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Water on silica and silicate surfaces. III. Hexamethyldisilazane-treated silica surfaces
Water on Silica and Silicate Surfaces III. Hexamethyldisilazane-treated Silica Surfaces 1 A. C. Z E T T L E M O Y E R AND H. H. H S I N G ~ Center for Surface and Coatings Research, Lehigh University, Bethlehem, Pennsylvania 18015 Received September 8, 1975; accepted December 18, 1975 Both a fully hydroxylated (HiSil) and a partially hydroxylated silica (Cab-O-Sil) were titrated to increasing coverageby HMDS. The hydrophobicity induced by the chemisorption of the (CH3)aSigroups was then monitored by water vapor adsorption isotherms as a function of ligand coverage. The Cab-O-Sil was made strongly hydrophobic whereas the underlying, unreacted OH's on the HiSil, even at full coverage of one ligand per 40 A*, remained available to the water molecules. Reflectance ir was employed to examine the HMDS and the water interaction with the modified silica surfaces; a new major revelation was the N-H band indicating conclusively that ammonia from the disilazane was also adsorbed. INTRODUCTION
[-growth of the (v + ~) combination band] can be monitored very effectively in the N I R . Application of the reflection N I R technique to silicas modified by the addition of organic ligands to the surface hydroxyls was a natural consequence of this earlier work. Hexamethyldisilazane (HMDS) was chosen for the first investigation because it can be handled in the vapor phase so that the extent of the adsorption and reaction with the surface hydroxyls can be ascertained. The reaction kinetics have already been explored by Hertl and Hair (5), and the reactivity of silicas generally even earlier by Kiselev and co-workers (6). I t was of interest in the present work to examine the resulting surfaces by reflection N I R and to learn whether the NHa produced:
Silicas possess a wide range of surface properties depending upon their mode of preparation and prior history. Flame hydrolyzed types such as Cab-O-Sil M-5 have only about 25% of the full quota of surface hydroxyls which are widely spaced over the surface. Frecipitated types such as HiSil 233 have a fully hydroxylated surface which upon heat treatment gradually lose surface hydroxyls. Bassett, Boucher, and Zettlemoyer (1, 2), Zettlemoyer (3) and Klier, Shen and Zettlemoyer (4) have reported on argon versus nitrogen adsorption on these silicas, on water vapor adsorption, and on the increased ice nucleation ability of partially dehydrated HiSil versus the poor nucleating ability of Cab-O-Sil. Reference (4) described the reflectance N I R technique employed so successfully to monitor water adsorption on residual surface hydroxyls. Both the titrations of the residual OH's (decrease in the 2v stretch band) and the H O H adsorption
H M D S + 2 ~ S i , OH ---, ~2Sis-O-Si (CH~)3 + NH3
a Presented at the 49th National Colloid Symposium, Potsdam, New York, June 16-18, 1975. 2Ashland Chemical Company, Columbus, Ohio.
[-1]
also reacts with surface hydroxyls. Sis is a surface silicon atom. The possibility of the NH3 adsorption was not taken into account by Hertl and Hair (5) or others. Water adsorption on the silicas reacted to various extents
637 Copyright ~ 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976
638
ZETTLEMOYER AND HSING
was also monitored by the reflection N I R technique coupled with direct water vapor adsorption isotherm measurements. With the Cab-O-Sil, if the OH's are about equally spaced, there is just enough room for the -Si(CH3)3 ligand per OH. With the HiSil, on the other hand, unreacted OH's would persist and their availability to water vapor, or lack thereof, would be of interest. Further interest in this work stems from the possibility of adding other ligands besides the hydrophobic -Si(CH3)a groups to learn of their interaction with water vapor. Interaction of water with organic ligands is of great interest in solution chemistry and in cell membrane chemistry. In the case of the silicas, the ligands can be tied to a solid tractable surface so that their interaction with water can be explored one at a time and studied by very revealing techniques. A further report on other ligands will be offered shortly (7). Reactions of silanes with silicas have been reported by numerous workers (8). The in'eraction of water with the products has been little investigated. And the ir work reported has been mostly performed in the transmission mode in which the