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Experimental study of the stability and metastability of palmitic acid
Thin Solid Films, 178 (1989) 93-101
93
EXPERIMENTAL STUDY OF THE STABILITY AND METASTABILITY O F P A L M I T I C ACID O. ALBRECHT Molecular Electronics Corporation, 4030 Spencer Street, M S 108, Torrance, CA 90503 (U.S.A.) (Received April 24, 1989; accepted May 25, 1989)
Palmitic acid is one of the best known monolayer- and multilayer-forming materials. However, there is no study that presents a comprehensive range of film balance curves under consistently controlled conditions. In the present paper experimental results are presented for palmitic acid on pure water at 20 °C under a variety of different conditions. One of the surprising results is of a general nature and concerns the construction of Langmuir-Blodgett instruments. It is found that an ideal analytical instrument should be constructed quite differently from an ideal deposition instrument.
1. INTRODUCTION During many years of work with film balances the present author found that some of the basic information in the field is scattered over many publications and much of it is obscured by problems with the technology or purity of the used materials. For a beginner in the field, it is often almost impossible to obtain up-todate information because some new findings are never explicitly stated. There are several papers about the interaction of fatty acid monolayers with ions and the influence of the pH (see refs. 1 and 2 and references therein), and also data can be found about the desorption, or stability, of shorter chain fatty acids (see ref. 3). It is difficult, however, to find from the literature how the isotherm of palmitic acid (PA) at room temperature on clean water really behaves, what is artefact and what is a material property. An attempt is made to clarify this with the emphasis on experimental results and not on theoretical explanations. It is not claimed that the used fatty acid is the purest sample available, but the measured curves clearly show the proper trends. 2. MATERIALS AND EXPERIMENTAL DETAILS The PA was purchased from Fluka (reference for gas chromatography), used without further purification and spread from a solution of about 0.2 mg m l - 1. The 0040-6090/89/$3.50
© ElsevierSequoia/Printedin The Netherlands
94
o. ALBRECHT
low concentration was chosen because the spreading of a larger amount (about 180 ~tl) is more reproducible than of a small amount. The spreading solvent was chloroform, obtained from EM Science (Omnisolv, for spectroscopy). It contains 1~o ethanol and is stabilized with a non-polar hydrocarbon. This solvent has been checked by spreading PA from solutions with different concentrations and spreading large amounts of pure solvent in addition to the spreading mixture and comparing the resulting isotherms. Even when 10 times more chloroform was spread than used for a typical isotherm no change in the shape was detectable. All curves have been measured using the same stock solution (in a 25 ml flask with ground glass stopper) to eliminate the inevitable small changes in area when a new solution is prepared. It is found that because of evaporation the area of the curves tends to increase with time. This is negligible for a duration of one day, but introduces an error of up to 1~ per day of storage. On some plots this small error has been compensated by an appropriate change in the constants that are used to calibrate the curves. The water for the subphase was taken from a semiconductor grade source and passed through a Millipore system immediately before use. The resistivity and total oxidizable organic content (TOC) of the water were checked with a T O C analyzer (Anatel, Boulder, CO). Typical values were a TOC of 12 ppb and a resistivity of 17.0M~cm. The resistivity might look poor, but the Anatal instrument consistently reads lower values than the instrument that is built into the Millipore system. Typically the Anatel readout varies from 16.5 to 17.4 MD cm while the Millipore meter keeps reading 18.3 MD cm. The procedures employed were the same as described in ref. 4 and the film balance used was a copy of that described therein. Modifications were of a type that enhances the user friendliness by expanding the software into an extensive overlay system and the use of floppy disks instead of a cassette unit for storing programs and data. One new feature, which will be used later, is the possibility of measuring isotherms in a "thermodynamic" mode (cf. ref. 5). In this mode the barrier advances one step (1/1000 of the total compression) and waits until the surface pressure has equilibrated, i.e. until the digitized pressure value is twice the same within 0.5 s. The resolution of the pressure digitizer is around 0.025 mN m - ~. This mode is very useful for quickly screening materials (e.g. polymers where the time constants for relaxation may be very long and unknown) because the barrier moves rapidly where the pressure does not change and slows down only where necessary. This simulates the discontinuous, point-by-point measurement that is often recommended in the literature, but even at the stated resolution and time constant the results are still far from thermodynamic equilibrium as will be demonstrated below. Generally this method allows the easy determination of unstable and stable phases. As long as the film is stable, the curve looks smooth. When the film is unstable the result is an irregular sawtooth curve. This happens beyond the film breakdown, but for contaminated materials also around the lift-offpoint or the main transition, if exhibited by the material, or generally at any phase change. Compared with a fast (less than 10min sweep time) isotherm measured in the normal mode these curves are at a slightly higher pressure in stable areas (because of the time constants of the film and the pressure pick-up system) and at a lower pressure in unstable areas.
