Langmuir 1994,10, 2830-2835
2830
Use of the Quartz Crystal Microbalance for the Study of Adsorption from the Gas Phase V. Tsionsky and E. Gileadi* School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Ramat-Aviv 69978, Israel Received February 7, 1994. In Final Form: May 16, 1994@ The quartz crystal microbalance has been used to study adsorption from the gas phase onto a gold surface. The commercial device used in our measurements has a resolution of 0.15 Hz, corresponding to 1.8 ng/cmz. Changes of frequency are caused by a change of mass, but factors such as the total pressure, the viscosity, the density, the surface roughness, the temperature, and the thermal conductivity of the medium may also cause a shift in frequency. A new method, which we call the supporting gas method (SGM),was developed to eliminate the effect of these factors on the frequency of the crystal. In this method the substance being studied is mixed with a large excess of an inert gas (Ar was mostly used, but HZand He yielded identical results). In the course of measurement, the total pressure is maintained constant, while the partial pressure of the substance being studied is varied. The adsorptionisotherms of a number of substances were determined. A comparison between benzene and pyridine showed that the former is adsorbed flat on the surface, while the latter is attached to it in an upright orientation, through the nitrogen atom. The saturated homologues ofthesetwo compounds, cyclohexane and piperidine, respectively, do not exhibit monolayer adsorption. Water, methanol, and 1-propanol are all adsorbed, occupying the same number of sites per molecule on the surface. It may be concluded that these compounds are attached to the surface through the OH group, with the other atoms facing away. The SGM can be a powerful tool in the study of submonolayer adsorptionfrom the gas phase. The sensitivity is high, on the order of a few percent of a monolayer, and can be increasedfurther. The stability is excellent, and seems to be determined by the parameters of the system as a whole, and not by the inherent properties of the quartz crystal microbalance itself. There are several sources of error which can lead to misinterpretation of the results. These can be reduced to an insignificant level by making measurements at a constant total pressure, employing an inert gas in large excess.
Introduction The high sensitivity and the simple relationship between mass and frequency make the quartz crystal microbalance (QCM) an ideal tool for the study of adsorption and as a selective chemical sensor in many application^.'-^ The change in frequency, Af, resulting from a change in mass per unit surface area, Am, was given by Sauerbref as Af= -C,Am
(1)
The constant C , in this equation is a property of the crystal used. It is given by
where fo is the fundamental resonance frequency of the crystal, pq = 2.947 x 10" gcm-l-s is the shear modulus, and eq= 2.648 g ~ m is- the ~ density. The parameter n is assigned a value of 1or 2, depending on whether one face of the crystal or both are exposed. For the commercial unit used in the experiments described below, the sensitivity of the QCM is 12.3 ng/cmz for a change of frequency of 1 Hz, and the resolution is 0.15 Hz. The fundamental resonance frequency is f o = 6 MHz. Equation 1is applicable only if one can assume that the adsorbed molecules are rigidly attached to the crystal surface and vibrate with it. This condition is evidently @Abstractpublished in Advance ACS Abstracts, July 1, 1994. (1)Warner, A. W.; Stockbridge, C. D. The Measurements of Mass using Quartz Crystal Resonators. Symposium on Vacuum Microbalance Techniques, Los h g e l e s , CA, 1962. (2) King, W.,Jr. Res./Deu. 1969,20, 28. (3) Hlavay, J.; Gullbault, G. G. Anal. Chem. 1977,49, 1890. (4) Buttry, D. A. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker Inc.: New York, 1991; Vol. 17, p 1. ( 5 ) Krim, J.; Chiarello, R. J . Vac. Sci. Technol. 1991, A9, 2566. ( 6 ) Sauerbrey, G.2.Phys. 1969, 155, 206.
