4588
J . Phys. Chem. 1990, 94, 4588-4595
Phase Diagram of Langmulr Monotayers of Pentadecanoic Acid: Quantitative Comparison of Surface Pressure and Fluorescence Microscopy Results Brian G. Moore, Charles M. Knobler,* Department of Chemistry and Biochemistry, University of California, Los Angeles. California 90024
Silvlre Akamatsu, and Francis Rondelez Laboratoire Structure et REactivitE aux Interfaces, UniversitP Pierre et Marie Curie (Paris VI), 1 I Rue Pierre et Marie Curie, 75231 Paris, Cedex 5. France (Received: May 31, 1989; In Final Form: January 12, 1990)
The phase behavior of Langmuir monolayers of pentadecanoic acid has been investigated by measurements of surface pressure-area isotherms and by fluorescence microscopy. The two techniques are in quantitative agreement and allow the phase boundaries between the gas (G), liquid expanded (LE), and liquid condensed (LC) phases to be determined with precision. The LE-LC and LE-G transitions are first-order. The corresponding critical temperatures, if any, are higher than 37 "C for the LE-LC transition and 70 "C for the LE-G transition.
Introduction Langmuir monolayers, the monomolecular films that form when amphiphilic molecules spread at the air/water interface, can be described as two-dimensional systems that can exist in several different surface phases.'P2 Harkins3 originated a systematic classification for the monolayer phases that is widely used. At low densities, monolayers exist in a gaseous state (G), which on compression at constant temperature can undergo a first-order phase transition to a more condensed, 'liquid-expanded" (LE) state. On further compression, the LE phase undergoes a transition to the "liquid-condensed" (LC) state. The nature of the LE-LC transition as been in doubt and we will address this question later. Harkins suggested that there was an intermediate phase (I) between the LE and LC phases and that the LE-I and I-LC transitions were second-order, but there is now a consensus that this phase does not exist. At high densities, the LC phase undergoes a first-order transition into an even more condensed phase, presumably solid ( S ) . As in three dimensions, the stability ranges of the phases depend on the temperature. At sufficiently low temperature a monolayer may go directly from the gaseous to the liquid-condensed state without passing through the intermediate LE state; at high temperature, the G-LE transition is expected to disappear at a critical point. It has also been suggested that the LE-LC transition could end at a critical or tricritical point. The identification of the monolayer phases and the boundaries between them has been based primarily on measurements of the surface pressure II as a function of the area per molecule. According to the phase rule, the slope of a JJ-A isotherm for a one-component system must be horizontal when two phases are in equilibrium; the limits of a two-phase region should therefore be detectable by changes in slope. In practice, however, there are a host of experimental factors-impurities present in the amphiphile, impurities that leach out of the trough or subphase or enter from the air, solubility of the amphiphile in the subphase, drifts in the pressure-measuring system, leakage of the barrier, systematic variations in the pressure associated with the spreading and technique by which the density is changed, etc.-that can distort the shape of the isotherm and complicate the interpretation. The location of phase boundaries from isotherm measurements can be difficult even in three-dimensional systems, for which the pressure can be measured with instruments of very high sensitivity ( I ) Gaines, G . L.. Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966. (2) Adamson, A. W. Physical Chemistry of Surfaces, 4th ed.;Wiley: New York, 1982; Chapter 4. (3) Harkins, W . D. The Physical Chemistry of Surface Films; Reinhold: New York. 1952; Chapter 2.
0022-3654/90/2094-4588$02.50/0
and stability and in which the relatively large size of the sample minimizes the effects of impurities leached from the apparatus. The problems are particularly severe in the neighborhood of critical point^:^ the limits of two-phase coexistence are not easily recognized because isotherms remain very flat well above the critical temperature, and the two-phase and one-phase segments of an isotherm do not meet at a sharp angle because there are sizeable fluctuations in one-phase regions near coexistence. As Fisher5 has pointed out, the interpretation of isotherms near two-dimensional critical points is especially questionable because of the extreme flatness of isotherms and coexistence curves in two dimensions. In fact, studies of phase boundaries by isotherm measurements are rarely carried out in three-dimensional systems. The coexistence regions are usually determined by visual observation. It had not been possible to observe monolayers directly, but recently McConnel16 and Mohwald' and their co-workers have demonstrated that the technique of fluorescence microscopy allows Langmuir monolayers to be seen, and this opens the possibility of direct determinations of their phase behavior. The full potential of the fluorescence technique for quantitative measurements of a monolayer phase diagram has not yet been realized. Most of the measurements that have been reported have been qualitative or concerned with pattern formation. Phospholipids have received the most attention, although there have been limited studies on stearic and pentadecanoic In this paper we report quantitative comparisons between fluorescence and isotherm measurements over a range of temperatures. The system examined is pentadecanoic acid (PDA), which is a fatty acid that has been studied in great detail by classical methods. Experiments on PDA were carried out over 60 years ago by Adam and Jessup;l0 extensive measurements were later performed by Harkins and co-workers."-12 More recently, PDA has been the subject of two unusually careful and detailed investigations. Kim and CannellI3 studied (4) See, e&: Simon, M.; Knobler, C . M . Ber. Bunsen-Ges. Phys. Chem.