95
STABILITY AND METASTABILITY OF PALMITIC ACID
3. RESULTS Figure 1 shows the isotherms of PA measured at different rates of compression but otherwise identical conditions. For clarity only four of the measured eight curves with 3, 12, 96 and 384min of sweep time are shown, but they indicate clearly the trend with longer sweep times. Table I gives the breakdown pressure for the different sweep times. The 6 min curve exhibits a maximum for the breakdown pressure and the pressure beyond breakdown tends to approach ever lower values with slower compression.
oS°1 3 min.
I0
110
120 .~2/motecute
130
area
Fig. 1. Isothermsmeasuredat differentrates of compression. TABLE I BREAKDOWN PRESSURE AS FUNCTION OF THE COMPRESSION RATE
Sweep time (min) Rate of compression (nm molecule- : m i n - :) Breakdown pressure(mNm-1)
3 0.1 45.9
6 0.5 47.8
12 0.26 47.3
24 0.13 46.2
48 0.06 40.5
96 0.03 33.5
192 384 0.016 0.008 31.5 23.1
Figure 2 shows an isotherm measured in the thermodynamic mode described above. What is especially striking is that the pressure does not go up as the area approaches low values, as in all published curves. What is also striking is that the Wilhelmy plate is always pushed slightly out of the vertical when the area decreases in the constant velocity mode from about the onset of the slow increase in the breakdown area. Thus the recorded pressures are actually too low. In the thermodynamic mode this effect does not occur; the Wilhelmy plate is always in the proper position. To obtain some idea of the final pressure of the collapsing film a hysteresis curve has been measured for compression of the film beyond breakdown.
96
o . ALBRECHT
so
mN
nl
io
110
120 ~2/mo t e e '
130
area
Fig. 2. Isotherm measured in a computer-controlled point-by-point measurement (ESP, equilibrium spreading pressure).
31" mH
f~
~0
g 2O
ESP
/
120 k~/motee,
110
ra0
area
Fig. 3. Hysteresis curves, measured in compression-expansion-recompression with different pressures per area for ending the first compression. Between the curves the barrier was stopped for 15 rain (ESP, equilibrium spreading pressure).
STABILITY AND METASTABILITY OF PALMITIC ACID
97
Figure 3 shows the hysteresis curve, with two more that were stopped at 10 and at 20 m N m - 1. At the end of the first compression curve and the expansion curve the barrier was stationary for 15 min. In all cases the recompression does not reveal any loss of material, but the recompressed curves show a slightly higher breakdown pressure than the first compression shows or would show. If the film balance is stopped in the breakdown area the pressure drops slowly to a value of about 11.5 m N m - 1 and when the barrier is moved slightly to larger areas the pressure drops initially and then approaches this same value from below within minutes. Figure 4 shows several area-time curves at different pressures. The curve for 1 0 m N m-1, clearly shows little or no decrease in area (the actual change was a decrease by 3 steps out of 734 steps within 60 rain). At 20 m N m - 1 there is a decrease with time and at 30 m N m - 1 the decrease is rather fast (not shown in Fig. 4). None of these curves demonstrates any relaxation effect, i.e. an initial decrease to a second, more stable value. The lowest curve was measured at 20 m N m - 1 again, but this time only half the previous amount of material was spread. What is striking is that the idecrease in area in absolute terms is the same as in the previous case, which means that the decay in area per molecule is twice as fast. After the area-time curves were measured, the film was expanded and an isotherm was measured, with a wait time of about 5 min only in the fully expanded position. The recompression curves showed a loss in area of less than 2~. The area-time curves are very sensitive to trace impurities. A typical sequence of several curves at 10 m N m-1 shows gradually less and less instability. After running a cleaning cycle or starting out after the balance has not been used for one night the initial decrease is typically something like 50 steps out of 700 within 60 min and improves after about three or four runs to the level
~o
10 rnNlm
lb"
J
2Jo
J
~Jo
i
do ~i~.
time
Fig. 4. Area-time curves at different pressures. The lowest curve was obtained with half the amount of material spread.
