ADSORPTION OF METHANOL VAPORON SILVERIODIDE
3373
The Adsorption of Methanol Vapor on Silver Iodide'
by Harry W. Edwards2and M. L. Corrin Department of Chemistry, The University of Ariwna, Tucson, Arizona
~~
(Received October 84, 1966)
~
The adsorption of methanol vapor on silver iodide was measured a t 9.77, 19.79, and 30.02' over the pressure range 0.24-108 mm. The silver iodide was prepared by the reaction of silver and iodine in vacuo with subsequent liquid ammonia treatment. The adsorption isotherms do not fit into the Brunauer classification. The shape of the isotherms indicates the absence of three-dimensional clustering in the adsorbed phase. The dependence of the isosteric heats of adsorption upon surface coverage reveals the dual nature of the silver iodide surface. The surface is heteroenergetic with approximately 12% of the surface consisting of higher energy sites located patchwise over the surface. Selection of methanol vapor as the adsorbate eliminated three-dimensional clustering in the adsorbed phase due to hydrogen bonding, and thus characterization of the silver iodide surface was straightforward.
Introduction Interest in silver iodide as a nucleant for supercooled water vapor stems from the report by Vonnegut3 that finely divided silver iodide in the form of a smoke produces ice crystals from supercooled water vapor at Although silver iodide was temperatures below -4'. originally selected on the basis of its similarity to ice in crystal structure, it is now generally recognized4 that the surface properties of the solid may play a major role in the mechanism of heterogeneous nucleation of supercooled water vapor. Attempts to characterize the silver iodide surface in terms of its interaction with water vapor are in disagreement both as to the nature and to the extent of the silver iodide surface.j-" In two case^,^^^^ where the adsorption of water vapor was measured on silver iodide prepared by aqueous precipitation, the presence of surface contaminants was reported. In fact, there is strong evidence that silver iodide prepared by aqueous precipitation is always contaminated to some extent with counterions present during precipitation. l 2 This result casts considerable doubt on the validity of measurements of water adsorption made on silver iodide samples prepared by aqueous precipitation; it is clear that the discrepancies may be due to the presence of varying amounts of surface contaminants. In an earlier paper,13 we reported the preparation of silver iodide free of hygroscopic impurities 2nd its
interaction with water vapor. Silver iodide was prepared in finely divided form by the reaction in vacuo between metallic silver and iodine and subsequent treatment with liquid ammonia. Comparison of the water vapor adsorption isotherm for this material at 30" with that of silver iodide prepared by aqueous precipitation reveals a gross difference in the behavior of the two preparations. The amount of water adsorbed per unit (1) This paper is based, in part, on the dissertation of H. W. Edwards submitted to the faculty of the Department of Chemistry in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate College of The University of Arizona, 1966. (2) Department of Mechanical Engineering, Colorado State University, Fort Collins, Colo. 80521. (3) B. Vonnegut, J. A p p l . Phys., 18, 593 (1947). (4) N. H. Fletcher, 2. Angev. Math. Phys., 14, 487 (1963). (5) L. V. Coulter and G. A. Candela, 2. Elektrochem., 56, 449 (1952). (6) S. J. Birstein, J. Meteorol., 12, 324 (1955). (7) F. E . Karasz, W. M. Champion, and G. D. Halsey, Jr., J . Phys. Chem., 60, 376 (1956). (8) N. M.Moskvitin, M. M. Dubinin, and A. I. Sakharov, Zze. A M . iVauk SSSR, Otd. Khim. Nauk, 122, 840 (1958); Bull. Acad. Sei. U S S R , Div. Chem. Sci., 23, 2080 (1959). (9) A. C. Zettlemoyer, N. Tcheurekdjian, and J. J. Chessick, Nature, 192, 653 (1961). (10) P. G. Hall and F. C. Tompkins, Trans. Faraday Soc., 58, 1734 (1962). (11) N. Tcheurekdjian, A. C. Zettlemoyer, and J . J. Chessick, J. Phys. Chem., 68, 773 (1964). (12) hl. L. Corrin and N. S. Storm, ibid., 67, 1509 (1963). (13) M. L. Corrin, H. W. Edwards, and John A. Nelson, J . Atmospheric Sei., 21, 565 (1964).
