Langmuir 1995,11, 204-210
204
The Powder Method for the Study of Condensed Langmuir Monolayers with Applications to Polymers Benjamin R. Malcolm Institute of Cell a n d Molecular Biology, University of Edinburgh, King's Buildings, Edinburgh EH9 3JR, U.K. Received February 25, 1994. In Final Form: October 4, 1994@ The method used to apply a net of powder to the surface of a condensed monolayer is described in detail. It is shown that photographic records of the flow can be analyzed to determine areas of under- and overcompression. By use of a polymer with a plateau in the pressure-area curve, the effect of pressure gradients can be studied in relation to the methods used to compress the film. Examples of the application of the technique are given for three polymers. Poly(buty1methacrylate), which behaves (approximately) as a Newtonian monolayer, is shown to compress fairly uniformly. In contrast polybalanine) and poly(y-ethylL-glutamate)do not flow as Newtonian monolayers and show marked deviations from uniformity of compression. The significance of these results is discussed in relation to the deposition of uniform LB films and in making reliable measurements of surface potential.
Introduction The standard methods for studying molecular monolayers have hardly changed in principle since the work of Pocke1s.l Still the most common instrument is based on a rectangular trough filled to the brim with water, with a barrier sliding over the sides of the trough to compress the monolayer. Its use for over a hundred years has shown the instrument's value but it has also led to a lack of critical thinking about its limitations.2 This is particularly evident in relation to studies of monolayers of polymers and other materials that form rather rigid monolayers in the condensed state. It is usually assumed that, provided the monolayer is compressed sufficiently slowly,the area per monomer unit remains uniform over the surface and that the surface pressure measured either along a line at the end of the trough, or a t a n arbitrary small area with a Wilhelmy plate, represents the pressure at all points on the surface. It is further usually assumed that the plate does not either affect the distribution of monolayer or introduce pressure gradients. Similarly while measurements of surface potential do not disturb the flow, it is often assumed that the potential measured at a n arbitrary part of the surface can be correlated directly with measurements of surface pressure made elsewhere on the surface. That these assumptions may not always be valid has led to recent experiments designed to show the existence of pressure gradients in the surface when monolayers are compressed, making use of Wilhelmy plates to measure surface p r e ~ s u r e This . ~ procedure can further complicate the situation. While it was noted that with condensed monolayers a freely suspended plate could be deflected, or even forced out of the ~ u r f a c e ,it~appears ,~ that it was not appreciated that it was manifestly affecting the flow. If the plate is mounted centrally in a rectangular trough and compression is symmetrical from both ends, it will of course not be deflected, but elsewhere in the surface a deflected plate shows that there is a pressure difference across its faces. This approach does not give any clear @Abstractpublished in Advance ACS Abstracts, December 1, 1994. (1)Pockels, A.Nature 1891,43,437. (2) Pethica, B. A. Thin Solid Films 1987,152, 3. (3) Peng, J. B.; Barnes, G. T. Langmuir 1991,7,1749. (4)Peng, J. B.; Barnes, G. T. Langmuir 1990, 6, 578. ( 5 ) Egusa, S.; Nakayama, T.; Gemma, N.; Miura, A.; Azuma, M. Thin Solid Films 1989,178, 165.