silicas were pressed into thin wafers under high pressure. Distortion of the surface properties can thus occur. Reflection ir from a bed of the silica particles avoids this problem and allow vapor reactants or water vapor to be added to the sample cell while the absorption bands are being measured. EXPERIMENTAL The two silicas employed were HiSil 233, a wet precipitated material from P P G Industries, and Cab-O-Sil M-5, from silicon tetrachloride flame hydrolyzeJ at ll00°C, produced by the Cabot Corp. B E T surface areas based on an argon co-area 3 at 16.0 A s were 123 and 178 mS/g, respectively. Details of their surface properties were given heretofore (1, 2) and elsewhere. H M D S was reagent grade, labeled Z6079, from Dow Corning. Both the H M D S and the 3 F. J. Micale would choose 16.6 X.., based on his very recent analysis (9). Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976
distilled water employed were freed of air by the freeze-thaw-vacuum technique. The silicas were freed of physically adsorbed water to 10.6 Torr for 12 hr at 170°C for the HiSil and at ll0°C for the Cab-O-Sil as determined necessary heretofore. The spring balance used to monitor the adsorptions has a sensitivity of 10.6 g and a loading capacity of 15 mg; this shallow bed allowed all the silica "to see" the vapor added. The Datametrics Inc. sensor measured pressures directly with a sensitivity of 0.01 Torr. The adsorption temperature of 25°C was controlled to -4-0.05°C; 0°C was obtained by an ice-water bath. The N I R measurements were recorded and plotted as described heretofore (4). However, the Datametrics sensor with a capacity of 100 Torr was employed. Thicknesses of the silica samples in the ir cell were 5 mm for the HiSil and 7.5 mm for the Cab-O-Sil. Whether H M D S or water vapor, 12 hr were allowed for equilibrium to be reached. Amounts ascribed to chemisorption were those remaining after pumping at room temperature. RESULTS AND DISCUSSION Graz,imetric Studies Adsorption isotherms for H M D S on both the HiSil and the Cab-O-Sil surfaces at 25°C in units of -Si(CHs)3 ligand groups/100 A2 are plotted in Fig. 1. From both B points, the Si,-O-Si(CH3)3 cross sectional area is 40A 2, in accord with the estimate from the atomic model. It is interesting that the B point for the initially fully hydroxylated HiSil is reached at a lower H M D S pressure than it is for the Cab-O-Sil, suggesting that the HiSil possesses the more reactive OH's. The fact that the B point for the initially 25% covered Cab-O-Sil is the same as that for the HiSil indicates that the surface OH's on Cab-O-Sil must be almost equidistantly spaced. Water vapor isotherms at 0 and 25°C for HiSil are plotted in Fig. 2 for different amounts of chemisorbed H M D S from 0 to 100%. Controlled amounts of H M D S were introduced
WATER ON SILICA
639
7
t 0
{
~
~
~
l'0
h
1'4
fs,
16
z0
p (Torr)
FiG. 1. The upper two curves are the HMDS isotherms for HiSil 233 and for Cab-O-Si[ M-5 /~ at 25°C. The lower four curves (discussed later) are water vapor adsorption isotherms for bare Cab-O-Sil [-], and HMDS-treated Cab-O-Sil: 0.92 ligands/100 ~ (~ and 1.75 ligands/100 ~2 V, all at 25°C. The latter coverage produces considerable hydrophobicity. and then the amounts retained were ascertained gravimetrically after exhaustive pumping at room temperature. The H M D S percentages are based on the coverage of one [igand per 40 As as being 100~o. The water isotherms are strongly Type I I and retain a sharp knee even after high H M D S coverage. I t is interesting to merge the H M D S and water vapor isotherms for HiSil as presented in Fig. 3. They coincide in the monolayer region b u t diverge at higher relative pressures as might be expected. The chemisorbed ligands restrict the growth of the second and subsequent layers, the more so at higher H M D S coverages. Table I gives these combined monolayer values based on the B E T model. (We shall learn below that the N I R results make this model doubtful, b u t comparison between isotherms are still useful.) The resultant totals of H.oO + ligands are quite close. However, the molecular area for the water molecules is only 6.5 tt s, considerably smaller than that reported from this Laboratory heretofore (2). (The earlier result, it should be noted, was based on the more doubtful volumetric method.) This apparently small value for the co-area of water supports the contention of Sing (9) that
140. Water on HMDS-HiSi[ 0 ° and 25 ° C
J
/j
120
/I
100
/ / / i .o"
.o.