fulfilled for metal films formed during sputtering in vacuum. For this reason the QCM is most commonly employed for monitoring metal thickness in such processes. Molecules chemisorbed on the surface can also be considered as being rigidly attached to it. A discussion of the interaction of a vibrating crystal with the fluid in contact with it is given
Factors Influencing the Frequency of a Quartz Crystal Weight of a Monolayer of Benzene. In order to estimate the influence of different factors affecting the frequency of the QCM, we estimate first the weight of a monolayer, using benzene as an example. The number of sites on a gold surface is about 1.4 x 1015 sites/cm2. Assuming that each benzene molecule occupies six sites,1° this yields 0.4 nmol/cm2,which corresponds to 31 ng/cmz. The resulting shift in frequency, for the system used in our experiments, is -2.5 Hz. Effect of Pressure. It was shown by Stockbridge7that the frequency of a QCM increases linearly with pressure: Af = C$
(3)
where the constant Cp is given by
c, = 1.4
10-~f~
(4)
if the pressure P is expressed in torr. The coefficient Cp (7) Stockbridge, C. D. In VacuumMicrobalance Techniques;Behmdt, K. H., Ed.; Plenum Press: New York, 1966; Vol. 5,p 147. (8) Kanazawa, K. K.; Gordon, J. Anal. Chim. Acta 1985,175,99. (9) Thompson, M.; Kipling, A. L.; Duncan-Hewitt, W. C.; Rajakovi, L.V.; Cavic-Vlasak, B. A. Analyst 1991, 116,881. (10) Heiland, W.; Gileadi, E.; Bockris, J. OM. J . Phys. Chem. 1966, 70,1207.
0743-7463/94/2410-2830$04.50/0 0 1994 American Chemical Society
Langmuir, Vol. 10, No. 8, 1994 2831
Study of Adsorption from the Gas Phase is independent of the nature of the gas. Equations 3 and 4 have been confirmed e~perimentally.~Jl For the case of benzene on gold, monolayer adsorption is observed a t a partial pressure of about 60 Torr. The corresponding shift in frequency, calculated from eqs 3 and 4, is 0.50Hz. The effect of increasing the pressure of benzene from 0 to 60 Torr is evidently an important effect for the study of submonolayer adsorption, as it gives rise to a frequency shift corresponding to about 20% of a monolayer. Effect of Viscosity and Density. Interaction of the vibrating crystal with a viscous medium leads to an additional loading of the crystal, expressed by a decrease in frequency, which is proportional to the square root of the product of viscosity, 7, and density, e:
0
0
nm
2000 O0
Figure 1. STM image of a typical gold surface used as the substrate in the present work.
where the constant C, is given by
and where Q Iand ql refer to the properties of the fluid in contact with the quartz crystal, while e, and p, refer to the properties of the crystal itself. Equation 6 was adopted from ref 8, where it was originally derived for a QCM in contact with liquids. A review of several approaches to explain this phenomenong shows that eqs 5 and 6 are applicable. It should be noted, however, that similar expressions had already been derived by Stockbridge7 20 years earlier for the behavior of the QCM in contact with gases. For gases the validity of these equations is limited to pressures above 300 Torr or so. This limitation is imposed by the relaxation time needed for the gas molecules to equilibrate thermally, after having gained (or lost) kinetic energy upon contact with the surface of the vibrating crystal. In liquids this relaxation time is evidently very short compared to the period of vibration of the crystal, but in gases it depends on pressure, and becomes small enough only when P 2 300 Torr. The viscosity of gases is typically in the range of 0.10.2 mP, and is independent of pressure (above ca. 0.1 Torr, where the mean-free path becomes of the order of magnitude of the container). For benzene at a pressure of 60 Torr eqs 5 and 6 yield a frequency shift of about 4 Hz. (Stockbridge has shown7that for krypton this effect decreases by about a factor of 2, when the pressure is below 300 Torr.) Evidently, the effect of viscosity and density on the frequency of the QCM is very important, and could be dominant, even at pressures much below those required for monolayer adsorption. It is interesting to note the difference in the application of eq 5 to gases and liquids. In the former, the viscosity does not change significantly in the range of pressure of interest for most applications, while the density can change over many orders of magnitude. In contrast, the density of most liquids is of the same order of magnitude, while the viscosity can change over several orders of magnitude. Effect of Roughness. The sensor of the QCM consists of a quartz crystal, covered by two thin gold electrodes, which are used both for electrical contacts, to induce vibrations in the crystal, and as the active surface, upon which adsorption takes place. The metal surface, formed by sputtering, is evidently not flat on the atomic scale, as shown in the scanning tunneling microscopy (STM)image in Figure 1. This roughness can have two effects. For the calculation of the fractional surface coverage from the added weight, one must know the real surface area, i.e., the geometrical (or projected) area, multiplied by the (11)Wade, W. H.; Allen, R. C. J.Colloid Interface Sci. 1968,27,722.
