4264
D. KEISERAND A. S. KANA'AN
Enthalpy and Entropy of Sublimation of Tetraphenyltin and Hexaphenylditin. The Bond Dissociation Energy of Sn-C and Sn-Snl by D. Keiser and A. S. Kana'an Department of Chemistry, Western Michigan University, Kalamazoo, Michigan 49001
(Received M a y 16, 196B)
The vapor pressures of solid tetraphenyltin and hexaphenylditin have been measured directly by measurements of the torsional recoil of a suspended effusioncell, P,, and indirectly by measurement of the mass effusion, PIC. The results of the sublimation pressures in atmospheres are: For tetraphenyltin the torsional recoil yields in the range 428-454'K loglo P = (12.942 =k 0.753) - (7963 =k 33)T-I; the mass effusion yields in the range 393461°K loglo PK = (12.816 f 0.131) - (7927 f 56)T-'. For hexaphenylditin the torsional recoil yields in the range 463-503'K log,, P = (14.087 rf 0.276) - (9373 f 42)T-I; the mass effusion yields in the range 444-503'K log10 PK = (14.082 f 0.124) - (9399 f 59)T-I. The cited errors are standard deviations generated in the least-squares analyses. The average ratio P J P K is of the order of 1.10; the deviation from unity represents a systematic difference between the two procedures. The enthalpies and entropies of sublimation derived from the measurementsare 36.3 f 0.3kcal mol-1 and 58.6 f 0.6 eu for tetraphenyltin at 433'K and 43.0 i: 0.3 kcal mol-I and 64.4 i: 0.6 eu for hexaphenylditin at 475'K, respectively. The average bond dissociation energy, E(Sn-C) is calculated to be 55.8 f 1.7 kcal mol-', and the bond dissociation energy of Sn-Sn in hexaphenylditin is calculated to be 36.3 5 2.4 kcal mol-'.
Introduction I n recent years there has been a notable increase in the accumulated thermochemical data for organometallic compounds. However, there is a lack of reliable and systematic study of the vapor pressures of related compounds to elucidate the relationship between their thermal and structural properties. I n this paper the thermochemical quantities associated with the sublimation of tetraphenyltin and hexaphenylditin are reported. The average bond dissociation energy of Sn-C in these compounds and the bond energy of Sn-Sn in hexaphenylditin are derived. I n the present investigation the vapor pressures of tetraphenyltin and hexaphenylditin have been determined from the simultaneous measurements of the torsional recoil and the rate of mass effusion in the temperature ranges 405-461 and 443-503"K1 respectively. The vapor pressure of SnPhr has been previously studied in the temperature range 298~ ~ E I ' K The . ~ vapor pressure of SnzPhe has not been cited in the literature in any temperature range. Experimental Section Apparatus. The apparatus is essentially similar to that described by McCreary and Thorna except for a n automatic semimicro recording-vacuum balance (Ainsworth, RV-AUI) and the furnace. Only details unique to the present study are mentioned. Torsion Fibers. Tungsten wires 0.038 mm in diameter and 35.6 cm long, served as torsion fibers. The torsional constant was determined by using the assembly as a torsional pendulum, according to the procedure described by M y l e ~ . The ~ measured conThe Journal of Physical Chemistry
stants and characteristics of the cylinders of known moment of inertia, used in the calibration, are listed in Tables I and 11. The cited errors in mass, diameter, and time are those estimated from the precision of these measurements. The propagated error in 7, d~0.01dyn cm rad-l, compares well with the differences obtained from repeated measurements. Table I: Measurements of the Torsional Constant Parameters of the Ring of the Torsion Pendulum Aluminum Parameter
Inside diameter, cm Outside diameter, cm Mass, g
Moment of inertia, g om2
ring"
Brass ringa
5.398 5.716 11.9604 92.411
5.392 5.720 39.4653 304.830
"Errors: mass, 3 X 10-6 g; diameter, 5 0.01 sec; torsional constant (propagated), 1 x
x 10-3
cm; time, dyn cm rad-'.