-.
1972. 76. 321.
Fisher, M. E. J . Chem. Soc., Faraday Trans. 2 1986,82, 1824. von Tscharner, V.; McConnell, H. M. Biophys. J . 1981, 36, 409. Losche, M.; Sackmann, E.; Mohwald, H. Ber. Bunsen-Ges. Phys. . 1983, 87, 848. Losche, M.; Mohwald, H . Reu. Sci. Instrum., 1984, 55, 1968. (8) Rondelez, F.; Baret, J. F.; Suresh, K. A.; Knobler, C . M. Proceedings of NATO Advanced Study Insritute on Physico-Chemical Hydrodynamics: Interfacial Phenomena: Velarde, M . G., Ed.; Plenum: New York, 1988; p 857. (9) Knobler, C. M. Adv. Chem. Phys. 1990, 77, 397. ( I 0) Adam, N . K.; Jessop, G. Proc. R. Soc. London, A 1926, 112, 364. ( I I ) Harkins, W. D.; Young, T. F.: Boyd, E. J . Chem. Phys. 1940,8,954. (12) Harkins, W. D.; Boyd, E. J . Phys. Chem. 1941, 45, 20.
0 1 990 American Chemical Society
Langmuir Monolayers of Pentadecanoic Acid the LE-G coexistence region with a very sensitive differential pressure measurement technique. They determined the coexistence curve and concluded that a critical point existed at 26 “C. (Previous isotherm measurements by Hawkins and BenedekI4gave a similar critical temperature, but the experiments were performed on an unacidified subphase and are therefore s ~ s p e c t . ’ ~ ) The other study was carried out by Pallas and Pethica,I5 who concluded from their isotherms that the G-LE transition in PDA had no critical point below 40 OC. The coexisting densities that they reported were also markedly different from those determined by Kim and Cannell. Pallas and Pethica16also studied the LE-LC transition in PDA. In contrast to all previous experimenters, they were able to obtain isotherms with a marked horizontal plateau, which indicates that the LE-LC transition is indeed first order, a point that has been disputed ever since the early measurements. Horizontal isotherms in the LE-LC transition region have now also been observed by Chen et al.,” Pegg and Morrison,’* and Hifeda,I9 each of whom used experimental protocols that differ somewhat from those employed by Pallas and Pethica. We will compare our fluorescence studies with these isotherm measurements and with a series of more closely spaced isotherms on PDA that we have carried out to higher pressures. Isotherm measurements have also been carried out on PDA-probe mixtures in order to assess the effect of the probe on the phase behavior. As we will show, the fluorescence studies are in excellent accord with the more classical measurements and allow precise determinations to be made of phase boundaries at the G-LE and LE-LC transitions.
Experimental Section The experiments were performed in two laboratories, at UCLA and at the University of Paris. Both groups carried out fluorescence and isotherm measurements. Although the procedures employed in the two laboratories differed somewhat, the results are consistent with each other (and, as we shall demonstrate, with the work of previous investigators). The availability of image analysis facilities at UCLA allowed a quantitative interpretation of the microscopic investigations that was not possible in Paris, so it is this work that is described in detail here. (An incompatibility between European and American television standards made it impractical to analyze tapes recorded in Paris at UCLA.) On the other hand, the isotherm measurements performed in Paris were much more extensive and precise than those at UCLA, and it is therefore the Paris data that are presented. The Langmuir trough for the isotherm studies was constructed of poly(viny1 chloride) and was cleaned with chromic acid. Films were deposited in a rectangular frame 15 X 15 cmz made of paraffin-coated mica that floats on the water surface. A mechanically driven paraffin-coated mica strip acts as a movable barrier and allows the monolayer to be compressed from 225 to 100 cm2. A platinum Wilhelmy plate, with a sensitivity of 10.1 mN m-I, was used for the pressure measurements; it was cleaned by heating it red hot in a gas flame. The temperature regulation ( f O . l K ) was by water circulation. The reproducibility of the isotherms was f0.2 mN m-I in the pressure and f0.3 A2/molecule in the area. Studies were carried out both by compression and expansion. The pressure and area at the kink in the isotherm at the start of the LE-LC transition were independent of the speed of compression over the range 0.3-3 A2 molecule-I m i d . If the monolayer is expanded, the kink occurs at the same pressure, but the area is smaller by about 0.6 A2; isotherms obtained by expansion are always flatter than those obtained by compression. All the data reported here have been (13) Kim, M. W.; Cannell, D. Phys. Reu. A 1976, 13,411. (14) Hawkins, G. A,; Benedek, G. B. Phys. Reu. Lorr. 1974, 32, 524. ( 1 5 ) Pallas, N. R.; Pethica, B. A. J . Chem. Soc., Faraday Trans. l 1987,
83. 585. Pallas, N. R. Ph.D. Thesis, Clarkson College of Technology, 1983. (16) Pallas, N . R.; Pethica, B. A. Langmuir 1985, I , 509. (17) Chen, Y.-L.; Sano, M.; Kawaguchi, M.; Yu, H.; Zografi, G. Lungmuir 1986, 2. 349. ( I 8) Pegg, 1.; Morrison, G. Unpublished.
