898
Langmuir 1992,8, 898-900
Adsorption Hysteresis, Capillary Condensation, and Melting in Multilayer Methane Films on Graphite Foam Mark J. Lysek, Marissa LaMadrid, Peter Day, and David Goodstein’ Condensed Matter Physics 114-36, California Institute of Technology, Pasadena, California 91125 Received September 12, 1991. In Final Form: November 15, 1991 We have studied adsorption hysteresis in the system multilayer methane adsorbed on graphite foam by measuring heat capacities on each branch of the hysteresis curve. We are able to show that, on the stable branch, capillary condensation begins, surprisingly, at 1.1 layers. These observations call into question previous conclusions about this and other multilayer adsorbed systems.
Introduction A great deal of effort has been expended studying the multilayer phase diagrams of methane and other adsorbates on graphite These studies have relied on some evidence and a variety of arguments indicating that capillary condensation does not occur in these systems, a t least not until the first five or so layers have been adsorbed. However, there has also been some evidence to the contrary.6 To test this important point, we have investigated the methane graphite system for adsorption1 desorption hysteresis. We have found that hysteresis is indeed present; it begins just after the completion of the first layer, and it can be unambiguously attributed to the effects of capillary condensation. The literature contains much experience, and even a standard vocabulary for discussing vapor pressure isotherms when capillary condensation is p r e ~ e n t . ~Figure -~ 1 shows a typical hysteresis curve with boundary and scanning curves. Isotherm data taken by incrementally adding gas to the porous substrate, starting with no adsorbate, trace out the “lower boundary curve”. The “upper boundary curve” is found by removing doses of gas, starting from a fully saturated system. The boundary curves may join at low coverage,where there is no capillary condensate, and at high coverage, where the pores in the adsorbent are completely filled. A “scanning curve” is any isotherm curve that lies between the upper and lower boundary curves. A typical upward scanning curve might start on the upper boundary curve, pass into the region between the boundary curves, and may or may not join the lower boundary curve.l0 Likewise,a typical downward scanning curve might start on the lower boundary curve, and may or may not join the upper boundary curve. At any given coverage, only the curve at lowest pressure, P (and hence the lowest chemical potential, p ) , can be stable. Thus, the upper boundary curve is the only one (1)Hamilton, J. J.; Goodstein, D. L. Phys. Rev. E 1983,28,3838. (2) Goodstein, D. L.; Hamilton, J. J.; Lysek, M. J.;Vidali, G. Surf. Sci. 1984,148,187. (3)Pettersen, M. S.;Lysek, M. J.;Goodstein,D. L. Surf. Sci. 1986,175, 141. (4)Pettersen, M. S.;Lysek, M. J.; Goodstein, D. L. Phys. Reu. E 1989, 40,4938. (5)Zhu, D. M.; Dash, J. G. Phys. Rev. Lett. 1986,57,2959. (6)Inaba, A.; Morrison, J. A. Chem. Phys. Lett. 1986,124,361. (7)Everett, D.H.Adsorption hysteresis. In The Gas Solid Interface; Flood, E. A., Ed.; Marcel Dekker Inc.: New York, 1967;Chapter 36. (8) Gregg, S. J.;Sing,K. S. W. Adsorption, Surface Area and Porosity, 1 s t ed.; Academic Press Inc.: London and New York, 1967. (9)Everett, D. H., Stone, F. S., Eds. The Structure and Properties of Porous Materials; Academic Press Inc. and Butterworths Scientific Publications: New York and London, 1958. (10) Rao, K. S. J. Phys. Chem. 1941,45,500,506,513,517.
I
I
Relative Pressure Figure 1. Typical boundary and scanning curves for adsorption on a porous substrate. that may represent the thermodynamic equilibrium state of the ~ y s t e m .We ~ shall show that this kind of behavior occurs in the methanelgraphite system and that, on the equilibrium or desorption branch, there is substantially more capillary condensation coexisting with the unsaturated film than there is on the adsorption branch.