indication of how pressure gradients are caused or detect variations in the mass per unit area over the surface. The perturbations caused by a Wilhelmy plate are avoided by using a capillary wave probe6 and it has been shown that studies of nonuniform films using this method can be correlated with observations with a fluorescence microscope with appropriate materials.' The method described here is however technically more simple and direct. Many types of observations on monolayers and their deposition as LB films involve flow of monolayer over the surface. Measurement of the surface viscosity would therefore appear to be a n appropriate tool by which to characterize them.8 However for condensed films, meaningful viscosity measurements are difficult and not necessarily directly applicable to the conditions of flow met either when depositing LB films or simply measuring pressure-area isotherms, particularly where nonuniformities may be present. The method described here, based on the use a net of powder applied to the surface, can be used both to see how the monolayer responds to rheological forces as it is manipulated and to measure how uniformly elements of the surface change their area as the surface is compressed. Provided that the monolayer is condensed, the method is straightforward and can be used to make measurements at a reasonable resolution over the whole surface. The use of a powder to study surface flow predates Pockels' invention of the Langmuir trough and was apparently first used by Aitkengto study oil films on water. He emphasized that the powder must be free from greasy matter and found well-burned ashes sifted through a wire gauze to be suitable. RayleighlO found sulfur powder to be preferable, while Pockels used sulfur or lycopodium powder.' A number of workers have used one or more lines of powder to observe flow, usually qualitatively, to study surface transport11J2 and the flow associated with a rotating cylinder in the i n t e r f a ~ e . ' ~ ,When '~ applied as a (6)Miyano, K Langmuir 1990, 6,1254. (7) Miyano, K,Tamada, K. Langmuir 1993,9, 508. (8)Buhaenko, M. R.; Goodwin,J. W.; Richardson, R. M. Thin Solid F i l m 1985,134,217. (9)Aitken, J. Proc. R. SOC.Edinburgh 1882,12,56. (10)Rayleigh, Lord Proc. R. SOC.London, A 1890,48,127. (11) Crisp, D.J. Trans. Faraday SOC.1946,42,619. (12) Dimitrov, D.S.;Panaiotov, I.; Richmond, P.; Ter-minassianSaraga, L. J. Colloid Interface Sci. 1978,65, 483. (13)Ries, H.E.;Gabor, G. Nature 1966,212,917.
0743-7463/95/2411-0204$09.00/00 1995 American Chemical Society
Study of Condensed Langmuir Monolayers
net over the whole surface, the powder method can be used, both qualitatively and quantitatively, in a number of ways on condensed monolayers: (1)To make measurements of the compression of small elements of the surface and see how these deviate from the average value, as described below. (2) To observe the flow of monolayer caused by manipulation (including LB film deposition) and to assess the effect of trough geometry.'S (3) To observe and measure the flow of condensed monolayer for comparison with that calculated for a model system with Newtonian viscosity.16 For a given set of boundary conditions, the pattern of Newtonian flow is the same for all materials and is independent of the absolute value of the surface viscosity, or the rate of flow. Deviations of the observed flow from that calculated can provide a useful insight into the nature of the monolayer. (4) To characterize routinely new monolayer materials as part of any study of their behavior in the condensed state, e.g. in relation to crack formation and monolayer collapse. Recently the flow of disks has been used as a way of studying monolayer flow. This can, in principle, give the same information as the techniques described here, but in practice it is less informative and can be misleading. Thus while Daniel and Hart,17 using this method, concluded that their flow pattern indicated parallel flow, the severe shearing of their monolayers by the velocity gradients was not evident. Similarly Schweigk et al.18 found convergent flow with LB deposition on a substrate small compared with the width of their trough, whereas parallel streamlines were recorded with a wide substrate. They concluded from their results that the observed orientation on a narrow substrate was a result of convergent flow conditions. Their results did not show the extent that their monolayer was being sheared in the course of deposition, which can produce orientation effects in monolayers. The powder method, a s described here, is limited to monolayers that are sufficiently condensed for a stable net to be applied. I t is however in these conditions that monolayers depart most from ideal behavior and are manipulated for LB film deposition. While brief accounts of the method have been given previo~sly,'~ its development and accuracy as a quantitative tool warrant a more detailed description. The method is applied to three polymers to illustrate some ofthe ways it can be employed and related to the more usual methods. Poly(buty1 methacrylate) has been used because it is well understood and has been shown to approximate to a Newtonian fluid, though not perfectly.16 The a-helical synthetic polypeptides, poly(L-alanine) and poly(y-ethyl L-glutamate), are examples of monolayers that depart markedly from ideal behavior because of their rigidity. Their properties are not however unique and some of the general conclusions drawn here are important in understanding the behavior of condensed monolayers of other materials.
Experimental Methods Choice of Powder. Ideally the powder should be inert, freeflowing,free from contamination,oflow density, and hydrophobic. In practice sulfur appears to work perfectly well though treatment (14)Ohkawa, K.; Morita, F.; Kanda, S. Nippon Kagaku Kaishi 1983 (part 6),910. (15) Malcolm, B. R. J.Phys. E: Sci. Instrum. 1988,21,603. (16)Byatt-Smith,J.B.; Malcolm,B. R. J.Chem.Soc., Faraday Trans.
1994,90,493. (17) Daniel, M. F.; Hart, J. T. T. J. Mol. Electron. 1986,1, 97. (18) Schweigk, S.; Vahlenkamp, T.; Wegner, G. Thin Solid Films 1992,210,6. (19)Malcolm, B. R. J. Colloid Interface Sci. 1985,104,520.