98
O. ALBRECHT
as shown (it should be noted, that for all curves the subphase is completely removed with an aspirator tip and is replaced with fresh water each time). The curves shown are the best obtained after several repetitions. It has been found that the compressibilities of a material in the solid condensed phase depend on the film balance they were measured with. To investigate this effect, several isotherms have been measured with different amounts of materials spread, but with the initial area per molecule and the compression rate per molecule kept constant. Table II shows the resulting compressibilities for two different pressures. These figures clearly show that the values at 30 m N m - 1 do not reflect a material constant. Even in the more compressible range at 1 0 m N m -1, a rather high machine-dependent contribution is evident. TABLE II COMPRESSIBILITIESOF ISOTHERMSMEASURED UNDER IDENTICAL COMPRESSIONRATES PER MOLECULE AND INITIALAREASPER MOLECULE~BUT WITHDIFFERENTAMOUNTSOF TOTALMATERIAL Amount spread
Compressibilities (m N - 1) at the following pressures l O m N m -1
30mNm- 1
100% 80~o 60~o 40~o
7.5 7.7 7.7 7.8
O.643 0.646 0.702 0.749
20%
8.5
1.05
The measured compressibility at 30 m N / m - ~ is several orders of magnitude larger than the real compressibdity, because of deformation of the meniscus at the edge of the film while the surface tension changes.
4. DISCUSSION The shape and the behavior of the isotherms suggests that the "true" shape of an isotherm is a curve that follows the measured curve at least up to the breakdown point (probably to an even higher value) and then drops to a constant value, which can be identified with the equilibrium spreading pressure (ESP) defined by the change in surface tension that is measured on a film balance after applying small crystallites of the material under investigation (see, for example, ref. 6). This pressure is expected to be maintained until the area is so small that the film thickness reaches macroscopic dimensions. The minimum areas shown are the smallest accessible areas with the film balance used, keeping the Wilhelmy plateat a safe distance from the rim of the trough and the moving barrier. The range between the ESP and breakdown is metastable if the system is allowed to escape into the third dimension. There is clearly a tendency to more stable films when the system is clean. In one case so far, it was even possible to stabilize PA on pure water in the condensed phase long enough to deposit multilayers on a hydrophobized slide 7. The curves between breakdown and minimum area are almost featureless. This appears to be an indication of a clean system. It has been noticed that these curves show characteristic structures, similar to those in Fig. 5, when the film balance had
99
STABILITY AND METASTABILITY OF PALMITIC ACID
5O mN -\
m
P,~
3o
P= ==
_
1_o
ESP 0
I10
--area
~20 ~,2/motec.
130
Fig. 5. Typical isotherm of PA on a cadmium-chloride-containing subphase (concentration, 1 g l-1). More details are visible beyond the breakdown than in the previous case on pure water. The equilibrium spreading pressure is below 0.2 m N m - 1.
