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Adsorption studies on hydrated and dehydrated silicas
Adsorption Studies on Hydrated and Dehydrated Silicas D. R. BASSETT, ~ E. A. B O U C H E R , ~ AND A. C. Z E T T L E M O Y E R
Center for Surface and Coatings Research, Lehigh University, Bethlehem, Pennsylvania Received March 4, 1968 Dehydration of colloidal silicas by heating at elevated temperatures improves their ice nucleating ability. Adsorption studies were performed to study their surface characteristics, on both hydroxylated and dehydroxylated samples. To monitor surface areas, argon was found to be a more reliable adsorbent than nitrogen. The latter displayed a specificity depending on hydroxyl concentration. The highly dehydrated amorphous silicas gave the same surface areas if 16.2 A~was used for the co-area of nitrogen and 16.6 ~2 for argon. Water adsorption was used to assess the surface coneentration of hydroxyls and isopropanol adsorption to determine their steric distribution. The dehydrated silica prepared by heating a wet, precipitated variety hydrated partiMly on cooling in air. Contrary to earlier reports, boiling in water did not yield a fully hydroxylaged surface. In the course of developing and studying silica-based ice nucleating agents, gas adsorption techniques were used to characterize the substrate surfaces as a means of investigating the surface chemistry of ice nucleation. Adsorption studies were undertaken to determine quantitatively what portion of a given surface was water-receptive, since the degree of surface hydrophilieity was thought to be a factor in the heterogeneous nucleation of ice from the v a p o r phase. Indeed, efficient ice-forming silica nueleants were generally found to possess predominantly hydrophobie surfaces. A major problem in dealing with silica surfaces is t h a t of arriving at an estimate of the concentration of surface hydroxyls, nor~ (expressed as the number of OH groups 02 per 100 A ). Methods of determining no~ include isotopic exchange, reaction with OH-specific compounds, and measurement of weight loss on heating. Agreement among 1 Present address: TechnieM Center, Union Carbide Corporation, South Charleston, West Virginia. 2 Present address: School of Chemistry, University of Bristol, Bristol, England. 649
the different methods, however, is generally poor (1). I t is sometimes difficult to tell under what conditions physically adsorbed molecular water is completely eliminated. I t is equally difficult, in m a n y cases, to be certain t h a t all loss in weight or~ heating silicas to high temperature is due exclusively to the condensation of surface hydroxyls. Surface hydroxyls, or silanol groups, act as primary adsorption sites for polar molecules (2). The amount of water adsorption per unit area should thus give a direct indication of the concentration of polar sites. In addition, the spatial distribution of silanols is of interest, since a given number of sites m a y be uniformMly distributed or m a y be clustered in patches. The difference between an isolated and patehwise arrangem e n t of polar surface sites should become apparent by comparing water-adsorption results with those obtained for a larger polar molecule such as isopropanol, the latter would tend to block adsorption on nearby sites. The purpose of this paper is to explore these hypotheses and to report results of initial studies of several finely divided silicas showing the applicability of adsorption Journal of Colloid and Interface Science, Vol. 27, No. 4, August 1968
650
BASSETT, BOUCHER, AND ZETTLEMOYER
methods to investigations of heterogeneous surfaces. EXPERIMENTAL PROCEDURE Two types of silica were studied: a precipitated variety, ttiSil 233 (Pittsburgh Plate Glass Company), and a pyrogenic variety, Cab0Sil (Cabot Corporation). Due to the nature of the manufacturing process, CabOSil is the purer silica and has a smaller particle size than. HiSil. The absence of hysteresis in the desorption branches of argon and nitrogen isotherms indicates that both silicas are nonporous. The Graphon sample was the same as that described previously (3). Argon and nitrogen isotherms were determined by using a glass volumetrie apparatus with a mercury manometer. Research-grade argon, nitrogen, and helium gases were further purified over hot coppershot and degassed charcoal before using. Water-adsorption isotherms were determined volumetrically by using a glass apparatus incorporating a manometer filled with Apiezon B oil, heat-treated to inhibit water adsorption. Pressure could be determined to 0.01 Tort. Argon isotherms in the low pressure range were also determined with this apparatus. Triply distilled water was used after degassing through five freeze-thaw cycles under high vacuum. Isopropanol isotherms were determined, as previously described (3), using a greaseless system featuring Teflon needle valve stopcocks and a Bourdon-type glass pressure gauge with a sensitivity of 0.01 Torr.
Argon and Nitrogen Adsorption Selection of a gas for use in specific surface area determinations presented unexpected problems. Agreement of surface areas could not be obtained when the usual adsorbate cross-sectional areas of 13.8 and 16.2 ~,2, calculated from liquid densities at -196°C, were used for argon and nitrogen, respectively. Nitrogen surface areas were commonly 15-20 % higher than corresponding argon areas. In every case for
which argon and nitrogen isotherms were compared using the same silica substrate, the amount of nitrogen adsorbed at a given relative pressure exceeded that for argon. In only one case were the relative positions of the isotherms reversed--that for adsorption on the graphitized carbon black, Graphon. Consistent surface areas might have been obtained for the silica surfaces simply by adjusting one of the cross-sectional areas to give agreement were it not for the fact that the difference appeared to vary with the degree of surface hydration. Since the overall aim of the study was to estimate the number of polar sites per unit area, it was important that surface area determinations be independent of surface polarity. There is a growing body of evidence (4-7) that nitrogen adsorption is affected by specific interactions between the quadrupole of nitrogen and polar surfaces such as hydroxylated silicas. Aristov and Kiselev (6, 7) compared argon and nitrogen adsorption on a series of silicas having progressively dehydrated surfaces and found that nitrogen adsorption fell sharply with increasing dehydration of the surface, whereas argon adsorption was virtually independent of the degree of surface hydration. These authors used 13.7 A: as the cross-sectional area of argon because of the agreement with nitrogen (o- = 16.2 ~2) on graphitized carbon. Pierce and Ewing (8, 9) have shown, however, that nitrogen exhibits an anomalously high co-area of approximately 20 ~2 on homogeneous carbon surfaces as a result of localization in preferred positions. It is probable, therefore, that the value of 13.7 A: for argon (corresponding to liquid packing) is too low. A nonpolar, nonporous, and amorphous substrate should be ideally suited for comparison studies of surface area measurements. Such a substrate can be obtained by dehydrating the proper silica. In the present study, HiSil was heated in a muffle furnace for 4 hours at 800°C, a sufficiently
Journal of Colloid and Interface Science, Vol. 27, No. 4, August 1968
ADSORPTION STUDIES ON HYDRATED AND DEHYDRATED SILICAS
651
S
100 O0
80
i
S0,
g
6O 6O b0
-g
u u >m 4 0
4O
20 u,NITROGEN o,ARGON
2O
o, ARGON n, NITROGEN
0.2
0.4
P/P.
0,6
0.8
FIO. 1. Adsorption of argon and nitrogen at -196°C on HiSil 233 (top pair of curves) and dehydrated HiSil 233 (bottom pair). Solid points indicate desorption. high temperature to remove most surface hydroxyl groups (10). Argon and nitrogen isotherms are shown for this sample and for untreated HiSil in Fig. 1. The isotherms are closer together in the case of the dehydrated HiSil, the surface area of which is lower than that of the untreated sample because of sintering. The general shapes and relative positions of the isotherms shown in Fig. 2 for Graphon indicate that this homogeneous surface is quite different from that of the heterogeneous silicas. B E T results for these isotherms are given in Table I. In the case of the dehydrated silica, argon must possess a cross-sectional area of 16.6 K s to give agreement with nitrogen using ¢N~ = 16.2 ,~2. These values of for argon and nitrogen are identical to those shown by Corrin (11) as required to give good agreement with the Harkin-Jura method of estimating surface areas. The value of Ca~ = 16.6 ~2 is also in good agree-
0.2
0.4
06
0,8
FIG. 2. Adsorption of argon and nitrogen at -196°C on Graphon. Solid points indicate desorption. ment with the recent results of Harris and Sing (12) for nonporous solids, although these authors caution against the indi scrimihate use of argon for surface area measurements.