Cells. Two sets of effusion cells machined from brass and aluminum were used. An aluminum cell, 12.7 mm in diameter and 38 mm long, consisted of three sections. The central section joined each of the two end sections by means of tapered joints. The central section, 16 mm diameter, had a tapered key(1) This paper is based on work presented by D. Keiser to the School of Graduate Studies, Western Michigan University in partial fulfillment of the requirements of ri M.A. degree in Chemistry. (2) A. 8. Carson, R . Cooper, and D. R. Stranks, Trans. Faraday Xoc., 58,2125 (1962). (3) J. R.McCreary and R. J. Thorn, J . Chem. Phys., 48,3290(1968). (4) K. M.Myles, Trans. A I M E , 230,736 (1964).
ENTHALPY AND ENTROPY O F SUBLIMATION O F SnPh4 AND SnzPh6
4265
ment was essential to eliminate direct contact with the fluidized sand, which otherwise could cause un(diameter, 0.038 mm; length, 35.6 cm) desirable vibration of the system. Also the brass cylinder served as a heat sink which improved thermal -Time per oscillation, seta- Torsional ExperiRing With Without constanta stabilization. ment material ring ring dyne om rad-1 Temperature Measurement. Temperatures of the 4 Brass 117.589 26.497 0.9169 effusion cell were considered to be those of a calibrated 4 Aluminum 66.119 19.518 0.9142 chromel-alumel thermocouple, suspended in a fixed 5 Aluminum 66.273 19.537 0,9097 6 Aluminum 66.608 19.593 0.9002 position inside the heating chamber. The thermo7 Aluminum 66.725 19.685 0.8976 couple hot junction was about 3 mm above the upper side of the cell. The relation between the potential a See footnote a, Table I. of this thermocouple (reference thermocouple) and the temperature inside the cell was established in the following way. A standard thermocouple was inserted way machined traversely across the upper side. A into an equivalent stationary cell, located in the same brass cell, of the same over-all dimensions, consisted position of the cell used in an actual experiment. With of one piece. The central part, 16 mm in diameter the apparatus evacuated to a pressure of about low6 and 12.5 mm long, accommodated the tapered lieyTorr, the reference and standard thermocouples were way. The cell end openings were fitted with tapered intercompared over a wide temperature range inplugs. I n both sets a cell was connected to the torsion cluding the temperatures reported in this work. The assembly by means of a dovetail matching the keystandard thermocouple was initially standardized way in the main. body of the cell. The characteristics against the freezing temperatures of lead, zinc, aluof the cells, listed in Table 111, were measured microminum, and copper. The resulting relation of the scopically. The transmission factors for mass effusion temperature us. emf of the thermocouple derived from and for torsional recoil are based on the tabulations the linear relations for both operations, the calibration of DeMarcus and Hoppersa and Schulz and S e a r ~ y , ~ ~ and standardization is respectively. T("K) = 279.26 24.3gVr (1) Table I1 : Calibration Measurements of Tungsten Wire
+
Table 111: Parameters of Effusion Cells Diameter of orifices, Cell no. 3B 4B
1A 2A
3A
Moment Clausing factor
Pressure factor
Set I: Brass Cells" 89.97 0.8982 90.86 0.9018 89.18 0.9196 0.9187 90.07
0.9289 0,9316 0.9447 0,9441
arm
crn X 10%
8.91 9.23 11.54 11.41
cm X 101
Set 11: Aluminum 8.23 158.25 154.40 8.21 156.21 9.86 160.63 9.79 160.52 12.15 12.04 158.20
a All orifice lengths were 10.16 X were 22.86 X cm.
Cellsb 0.7847 0.7844 0.8131 0.8121 0.8427 0.8413 cm.
