Langmuir 1989, 5 , 531-533
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derived from dynamic surface and inferfacial tension measurements by using the dynamic drop-volume method.6
(research assistant). P.J. is indebted to the Akademie der Wissenschaften der D.D.R. for an invitation to the Zentralinstitut fur Organische Chemie in Berlin.
Acknowledgment. J.V.H. is indebted to the Belgian National Fund for Scientific Research for a research grant
Registry No. Brij 58, 9004-95-9; SDS, 151-21-3; sodium chloride, 7647-14-5; n-hexane, 110-54-3.
Letters Demonstration of a Two-Dimensional Mesophase in Polypeptide Films at the Air-Water Interface R. H. Tredgold and R. Jones* Department of Physics, University of Lancaster, Lancaster, England LA1 4YB Received October 14, 1988. In Final Form: November 30, 1988 Surface pressure changes and surface pressure anisotropy have been studied in compressed Langmuir films by viewing ripples on the water surface generated by a vibrating probe. The expected dependence of ripple wavelength on surface tension has been verified for isotropic films of arachidic acid. Anisotropy has been detected in compressed films of synthetic polypeptides, in the form of elliptical ripple patterns. The anisotropy is associated with the formation of an ordered bilayer with the long molecular axes parallel to the compressing barrier. This simple method should prove useful in studying anisotropy in films of other rigid rod polymers.
Introduction In-plane anisotropy and molecular ordering in Langmuir films have not received much attention until recently despite the fact that these two-dimensional systems are of considerable interest in their own right and might be the precursors for Langmuir-Blodgett (LB) films with useful electronic or optical properties. Monomeric materials have been studied directly as spread monolayers by using sophisticated optical techniques such as multiple pass absorption, resonance Raman spectroscopy, second harmonic generation, or ellipsometry.1$2 Polymeric materials offer much more scope for molecular ordering in monolayers; for example, synthetic polypeptides adopt an a-helical conformation under suitable conditions, giving rigid rods which are known to form lyotropic mesophases in the bulk3 and rigid Langmuir films which can be orientated by the compressing b a ~ ~ i e r .Several ~ , ~ previous studies have hinted a t the possibility of forming two-dimensional mesophases with polymer monolayers." However, they used indirect evidence from collapsed films or LB films. A novel method for studying this ordering in Langmuir films occurred to us which is both simple and direct. I t is based on the fact that the wavelength of water ripples generated by a vibrating prove is dependent on surface tension, which in turn is expected to be anisotropic in the ordered, rigid polymeric systems of the kind under con(1)Mobius, D.; Orrit, M.; Gruniger, H.; Meyer, H. Thin Solid Films 1985,132,41. (2)Rasing, T.; Shen, Y. R.; Kim, M. W.; Grubb, S.; Bock, J. Springer Ser. Opt. Sci. Laser Spectrosc. 1985,49,307. (3)Dupr6, D. B. J. Appl. Polym. Sci. 1985, Applied Polymer Symposium 41,68. (4)Malcolm, B. R.Proc. R. Soc., London A 1968,305,363. (5)Jones,R.; Tredgold, R. H. J. Phys. D AppZ. Phys. 1988,21,449. (6)Orthmann, E.; Wegner, G. Angew. Chem.,Int. Ed. Engl. 1986,25, 1105.
0743-7463/89/ 2405-053l$01.50/0
sideration.4~~Thus molecular ordering should be immediately apparent as a deviation from circularity of ripple patterns. Water waves are classified as gravity waves or capillary waves according to whether their phase velocity is dominated by mass transport or surface tension effects.8 For wavelength X 5 5 mm, surface tension effects dominate, and Kelvin's equation may be simplified to
where S is the surface tension, n is the frequency of ripple formation, and p is the density of water. This is a linearized solutim for small-amplitude capillary waves. However, Crapper8$has provided an exact solution for the nonlinear case of waves of finite height, which predicts wave crests that are broad and rounded and troughs that get progressively sharper with increasing amplitude. Waves of this form have been experimentally verified and for our purpose have the useful property that the dispersion relation between velocity and wavelength for the linear system holds with only a small correction factor up to very large values of amplitude and wave steepness (ref 8, section 5.3). Thus, measuring the wavelength of ripples of an amplitude large enough to be viewed by a simple optical arrangement is a plausible method for studying the surface tension change, and its possible anisotropy, due to a spread monolayer. However, difficulties could arise because (i) vibrations of an amplitude sufficiently large to be viewed might disrupt ordering in the monolayer, since the stability (7) Malcolm, B. R. J. Colloid Interface Sci. 1985,104,520. (8)Kinsman, B. Wind Waoes; Prentice-Hall: Englewood Cliffs, NJ, 1965. (9)Crapper, G. D. J.Fluid Mech. 1957,2,532.
