Langmuir 1994,10, 3303-3310
3303
Interaction of CCl, with the Surface of Amorphous Ice Jason L. Blanchard and Jeffrey T. Roberts* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455-0431 Received April 22, 1994. In Final Form: June 24, 1994@ The temperature programmed desorption of C c 4 from amorphous ice films is described. Two distinct desorption states are observed, near 130 and 145 K. In contrast to most thermal desorption systems, the lower temperature state fills exclusively at low coverages, followed by the 145 K state at higher coverages. The two states are assigned to thermal desorption of ccl4 from ice and to sublimation of the condensed cc14 phase, respectively. Both the frequency factor and the activation energy for cc14 desorption from ice are greater than the frequency factor and activation energy for cc14 sublimation. This difference is attributed to hydrophobic interactions between adsorbed cc14 and the underlying ice layer.
Introduction easily controlled and well defined conditions present in a n ultrahigh vacuum (UHV) environment, Our work is Water has long fascinated chemists and physicists.' It partly motivated by a desire to gain insight into heterois ubiquitous in the environment and essential for life on geneous atmospheric chemistry, but also by a more Earth. Additionally, water has many unusual properties fundamental interest in the surface chemistry of ice in (high boiling point, negative volume change upon melting, particular and of molecular solids in general, since very etc.), the understanding of which continues to stimulate little is known about the surface chemistry of molecular much experimental and theoretical work. Interest in the solids compared to what is known about metals, semisurface properties of ice can be traced as far back as conductors, and oxide materials. Our approach is to grow Faraday's suggestion that there exists a liquid like ultrathin (5200 A thick) ice films in situ by depositing transition layer a t a n ice surface which mediates the solid water from the gas phase onto a single crystal transition and gas phases,2 but it was not until the late 1960s that metal surface. These ice films serve as substrates upon the surface chemistry (as opposed to physics) of ice received which adsorption and desorption kinetics and adsorbate significant attention, principally by Adamson and costructure can be investigated. The advantages of studying workers, who determined isosteric heats of adsorption and such films under UHV are manifold. First, the high contact angles of many substances, among them nitrogen, carbon dioxide, and various hydrocarbons, on i ~ e . ~ - ~surface area to volume ratio of a very thin film makes it a relatively simple matter to investigate both surface and Subsequent progress in the surface chemistry of ice was bulk phenomena using, for instance, temperature proslow, primarily because of a lack of suitable spectroscopic grammed desorption and infrared spectroscopic methods. probes. However, the recognition in 1987that ice surfaces Second, surface charging is eliminated if films of sufficient may catalyze chemical transformations which are instruthinness are deposited on a conducting material. This mental in opening the stratospheric ozone "hole" over the allows the straightforward application of electron-based Antarctic made the surface chemistry of ice a matter of probes, such as X-ray photoelectron spectroscopy (XPS). considerably more than academic i m p ~ r t a n c eand , ~ the Finally, under UHV,the sublimation rate of the film is subject has since come under very active scrutiny.1°-15 low, and surface lifetimes are on the order of several hours We have been interested for some time in developing or more. This eliminates the need to include considermethods to study the surface chemistry of ice under the ations ofthe solid-gas equilibrium in data interpretation. Here we consider the temperature programmed de* Author to whom correspondence should be addressed: telesorption of carbon tetrachloride (CC14)from amorphous phone, (612)625-2363;fax,(612)626-7541. ice films grown on W(100). We show that adsorbed CC14 Abstract published inAdvance ACSAbstracts, August 15,1994. desorbs from ice a t ~ 1 3 K, 0 a temperature below that a t (1)Eisenberg, D.; Kauzmann, W. The Structure and Properties of Water; Oxford University Press: London, 1969. which condensed C C 4sublimes ( ~ 1 4 K). 5 The desorption (2)Faraday, M. Proc. R . SOC.(London) 1860,10,440-450. temperature of adsorbed CC14 increases with exposure, (3)Adamson, A. W.; Dormant, L. M. J . Am. Chem. SOC.1966,88, implying that the desorption kinetics are zeroth order. 2055-2057. (4)Adamson, A. W.; Bormant, L. M.; Orem, M. J . Colloid Interface On the basis ofthese results, we suggest that CC14adsorbs Sci. 1967,25,206-217. as metastable two-dimensional islands, from which de(5)Orem, M. W.; Adamson, A. W. J. Colloid Interface Sci. 1969,31, sorption is more rapid than condensation into the ther278-286. modynamically more stable three-dimensional solid. (6)Nair, N. K.; Adamson, A. W. J . Phys. Chem. 1970,74,22292230. When analyzed in the context of a zeroth-order kinetic (7)Adamson, A. W.; Shirley,F. P.; Kunichika,K. T. J. Colloid Interface model, the activation energy for desorption from twoSci. 