Specific
surface
areas
in
Table
I
are given for comparison purooses on the basis of <~ = 13.8 A as we]] as zA, = 16.6 ~s. We continue to accept 20 i s or the appropriate co-area for nitrogen to assess surface areas of graphitic powders. For amorphous, dehydrated silicas, it is recommended that argon at 16.6 ~2 or nitrogen at 16.2 A: should be used. Apparently, argon is less specific than nitrogen to the surface concentration of hydroxyls. In part (b) of Table I, the required cross-sectional areas of nitrogen are calculated to give agreement with zA~ = 16.6 A~. Assuming argon to be unaffected b y the degree of surface hydration, the change in the effective packing of nitrogen can thus be .
o
2
Journal of Colloid and lnlerface Science, ¥ol. 27, No. 4, August 1968
652
BASSETT, B O U C t t E R , AND Z E T T L E M O Y E R
shown by comparing the cross-sectional areas on dehydrated and fully hydraeed HiSil. The decrease from 16.2 to 15.7 ~2 indicates that nitrogen is more tightly packed on the hydrated surface, possibly a result of the added quadrupole interaction of nitrogen with a surface covered with polar hydroxyls. It is interesting that the relative variation of v,, in comparing hydrated and strongly dehydrated silica in this study is the same as that found by Aristov and Kise. lev (6, 7). The difference in values of the cross-sectional areas found by Kiselev and those found in the present study arise as a consequence of the manner in which the cross-sectional area of argon is fixed. In view of the nature of homogeneous graphitized carbon surface and the uncertainties involved in assessing the surface area of such solids, it is concluded here that a dehydrated, amorphous, nonporous silica offers advantages for comparison studies. On the basis of these results, argon adsorption, with car = 16.6 K2 is used as the basis of surface area determination in the next section. Water and Isopropanol Adsorption Water Adsorption on HiSil. Before undertaking water adsorption measurements on a solid, it is important that all molecular water be removed from th~ surface. Figure 3 shows the variation in water adsorption with outgassing temperatures for HiSil. The slight increase in adsorption with increase in activation temperature indicates that not all physically adsorbed water was removed by evacuation at 25°C. A gravimetric study between 25° and 220°C was undertaken to determine the loss in weight of HiSil on heating in vaeuo. The curve in Fig. 4 was obtained using a Cahn vacuum mierobalance with a sensitivity of better than 5 X 10-6 gin. As expected, most loss in weight occurred on outgassing at room temperature. There was, however, a significant loss between 25° and 110°C which amounted to 4.614 mg/gm corresponding to 5.74 ec(STP)/gm. This loss is attributed
120
100
80
>~4o A 12 hr. at, 2 5 ° C
20 0 0
t
o.2
o.3
,,
L
o:4
&
,
o.6
PffPo FIG. 3. Effect of outgassing t e m p e r a t u r e on the adsorption of w a t e r v a p o r on ttiSil at 25°C.
to adsorbed water held in positions favorable for stronger bonding. While equilibrium was attained within 12 hours at 150°C and below, the evolution of water at 180° and 220°C was still incomplete after 24 hours, most probably a result of the slow removal of silanol groups above 180°C as proposed by Young (13). In general, B E T plots for water adsorption on silica were linear, and the monolayer volumes determined in this way agreed reasonably well with visual estimates of the isotherm knees (Point B). While the observance of straight line plots does not necessarily prove the applicability of the theory, the B E T method provided a consistent means of estimating v~. Examination of volumetric results given in Table II shows that outgassing temperature had very little effect on argon adsorption. On the other hand, outgassing for 12 hours at IIO°C caused an increase in the water v~ of 4.6 c c ( S T P ) / g m (at 25°C) over that for the 25°C outgassing, which is equal to the weight lost during outgassing over this range. Outgassing treatment for 12 hours at 110°C was adopted as standard procedure prior to adsorption measurements. Considering further the results listed in Table II, the ratio of the specific surface
Journal of Colloid and Interface Science, Vol, 27, :No.4, August 1968
ADSORPTION STUDIES ON HYDRATED AND DEHYDRATED SILICAS
653
225
Initial Weight 220
215 210 205 I
20.,0,..
2
r
I
I
I
50
75
100
125
"I
1'20
I
I
175
2 0 1 3 "225
FIG. 4. Gravimetrie weight-loss study of HiSil TABLE I A D S O R P T I O N OF A R G O N AND ~ I T R O G E N
Substrate
(a) Argon adsorption at -196°C HiSil, untreated HiSil, 4 hours at 800°C Graphon
vm
ON I°OLAR AND ~ O N P O L A R S U B S T R A T E S
(co(ST-P)~ g)
(~Ar(.~ ~)
(¢rNe = 16.2)
~(m~/g) (O'Ar = 16.6)
27.6 15.1
102 55.9
17.1 16.6
123 67.5
20.9
77.5
15.0
93.3
(c%'2 = 16.2
~rN~ = 20.0
(eAr = 16.6)
vrn (cc(STP)/g)
(b) Nitrogen adsorption at -196°C HiSil, untreated tIiSil, 4 hours at 800°C Graphon
F.(me/g) O'Ar = 13.8)
29.1 15.5 19.4
areas of water and argon gives an indication of the fraction of the surface which is water receptive. This procedure is not entirely satisfactory since this ratio is dependent on the size of the polar adsorbate molecule, water. A better measure of the degree of hydration would be the average site concentration. Since the total area is known from argon adsorption, and the number of water molecules in the monolayer is known from v,~, the area per site (or effective area per molecule) can be calculated. In the case of untreated HiSil, the number of molecules adsorbed is 12.5 per 100 ~2 yielding an area per molecule of 8.0 ~J. Since the cross-sectional area of water calculated from the density at 25°C is 10.5 ,~2, the molecules must be compressed
127 67.6 84.6
104
15.7 16.2 17.9
laterally due to a dense hydroxyl population. The apparent decrease in molecular diameter of water in this ease is about 13 %. Assmning one water molecule per hydroxyl, no~ = 12.5. From structural considerations, estimates of the number of silicon atoms in a silica surface range from 4.6 (10, 14) to 7.9 (15) per 100 ~k~. If a substantial fraction of the surface silicons hold two hydroxyls, quite probable in the ease of the precipitated HiSil, the emTesponding estimates of nou would range from 9.2 to 15.8. The value of i2.5 cMeulated from adsorption lies within this range. One objection to measuring the number of available adsorption sites by the isotherm technique is that initially adsorbed water molecules might provide additional sites Journal of Colloid and Interface Science, Vol. 27, iYo. 4, Augusf~ 1968
654
BASSETT, BOUCHER,
AND ZETTLEMOYEI~
TABLE
II
ADSORPTION Or ARGON AND WATER ON HiSil Argon (at -t96°C)
Substrate treatment None None None None 2 h o u r s a t 650 2 h o u r s a t 800 4 h o u r s at 800 Repeat
Activation temp.(OC) 25 25 110 110 110 110 110 110
Water Water isotherm 2 (H~O) vra (ec ~(¢ = 15.6) temp. (°C) (cc(STP)/Vm'2(¢ = 10.5) Z(Ar) (STP)/g) (mV~) g) (roVe) 27.0
120
27.6
123
25.0 19.5 15.1
110 87.0 67.5
0 25 0 25 25 25 25 ( l s t ) 25 (2nd)