0.8365 0.8363 0.8609 0.8600 0.8854 0.8842
All orifice lengths
Heating Chamber and Furnace. A Pyrex tube, 67 mm i.d. and 40 cm long, enclosed the suspended cell. The tube was connected to the main apparatus via an O-ringed Pyrex flange. This heating chamber was immersed in an air-fluidized sand bath (Tecam, type SBS4). The temperature of the bath was controlled through a relayed thermoregulator immersed in the sand bath. The heating chamber was jacketed with a brass cylinder, without direct contact. This arrange-
where Vr is the emf in millivolts for the reference thermocouple. During a typical experiment lasting 2 hr, the temperature was constant within *0.2°. Chemicals. Samples of Alfa's tetraphenyltin and hexaphenylditin were purified by repeated recrystallization, from benzene solutions, prior to use. The melting points of the purified samples actually used were 225-227" for SnPhr and 230-232" for SnJ'hs. Procedure. Prior to an experiment the effusion cell was outgassed under reduced pressure of the order of Torr a t 250". The weight of the suspension system excluding the cell was recorded. This was necessary to determine the fraction of effusate which condensed on the suspension system and to correct the rate of effusion accordingly. For such correction it was assumed that the condensed fraction was constant for all times and temperatures involved. I n a typical experiment of a total weight loss of 370 mg the weight of the suspension system increased by about 20 mg or what corresponds to less than 6% of the total weight loss. The amount of starting material was 0.6-0.8 g. After the system was evacuated to a pressure of the order of Torr, the sample was degassed by raising the temperature gradually, first to about 425°K for 2 hr and finally to the highest temperature of (5) (a) W.C.DeMarcus and E. H. Hopper, J. Chem. Phya., 2 3 , 1344 (1955); (b) D.A. Schulz and A. W. Seamy, ibid., 36, 3099 (1962). Volume 73, Number 12 December 1969
D. KEIBERAND A. S. KANA’AN
4266
interest. After 10 min a t this temperature, during which the weight loss was approximately 20 mg, the temperature was lowered and vapor pressure measurements were performed a t randomly selected temperatures. The rate of effusion was recorded a t each temperature after equilibration had occurred as indicated by the steady rate of weight loss. The angular deflection at the same temperature was obtained from measurements of the angular displacement of the reflected light beam on the scale of the optical lever ~ y s t e m . ~The deflection and temperature were recorded every 15 min during the time of an effusion measurement a t a given temperature. The residual pressure in the system was maintained between and lov6Torr at all times. During the course of investigation of each compound the melting points and the infrared spectra of the original sample, the condensed effusate and the residue were compared. Such observations confirmed the simple sublimation of the samples.
Results and Treatment of Data Calculations. The saturated vapor pressures in equilibrium with solid tetraphenyltin and hexaphenylditin were determined from the simultaneous measurements of the rate of effusion and the torsional
Torsion
.*
Expt. 5 Expt. 6
.-.-.A
Knudsen _. Expt. 4 Expt. 5 Expt. 6
x o e
-6.1
z
-me .IS
-7.0
-7.6
2.0
2.1
2.2
I 23
T.’Ylr? l V J I
Torsion Expt. 3 Knudsen -4.
ExDt. 1
Figure 2. Torsion- and Knudsen-effusion data for hexaphenylditin.
-.-.-
r\
o
recoil. The vapor pressures were calculated according to the familiar equations of mass effusion6and torsional recoil.’ In the calculation of PK the molecular weight of the monomer of the corresponding compound has been employed. The calculated vapor pressures were analyzed by the least-squares method to establish the linear Clausius-Clapeyron equations. The secondlaw enthalpies and entropies of sublimation were calculated from the parameters A and B of these equations. Results. The results of Knudsen-effusion and torsional-recoil measurements of the sublimation pressures are presented in Table IV and Figures 1 and 2. The cited errors are the standard deviations obtained in the least-squares analyses. Also listed in Table IV are the results of the least-squares analysis of the combined points of the indicated sets of experiments, I n this averaging approach the points having the largest residuals were successively rejected in each analysis. In the case of SnPh4 five points were removed (two points from each of experiments 1 and 2
.5
c
?.