/
7
e
.v"
,
Jl
/
4O
'2 0.0 0.1 02 03 0.4 0.5 0.6 0.7 0.8 0.9 p/po FIG. 2. Water vapor isotherms on HMDS-treated HiSil at four coverages: 0% HMDS O, 29.6% V, 52.2% El, and 100% (D at 25°C; solid points are for 0°C. The lowest curve (D for 100% HMDS was determined earlier by a volumetric technique; the apparent 25% lower adsorption is believed to be due to experimental problem such as adsorption on the sensor. Journal of Colloid and Interface Science,
Vol.55, No. 3, June 1976
640
ZETTLEMOYER AND HSING 35~
1 30.
.25"
20.
15' /¢ .a
HMDS-I- H20 on HiSil
o.o
o:l
o:z
o:3
o:a
o:s
oI~
o:7
o:8
: o:9
l.o
p/pO
FIG. 3. Composite isotherms for water plus HMDS ligands on HiSil. The similar B points and repressed multilayer adsorption, especially at the higher ligand coverage, is expected. Composite monolayer values are also shown to agree in Table I. HiSil contains a minor amount of micropores. While water would enter the micropores, of course the H M D S ligands would not. Isosteric heats for water adsorption at three different H M D S coverages on HiSil are presented in Fig. 4. These curves overlap and only approach the heat of liquefaction at 0 = 2. The interaction of water with the underlying OH's is independent of the H M D S coverages. These values are higher than have been reported at 0.5 to 1.0 0 for partially dehydrated HiSil (1, 2). Micropores may explain the high heats and these may be eliminated by the heat treatments used to produce the partially dehydrated samples. Water adsorption isotherms for H M D S TABLE I HMDS + H~O on HMDS-HiSil HMDS Groups/ 100 .~2
0
HMDS %
H~O's/ 100 . ~
HMDS + H20/ 100 .~2
0
15.8
15.8
14.8 14.1 13.0
15.6 15.4 15.5
0.73
29.6
1.28 2.45
52.2 100
Journal of Colloid and Interface Science, Vol. 55, No. 3, J u n e 1976
treated Cab-O-Sil are compared with that for the bare Cab-O-Sil in Fig. 1. At the highest H M D S coverage (unpumpable at 25°C), there are 1.75 ligands/100/~2 and the remaining OH's yield a water monolayer value of 0.58 by the B E T model. The latter value has been corrected to about 0.7 from the N I R studies reported below. The total of 2.45/100/~ gives about 40/~2 per adsorbate unit as expected. This total is somewhat smaller, however, than the N I R monolayer value of 2.9 for the bare Cab-O-Sil reported below.
Near Infrared Reflectance Spectroscopy H M D S interaction with the silica surfaces. Examples of the spectra obtained for HiSil at several increasing H M D S pressures are presented in Fig. 5. The peak intensities of the several bands are plotted against H M D S pressure in Fig. 6. The C - H stretch frequencies at 5730 and 5880 cm-1 produce curves s'milar to the adsorption isotherms as might be expected. The Si~OH band (2v) is shifted slightly frem 7289 to 7241 cm 1 apparently due to perturbation by the adsorbed ligands; this band diminishes with pressure and then levels off
WATER ON SILICA HEATS:
ISOSTERIC B
13
WATER
O
Q
ov~v~
641
ON
HMDS--H~Sil
v I •
v~ v~
oD ~ o
t~
lZ
7 &HL
,
9-
8 FIC. 4. Isosteric heats of adsorption on HMDS-treated HiSil: 0% O, 29.6% V, 52.2% [~, showing almost no effect of the presence of ligands on the energetics of water adsorption. Band assignments are indicated here and in Table IL above 2 Torr in accord with the H M D S isotherms. The band at 6530 cm -~ is assigned to the N - H strecth frequency. The development of this band parallels the diminution in the Si,OH band. This evidence for the ammonia adsorption is, we believe, the first that has been provided. The shoulder which develops at 6610 cm -~ is ascribed to the build up of the second layer of physically adsorbed HMDS. Thus, the complete picture of chemisorbed and physisorbed H M D S on HiSil is established by this spectral analysis. The HMDS-treated HiSil was further 9.9 HMDS
ON
HMDS ( t o r r )
HISIL
8.8
+
0
7.7
m
1.33
6.6 X
studied by obtaining spectra after evacuating to 10 ~ Tort at room temperature and then at a sequence of elevated temperatures. Evacuation at room temperatures reduces all the bands except for a slight increase in the Si,OH (2v) band. The N - H (NH3) band at 6530 cm -~ decreases only slightly indicating the strong adsorption of NH8 to the surface OH's. Apparently, the main event upon evacuation at room temperature is the loss of physically adsorbed HMDS. Furthermore, little change in band intensities occurs up to 50°C. Large changes then occur up to 140°C which plateau at 170°C. The complete elimination of the
(9
1.72
A
3.54
X
5.45
O-H
5.5
%
4,4
t C.