been used with ions several days before measurement. Although no influence on the curves in the prebreakdown region is apparent, traces of remaining ions seem to produce such structures. The experimentally found equilibrium spreading pressure of PA on CdC12 is below 0.2 m N m - 1, and thus the measurement of the ESP might be a good indicator for the absence of the rather elusive ions (cf. ref. 8). No experiments with regard to this have been tried so far. The described method of determining the ESP is much faster than waiting for several hours after sprinkling the surface with crystallites. This is partially due to the fact that the area can be expanded and compressed slightly to bracket the final value rapidly until the desired precision is reached, since the film balance surface is covered with a very large number of very fine crystallites. A very noticeable feature of the area-time curves is that the decrease in area with time is not proportional to the area of the film. This indicates that the curve is to a large extent dependent on the film balance. The following might be a clue to understanding this behaviour. The slope of the curves increases with time. This implies that the broken-down film itself induces further breakdown. Examination of the film during compression shows that, after a certain area is reached, the film begins to become visible as a milky, whitish "scum" layer on the water. From about half the breakdown area onward, the development of this layer can be seen as starting in streaks emanating diagonally from the corner between trough edge and moving barrier. Because the film is stressed most in these corners, the film first breaks down there. The resulting defects then produce more and more "crystallization" centers for further breakdown until the surface coverage is self-limiting at very low areas. An indication for this may
100
o. ALBRECHT
be obtained from the change in slope of the second 20 m N m - 1 curve at longer times. The assumption that the breakdown is not self-enhancing would result in a curve with opposite curvature because the area of the film decreases and therefore also the absolute decrease should be decreasing proportionally. This effect is overlaid in any case and makes the breakdown even more dramatic. If it is assumed that other films are similar, this behavior has significant consequences for the deposition of monolayers. What has been found many times in the past is that a film appears to be stable enough to allow deposition but, at the moment the deposition process begins, the film collapses at an increasing speed. Despite this, many times the deposited films are of surprisingly reasonable quality because the breakdown occurs where the film is stressed most, not around the object to be coated. There the film is generally relaxed, not overcompressed. The findings about the compressibility show again that, in the case of the condensed phase, the measured values are an instrument property. What is measured is the change in the shape of the meniscus along the rim of the film balance and the moving barrier when the surface tension (or film pressure) changes. The construction of the film balance used represents an attempt to minimize these errors, but obviously not very efficiently. The moving barrier is a rectangular slab of Teflon and the sides of the trough also have a rectangular cross-section (Fig. 6). To minimize the above effects, the water is filled in until the meniscus is shaped like the dotted lines in Figs. 6(a) and 6(b). If the water level is too low the water can detach from the moving barrier, causing a leak, and the line of contact along the rim is poorly defined. This causes a large error in area. If there is too much water, the film tends to deposit on the barrier and to creep beyond the edge of the rim. At the correct level, the changing surface tension leads to a change in curvature of the meniscus only, which causes small uncertainties in area but contributes a large error at low compressibilities. One way out of this is to use film balances with large surfaces, ideally combined with a geometry as proposed in Fig. 6(c). The disadvantage of this shape is that it is very liable to damage and not easy to machine. A compromise might be to use floating thin films or hydrophobic threads to contain the film, carefully controlling the height with respect to the water surface. In any case, repeating the experiment with two different amounts of material can help at least to find these problems.
(a)
water Level
.......... .
.
.
.
.
.
.
.
.
.
.
.
.
.
right Low
(b) (c) Fig. 6. The differentgeometriesof the rim of a conventionalfilm balance: (a) meniscus at the rim; (b) meniscus at the movingbarrier; (c) shape of the edge of the trough and movingbarrier for an improved analytical filmbalance.
STABILITY AND METASTABILITY OF PALMITIC ACID
101
5. CONCLUSION
A new method is proposed for measuring the ESP rapidly and to check the cleanliness of a film balance (or of the water system) for the presence of ions. The latter may be achieved by measuring the ESP or by evaluating the structure of a film beyond breakdown. The presented data clearly suggest that a film balance that is optimized for analytical work will work poorly when used for deposition work. It is assumed that the compressibility of most materials is at least as low as that of PA on pure water. It seems almost impossible to design a feedback system that can handle such high compressibilities and respond properly while an object is dipped, which causes an apparent compressibility probably more than five decades higher. For deposition the "faults" that result in high apparent compressibilities can be used deliberately to improve on the feedback parameters. What is also clear from the data is that a film balance should have as few stress points as possible, and they should be far from the place where the film is transferred. A design with one rigid moving barrier has two only whereas the popular continuous ribbon exhibits two per roller used for tensioning the ribbon. In all cases a very large surface area of the film balance is helpful. ACKNOWLEDGMENT
The help of Dr. Tariq Ginnai is gratefully acknowledged for helpful discussions as well as for correcting my German English. REFERENCES 1 2 3 4 5 6
G. Patil, R. H. Matthews and D. G. Cornwell, J. Lipid Res., 13 (1973) 574. H. Steinbach and C. Sucker, Kolloid-Z. Z. Polym., 250 (1972) 812. L. Ter Minassian-Saraga, J. Chem. Phys., 52 (1955) 14. O. Albrecht, Thin Solid Films, 99 (1983) 227. O. Albrecht, W. Prass and H. Ringsdorf, Eur. Biophys. J., 14 (1986) 97. S.R. Middleton, M. Iwasaki, N. R. Pallas and B. A. Pethica, Proc. R. Soc. London, Ser. A, 396 (1984) 143. 7 A. Laschewsky, University of Mainz, personal communication, 1983. 8 M.B. Biddle, S. E. Rickert and J. B. Lando, Thin Solid Films, 134 (1985) 121.