Volume 71, ,Vumber 1 1
October 1967
3374
surface area at relative pressures exceeding 0.5 is considerably greater for the silver iodide prepared by aqueous precipitation. This result shows that the interaction between the silver iodide surface and water vapor is highly sensitive to small amounts of surface impurities. The measurements reported in this paper were carried out to characterize further the silver iodide surface resulting from the direct reaction preparation. Interpretation of water vapor adsorption on silver iodide in terms of the silver iodide surface is complicated by three-dimensional clustering of adsorbate molecules in the adsorbed The stacking effect for water is attributed to intermolecular hydrogen bonding in the adsorbed phase. I n this investigation methanol vapor was selected as the adsorbate to simplify interpretation of the thermodynamic data. The simplification results in effect from the substitution of a methyl group (methyl hydrogens are essentially inactive with respect to hydrogen bonding) for a hydrogen atom in the water molecule. The orientation of a polar molecule in an adsorbed phase has been examined in detail by Harkins for a large number of system^.'^ The principle governing the orientation of polar molecules a t an interface is that molecules are arranged to provide the least abrupt energetic transition between phases. For the adsorption of a gas on a solid, this principle rules out as thermodynamically unfavorable the formation of a surface of higher free energy than the bare surface. A similar argument was used by Langmuir to determine the orientation of molecules a t an interface by attributing a local surface free energy t o each portion of the molecule. l5 The orientation of adsorbed methanol on a solid surface is dependent upon the nature of the solid surface. On the hydrophobic graphitized carbon black surface, Avgul’, et al., reported a planar orientation for adsorbed methanol.16 In the case of a polar ionic crystal, however, the adsorbed methanol molecule is oriented perpendicularly to the surface with the hydroxyl function adjacent to the surface.” In neither of the two cases cited, which represent relative extremes of adsorbent surface structure and energy, was threedimensional adsorbate cluster formation observed. It is important to note that perpendicular orientation of the methanol molecule with the methyl group adjacent to the surface does not occur even for the graphitized carbon black since the surface resulting from such an arrangement would be of higher free energy than the bare surface. Stacking of methanol molecules a t !ess than monolayer coverages is similarly ruled out as an energetically unfavorable configuration. Variations The Journal of Physical Chemistry
HARRY W. EDWARDS AND M. L. CORRIN
in the thickness of the adsorbed phase, however, could be expected after completion of the first monolayer.18 The choice of methanol vapor as the adsorbate thus provides a means for characterizing the slver iodide surface in terms of its interaction with the hydrophilic hydroxyl group in the absence of three-dimensional clustering effects which have been reported for the adsorption of water vapor on silver iodide. The simplicity of methanol adsorption a t less than monolayer coverages apparently results from the fact that methanol is capable of forming only one intermolecular hydrogen bond per molecule.
Experimental Section Two separate volumetric systems were constructed to obtain the methanol adsorption isotherm data. The low-pressure system employing a NcLeod gauge was used for pressure measurements below 5 nim. The difference in height between the columns of mercury in the gauge was measured with a Gaertner telemicroscope fitted with a bifilar eyepiece. A precision-divided stainless steel scale was rigidly mounted directly behind the arms of the ?tlcLeod gauge; the smallest division on the scale was 1 mm. The bifilar eyepiece provided approximately 100 divisions between two adjacent 1-mni divisions inscribed on the scale. Using this arrangement, heights could be estimated to the nearest 0.002 mm . Pressure measurements above 5 mm were made with the high-pressure system which employed a wide-bore mercury manometer as the primary pressure-measuring device. The wide-bore manometer was joined to the adsorption system through a capillary null rnanonieterlg so that the volume of the adsorption system could be adjusted to the same value for all high pressure measurements. The difference in height between the columns of mercury in the wide-bore manometer was measured with a cathetometer which could be read to 0.01 mm. In both systems mercury float valves and mercury cutoffs were employed. Prior to adsorption measurements, the systems were thoroughly outgassed for a period of at least 1 week until the system pressure de(14) W. D. Harkins, “The Physical Chemistry of Surface Films,” Reinhold Publishing Corp., New l’ork, N. Y., 1952. (15) I. Langmuir, “Colloid Symposium Monograph,” The Chemical Catalog Co., Inc., New Tork, N. Y., 1925, p 48. (16) N. N. Avgul’, G. I. Berezin, and A. V. Kiselev, Irc. Akad. B a u k SSSR, Otd. Khim. Naulz, 205 (1961). (17) V. E. Vasserberg, A. A . Balandin, and XI. P. Maksimova, Zh. Fiz. Khim., 35, 858 (1961). (18) W. D. Harkins, “The Physical Chemistry of Surface Films,” Reinhold Publishing Corp., New York, N. T., 1952, p 245. (19) W. D. Hai,kins and G. Jura, J. Am. Chem. SOC.,6 6 , 1366 (1944).