3.3
c-H
4
1
2.2 1.1 0.0 4SO0 ENERGY {WAVE NOS.)
Fla. 5. Typical NIR reflectance spectra at a variety of pressures of HMDS adsorbed on HiSil at 25°C. Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976
642
ZETTLEMOYER
AZ;D
HSING
0.8,
HMDS
- HiSil 25’C
10
8 15 % x f ‘L
6 IQ a .x 5
4 05
2
0
00 0
2
6
4
8
10
12
14
P norr1
FIG. 6. Intensity of bands (Schuster-Kubelka-Monk function) Fig. 5: 0 Si,OH(2 Y) band at 7289 cm-l; A and V C-H bands at 6530 cm-l and 0 N-H (HMDS) at 6610 cm-l.
N-H (NHB) band at 170°C is of interest because the same temperature is required at 1OV Torr to remove physically adsorbed water from HMDS
ON
versus HMDS pressure at 25’C from at 5730 and 5880 cm-l; 0 N-H(NHJ
HiSil. The shift of some 80 cm-’ (6610 to 6530 cm-l) suggests a strong perturbation in the decrease of the N-H vibration frequency
CABOSIL
HMDS
O-H
i
(tow-)
+
bare
0
0.22
A
2.17
0
4.13
4
5.54
Y
0 (pumped
out)
2.4
FIG. Journal
of
7. Typical
Colloid
and
NIR
reflectance
I?zlrvfacc Science,
spectra
at a variety
Vol. 55, No. 3, June
1976
of HMDS
pressures
on Cab-O-%1
at 25°C.
643
WATER ON SILICA TABLE II NIR Bands HMDS Silicas Freq. cm-I
Surface
Group A s s i g n m e n t s ~ v e r t o n e s
4975 5730 & 5880 6530 6580 6610 7246 7289 7331
HiSil & Cab-O-Sil HiSil & Cab-O-Sil HiSil Cab-O-Sil HiSil Cab-O-Sil HiSil Cab-O-Sil
Adsorbed NH3--bending 4- assymetric stretch C-H stretch N-H stretch--chemi. NH3 N-H stretch--chemi. NH3 N-H stretch--physi. NH3 O-H stretch--hydrogen bonded O-H stretch (2p)--free OH's O-H stretch--free OH's
from HMDS to NH3 Adsorption. Two N - H bands, for symmetric and asymmetric vibrations, were not found in the overtone region as has been reported for the fundamental region using the transmission method. The N I R reflectance technique was also applied to the bare and HMDS-treated Cab-OSil surfaces. The spectra are displayed in Fig. 7. The assignments made for the bands are tabu-
0.3
HMDS
--
Cab-
lated in Table II. The most striking discovery is that two distinct bands due to Si-OH are found at 7246 and 7331 cm -1. I n the transmission mode, only a mixed (3665 and 3749 cm -1) band has been reported [ 8 ( a ) ] with a wider shoulder on the low frequency side attributed to hydrogen bonded OH's or bulk OH groups. The results presented here indicate that hydrogen bonded OH's either do not react with
o-
Sil
25°C
%
0.2~ r r l ol
-1
~\%"\\\\ x k\
0.0
2.2" 2"0"~S ].5"
i-ON (2v)
53~*'~""-_
'~.,~
1
2
5880
3 4 P(Torr)
5
06
~o.4
L
6 evacuati at25°C on
FIG. 8. Band intensities for HMDS adsorption on Cab-O-Silfrom Fig. 7 : O free OH band at 7331 cm-1; /k and V C-H bands at 5730 and 5880 cm-t; [] N-H (NHs) band at 6580 cm-z. Dashed fines show changes caused by evacuation at room temperature to 10-6 Torr. Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976
644
ZETTLEMOYER AND HSING
W, mg/g
7I I
10
20
t
I
SiOH ( 2 u ) Intensity vs H20 Adsorbed
61 5 ¢q
O x4 O
HMDS- HiSII
0- 3 h_
• k
OQ \
•
\ \
Cabos~l
•
\
•
N o
I0
•
L
2O
3u
~0'\
5'0
6'0
73
V, cc/g
FIG. 9. Band intensities (S-K-M function) for water on 100% HMDS-treated HiSil O and on bare Cab-OSil. The tailing off is due to clustering of water molecules around those first adsorbed.