roughness factor. In addition, there may be some liquid or gas entrapped in the recessed regions on the surface. This material, or part of it, may vibrate with the crystal, adding to its virtual weight. The effect of entrapped liquid has been discussed qualitatively in the literature.12 Further work on the theory of movement of viscous fluids on rough surfaces is now underway.13 Here we shall show that, for a QCM in contact with a gaseous medium at moderate pressures, the effect is negligible. To do this, assume a surface with a rectangular roughness of 10-nm width, depth, and distance apart. The roughness factor of such a surface is 2, doubling the frequency change for a monolayer (from Af= 2.5 to Af= 5.0Hz in the case ofbenzene). The total volume of the recessed region, per square centimeter of cm3, and the weight of projected surface, is 5 x benzene entrapped in this volume, at a pressure of 60 Torr, is only 0.14 ng/cmz, compared to 62 ng/cmz of projected area on this rough surface. Moreover, the gas entrapped can hardly be considered to be “rigidlyadhering” to the surface, so that the number calculated here is an overestimate. If adsorption of benzene from a dilute aqueous solution is studied, the effect of the above roughness could be quite different. The weight of entrapped water in the recesses would be 5 x lo2 ng, which corresponds to about 8 molecular layers of benzene, for a surface having a roughness factor of 2. Thus, complications resulting from surface roughness can represent a major source of error when measurements are taken in liquids, but are quite unimportant for measurements in the gas phase. Effect of Temperature. The temperature dependence of the frequency of an AT-cut quartz crystal, of the type commonly used for the QCM, can be described by a cubic parabola. Over a wide range of temperature the coefficient CT,determining the change of frequency with temperature, is large and positive. Yet, for a limited range of temperature (0-80 “C), the dependence is relatively small, and can be described by the equation Af = -C&AT
(7)
The numerical value of CT depends on the design and manufacturing procedure of a given crystal.14 For acrystal of typical dimensions ( r = 5 mm, thickness 0.2 mm) CT has a fairly constant value of 4 x deg-l. For a fundamental frequency of fo = 6 MHz, this leads to a frequency change of -2.4 Hddeg, which is equivalent to an apparent change of weight of 30 ng/cmz. Thus, the (12)Schumacher, R.; Borges, G.; Kanazawa, K. K. Surf. Sci. 1986, 163, L612. (13)Urbakh, M.;Daikhin, L.Phys. Rev.B 1994,49,4866. (14)Zelenka, J. Piezoelectric Resonators and their Applications; Elsevier: Amsterdam-Oxford-New York-Tokyo, 1986;Chapter 6.
Tsionsky and Gileadi
2832 Langmuir, Vol. 10, No. 8, 1994 temperature of the QCM must be controlled (or at least maintained constant during a given measurement) to at least f0.05"C, in order to obtain meaningful results on adsorption in the submonolayer region. The vibrating quartz crystal dissipates heat. An equivalent circuit describing this device contains a resistance between the electrodes exciting the vibrations in the crystal. A typical value of the resistance, for an For the crystal electrode in contact with air, is 100 used in our work a value of R = lo3 Q is specified by the manufacturer, for operation in vacuum. (The specific value of this resistance depends on the characteristics of the crystal and on the nature of the fluid in contact with it.) For a voltage of 0.2 V, needed to run the QCM, the thermal energy dissipated is hence 0.04-0.4 mW. The thermal conductivity of air is about 0.2 mW/(cmdeg). This would lead to an increase of temperature of AT = 2 "C with respect to the ambient temperature of the chamber containing the crystal. The above rough estimate of AT is valid for measurement in air at ambient pressure. It represents an upper limit, since heat transport by convective flow in the gas and through the crystal holder has been ignored. However, the effective thermal conductivity ofgases in contact with a vibrating crystal decreases below about 300 Torr, for the same reason that the viscosity decreases, as discussed above. Thus, in the range of pressures needed to determine the adsorption isotherm for benzene, for example (P= 1-60 Torr), AT may change significantly with pressure, leading to an apparent change in weight, which would be difficult to separate from the real effect being measured.