93
EXPERIMENTAL STUDY OF THE STABILITY AND METASTABILITY O F P A L M I T I C ACID O. ALBRECHT Molecular Electronics Corporation, 4030 Spencer Street, M S 108, Torrance, CA 90503 (U.S.A.) (Received April 24, 1989; accepted May 25, 1989)
Palmitic acid is one of the best known monolayer- and multilayer-forming materials. However, there is no study that presents a comprehensive range of film balance curves under consistently controlled conditions. In the present paper experimental results are presented for palmitic acid on pure water at 20 °C under a variety of different conditions. One of the surprising results is of a general nature and concerns the construction of Langmuir-Blodgett instruments. It is found that an ideal analytical instrument should be constructed quite differently from an ideal deposition instrument.
1. INTRODUCTION During many years of work with film balances the present author found that some of the basic information in the field is scattered over many publications and much of it is obscured by problems with the technology or purity of the used materials. For a beginner in the field, it is often almost impossible to obtain up-todate information because some new findings are never explicitly stated. There are several papers about the interaction of fatty acid monolayers with ions and the influence of the pH (see refs. 1 and 2 and references therein), and also data can be found about the desorption, or stability, of shorter chain fatty acids (see ref. 3). It is difficult, however, to find from the literature how the isotherm of palmitic acid (PA) at room temperature on clean water really behaves, what is artefact and what is a material property. An attempt is made to clarify this with the emphasis on experimental results and not on theoretical explanations. It is not claimed that the used fatty acid is the purest sample available, but the measured curves clearly show the proper trends. 2. MATERIALS AND EXPERIMENTAL DETAILS The PA was purchased from Fluka (reference for gas chromatography), used without further purification and spread from a solution of about 0.2 mg m l - 1. The 0040-6090/89/$3.50
© ElsevierSequoia/Printedin The Netherlands
94
o. ALBRECHT
low concentration was chosen because the spreading of a larger amount (about 180 ~tl) is more reproducible than of a small amount. The spreading solvent was chloroform, obtained from EM Science (Omnisolv, for spectroscopy). It contains 1~o ethanol and is stabilized with a non-polar hydrocarbon. This solvent has been checked by spreading PA from solutions with different concentrations and spreading large amounts of pure solvent in addition to the spreading mixture and comparing the resulting isotherms. Even when 10 times more chloroform was spread than used for a typical isotherm no change in the shape was detectable. All curves have been measured using the same stock solution (in a 25 ml flask with ground glass stopper) to eliminate the inevitable small changes in area when a new solution is prepared. It is found that because of evaporation the area of the curves tends to increase with time. This is negligible for a duration of one day, but introduces an error of up to 1~ per day of storage. On some plots this small error has been compensated by an appropriate change in the constants that are used to calibrate the curves. The water for the subphase was taken from a semiconductor grade source and passed through a Millipore system immediately before use. The resistivity and total oxidizable organic content (TOC) of the water were checked with a T O C analyzer (Anatel, Boulder, CO). Typical values were a TOC of 12 ppb and a resistivity of 17.0M~cm. The resistivity might look poor, but the Anatal instrument consistently reads lower values than the instrument that is built into the Millipore system. Typically the Anatel readout varies from 16.5 to 17.4 MD cm while the Millipore meter keeps reading 18.3 MD cm. The procedures employed were the same as described in ref. 4 and the film balance used was a copy of that described therein. Modifications were of a type that enhances the user friendliness by expanding the software into an extensive overlay system and the use of floppy disks instead of a cassette unit for storing programs and data. One new feature, which will be used later, is the possibility of measuring isotherms in a "thermodynamic" mode (cf. ref. 5). In this mode the barrier advances one step (1/1000 of the total compression) and waits until the surface pressure has equilibrated, i.e. until the digitized pressure value is twice the same within 0.5 s. The resolution of the pressure digitizer is around 0.025 mN m - ~. This mode is very useful for quickly screening materials (e.g. polymers where the time constants for relaxation may be very long and unknown) because the barrier moves rapidly where the pressure does not change and slows down only where necessary. This simulates the discontinuous, point-by-point measurement that is often recommended in the literature, but even at the stated resolution and time constant the results are still far from thermodynamic equilibrium as will be demonstrated below. Generally this method allows the easy determination of unstable and stable phases. As long as the film is stable, the curve looks smooth. When the film is unstable the result is an irregular sawtooth curve. This happens beyond the film breakdown, but for contaminated materials also around the lift-offpoint or the main transition, if exhibited by the material, or generally at any phase change. Compared with a fast (less than 10min sweep time) isotherm measured in the normal mode these curves are at a slightly higher pressure in stable areas (because of the time constants of the film and the pressure pick-up system) and at a lower pressure in unstable areas.