-6.
.7
.7
I 2.0
2.1
2.2
2.3
2.4
~ ’ ~ (K,’) 1 0 ~
Figure 1. Torsion- and Knudsen-effusion data for tetraphenyltin. The Journal of Phvsical Chemistry
2.5
2.6
(6) See, for instance, J. L. Margrave in “Physicochemical Measure ments at High Temperatures,” J. O’M. Bockris, J. L. White, and J. D. Mackenzie, Ed., Butterworths Scientific Publications, London, 1959, pp 225-246.
(7) See, for instance, It. D. Freeman, in “The Characterization of High Temperature Vapors,” J. L. Margrave, Ed., John Wiley & Sons, Inc., New York, N. Y., 1967, pp 152-192.
ENTHALPY AND ENTROPY OF SUBLIMATION OF SnPh4AND SnzPhs Table IV:
Torsion- and Knudsen-Effusion Results (log,, P ( a t m ) = A
4267
- BT-1)
ExptMethod
P?
Temp.
cell no.
range,
B
A
Knudsen Knudsen Knudsen Torsion Knudsen
1-3B 2-4B 3-1A 3-1A 1 1 2, 3
12.737 4 12.579 4 12.591 4 12.942 rt 12.816 4
0.264 0.213 0.435 0.753 0.131
Tetraphenyltin 7882 f- 114 7837 f- 88 7829 i 193 7963 I 33 7927 4 56
Knudsen Knudsen Torsion Knudsen Torsion Knudsen Torsion
4-2A 5-3A 5-3A 6-1A 6-lA 4, 5, 6 5, 6
13.690 =k 0.217 14,068 zk 0.232 14.178 4 0.944 14.296 i 0.222 14,006 4 0.220 14.082 f- 0.124 14.087 rt 0.276
Hexaphenylditin 9222 zk 32 9392 f 109 9431 I 454 9493 4 107 9330 f 107 9399 rt 59 9373 f 42
x
108,
!?, OK
atm
OK
407-454 394453 424-461 428-454 393-461
3.418 3.019 3.236 3.568 3.358
433
444-500 451-495 469-495 457-503 463-503 444-503 463-503
1.879 1.978 2,107 2.047 2,312 1.971 2.262
47.5
Table V : Thermodynamic Properties of Tetraphenyltin and Hexaphenylditin -----Tetraphenyltin-Knudsen
T, "K
ART',kea1 mol-' AST",eu AR29801 kcal mol-'
433 36.3 I 0 . 3 58.6 i 0 . 6 38.5 i. 1 . 0
Hexaphenylditin
--c
Torsion
Knudsen
Torsion
433 36.4 f 0.2 59.2 4 3 . 4 38.6 i 0 . 9
476 43.0 I 0 . 3 64.4 I 0 . 6 45.0 f 1 . 0
475 42.9 1 0 . 2 64.5 I 1 . 3 44.9 f 0 . 9
and one point from experiment 3). In the case of SnzPh6 two points were removed (one point from each of experiments 5 and 6). Such a test indicated that all experiments for a given compound are reliable, even though systematic errors are probably present. Such systematic errors are encountered in the various measurements of vapor pressures. For both compounds there occurs a linear correlation of ASo vs. AH", both derived from the linear least-squares analysis of log P us. T-l. Thorn and coworkersB-l0have demonstrated the same linear correlation in their vapor pressures measurements. The underlying principle discussed by Thorn" reveals that the errors encountered in the vapor pressure measurements are, in the sense of AH" vs. AS", systematic rather than random. Thermodynamic Properties. Thermodynamic properties derived from the linear least-squares analyses of the data, summarized in Table IV, are given in Table V. Additional thermodynamic properties derived from the measurements associated with sublimation are the enthalpies of formation of the gaseous molecules, the average bond dissociation energy, E(Sn-C6H6), and the bond dissociation energy, E(SnSn). To calculate these quantities the following auxiliary data were employed. Enthalpy of Formation of SnPhr. The heat of combustion of SnPhe, a t 298"K, was measured by Pope 0.8 kcal/mol-'. For and Skinner12 to be -3177.7