1970,34, 461-468. dimensional islands appears to be greater than the (8)Adamson. A. W.: Jones, B. R. J. Colloid Interface Sei. 1971.37, 831-835. sublimation energy. Desorption from the two-dimensional (9) Solomon, S. Rev. Geophys. 1988, 26, 131-148. phase is more rapid than sublimation because of the (10)Wofsy,S. C.;Molina,M. J.;Salawitch,R. J.;Fox,L. E.;McElroy, different frequency factors in the two desorption rate M. B. J . Geophys. Res. D 1988,93,2442-2450. expressions. (11)Leu, M. Geophys. Res. Lett. 1988,15, 851-854. (12)Tolbert, M. A.; Middlebrook, A. M. J . Geophys. Res. D 1990,13, 22423-22431. _ ~ ~ ~ . Experimental Section (13)Elliot, S.; Turco, R. P.; Toon, 0. R.; Hamill, P. J . Atmos. Chem. 1991,13,211-224. Experimentswere conducted in a stainless steel UHV chamber (14)Abbatt, J. P. D.; Beyer, K. D.; Fucaloro, A. F.; McMahon, J. R.; of base pressure =l x 10-8 Pa. The chamber was pumped Wooldridge, P. J.;Zhang, R.; Molina, M. J . Geophys. Res. D 1992,97, continuously by a 300 Less* ion getter pump and intermittently 15819-15826. by a titanium sublimation pump (bothfrom Physical Electronics). (15)Horn, A. B.;Chesters, M. A,; McCoustra, M. R. S.; Sodeau, J. R. J . Chem. SOC.,Faraday Trans. 1992,88,1077-1078. The vacuum system was equipped with a quadrupole mass @
~~~~~
0743-746319412410-3303$04.50/00 1994 American Chemical Society
Blanchard and Roberts
3304 Langmuir, Vol. 10, No.9, 1994 spectrometer (Extrel (2501, a double pass cylindrical mirror electronenergy analyzer (PhysicalElectronics15-255G),an X-ray source (PhysicalElectronics 04-548(3),and low energy electron diffractionoptics (PhysicalElectronics10-120). The X-raysource is equippedwith a magnesium anode;the X-rayenergy is 1253.6 eV. Gases were admitted into the chamber using leak valves (VacuumGenerators Ltd. MD6)and directed dosers. Exposures are given in units of Pa-s,the product ofthe background pressure rise upon opening the doser and the exposure time. Exposures are uncorrected for the enhancementfactors of the dosers,which are approximately 100. Water layers were grown via depositionof water onto a W(100) surface. The tungsten substrate was mounted on a sample manipulator which allowed for rotation and translation within the vacuum chamber. The substrate was in thermal contact with a liquid nitrogen-cooled reservoir and could be cooled to approximately 90 K. A tungsten filament positioned ~2 mm behind the substrate was used for radiative heating. The substrate could also be biased to +500Vforelectronbeam heating as required. Temperatures were recorded with a W-5% Re/ W-26% Re thermyouple junction spot-weldedt o the edge of the tungsten crystal; an electronic ice point (Omega MCJ-C) substituted for a referencejunction. Because the ice layers were so thin, thermal gradients between W(100)and a thin film surface were negligible,and the temperature at the surface of an ice film can be taken as equivalent to that of the W(100) substrate. Temperatureprogrammed desorptiondata were acquiredwith the W(100) surface positioned in line of sight of the mass spectrometer,approximately20 mm from the ionizer. The mass spectrometer was encased in a stainless steel shield similar to one described previously.16 The shield allowed for interposition of a collimator 2 mm in diameter between the W(100)substrate and the mass spectrometer. The collimator-surface distance was typically~2 mm during a temperature programmed reaction experiment. This arrangement results in the preferential detectionofproductsfrom the center ofthe W(100)surface,where the defect density is lowest. Furthermore, contributions to the temperature programmed desorptionspectra of products evolving from the crystaledgesand backside,the supportwires,and other parts of the sample manipulator are thereby minimized. The mass spectrometer was interfaced to an IBM-AT clone PC via a data acquisition board (Keithly DAS-HRES). Data acquisition sofiwareallowed for the collection of up to seven ion-temperature profiles during a single experiment. The 100-orientedtungsten single crystal was obtained from Metal Crystals Limited (Cambridge,U.K.)and cleaned in uucuo according to established methods.17 A typical cleaning cycle involved exposingthe hot surface (2' = 1400 K) to oxygen gas (P = 10+ Pa) for a period of approximately5 min. The crystal was then flashed to 2300 K for 30 s, thereby removing oxygen as a volatile tungsten oxide. The crystal was subsequentlycooled to approximatelyroom temperatureand then briefly flashedto 1100 K to remove any residual CO. Subsequent Auger analysis typically showed the surface to be free of impurities within the , and 0.04 limits of detectability of our apparatus ( ~ 0 . 0 20.01, monolayers for C, 0,and C1, respectively). Water was deionized and triply distilledbefore use. It was degassedviaseveralfreezepump-thaw cycles before use each day. Carbon tetrachloride (SpectrumChemicals,ACS grade)was degassed before use each day and otherwise used as received. Oxygen (Matheson,extra dry, 99.6% minimum purity) was used as received.