95
STABILITY AND METASTABILITY OF PALMITIC ACID
3. RESULTS Figure 1 shows the isotherms of PA measured at different rates of compression but otherwise identical conditions. For clarity only four of the measured eight curves with 3, 12, 96 and 384min of sweep time are shown, but they indicate clearly the trend with longer sweep times. Table I gives the breakdown pressure for the different sweep times. The 6 min curve exhibits a maximum for the breakdown pressure and the pressure beyond breakdown tends to approach ever lower values with slower compression.
oS°1 3 min.
I0
110
120 .~2/motecute
130
area
Fig. 1. Isothermsmeasuredat differentrates of compression. TABLE I BREAKDOWN PRESSURE AS FUNCTION OF THE COMPRESSION RATE
Sweep time (min) Rate of compression (nm molecule- : m i n - :) Breakdown pressure(mNm-1)
3 0.1 45.9
6 0.5 47.8
12 0.26 47.3
24 0.13 46.2
48 0.06 40.5
96 0.03 33.5
192 384 0.016 0.008 31.5 23.1
Figure 2 shows an isotherm measured in the thermodynamic mode described above. What is especially striking is that the pressure does not go up as the area approaches low values, as in all published curves. What is also striking is that the Wilhelmy plate is always pushed slightly out of the vertical when the area decreases in the constant velocity mode from about the onset of the slow increase in the breakdown area. Thus the recorded pressures are actually too low. In the thermodynamic mode this effect does not occur; the Wilhelmy plate is always in the proper position. To obtain some idea of the final pressure of the collapsing film a hysteresis curve has been measured for compression of the film beyond breakdown.
96
o . ALBRECHT
so
mN
nl
io
110
120 ~2/mo t e e '
130
area
Fig. 2. Isotherm measured in a computer-controlled point-by-point measurement (ESP, equilibrium spreading pressure).
31" mH
f~
~0
g 2O
ESP
/
120 k~/motee,
110
ra0
area
Fig. 3. Hysteresis curves, measured in compression-expansion-recompression with different pressures per area for ending the first compression. Between the curves the barrier was stopped for 15 rain (ESP, equilibrium spreading pressure).
STABILITY AND METASTABILITY OF PALMITIC ACID
97
Figure 3 shows the hysteresis curve, with two more that were stopped at 10 and at 20 m N m - 1. At the end of the first compression curve and the expansion curve the barrier was stationary for 15 min. In all cases the recompression does not reveal any loss of material, but the recompressed curves show a slightly higher breakdown pressure than the first compression shows or would show. If the film balance is stopped in the breakdown area the pressure drops slowly to a value of about 11.5 m N m - 1 and when the barrier is moved slightly to larger areas the pressure drops initially and then approaches this same value from below within minutes. Figure 4 shows several area-time curves at different pressures. The curve for 1 0 m N m-1, clearly shows little or no decrease in area (the actual change was a decrease by 3 steps out of 734 steps within 60 rain). At 20 m N m - 1 there is a decrease with time and at 30 m N m - 1 the decrease is rather fast (not shown in Fig. 4). None of these curves demonstrates any relaxation effect, i.e. an initial decrease to a second, more stable value. The lowest curve was measured at 20 m N m - 1 again, but this time only half the previous amount of material was spread. What is striking is that the idecrease in area in absolute terms is the same as in the previous case, which means that the decay in area per molecule is twice as fast. After the area-time curves were measured, the film was expanded and an isotherm was measured, with a wait time of about 5 min only in the fully expanded position. The recompression curves showed a loss in area of less than 2~. The area-time curves are very sensitive to trace impurities. A typical sequence of several curves at 10 m N m-1 shows gradually less and less instability. After running a cleaning cycle or starting out after the balance has not been used for one night the initial decrease is typically something like 50 steps out of 700 within 60 min and improves after about three or four runs to the level
~o
10 rnNlm
lb"
J
2Jo
J
~Jo
i
do ~i~.
time
Fig. 4. Area-time curves at different pressures. The lowest curve was obtained with half the amount of material spread.