(19) Hifeda. Y . M. Ph.D. Thesis, University of Oregon, 1988.
The Journal of Physical Chemistry, Vol. 94, No. 11, 1990 4589 obtained by continuous compression. A Reichert-Jung Polyvar Met microscope equipped for epifluorescence was used for the fluorescence microscopy studies. Images were detected with a SIT television camera and recorded on videotape for later analysis. The Teflon trough, which was 20 cm2 in area, was mounted on the microscope stage. The bottom of the trough consisted of a Teflon sheet 0.1 mm thick, whose lower surface was in direct contact with circulating thermostated water. A motorized Teflon barrier could be used to compress the film, but in all of the quantitative measurements made at UCLA, ”one-shot” additions were made at constant area. In order to reduce evaporation and convection, the trough was covered with a thin glass plate. For measurements made above room temperature, the plate was heated by passing a current through an indium/tin oxide coating on its surface. The current was adjusted to bring the plate to the minimum temperature at which no condensation occurred on the glass (1-2 K above the trough temperature). The water for the monolayer substrate utilized by both groups was distilled water that had been additionally purified by passing it through a Milli-Q system that included an Organics filtering cartridge. Measurements with a Wilhelmy plate of the surface tension of the water treated this way gave 72 f 1 mN m-l at 25 OC (UCLA) and 72.9 f 0.2 mN m-I at 20 OC (France), in agreement with the literature values.20 After full compression of a clean surface in the Langmuir trough the surface tension was 72.6 mN d. The PDA was obtained from Nu-Chek Prep (UCLA) and Fluka Chemicals (Paris), both with stated purity >99%. The fluorescent probe, NBD-HDA [4-(hexadecylamino)-7-nitrobenz-2-oxa- 1,3-diazole] was supplied by Molecular Probes, Inc. Both the acid and the probe were used without further purification. Stock solutions of PDA and NBD-HDA were prepared in chloroform (Fisher, reagent Grade) at UCLA and in a 9:l chloroform/ethanol mixture (Merck, spectroscopic grade) in Paris, and then mixed. Experiments were carried out at probe concentrations as low as 10-3 mol % and as high as 2 mol %. Monolayers were formed by depositing a small amount (1-50 pL) of the solutions onto the surface of the water with a Hamilton syringe and allowing the solvent to evaporate. No solvent retention was observed in the isotherm measurements. Fluorescence studies with myristic acid have been carried out in Paris with a variety of spreading solvents-chloroform, chloroform/ethanol, and hexane/ethanol. They give identical results as long as the solvents are clean and free of impurities. We therefore do not believe that the nature of the solvent has any influence on our pentadecanoic acid experiments. As will be demonstrated in the Results and Analysis section, the fluorescence technique allows the monolayer phases to be distinguished. Thus, it becomes possible to carry out many of the thermodynamic studies of phase equilibria that are commonly employed in investigations of three-dimensional systems. One can, for example, locate phase boundaries by compressing a fixed amount of an amphiphile at constant temperature or by changing the temperature along an isochore. In both cases the crossing of a phase boundary is detectable by the appearance or disappearance of a phase. Phase boundaries can also be determined by application of the “lever rule”, the technique that we have utilized in most of the quantitative measurements. In such experiments, which are performed at a fixed average density, the molar densities of the coexisting phases are obtained from measurements of the fraction of the total area occupied by each phase. The analogous threedimensional measurements are usually carried out on fluid phases, and the relative volumes are then easily determined because the system separates into two homogeneous phases. Gravity does not enhance phase separation in monolayers or define a special direction, so the film remains very heterogeneous. Moreover, we often observe that the scale of the heterogeneity is not uniform. Thus, it is essential that one be able to scan the entire surface (20) Pallas, N . R.; Pethica, B. A. Colloids Sur/. 1983, 6, 221.
