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ADSORPTION ON’ HYDROXYLATED SILICASURFACES derivation of the bond dissociation energy, E(Sn-Sn) in hexaphenylditin gave a value of 36.3 2.4 kcal mol-l. This value is in agreement with that expected in organometallic compounds of tin. Cottrel122 cited a value of E(Sn-Sn) = 39.0 kcal mol-1 in hexamethylditin based on the work of Pedley, et a1.23 The differences between P, and PK may be attributable to systematic difference between the two methods. The average ratio P,/PK is of the order of 1.10 for the present system. This value is within the range of 1.07-1.15 found by other investig a t o r ~ ~ for , ~ ~the - ~ simultaneous ~ measurements of the vapor pressure of substances known to vaporize as monomers.
Acknowledgments. The authors wish to acknowledge partial support for equipment and supplies through grants to A.S.K. by Western Michigan University, Faculty Research Funds, and the Petroleum Research Funds administered by the American Chemical Society.
(22) T. L. Cottrell, “The Strengths of Chemical Bonds, 2nd ed, Butterworths Scientific Publications, London, 1958, p 259. (23) J. B. Pedley, H. A. Skinner, and C. L. Chernick, Trans. Faraday Soc., 53,1612 (1957). (24) R. D. Freeman and A. W. Searoy, J . Chem. Phys., 22, 762 (1954). (25) G. M. Rosenblatt and C. E. Birchenall, ibid., 35, 788 (1961). (26) J. H. Kim and A. Cosgarea, Jr., ibid., 44,806 (1966).
Adsorption on Hydroxylated Silica Surfaces by M. L. Hair and W. Hertl li!esearch and D e v e l o p e n t Laboratory, Corning Glass Works, Corning, New York 14880 (Received May 19, 1363)
Adsorption isotherms have been measured by volumetric, gravimetric, and spectroscopic techniques on silica surfaces which have been modified so as to contain only freely vibrating hydroxyl groups, or only H-bonded hydroxyl groups, or no hydroxyl groups. For most of the adsorbates the freely vibrating hydroxyl group is the strongest surface adsorption site, and adsorption on this site accounts for the major part of the adsorption which takes place a t low pressures. Hydrocarbon adsorbates interact with these free OH groups an a 1: 1 basis. The nonhydrocarbon compounds studied interact on a 1: 1 basis when only a single OH is connected to a surface silicon atom, but on a 1:2 basis when the freely vibrating surface hydroxyl groups have a geminal configuration. Mutually H-bonded surface OH groups interact only slightly with lone-pair and hydrocarbon adsorbates. This slight interaction takes place only with the weakly H-bonded OH groups. Water behaves differently from the other adsorbates in that it interacts strongly with the H-bonded OH groups. The formation of the first and second layers of adsorbed water can be observed as plateaus in the absorption isotherm. The adsorption properties of silica containing a meadsorbed laver of water were also studied. When adsorption takes place on this “wet” silica, the amount adsorbed is increased. This adsorbed water thus acts as a specific adsorption site for other molecules.
Introduction The surface of silica and adsorption on that surface has been the subject of many investigations. Infrared studies, in particular, have led to an acceptable picture of that surface as containing siloxane groups, hydrogenbonded hydroxyl groups, and free hydroxyl groups which may be either single or geminal. It has been shown that specific adsorption occurs on the surface silanol groups. 2 , 3 The specific adsorption on these groups has been measured spectroscopically and heats of adsorption have been ~ b t a i n e d . ~These measurements, however, apply only to the free hydroxyl groups and ignore any other adsorption which may place on the surface, such as on the H-bonded OH groups. Specific adsorption refers here to an adsorbed
molecule which has a relatively high heat of adsorption and is probably localized on a given adsorption site. Under normal atmospheric conditions the silica surface also contains adsorbed water. I t has been suggested that water adsorbs preferentially on the Hbonded OH gr0ups.l For practical reasons it is important to know what effect this adsorbed water has on the adsorbent properties of silica, since small quantities of adsorbed water are known to have a great effect in either accelerating or poisoning the rate of catalytic (1) LM.L. Hair, “Infrared Spectroscopy in Surface Chemistry,” Marcel Dekker, Inc., New York, N. Y . , 1967. (2) A. V. Kiselev, Quart. Rev. (London), X V , 99 (1961). (3) M.R. Basila, J . Chem. Phys., 35,1151 (1961). (4) W. Hertl and M. L. Hair, J. Phys. Chem., 72, 4676 (1968).