ADSORPTION OF METHANOL VAPORON SILVER IODIDE
creased to 2 X 10-7 mm. Since it has been reported that silver iodide sinters appreciably a t 50°, l2 heating could not be employed to accelerate the rate of outgassing. Adsorption measurements mere carried out at 9.77, 19.79, and 30.02". Three Philadelphia differential thermometers were used for temperature measurements, one for each of the three isotherm temperatures. Each of the thermometers was calibrated against a platinum resistance thermometer certified by the National Bureau of Standards. The uncertainty in reading the differential thermometers was 0.005". The temperature of the adsorbent cell mas regulated with a water bath equipped with a heating unit and a refrigeration unit. The control unit for the bath was fitted with a thermistor temperature transducer which was found sufficient for temperature control of lt0.01". The silver iodide was prepared by the direct reaction of silver and iodine in vucuo with subsequent liquid ammonia treatment by the method reported earlier.I3 The specific surface area of the silver iodide was determined by application of the BET relation to krypton adsorption isotherm data at 76.7"K. At the time of the lowpressure meamrements the surface area was found by this method t3 be 1.14 ni2/g. After 3.5 months, at the time of the high-pressure measurements, the BET surface area of the silver iodide was found to have decreased to 0.84 ni2/g. The silver iodide was stored in a lowactinic Pyrex flask which was kept in a blackened vacuum desiccator. After each opening, the desiccator was promptly evacuated through a trap cooled to liquid nitrogen temperature. The niethaiiol was obtained as Xallinckrodt anhydrous methanol (acetone free) which the manufacturer specified t o be greater than 99.5'% CH30H and less than 0.05% H20. This material was refluxed over magnesium turnings in n closed system fitted with a mercury blow-off tube for hydrogen venting. The middle fraction of the dijtillate was thoroughly degassed and retained as the methanol source. A volumetric method was used to obtain both the low- and high-pressure adsorption data. In the lowpressure system the 1IcLeod gauge was used both as a doser and to mensure the equilibrium pressure in the adsorption system. The temperature variation method was employed in both systems so that a single sample of silver iodide vould be used to obtain the adsorption data at all three isotherm temperatures. After admission of a dose of methanol to the adsorption system, a period of at least 6 hr was allowed for the pressure in the system t o attain a constant value. The 6-hr period was adequate for attainment of equilibrium since in every case the pressure became
3375
0.05
sE
\0.04
-
-
4-
x
*)
E
Y
003
0
-
1 0
a goo2
-
3 -I 0
>
001
-
1.0
2.0
30
5
40
PRESSURE ( m m )
Figure 1. Low-pressure methanol adsorption isotherms.
invariant with time within a few minutes after admission of each dose of methanol vapor. The low-pressure adsorption data were corrected for the amount of methanol taken up by the adsorption apparatus. Low-pressure blank isotherms were measured for determination of this correction. The correction was found to be negligible with respect to the high-pressure adsorption. The points plotted in Figures 1 and 2 represent the experimental adsorption points; desorption points fell on the adsorption curves within the precision of the experiment.
Results and Discussion The amount of methanol adsorbed as a function of pressure and temperature is shown in Figures 1 and 2 . Although the adsorption at low pressures shown in Figure 1 is typical of physical adsorption at low coverages, the adsorption at higher pressures shown in Figure 2 does not fit into any of the five general categories described by Brunauer.20 Type I1 adsorption, characterized by an initially decreasing and then increasing isotherm slope, corresponds t o multilayer adsorption or three-dimensional stacking of adsorbate molecules. The concavity of the methanol adsorption (20) 9. Brunauer, "The Adsorption of Gases and Vapors," Vol. I, Princeton University Press, Princeton, N. J., 1945.
Volume 71, hTumber11
October 1967
HARRY W. EDWARDS AND M. L. CORRIN
3376
made for silver iodide samples resulting from solution preparation which were contaminated to some extent by coprecipitated ionic compounds. Tcheurekdjian, et al.," reported a 56-fold decrease in surface area for a silver iodide sample with no change in the number of polar sites. However, the work reported herein indicates that the higher energy sites on the silver iodide surface resulting from the direct reaction preparation are unstable with respect to time. The decrease in the number of higher energy sites is shown by the adsorption data. The fact that the amount adsorbed per unit area for the high-pressure data falls below the extrapolated low-pressure isotherms indicates a decrease in the number of high energy sites which are responsible for adsorption at low coverages. Isosteric heats of adsorption were calculated in the usual manner by application of a form of the ClausiusClapeyron relation to the adsorption isotherm data.*l The heat of adsorption is shown as a function of amount adsorbed in Figures 3 and 4. The estimated standard deviation for the low-pressure heats is 0.07 kcal/mole
0.35
0.30
CU-
E
I. 0.25
n
f m
E
u
o.2a
W
m
a
2 w 045
3
$ 04 0 9.77.C D 19.79.C
005
0
Figure 2.
