the dashedline in the figure) the monolayer is composed of ordered islands in which the collective tilt of the amphiphile molecules is about 18" and that along the limb in which the surface pressure increases as the area per molecule decreases (to the left of the dashed line in the figure) the collective tilt of the amphiphile molecules decreases continuously to nearly 00.
We show in Figure 2a the frequencies of the symmetric and antisymmetric CH2 stretches, and in Figure 2b the corresponding ratios of the CH2 absorption intensities perpendicular to and parallel to the plane of incidence, as functions of the area per molecule; both data sets were taken along the isotherm shown in Figure 1. The change in peak frequency of the antisymmetric stretch as a function of area per molecule is consistent with continuous reduction of the gauche defect concentration as the area per molecule is decreased, while the insensitivity of the polarization ratio to area per molecule implies that there is no change in the collective tilt of the amphiphile molecules as the area per molecule is decreased. Clearly, our interpretation of these data differs from the interpretation of the GIXD data from unrelaxed monolayers of heneicosanol. However, our interpretation is consistent with the results of recent GIXD studies2' of fully relaxed monolayers of behenic acid on water at 20 "C, at which temperature the monolayer isotherm is like that shown in Figure 1. The results obtained indicate that a t zero surface pressure the behenic acid molecules have a collective tilt of 33", which decreases to only 28" as the area per molecule is decreased to the kink in the isotherm. Our deductions from the experimental observations reported are supported by the results of molecular dynamics simulations. As a continuation of previous theoretical studies,%molecular dynamics simulations have been used to study the correlation between collective tilt and conformational disorder in a model monolayer of amphiphile molecules. The model amphiphile molecules contain 19 pseudoatoms, each representing an internal methylene group, a terminal methyl group, or a carboxylate head group. Amphiphile-amphiphile interactions were represented using an anisotropic united atom model that accounts implicitly for the hydrogen atoms bonded to the carbon atoms; water-amphiphile interactions were represented using external potentials that do not constrain the head groups to the interface, that allow methylene segments to enter the water, and that define a nonzero width interface similar to the observed air-water interface%. (26) Shih, M. C.; Bohannon, T. M.; Mikrut, J. M.; Zschack, Dutta, P. J. Chem. Phys. 1992,97,4485. (27) Schloeman, M. L. Private communication. (28) Karabomi, S.;Toxvaerd, S. J. Chem. Phys. 1992,96,5505; 1992, 97,5876. Karabomi, S.; Toxvaerd, S.; Olsen, 0.H. J. Phys. Chem. 1992, 96,4965. Karaborni, S. Langmuir, in press.
Notes
1606 Langmuir, Vol. 9,No. 6, 1993 0 1
t-
9
6
m
e % 092
e W
4 '"18 U
20
22
Area molecule
26O
(A2)
Figure 3. Mean collective tilt (0) and mean fraction of trans conformations (m) as a functionof surfacecoverage at 27 O C . The
2849.0 2848.6
24
1
continuous line represents the collective tilt inferredfrom X-ray reflectivitymeasurements of monolayers of Cl&I&OOH." Note that the difference between the simulated and experimentaltilt angles grows with increasing area per molecule and appears correlated with the decreasing fraction of trans conformations.
1
12
n
6.0
1
i 0
3.01
19
'
I
20
'
'
21
'
'
22
I '
I
'
23
'
I
24
'
I
25
'
J
26
i2/ molecule Figure 2. (a, top) CH2 antisymmetric stretch frequency (top) and symmetricstretchfrequency (bottom)as a functionof surface
coverage for p-polarization( 0 )and e-polarization(0). (b,bottom) Dichroic ratios as a function of surface coverage for the CH2 antisymmetric ( 0 )and symmetric (0) stretching modes (at the respective peak frequencies).
Eight NVT simulations were carried out a t 27 "Cin the surface coverage range one molecule per 18.5 A2 to one molecule per 25 A2. Figure 3 displays the calculated mean collective tilt angle and the mean fraction of trans state conformations, as well as the collective tilt angle determined from X-ray reflectivity m e a s ~ r e m e n t sas , ~ func~ tions of the area per molecule. For surface coverages greater than one molecule per 20 A2 the simulations generate a monolayer with sensibly untilted molecules, in good agreement with the experimental data. However, for the monolayers with lower surface coverages, there is a systematic difference between the tilt angles calculated from the simulations and those estimated from the experiments. This difference grows with decreasing surface coverage. Note also that the simulation results hint that the collective tilt may converge to a constant value for area per molecule larger than, say, 23 A2. The difference between the simulation tilt angles and the experimentally inferred tilt angles appears to be correlated with the concentration of gauche defects (see Figure 3). (29) Kjaer, K.; Ala-Nielsen, J.; Helm, C. A.; Tippman-Krayer, P.; Mohwald, H.J. Phya. Chem. 1989,93, 3200.
-
0
100 200 300 Time (picoseconds1
Figure 4. Running average collectivetilt angle (0) and fraction of trans conformations (B) at 22 A2 per molecule and 47 O C as a function of time. The origin of the time scale coincides with the end of the expansion of the monolayer from 20A2per molecule to 22 A2per molecule. During the first 12 ps following cessation of expansion, the collective tilt rapidly increases; as more and more gauche defects are slowly introduced into the amphiphile chains (relaxationtime large compared with 12 ps) the collective tilt decreases.