0 1989 American Chemical Society
Letters
532 Langmuir, Vol. 5, No. 2, 1989
Results and Discussion Light- t i g h t box 50"
fA.8 lens
compressing barrier
I I'
Langmuir trough
ID -
1
I
Figure 1. Schematic diagram of the arrangement for viewing ripples on a Langmuir trough: S, stroboscopic light source; M, lightly aluminized mirror; V, vibrating probe; P, pressure sensor (phosphor bronze strip); D, dark background for viewing ripples (silicon plate).
of Langmuir films is generally acknowledged to be vibration-sensitive, and (ii) eq 1 shows that X is rather insensitive to changes in S and so must be measured accurately t o be of use. We show in this letter that measurements of reasonable accuracy are possible without disrupting the monolayer structure and t h a t interesting anisotropic properties of polypeptide films can indeed be studied in this way.
Experimental Section Polypeptide monolayers were studied essentially as previously de~cribed.~ The polypeptides were poly(y-benzyl L-glutamate) (PBLG) (molecularweights 28000 and 260000) and poly(?-methyl L-glutamate) (PMLG) (molecular weight 46 000) from Sigma Chemical Co. Spreading solutions were made up using dichloromethane; in the case of PMLG the solid polymer was first dissolved in a small amount of dichloroacetic acid. Arachidic acid (also from Sigma) was dissolved in ethyl acetate and used as a comparison material, which was expected to give isotropic monolayers. The Langmuir trough was an alLpoly(tetrafluoroethy1ene) design with a single compressing barrier and a pressure-sensing mechanism based on the lateral displacement of a phosphorbronze strip aligned parallel to the moving barrier.5s'0 This arrangement means that only the component of force perpendicular to the barrier (Le.,parallel to the direction of compression) is detected. Conventional surface pressure readings with rigid films are generally unreliable and dependent on the orientation of the Wilhelmy Monolayers over a distilled water subphase were slowly compressed once only, surface pressure readings and ripple photographs were taken simultaneously,and then the trough was cleaned thoroughly before spreading another monolayer. The arrangement for generating and viewing ripples is shown schematically in Figure 1. Good results were obtained with a metal probe of circular cross section (-0.5-mm diameter) immersed -3 mm into the subphase and attached to a Gearing and Watson GWV2 vibrator unit driven at -60 Hz by using the oscillator of a Brookdeal lock-in amplifier. Light from a stroboscopic source adjusted to this frequency, giving a stationary ripple pattern, was reflected vertically down onto the ripples by a lightly aluminized diagonal glass plate and then passed vertically s at f/1.8) up through the plate to be viewed or photographed against a dark background on the bottom of the trough. By experimenting with probe shape, depth of immersion, amplitude of vibration, and optimizing illumination conditions, we could observe good ripples. The vibrator power was turned up just sufficiently to give clearly visible ripples without the distortion and breakup of the simple circular pattern, which occurred at higher power levels. At these moderate power levels, interference effects due to reflection from walls of the trough were minimal despite the small width (-6 cm). Ripple photographs were measured under a low-power microscope to determine wavelength. (10)Malcolm, B. R.;Davies, S. R. J. Sci. Instrum. 1965, 42, 359.
Slow compression of a monolayer of arachidic acid from 0 t o 50 m N m-l (corresponding to a decrease of surface tension from 73 to 23 m N m-l) shows an obvious decrease of ripple wavelength, but the ripples remain perfectly circular (Figure 2a,b). This confirms that there is no significant anisotropy of surface pressure for the fatty acid monolayer, as expected. A plot of ripple wavelength against surface tension derived from pressure sensor readings confirms the cube root dependence of eq I, at least over the accessible range of surface tension (Figure 3 ) . It is interesting that a plot of eq 1 for p = 1000 kg m-3 and n = 58 Hz, the vibrator frequency used for the experimental points (dotted line), shows a large discrepancy, but a plot a t exactly twice this frequency (continuous line) shows a good fit. This indicates that the ripples are not propagated exactly in phase with the vertical motion of the probe but are initiated as a discontinuous disturbance each time the probe changes direction. The random errors in measuring X and S are both of the order &3%. In addition, there are likely to be small systematic errors in Figure 3 due to the frequency setting of the oscillator and estimating the scale of the photographic negatives (very close to 1:l actual size). T h e latter errors might account for the experimental points all lying slightly below the calculated line. On compression of a monolayer of poly(ybenzy1 Lglutamate), the ripple wavelength changes are much less noticeable than for the fatty acid since the surface pressure reaches a maximum value of -30 m N m-l, at which irreversible collapse of the films occurs. However, a t pressures above 10 m N m-l the ripples become distinctly elliptical with the long axis ( a ) perpendicular t o the direction of compression (Figure 2c). This result corresponds t o the infrared dichroism observed by us in films of PBLG formed in this pressure range by the horizontal touching m e t h ~ d Measuring .~ the ratio of the long axis to short axis ( a / b ) as a function of surface pressure in several experiments shows clearly that for PBLG the Langmuir film is isotropic u p to 10 m N m-l and distinctly anisotropic at pressures only slightly above this (Figure 4). This corresponds closely to the pressure at which a plateau occurs in the isotherm associated with an orderly monolayerbilayer phase ~ h a n g e . The ~ , ~exact ~ ~ pressure and flatness of the plateau are somewhat dependent on temperature and molecular eight.^,^ Because the phase change is initiated at the compressing barrier (not uniformly over the surface) and then propagates regularly along the t r ~ u g h , ~ it is important that the ripples are observed close t o the pressure sensor. T h e ratio a / b for the ripples (hence surface pressure anisotropy) rises rapidly over a small pressure range (between 10 and 15 m N m-l), presumably because of the rapidity of the phase change in the vicinity of the vibrating probe and the ordering of the polymer axes parallel to the compressing barrier suggested by other studies, including those of and our recent infrared ~ t u d i e s .In ~ some cases the ripples appear asymmetric, indicating that the monolayer-bilayer transition is crossing the ripple pattern. As the bilayer is compressed further, a n upper limit of a / b -1.14 is reached, and the measured surface pressure rises t o -25 m N m-l before the film becomes completely unstable. At the same time the ripples become less uniformly smooth, with a ragged appearance. This is consistent with further collapse of the bilayer t o thicker regions, as suggested by Malcolm,' but now in a random, patchy manner over the whole surface with the ordering being partly destroyed.