Results 1. Characterization of water adsorption on W(100). We describe here the growth of multilayer ice films on W(100). A thorough description of water adsorption on W(lOO), including complete characterization of both the adsorbed and multilayer phases, appears elsewhere.l8 Films were grown via the adsorption of gaseous water onto a clean W(100) surface. During film growth, the substrate was positioned x 3 mm in front of a directed (16)Roberts,J. T.;Friend, C. M. J.Am. Chem. Soc. 1986,108,7204. (17)Pearlstine, K.A.;Friend, C. M. J.Phys. Chem. 1986,90,43444347. (18)Blanchard, J. L.;Roberts, J. T. To be published.
p i m a t i o n ice
100
150 200 250 temperature / K
I
300
Figure 1. Temperature programmed desorption of a water Pws) from W(100).Water was multilayer (exposure 1.0 x detected as m / e 18 (HzO+);the heating rate was x5 Kes-'.
doser designed so that the inside diameter of the doser tube (=Emm) is significantly larger than the diameter of the W(100) substrate ( ~ mm). 6 This arrangement ensures that the water flux across the substrate surface is nearly constant, thereby allowing the deposition of uniform water films. The water adsorption rate was ~ 0 . 1 monolayers-l. Films were grown at a substrate temperatures of 95 K under conditions which lead to the deposition of amorphous ice.lg The temperature programmed desorption of a water multilayer on W(lO0) is shown in Figure 1. The ice film represented by Figure 1is quite thin; clear evidence for chemisorbed states can also be discerned, at 190 K and as a high temperature tail. The sublimation kinetics of much thicker films were studied, to prevent interference by the chemisorbed states. Sublimation proceeds with approximately zeroth-order kinetics, as expected.20 Plots of ln(desorption rate) versus reciprocal temperature along the leading edge of the sublimation spectrum are linear; from the slope of these plots we infer an activation energy for sublimation (&b) of 45.8 f 0.6 kJ.mol-', where the uncertainty represents the standard deviation in Esubfrom five separate spectra. This value for Esub compares favorably with those for water sublimation from many other single-crystal transition-metalsurfaces21and is very close to the sublimation energy of bulk ice (50 kJ*mol-l at 170 K1.l The low uncertainty in Esub indicates that temperature measurements are not subject to significant random error. Rather, errors in temperature measurement are of a systematic nature. Film thicknesses were estimated using two methods, one based upon the water desorption yield and the other on the attenuation of the W(40 photoelectron spectrum. Both methods assume that deposited water films are relatively uniform, an assumption for which justification will be presented below. In any case, thicknesses cited in this work are intended to provide only a rough gauge of film thickness, and it should be understood that films may exhibit some roughness. Thicknesses are given in units of water monolayer equivalents (ML). From the density of ice and simple geometrical considerations, we estimate that a monolayer is approximately 3.2 thick. Figure 2 shows the W(4f7 photoelectron spectra of clean W(100) and of W(lO0) exposed to 5.0 x Pa-sof water. In the presence of this ice film, the integrated intensity of the W(40 feature is 62% of that of the clean surface. The attenuation of the W(40 spectrum can be related to thin film thickness via
a
(19)Schaff,J.E.;Roberts, J. T. J.Phys. Chem. 1994,98,6900-6902. (20) Yates, J. T. J. Methods Exp. Phys. 1985,22,425-465. (21)Thiel, P. A.;Madey, T. E. Surf Sci. Rep. 1987,7,211-385.
Interaction of CCZ4 with Ice
Langmuir, Vol. 10, No. 9, 1994 3305 (a) clean W(100)
where Z is the integrated intensity of the W(40 feature in the presence of a water thin film, IOis the intensity from the clean surface, d is the film thickness, 1 is the photoelectron inelastic mean free path, and 8 is the photoelectron take-off angle with respect to the surface normal.22 In these experiments, the sample was positioned so that the W(100) surface plane was perpendicular to the axis of the electron energy analyzer, resulting in a takeoff angle of 42 f 6". For the data in Figure 2, we estimate a thickness of 9 f 1ML, assuming a n inelastic mean free path of 24 ML. The uncertainty in this value is primarily associated with the uncertainty in 1, an energy- and material-dependent quantity which has not been measured in ice near kinetic energies of 1200 eV, the approximate energy of the W(40 photoelectrons. Our estimate of the mean free path was obtained by extrapolating from A in ice a t 58 eV3to 1200 eV along the universal curve,24and is probably correct within a factor of 2. Film thicknesses were also obtained from the timeintegrated water sublimation spectra using the following procedure. The water desorption yield from a heavily oxidized W(100) surface which was completely inactive for water decomposition was determined and assigned a value of 1monolayer equivalent. Thicknesses of multilayer films were defined as the ratio of the total water desorption yield to the monolayer desorption yield. Thicknesses determined using this method could be in error by as much as a factor of 2; the most significant potential source oferror is the assumption that the packing density of water adsorbed a t W(100) is identical to that of water in the condensed phase. However, thicknesses estimated from the desorption yields are in good agreement with those derived from the XPS spectra. For the XPSbased determination given above (9 ML), the corresponding thickness estimate from the water desorption yield is 10 ML, a difference of approximately 10%. Because desorption spectra are much more rapidly obtained than XPS spectra and because X-radiation may result in damage of a n ice film (via either photon- or secondary electron-induced processes), thicknesses cited below were estimated from the water desorption yields. 2. Adsorption of CCl, on amorphous ice. Carbon tetrachloride was adsorbed on water films which were thicker than 10 ML, enough to evolve a significant water multilayer during temperature programmed desorption. Figure 3 shows a series of CC14temperature programmed desorption spectra from water deposited a t 95 K. In this set of experiments, films were 50 f 5 ML thick, and the CC14 exposure was varied between 8.0 x lo-' and 4.0 x Pa-s.A representative HzO desorption spectrum is also shown. C c 4 was detected as m / e 117 (C35C13+),the most abundant CC4-derived ion in the mass spectrometer, and HzO as m / e 16 (O+), the least abundant HzO-derived ion. This detection scheme prevented saturation of the mass spectrometer electron multiplier and preserved as much of the dynamic range of the instrument as possible for detection of CC14. Two CC14 desorption states are observed, with the spectra exhibiting a complex dependence on C c 4 coverage. Pa*s)a single desorption At very low exposures ( < 1.6 x state, a-CC14,is observed. The a desorption temperature (22)Practical Surface Analysis, 2nd ed.;Briggs,D., Seah, M. P., Eds.; John Wiley & Sons, Inc.: New York, 1990. (23) Kurtz, R. L.; Usuki, N.;Stockbauer,R.; Madey, T. E. J.Electron Spectrosc. Relat. Phenom. 1986,40, 35-58. (24) Seah, M. P.; Dench, W. A. Su$. Interface Anal. 1979,1, 2-11.