-
Langmuir, Vol. 11, No. 1, 1995 205 BLACK SCREEN
CAMERA
I GRID
_.
__
~
-SCALE WATER TROUGH BLACK GLASS
Figure 1. Diagram (not t o scale) showing the arrangement for applying lines of powder and recording the flow. The trough is shown in cross section with a grid in place for applying lines parallel to the direction of compression. with solvent t o remove any surface-active contamination may be necessary. Alternatives that might be used include fine glass balls, treated with a silicone to render them hydrophobic, and MgO powder from a burning magnesium ribbon. The latter is valuable for studies at high resolution because it is particularly fine. It is however difficult to controland with both these methods dark-ground illumination of the surface is desirable. It is supposed that the use of a powder to study flow does not impede or affect the movement of the monolayer. This is probably true, in the writer's limited experience under the conditions described here, but it is difficult to be certain a priori. The flow of monolayer, as observed by photographing the movement of powder, appears independent of the amount of powder in that part of the surface; a few particles and a relatively thick line move to the same extent, as shown below. Additionally surface pressure-area curves do not appear to be affected by the presence of powder. These observations suggest that the powder does not perturb the system under investigation. Crispll has considered the same point in relation to more fluid systems, where it may be that the powder does not always move with the monolayer. Application of Lines of Powder to the Surface. The surface pressure-area curve of the monolayer is first established in the usual way, with observation of traces of powder on the surface to determine a t what stage the monolayer becomes sufficiently condensed for a stable net of powder to be applied. In practice this means when powder on the surface remains almost immobile when gently blown. Compressionof monolayer to the same condensed area is then repeated with obstructions to flow and access to the surface (Wilhelmybalance, etc.) removed. If necessary, the surface pressure can be monitored using symmetrical compression, with sulfur applied to one half only; the Wilhelmyplate is then used close to the center on the powderfree side. Powder is applied to the surface through a grid placed above the trough (Figure 1). The grid is made of parallel strips of transparent plastic, 5 mm or more thick, with gaps about 1 mm wide between them. The strips are held together by two further strips cemented at right angles above them. The width of the strips is determined by the net spacing required; 9 mm wide strips for a 10-mm grid spacing is convenient. The grid is held about 3 mm above the water surface by inverted L-shaped blocks, which fit over the sides of the trough. The grid can then be slid along the trough to extend the area over which the net is to be applied. To form a square net, rather than a set of lines, a second set is applied using a grid with slots running a t right angles to the first one. The powder can be applied through the grid by using a container with a fine hole in the bottom. This is tapped as it is moved along above the slots. A n alternative method which is much quicker, is first to sprinkle powder on the grid and then, when required, to set it in place and brush the powder through with a fine brush. It is not necessary for the lines to be continuous or uniform for most purposes; the distortion of the net gives an overallpicture ofthe flow ofthe monolayer,but when quantitative measurements are to be made, the primary function of the net is to facilitate followingselectedparticles on photographicrecords. Recording the Flow. The grid is removed after application of the powder and the flow recorded during the experiment with a 35-mm camera mounted above the trough. Provided that the monolayer is sufficiently condensed, it is unnecessary to cover the surface with a transparent cover to avoid the effect of drafts.