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of the trough, and for this reason we have chosen to work with such a small area. Image analysis was performed at UCLA and could be employed to determine the phase boundaries with the lever rule. Images were recorded at a number of average monolayer densities in a two-phase region. After smoothing and subtracting a background in order to correct for the nonuniformity of the field of the microscope, a cutoff intensity value was chosen for which the two phases were clearly distinguishable. A “binary image” could then be generated in which one phase was completely black and the other completely white. The cutoff was chosen by viewing binary images Corresponding to a series of cutoff values and then choosing the value that produced the binary image which best reproduced the original. A check of the histograms of a few images showed that the histogram (after background subtraction) was distinctly bimodal and that the pixel intensity cutoff chosen visually corresponded to the minimum between the two peaks in the histogram. Then @, the fraction of the pixels above the cutoff, which is the fraction of the area occupied by one of the phases, was plotted against the overall number density, p. The plots were linear and an unweighted least-squares fit could be extrapolated to @ = 1 and Q = 0 to obtain the densities at the boundaries of the coexistence region. One should note that the slope and intercept determined from this fitting procedure are necessarily correlated, so it is essential that the covariance2’ be included in determining the uncertainty in the densities. For example, if the intercept and slope are denoted by a and b, respectively, then the relative variances in the densities of the LE and LC phases are given by ma2
0 P L E2
-=-
pLE2
( I -a)*
02
+ -b2+ -
2uab2 b(1 - a )
-u p-~ c 2 - -0:+ - - -ab2 20ab2 pLc2
a2
b2
ab
where ua is the standard deviation of the intercept, a b is that of the slope and uab is the covariance. Estimates of errors can be very misleading if the covariances are not included. For example, in a typical data set, u p / p for the LC phase was 0.25 if the covariance was neglected but 0.03 when it was included.
Results and Analysis A. General Features of the Phase Diagram. Figure 1 shows an isotherm measured at 25 OC by continuous compression. Its shape is typical of all of the isotherms studied (for temperatures above the monolayer triple point, which is about 17 OC for PDA). The pressure becomes detectable with our apparatus at a molecular area of -43 A2 and then rises smoothly until A L E = 31.6 f 0.2 A2and l l L E = 8.0 f 0.1 mN m-l, where there is an abrupt change in slope. The new slope value is about 0.14 mN m-l A-2 until -26 A2, where the pressure rises steeply. A small kink appears at a molecular area, A ~of, -20 A2. Also plotted in Figure 1 are the 25 OC isotherm measurements obtained by Harkins and BoydI2 using continuous compression and those found in discontinuous compression studies by Pallas and Pethica.16 The latter measurements differ from the flatter “single-shot” isotherms also reported by Pallas and Pethica but are shown here because the technique used, and perhaps the systematic errors, are closer to ours. Neither Harkins and Boyd nor Pallas and Pethica carried out their studies to sufficiently high pressure to observe the kink that can be detected at high compression in our isotherms. The fluorescence images that correspond to the various stages of compression in a 25 OC isotherm are shown in Figure 2. At a molecular area of 61 A2, which according to the classical description of Adam and Harkins is in the L E 4 region, one observes a bright field containing dark circular regions. The dark regions are bubbles of the two-dimensional gas surrounded by the liquid (21) Bevington, P. Data Reduction and Error Analysis f o r the Physical Sciences, McCraw-Hill: New York, 1968; p 154.
I
I 5t
1
I
+
+\A
J
A r e a (A2/Molecule) Figure 1. II-A isotherms of P D A at 25 OC. Solid line, continuous compression results; A, results of Harkins et al. (ref 11); +, Pallas and Pethica (ref 16).
phase. The contrast between the two phases is the result of both the difference in density and the quenching of the dye in the gas phase.22 As the density of the monolayer is increased, either by compression or by addition, the gas bubbles become smaller and eventually disappear, leaving a uniformly bright film corresponding to a single phase. This occurs at -43 A2, the same point where the pressure rise signals the end of the L E 4 coexistence region in the isotherm measurements. The film remains homogeneous from 43 until -32 A2, the start of the LE-LC transition, ALE. At higher compression, the image once again is heterogeneous, the dark circular regions now being associated with the LC phase. In this case the contrast is due to the lack of solubility of the dye in the more condensed phase. The direct observation of coexisting domains in the LE-LC transition constitutes a convincing confirmation that this transition is first-order. With further compression, the dark LC regions grow and, if the dye concentration is very low (S10-3 mol %), a nearly completely dark field can be obtained, corresponding to the disappearance of the LE phase at about 20 A*/molecule. We have shown that the fluorescence images correspond to the progression of phases that has been deduced from the isotherms. In the next sections, we will demonstrate that there is quantitative agreement between the two types of measurements. B. Effect of the Probe. Since the probe is an impurity, it is necessary to examine its effect on the phase behavior before making quantitative comparisons. A comparison is made between an isotherm of pure PDA at 20.4 OC and that of a mixture of 2 mol % NBD-HDA in PDA (Figure 3). At the start of the LE-LC region, the isotherm is shifted to higher pressure by 0.4 mN m-’ and the area at which the discontinuity in the slope occurs is shifted to higher values by about 0.8 A2/molecule. The study of the influence of the probe on PDA is confirmed by extensive independent measurements on tetradecanoic (myristic) acid.23 It is found that the pressure changes by 0.17 f 0.02 mN m-l per mol % of probe added, and that there is no change in area for probe concentrations below 0.5 mol % (within the experimental error of f0.3 A2/molecule). Fluorescence studies have been carried out for samples with probe concentrations ranging from 2 to IO-’ mol % in PDA. The same pattern of phases is always observed, and the size of the (22) Moore, B.; Knobler, C. M.; Broseta, D.; Rondelez, F. J . Chem. SOC., Faraday Trans. 2 1986, 82, 1753. (23) Akamatsu, S.:Rondelez, F. Unpublished results.