Experimental Section Instrumentation. The quartz crystals employed in this study are of the "AT-cut" type, coated on both sides with gold, which serves both as the electrodes and the adsorbing surface. The diameter of the crystals is 1.4 cm, and their fundamental resonance frequency,fo,is 6 MHz. A"deposition controller" type XTCI2, manufactured by Leybold Inficon Inc., is usedto measure the frequency. The change in frequency can be read through an RS232 output in two modes: in units of frequency or in units of thickness. The resolution on the former is -0.4 Hz. The resolution on the thickness scale, recalculated in units of frequency, amounts to 0.15 Hz. This corresponds to 1.9 ng/cmz. This mode was used in all our measurements. The stability of the system during measurement (up to several hours) is better than 0.5 Hz, as seen in Figure 2. The crystal holder has the form of a pair of tweezers, making several electrical contacts at the periphery of the crystal on both sides. The temperature of the air thermostat containing the chamber with the quartz crystal is controlled to within h0.05 "C, corresponding to a frequency fluctuation of &0.12 Hz. The temperature fluctuations of the crystal itself are smaller, due to the thermal inertia of the chamber in which it is placed. The only construction materials in contact with the tested substances are gold, Teflon, and glass. Experimental Procedure. Two methods have been applied to measure changes of the frequency, resulting from adsorption of molecules on the gold surface. In the stationary method, the crystal is placed in a vacuum chamber and its frequency is determined. Next, an inert gas is admitted at a given pressure, and the new value of the frequency is measured, after the steady state has been reached. (The need for the use of an inert gas will be discussed below.) Subsequently, a mixture of the inert gas and the substance being studied is admitted. This procedure is repeated many times, using different partial pressures of the substance being studied, maintainingthe total pressure constant. In the flow method, an inert gas is passed at a constant rate of flow through a highly efficient saturator, containing the substance under investigation. The partial pressure of this substance in the gas flow emerging from the saturator is controlled by varying its temperature. The vapor pressure as a
I
-'OL----0
2doo
I
4000
TIME, sec
Figure 2. Frequency shift due to adsorption of benzene: (a) argon as the supporting gas; (b) helium as the supporting gas. function of temperature is calculated, on the basis of literature data,15J6but is not measured directly. Argon is used as the supporting inert gas in all experiments reported here. Tests using helium or hydrogen instead lead to the same results, as shown in Figure 2. All measurements were taken at 25 "C. In all experiments the frequency in the pure inert gas was measured before and after measuring the frequency in the mixture, to eliminate complications which may be cause by a drift in frequency or by "memory" effects in the system. The frequency shifts Af given below all refer to the difference in frequency measured in the inert gas and in its mixture with the substance under investigation, both taken at the same total pressure. The surface of the QCM was cleaned by rinsing consecutively in methanol, acetone, and petroleum ether. The lack of residue in petroleum ether was tested by evaporating more than 10drops on a crystal and observing no change in frequency, within the limits of detection.
Results and Discussion Inert Gas. Use of a large excess of an inert gas eliminates most of the complications discussed above, which might render the interpretation ofthe data obtained by the QCM questionable. The effect of pressure can be ignored, since all measurements are conducted at the same total pressure. In Figure 3 we compare the frequency shiR obtained as a function of the pressure of water alone (line 1)and as a function of the partial pressure of water in the presence of argon (line 2). The intercept of this line at P = 0 represents the frequency shift caused by the inert gas itself. The variation of Af with pressure is evidently predominant when water alone is used, a n d one
cannot obtain information concerning the adsorption of this species under such conditions. When argon is added in large excess and the total pressure is kept constant at P = 470 Torr, independent of the partial pressure ofwater, a plateau corresponding to monolayer adsorption can be discerned, with a frequency shift of about 1.8 Hz (32 ngl cm2). The effects of viscosity and density are maintained constant throughout such an experiment, for exactly the same reasons, when a large excess of an inert gas is used. (15)Handbook of Chemistry;Nikolsky, B. P., Ed.;Khimiya: MoscowLeningrad, 1962; Vol. 1. (16)Handbook of Chemistry and Physics; Weast, R. C . , Ed.; CRC Press: Cleveland, OH, 1976.