for further adsorption. This argument is correct in certain eases; for instance, the rehydration of strained siloxane bridges yields two hydroxyls. But for physical adsorption, there should be a difference in adsorption energy at primary sites, such as bare hydroxyls, and secondary sizes such as already adsorbed water molecules. The isotherm knee, from which v~ is obtained, should thus indicate the number of molecules adsorbed at primary sites. The validity of this interpretation is partially shown in that the small amount of water, measured gravimetrieally, which desorbed from HiSil between 25 ° and 110°C. Thereby, additional primary adsorption sites were uncovered reflected in an increase in v~ obtained from volumetric isotherms. Were the distinetion between primary and secondary sites nonexistent, then the shape of the isotherm would be largely independent of the amount of water preadsorbed which is clearly not the ease. When the HiSil surface is dehydrated in air at 650°C, the site concentration drops to 6.06, less than half the original value. Part of the v~ in this ease is undoubtedly due to rehydration of strained siloxanes as indicated by a shift in the desorption curve to the left of the adsorption isotherm. A more severe ease of desorption hysteresis caused by rehydration of the surface occurs in the ease of water adsorption on HiSil heated in air at 800°C. In contrast to water adsorption on untreated HiSil, in which Journal of Colloid and InterfaCe Science, ¥ol. 27, No. 4, August 1968
53.0 52.6 57.8 57.2 25.2 8.81 5.01 8.58
149 148 163 161 71.0 24.9 14.1 24.2
1.24 1.24 1.33 1.31 0.63 0.29 0.21 0,36
Sites 100 ,~2 11.9 11.8 12.6 12.5 6.06 2.72 2.00 3.41
A: Site 8.42 8.48 7.91 8.00 16.5 36.7 50.1 29.3
equilibrium was rapidly attained at each relative pressure, adsorption on HiSil dehydrated at 800 ° was characterized by long equilibrium times. Apparent equilibrium was not complete within an hour. An indication of the extent of rehydration is given by the large hysteresis in the desorption curve. After the first desorption was completed, the sample was activated again for 12 hours at 110°C and the isotherm repeated. The second run showed a substantial increase in amount adsorbed and only slight changes in pressure during overnight pauses. These results indicate that strongly dehydrated HiSil reacts with water vapor more easily than expected from previous studies (10, 13, 14) and the density of residual hydroxyl or rehydration sites, or both, is much higher than anticipated for a strongly dehydrated silica surface. Isopropand Adsorption on HiSil. As mentioned earlier, a limited number of surface sites can assume either of two basically different arrangements: (i) an isolated configuration in which the sites are distributed in a random array so that the distance of separation is essentially unifonl~; or (ii) a crowded configuration in which the available sites are clustered in separate patches on the surface. It is of interest, therefore, to know not only the average concentration of sites, but also their spatial distribution. One way of investigating the problem of site distribution is through adsorption
ADSORPTION STUDIES ON HYDRATED AND DEHYDRATED SILICAS studies using molecules of different size. Assuming that adsorption is limited to specific sites, and that the adsorption mechanism is independent of adsorbate size, adsorption of large molecules might be expected to block further adsorption on near neighbor sites arranged in a patehwise configuration even though these sites are avilable for the adsorption of smaller molecules. In the ease of an isolated site distribution, if the distance of separation of the sites is greater than the diameter of the large molecule, then few sites will be excluded by steric hindrance and the number of molecules in a nominal monolayer will be the same as that for smaller molecules. In other words, close packing of sites leads to reduced adsorption for large molecules eompared with the adsorption of smaller adsorbates, whereas for isolated sites, adsorption is independent of adsorbate size. Since alcohols adsorb on silica in a manner similar to water (2), i.e., through hydrogen bonding to surface hydroxyls, it was thought. that a comparison of water and alcohol adsorption might provide a means of exploring site distributions on various treated and untreated silicas in the present study. Isopropyl alcohol was chosen as the adsorbate for several reasons. Its vapor pressure is such that isotherms could be easily determined in the temperature range 0°-25°C. It is roughly spherical with the hydroxyl flanked by two bulky methyl groups. And its cross-sectional area is roughly three times that of water. Determination of effective cross-sectional areas of molecules used i n sterie hindrance studies is of some importance since conclusions regarding site distribution are directly related to these quantities. Water has been used as adsorbate in many surface studies. Although molecular models indicate that water should have a cross-sectional area of 0 2 6-8 A , the general opinion is that a value 0 2 of 10.5 A , derived from the density at 25°C, is a reasonable value in agreement with other adsorption results.
655
The task of fixing the cross-sectional area of isopropanol is more difficult. Few adsorption studies have been made using this molecule as adsorbate. Furthermore, orientation of the molecule at the surface significantly affects ~, although to a lesser extent than for straight-chain alcohols. The cross-sectional area calculated from o 2 liquid density at 25°C is 27.7 A . The 40 X2 arrived at from isopropanol adsorption on Graphon, reported earlier (3), seems too large compared with these estimates, although similarly high values of the co-area have been found on two of the dehydrated samples in Table II. Results of isopropanol adsorption on HiSil are given in Table III. The effective cross-sectional area on untreated HiSil ranges from 33.1 to 33.9 ~2. For isopropanol adsorption on four precipitated aluminas, Vasserberg et al. (17) obtained an average of 31.4 ~2. In view of the uncertain nature of adsorption on graphitized carbons and the controversy surrounding the true area of those surfaces, the estimate of 40 A~ from adsorption on Graphon is viewed with some doubt. Therefore, the cross-sectional area of isopropanol was taken to be 33 A~ in the present studies. As with water adsorption on untreated HiSil, isotherm temperatures had very little effect on isopropanol adsorption, and only a slight increase in adsorption was observed on increasing the activation temperature from 25 ° to 110°C (both 12-hour outgassing). In general, isopropanol isotherms exhibited sharper knees in the monolayer region than did water isotherms on the same surfaces. The sharp B points allowed more precise visual estimation of monolayer volumes. In some cases, the BET plots were linear only up to P s / P o = 0.15, above which positive deviation was observed. In eases where linear BET plots could not be obtained over an appreciable pressure range, B points were employed. Isotherms were reversible up to relative pressures of at least 0.5. The tendency to ehemisorb on highly Journal of Colloid and Interface Science,
VoI.