Langmluir Monolayers of Pentadecanoic Acid
a
The Journal of Physical Chemistry, Vol. 94, No. 11, 1990 4591
b
C
I
e f Figure 2. Fluorescence microscopy images of PDA at 25 OC. Molecular areas: (a) 60.9 A2; (b) 50.0 A2; (c) 36.5 Biz; (d) 27.1 A2; (e) 24.4 A2; (f) 19.3 A2. Images (a) and (b) are coexistence of LE and G phases, (c) is the LE phase only, and (d)-(f) are LE-LC coexistence.
nucleating domains and their subsequent evolution upon compression are also independent of the probe concentration. The phase boundaries, visible only with probe concentrations of at least lo-* mol %, are unaffected, in agreement with the isotherm results. mol %, the contrast between the At a probe concentration of LE and LC phases is too low to allow the observation of the phase boundary on the low-density side, but upon further compression the probe-enriched LE phase becomes bright enough to make the two phases visible in the coexistence region. The partitioning the probe between the LE and LC phases accounts for the difference in contrast. The solubility of the probe
in the LC phase is not negligibly small, however, as demonstrated by the observation that in rapid expansions the LE phase immediately appears in the middle of dark LC regions: the diffusion of the probe does not control the rate at which the bright phase appears. Unless the amount of probe employed is very small, determinations of the density of the LC phase by compressing a two-phase system until the LE phase disappears are subject to large systematic errors. On the other hand, if the density is determined by the lever rule, measurements can be carried out for relative areas of the phases at which the increase in the probe concentration in the LE phase is not significant, as demonstrated
4592
The Journal of Physical Chemistry, Vol. 94, No. 1I, I990
Moore et al.
i
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imolecule/A2) Figure 5. Ratio of the area of the LC phase to the total area of the image (digitized from the fluorescence microscope) vs p , the surface molecular density. 0,30 OC; +, 23 "C; A, 17 "C. Solid lines are the linear least-squares fits to the data. P
Figure 3. Two ll-A isotherms of PDA at 20.4 OC. Solid line, PDA with no probe; dashed line, PDA with 2 mol 9% probe added. 1
1
-
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Figure 4. Film pressure llLE, at the onset of the LE-LC transition as a function of temperature. Solid line is the linear least-squares fit to the
data. by the linearity of plots of Q, against p. C. Quantitatiue Comparisons. From the fluorescence microscopy measurements, the break in the isotherm at the start of the LE-LC transition can definitely be associated with a phase boundary. The location of this boundary can be determined with precision from the isotherm measurements. The variation of nLE with temperature from 18 to 30 OC is shown in Figure 4. The pressure changes approximately linearly with temperature, a behavior typical of many amphiphiles. Kellner et al.24suggested that for fatty acids one could represent the transition pressure by the relation n L E = m( T - To) where, according to Bouloussa and D ~ p e y r a tm, ~is~ a constant (24) Kellner, M . J.; Mhller-Landau, F.; Cadenhead, D. A. J . Colloid Interface Sei. 1978,82,597. (25) Bouloussa, 0.; Dupeyrat, M. Biochim. Biophys. Acta 1987,896, 239.
I,, ,
2616
20
, 24
,
,*!I
28
32
T ('CC) Figure 6. Molecular area, ALE,at the onset of the LE-LC transition as a function of temperature. from ll vs A isotherms; 4, from fluores-
+,
cence microscopy using the lever rule technique described in the text. and Todepends on the chain length. In the case of PDA, we found from least-squares analysis m = 1.14 f 0.05 mN m-I K-I and To = 17.9 f 0.1 OC, in good agreement with the results of Bouloussa and Dupeyrat. The values of ALE( from the isotherms can be directly compared with the position of the LE-LC phase boundary located by fluorescence microscopy using image analysis and the "lever-rule" technique. Sample plots of 9 [=ALc-(ALc + ALE)] vs p for three temperatures (17, 23, and 30 " C ) are shown in Figure 5. Each data point represents an average of three to five images taken in different parts of the trough. The results for the phase boundary ALE,as determined by fluorescence microscopy and by the isotherm measurements, are plotted in Figure 6. The agreement is excellent, better than one should expect given the systematic differences of 1-2 A2 that are often found between
The Journal of Physical Chemistry, Vol. 94, No. 11. 1990 4593
Langmuir Monolayers of Pentadecanoic Acid
-
39 -
A
35 -
A
A
-m
:3 8
0.6
A
A
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- = L P
27
1
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24
30
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36
42
48
(i2/Molecule)
Figure 7. T vs A phase diagram for PDA. The leftmost band of data
points represents AK and ALC.those data poinis at the center represent ALE, those data points to the right represent ALE. 0,fluorescence microscopy results, +; isotherm results; 0, results of Pallas and Pethica (refs 15 and 16); +, results of Middleton and Pethica (ref 26); A, results of Harkins and Boyd (ref 12).