We note that the tilt angles obtained from the X-ray measurements are calculated assuming the amphiphile molecules are in the all trans state, which assumption is certainly not supported by the simulation results. We have also examined the time evolution of the mean fraction of trans conformations and of the mean collective tilt following expansion of the model monolayer described above; these molecular dynamics simulation were carried out a t 42 "C. As shown in Figure 4, the results obtained suggest that the monolayer initially responds to expansion by a rapid increase in collective tilt. We also observe that on a much longer time scale there is an increase in the concentration of gauche defects which leads to a slow decrease in collective tilt. The source of the coupling of tilt with gauche concentration is the following: An increase in the concentration of gauche defects results in an increase in the effective diameter of the tail of the amphiphile molecule which, for a given surface coverage, favors a less tilted packing of the molecules. A similar couplingbetween collective tilting and density of gauche defects is found in Monte Carlo simulations of self-assembled monolayers.21 Of course, given the limited system size and the use of cyclic boundary conditions, a molecular dynamics simulation cannot give an accurate description of very slow relaxation processes in macroscopic systems, so it is not possible to assert that the time-dependent structural changes found in the simulations necessarily model those
Notes
observed on a much longer time scale in real experiments. In this sense the Monte Carlo simulations mentioned provide important complementary information, since the methodology used provided efficient sampling of the molecular conformational phase space, and thereby accounted for those processes which evolve most slowly on the natural time scale. Perhaps one can account for the difference in relaxation time scales observed in real monolayers and in the simulations by supposing that in a real monolayer the domains adjacent to the movable barrier respond first (relatively quickly) to a change in surfacearea per molecule, which response then propagates slowly to the boundaries of the system by quasihydrodynamic processes. We believe that the results of the computer simulations support the suggestion that the equilibration of the gauche defect concentration is the slow step in the relaxation of a monolayer in response to a change in area per molecule. Our experimental studies of the symmetric and antisymmetric CH2 stretching frequencies as a function of surface coverage and temperature do not reveal the presence of a liquid phase. In this way the long chain alkanol monolayer differs from the phospholipid monolayers studied by Dluhy. We find that even at sensibly zero surface pressure and low surface coverage the long chain alkanol is in a mesophase with collective tilt order, possibly as a heterogeneous assembly of islands without any other phase present, and possibly coexisting with a disordered phase that we cannot detect. The island formation inferred from the infrared and the GIXD experimental data4 is found in molecular dynamics simulations of the model monolayer described earlier in this report, e.g. at 0 OC when the surface coverage is 40 A2per molecule. The average lattice spacingin these islands is found to be 5.04 A, the same as the lattice spacing in the homogeneous monolayer with area per molecule of (30) See, for example, Harris, J. G.;Rice, 5.A. J. Chem. Phys. 1988, 89,6898. Shin, S.; Collazo, N.; Rim, S. A. J. Chem. Phys. 1992,96,1362.
Bareman, J. P.;Klein, M.In Atomic Scale Calculations in Materials Science; Ternoff, J., Vanderbilt, D., Witek, V., Ede.; Material Regearch Society Symposium Seriea No. 141; Elsevier: New York, 1989; p 411.
Langmuir, Vol. 9, No. 6,1993 1607 22 A2. Island formation has also been found in other molecular dynamics simulationsm and in Monte Carlo simulations.n We suggest that our experimental data and our Molecular Dynamics simulations strongly support the inference that the packing structures of relaxed and unrelaxed monolayers, at the same temperature and area per molecule, can be different. One way to rationalize this differenceis to adopt a kinetic model in which the collective tilting of the molecules can respond to a change in area much more quickly than can the gauche conformer concentration.22 Certainly, the results of the molecular dynamics simulations support the suggestion that the equilibration of the gauche defect concentration is the slow step in the relaxation of a monolayer in response to a change in area per molecule. This model is consistent with the analysisof Shinand RiceF1who show that uniaxial stress on a monolayer of rodlike molecules can induce a tilting transition, and with the experiments of Lin et al.,S2 who observed this effect. Thus, when intramolecular gauche-trans equilibration cannot be achieved on the time scale of the change in monolayer area, the stress associated with the change in area is relieved by generating a collective tilt of the molecules. But if the change in area of the monolayer is carried out slowly enough that there is time for the internal conformations of the long chain molecules to equilibrate, the initial collective tilt of the molecules changes very little.
Acknowledgment. The part of this research carried out at the University of Chicago was initially supported by a grant from the Petroleum Research Fund and later supported by a grant from the National Science Foundation. J.T.B. and S.A.R. have also benefited from the w e of facilities provided by the National Science Foundation for materials research at the University of Chicago. (31) Shin, 5.;Rice, S . A. J. Chem. Phys. 1990,92, 1495. (32) Lm,B.; Peng, J. B.;Ketbraon, J. B.; Dutta, P.; Thomas,B. N.; Buontempo, J.; Rice, S . A. J. Chem. Phys. 1989,90,2393.