Study of Adsorption from the Gas Phase
-20
c
6 -
0
I
A-
4
D-
3
1
I
00
5
10
PRESSURE, t o r r
15
Figure 4. Comparison of the isotherm for adsorption of water as measured with different inert gases by the flow method (1, Hz;2, Ar) and by the stationary vacuum method (3, He; 4,Ar). 7 ,
OY 0
I
I
I
I
I
I
I
2
4
6
8
IO
12
14
I
!
I
I
I
0.2
0.4
I 0.6
PRESSURE, torr
Figure 3. Frequency shift, Af, as a function of pressure: line 1,only water vapor is admitted into the chamber (totalpressure is equal to the pressure ofwater vapor);line 2, partial pressure of water in the presence of an inert gas. (Total pressure 470 Torr). 0.0
The effect of surface roughness is somewhat more complex. First, one needs to know the real surface area in order to calculate the surface concentration. This can be partially circumvented by using the fractional coverage, 8, and assuming that 8 = 1at the plateau. The effect of gas molecules entrapped in the recessed regions may be higher (since the total pressure is higher), but this will not be affected significantly by the increase of the partial pressure of the substance being studied. Use of a relatively high total pressure (470 Torr in the stationary system and 800 Torr in the flow system) allows for more efficient heat transfer. Thus, the heat generated by the vibration of the crystal will cause a smaller difference AT between it and the temperature of the chamber. Moreover, heat transfer conditions will be constant, irrespective of the partial pressure of the substance studied, maintaining AT constant. Results similar to those shown in line 1 of Figure 3 were reported by Krim et al.17 for the variation of frequency with the pressure of water vapor (without a supporting inert gas), on a QCM coated with gold. Finally, it is necessary to show that the presence of the inert gas does not affect the measurement itself, e.g., by competing with the adsorbed molecules for sites on the surface. In Figure 2 we show results of such an experiment for the adsorption of benzene, where argon or helium is used as the inert supporting gas. Starting with the QCM in vacuum, we first add argon (Figure 2a), causing a decrease in the frequency of nearly -3.8 Hz. When the gas is pumped off, the frequency returns to its initial value, within the resolution of the instrument (0.15 Hz). Note that this is observed about 1h after the initial measurement, attesting to the high stability of the whole system. Admitting a mixture of argon and benzene, at the same total pressure, leads t o a greater change in frequency of about -11.8 Hz. The value of Af relevant for the calculation of the amount ofbenzene adsorbed is, of course, the difference between these numbers, namely, about 8.0 Hz. Replacing argon by helium and repeating the same sequence, we find first an increase in the frequency of the (17)Chiarello, R. P.; Krim, J.; Thompson, C. Suqf Sci. 1994, 306, 359.
P/P"
Figure 5. Frequency shift as a function of the normalized pressure for three similar compounds: line 1, water; line 2, methanol; line 3, 1-propanol. The total pressure was maintained at 470 Torr. crystal (the reason for the difference in behavior between
Ar and He will be discussed in a subsequent publication1*), but adding a mixture ofHe and benzene leads to a negative value of Af, as seen in Figure 2b. While the two inert gases affect the frequency t o different extents and in different directions, the additional change in frequency caused by the presence of benzene in the gas mixture is remarkably close, to within about 2%)which is the limit of resolution in our measurement. In other words, the extent of adsorption of benzene is found to be the same, within the resolution of the instrument, employing two very different inert gases. Stationary vs Flow Method. The results obtained for water adsorption by the stationary method are compared to those obtained by the flow method in Figure 4. The data are indistinguishable, within experimental error, showing that either method can be used. In the work presented here we used mostly the stationary method. Experiments with the flow method served as an independent test for the validity of the measurements to detect systematic errors which may exist. The agreement found between results obtained by the two methods ensures that no such sources of error exist. Water and Alcohols. The frequency shift observed upon the adsorption of water, methanol, and 1-propanol is shown in Figure 5, as a function of partial pressure. We use here a normalized scale of pressure, PIP", where P is the partial pressure of the relevant substance and Pois its vapor pressure in the pure state, at 25 "C. The weight of a monolayer clearly increases with the molecular weight. In Figure 6 we have normalized the results by dividing the change in mass, Am (calculated from hf), for each compound by its molecular weight, and multiplying by Avogadro's number, to yield the number of molecules per (18) Tsionsky, V.; Daikhin, L.; Urbakh, M.; Gileadi, E. Langnuir, submitted for publication.