I
I
20
40
I
60 PRESSURE
30.02.C
I
80
I
100
(mm)
High-pressure methanol adsorption isotherms.
isotherms toward the pressure axis, i.e., ( b 2 v / ? l p 2 )< ~ 0, persists a t higher coverages indicating that the adsorption is not type 11. Multilayer formation and three-dimensional stacking of adsorbate molecules evidently do not occur in the range of pressures measured. From an energetic point of view, this behavior indicates that adsorption of methanol vapor upon the partially covered silver iodide surface does not favorably alter the free energy lowering of the system for additional methanol adsorption. This result is in sharp contrast with the usual case of type I1 adsorption. The adsorption data shown in Figures 1 and 2 are based on the specific surface areas of the silver iodide a t the time of adsorption measurements, 1.14 m2/g for the low-pressure measurements and 0.84 m2/g for the high-pressure measurements. Since the low- and highpressure isotherms do not join, e.g., the high-pressure points falling below the extrapolated low-pressure isotherms, a concomitant change in surface properties occurred with the decrease in surface area. Although the decrease in surface area of silver iodide with time has been reported previously, 11*12 the observations were The Journal of Physical Chelni8tTy
1
0~010 VOLUME ADSORBED 0 ~ 0 2 0 (em' S.T,P,/m210 , 0 3 0
Figure 3.
Isosteric heats of adsorption; low-pressure data.
(21) D. M. Young and A. D. Crowell, "Physical Adsorption of Gases," Butterworth and Co. Ltd., London, 1962.
ADSORPTION OF METHANOL VAPORON SILVER IODIDE
3377
and that for the high-pressure heats is 0.1 kcal/mole. The minimum in the low-pressure heat of adsorption curve occurs a t 0.016 cm3/g and presumably results from the interplay of two opposing effects. As the higher energy sites are filled on a heterogeneous surface, the heat of adsorption decreases with increasing coverage. However, if the higher energy sites are located adjacent to one another on the surface, the cooperative effect in the adsorbed phase makes a positive contribution to the heat of adsorption. Thus the minimum in the isosteric heat of adsorption at this coverage results from the influence of the two opposing effects and constitutes strong evidence for a heteroenergetic silver iodide surface. A maximum is observed in the low-pressure heat curve at 0.024 cm3/g. The maximum may also be interpreted in terms of the result of an interplay between two opposing effects. Although the contribution due to lateral interaction tends to increase the heat of adsorption, the number of lateral interactions decreases as the higher energy sites become completely occupied. The maximum in the low-pressure heat curve corresponds t o an area per adsorbate molecule of 155 A2; it is thus highly unlikely that monolayer formation has occurred over the entire surface. Evidently, pseudo-monolayer formation has occurred over the higher energy sites. The higher pressure heat of adsorption curve given in Figure 4 shows a maximum a t a coverage of approximately 0.20 cm3/g which corresponds to an area per adsorbate moleculk of 19 A2. This value is in good agreement with the value of 17 A2 for the area of projection of an isolated methanol molecule calculated by hvgul', et uZ.,'~ from van der Waals' dimensions. This result suggests that the methanol monolayer on silver iodide is less tightly packed than that for graphitized carbon black. Evidently, monolayer formation over the entire surface has occurred at 0.20 cm3/g. The
heat of adsorption then diminishes in the usual manner, presumably approaching the heat of liquefaction as the amount adsorbed increases. A quantitative interpretation of physical adsorption on a heterogeneous surface in terms of molecular parameters requires evaluation of integral thermodynamic functions for the adsorption process. The spreading pressure was not calculated since the data were considered insufficient to permit meaningful extrapolation of the low-pressure isotherms to zero coverage. However, since the isosteric heat of adsorption is particularly sensitive to changes in the degree of lateral interaction in the adsorbed phase, it is concluded that the silver iodide surface has been fairly well characterized by this study. Selection of methanol vapor as the adsorbate eliminated three-dimensional clustering in the adsorbed phase due to hydrogen bonding. Interpretation of the thermodynamic data was therefore straightforward. Perhaps the most significant conclusion that may be drawn from the work herein reported is that the silver iodide surface free of hygroscopic contaminants resulting from solution preparation is highly heteroenergetic, with about 12% of the surface consisting of the higher energy sites. It is also shown that the higher energy sites are not randomly distributed over the surface but occur patch-wise on the surface, as evidenced by the shape of the low pressure isosteric heat of adsorption curve. This work indicates that the hydrophilic sites resulting from the direct reaction preparat!ion of silver iodide are very different from those resulting from contamination.
Acknowledgments. This research was supported by the Atmospheric Sciences Program, Sational Science Foundation, NSF Grant GP-5173. The award of a General Electric Foundation Fellowship in Chemistry to H. W. E. is gratefully acknowledged.
Volume 71,Number 11 October 1967