98
O. ALBRECHT
as shown (it should be noted, that for all curves the subphase is completely removed with an aspirator tip and is replaced with fresh water each time). The curves shown are the best obtained after several repetitions. It has been found that the compressibilities of a material in the solid condensed phase depend on the film balance they were measured with. To investigate this effect, several isotherms have been measured with different amounts of materials spread, but with the initial area per molecule and the compression rate per molecule kept constant. Table II shows the resulting compressibilities for two different pressures. These figures clearly show that the values at 30 m N m - 1 do not reflect a material constant. Even in the more compressible range at 1 0 m N m -1, a rather high machine-dependent contribution is evident. TABLE II COMPRESSIBILITIESOF ISOTHERMSMEASURED UNDER IDENTICAL COMPRESSIONRATES PER MOLECULE AND INITIALAREASPER MOLECULE~BUT WITHDIFFERENTAMOUNTSOF TOTALMATERIAL Amount spread
Compressibilities (m N - 1) at the following pressures l O m N m -1
30mNm- 1
100% 80~o 60~o 40~o
7.5 7.7 7.7 7.8
O.643 0.646 0.702 0.749
20%
8.5
1.05
The measured compressibility at 30 m N / m - ~ is several orders of magnitude larger than the real compressibdity, because of deformation of the meniscus at the edge of the film while the surface tension changes.
4. DISCUSSION The shape and the behavior of the isotherms suggests that the "true" shape of an isotherm is a curve that follows the measured curve at least up to the breakdown point (probably to an even higher value) and then drops to a constant value, which can be identified with the equilibrium spreading pressure (ESP) defined by the change in surface tension that is measured on a film balance after applying small crystallites of the material under investigation (see, for example, ref. 6). This pressure is expected to be maintained until the area is so small that the film thickness reaches macroscopic dimensions. The minimum areas shown are the smallest accessible areas with the film balance used, keeping the Wilhelmy plateat a safe distance from the rim of the trough and the moving barrier. The range between the ESP and breakdown is metastable if the system is allowed to escape into the third dimension. There is clearly a tendency to more stable films when the system is clean. In one case so far, it was even possible to stabilize PA on pure water in the condensed phase long enough to deposit multilayers on a hydrophobized slide 7. The curves between breakdown and minimum area are almost featureless. This appears to be an indication of a clean system. It has been noticed that these curves show characteristic structures, similar to those in Fig. 5, when the film balance had
99
STABILITY AND METASTABILITY OF PALMITIC ACID
5O mN -\
m
P,~
3o
P= ==
_
1_o
ESP 0
I10
--area
~20 ~,2/motec.
130
Fig. 5. Typical isotherm of PA on a cadmium-chloride-containing subphase (concentration, 1 g l-1). More details are visible beyond the breakdown than in the previous case on pure water. The equilibrium spreading pressure is below 0.2 m N m - 1.