(b) 5.0xlO'Pas
45
H20
40 35 30 binding energy / eV
25
Figure2. W(4OX-rayphotoelectronspectraof (a)clean W(lOO), and (b)W(100)after deposition of an ultrathin water film (5.0 x Paes water exposure).
100
125 150 175 temperature I K
2 IO
Figure 3. Temperature programmed desorption of CCld from 50 f 5 ML thick water films on W(100). CC14 exposures, determined as described in the text, were as follows: (a)8.0 x (b) 1.6 x (c) 2.4 x (d) 3.2 x (e)4.0 x and (04.8 x Pa-s. A representative water sublimation spectrum is shown in (g).The heating rate was =5 Kes-l. CC1, was detected as m l e 117 (C35C13+)and HzO as m l e 16 (0'9.
increases with coverage, from 130 K a t the lowest coverages investigated to 136 K a t the highest attainable a-CC14coverages. The desorption yield increases roughly linearly with exposure until e2.4 x Pa*s, reaching a maximum a t approximately 125% of the exposure required to saturate the clean W(100) surface with P a s , the a-CC14 yield adsorbed CC14. Above 4.0 x decreases with increasing exposure, and a second desorption state appears in its place at e145 K. This state does not saturate and is attributed to the sublimation of a carbon tetrachloride multilayer. Assignment of the higher temperature state as a multilayer was confirmed Pa-sdesorption spectrum by comparing the 6.4 x from a 50 ML thick water film to that of a true CC14 multilayer deposited on a clean W( 100) surface (Figure
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3306 Langmuir, Vol. 10,No. 9,1994
condensed CCI4/ W( 100)
(b)
I .k
(a)
I
A
'
condensed CC14/ water thin film
cc14/ W(100)-2 ML H20
f-- atomic
CI difference spectrum
100
125 150 175 temperature / K
200
Figure 4. Temperature programmed desorption of (a) condensed CC4multilayer from a 50 ML thick water film and (b) condensed CC4from W( 100). cc14 exposures were 6.4 x Pas.CC4was detected as mle 117 (C35C13+),and the heating rate was 6 K0s-l. The desorptionstate near 165Kin (b)is due to the desorptionof chemisorbed Cc4 from the W(100)surface.
4). An intriguing aspect of the CC14/H20 desorption spectra is the decreasing a-CC14 yield above 2.4 x Pps. Because the a and multilayer states overlap, it is not possible to determine the precise dependence of the a-CC14 yield on the multilayer yield. However, as shown in Figure 4, a t a CC14 exposure of 6.'4 x Pa-s, the leading edge of the desorption spectrum is superimposable with the leading edge of a true multilayer, demonstrating that the a state vanishes at or just below an exposure of 6.4 x Paas. Auger electron spectra recorded after temperature programmed desorption of C C 4 on films thicker than 10 ML show that essentially no chlorine (x0.04 ML) is deposited on W(100) during desorption. Spectra of the post-desorption and clean surfaces are identical within experimental error between 100and 250 eV kinetic energy (parts a and b of Figure 5, respectively). This is a critical point because W(100) is active for C-C1 bond scission, even in the presence of a passivating water layer 2 ML thick. The reactivity of such a surface may be readily appreciated by comparing the Auger electron spectrum of a clean W(100) surface to that obtained after thermally desorbing C c 4 from a surface precovered with 2 ML of water (parts b and c of Figure 5). The different line-shape of the 163/169/179 eV triplet in Figure 5c is due to the superposition of the LMM transition in chlorine a t 181eV with the tungsten peaks. The chlorine feature is more clearly seen in Figure 5d, the difference between the spectra in parts c and b of Figure 5. The peak intensity corresponds to a chlorine coverage of approximately 0.05 ML. Given the reactivity of W(lO0) toward C C 4 in the presence of coadsorbed water, the absence of a chlorine feature in Figure 5a argues against any interaction between carbon tetrachloride and W( loo), provided that the intervening water films are thicker than 10 ML. We conclude from these data that films '10 ML thick are dense and free of micropores and related defects which, if present, would expose the underlying W(100) substrate to gaseous CC4. For films thicker than 10 ML, thickness has no discernible influence on the a-CC1, desorption temperature
100
150 200 250 kinetic energy / eV
Figure 5. Auger electron spectra of W(100). Spectra refer to (a)the surface after exposing a 10 ML thick water film to CC14 (%4.0 x Pas)at 100 K and then heating to 700 K, (b) the clean surface, (c)the surface after exposing a 2 ML thick water x Pas)at 100 K and then heating to film to CC14 (~4.0 700 K, and (d)the difference spectrumc-b. The primary electron beam energy was 3 kV.