the enthalpy of formation they calculated the value AHrO(SnPh4, s, 298°K) = 98.5 f 0.9 kcal/mol-l. The above value of the enthalpy of formation and the corrected enthalpy of sublimation AHsubo(SnPhr, 298°K) = 38.5 f 1.0 kcal mol-l, yield the enthalpy of formation of the vapor, AHfO(SnPh4, g, 298°K) = 137.0 f 2.0 kcal mol-'. Enthalpy of Formation of Phenyl Radical and Sn(g). Fielding and Pritchard18 reported for the enthalpy of formation of the phenyl radical the value AHt"(Ph., g) = 72.0 f 2.0 kcal mol-'. The value for the enthalpy of formation of Sn(g) is AHto(Sn, g) = 72.2 f 0.5 kcal m01-l.l~ Enthalpy of Formation of SnzPhe. The heat of combustion of Sn2Ph6 was measured by Lautsch, et a1.,16 to be -4848.5 f 7.0 kcal mol-'. For the enthalpy of formation they reported the value AHfO(8) J. R. McCreary and R. J. Thorn, J. Chem. Phys., 50, 3725
(1969). (9) A. S. Kana'an, J. R. McCreary, D. E. Peterson, and R. J. Thorn, High Temp. Sci., 1 , 222 (1969). (10) J. R. McCreary, S. A. Russoul, and R. J. Thorn, ibid., in press. (11) R. J. Thorn, ibid.,in press. (12) A. E. Pope and H. A. Skinner, Trans. Faraday Soc., 60, 1402 (1964). (13) W. Fielding and H. 0. Pritchard, J. Phys. Chem., 66,821 (1962). (14) R. Hultgren, 12. L. Orr, P. D. Anderson, and K. K. Kelley, "Selected Values of Thermodynamic Properties of Metals and Alloys," John Wiley &Sons, Inc., New York, N. Y., 1963, p 262. Volume Y.9, Number 13 Decembm 1969
4268
D. KEISERAND A. S. KANA'AN
(SnzPhe, s) = 160.3 kcal mol-'. Using this value and the corrected enthalpy of sublimation AHsubo(SnzPhs, 298°K) = 45.0 1.0 kcal mol-', the enthalpy of formation of the vapor is calculated to be AHfo(Sn2Phe, g) = 205.3 f 1.0 kcal mol-'. The Average Bond Dissociation EneiAgy of Sn-C. The average bond dissociation energy, E(Sn-C) in tetraphenyltin is calculated from the enthalpy change for the process
*
SnPhe(g, 298°K) +Sn(g, 298°K)
+ 4Ph. (g, 298°K)
The calculated value, using the above thermodynamic data, is B(Sn-C) = 55.8 f 1.7 kcal mol-'. The Bond Dissociation Energy of Sn-Sn in Hexaphenylditin. The bond dissociation energy E(Sn-Sn) is calculated from the enthalpy change for the process 2SnPh4(g, 298°K) +SnzPhe(g, 298°K) f 2Ph. (g, 298°K) The above enthalpy change is represented approximately by
AH" = 2B(Sn-C) - E(Sn-Sn) =
75.3
f
1 kcal mol-'
The bond dissociation energy of Sn-Sn is calculated to be E(Sn-Sn) = 36.3 & 2.4 kcal mol-'.