Experimental Section The substrate for these experiments is Union Carbide Grafoamll which has been prepared by outgassing at lo00 “C until the pressure is below 1X 10” Torr, after an initial 1500 O C heating in a chlorine atmospherewhen it is manufactured. The adsorbate is 99.995% pure methane gas. The heat capacity and vapor pressure isotherm curves were performed on a home-made automated differential scanning calorimeter. The calorimeter is similar to a design implemented by Bu~kingham~~J~ and is particularly useful for finding small heat capacity features on the large Grafoam background. The sample heat capacity, C-,, is found by comparingit to the heat capacity of a standard thermal mass, CBu.During operation, the standard’s heat input, WBa,is constant, causing ita temperature to rise linearly with time. The sample’s heat input, W-,, is varied to make ita temperature closelyfollow the standard. Ignoring stray heat leaks, the sample heat capacity is
where 6p is the time derivative of the temperature difference between the sample and the standard. The heat capacity data are reproducible to i0.1% ,and the noise for data pointa separated ~~
(11)Grafoam is a registered trademark of the Union Carbide Corp. for expanded graphite products. (12)Buckingham, M. J.; Edwards, C.; Lipa, J. A. Reu. Sci. Iwtrum. 1973,44, 1167. (13)Loram, J. W.J. Phys. E 1983,16, 367.
0743-7463/92/2408-0898$03.00/00 1992 American Chemical Society
Multilayer Methane Films on Graphite Foam by 0.1 K is less than &0.04% of the background (including the Grafoam and heat capacitycell),for 2 K/h scanning rates. More points per degree can be obtained by operating at slower drift rates, with similar calorimetric accuracy. Heat capacity scans were performed from 65 to 120 K over 28 h of continuous operation. The heat capacity traces presented were performed with a scanning rate always less than 2 K/h, and each point is calculated as a fit to a least-squares cubic curve, for data taken over 3 min. Points are reported every 3 min, so the data are statistically independent;there should be no visible smoothing in the data. The vapor pressureswere measured with a 1000-Torrcapacitive manometer, typically linear to within 0.05 % of full scale. Each dose of gas was taken from a 2-Lreservoir of methane gas at low pressure, controlled by an air-actuated bellows valve that was modified to leak at a fixed rate when open. Doses of gas were withdrawn using bellows valves that connected to a liquid nitrogen trapped turbopump. The entire apparatus was computer controlled duringcalorimetryand vapor pressure isotherms. A more completedescriptionof the experimentalapparatuswill be given in a future paper.
Vapor Pressure Isotherms Inaba and Morrison6have previously reported hysteresis in vapor pressure isotherms in the methane/graphitefoam system for temperatures above 84.5 K (the triple point of bulk methane is 90.685 K). Because the nature of the hysteresis was different from that normally attributed to capillary condensation, the cause of the hysteresis was ~nc1ear.l~ Along with the vapor pressure hysteresis, Inaba, Koga, and Morrison also found a wetting transition at 75.5 K (also for krypton and xenon15). However, other vapor pressure studies of methane on graphite did not find a wetting transition in this temperature range.16Thus, in spite of ref 6, several papers from our laboratory and others have assumed that there is no capillary condensation for methane films several layers thick. More recently, Ser et al.17 and Meichel et al.'* have reexamined the wetting transition, and a neutron scattering study has found capillary condensation in argon films.lg An X-ray scattering study of xenon films on exfoliated graphite on both the adsorption and desorption branches found capillary condensate coexisting with films as thin as a monolayer on the desorption branch.20 To test for hysteresis in this system, we performed an isotherm at 95 K, shown in Figure 2. The upper and lower bounding curves join at 1.1layers, corresponding to 0.27 relative pressure, not surprising for capillary condensation. It has long been known that the point of closure is more related to the properties of the adsorbate than to the nature of the substrate.21~22~8 The bounding curves would presumably join again at a much higher coverage, when the 2000-A average spacing between the Grafoil platelets is filled. Figure 2 shows two downward isotherms, starting at nominal 11 and 5 layers adsorbed. The 5-layer curve was not continued to coverages below 2 layers, where it would eventually join with the other curves, possibly at a point before where the hysteresis loop closes. Another downward isotherm that started at 14 layers adsorbed (14)Inaba, A.;Koga, Y.;Morrison, J. A. J.Chem. Soc., Faraday Trans. 2 1986,82,1635 also see discussion on pp 182C-1826. (15)Inaba, A.; Morrison, J. A.; Telfer, J. M. Mol. Phys. 1987,62,961. (16)Larher, Y.; Angerand, F. Europhys. Lett. 1988,7,447. (17)Ser, F.;Larher, Y.; Gilquin, B. Mol. Phys. 1989,67,1077. (18)Meichel, T.;Dawson, P. T.; Morrison, J. A.; Sullivan, D. E.; Koga, Y. Langmuir 1990,6, 1579. (19)Larese, J. 2.; Zhang, Q.M. Phys. Reu. Lett. 1990,64,922. (20)Morishige, K.; Kawamura, K.; Yamamoto, M.; Ohfuji, I. Langmuir 1990,6,1417. (21)Foster, A. G.J. Chem. SOC.1952,1806. Whitton, W. I. Proc. R . SOC.London 1955,A230, (22)Everett, D. H.; 91.
