Thin Solid Films, 134 (1985) 201 208
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GENERAL FILM BEHAVIOUR
STUDIES OF THE FLOW OF MOLECULAR MONOLAYERS DURING COMPRESSION AND THE EFFECT OF A PLATEAU IN THE PRESSURE-AREA CURVE* B. R. MALCOLM
Department of Molecular Biology, University of Edinburgh, Edinburgh EH9 3JR (Gt. Britain) (Received May 30, 1985; accepted July 23, 1985)
Surface pressure-area measurements are reported for a number of polymers and for 4-cyano-4"-n-pentyl-p-terphenyl (T15). Additionally the flow of the condensed films has been measured by applying lines of sulphur powder to the surface. This shows whether or not the film is homogeneous, and how it can be affected by a plateau in the pressure-area curve and by a glass plate in the interface. Measurements on poly(7-methyl-L-glutamate) confirm earlier work on 0t-helical synthetic polypeptides. Poly(butyl methacrylate) monolayers are reversible in their behaviour, but are affected by a plate in the interface. T15 shows non-homogeneous behaviour over the range of the plateau in the pressure-area curve, followed by collapse of the film. Some implications of the observations in relation to instrument design and experimental studies are pointed out.
1. INTRODUCTION
Molecular monolayers of m a n y organic materials become condensed when compressed at the air-water interface on a Langmuir trough. Under further compression, in some instances, a transition is observed characterized by a fiat plateau in the pressure-area curve. Two states may then coexist in the surface with different physical properties. Such transitions are often presumed to be homogeneous ~, i.e. they appear to take place uniformly over the surface, although it has been pointed out that they m a y be induced by flow 2. Recent work on a-helical synthetic polypeptides has shown that a transition from monolayer to bilayer, associated with a fiat plateau in the pressure-area curve, starts initially in the region of the barrier and is then propagated across the surface as the monolayer is compressed 3. It appears to be possible that quite often, when monolayers are condensed, transitions are non-homogeneous. Shear effects and obstructions to the flow of the monolayer, as may be caused by a plate in the interface, can then have a considerable effect on its properties. * Paper presented at the Second International Conference on Langmuir-Blodgett Films, Schenectady, NY, U.S.A.,July 1-4, 1985. 0040-6090/85/$3.30
© Elsevier Sequoia/Printed in The Netherlands
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In this paper the work on s-helical synthetic polypeptides is extended to other polymers and to 4-cyano-4"-n-pentyl-p-terphenyl (T 15) which forms very condensed monolayers and exhibits a plateau in the pressure-area curve 4. The method used to observe the flow of the monolayer is an extension of the well-known technique for observing the movement of a monolayer by applying a powder to the surface. To enable quantitative measurements to be made and the flow of the whole monolayer to be observed, regularly spaced lines of sulphur powder are applied to the condensed monolayer through a mask. Their movement can then be measured directly and changes in area obtained by photographing the surface. 2.
EXPERIMENTAL DETAILS
Surface pressures were normally measured using a surface balance based on a flexible metal strip in the interface 5 that measured the horizontal force by means of a displacement transducer 3. This system has a rapid response so that instabilities in the film, associated with collapse, can be more readily observed than if a Wilhelmy plate is used. The trough, 15 cm wide, was constructed of polytetrafluoroethylene with a sheet of black glass fitted in it to observe clearly the lines of powder. The mask used to apply the sulphur was made of transparent plastic with slots 1 m m wide every 1 cm. It was placed about 2 m m above the water surface and lifted offafter the powder had been applied. Changes in the spacings of the lines were measured with a scale mounted above the monolayer on a transparent cover, and changes in the areas defined by the lines were measured by using enlarged photographs of the surface. In other respects the procedures were as described previously 3. A number of s-helical synthetic polypeptides were investigated: poly(y-methylL-glutamate) (PMG), side-chain R - C H / C H z C O O M e ; poly(L-methionine), sidechain R = - C H 2 C H z S C H 3 ; poly(L-norleucine), side-chain R - (CH2)3CH 3. The P M G (molecular weight, 100000) and poly(L-methionine) (molecular weight, 40 000) were obtained from Sigma Chemical C o m p a n y ; poly(L-norleucine) was as used previously 6. Poly(butyl methacrylate) (PBMA) (molecular weight, 320 000) was obtained from Aldrich Chemical Co. Ltd. T15 was kindly provided by Dr. M. F. Daniel (Royal Signals and Radar Establishment, Malvern). The spreading solvent used was chloroform, with the addition of 10 vol.~o dichloroacetic acid in the case of the polypeptides. 3. RESULTS
3.1. Monolayers of polymers The behaviour of the polypeptides followed the general pattern of those already reported3; only the case of P M G will be described in detail for comparison with PBMA. Figure 1 shows the pressure-area curve for the monolayer and how the spacings of lines of sulphur, measured down the mid-line of the trough, change as the monolayer is compressed. Up to line A all parts of the monolayer are displaced, with the lines extrapolating to zero area per m o n o m e r unit. This is therefore a region of homogeneous behaviour. The region between lines A and B corresponds to the formation of the plateau, with a less than proportionate displacement of the monolayer, which is most pronounced in the region furthest from the moving
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FLOW OF MOLECULAR MONOLAYERS DURING COMPRESSION
barrier. At higher compressions the film then reverts to homogeneous behaviour, and the lines become evenly spaced. In the region of line B the monolayer is undergoing a transition, starting at the moving barrier, which then extends across the surface as the monolayer is compressed to (approximately) half its initial area.