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reactions on surfaces. Also, water has been used as a liquid phase in gas-liquid chromatography.5 In order to understand properly the nature of the silica surface, three questions remain to be answered: (1) to what total extent does adsorption take place on silica, (2) is the additional adsorption specific to a given site and, if so, what site, and (3) what effect does adsorbed water have on the absorption properties of silica? To answer these questions, adsorption isotherms have been measured by volumetric, gravimetric, and spectroscopic techniques on silica surfaces which have been modified in a variety of mays. These modifications have been arranged so as to allow adsorption studies on either the freely vibrating or the H-bonded OH groups on the surface, individually, without the complication arising from the simultaneous presence of both types of OH groups. The treatments result in a sparse covering of the specific adsorption sites and the population of the surface OH groups is less than one-third of that associated with a completely hydrated silica. This minimizes the effects of lateral interactions between adsorbate molecules and allows the data to be easily interpreted. The silica used is a nonporous material, selected to minimize any capillary effects. It should be emphasized that most silicas employed in adsorption studies are microporous and contain many more hydroxyl groups on the surface than the simplified silica employed here.
M. L. HAIRAND W. HERTL
Si02 HEATED TO 8W'C a METHYLATED
w z u
2 8
3
SO2 HEATED TO 520° a METHYLATED
(C)
/
3800
,
I
I
3600
I
,
3400
I
3200
,
I
3000
I
I
2800
CM'I
Figure 1. Spectra of silicas with modified surfaces which were used in adsorption experiments: A, silica heated to 800°, surface contains free OH groups; B, silica heated to 800" and methylated, surface contains methyl groups but no OH groups; C, silica heated to 520" and methylated, surface contains Hbonded OH groups and methyl groups.
DIETHYL
ETHER
ADSORPTION ( T = 22'C)
ON SILICA
Experimental Section 1. Measurement of Isotherms. The volumetric isotherms were obtained on a standard instrument used for measuring BET surface areas. The gravimetric isotherms were obtained on an Ainsworth recording balance, connected to a conventional vacuum rack. Spectroscopic isotherms were obtained by measuring the decrease in intensity of the band due to the free OH groups. The silica sample was mounted in a compensated cell, so as to cancel out any absorption due to gas phase bands. 2. Modification of Silica Surface. The silica used in these experiments was Cab-O-Si1 (Cabot Co., Boston) and had a BET surface area of 160 m2/g. N o significant surface area changes occur on heating this silica to 800". The following pretreatments were used. (a) The silica was heated to 800". The surface contains only free OH groups (Figure la). (b) The silica was heated to 800" and then methylated with hleaSiC1. The surface contains no OH groups; only siloxane linkages and the organic methyl groups (Figure lb). (c) The silica was soaked in HN03 for 16 hr, heated to 520") and then methylated with Il!te&3iCl. This surface contains vicinal H-bonded OH groups but no free OH groups (Figure IC). (d) The untreated silica was exposed t o several Torr of water vapor before the desired adsorbate was added t o the system. The surface The Journal of Physical Chemistry
P (torr1
Figure 2. Diethyl ether adsorption isotherms on silica a t 24": A, volumetric isotherm on silica containing only free OH groups; B, volumetric isotherm on dehydroxylated silica; D, spectroscopically measured isotherm on free OH groups.
contains free OH groups, H-bonded OH groups, and adsorbed water.
Results and Discussion Figures 2 , 3, and 4 give the volumetric and spectroscopic isotherms for diethyl ether, acetone, and benzene obtained on the surfaces containing only free OH groups (sample a) and on completely dehydroxylated surfaces (sample b). Figures 5-9 give the gravimetric isotherms obtained for hexane, triethylamine, pyridine, methanol, and ( 5 ) B. 1;. Karger and A. Hartkopf, Anal. Chem., 40,215 (1968).
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TRIETHYLAMINE
ADSORPTION ON SILICA ( T 22'C)
t