Langmuir, Vol. 8, No. 3, 1992 899
30
0
60
90
120
150
Pressure (torr)
Figure2. Vapor pressureisotherm. Central downwardscanning curve is desorption path for heat capacity case two in text. The first layer point of inflection occurs with 194STPCC's adsorbed. The inset is an expanded view of the region where the upper and lower boundary curves come together,
77
80
83 86 Temperature
89
92
Figure 3. Heat capacity for three films with the same amount adsorbed, but at different locations on the hysteresis curve, uncorrected for the heat of desorption. Upper two curves are displaced upward by 0.4 and 0.8 J/K, respectively.
(not shown) demonstrates that the methane/Grafoam system differs from the classical boundary and scanning curve behaviorlo in that the downward scanning curves do not join the upper boundary curve until a large fraction of the film is desorbed. It is possible that the scanning curves do not join until the point where the boundary curves meet, as implied by Inaba and Morrison. Similar hysteresis curves have been observed for adsorption on materials comprised of parallel plate^.^
Heat Capacity Scans Initially, four cases were studied. For all, the total numbers of molecules in the system were identical during the heat capacity measurements, twice the monolayer coverage. However, the films were formed by different methods. The total amount in the system includes molecules in the uniform film, capillary condensate, and the 3-D vapor. The heat capacity data are shown with the heat capacity of the Grafoam and the aluminum heat capacity cell subtracted. The first case studied (the lowest curve in Figure 3) was formed by increasing the total amount in the system from zero to two effective layers while cooling from 120 to 108 K. It should therefore be on the lower boundary curve
900 Langmuir, Vol. 8, No. 3, 1992 (i.e., it was formed by adsorption). The heat capacity has two small, sharp peaks between 80 and 83 K in the temperature range shown, 75-92 K. The low-temperature peak may be a signal of an incommensurate to commensurate phase transition, as noted by a recent neutron scattering s t ~ d y . ~The 3 higher temperature feature is part of a complicated arrangement of phase boundaries that will be discussed in a later paper. Tentatively, we associate this peak with the melting of the second adsorbed layer. To test for reproducibility, these data were taken three times, starting with an empty cell. The heat capacities agreed to within 0.1 7%. The second case studied followed the central downward scanning curve in Figure 2, as described above. The equivalent of 5.5 layers was admitted into the system with the Grafoam at low temperature (94 K). Then the cell was warmed to 120 K, with the amount of gas in the vapor phase limited to '12 layer by closing a cryogenic valve at the mouth of the heat capacity cell. After cooling to 95 K, gas was removed from the system in ' 1 3 layer doses until 2 layers remained in the system, including molecules in the uniform film, capillary condensate, and vapor phase. This case, formed by desorption from 5.5 to 2.0 layers, was thus in the intermediate region, between the upper and lower boundary curves. The heat capacity for this case was radically different from the first, as shown in the middle curve of Figure 3. The two low-temperaturepeaks present in the first case were translated to lower temperature and another peak appeared, centered a t 85 K. The third case studied was formed by desorption from very high coverage. The equivalent of 11layers was dosed into the system, and then it was annealed at 120 K as in the second case above. After cooling to 95 K, gas was removed from the system in doses less than or equal to l/3 layer. The uppermost curve in Figure 3 shows the heat capacity when there were two effective layers remaining in the system, the same amount as in the other curves in Figure 3. This case was very nearly on the upper boundary curve (desorption branch). The position and size of the low-temperaturepeaks are different from those in the other cases, and the high-temperature peak is much larger. We performed a complete study of 20 different coverages on the adsorption branch and 12 coverages on the desorption branch. As the coverage is increased on the desorption branch, the peak at 85 K grows into what was previously believed to be the melting of the thick film. This peak disappears for coverages below 1.1layers, just where the hysteresis loop closes. This evidence shows clearly that this peak is due to the melting of capillary condensed bulk in the narrow crevices and interconnecting channels of the graphite foam. On the adsorption branch, a similar peak occurs for coverages greater than three layers. This is presumably the point where capillary condensate is nucleated on the lower boundary curve of the hysteresis loop. While attempting to form a two-layer film on the desorption branch, we found that it was possible to create a film in a different state than the first case discussed above, but with the same pressure. This statewas prepared starting with a total system number equivalent to 5.5 layers of methane. With the calorimeter at 120 K, 3.5 layers of (23) Larese, J. Z.; Zhang,Q.M. Private communication.