27,
N o . 4, A u g u s ~ 1968
656
BASSETT, BOUCHEt~, AND ZETTLEMOYER
dehydrated silica surfaces was also less than that shown for water. Adsorption times were short in most cases, on the order of 15 minutes, although at least 1 hour was usually allowed for equilibrium to be attained at each pressure. The effect of adsorbate size is best seen by comparing adsorption results for water in Table II and corresponding results for isopropanol in Table III. Comparing monolayer volumes for the same substrate treatment, water adsorption drops sharply when HiSil is heated to 650°C. In contrast, isopropanol adsorption drops only slightly. The reason for this difference is that for a given available surface area, there are still sufficient polar sites (given by the water data) for isopropanol to adsorb in a more
or less close-packed array due to its greater size. Only in the case of the 800°C treatment does this area per site exceed the assumed cross-sectional area of isopropanol. In this case, the v~'s for water and isopropanol should be identical with identical estimates of the area per site. It is seen that the isopropano] v~ is lower than the water v~ leading to the conclusion that some of the sites must be near neighbors such that isopropanol adsorption on these sites is somewhat hindered. This example is a borderline ease, however, since the area per site (from water adsorption) is only slightly greater than the area of isopropanol itself. Adsorption Studies on CabOSil. Methods developed in the last two sections were applied to an investigation of CabOSil,
TABLE III ADSORPTION Ot~ ISOPROPANOL ON HiSil
Activation Isotherm vm (cv(STP) Substrate treatment Temp. (°C) temp. (°C) /g) None None None 2 h r a t 650 2 h r at 800
25 110 110 110 110
25 0 25 25 25
13.5 13.8 13.8 12.8 8.4
~(~ = 33) (m ~/g)
Sites 100 ~
~-~ Sit-~
122 122 122 113 74.5
2.95 3.02 3.02 3.08 2.60
33.9 33.1 33.1 32.5 38.5
:~(i-PrOH) :~(Ar)
vm(i-PrOH)
vm(H~O)
0.98 1.00 1.00 1.00 0.85
0.24 0.24 0.26 0.51 0.948
T A B L E IV ADSORPTION OF
ARGON, WATER
AND ISOPROPANOL
ON CabOSil
Argon ( at -- 196°C) Substrate treatment Activation temp (°C)
None None None Rehydrated Rehydrated
25 110 110 110 110
vr~ (co (ST~')/g)
25 110 110 110
(m~/g)
40.9 41.9
182 187
42,6
190
Substrate treatment Activation (Argon) temp. (°C) ~(mS/g) None None None Rehydrated
Water Water 2~(~ = 16.6) tisotherm ~ _ +oo~Vm (6c(STP)/ Z(~ = 10.5)
182 187 190
.... v.~ ~+
25 0 25 0 25
g)
(m~/g)
~(HlO) 21(At)
15.3 15.8 15.2 26.6 23.4
43.1 44.5 42.9 75.0 66.0
0.237 0.238 0. 229 0.395 0.347
i-PrOH isothermi-PrOHv +cc+STP w temp.(OC) m ~ g/ ++ 2~(~ = 33) (uVg) 25 0 25 25
16.9 17.5 16.6 21.0
Journal of Colloid and Interface Science, Vol. 27, No. 4, August 1968
150 155 147 186
:~(i-PrOH) 2~(Ar) 0.824 0.829 0.786 0.979
~m(i-PrOH) vm(H~O) 1.10 1.11 1,09 0.90
Sites 100 fi3
A~ Site
2.26 2.27 2.19 3.76 3,31
44.2 44.0 45.7 26.6 30.2
Sites IOOA~
Site
2.49 2.51 2.39 2,98
40.2 39.8 41.9 33.6
A2
ADSORPTION STUDIES ON HYDRATED AND DEHYDRATED SILICAS the results of which are presented in Table IV. T h a t the surface properties of CabOSil are quite different, from those of HiSil is immediately evident on comparing specific surface areas determined by argon and water adsorption. While the apparent water area of untreated HiSil was found to be greater than the argon area (Table II), the water area of CabOSil is only about 25 % of the argon area (eighth column of Table IV). Several other differences are also apparent. Activation at 110°C instead of 25°C had a negligible effect on argon, water or isopropanol adsorption on untreated CabOSil at 25°C. Physically adsorbed water is apparently held less strongly on CabOSil than on the more highly hydrated HiSil. CabOSil was rehydrated by the method of deBoer (14) to see if a site density approaching that exhibited by HiSil could be obtained. This procedure consisted of heating the silica in distilled water for at least 6 hours at 90°-95°C, then drying at 110°C. The average polar site concentration on untreated CabOSil (2.19/100 A~) corresponds to that of HiSil heated to 800°C (2.00/100 A~). Even when rehydrated, the site concentration remains below 4, a surprising result in view of the fact that the deBoer method is widely used for rehydrating previously dehydrated silica surfaces to a "fully h y d r a t e d " condition. It could be argued that each adsorption "site" represents two silanol groups so that the 2.19 e 2 sites per 100 A means that no~ = 4.4, very close to deBoer's (14) and Hockey and Pethieas's (10) estimate of 4.6 for an annealed silica surface. This argument is untenable, however, when results given in Table IV are examined. The average area per site from water adsorption is about 45 o 2 A , more than enough to accommodate one isopropanol molecule per site if the sites are isolated. Two
facts indicate that the
hydroxyls on CabOSil are indeed isolated. The v~'s for isopropanol and water at 25°C are about equal indicating little st.erie hindrance. In addition, the specific surface area
657
Z computed from the isopropanol v~ is less than that of the argon area. Thus, the hydroxyls are separated to such an extent that adsorption according to a bridge mechanism in which each water molecule is adsorbed on two sites is improbable. The conclusion that silanol groups on CabOSil are isolated was reached earlier by McDonald (18) from infrared measurements. The agreement provides, however, some independent verification of the steric hindrance method of investigating surface site distribution. For an isolated site distribution, the ratio of v~ (i-PrOH)/v~(HsO) will be near unity, while for a patchwise site distribution this ratio will be less than unity. In both cases, the ratio of E(iP r O H ) / E ( H ~ O ) must be less than unity to satisfy the condition that the area per site is greater than the cross-sectional area of isopropanol. CONCLUSIONS Adsorption studies on untreated and thermally modified silica surfaces have shown that variations in polar site concentration can be followed by water adsorption, and site distributions can be studied by a sterie hindrance method using water and isopropanol as adsorbates. Outgassing at room temperature is sufficient to remove physically adsorbed water from pyrogenie silica (CabOSil), but evacuation at 110°C is required for the more highly hydrated surface of precipitated silica (HiSil). I t is clear that a highly dehydrated silica surface ehemisorbs water from the vapor in the process of rehydrating siloxane groups t e produce silanols. Very low values of no~ were not obtained in the present study by heating silica in air. No doubt rehydration occurred on cooling. It appears that a completely dehydrated, and therefore hydrophobie, silica surface can be maintained only under vacuum, tt.ehydration of a par tially hydrophilic pyrogenie silica, CabOSil, by immersion in water near boiling is insufficient to produce a surface with the de-
658
BASSETT,
BOUCHER,
gree of h y d r a t i o n possessed b y the original surface of precipitated silica. T h e use of the t e r m "fully h y d r a t e d , " therefore, should be used only after careful consideration of the past history of the sample. Evidence has been presented which indicates that nitrogen adsorption is affected to some degree b y the degree of h y d r a t i o n of a silica surface. T h e effect is p r o b a b l y due to specific interations between the nitrogen quadrupole m o m e n t and surface hydroxyls on silica and should be considered if the variation in polarity of a surface is of interest, in which case a less specific gas such as argon should be used for surface area determinations. T h e authors prefer a value nf 16.6 ~2 for the co-area of argon. ACKNOWLEDGMENT This research was supported by the National Science Foundation under grants G.P. 1590 and G.A. 560X. REFERENCES 1. BOE~, H. P., Advan. Catalysis 16,179 (1966). 2. KISEL~V, A. V., "The Structure and Properties of Porous Materials," (D. H. Everett and F. S. Stone, eds.), p. 195. Butterworths, London (1958). 3. BASSETT, ]). R., BOUCHER, E. A., AND ZET-
AND
ZETTLEMOYEI~
~'LE~OYER, A. C., J. Phys. Chem. 71, 2787 (1967). 4. FROHNSDORFF, G. J. C., AND KINGTON, G. L., Trans. Faraday Soc. 55, 1173 (1959). 5. KISELEV, A. V., Disc. Faraday Soc. 40, 205 (1965). 6. ARISTOV,B. O., AND KISELEV, A. V., Zh. Fiz. Khim. 37, 2520 (1963). 7. ARISTOV,B. G., AND KISnLEV, i . V., Zh. Fiz. Khim. 38, 1984 (1964). 8. PIERCe, C., ANDEWINe, B., J. Am. Chem. Soc. 84, 4070 (1962). 9. PIERCE, C., AND EWING, B., J. Phys. Chem. 68, 2562 (1964). 10. HOCKEY, J. A., AND PETHICA, B. A., Trans. Faraday Soc. 57, 2247 (1961). 11. CORRIN, M. L., Y. Am. Chem. Soc. 73, 4061 (1951). 12. HARRIS, M. R., AND SING, K. S. W., Chem. Ind. 757 (1967). 13. YOUNG,G. J., J. Colloid Sci. 67 (1968). 14. DEBoER, J. H., HI,]RMANS, M. E. A., AND VLE~S~ENS, J. M., Proc. Kon. Ned. Akad. Wet. B60, 45, 54 (1957) ; B61, 2 (1958). 15. ILER, R. K., "The Colloid Ch emistry of Silica and Silicates," p. 242. Cornell Univ. Press, Ithaca, New York. (1955). 16. CHESSIC~:, J. J., ZETTLEMOYER, A. C., AND YTJ, Y.-F., J. Phys. Chem. 64, 530 (1960). 17. VASS~.RBERG, V. E., BALANDIN, A. A., AND MAKSI~OVA, M. P., Zh. Fiz. Khim. 35, 858 (1961). 18. McDONALD, ]~. S., J-. Am. Chem. Soe. 79, 850 (1957).