and very little bearing on the desorption yield: the a-CC14 yield from a 2 x lod6Pes exposure on a 20 ML thick film is only 5%larger than the yield from an identical exposure on a 10 ML thick film. The significance of this result is 2-fold. First, it provides further evidence that a-CC14is derived from a state a t the thin film surface. Second, it implies that the deposited water films are smooth, since the accessible surface area of a rough film is expected to increase with thickness. It is also noteworthy that the maximum a-CC14 yield is comparable to the yield of reversibly adsorbed C c 4 on a clean W(100)surface. This suggests that the the microscopic surface area of a thin film is similar to that of the single crystal substrate; films are smooth on the molecular scale. This conclusion is further supported by the fact that the desorption yields of several other substances, namely acetone,lg toluene,25 and hydrogen chloride,26are also nearly independent of water film thickness. We briefly examined the temperature programmed desorption of sandwich layers, i.e. layers formed by adsorbing water on top of a n a-CC4-saturated thin film. Results of one such experiment, correspondingto a 20 ML 1.6 x Paos 40 ML sandwich, are presented in Figure 6. Significantly, a-CC14 desorption is completely absent during the thermally induced decomposition of a sandwich. Rather, all C c 4 evolves from a single state a t 170 K, a temperature higher than that from either the a or multilayer states. In fact, C c 4 desorption does not commence until a significant fraction of the ice layer has sublimed. Once C C 4desorption begins, it is exceedingly rapid, as indicated by the sharply rising leading edge on the CCl4 desorption spectrum. This disappearance of a-CC14 upon sandwich formation further supports our assignment of a-CC4as an adsorbed phase. We attribute the desorption temperature shift to the imposition of a diffusional barrier by the top water layer which prevents
+
+
(25) Schaff, J.; Roberts, J. T. To be submitted for publication. (26) Graham, J. D.; Roberts, J. T. J.Phys. Chem. 1994,98,59745983.
Interaction of CCZd with Ice
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II
I
6.8
7.2
7.6 T-’/ 10-3.~
100
125 150 temperature /
175
200
K Figure 6. Temperature programmed desorption of a “sandwich”layer: a 20 ML thick water film was deposited onto CC4 (exposure1.6 x P a 4 which had been adsorbed on a 40 ML thick water film. The heating rate was %7 KW’. cc14 was detected as m l e 117 (C35C13+)and HzO as m l e 16 (O+).
release of C C 4 into the gas phase. Upon sublimation of enough water to form a “hole”in the top layer, the barrier for C C 4 desorption vanishes, resulting in immediate evolution into the gas phase. I t should be noted that the limiting step for CC14desorption here is not the diffusion of C C 4through ice. Were that the case, C C 4would diffise toward the W(100)surface and the ice-vacuum interface. The Auger spectra in Figure 5 show that this is not so. Havingestablished that a-CC14is derived from the thin film surface and that the W(100) substrate exerts no influence on the CCL desorption spectra, we now consider the kinetics for CCL desorption. The a-CC14desorption temperature increases with coverage, implying that the desorption order is less than unity. We attempted to model a-CC14desorption using both half-order and zeroth-order rate expressions. A zeroth-order expression yields the most satisfactory agreement with the data. Evidence for a zeroth-order expression is %fold. First, the logarithm of the a-CC4 desorption rate depends linearly on reciprocal temperature along the leading edge of the desorption spectra (Figure 7hZ0 Second, leading edges of the desorption curves are independent ofheating rate when only the a phase is present. Finally, no reasonable set of halforder rate parameters could explain the observed increase in desorption temperature between of 0.8 and 2.4 x
Pes. The activation energy for a-CCL desorption, determined from the slope of the least-squares fit of the zeroth-order plot (Figure 7), is 47 f 3 kJ-mol-’. The data plotted in Figure 7 are derived from five low coverage desorption spectra, in which no multilayer formation occurred. This may be compared to the activation energy for ccl4 sublimation of 36 f 1 kJ-mol-l, which was also determined from a zeroth-order plot of the multilayer evaporation rate (Figure 7) and which is in excellent agreement with the literature value for the sublimation energy (38 k J . m ~ l - l ) . Note ~ ~ that the activation energy for a-CC14 evolution is greater than that for multilayer sublimation, despite the fact that a evolution occurs at a lower temperature. The two other rate parameters for a-CC14 (27) Nita, I.; Seki, S. J. Chem. SOC.Jpn. 1948, 69,85-87.