02//
oboOOO
isotherm measurements from different laboratories. Figure 7 is a plot of T vs A, showing three phase boundaries obtained from the fluorescence measurements and several different isotherm studies: 1 . ALE, the low-density side of the LE-LC coexistence curve that has already been plotted in Figure 6. 2. ALE, the high-density side of the LE-G coexistence curve. This boundary was located in the fluorescence measurements by making repeated single additions and observing the number of phases present. One point, indicated by a solid square, was determined by image analysis. Although the pressures at the LE-G transition are extremely small, which makes isotherm measurements very difficult, three detailed studies on PDA have been carried out, and the molecular areas of the coexisting phases have been determined from the discontinuities at the ends of the plateau region. The values of A L E reported by Pallas and PethicaIs and Harkins and Boyd12 are plotted in the figure; they are in fair agreement-with the fluorescence studies. Kim and CannellI3 found values of A L E that lie outside the range of areas shown in Figure 7. Moreover, although they located the LE-G critical point at 26.27 OC, Pallas and Pethica15 have reported that isotherms are horizontal up to 40 OC. We have observed distinct coexistence between LE and G domains to temperatures approaching 70 OC. Evaporation and strong convection do not allow us to make observations at higher temperatures. 3. ALC,the high-density side of the LE-LC coexistence region, which has been obtained from the lever-rule analyses and confirmed by studies with very low dye concentrations. This boundary cannot be obtained from the isotherm measurements; Middleton and Pethica,26however, have estimated its position at a single temperature from surface potential measurements. This point is indicated in Figure 7. If we interpret the kink in the isotherms at high pressure to be the result of a transition of the LC phase to another condensed phase, then the area A is a lower bound to ALC. As seen in the figure, ALc is roughly 1 i2/molecule larger than AK. We have observed LE-LC coexistence in fluorescence studies at temperatures as high as 37 OC at molecular areas of 20-22 A2, but we have not been able to perform quantitative measurements at these elevated temperatures. In isotherm measurements above 30 OC there is a slight decrease in pressure with time for II L 15 mN m-I at fixed monolayer area. Fluorescence studies performed under the same conditions confirm this effect, since we observe a gradual decrease in the total area occupied by LC domains in the LE-LC coexistence region. At the same time, small ( inconsistent with the result from isotherm measurements of AG = 1500 Az/molecule from Pallas and Pethica.I5 E. The Triple Point. From the phase diagram shown in Figure 7, it is evident that there is a triple point at which the LC, LE, and G phases coexist. The triple point can be located from isotherm measurements by determining the point at which the LE-LC plateau disappears at low temperature. According to Harkins and Boyd12 (their Figure 4), the triple-point temperature is 17 OC; Pallas and Pethica15 report 17 f 2 OC. If we associate To with the triple-point temperature, we obtain 17.9 f 0.1 OC. The triple-point temperature can be determined directly with the fluorescence technique by observing the evolution of the film when it is quenched into the LC-G region from the L E 4 region, Figure 9. Dark LC domains begin to form in the bright liquid regions, and as they form they provide nucleation sites for the gas phase, which appears in the form of bubbles that have a cloverleaf form. The amount of the LE phase decreases with time, as shown in the progression of images in Figure 9, but the evolution slows and the LE Dhase does not com~letelvdisaDDear for times as long as 48 h. When the temperaturk is raised, ihe three-phase region disappears in the range 16-17 OC, in good agreement with the isotherm measurements. Pallas and PethicaIs and Harkins and
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The Journal of Physical Chemistry, Vol. 94, No. I I , I990
TABLE I: Molecular Areas (A*/molecule) at Coexistence in PDA Monolayers L E 4 Transition TIOC 15 20 25
30 40
Ar.