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(a) (b) Fig. 1. (a) Surface pressure-area curves for PMG (19°C): - - , monolayer compressed to 5 A z per residue; - - -, monolayer compressed toj ust below 10 ~a per residue and re-expanded. On recompression the pressure follows close to the initial curve. (b) Measurements down the centre-line of the trough showing movements of lines of sulphur powder on PMG as the area is reduced from 20 to 10 A 2 per residue. ,
It will be seen (Fig. 2) that the profile of the flow has a marked curvature, away from the moving barrier. In consequence, measurements of the movement of the monolayer along the centre line underestimate the extent of decrease in area associated with the transition; measurements of the areas involved are more exact. Such measurements have been made for P B M A and T15 with the monolayer divided into five sections (below). The P M G monolayer can be expanded and recompressed, with little decrease in the observed monolayer area, giving rise to what has been described as a hysteresis cycle 7. It is quite clear from Fig. 2(b) that what appears as a uniform process, as judged by the shape of the pressure-area curve, is quite inhomogeneous when the flow is observed. Qualitatively similar results were found for poty(L-norvaline) and poly(L-methionine), although the latter shows an inflection rather than a flat plateau in the pressure-area curve 6. The flow profile of P B M A shows a curvature similar to that of P M G but in this instance there is little hysteresis when the monolayer is expanded, and lines of sulphur return to their initial straight uniformly spaced lines (Fig. 3(a)) except near the sides. To test whether the decrease in area was uniform over the surface, four lines of sulphur, 3 cm
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(b)
Fig. 2. Lines of sulphur powder on PMG: (a) applied at 20 ~2 per residue and compressed to 12.5 ~2 per residue; (b) further compressed to 10 A2 per residue and then expanded to 17.5 ~2 per residue. The moving barrier is on the right-hand side in both photographs.
(a) (b) (c) Fig. 3. Lines of sulphur powder on PBMA : (a) sulphur powder was applied at 22.5/~2 per monomer unit, the monolayer was then compressed to 10 ~2 per monomer unit and expanded to the initial area; (b) sulphur powder was applied at 22.5 ~2 per monomer unit, a glass plate was inserted into the interface and the monolayer was then compressed to 12.5 A2 per monomer unit; (c) the same experiment without a glass plate. The moving barrier is on lhe right-hand side in all photographs. a p a r t , w e r e u s e d to d i v i d e the a r e a i n t o f r a c t i o n s a l , a2,. •., as. T o relate the d e c r e a s e in a r e a to the p o s i t i o n o n the m o n o l a y e r , lines r e p r e s e n t i n g a l , a l + a2, a l + a2 + a3 etc. w e r e p l o t t e d a g a i n s t the ( a v e r a g e ) a r e a per residue, for e i g h t stages of c o m p r e s s i o n f r o m 25 to 11/~2 p e r m o n o m e r unit (Fig. 4). It will be seen that, a l t h o u g h the f l o w profile is c u r v e d , the d e c r e a s e in a r e a is linear a n d h o m o g e n e o u s as j u d g e d by this m e t h o d .