8.0
‘ 1
Figure 7. Zeroth-order plots for desorption of a-CC14 and sublimation of condensed CC4. Lines are derived from linear regression analyses of the data points. Spectra which were analyzed for the a rate parameters corresponded to C c 4 exposures below 2.4 x Pa-s; i.e. no evolution of the condensed phase was observed during temperature programmed desorption.
desorption, namely v, the frequency factor, and N , the number density of sites from which desorption can occur, can in principle be extracted from the y-intercept of the zeroth-order plot y = ln(v)
+ ln(N) + ln(c)
(2)
where y is the intercept and c is a proportionality constant which relates the desorption rate in arbitrary units to the absolute desorption rate. Because c and N are unknown, it is not possible to determine the frequency factor for a-CC14 desorption. However, the y-intercept for a-CC14 desorption can be compared to that for C C 4 sublimation
where the subscripts refer to the a desorption and sublimation processes. From linear regression analyses of the zeroth-order plots, we find that yu - ysub is 10 f 2. The fact that a-CC14desorbs below the temperature at which condensed CCL sublimes indicates that an adsorbed phase of C C 4is trapped a t the ice surface and kinetically prevented from forming the thermodynamically more stable condensed phase. This may be confirmed by investigating the influence of heating rate on the C C 4 desorption spectra (Figure 8). We show the C C 4 desorption spectra for three equivalent C C 4exposures (2 x Paas) at heating rates of 3,7, and 13 K c l . Note that a evolution is preferred over multilayer sublimation at high heating rates; indeed, for these CC14 exposures, multilayer evolution is almost completely absent a t a heating rate of 13 K-s-’. At low heating rates, however, conversion from the a into the condensed phase is competitive with a-CC14 desorption. Discussion We begin with the assertion that the data shown in Figure 3 represent the temperature programmed desorption of CC14from ice. a-CC14is clearly confined to the surface of a thin water film; this is amply demonstrated by the thickness dependence measurements, the sandwich experiments, and the post-desorption Auger electron spectra. Furthermore, the surface of a n ultrathin water film on W(100) is almost certainly essentially identical to that which would be present on bulk amorphous ice.
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3308 Langmuir, Vol. 10,No. 9,1994
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I
(a) Displacement by water as it sublimes:
(b) Sublimation of high vapor pressure CCI4droplets:
a
water
U U (a) p = 3 K.s"
100
125 150 175 temperature / K
200
Figure 8. The influence of heating rate on cc14 desorption. Ice films were M O ML thick, CC4 exposures were 2 x Pass, and CC4 was detected as m / e 117 (C36Cls+). Spectra were recorded at heating rates of (a) 3, (b) 7, and (c) 13 K9-l. Intermolecular forces in molecular solids like water are quite localized, and it is extremely unlikely that the underlying tungsten substrate exerts a n effect on the surface properties of a n ice layer greater than several monolayers thick. The close correspondence between Esub for water reported here and those reported by others on other surfacesz1 provides strong experimental evidence that the properties of a n ultrathin water layer are unaffected by the underlying substrate. The fact that the activation energy for sublimation of the ultrathin layers is equal within experimental error to the actual sublimation energy of ice also argues against any significant influence of the substrate on the properties of the ice layer. Two states are observed during the temperature programmed desorption of C C 4 from ice: the a state near 130 K and the normal condensed phase a t 145 K. Noteworthy aspects of the a-CC14state are the desorption kinetics, which are well described by a zeroth-order rate expression, and the desorption temperature, which is below the sublimation temperature of the condensed phase. The latter observation implies that a-CC14 is metastable, i.e. it is kinetically prevented from forming the thermodynamically more stable condensed phase. Barriers to nucleation of a three-dimensional phase are well known.28 For CC14 adsorbed on ice, it is possible to surmount the barrier for three-dimensional C C 4 nucleation during temperature programmed desorption, but only at low heating rates (Figure 8). Analogous behavior was previously reported for benzene and water desorption from R U ( O O ~ )Enhanced .~~ formation of the condensed phase at low heating rates implies that the activation energy barrier for conversion from islands into the condensed phase is greater than the barrier for desorption from a phase. We considered three mechanisms for a-CC4 desorption from ice, schematically depicted in Figure 9, all of which predict zeroth-order desorption kinetics: displacement of adsorbed C c 4 by the underlying water film as it sublimes, (28) Kern, R.;L e Lay, G.;Metois,J. J. Basic Mechanisms in the Early Stages of Epitazy; North-HollandPublishing Co.:Amsterdam, 1979; VOl. 3. (29) Polta, J.A.;Schmitz,P. J.;Thiel, P. A. Langmuir 1987,3,11781180.