Malcolm
206 Langmuir, Vol. 11,No. 1, 1995 Reflections from the surface can be avoided if a sheet of black material is mounted above the camera and the surface illuminated obliquely. A sheet ofblack glass that fits in the bottom of the trough gives good contrast for photography. To record barrier movement,it is convenientto photograph simultaneously a scale fixed along the side of the trough. With good laboratory illumination, additional lighting is unnecessary and photographs can be taken with fast black-and-white film (e.g. Ilford Delta IS0 400) a t about '/e0 s at f2.8. For accurate quantitative studies, enlarged prints are made to the full object size or larger, ensuring that the degree of enlargement is fixed throughout. A final stage of enlargement with a well-adjusted office photocopier is economical and convenient. To make measurements of changes of area during compression, a square net is first drawn over the lines of the photocopy of the initial net with a fine ball point pen. For most purposes it may be sufficient to use every third line to give a 3 x 3 cm2square net as in the experiments reported here. By use of carbon paper, a tracing ofthis net is obtained. Similar tracings are then made on subsequent photocopies, but now following the paths of the lines drawn to form the 3-cm net, using individual corresponding particles of powder as reference points. The accuracy of this procedure is limited largely by the degree of enlargement used, unless cracking ofthe monolayer has occurred, which can be seen from discontinuities in the lines of powder. The areas ofthe elements can be measured by a computer system or simply by a weighing method. Thus for a trough 15 cm wide, an initial area 18 x 15 cm2 gives 30 measurements on each photograph. The 3-cm net uses only one-third of the lines, so that for increased accuracy other nets may be drawn. The 1cm net is however useful in locating individual particles of powder, for detecting cracking, and recording the overall flow characteristics of the monolayer. The amount of powder required to analyze changes in area of an element of the monolayer is not critical but ideally rather less than is required for clear photographic reproduction for publication. For this reason some of the patterns shown here have had flow lines overdrawn for the purposes of illustration. Analysis of the Compression. Measurements of the areas of the (approximately) 3 3 cm2 elements before and after compression show the extent of departure from uniform compression. Let A0 be the area of the condensed monolayer when the net was applied and over which measurements are to be made and A be the area after compression; similarly let a0 and a be corresponding values for an individual element. The differencebetween the overall fractional decrease in area (A /AD) and the fractional decrease of an element (alao) is (MA0 - a/%). This is then a measure of any deviation from uniformity for that element. The deviations are then plotted as a set to show the distribution of deviations over the surface for each stage of compression so that a point to the right of the local zero (uniform compression)represents a measure of overcompressionand one to the left represents a measure of undercompression. General Experimental Details. Poly(buty1 methacrylate) (Aldrich secondary standard, typical M W 320 000) is the same sample as used previously for studies of flow,'g and was spread from solution in chloroform. Poly(y-ethyl L-glutamate) (Sigma, MW > 100 000) and poly(L-alanine)are the same as used for flow studies p r e v i o ~ s l y . ~ 9 ~The ~ o Jspreading ~ solvent was chloroform containing 5% (dv) dicloracetic acid. Monolayers were compressed symmetricallyon double distilled water in a PTFE trough 60 cm x 15 cm. Surface pressures were measured by a 2 cmwide Wilhelmyplate mounted centrally perpendicular to the direction of compression. The barrier speed was 2 mm/min.
Applications with Polymers Poly(buty1 methacrylate). Figure 2 shows a small part of the central area of a final photocopyof a grid applied to a monolayer. The scale shows that where necessary a 3 x 3 cm2 area of monolayer can usefully be enlarged to about 9 x 9 cm2to increase the accuracy of measurement. For most of the results reported here the enlargement was approximately twice full size. The blank parts of the Malcolm, B. R. Thin Solid Films 1989,178, 191. (21) Malcolm, B. R. Thin Solid Films 1989,178, 17. (20)
IIII(I
0
5cm
Figure 2. Part of a final photocopy enlargement of sulfur on a monolayer of poly(buty1 methacrylate): (a) as applied to the condensed monolayer at a pressure of 8 mN m-l; (b) after compression to approximately half the area; (c) re-expanded to approximately the initial area. Compression was from left to right.
lines are caused by the support strips of the grids. This polymer shows almost complete reversibility in its compression from 0.22 nm2/monomer unit to 0.11 nm2/ monomer unit as judged from measurements of the pressure-areaisotherm.16 The reversibility is also shown in Figure 2 and necessarily applies also to the sulfur on its surface. This shows therefore that irrespective of the amount of sulfur present, over the range shown, the powder particles do not appear to interact when compressed, or affect the flow of the monolayer. It should be noted that, for this part of the monolayer, compression involved its transfer over the water surface for a distance of approximately 5 cm. Quantitative measurements of the flow of this polymer have shown that it behaves approximately as a Newtonian flUid.16 This is also shown by the pronounced inward flow (Figure3)in front of the advancing barrier, which is similar (but not identical) to that calculated for the flow pattern using a single barrier. Measurements of the compression of 3-cm elements for half of the monolayer have been made and plotted as described above, to show the distribution a n d extent of departures from ideal compression at various stages of compression. It will be seen that in this case there is a small, but probably significant, overcompression a t the end close to the moving barrier. Elsewhere the deviations probably do not differ within experimental error from uniform compression. The error of the measurements isjudged to be approximately f0.02,but this varies from one element to another, depending on the quality of the local data. It should be noted that, because it is necessary to first compress the monolayer until it is sufficiently condensed to apply powder, the overall nonuniformity caused by compression may be greater than that measured. These results show that in the case of this polymer, approximately uniform compressionis associated with flow
Langmuir, Vol. 11, No. 1, 1995 207
Study of Condensed Langmuir Monolayers
0 1 2 3 4 5 c m I
1
I
"
0 I
'
0 I
0
I
I
0
I I
C I
I I
1
0-
0
d
Figure 3. Half of a symmetrically compressed monolayer of poly(buty1methacrylate) showing marked inward flow. NAo= 0.48. The curvature of the lines on the left is caused by a Wilhelmy plate, to the left of the trough center, outside the field of view. The temperature was 20 "C.