3.2. Monolayersof 4-cyano-4"-n-pentyl-p-terphenyl T h e p r e s s u r e - a r e a c u r v e o f T15 was f o u n d to rise at a b o u t 30 ~2 m o l e c u l e 1 a n d the m o n o l a y e r t h e n b e c a m e c o n d e n s e d , as has b e e n r e p o r t e d in e a r l i e r w o r k 4. W i t h s l o w c o n t i n u o u s c o m p r e s s i o n a p e a k in the p r e s s u r e - a r e a c u r v e was o b s e r v e d , f o l l o w e d by a flat p l a t e a u e x t e n d i n g d o w n to 15 ~ 2 m o l e c u l e 1 (Fig. 5). F u r t h e r c o m p r e s s i o n of the film c a u s e d the p r e s s u r e to rise a n d the m o n o l a y e r b e c a m e i n c r e a s i n g l y u n s t a b l e , as s h o w n by p r o n o u n c e d f l u c t u a t i o n s in the p r e s s u r e a s s o c i a t e d w i t h c o l l a p s e o f e l e m e n t s of the film. In m e a s u r e m e n t s w i t h m o r e r a p i d c o m p r e s s i o n , at 5 ~ 2 m o l e c u l e 1 rain 1, the t r a n s i t i o n p r e s s u r e was raised f r o m 12.5 to 15 m N m 1, a n d to 16 m N m i w h e n m e a s u r e d w i t h a W i l h e l m y p l a t e p a r a l l e l to
FLOW OF MOLECULAR MONOLAYERS DURING COMPRESSION
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Fig. 4. Surface pressure area curve for PBMA (20°C) (lower curve); expansion and recompression follows very nearly the same curve. The upper lines show uniform decrease in area of the monolayer, divided into five sections, initially 3 cm wide, as the area is reduced by the barrier from 25 to 10/~2 per monomer unit, expressed as fractions of the initial area. Fig. 5. Similar measurements to Fig. 4 for T15 (19°C). In this instance the measured areas show appreciable deviations from ideal homogeneous behaviour (indicated by broken lines). the m o v i n g b a r r i e r w i t h a c o m p r e s s i o n r a t e of 15/~z m i n - 1. T h i s is n o t in a g r e e m e n t w i t h e a r l i e r w o r k w h e r e at s i m i l a r h i g h rates o f c o m p r e s s i o n the t r a n s i t i o n was f o u n d at 22 m N m - 1 a n d a s e c o n d c o l l a p s e p o i n t at 70 m N m - 1 4 L i n e s of s u l p h u r a p p l i e d to the m o n o l a y e r at an a r e a of 25 ~ 2 m o l e c u l e 1 s h o w e d that, at least o v e r the l e n g t h of the p l a t e a u , the film was n o n - h o m o g e n e o u s . T h e lines of s u l p h u r b e c a m e closest (Fig. 6) a n d the a r e a s t h e y d e f i n e d w e r e m o s t r e d u c e d in the r e g i o n of the film n e a r to the m o v i n g b a r r i e r (Fig. 5). B e l o w a b o u t 10/~z m o l e c u l e - 1 , the c o l l a p s e t a k e s p l a c e u n i f o r m l y o v e r the surface a s s o c i a t e d w i t h l a r g e - s c a l e e l e m e n t s o f the film.
(a)
(b)
Fig. 6. (a) Lines of sulphur powder applied to a monolayer of T15 at 25 A 2 molecule- 1 and compressed to 15/~z molecule-l; (b) a similar experiment with a glass plate inserted after the sulphur has been applied. The moving barrier is on the right-hand side in both photographs.