G
CC14droplet
(c) Desorption from a two dimensional adsorbed phase:
O O
0
0= CCl4
water
Figure 9. Three possible mechanisms for desorption of CC4 from an ultrathin water film: (a) displacement of adsorbed CCL by sublimationof the underlying water layer; (b) evaporation of high vapor pressure CCLQ microdroplets; (c) simple desorption of a two-dimensional adsorbed phase. sublimation of high vapor pressure C C 4 microdroplets, and desorption of adsorbed C C 4 from two-dimensional islands on ice. Displacement can be discounted because a-CC14desorption is '99% complete before even 0.1 water monolayers have evolved into the gas phase. The evaporation of high vapor pressure microdroplets can be excluded as well, since (i) the apparent activation energy for a-CC14desorption is independent of coverage, (ii)the leading edges of the a desorption curves are superimposable, and (iii) the a desorption curves are readily distinguished from the condensed state. One would expect microdroplets to increase in size and therefore decrease in vapor pressure with increasing CCl, exposure, resulting in smooth conversion from the a state into the normal condensed phase. Given that neither displacement nor microdroplet evaporation can account for a-CC14evolution, we are left with a mechanism involving desorption of a n adsorbed phase. The adsorbed C C 4phase cannot consist ofisolated and noninteracting molecules; the fact that the desorption temperature increases with coverage implies that attractive interactions are important. Rather, we propose that desorption proceeds from the edges of CCl, islands. Island formation is driven by dispersive CCl4-CCl4 interactions, which at these temperatures are normally sumcient to stabilize the solid phase. The intermolecular structure of the CC4 islands cannot be determined from these experiments, but presumably resembles the structure of condensed CCL. The a-CC14desorption mechanism involves reversible dissociation from a n island edge (eq 4) followed by desorption into the gas phase (eq 5): CCl,(island edge)
2
CC14(2Dgas)
CC14(2Dgas)
k2
CC14(g)
(4)
(5)
where kl, k-1, and k z are the Arrhenius rate constants. Island formation is consistent with the observation that the a-CC14state disappears at very large exposures, as all CC14is perforce incorporated into the three-dimensional
Interaction of CC14 with Ice
Langmuir, Vol. 10, No. 9, 1994 3309
Equation 8, which applies in the very low coverage limit, describes a half-order process. We attempted to identify a half-order regime by examining the desorption spectra of extremely low CC14 coverages, but zeroth-order kinetics were observed even for exposures which resulted in a desorption signal at the threshold of detectability. We also tried to find evidence for half-order desorption kinetics a t the trailing edges of the desorption spectra. These efforts were unsuccessful, possibly because the rate of decrease in the desorption signal is controlled by the pumping speed of the vacuum chamber. Despite the fact that we could not observe half-order kinetics, we believe that the above mechanism accounts for the observed desorption behavior. Models similar to the one presented here are often used to analyze sublimation kinetic^.^^,^^ They predict fractional-order desorption kinetics in the limit of slow diffusion of the two-dimensional gas and zeroth-order kinetics in the limit of rapid diffusion. Although zeroth-order sublimation kinetics have often been reported, to the best of our knowledge, fractionalorder sublimation kinetics have never been observed in a simple thermal desorption experiment. We measure a value for E , of 47 f 3 kJ.mo1-l) which in the zeroth-order regime corresponds to the energy difference between C C 4 adsorbed a t an island edge and the desorption transition state. This may be compared to the sublimation energy of bulk CCl4 (36 f 1 kJ*mol-l). This is a surprising difference, given that a-CC14evolves into the gas phase a t a temperature below that a t which the bulk phase sublimes. The higher rate of a-CC14 desorption arises from a greater frequency factor in the a desorption rate expression. This is well demonstrated by extrapolation of the zeroth-order plots in Figure 7 to their y-intercepts: the difference between y, and ysub is 10 f 2. This difference is equal to ln(v,/v,,b) + ln(N,/Nsub) (eq 3)) where N, and Nsubrefer to the number densities of sites from which desorption or sublimation can occur.