2834 Langmuir, Vol. 10, No. 8,1994 2.0
,
m
;-. I..=
?k.
2 0.5 0
I
I
1
w.?:'
.
*
.
m
4i
2.0
,
0.0
Tsionsky and Gileadi 1
I
0.1
0.2
I
I
I
I
0.3
0.4
0.5
0.6
P/PO
Figure 6. Same data as in Figure 5,presented as the number of molecules per square centimeter. 2.0
,
1
I
I
1
/i 0.0
0.4
0.2
0.6
P/PO
Figure 7. Comparison of the adsorption of benzene (line 1)t o that of pyridine (line 2) on a gold surface.
square centimeter of geometrical surface area: N(molecules/cm2) =
Am(glcm2) N&molecules/mol) M(g/mol)
The plateau for all three compounds shown here is the same, within experimental error. We conclude that water, methanol, and 1-propanol all occupy the same number of sites on the surface; i.e., they all have the same effective size on the surface, and so are probably attached to it through the OH group. This is what one might expect, and it is an important indication that the method works correctly. It also shows that the configuration ofmolecules at the interface can be determined by measuring accurately the weight of a monolayer. Benzene and Pyridine. Next we compare, in Figure 7, the adsorption of benzene to that of pyridine. We note that, although the size of these two molecules is almost the same, a monolayer of pyridine contains nearly twice as many molecules per square centimeter as a monolayer of benzene. The most likely conclusion is that benzene is adsorbed flat on the surface, while pyridine is adsorbed in an upright position. We note also that the coverage by pyridine rises with its partial pressure much more rapidly than that of benzene. It was also observed that benzene could be pumped off much more rapidly than pyridine. We conclude that the energy of adsorption of pyridine on gold is significantly higher than that of benzene. It follows also that pyridine is most likely adsorbed through the nitrogen atom. Had it been adsorbed through one of the CH groups, it would be hard to rationalize why the energy of adsorption would be higher than that ofbenzene, which has very similar CH groups in the aromatic ring. It may seem somewhat surprising that a benzene molecule adsorbed flat on the surface, with six n orbitals perpendicular to the metal surface, is less strongly adsorbed than a pyridine molecule adsorbed in an upright position, through the nitrogen atom. This may be due to specific interactions between the nitrogen atom and the gold surface. Alternatively, it may be caused by the misfit
Figure 8. Effectof aromaticity on adsorption: benzene (line 1) compared to cyclohexane (line 2) and pyridine (line 3) compared to piperidine (line 4).
between the atomic distances in the benzene ring and those between the metal atoms on the metal surface. The adsorption of aromatic substances on gold electrodes from aqueous solution has been discussed in the literature.19-22 This process, referred to as electro~orption,~~ differs from chemisorption from the gas phase in two respects: (i)The surface coverage is potential dependent. This makes it possible to detect changes in the configuration of the molecules on the surface as a function of potential. It was shown, for example, by Lipkowski et al.21that pyridine electrosorbedon gold in a flat orientation can reach a maximum surface concentration of 0.18 x 1015 molecules/cm2, while in a vertical orientation its maximum surface concentration is about 0.42 x 1015 molecules/cm2. A similar ratio of surface concentrations between benzene and pyridine was observed in our measurements (cf. Figure 71, indicating that the former is adsorbed flat on the surface while the latter is in an upright position. (ii) Unlike chemisorption from the gas phase, electrosorption is a replacement reaction. Since a certain number of water molecules have to be desorbed from the surface for each substrate molecule adsorbed, a configuration occupying a smaller area on the surface may be energetically favored. Aromatic vs Alicyclic Compounds. In Figure 8 we compare the adsorption behavior of benzene to that of cyclohexane and of pyridine to that of piperidine (hexahydropyridine). Aromaticity clearly plays a major role in the adsorption behavior. Benzene and cyclohexane are adsorbed almost to the same extent at low normalized pressure, but unlike for benzene, no plateau is observed in the case of cyclohexane. This is hardly surprising. Adsorption on a surface is driven by two factors: the free energy of interaction between the adsorbate and the metal surface and that between molecules of the adsorbate, which lead eventually to condensation. If the free energy of adsorption is significantly more negative than that of condensation, distinct monolayer adsorption is expected, followed by multilayer formation, as the partial pressure increases. If the two free energies are about equal, there will be no clear transition from mono- to multilayer adsorption. The former is the case for benzene while the latter is the case for cyclohexane, in which all chemical bonds are fully saturated. The difference between pyridine and its saturated homologue piperidine is similar, for the same reasons. (19) Dahms, H.; Green, M. J . Electrochem. SOC.1963, 110, 1075. (20)Ping Gao; Weaver, M. J. J . Phys. Chem. 1986,89,5040. (21) Stolberg,L.;Richer, J.;Lipkowski, J.;Irish, D. E.J . Electround. Chem. 1986,207,213. (22) Zelenay,P.;RiceJackson, L.M.;Wieckowski,A. Langmuir 1990, 6 , 974. (23)Electrosorption; Gileadi,E.,Ed.;Plenum Press: New York, 1967; Chapter 1.