JouTnal of Colioidand Interface Science, Vo].27, No. 4, August 1968
Center for Surface and Coatings Research, Lehigh University, Bethlehem, Pennsylvania Received March 4, 1968 Dehydration of colloidal silicas by heating at elevated temperatures improves their ice nucleating ability. Adsorption studies were performed to study their surface characteristics, on both hydroxylated and dehydroxylated samples. To monitor surface areas, argon was found to be a more reliable adsorbent than nitrogen. The latter displayed a specificity depending on hydroxyl concentration. The highly dehydrated amorphous silicas gave the same surface areas if 16.2 A~was used for the co-area of nitrogen and 16.6 ~2 for argon. Water adsorption was used to assess the surface coneentration of hydroxyls and isopropanol adsorption to determine their steric distribution. The dehydrated silica prepared by heating a wet, precipitated variety hydrated partiMly on cooling in air. Contrary to earlier reports, boiling in water did not yield a fully hydroxylaged surface. In the course of developing and studying silica-based ice nucleating agents, gas adsorption techniques were used to characterize the substrate surfaces as a means of investigating the surface chemistry of ice nucleation. Adsorption studies were undertaken to determine quantitatively what portion of a given surface was water-receptive, since the degree of surface hydrophilieity was thought to be a factor in the heterogeneous nucleation of ice from the v a p o r phase. Indeed, efficient ice-forming silica nueleants were generally found to possess predominantly hydrophobie surfaces. A major problem in dealing with silica surfaces is t h a t of arriving at an estimate of the concentration of surface hydroxyls, nor~ (expressed as the number of OH groups 02 per 100 A ). Methods of determining no~ include isotopic exchange, reaction with OH-specific compounds, and measurement of weight loss on heating. Agreement among 1 Present address: TechnieM Center, Union Carbide Corporation, South Charleston, West Virginia. 2 Present address: School of Chemistry, University of Bristol, Bristol, England. 649
the different methods, however, is generally poor (1). I t is sometimes difficult to tell under what conditions physically adsorbed molecular water is completely eliminated. I t is equally difficult, in m a n y cases, to be certain t h a t all loss in weight or~ heating silicas to high temperature is due exclusively to the condensation of surface hydroxyls. Surface hydroxyls, or silanol groups, act as primary adsorption sites for polar molecules (2). The amount of water adsorption per unit area should thus give a direct indication of the concentration of polar sites. In addition, the spatial distribution of silanols is of interest, since a given number of sites m a y be uniformMly distributed or m a y be clustered in patches. The difference between an isolated and patehwise arrangem e n t of polar surface sites should become apparent by comparing water-adsorption results with those obtained for a larger polar molecule such as isopropanol, the latter would tend to block adsorption on nearby sites. The purpose of this paper is to explore these hypotheses and to report results of initial studies of several finely divided silicas showing the applicability of adsorption Journal of Colloid and Interface Science, Vol. 27, No. 4, August 1968
650
BASSETT, BOUCHER, AND ZETTLEMOYER
methods to investigations of heterogeneous surfaces. EXPERIMENTAL PROCEDURE Two types of silica were studied: a precipitated variety, ttiSil 233 (Pittsburgh Plate Glass Company), and a pyrogenic variety, Cab0Sil (Cabot Corporation). Due to the nature of the manufacturing process, CabOSil is the purer silica and has a smaller particle size than. HiSil. The absence of hysteresis in the desorption branches of argon and nitrogen isotherms indicates that both silicas are nonporous. The Graphon sample was the same as that described previously (3). Argon and nitrogen isotherms were determined by using a glass volumetrie apparatus with a mercury manometer. Research-grade argon, nitrogen, and helium gases were further purified over hot coppershot and degassed charcoal before using. Water-adsorption isotherms were determined volumetrically by using a glass apparatus incorporating a manometer filled with Apiezon B oil, heat-treated to inhibit water adsorption. Pressure could be determined to 0.01 Tort. Argon isotherms in the low pressure range were also determined with this apparatus. Triply distilled water was used after degassing through five freeze-thaw cycles under high vacuum. Isopropanol isotherms were determined, as previously described (3), using a greaseless system featuring Teflon needle valve stopcocks and a Bourdon-type glass pressure gauge with a sensitivity of 0.01 Torr.
Argon and Nitrogen Adsorption Selection of a gas for use in specific surface area determinations presented unexpected problems. Agreement of surface areas could not be obtained when the usual adsorbate cross-sectional areas of 13.8 and 16.2 ~,2, calculated from liquid densities at -196°C, were used for argon and nitrogen, respectively. Nitrogen surface areas were commonly 15-20 % higher than corresponding argon areas. In every case for
which argon and nitrogen isotherms were compared using the same silica substrate, the amount of nitrogen adsorbed at a given relative pressure exceeded that for argon. In only one case were the relative positions of the isotherms reversed--that for adsorption on the graphitized carbon black, Graphon. Consistent surface areas might have been obtained for the silica surfaces simply by adjusting one of the cross-sectional areas to give agreement were it not for the fact that the difference appeared to vary with the degree of surface hydration. Since the overall aim of the study was to estimate the number of polar sites per unit area, it was important that surface area determinations be independent of surface polarity. There is a growing body of evidence (4-7) that nitrogen adsorption is affected by specific interactions between the quadrupole of nitrogen and polar surfaces such as hydroxylated silicas. Aristov and Kiselev (6, 7) compared argon and nitrogen adsorption on a series of silicas having progressively dehydrated surfaces and found that nitrogen adsorption fell sharply with increasing dehydration of the surface, whereas argon adsorption was virtually independent of the degree of surface hydration. These authors used 13.7 A: as the cross-sectional area of argon because of the agreement with nitrogen (o- = 16.2 ~2) on graphitized carbon. Pierce and Ewing (8, 9) have shown, however, that nitrogen exhibits an anomalously high co-area of approximately 20 ~2 on homogeneous carbon surfaces as a result of localization in preferred positions. It is probable, therefore, that the value of 13.7 A: for argon (corresponding to liquid packing) is too low. A nonpolar, nonporous, and amorphous substrate should be ideally suited for comparison studies of surface area measurements. Such a substrate can be obtained by dehydrating the proper silica. In the present study, HiSil was heated in a muffle furnace for 4 hours at 800°C, a sufficiently
Journal of Colloid and Interface Science, Vol. 27, No. 4, August 1968
ADSORPTION STUDIES ON HYDRATED AND DEHYDRATED SILICAS
651
S
100 O0
80
i
S0,
g
6O 6O b0
-g
u u >m 4 0
4O
20 u,NITROGEN o,ARGON
2O
o, ARGON n, NITROGEN
0.2
0.4
P/P.
0,6
0.8
FIO. 1. Adsorption of argon and nitrogen at -196°C on HiSil 233 (top pair of curves) and dehydrated HiSil 233 (bottom pair). Solid points indicate desorption. high temperature to remove most surface hydroxyl groups (10). Argon and nitrogen isotherms are shown for this sample and for untreated HiSil in Fig. 1. The isotherms are closer together in the case of the dehydrated HiSil, the surface area of which is lower than that of the untreated sample because of sintering. The general shapes and relative positions of the isotherms shown in Fig. 2 for Graphon indicate that this homogeneous surface is quite different from that of the heterogeneous silicas. B E T results for these isotherms are given in Table I. In the case of the dehydrated silica, argon must possess a cross-sectional area of 16.6 K s to give agreement with nitrogen using ¢N~ = 16.2 ,~2. These values of for argon and nitrogen are identical to those shown by Corrin (11) as required to give good agreement with the Harkin-Jura method of estimating surface areas. The value of Ca~ = 16.6 ~2 is also in good agree-
0.2
0.4
06
0,8
FIG. 2. Adsorption of argon and nitrogen at -196°C on Graphon. Solid points indicate desorption. ment with the recent results of Harris and Sing (12) for nonporous solids, although these authors caution against the indi scrimihate use of argon for surface area measurements.