Given the coverages investigated in this work, N, and conceivably differ by more than a n order of magnitude. The difference between the two y-intercepts is therefore almost entirely associated with the ln(Ya/Ysub) term. Within the context of a n Arrhenius analysis,20we can relate the frequency factor ratio to AS*, - ASSsub,the difference in activation entropies. We estimate a value for AS*a - AS*subof 83 J-mol-'0K-l. CC4 adsorption probabilities are high (>0.8) and independent of coverage on ice and on condensed CC4. Furthermore, for multilayer sublimation, the kinetic and thermodynamic desorption energies are equal within experimental error. From these facts, we conclude that the barrier for CC14 adsorption is low on both ice and condensed cc4. This in turn implies that the activation barrier for the reverse process, i.e. desorption, is nearly equal to the depth of the adsorption potential energy well. The transition states for a-CC14 desorption and C C 4 multilayer sublimation therefore both resemble gaseous c c 4 . Because the transition states are identical, the difference between ASs, and A S l s u b can be attributed to ground-state effects: the entropy of CC14 a t the edge of a two-dimensional island on ice is lower than the entropy of C C 4 in the multilayer state by ~ 8 J.mol-'-K-l. 3 The forces between C c 4 and amorphous ice are entirely dispersive in nature; there is no chemisorption bond. In particular, there is no evidence for hydrogen bonding, since the free OH stretch a t the surface of amorphous ice persists upon C c 4 adsorption.15 The interactions between adsorbed CC14 and ice can be interpreted in terms of a phenomenon analogous to the well-known hydrophobic effect in aqueous solution^.^^ With respect to the present work, a n important aspect of the hydrophobic effect is that the immiscibility of nonpolar substances in water is often entropic rather than enthalpic in origin. For instance, AS" for transfer of 1 mol of pentane from its pure liquid phase into liquid water is -105 J-mol-l-K-l, while AH' is -2100 Jsmol-'; for ethane AS" is -88 Jsmol-'.K-l and AH' is -10 500 J - m ~ l - ' . ~Note ~ the qualitative and quantitative similarities between AS' and AH' for dissolution ofnonpolar substances in liquid water and h s * s u b - AS*, and E s u b - E, for CC14 desorption. As is the case for dissolution of many nonpolar substances in water, AE and A S for conversion from the condensed to the adsorbed phase are negative, with A E on the order of -10 kJ-mol-l and A S near -100 J*mol-l*K-l. Negative entropy changes for dissolution of nonpolar substances in liquid water are rationalized by postulating that water solvates a nonpolar moleculevia reorganization of the intermolecular hydrogen bonding network to create a cavity in which the nonpolar solute resides. Such reorganization results in a higher degree of solvent organization and therefore a lower entropy. As long as the ice surface is sufficiently flexible, similar effects might be expected to be observed in an adsorbed layer ofnonpolar molecules, like CC14. Specifically, we assert that the underlying ice layer adjusts in such a way as to minimize unfavorable interactions with CC4. This could force the ice surface into a n entropically unfavorable state, thereby resulting in an unexpectedly high frequency factor in the rate expression for a-CC14 desorption. Reconstruction implies nonrigidity of the ice surface. Although the temperatures investigated in this work are well below the ice melting point, simple energetic considerations suggest that surface reconstruction is possible. The adsorption energy of CC14on ice (47 kJ-mol-l) is nearly as
(30) Ruiz-Suarez, J. C.; Vargas, M. C.; Goodman, F. 0.;Scoles, G. Surf. Sci. 1991,243,219-226. (31)Asada, H.; Sekito, H. Surf. Sci. 1992,273,139-146.
(32) Tanford, C. The Hydrophobic Effect: Formation ofMicelles and Biological Membranes., 2nd ed.; John Wiley & Sons, Inc.: New York, 1980.
condensed phase. The above mechanism is similar to that proposed for xenon desorption from graphite,30 and for the desorption of certain adsorbed metal layers from dissimilar metallic surfaces.28 If the coverage of CC4 (2D gas) is small and unchanging during desorption (i.e. if steady-state kinetics are assumed), then the desorption rate expression is
where Nedgeis the number density of C C 4 molecules a t island edge sites and N, is the number density of sites from which the two-dimensional gas may desorb. Equation 6 can be analyzed in terms of two limiting cases, corresponding to small and large coverages. In the latter case, k-l*Nedge>>kz-N,, the coverage dependence vanishes, and desorption is zeroth order:
(7) As CC14 desorption proceeds and the coverage decreases, the rate of CC14(2D gas) reattachment to a n island edge becomes negligibly small. Now kz-N, >> JZ-l'Nedge, and the desorption rate expression reduces to
Nsubcannot
3310 Langmuir, Vol. 10, No. 9, 1994 large a s the cohesive energy ofice (the sublimation energy of water is 48 kJ.mol-’). As was pointed out long when such a situation exists, the heat released during adsorption could provide the driving force for reconstruction of a n otherwise rigid layer. There is also spectroscopic evidence which points to the adsorption-induced reconstruction of amorphous ice. An amorphous ice surface has a high density of free (i.e. non-hydrogen bonded) O-H groups.34 Horn et al. have examined the infrared spectra of C C 4 adsorbed on ice-&. They reported a shift in the frequency of the free O-D stretch of deuterated ice upon C C 4 adsorption, from 2728 to 2695 cm-I.l5 The nature of the shift in ~ ( 0 - D ) can be interpreted in terms of a structural change of ice. (33)Adamson, A. W. Physical Chemistry of Surfaces; 5th ed.; John Wiley & Sons, Inc.: New York, 1990. (34) Callen, B. W.; Griffiths,K.; Norton, P. R. S u q . Sci. 1992,261, L44-J.M.
Blanchard and Roberts
Conclusion A significant finding of this work is that temperature programmed desorption methods can be employed to investigate adsorption at an ice surface, and, by extension, at the surfaces of many molecular solids. This is true despite the narrow temperature range which can be studied (~90-170 K for water) due to vapor pressure limitations. The intermolecular forces between cc14 and ice are quite weak, as one might expect, and the interaction would normally be classified a s physisorption. However, the adsorption forces are specific enough that the adsorbed state can be distinguished from the multilayer state and the a-CC14desorption kinetics relatively easily studied.
Acknowledgment. “his work was supported by the National Science Foundation through Grant No. CHE9200108, by the University of Minnesota through the McKnight-Land Grant Assistant Professor program, and by the Dreyfus Foundation through a New Faculty Award.