Lysek et al.
gas were rapidly removed from the system, leaving 1layer of gas, and 1layer adsorbed. Note that this coverage is below the point at which hysteresis begins in Figure 2. The cell was then cooled, causing nearly all the gas to condense. The resulting pressure a t 95 K was equal to that of the first case above, on the adsorption branch. However, the heat capacity scan was not identical to that of the first case studied. The second low-temperaturepeak moved to higher temperature, and the overall heat capacity was lower. This behavior is consistent with the formation of a metastable state. Since all states but those on the upper boundary curve (desorption branch) are metastable, the history of the system may cause a wide variation in the heat capacity signal, even for systems with equal vapor pressure. To obtain reproducible results with systemsthat are not in the equilibrium state, they must all be formed by the same method. However, by using identical procedures to create systems on the adsorption branch, we have found the data to be reproducible to within the resolution of the instrument, 0.04% of the background heat capacity. Other data have been published that examined capillary condensation and melting on saturated porous subs t r a t e ~ . ~To ~ our - ~ ~knowledge, however, these are the first heat capacity measurements that have been reported at several locations on the hysteresis loop.
Conclusion We have shown that, in the methanelgraphite foam system, capillary condenaate coexists with the adsorbed film in thermodynamic equilibrium at all coverages above 1.1layers, correspondingto a partial saturation pressure of 0.27. We have also shown that it is possible to create systems in metastable states with far less capillary condensation that are nevertheless extremely reproducible. Finally, we have also studied systems of the same coverage and vapor pressure that are, however, observably not in the same physical state. The inescapable conclusion to be drawn from these results is that multilayer film studies must be accompanied by far more detailed accounts of the preparation of the system than has been customary in the past, including the equivalent of our Figure 2. A second, no less important, conclusion is that heat capacity peaks that were interpreted as due to melting of uniform films in our own previous work on the methane/graphite s y ~ t e m ' - ~were 9 ~ ~ due to melting of capillary condensate instead. If, as seems very likely, similar behavior occurs for other gases (argon, neon, krypton, etc.) adsorbed on graphite, a very large body of work in the current literature will have to be reevaluated. Acknowledgment. We thank the Union Carbide Corp. for supplying us with the Grafoamsample. We are grateful to Professor M. W. H. Chan, Professor J. M. Phillips, Professor J. G. Dash, Dr. J. Larese, and Dr. M. Bienfait for many helpful discussions. This work was supported by Department of Energy (DOE) Contract No. DE-FGOB85ER45192. Registry No. methane,74-82-8; graphite, 7782-42-5. (24) Awschalom, D. D.; Warnock, J. Phys. Rev. B 1987,35,6779. (25) Antoniou, A. A. J. Phys. Chem. 1964, 68, 2754. (26) Tell, J. L.; Maris, H. J. Phys. Rev. B 1983, 28, 5122. (27) Lysek, M. J.; Pettersen, M. S.; Goodstein, D. L. Phys. Lett. A 1986, 115, 340.