Specific
surface
areas
in
Table
I
are given for comparison purooses on the basis of <~ = 13.8 A as we]] as zA, = 16.6 ~s. We continue to accept 20 i s or the appropriate co-area for nitrogen to assess surface areas of graphitic powders. For amorphous, dehydrated silicas, it is recommended that argon at 16.6 ~2 or nitrogen at 16.2 A: should be used. Apparently, argon is less specific than nitrogen to the surface concentration of hydroxyls. In part (b) of Table I, the required cross-sectional areas of nitrogen are calculated to give agreement with zA~ = 16.6 A~. Assuming argon to be unaffected b y the degree of surface hydration, the change in the effective packing of nitrogen can thus be .
o
2
Journal of Colloid and lnlerface Science, ¥ol. 27, No. 4, August 1968
652
BASSETT, B O U C t t E R , AND Z E T T L E M O Y E R
shown by comparing the cross-sectional areas on dehydrated and fully hydraeed HiSil. The decrease from 16.2 to 15.7 ~2 indicates that nitrogen is more tightly packed on the hydrated surface, possibly a result of the added quadrupole interaction of nitrogen with a surface covered with polar hydroxyls. It is interesting that the relative variation of v,, in comparing hydrated and strongly dehydrated silica in this study is the same as that found by Aristov and Kise. lev (6, 7). The difference in values of the cross-sectional areas found by Kiselev and those found in the present study arise as a consequence of the manner in which the cross-sectional area of argon is fixed. In view of the nature of homogeneous graphitized carbon surface and the uncertainties involved in assessing the surface area of such solids, it is concluded here that a dehydrated, amorphous, nonporous silica offers advantages for comparison studies. On the basis of these results, argon adsorption, with car = 16.6 K2 is used as the basis of surface area determination in the next section. Water and Isopropanol Adsorption Water Adsorption on HiSil. Before undertaking water adsorption measurements on a solid, it is important that all molecular water be removed from th~ surface. Figure 3 shows the variation in water adsorption with outgassing temperatures for HiSil. The slight increase in adsorption with increase in activation temperature indicates that not all physically adsorbed water was removed by evacuation at 25°C. A gravimetric study between 25° and 220°C was undertaken to determine the loss in weight of HiSil on heating in vaeuo. The curve in Fig. 4 was obtained using a Cahn vacuum mierobalance with a sensitivity of better than 5 X 10-6 gin. As expected, most loss in weight occurred on outgassing at room temperature. There was, however, a significant loss between 25° and 110°C which amounted to 4.614 mg/gm corresponding to 5.74 ec(STP)/gm. This loss is attributed
120
100
80
>~4o A 12 hr. at, 2 5 ° C
20 0 0
t
o.2
o.3
,,
L
o:4
&
,
o.6
PffPo FIG. 3. Effect of outgassing t e m p e r a t u r e on the adsorption of w a t e r v a p o r on ttiSil at 25°C.
to adsorbed water held in positions favorable for stronger bonding. While equilibrium was attained within 12 hours at 150°C and below, the evolution of water at 180° and 220°C was still incomplete after 24 hours, most probably a result of the slow removal of silanol groups above 180°C as proposed by Young (13). In general, B E T plots for water adsorption on silica were linear, and the monolayer volumes determined in this way agreed reasonably well with visual estimates of the isotherm knees (Point B). While the observance of straight line plots does not necessarily prove the applicability of the theory, the B E T method provided a consistent means of estimating v~. Examination of volumetric results given in Table II shows that outgassing temperature had very little effect on argon adsorption. On the other hand, outgassing for 12 hours at IIO°C caused an increase in the water v~ of 4.6 c c ( S T P ) / g m (at 25°C) over that for the 25°C outgassing, which is equal to the weight lost during outgassing over this range. Outgassing treatment for 12 hours at 110°C was adopted as standard procedure prior to adsorption measurements. Considering further the results listed in Table II, the ratio of the specific surface
Journal of Colloid and Interface Science, Vol, 27, :No.4, August 1968
ADSORPTION STUDIES ON HYDRATED AND DEHYDRATED SILICAS
653
225
Initial Weight 220
215 210 205 I
20.,0,..
2
r
I
I
I
50
75
100
125
"I
1'20
I
I
175
2 0 1 3 "225
FIG. 4. Gravimetrie weight-loss study of HiSil TABLE I A D S O R P T I O N OF A R G O N AND ~ I T R O G E N
Substrate
(a) Argon adsorption at -196°C HiSil, untreated HiSil, 4 hours at 800°C Graphon
vm
ON I°OLAR AND ~ O N P O L A R S U B S T R A T E S
(co(ST-P)~ g)
(~Ar(.~ ~)
(¢rNe = 16.2)
~(m~/g) (O'Ar = 16.6)
27.6 15.1
102 55.9
17.1 16.6
123 67.5
20.9
77.5
15.0
93.3
(c%'2 = 16.2
~rN~ = 20.0
(eAr = 16.6)
vrn (cc(STP)/g)
(b) Nitrogen adsorption at -196°C HiSil, untreated tIiSil, 4 hours at 800°C Graphon
F.(me/g) O'Ar = 13.8)
29.1 15.5 19.4
areas of water and argon gives an indication of the fraction of the surface which is water receptive. This procedure is not entirely satisfactory since this ratio is dependent on the size of the polar adsorbate molecule, water. A better measure of the degree of hydration would be the average site concentration. Since the total area is known from argon adsorption, and the number of water molecules in the monolayer is known from v,~, the area per site (or effective area per molecule) can be calculated. In the case of untreated HiSil, the number of molecules adsorbed is 12.5 per 100 ~2 yielding an area per molecule of 8.0 ~J. Since the cross-sectional area of water calculated from the density at 25°C is 10.5 ,~2, the molecules must be compressed
127 67.6 84.6
104
15.7 16.2 17.9
laterally due to a dense hydroxyl population. The apparent decrease in molecular diameter of water in this ease is about 13 %. Assmning one water molecule per hydroxyl, no~ = 12.5. From structural considerations, estimates of the number of silicon atoms in a silica surface range from 4.6 (10, 14) to 7.9 (15) per 100 ~k~. If a substantial fraction of the surface silicons hold two hydroxyls, quite probable in the ease of the precipitated HiSil, the emTesponding estimates of nou would range from 9.2 to 15.8. The value of i2.5 cMeulated from adsorption lies within this range. One objection to measuring the number of available adsorption sites by the isotherm technique is that initially adsorbed water molecules might provide additional sites Journal of Colloid and Interface Science, Vol. 27, iYo. 4, Augusf~ 1968
654
BASSETT, BOUCHER,
AND ZETTLEMOYEI~
TABLE
II
ADSORPTION Or ARGON AND WATER ON HiSil Argon (at -t96°C)
Substrate treatment None None None None 2 h o u r s a t 650 2 h o u r s a t 800 4 h o u r s at 800 Repeat
Activation temp.(OC) 25 25 110 110 110 110 110 110
Water Water isotherm 2 (H~O) vra (ec ~(¢ = 15.6) temp. (°C) (cc(STP)/Vm'2(¢ = 10.5) Z(Ar) (STP)/g) (mV~) g) (roVe) 27.0
120
27.6
123
25.0 19.5 15.1
110 87.0 67.5
0 25 0 25 25 25 25 ( l s t ) 25 (2nd)