HMDS, as suggested previously (5), or, as seems more likely to us, that bulk OH's are responsible for the low frequency peak. Additionally, the N - H (NH3) is both weaker on the Cab-O-Sil surface and is totally removed by evacuation at room temperature; its appearance at a higher frequency (6580 cm-1) also suggest weaker interaction. Furthermore, the Si,OH band (7331 cm-1) is almost entirely quenched again indicating the rather equal spacing of the OH groups on the Cab-O-Sil. The band intensities versus the pressure are plotted in Fig. 8. In accord with the isotherms, these curves are not as sharp as for HiSil. The dashed lines refer to the changes on evacuation at room temperature. Both the C-H bands and the N - H band diminish on evacuation indicating weaker interaction with the Cab-OSil surface. Adsorption of water. N I R reflectance spectra were also measured in this work on the bare and HMDS-treated silica surfaces. Since similar spectra were reported on the bare surfaces heretofore (4), these spectra are not displayed here. As noted before, water adsorption on the HMDS-treated HiSil is little affected by the HMDS treatment except for a reduction in the multilayer region. The intensity curve (SKM function) for
the 100% HMDS-treated HiSil is given in Fig. 9. The tailing off suggests clustering of the water molecules around molecules adsorbed at lower pressures (4). And the extrapolated value is believed to be a more appropriate measure of the residual surface OH concentration than is given by the B E T model or B point value from the isotherm. Of course, the diminution of the Si,OH(2v) peak indicates that adsorption occurs on the OH's below the silazane ligands; apparently about 50% of the OH's are titrated before clustering becomes important. Even at high water vapor relative pressures, the spectra indicate considerable difference of the adsorbed water from bulk water. Water adsorption is too low on 100% HMDS-treated Cab-O-Sil to produce very useful results from the spectra. The intensity results on bare Cab-O-Sil are plotted in Fig. 9 for comparison. The extrapolated monolayer value is 15.9 mg/g. (2.9 molecules/100 A~). This 18% difference is not too surprising; the clustering effect takes place here as well, REFERENCES 1. BASSETT, D. R., BOUCtIER, E. A., AND ZETTLE~ MOYER, A. C., J. Colloid Interface Sci. 27, 649 (1968). 2. BASSETT, D. R., BOUCHER~ E. A., AND ZETTLEMOYER, A. C., J. Colloid Interface Sci. 34, 436 (1970). 3. ZETXLEMOYER, A. C., J. Colloid Interface Sci. 28, 4 (1968). 4. KLIER, K., SI:IEN,J. H., AND ZETTLEMOYER, A. C., J. Phys. Chem. 77, 1458 (1973). 5. HERTL, W. AND HAIR, M. L., J. Phys. Chem. 75,
Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976
181 (1971). 6. (a) KISELEV, A. V., (1958), in "Structure and Properties of Porous Materials," (Everett, D. H. and Stone, F. S., Eds.), p. 210; (b) DAVYDOV, V. Y., KISELEV, A. V., AND ZHURAVLEV, L. T., Trans. Faraday Soc. 60, 2254 (1964). 7. HSING, H. H. AND ZETTLEMOYER, A. C., "Water on Silica and Silicate Surfaces. IV. Silane Treated Silicas," International Conference on Colloid and Surface Science, Budapest, Sept. 15-20, 1975, IUPAC sponsored. 8. (a) HAIR, M. L. "Infrared Spectroscopy in Surface Chemistry," Dekker, New York, 1967; (b) HAIR, M. L. AND HERTL, W., J. Phys. Chem. 73, 2372 (1969). 9. MICALE, F. J., Paper R6, 49th Nat. Colloid Sym., Potsdam, New York, June 16-18, 1975, to appear.