patterns that approximate to Newtonian flow and a pressure-area curve that is nearly reversible. Poly(y-ethyl L-glutamate). Monolayers of highmolecular-weight a-helical synthetic polypeptides with a sufficiently long and flexible side chain usually have a plateau in the pressure-area isotherm, associated with the formation of a second layer of molecules.22 By application of lines of powder to the surface and measurement of their change of spacing along the center line of the trough, it has been shown that this transition starts close to the advancing barrier and then extends along the trough.lg More detailed information can now be obtained using the improved technique described here. Figure 5 shows a typical photograph used to analyze the compression. The results (Figure 6 ) show that there is a high degree of overcompression at the end near to the moving barrier, particularly at the sides. These are the areas where, in the case of a monolayer with Newtonian viscosity, compression and the associated induced flow produce the largest deformations in the monolayer.16In the present instance the flow is clearly not Newtonian (compare Figures 3 and 5). It appears therefore that failure to respond to the rheological forces, caused by the rigidity of the surface, creates overcompression close to the banier and associated undercompression at the center, which is greatest close to the sides. This last observation can be correlated with measurements of the pressure with a centrally mounted Wilhelmy plate (Figure 7). It will be seen that the pressure recorded is higher when the plate is in the middle of the trough than when it is close to the side, where the extent of undercompression is greatest. (22) Malcolm,
B. R.Proc. R. Soc. London, A 1968,305,363.
I
0
I
0
0.71
P 0
0 I
0 I
0
0
l
$, !o I
0 I 0
0
(a)
P
0
l 0
P
I
0
I 0
P
0 I 0 I
0 I 0
0 I
0 0
do
(b)
L 0.72 I
I
0
0.48
I
0 I
0
I
I,
0
0
I
0
'0
0 I 0
:0
0
0
0
I
0
P
0.97
I
Figure 4. Diagrams showing the distribution of deviations from uniform compression for poly(buty1 methacrylate). The values for NAo are given for (a) compression and (b) reexpansion. Deviations are plotted (circles) for each element, initially approximately 3 cm x 3 cm, from the local true value (broken vertical lines) accordingto the scale below. Compression was symmetrical, but only the right half has been measured.
Figure 5. Symmetrical compression of a monolayer of poly(y-ethyl bglutamate) forNAo =0.53,showing much less inward flow than would be expected for Newtonian flow (compare with Figure 3).
While this work confirms and adds detail to the earlier studies, the results also show how a material with a flat plateau in the pressure-area curve can be used as a tool to detect pressure gradients arising from the manner in which the monolayer is compressed, by making use of the transition. While such a material is particularly sensitive to pressure gradients when close to the transition pressure, the rheological forces causing the observed deviations arise from the boundary conditions associated with the method used to compress the monolayer and will apply whenever this method is used. Use of a sensitive polymer to detect pressure gradients has shown that the compression of a monolayer is far more uniform in a folding rectangular frame than with
Malcolm
208 Langmuir, Vol. 11, No. 1, 1995 p
o
0
)
b
lo
0
I 0
7
o
0 0I
o
o
I
b
0
o ' 0
0
l I0
I
0
o
p
1
y
0
I
0
0
0
l
0
{ 1
b
SCALE :
- O - O i - O
'-0 '4 e L 'I O 1L
0
01
/'
0 1
o -0
0'
OI
0
0
0
I?
9
0
1
o
0.82
d
I+ 0
0.34~
d
0.1
Figure 8. Analysis of the results of Malcolmz0for poly(y-ethyl
L-glutamate) compressed in a folding frame under conditions similar to those in Figures 6 and 7. The right half of the monolayer only, initially forming a rectangle approximately21 cm x 12 cm, has been measured. The frame folded to form a parallelogram with the acute angle at the top right corner of the field.