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Letters
Langmuir, Vol. 5, No. 2,1989 533
surfare pressure. mNm/
Figure 4. Anisotropy in surface pressure for Langmnir films of various materials as a function of the surface pressure measured parallel to the direction of compression. The ratio of the long to short axes (alb) of elliptical ripple patterns is used as a measure of anisotropy for compressed films of arachidic acid (A),poly(7-benzyl L-glutamate) (O),and poly(y-methyl&glutamate) (0). The broken line is the result expected for a perfectly anisotropic system, in which the surface pressure pardel to the compression direction is as indicated, and perpendicular to the compression direction is always zero. The data points corresponding to the photographs of Figure 2 are indicated. The effect of brief exposure to high vibrator power is also indicated for one data point for PBLG.
Figure 2. Klppie photographs at a vlbrator frequency of 58 Hz: (a, top) clean water surface; (b, middle) arachidic acid compresd to 50 mN m-'; (e, bottom) poly(y-benzylL-glutamate)(molecular weight 260000) compressed to 14.5 mN m-'. The compression direction is horizontal.
elliptical ripples become almost circular (Figure 4) whilst the pressure recorded by the sensor falls only slightly. This is good evidence that elliptical ripples are due to surface pressure anisotropy arising from an ordered film structure, which can be disrupted by sufficiently strong vibrations. The dotted line in Figure 4 shows the expected ratio a / b as a function of pressure parallel to the direction of compression (i.e., the measured pressure) for a hypothetical perfectly ordered system in which the pressure perpendicular t o this is always zero. It thus appears that on compression the real PBLG system goes from a n unordered system (monolayer) to a well-ordered system (bilayer) t o a less well ordered system (thicker collapsed regions). In our previous work on IR dichroism of LB films: we suggested that the monolayer at 5-10 mN m-l might have order perpendicular to that of the bilayer. This would he indicated by data falling slightly below the horizontal line in Figure 4 over this range, as in fact occurs, hut the accuracy of the measurements is insufficient to provide fiim confirmation on this point. Poly(y-methyl L-glutamate) behaves rather similarly to PBLG except that the ripples only become elliptical at 3C-40 mN (Figure 4), and the film is stable to collapse a t higher pressures. This is consistent with the monolayer-bilayer transition, characterized by a very flat plateau in the isotherm, occurring at a higher pressure than with PBLG (-20 mN m-I), and provides further support for the view that the unordered monolayer gives an ordered bilayer on compression.
Conclusions
surface tension, mNm-l
Figure 3. Comparison of observed and expected ripple wavelengths as a function of surface tension for an isotropic arachidic acid monolayer. The expected dependence of h on S for p = loo0 kg mJ is plotted for n = 58 Hz,the experimental vibrator frequency (---), and for n = 116 Hz,twice the experimental frequency ( e ) .
At this point the compressing harrier is sometimes close to the vibrating prohe, and it is pxsible that the restricted trough area might affect the ripple shape. However, if the vibrator power is briefly tumed up and then down again,
Previous work on molecular ordering in Langmuir films has been limited in scope by being mostly confined to monomeric systems, limited in availability because of the complex optical systems required, or limited in validity because it involved extrapolating from the properties of deposited films. We have shown that a simple method based on the observation of ripples gives a direct picture of anisotropy and ordering in polypeptide films at the air-water interface and thus provides much clearer evidence for the existence of two-dimensional mesophases in such films. Registry No. PBLG, 25014-n-1; PMW, 25086-162;arachidic
acid, 506-30-9.