AI P
ref
614 1500
82 42 44 42 146 44 46 42 47 47 44 51
a
403 300 200 850
b C
d a
b C
d b C
d b
LE-LC Transition TIOC 20 25 30
A
ref
A1.c
P
37 (36) 38 31 (32) 33 31 28 (29) 31 28
23 22 22 22 21 21
Triple Point 17 17 16-17
b
C
Figure 9. Fluorescence microscope images of PDA at 14.3 OC, 52 A2/ molecule, after a temperature quench from an initial addition at 19.7 "C. The dark central circular domains are the LC phase, surrounded by dark G bubbles; the LE phase is white. The three images (a)-(c) represent the evolution of the film over time.
Boyd', give 38 f 4 and 44 A2/molecule, respectively, for A L E at the triple point; the fluorescence value is 43 A2/molecule. A summary of the results of this section is provided in Table I.
Discussion Our fluorescence studies and surface pressure measurements are in excellent agreement, a demonstration of the reliability of the fluorescence technique for locating phase boundaries in monolayers. The phase diagram that we have determined by the fluorescence studies is remarkably consistent with the general diagram deduced by Harkins from studied2 of the CI3-Cl6fatty acids. Our failure to observe any critical points is also in
38 44 43
b C
d
"Kim and Cannell, ref 13. bPallas and Pethica, ref 15. CHarkins and Boyd, ref 12. dThis work; values in parentheses from isotherms. e Pallas and Pethica, ref 16.
agreement with this early work, which showed no evidence of a critical point near room temperature in PDA. A linear extrapolation of the LE-LC phase boundaries would suggest that the coexistence region ends near 40 "C. This is consistent with the estimate by Suresh et aI.*' that there is a critical point in tetradecanoic (myristic) acid at about 3 1 "C if one applies the rule of thumb' that, in an homologous series of long-chain fatty acids, there is an 8-10 K shift in the phase diagram for each CH, group. (This correlation has been carefully investigated by two of us and will be discussed elsewhere.28) Our data are too imprecise and taken too far from the critical point to provide any information about the asymptotic shape of the coexistence curve. We therefore cannot decide whether the coexistence curve ends at a critical point or a tricritical point. The suggestion by Der~ichian,~ that the LE-LC transition changes from first order to second order at a molecular area of about 32 A2 is not supported by our measurements. The values of A L E determined by our fluorescence and isotherm measurements are essentially identical over the temperature range that we have studied; Le., the discontinuity in the slope of the isotherms is always associated with the appearance of a new phase. Pallas and PethicaI5 estimated that the G-LE T, was at 50-60. "C,at least 25 K higher than the critical temperature found by Kim and Cannell. Our observation of coexistence at about 70 "C seems to indicate that the critical point must be even higher. It is possible, however, that the probe has a significant effect on the critical temperatures. Indeed, the critical solution temperatures in some binary mixtures are known30 to increase by 30 K with the addition of 1% of impurity. On the other hand, the change in T, is likely to be at least roughly linear in impurity concentration, and measurements on PDA with probe concentrations of (27) Suresh, K. A.; Nittman, J.; Rondelez, F. Europhys. Lett. 1988,6,437. (28) Akamatsu, S.; Rondelez, F. To be published. (29) Dervichian, D. G. J . Colloid Interface Sci. 1982, 90, 71. (30) See, e.g.: Tveekram, J. L.; Jacobs, D. T. Phys. Reu. A 1983,27,2773.
J . Phys. Chem. 1990, 94, 4595-4599
4595
0.2% and 1% behave similarly, so such a pathologically large shift is unlikely. Thus, as in the case of the density of the LE phase, our results are more consistent with those of Pallas and Pethica than with Kim and Cannell’s. The mean-field exponents that Kim and Cannel1 measured must now be seriously doubted. Neither our data nor those of Pallas and Pethicals are sufficiently close to the critical point or sufficiently precise to allow distinguishing between mean-field and lsing exponents.
and light-scattering measurements of Winch and Earnshaw.jl
Note Added in ProoJ The first-order nature of the LE-LC transition in PDA has also been demonstrated by the isotherm
(31) Winch, P. J.; Earnshaw, J . C. J . Phys.: Condens. Matter 1989, I , 7 187.
Acknowledgment. This work was supported by National Science Foundation Grants CHE-8902354 and INT-8413698 and by a CNRS-Elf scholarship for S.A. We thank Dr. Keith Stine for helpful discussions and for his assistance with the image analysis of the LE-G transition. We are indebted to Prof. R. S. Williams for the use of the image analysis system.
Carbon Cluster Emission from Polymers under Kiloelectronvolt and Megaelectronvolt Ion Bombardment H. Feld,* R. Zurmiihlen, A. Leute, and A. Benninghoven Physikalisches Institut der Universitat Miinster, Wilhelm-Klemm-Strasse IO, 0-4400 Miinster, FRG (Received: July 18, 1989; In Final Form: January 11, 1990)
Experiments have been performed to compare secondary ion emission from polymer substrates under kiloelectronvolt ion (secondary ion mass spectrometry, SIMS) and megaelectronvolt ion (plasma desorption mass spectrometry, PDMS) bombardment. The yield of carbon cluster ions has been determined for different polymers. A positive PDMS spectrum of poly(viny1idene fluoride) showing even-numbered carbon clusters to C200is presented. The emission of these carbon cluster ions is explained by the strong degradation of the polymer under megaelectronvolt ion bombardment. This is discussed in comparison with the results in SIMS and laser mass spectrometry.