= +0.20
SCALE
Figure 6. Distributionof deviationsfrom uniform compression for symmetricalcompression of poldy-ethylbglutamate) drawn as in Figure 4, for four values ofA/Ao, Compression is from the right; the central area of the monolayer is on the left.
0.40
0.53
0.66
0.79
1.0 AIA.
301 'i 20E
z
E
Y
a =I
v)
fi a
10-
n.
01
,
1
will be seen that while the region close to the acute angle in the folding frame is overcompressed, overall departures from uniformity are significantly less in the folding frame than in a conventional trough with sliding barriers. Poly(L-alanine). Like the previous example, this a-helical polymer has a much more rigid backbone than poly(buty1 methacrylate) and forms monolayers that depart significantly from the Newtonian model. In the example shown (Figure 91, the flow on compression is not symmetrical down the center line of the trough, with more inward flow in the upper half than in the lower, and the flow profile is not smooth. In contrast to a monolayer with Newtonian flow, the monolayer at the sides is not stationary, until the barrier reaches it, but moves forward as in plug flow, though the curvature of the flow lines shows that it is not perfect plug flow either. Analysis of the compression (Figure 10) shows that the pattern of departure from uniformity is similar to the previous case though, without a flat plateau in the pressure-area curve, less pronounced. That the overall pattern should be similar, emphasizes that primarily it is the boundary conditions that determine the flow behavior, not the polymer. The pressure-area curve is not reversible (Figure 11). This is probably related to holes developing in the surface, close to the sides at the barrier end, as is suggested by the marked nonuniformities shown in the analysis of the powder pattern. Early work on this polymer showed a correlation between inflections in the surface pressure-area and the surface potential-area c u r ~ e s . However ~ ~ , ~ ~ Lavigne et al. have found in their measurements on poly(L-alanine) and poly(L4eucine) inflections in the surface potential curves at areas significantly greater thanin the pressurearea curves.z4 The probable reason for their lack of correlation is simply that their monolayers were not uniform. If we consider Figure 10 to represent compression using a single barrier and a Langmuir balance a t the left hand end, as in the early work, and as used by Lavigne et al., it will be seen that a potential measuring electrode mounted in front of the balance will be responding to a monolayer more compressed than at the balance. In the early work it was stated that the potential rises t o a peak (23) Malcolm, B. R. J.Polymer Sci.: Part C 1971,No.34, 87.
(24) Lavigne, P.; TancrBde,P.; Lamarche, F.; Max,J.-J. Langmuir
1992,8,1988.
Langmuir, Vol. 11, No. 1, 1995 209
Study of Condensed Langmuir Monolayers
SCALE :
--o =+O.l
Figure 10. Analysis of compression of polybalanine), drawn
to the same scale as the previous figures. Half the monolayer has been measured, with compression from the right: (a) compression; (b) re-expansion, showing the effect of “holes” developing at the sides. 0.72
0.59
0.49
1.0 AIA,
Figure 9. Symmetrical compression of a monolayer of poly-
(L-alanine)showingpowder appliedto half only: (a)as applied, MA0 = 1.0; (b)MA0 = 0.49. close to the area at which a transition starts in the pressure-area “his was based on measurements on a wide range of polymers and is still valid. Because the surface potential was never perfectly uniform over the surface, it was considered prudent to place the electrode as close as possible to the pressure-measuring device, thereby minimizing this error. When, by using the powder method, these nonunifonnities could be clearly demonstrated, the implications for surface potential , ~ evidently ~ ~ ~ they measurements were pointed O U ~though have not been fully understood. If symmetrical compression is used with a Wilhelmy plate and a potential measuring electrode mounted symmetrically about the center line, the measurements can be correlated, though the problem of nonuniform compression remains. If the electrode is movable, as recommended by it is a further method for detecting a nonuniform monolayer.
General Comments
It will have been noted that the flow and compression results are less than perfect, as evidenced by discontinuities and lack of symmetry in the patterns, suggesting that the procedures used were capable of improvement. Factors such as film cracking and nonuniform application of the monolayer may have played a part. Improvements (25) Malcolm, B. R. Thin Solid Films 1985,134, 201. (26) Adam N. K. Physics and Chemistry of SuTfaces; Oxford University Press: Oxford, 1941; p 34.