Discussion and Conclusions The vapor pressure and enthalpy of sublimation of tetraphenyltin at 298°K reported by Carson, et are 8.32 x 10-l2 atm and 15.85 kcal mol-', respectively. When the above values are inserted into the Clausius-Clapeyron equation In P(atm) = A S " / R
- AH"/RT
(2)
the entropy of sublimation is calculated to be 2.49 eu and the Clausius-Clapeyron equation parameters A and B are 0.544 and 3464, respectively. A comparison of these parameters with those derived from the least-squares analysis of the combined data of the present work shows a marked disagreement between the data of these two investigations. This disagreement is not explainable in terms of dissociative sublimation in the present study. The agreement, within experimental errors, between the simultaneous measurements of the vapor pressure by Knudsen effusion, assuming monomer vapor species, and by torsional recoil rules out such possibility. The enthalpy of sublimation of tetraphenyltin reported by Carson, et u Z . , ~ is too low relative to the corresponding values for polyphenyl compounds of group 111. The enthalpy of sublimation reported in this paper is in good agreement with that expected for tetraphenyl compounds of group IV according to the discussion below. Also the low value of the entropy of sublimation derived from their data is The Journal of Physical Chemistry
practically an impossible value for a process such as SnPh&) --t SnPh4(g). Probably they encountered an error associated with incomplete condensation of the effusate. The enthalpies of sublimation of planar hydrocarbons such as anthracene and pyrene correspond to a value of ca. 1.75 kcal mol-' per carbon atom.'6 The corresponding value for nonplanar molecules of related symmetry is expected to be lower than the cited value of planar molecules. Greenwood, et aZ.,17 reported relatively high values for the enthalpies of sublimation of triphenylgallium (32.9 f 0.9 kcal mol-') and triphenylindium (33.6 f 0.4 kcal mol-') and a low value for triphenylboron (26.6 f 0.3 kcal mol-'). They inferred that in triphenylgallium and triphenylindium it is possible that a crystal structure similar to that of the planar organic inolecules exists and that in triphenylboron steric interference of the three phenyl rings may be responsible for their nonplanarity with the B-C skeleton. Other nonplanar molecules of low enthalpy of sublimation are triphenylarsenic (23.5 f 1.0 kcal mol-'), triphenylantimony (25.4 f 1.0 kcal mo1-1),18 and triphenylmethane (24.1 f 1.0 kcal From these limited examples it is surmised that for such nonplanar molecules the enthalpy of sublimation corresponds to a value of ca. 1.4 f 0.1 kcal mol-' per carbon atom. In accordance with this conclusion, tetraphenyltin is expected to have AHaub" = 33.6 f 2.4 kcal mol-'. This value agrees with the enthalpy of sublimation obtained from the sublimation studied of this work. Other approximation rules which lead to values for the enthalpy of sublimation of tetraphenyltin which are in good agreement with the values reported in this paper are the additivity rule for lattice energy of organic crystals20 and the additivity of the enthalpy of sublimation in terms of uniquely defined group increments.21 According to the first Aihara20 assigned an allotment of 9 kcal mol-' per phenyl group. Bondi2' assigned a value of 10 kcal mol-' per phenyl group and an approximate value of 1.5 kcal mol-l per tetravalent tin in alkyl compounds. However, Aondi2' pointed out that his rule is expected to err consistently on the high side The derived average bond dissociation energy, l?(Sn-C) in tetraphenyltin when incorporated in the (16) W. F. Lautsch, A. Trober, W. Zimmer, L. Mehnen, W. Linck, H. M. Lehmann, H. Brandenburger, H. Korner, H. J. Metzschker, K. Wagner, and R. Kaden, 2.Chem., 3,415 (1963). (16) R. S. Bradley and T. G. Cleasby, J. Chem. Soc., 1690 (1953). (17) N. N. Greenwood, P. G. Perkins, and M. E. Twentyman, ibid., 2109 (1967). (18) H. A. Skinner in "Advances in Organometallic Chemistry," Vol. 2, F. G. A. Stone and R. West, Ed., Academic Press, New York. N. Y . , 1964, pp 49-114. (19) G. R. Cuthbertson and H. A. Beat, J. Amer. Chem. Soc., 58,2000 (1936). (20) A. Aihara, Bu2l. Chem. Soc. Jap., 3 2 , 1242 (1959). (21) A. Bondi, J. Chem. Eng. Data, 8,371 (1963).
4269
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).
Volume ‘78,Number 12 December 1969