Introduction During the past years the investigation of carbon clusters has been of increasing interest. Clusters play an important role for transition stage studies from gaseous to solid or liquid state of matter, because they possess properties of an intermediate aggregate state.’ They can be produced by different techniques, e.g., in flames or by SIMS, PDMS, and laser vaporization. There have been interesting developments in recent years in the study of carbon clusters generated by laser vaporization. Up to 1984 only clusters containing 33 carbon atoms had been observed by this technique. Then Kaldor et a1.2 exceeded this limit for the first time by laser vaporization of a graphite rod and subsequent photoionization of the neutral clusters on a time-offlight mass spectrometer. Even-numbered carbon clusters up to C I w +could be detected. The next step was to change the sample and to use polycyclic aromatic hydrocarbons (PAH) or polymers instead of graphite as carbon source. There have been systematic investigations of such samples by laser vaporization and subsequent detection of the produced cluster ions with a LAMMA-1000 TOF-MS3 or a FTMS4 instrument. The highest cluster ions registrated have been about C600+from benzene soot sample^.^ Most spectra taken by laser mass spectrometry show several distributions. Distributions with even- and odd-numbered cluster ions have been observed mainly in the lower mass range whereas in the higher mass range mostly only even-numbered cluster ions are registered. The main difference in these spectra is a strong enhancement of Cm+ for graphite samples compared with PAH or polymer samples. The structure of this superstable Ca+ cluster is assumed to be a polygon with 60 vertices and 32 faces, whereby ~~
( I ) Mfirk, T. D. Int. J. Mass Spectrom. Ion Processes 1987, 79, 1-59. (2) Rohlfing, E. A,; Cox, D. M.; Kaldor, A. J . Cbem. Pbys. 1984, 81, 3322-3330. (3) Lineman, D.; Viswanadham,
S.K.; Sharkey, A . G.; Hercules, D. M. J . Phys. Chem., submitted for publication. (4) So, H. Y.; Wilkins, C. L. J. Pbys. Cbem. 1989, 93, 1184-1187. ( 5 ) Zoeller. J. H. Jr.; Zingaro, R. A,; Macfarlane, R. D. I n i . J. Mass Spectrom. Ion Processes 1987, 77, 21-30. (6) Niehuis, E.; Heller, T.; Feld, H.; Benninghoven. A. J. Vac. Sci. Technol. A 1987, 5, 1243-1 246. 0022-3654/90/2094-4595!$02.50/0
12 faces are pentagonal and 20 hexagonal (truncated icosahedron).’ In the case of atomic particle bombardment of solid substrates the influence of primary ion energy and mass on cluster emission has been investigated. For primary ions of low energy (SIMS) impinging onto a carbon sample a strong emission of negatively charged clusters up to C1f was observed8 The situation is similar for megaelectronvolt particle impact, where positive and negative charged carbonaceous ions occur: (C,H,)- and (C,H,)+ for n < 25 and 0 -< x I 3. The cations could be observed from nearly all PAHs but not from highly aliphatic substrates.s A strong odd-even abundance rule was found in negative spectra of coronene for (C,H)- up to 300 u . ~ Up to now there has been no observation of carbonaceous cations above m = 360 u comparable to those obtained by laser vaporization. We now for the first time report about high mass carbon clusters produced by atomic particle bombardment of a substrate.
Experimental Section All spectra presented have been obtained with a new combination instrument. We developed a time-of-flight instrument that can be operated in the SIMS as well as in the PDMS mode, at the same sample position without breaking the vacuum. The instrument is based upon a design similar to a number of TOF instruments in our laboratory? Both particle sources may irradiate targets from the same side, allowing for the comparison of several substrates. In the SIMS mode the instrument is operated with mass-selected primary ion pulses (pulse width below 1.5 ns, about 5000 ions/pulse, frequency 5 kHz). Primary ion species (Ne, Ar, Xe) (7) Kroto, H. W.; Heath, J . R.; O’Brien, S.C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162-163. (8) Blaise, G. Summer School on Material Characterization Using Ion Beams Aleria, Corsica; Plenum: New York, 1976; p 167. (9) Della-Negra, S . ; Depauw, J.; Joret, H.; LeBeyec, Y. Presented at the Second International Workshop on MeV and keV Ions and Cluster Interactions with Surfaces and Materials, Orsay, France, 1988.
0 1990 American Chemical Society