[-growth of the (v + ~) combination band] can be monitored very effectively in the N I R . Application of the reflection N I R technique to silicas modified by the addition of organic ligands to the surface hydroxyls was a natural consequence of this earlier work. Hexamethyldisilazane (HMDS) was chosen for the first investigation because it can be handled in the vapor phase so that the extent of the adsorption and reaction with the surface hydroxyls can be ascertained. The reaction kinetics have already been explored by Hertl and Hair (5), and the reactivity of silicas generally even earlier by Kiselev and co-workers (6). I t was of interest in the present work to examine the resulting surfaces by reflection N I R and to learn whether the NHa produced:
Silicas possess a wide range of surface properties depending upon their mode of preparation and prior history. Flame hydrolyzed types such as Cab-O-Sil M-5 have only about 25% of the full quota of surface hydroxyls which are widely spaced over the surface. Frecipitated types such as HiSil 233 have a fully hydroxylated surface which upon heat treatment gradually lose surface hydroxyls. Bassett, Boucher, and Zettlemoyer (1, 2), Zettlemoyer (3) and Klier, Shen and Zettlemoyer (4) have reported on argon versus nitrogen adsorption on these silicas, on water vapor adsorption, and on the increased ice nucleation ability of partially dehydrated HiSil versus the poor nucleating ability of Cab-O-Sil. Reference (4) described the reflectance N I R technique employed so successfully to monitor water adsorption on residual surface hydroxyls. Both the titrations of the residual OH's (decrease in the 2v stretch band) and the H O H adsorption
H M D S + 2 ~ S i , OH ---, ~2Sis-O-Si (CH~)3 + NH3
a Presented at the 49th National Colloid Symposium, Potsdam, New York, June 16-18, 1975. 2Ashland Chemical Company, Columbus, Ohio.
[-1]
also reacts with surface hydroxyls. Sis is a surface silicon atom. The possibility of the NH3 adsorption was not taken into account by Hertl and Hair (5) or others. Water adsorption on the silicas reacted to various extents
637 Copyright ~ 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976
638
ZETTLEMOYER AND HSING
was also monitored by the reflection N I R technique coupled with direct water vapor adsorption isotherm measurements. With the Cab-O-Sil, if the OH's are about equally spaced, there is just enough room for the -Si(CH3)3 ligand per OH. With the HiSil, on the other hand, unreacted OH's would persist and their availability to water vapor, or lack thereof, would be of interest. Further interest in this work stems from the possibility of adding other ligands besides the hydrophobic -Si(CH3)a groups to learn of their interaction with water vapor. Interaction of water with organic ligands is of great interest in solution chemistry and in cell membrane chemistry. In the case of the silicas, the ligands can be tied to a solid tractable surface so that their interaction with water can be explored one at a time and studied by very revealing techniques. A further report on other ligands will be offered shortly (7). Reactions of silanes with silicas have been reported by numerous workers (8). The in'eraction of water with the products has been little investigated. And the ir work reported has been mostly performed in the transmission mode in which the silicas were pressed into thin wafers under high pressure. Distortion of the surface properties can thus occur. Reflection ir from a bed of the silica particles avoids this problem and allow vapor reactants or water vapor to be added to the sample cell while the absorption bands are being measured. EXPERIMENTAL The two silicas employed were HiSil 233, a wet precipitated material from P P G Industries, and Cab-O-Sil M-5, from silicon tetrachloride flame hydrolyzeJ at ll00°C, produced by the Cabot Corp. B E T surface areas based on an argon co-area 3 at 16.0 A s were 123 and 178 mS/g, respectively. Details of their surface properties were given heretofore (1, 2) and elsewhere. H M D S was reagent grade, labeled Z6079, from Dow Corning. Both the H M D S and the 3 F. J. Micale would choose 16.6 X.., based on his very recent analysis (9). Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976