been used with ions several days before measurement. Although no influence on the curves in the prebreakdown region is apparent, traces of remaining ions seem to produce such structures. The experimentally found equilibrium spreading pressure of PA on CdC12 is below 0.2 m N m - 1, and thus the measurement of the ESP might be a good indicator for the absence of the rather elusive ions (cf. ref. 8). No experiments with regard to this have been tried so far. The described method of determining the ESP is much faster than waiting for several hours after sprinkling the surface with crystallites. This is partially due to the fact that the area can be expanded and compressed slightly to bracket the final value rapidly until the desired precision is reached, since the film balance surface is covered with a very large number of very fine crystallites. A very noticeable feature of the area-time curves is that the decrease in area with time is not proportional to the area of the film. This indicates that the curve is to a large extent dependent on the film balance. The following might be a clue to understanding this behaviour. The slope of the curves increases with time. This implies that the broken-down film itself induces further breakdown. Examination of the film during compression shows that, after a certain area is reached, the film begins to become visible as a milky, whitish "scum" layer on the water. From about half the breakdown area onward, the development of this layer can be seen as starting in streaks emanating diagonally from the corner between trough edge and moving barrier. Because the film is stressed most in these corners, the film first breaks down there. The resulting defects then produce more and more "crystallization" centers for further breakdown until the surface coverage is self-limiting at very low areas. An indication for this may
100
o. ALBRECHT
be obtained from the change in slope of the second 20 m N m - 1 curve at longer times. The assumption that the breakdown is not self-enhancing would result in a curve with opposite curvature because the area of the film decreases and therefore also the absolute decrease should be decreasing proportionally. This effect is overlaid in any case and makes the breakdown even more dramatic. If it is assumed that other films are similar, this behavior has significant consequences for the deposition of monolayers. What has been found many times in the past is that a film appears to be stable enough to allow deposition but, at the moment the deposition process begins, the film collapses at an increasing speed. Despite this, many times the deposited films are of surprisingly reasonable quality because the breakdown occurs where the film is stressed most, not around the object to be coated. There the film is generally relaxed, not overcompressed. The findings about the compressibility show again that, in the case of the condensed phase, the measured values are an instrument property. What is measured is the change in the shape of the meniscus along the rim of the film balance and the moving barrier when the surface tension (or film pressure) changes. The construction of the film balance used represents an attempt to minimize these errors, but obviously not very efficiently. The moving barrier is a rectangular slab of Teflon and the sides of the trough also have a rectangular cross-section (Fig. 6). To minimize the above effects, the water is filled in until the meniscus is shaped like the dotted lines in Figs. 6(a) and 6(b). If the water level is too low the water can detach from the moving barrier, causing a leak, and the line of contact along the rim is poorly defined. This causes a large error in area. If there is too much water, the film tends to deposit on the barrier and to creep beyond the edge of the rim. At the correct level, the changing surface tension leads to a change in curvature of the meniscus only, which causes small uncertainties in area but contributes a large error at low compressibilities. One way out of this is to use film balances with large surfaces, ideally combined with a geometry as proposed in Fig. 6(c). The disadvantage of this shape is that it is very liable to damage and not easy to machine. A compromise might be to use floating thin films or hydrophobic threads to contain the film, carefully controlling the height with respect to the water surface. In any case, repeating the experiment with two different amounts of material can help at least to find these problems.
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(b) (c) Fig. 6. The differentgeometriesof the rim of a conventionalfilm balance: (a) meniscus at the rim; (b) meniscus at the movingbarrier; (c) shape of the edge of the trough and movingbarrier for an improved analytical filmbalance.
STABILITY AND METASTABILITY OF PALMITIC ACID
101
5. CONCLUSION
A new method is proposed for measuring the ESP rapidly and to check the cleanliness of a film balance (or of the water system) for the presence of ions. The latter may be achieved by measuring the ESP or by evaluating the structure of a film beyond breakdown. The presented data clearly suggest that a film balance that is optimized for analytical work will work poorly when used for deposition work. It is assumed that the compressibility of most materials is at least as low as that of PA on pure water. It seems almost impossible to design a feedback system that can handle such high compressibilities and respond properly while an object is dipped, which causes an apparent compressibility probably more than five decades higher. For deposition the "faults" that result in high apparent compressibilities can be used deliberately to improve on the feedback parameters. What is also clear from the data is that a film balance should have as few stress points as possible, and they should be far from the place where the film is transferred. A design with one rigid moving barrier has two only whereas the popular continuous ribbon exhibits two per roller used for tensioning the ribbon. In all cases a very large surface area of the film balance is helpful. ACKNOWLEDGMENT
The help of Dr. Tariq Ginnai is gratefully acknowledged for helpful discussions as well as for correcting my German English. REFERENCES 1 2 3 4 5 6
G. Patil, R. H. Matthews and D. G. Cornwell, J. Lipid Res., 13 (1973) 574. H. Steinbach and C. Sucker, Kolloid-Z. Z. Polym., 250 (1972) 812. L. Ter Minassian-Saraga, J. Chem. Phys., 52 (1955) 14. O. Albrecht, Thin Solid Films, 99 (1983) 227. O. Albrecht, W. Prass and H. Ringsdorf, Eur. Biophys. J., 14 (1986) 97. S.R. Middleton, M. Iwasaki, N. R. Pallas and B. A. Pethica, Proc. R. Soc. London, Ser. A, 396 (1984) 143. 7 A. Laschewsky, University of Mainz, personal communication, 1983. 8 M.B. Biddle, S. E. Rickert and J. B. Lando, Thin Solid Films, 134 (1985) 121.