Study of Adsorption from the Gas Phase
Conclusions Calculation of the mass of material adsorbed from the decrease of frequency of the QCM can be ambiguous, due to a number of factors which influence the frequency of the crystal. These include pressure, viscosity, density, temperature, and thermal conductivity. This may be the reason that this technique has not been widely used for adsorption studies. We have developed a new method, which we call the supportinggas method (SGM),to study adsorption in the gas phase, employing the QCM. The substance being studied is mixed with an excess of an inert gas, which is not adsorbed measurably under the experimental conditions chosen. The total pressure is kept constant while the partial pressure of the substance being studied is changed, for the determination of its adsorption isotherm. When measurement is conducted in this manner, all the complications mentioned above are eliminated, or at least reduced below the detection limit. Two experimental techniques were developed: (i) a stationary method, in which the probe is kept in a vacuum chamber and the desired mixture of gases is introduced, and (ii) a flow method, in which the same mixture of gases is passed over the crystal. The two methods were shown to yield the same results, within experimental error. In view of its high sensitivity, the quartz crystal microbalance can be used to study submonolayer adsorption and to determine adsorption isotherms. Moreover, it can be used to gain information concerning the configuration of adsored species on the surface. The adsorption of several substances on a gold surface was studied. (i) Comparison of isotherms measured for water, methanol, and 1-propanol showed that all these compounds occupy the same number of surface sites per molecule. This result implies that they are all adsorbed through the OH group, with the remaining carbon atoms facing away from the surface. (ii)A monolayer of pyridine
Langmuir, Vol. 10,No. 8,1994 2835 adsorbed on the surface weighs about twice as much as a monolayer of benzene, even though their sizes and their molecular weights are almost the same. We conclude that benzene is adsorbed flat on the surface, while pyridine is adsorbed in an upright configuration, with the nitrogen atom facing the surface. The energy of adsorption seems to be higher for pyridine. This is due either to specific interactions of the nitrogen atoms with the metal atoms on the surface or to the mismatch between the structure of the benzene ring and the atoms on the surface of the metal. (iii)Comparison of two pairs of compounds confirms that aromaticity plays a major role in adsorption on metals. While benzene is strongly adsorbed, cyclohexane does not show monolayer adsorption. The behavior observed for this material indicates that the energy of adsorption on the first layer is similar to that in the second and subsequent layers. In other words, this compound is probably condensed on the surface, to an extent depending on its partial pressure, but it is not chemisorbed. A similar difference is observed between the behavior of pyridine and its saturated homologue, piperidine. The method proposed here is very promising, and it opens the way to gain information concerning the structure of the gas-solid (and later also of the liquid-solid) interface.
Acknowledgment. Financial support for this work by the Ministry of Science and Development, under Contract Number 349 4191, and by the Israel Science Foundation of the Israel Academy of Science and Humanities is gratefully acknowledged. V.T. wishes to thank the Ministry of Absorption for partial support during the execution of this project. The authors wish to thank Professor M. Urbach and Dr. L. Daikhin ofthis department for useful discussions. We also thank Mr. M. Brand and B. Agam for their diligent and skillful technical assistance.