for further adsorption. This argument is correct in certain eases; for instance, the rehydration of strained siloxane bridges yields two hydroxyls. But for physical adsorption, there should be a difference in adsorption energy at primary sites, such as bare hydroxyls, and secondary sizes such as already adsorbed water molecules. The isotherm knee, from which v~ is obtained, should thus indicate the number of molecules adsorbed at primary sites. The validity of this interpretation is partially shown in that the small amount of water, measured gravimetrieally, which desorbed from HiSil between 25 ° and 110°C. Thereby, additional primary adsorption sites were uncovered reflected in an increase in v~ obtained from volumetric isotherms. Were the distinetion between primary and secondary sites nonexistent, then the shape of the isotherm would be largely independent of the amount of water preadsorbed which is clearly not the ease. When the HiSil surface is dehydrated in air at 650°C, the site concentration drops to 6.06, less than half the original value. Part of the v~ in this ease is undoubtedly due to rehydration of strained siloxanes as indicated by a shift in the desorption curve to the left of the adsorption isotherm. A more severe ease of desorption hysteresis caused by rehydration of the surface occurs in the ease of water adsorption on HiSil heated in air at 800°C. In contrast to water adsorption on untreated HiSil, in which Journal of Colloid and InterfaCe Science, ¥ol. 27, No. 4, August 1968
53.0 52.6 57.8 57.2 25.2 8.81 5.01 8.58
149 148 163 161 71.0 24.9 14.1 24.2
1.24 1.24 1.33 1.31 0.63 0.29 0.21 0,36
Sites 100 ,~2 11.9 11.8 12.6 12.5 6.06 2.72 2.00 3.41
A: Site 8.42 8.48 7.91 8.00 16.5 36.7 50.1 29.3
equilibrium was rapidly attained at each relative pressure, adsorption on HiSil dehydrated at 800 ° was characterized by long equilibrium times. Apparent equilibrium was not complete within an hour. An indication of the extent of rehydration is given by the large hysteresis in the desorption curve. After the first desorption was completed, the sample was activated again for 12 hours at 110°C and the isotherm repeated. The second run showed a substantial increase in amount adsorbed and only slight changes in pressure during overnight pauses. These results indicate that strongly dehydrated HiSil reacts with water vapor more easily than expected from previous studies (10, 13, 14) and the density of residual hydroxyl or rehydration sites, or both, is much higher than anticipated for a strongly dehydrated silica surface. Isopropand Adsorption on HiSil. As mentioned earlier, a limited number of surface sites can assume either of two basically different arrangements: (i) an isolated configuration in which the sites are distributed in a random array so that the distance of separation is essentially unifonl~; or (ii) a crowded configuration in which the available sites are clustered in separate patches on the surface. It is of interest, therefore, to know not only the average concentration of sites, but also their spatial distribution. One way of investigating the problem of site distribution is through adsorption
ADSORPTION STUDIES ON HYDRATED AND DEHYDRATED SILICAS studies using molecules of different size. Assuming that adsorption is limited to specific sites, and that the adsorption mechanism is independent of adsorbate size, adsorption of large molecules might be expected to block further adsorption on near neighbor sites arranged in a patehwise configuration even though these sites are avilable for the adsorption of smaller molecules. In the ease of an isolated site distribution, if the distance of separation of the sites is greater than the diameter of the large molecule, then few sites will be excluded by steric hindrance and the number of molecules in a nominal monolayer will be the same as that for smaller molecules. In other words, close packing of sites leads to reduced adsorption for large molecules eompared with the adsorption of smaller adsorbates, whereas for isolated sites, adsorption is independent of adsorbate size. Since alcohols adsorb on silica in a manner similar to water (2), i.e., through hydrogen bonding to surface hydroxyls, it was thought. that a comparison of water and alcohol adsorption might provide a means of exploring site distributions on various treated and untreated silicas in the present study. Isopropyl alcohol was chosen as the adsorbate for several reasons. Its vapor pressure is such that isotherms could be easily determined in the temperature range 0°-25°C. It is roughly spherical with the hydroxyl flanked by two bulky methyl groups. And its cross-sectional area is roughly three times that of water. Determination of effective cross-sectional areas of molecules used i n sterie hindrance studies is of some importance since conclusions regarding site distribution are directly related to these quantities. Water has been used as adsorbate in many surface studies. Although molecular models indicate that water should have a cross-sectional area of 0 2 6-8 A , the general opinion is that a value 0 2 of 10.5 A , derived from the density at 25°C, is a reasonable value in agreement with other adsorption results.
655
The task of fixing the cross-sectional area of isopropanol is more difficult. Few adsorption studies have been made using this molecule as adsorbate. Furthermore, orientation of the molecule at the surface significantly affects ~, although to a lesser extent than for straight-chain alcohols. The cross-sectional area calculated from o 2 liquid density at 25°C is 27.7 A . The 40 X2 arrived at from isopropanol adsorption on Graphon, reported earlier (3), seems too large compared with these estimates, although similarly high values of the co-area have been found on two of the dehydrated samples in Table II. Results of isopropanol adsorption on HiSil are given in Table III. The effective cross-sectional area on untreated HiSil ranges from 33.1 to 33.9 ~2. For isopropanol adsorption on four precipitated aluminas, Vasserberg et al. (17) obtained an average of 31.4 ~2. In view of the uncertain nature of adsorption on graphitized carbons and the controversy surrounding the true area of those surfaces, the estimate of 40 A~ from adsorption on Graphon is viewed with some doubt. Therefore, the cross-sectional area of isopropanol was taken to be 33 A~ in the present studies. As with water adsorption on untreated HiSil, isotherm temperatures had very little effect on isopropanol adsorption, and only a slight increase in adsorption was observed on increasing the activation temperature from 25 ° to 110°C (both 12-hour outgassing). In general, isopropanol isotherms exhibited sharper knees in the monolayer region than did water isotherms on the same surfaces. The sharp B points allowed more precise visual estimation of monolayer volumes. In some cases, the BET plots were linear only up to P s / P o = 0.15, above which positive deviation was observed. In eases where linear BET plots could not be obtained over an appreciable pressure range, B points were employed. Isotherms were reversible up to relative pressures of at least 0.5. The tendency to ehemisorb on highly Journal of Colloid and Interface Science,
VoI.