30-
c
IE 20-
z
E
W
a
3 10a W
n
I1 0.05
1
1
1
1
\ I
I
-
I
\ t
0.10 AVERAGE AREA nm’/monomer
’
I
0.15
Figure 11. Graphs showing the relation between the surface pressure, as measured by a centrally mounted Wilhelmy plate, and average area per monomer unit for polybalanine): full line, compression; broken line, re-expansion. The values ofAIA0 shown relate to the separate study of Figure 10.
to the flow patterns might therefore be regarded as a test of improving experimental quality and skills. However flow patterns have a general tendency to become nonsymmetrical because of stresses causing cracking and flow at some point. Once this happens, the symmetry of the stresses in the monolayer is broken and the probability of symmetrical flow is reduced. Rather than present detailed compression diagrams for every experiment, the measurements can be summarized, as in Table 1, by giving standard deviations calculated from each set of results. While a figure for the standard deviation does not give as much spatial information as plotting individual deviations does, it makes a useful
210 Langmuir, Vol. 11, No. 1, 1995 Table 1. Standard Deviations . a for Individual Polymers at Various Stages of Compression WAo), Calculated from WAo - U/UO) for n Elements; e Denotes Re-emansion Poly(buty1methacrylate). Symmetrical Compression AAo 0.71 0.48 0.72e 0.97 e 0.03 0.02 0.02 0.04 Poly( y-ethyl L-glutamate). Symmetrical Compression NAo 0.79 0.65 0.53 0.40 030 0.14 0.15 0.09 0.07 Poly(?-ethyl L-glutamate). Compression in Folding Frame 0.68 0.50 0.34 MA0 0.83 0.05 0.03 0.03 028 0.03 Poly(L-alanine). Symmetrical Compression 0.49 0.99e 0.72 0.59 NAo 0.06 0.06 0.04 030 0.05 030
summary and could serve as a n index of the uniformity of a monolayer under defined conditions of compression.
Conclusion In descriptions of experimental work on monolayers, it is customary (and on occasions important) to describe in detail the experimental apparatus and procedures to establish the validity of the work. However rarely do the problems addressed here receive proper consideration. It is essential that they do so in appropriate cases. The methods described are simple, inexpensive, and informative. We now understand how a n ideal monolayer with Newtonian viscosity should flow on a rectangular trough. It is systems that depart from this ideal that appear to give problems and for which the methods described here are particularly informative. Those monolayers that do not show uniform compression depart most from Newtonian flow. Thus it may be that by simply inspecting the flow pattern on compression, or transfer a t constant pressure,16 the possible extent of nonuniformity may be gauged. The polymers used here are not exceptional, and
Malcolm it is probable that many other materials will be found that behave in a similar way. Where necessary, compressing a monolayer in a folding frame should produce a more uniform compression. Alternatively the use of a rectangular frame with two opposite sides made of rubber bands might be considered. This arrangement gives true uniaxial c o m p r e s s i ~ n . ~ ~ An important target in producing good quality LB films by the vertical dipping method must be developing materials that flow in the required manner. Nevertheless it is common to find no explicit reference to the flow properties of the material under study. Where studies have been made, there is frequently confusion between flow phenomena associated with the boundary conditions imposed by the apparatus and the intrinsic properties of the material. The use ofthe powder method to study flow, coupled with a better understanding of how a n ideal monolayer with Newtonian viscosity should flow, can help to resolve some of these problems. In particular studies of flow a t constant pressure, by observing flow patterns when the monolayer is simply transferred a t constant area,16 is a logical step prior to seeking to transfer monolayer onto a LB plate. It is of interest to note that promising LB films of synthetic polypeptides can be preparedz8 by designing polymers to have good flow properties. Being able to observe and understand the flow process is a n important objective in the production ofgood LB films. Registry No. Supplied by Author. Poly(buty1methacrylate) 9003-63-8; poly(y-ethyl L-glutamate) 25189-520; polybalanine) 25191-17-7. LA940175K (27)Bohanon,T.M.; Lee,A. M.; Ketterson,J. B.; Dutta, P.Langmuir 1992,8, 2497.
(28)Mathauer, K; Mathy, A.; Bubeck, C.; Wegner, G.; Hickel, W.; Scheunemann, U.Thin Solid Films 1992,210, 449.