3.3. The effect of a plate in the interface A f t e r lines of s u l p h u r h a d b e e n s p r e a d o n the surface of the c o n d e n s e d m o n o l a y e r a glass p l a t e 3 c m w i d e w a s l o w e r e d i n t o the i n t e r f a c e a b o u t 4 c m f r o m
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the end of the trough and the monolayer compressed. The observed patterns were in all cases of the same form and indicated a significant deficit in the amount of material to the rear of the plate. The results on the polypeptides were all very similar and comparable with those of poly(~,-ethyl-L-glutamate) 3. Despite the near-ideal compression and expansion observed in PBMA, comparison of the photographs for a monolayer compressed to 12 from 25 ~z per m o n o m e r unit, with and without a plate in the interface (Figs. 3(b) and 3(c)), shows perturbations extending over half the width of the trough, with only about half the amount of polymer present to the rear of the plate compared with the surface without a plate. If a Wilhelmy plate is used to measure the surface pressure it is deflected in the direction of compression; the pressure excess causing this deflection is considerable and has been measured in earlier work on polypeptides 3. This is clearly related to the observations on flow. Figure 6 shows clearly that the effect of a glass plate in the interface is not confined to polymer monolayers. Both the curvature of the flow profile and the deficit of material in the region to the rear of the plate are evident. 4. DISCUSSION
4.1. The monolayer properties of poly(7-methyl-L-glutamate) methacrylate)
and poly(butyl
Crisp 8 first drew attention to the similarities between the monolayer properties of the synthetic polypeptides and the polymethacrylates and suggested the plateaux represent collapse of a monolayer to a three-dimensional state. It is now generally agreed that this is an ordered bilayer in the case of the s-helical polypeptides that exhibit a plateau and a consequence of the order imposed on the monolayer by the rod-like nature of the ~-helix 9 12. It has recently been suggested, however, that the plateau and appearance of a hysteresis cycle in the isotherms of P M G and poly(Lmethionine) are associated with a progressive interdigitation of the side-chains during compression and a recovery of the original structure on decompression v. It is clear from the flow patterns that the transition on compression, while not homogeneous, is a progressive change. In contrast, while expansion leaves the monolayer in very nearly its initial state, as judged by the isotherm, the flow pattern on expansion shows it to be a much less regular process. The hysteresis observed with the surface balance located at one end of the trough is clearly directly related to this very non-uniform behaviour on expansion. In so far as the proposed interdigitation theory rests on the observation of hysteresis, there would appear to be no direct connection. Poly(L-norleucine) resembles P M G in its behaviour rather than P B M A although in both cases there is a four-carbon side-chain. This supports the view that it is the difference in the rigidity of the backbone rather than the nature of the sidechains that underlies the differences in their behaviour. However, if a side-chain is sufficiently long and predominantly hydrocarbon, it will be the principal factor in determining the strength of interaction between the polymer and subphase, and the free energy of the p o l y m e r - v a p o u r surface. Thus, to a first approximation, the transition to a three-dimensional structure (or bilayer) will occur at around the same surface pressure for all such polymers. This pressure, around 14_+4 m N m -1,
F L O W OF M O L E C U L A R M O N O L A Y E R S D U R I N G C O M P R E S S I O N
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represents the work to be done on the monolayer for the poiymer-polymer interactions at the p o l y m e r - v a p o u r interface to cause the adhesive forces between polymer and subphase to be broken, leading to a more compact three-dimensional state. If there is a high degree of order in the monolayer, this gives rise to a flat plateau 9; disorder in the monolayer is to be expected to cause the transition to take place over a range of pressures. Thus for poly(L-norleucine) the transition pressure is at 10.5 m N m 1, for poly(7-n-decyl-L-glutamate) 6 it is at 17.5 m N m -1 with flat plateaux in both cases, while PBMA has a less-well-defined transition at 1 4 m N m -1.