27,
N o . 4, A u g u s ~ 1968
656
BASSETT, BOUCHEt~, AND ZETTLEMOYER
dehydrated silica surfaces was also less than that shown for water. Adsorption times were short in most cases, on the order of 15 minutes, although at least 1 hour was usually allowed for equilibrium to be attained at each pressure. The effect of adsorbate size is best seen by comparing adsorption results for water in Table II and corresponding results for isopropanol in Table III. Comparing monolayer volumes for the same substrate treatment, water adsorption drops sharply when HiSil is heated to 650°C. In contrast, isopropanol adsorption drops only slightly. The reason for this difference is that for a given available surface area, there are still sufficient polar sites (given by the water data) for isopropanol to adsorb in a more
or less close-packed array due to its greater size. Only in the case of the 800°C treatment does this area per site exceed the assumed cross-sectional area of isopropanol. In this case, the v~'s for water and isopropanol should be identical with identical estimates of the area per site. It is seen that the isopropano] v~ is lower than the water v~ leading to the conclusion that some of the sites must be near neighbors such that isopropanol adsorption on these sites is somewhat hindered. This example is a borderline ease, however, since the area per site (from water adsorption) is only slightly greater than the area of isopropanol itself. Adsorption Studies on CabOSil. Methods developed in the last two sections were applied to an investigation of CabOSil,
TABLE III ADSORPTION Ot~ ISOPROPANOL ON HiSil
Activation Isotherm vm (cv(STP) Substrate treatment Temp. (°C) temp. (°C) /g) None None None 2 h r a t 650 2 h r at 800
25 110 110 110 110
25 0 25 25 25
13.5 13.8 13.8 12.8 8.4
~(~ = 33) (m ~/g)
Sites 100 ~
~-~ Sit-~
122 122 122 113 74.5
2.95 3.02 3.02 3.08 2.60
33.9 33.1 33.1 32.5 38.5
:~(i-PrOH) :~(Ar)
vm(i-PrOH)
vm(H~O)
0.98 1.00 1.00 1.00 0.85
0.24 0.24 0.26 0.51 0.948
T A B L E IV ADSORPTION OF
ARGON, WATER
AND ISOPROPANOL
ON CabOSil
Argon ( at -- 196°C) Substrate treatment Activation temp (°C)
None None None Rehydrated Rehydrated
25 110 110 110 110
vr~ (co (ST~')/g)
25 110 110 110
(m~/g)
40.9 41.9
182 187
42,6
190
Substrate treatment Activation (Argon) temp. (°C) ~(mS/g) None None None Rehydrated
Water Water 2~(~ = 16.6) tisotherm ~ _ +oo~Vm (6c(STP)/ Z(~ = 10.5)
182 187 190
.... v.~ ~+
25 0 25 0 25
g)
(m~/g)
~(HlO) 21(At)
15.3 15.8 15.2 26.6 23.4
43.1 44.5 42.9 75.0 66.0
0.237 0.238 0. 229 0.395 0.347
i-PrOH isothermi-PrOHv +cc+STP w temp.(OC) m ~ g/ ++ 2~(~ = 33) (uVg) 25 0 25 25
16.9 17.5 16.6 21.0
Journal of Colloid and Interface Science, Vol. 27, No. 4, August 1968
150 155 147 186
:~(i-PrOH) 2~(Ar) 0.824 0.829 0.786 0.979
~m(i-PrOH) vm(H~O) 1.10 1.11 1,09 0.90
Sites 100 fi3
A~ Site
2.26 2.27 2.19 3.76 3,31
44.2 44.0 45.7 26.6 30.2
Sites IOOA~
Site
2.49 2.51 2.39 2,98
40.2 39.8 41.9 33.6
A2
ADSORPTION STUDIES ON HYDRATED AND DEHYDRATED SILICAS the results of which are presented in Table IV. T h a t the surface properties of CabOSil are quite different, from those of HiSil is immediately evident on comparing specific surface areas determined by argon and water adsorption. While the apparent water area of untreated HiSil was found to be greater than the argon area (Table II), the water area of CabOSil is only about 25 % of the argon area (eighth column of Table IV). Several other differences are also apparent. Activation at 110°C instead of 25°C had a negligible effect on argon, water or isopropanol adsorption on untreated CabOSil at 25°C. Physically adsorbed water is apparently held less strongly on CabOSil than on the more highly hydrated HiSil. CabOSil was rehydrated by the method of deBoer (14) to see if a site density approaching that exhibited by HiSil could be obtained. This procedure consisted of heating the silica in distilled water for at least 6 hours at 90°-95°C, then drying at 110°C. The average polar site concentration on untreated CabOSil (2.19/100 A~) corresponds to that of HiSil heated to 800°C (2.00/100 A~). Even when rehydrated, the site concentration remains below 4, a surprising result in view of the fact that the deBoer method is widely used for rehydrating previously dehydrated silica surfaces to a "fully h y d r a t e d " condition. It could be argued that each adsorption "site" represents two silanol groups so that the 2.19 e 2 sites per 100 A means that no~ = 4.4, very close to deBoer's (14) and Hockey and Pethieas's (10) estimate of 4.6 for an annealed silica surface. This argument is untenable, however, when results given in Table IV are examined. The average area per site from water adsorption is about 45 o 2 A , more than enough to accommodate one isopropanol molecule per site if the sites are isolated. Two
facts indicate that the
hydroxyls on CabOSil are indeed isolated. The v~'s for isopropanol and water at 25°C are about equal indicating little st.erie hindrance. In addition, the specific surface area
657
Z computed from the isopropanol v~ is less than that of the argon area. Thus, the hydroxyls are separated to such an extent that adsorption according to a bridge mechanism in which each water molecule is adsorbed on two sites is improbable. The conclusion that silanol groups on CabOSil are isolated was reached earlier by McDonald (18) from infrared measurements. The agreement provides, however, some independent verification of the steric hindrance method of investigating surface site distribution. For an isolated site distribution, the ratio of v~ (i-PrOH)/v~(HsO) will be near unity, while for a patchwise site distribution this ratio will be less than unity. In both cases, the ratio of E(iP r O H ) / E ( H ~ O ) must be less than unity to satisfy the condition that the area per site is greater than the cross-sectional area of isopropanol. CONCLUSIONS Adsorption studies on untreated and thermally modified silica surfaces have shown that variations in polar site concentration can be followed by water adsorption, and site distributions can be studied by a sterie hindrance method using water and isopropanol as adsorbates. Outgassing at room temperature is sufficient to remove physically adsorbed water from pyrogenie silica (CabOSil), but evacuation at 110°C is required for the more highly hydrated surface of precipitated silica (HiSil). I t is clear that a highly dehydrated silica surface ehemisorbs water from the vapor in the process of rehydrating siloxane groups t e produce silanols. Very low values of no~ were not obtained in the present study by heating silica in air. No doubt rehydration occurred on cooling. It appears that a completely dehydrated, and therefore hydrophobie, silica surface can be maintained only under vacuum, tt.ehydration of a par tially hydrophilic pyrogenie silica, CabOSil, by immersion in water near boiling is insufficient to produce a surface with the de-
658
BASSETT,
BOUCHER,
gree of h y d r a t i o n possessed b y the original surface of precipitated silica. T h e use of the t e r m "fully h y d r a t e d , " therefore, should be used only after careful consideration of the past history of the sample. Evidence has been presented which indicates that nitrogen adsorption is affected to some degree b y the degree of h y d r a t i o n of a silica surface. T h e effect is p r o b a b l y due to specific interations between the nitrogen quadrupole m o m e n t and surface hydroxyls on silica and should be considered if the variation in polarity of a surface is of interest, in which case a less specific gas such as argon should be used for surface area determinations. T h e authors prefer a value nf 16.6 ~2 for the co-area of argon. ACKNOWLEDGMENT This research was supported by the National Science Foundation under grants G.P. 1590 and G.A. 560X. REFERENCES 1. BOE~, H. P., Advan. Catalysis 16,179 (1966). 2. KISEL~V, A. V., "The Structure and Properties of Porous Materials," (D. H. Everett and F. S. Stone, eds.), p. 195. Butterworths, London (1958). 3. BASSETT, ]). R., BOUCHER, E. A., AND ZET-
AND
ZETTLEMOYEI~
~'LE~OYER, A. C., J. Phys. Chem. 71, 2787 (1967). 4. FROHNSDORFF, G. J. C., AND KINGTON, G. L., Trans. Faraday Soc. 55, 1173 (1959). 5. KISELEV, A. V., Disc. Faraday Soc. 40, 205 (1965). 6. ARISTOV,B. O., AND KISELEV, A. V., Zh. Fiz. Khim. 37, 2520 (1963). 7. ARISTOV,B. G., AND KISnLEV, i . V., Zh. Fiz. Khim. 38, 1984 (1964). 8. PIERCe, C., ANDEWINe, B., J. Am. Chem. Soc. 84, 4070 (1962). 9. PIERCE, C., AND EWING, B., J. Phys. Chem. 68, 2562 (1964). 10. HOCKEY, J. A., AND PETHICA, B. A., Trans. Faraday Soc. 57, 2247 (1961). 11. CORRIN, M. L., Y. Am. Chem. Soc. 73, 4061 (1951). 12. HARRIS, M. R., AND SING, K. S. W., Chem. Ind. 757 (1967). 13. YOUNG,G. J., J. Colloid Sci. 67 (1968). 14. DEBoER, J. H., HI,]RMANS, M. E. A., AND VLE~S~ENS, J. M., Proc. Kon. Ned. Akad. Wet. B60, 45, 54 (1957) ; B61, 2 (1958). 15. ILER, R. K., "The Colloid Ch emistry of Silica and Silicates," p. 242. Cornell Univ. Press, Ithaca, New York. (1955). 16. CHESSIC~:, J. J., ZETTLEMOYER, A. C., AND YTJ, Y.-F., J. Phys. Chem. 64, 530 (1960). 17. VASS~.RBERG, V. E., BALANDIN, A. A., AND MAKSI~OVA, M. P., Zh. Fiz. Khim. 35, 858 (1961). 18. McDONALD, ]~. S., J-. Am. Chem. Soe. 79, 850 (1957).
JouTnal of Colioidand Interface Science, Vo].27, No. 4, August 1968