4.2. The behaviour of 4-cyano-4"-n-pentyl-p-terphenyl The peak in the pressure-area curve at 25/~2 molecule- 1 is a characteristic of a nucleation phenomenon, and the excess decrease in area of the monolayer in the region of the moving barrier shows that the pressure caused by the movement of the barrier acting against the rigidity of the film produces an above-average amount of collapse in this region. Given that the pressure rises further at half the initial monolayer area, as has been observed in m a n y synthetic polypeptides, and that it is independent of compression rate, suggests that at 15 /~2 molecule-1 the rise in pressure represents the completion of a transition to a bilayer. The instability of the film under further compression well illustrates the point made by Gaines that such films are not thermodynamically stable 13. The surface pressure then is the sum of two contributions, one is the thermodynamic reduction in surface pressure by the film and the second is a true two-dimensional compression forcing the molecules together. However, for both T 15 and the polypeptides the rigidity of the monolayers, as shown by the deflection of a Wilhelmy plate, indicates that such a compression may exist even below the monolayer stability limit. 4.3. Some experimental considerations Gaines I has emphasized that measurements on condensed films represent the average behaviour of the film and that "some films are so rigid that when spread across a trough they may withstand a pressure of several dynes per centimetre on one side, with the other side completely unsupported". The photographs of monolayer flow show some of the consequences of this relevant to the design of Langmuir troughs. In all cases the flow of the monolayer has a curved profile and even in the case of P B M A a plate in the interface obstructing the flow produced a very ill-defined area with a considerable deficit of polymer to the rear of the plate. If this is a Wilhelmy plate, clearly it is seriously perturbing the system it is desired to observe. This can account for differences in the pressure-area curves for P M G reported in the literature. Thus both Loeb and Baier 14 and the present author find, measuring the horizontal force, a flat plateau; a more curved transition with a pronounced gradient is reported by Baglioni et al. 15.16, using a Wilhelmy system. It is further evident that trough geometry needs to be well defined, simple and of' adequate size. Recent elegant systems, for instance a circular trough 17, or a constant-perimeter trough with a re-entrant region as in the Joyce-Loebl design, combined with a Wilhelmy plate, are far from ideal for studying monolayers which are condensed. Measurements of surface potentials on an inhomogeneous mono-
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layer cannot be expected to correlate exactly with surface pressures measured either with a Wilheimy plate elsewhere in the trough or with a film balance forming a boundary to the monolayer. Similarly, if a Wilhelmy plate is used to monitor the surface pressure during deposition of a Langmuir-Blodgett (LB) film, it should be placed so that it occupies an equivalent position in the trough to the LB plate. If studies of interactions between polymers and small molecules in a monolayer are to be made, using the two-dimensional phase rule developed by Crisp 18, the consequences of making measurements on films that are condensed needs to be carefully considered. It would appear to be possible that the small molecules could affect the extent of non-ideal behaviour simply by acting as a plasticizer and affecting the fluidity of the monolayer. This could result in changes both to the observed monolayer area and to the pressure required to produce a transition. 5. CONCLUSION
It is simple and instructive to study the flow of condensed monolayers with the method described here, both in order to make more meaningful measurements and to interpret the observations. It is not claimed that the method is particularly novel, but the application of lines of powder through a grid does have the merit of enabling observations and quantitative measurements to be made of the flow of the whole surface. REFERENCES 1 G . L . Gaines, Insoluble Monolayers at Liquid-Gas Interfaces, Wiley-Interscience, New York, 1966, p. 155. 2 M. Joly, in J. F. Danielli, K. G. A. Pankhurst and A. C. Riddiford (eds.), Surface Phenomena in Chemistry and Biology, Pergamon, London, 1958, p. 88. 3 B.R. Malcolm, J. Colloid Interface Sci., 104 (1985) 520. 4 M . F . Daniel, O. C. Lettington and S. M. Small, Thin Solid Films, 99 (1983) 61. 5 B.R. Malcolm and S. R. Davies, J. Sci. Instrum., 42 (1965) 359. 6 B. R Malcolm, Adv. Chem. Ser., 145 (1975) 338. 7 A. Albert and J. Cordoba, Colloid Polym. Sci., 262 (1984) 811. 8 D.J. Crisp, in J. F. Danielli, K. G. A. Pankhurst and A. C. Riddiford (eds.), Surface Phenomena in Chemistry and Biology, Pergamon, London, 1958, p. 23. 9 B.R. Malcolm, Polymer, 7(1966) 595. 10 B.R. Malcolm, Prog. Surf. Membr. Sei., 7(1973) 183. 11 F. Takeda, M. Matsumoto, T. Takenaka and Y. Fujiyoshi, J. Colloid Interface Sci., 84 (1981) 220. 12 F. Takeda, M. Matsumoto, T. Takenaka, Y. Fujiyoshi and N. Uyeda, J. Colloidlnterface Sci., 9l (1983) 267. 13 G . L . Gaines, Insoluble Monolayers at Liqui&Gas Interfaces, Wiley-lnterscience, New York, 1966, p. 150. 14 G.I. Loeb and R. E. Baier, J. Colloid Interface Sci., 27 (1968) 38. 15 P. Baglioni, E. Gallori, G. Gabrielli and E. Ferroni, J. Colloid Interface Sci., 88 (1982) 221. 16 P. Baglioni, L. Dei and G. Gabrielli, J. Colloid Interface Sei., 93 (1983) 402. 17 D. Fromherz, Rev. Sci. Instrum., 46 (1975) 1380. 18 D.J. Crisp, Research (London), Surf. Chem. Suppl., (1949) 17.