distilled water employed were freed of air by the freeze-thaw-vacuum technique. The silicas were freed of physically adsorbed water to 10.6 Torr for 12 hr at 170°C for the HiSil and at ll0°C for the Cab-O-Sil as determined necessary heretofore. The spring balance used to monitor the adsorptions has a sensitivity of 10.6 g and a loading capacity of 15 mg; this shallow bed allowed all the silica "to see" the vapor added. The Datametrics Inc. sensor measured pressures directly with a sensitivity of 0.01 Torr. The adsorption temperature of 25°C was controlled to -4-0.05°C; 0°C was obtained by an ice-water bath. The N I R measurements were recorded and plotted as described heretofore (4). However, the Datametrics sensor with a capacity of 100 Torr was employed. Thicknesses of the silica samples in the ir cell were 5 mm for the HiSil and 7.5 mm for the Cab-O-Sil. Whether H M D S or water vapor, 12 hr were allowed for equilibrium to be reached. Amounts ascribed to chemisorption were those remaining after pumping at room temperature. RESULTS AND DISCUSSION Graz,imetric Studies Adsorption isotherms for H M D S on both the HiSil and the Cab-O-Sil surfaces at 25°C in units of -Si(CHs)3 ligand groups/100 A2 are plotted in Fig. 1. From both B points, the Si,-O-Si(CH3)3 cross sectional area is 40A 2, in accord with the estimate from the atomic model. It is interesting that the B point for the initially fully hydroxylated HiSil is reached at a lower H M D S pressure than it is for the Cab-O-Sil, suggesting that the HiSil possesses the more reactive OH's. The fact that the B point for the initially 25% covered Cab-O-Sil is the same as that for the HiSil indicates that the surface OH's on Cab-O-Sil must be almost equidistantly spaced. Water vapor isotherms at 0 and 25°C for HiSil are plotted in Fig. 2 for different amounts of chemisorbed H M D S from 0 to 100%. Controlled amounts of H M D S were introduced
WATER ON SILICA
639
7
t 0
{
~
~
~
l'0
h
1'4
fs,
16
z0
p (Torr)
FiG. 1. The upper two curves are the HMDS isotherms for HiSil 233 and for Cab-O-Si[ M-5 /~ at 25°C. The lower four curves (discussed later) are water vapor adsorption isotherms for bare Cab-O-Sil [-], and HMDS-treated Cab-O-Sil: 0.92 ligands/100 ~ (~ and 1.75 ligands/100 ~2 V, all at 25°C. The latter coverage produces considerable hydrophobicity. and then the amounts retained were ascertained gravimetrically after exhaustive pumping at room temperature. The H M D S percentages are based on the coverage of one [igand per 40 As as being 100~o. The water isotherms are strongly Type I I and retain a sharp knee even after high H M D S coverage. I t is interesting to merge the H M D S and water vapor isotherms for HiSil as presented in Fig. 3. They coincide in the monolayer region b u t diverge at higher relative pressures as might be expected. The chemisorbed ligands restrict the growth of the second and subsequent layers, the more so at higher H M D S coverages. Table I gives these combined monolayer values based on the B E T model. (We shall learn below that the N I R results make this model doubtful, b u t comparison between isotherms are still useful.) The resultant totals of H.oO + ligands are quite close. However, the molecular area for the water molecules is only 6.5 tt s, considerably smaller than that reported from this Laboratory heretofore (2). (The earlier result, it should be noted, was based on the more doubtful volumetric method.) This apparently small value for the co-area of water supports the contention of Sing (9) that
140. Water on HMDS-HiSi[ 0 ° and 25 ° C
J
/j
120
/I
100
/ / / i .o"
.o.
/
7
e
.v"
,
Jl
/
4O
'2 0.0 0.1 02 03 0.4 0.5 0.6 0.7 0.8 0.9 p/po FIG. 2. Water vapor isotherms on HMDS-treated HiSil at four coverages: 0% HMDS O, 29.6% V, 52.2% El, and 100% (D at 25°C; solid points are for 0°C. The lowest curve (D for 100% HMDS was determined earlier by a volumetric technique; the apparent 25% lower adsorption is believed to be due to experimental problem such as adsorption on the sensor. Journal of Colloid and Interface Science,
Vol.55, No. 3, June 1976
640
ZETTLEMOYER AND HSING 35~
1 30.
.25"
20.
15' /¢ .a
HMDS-I- H20 on HiSil
o.o
o:l
o:z
o:3
o:a
o:s
oI~
o:7
o:8
: o:9
l.o
p/pO
FIG. 3. Composite isotherms for water plus HMDS ligands on HiSil. The similar B points and repressed multilayer adsorption, especially at the higher ligand coverage, is expected. Composite monolayer values are also shown to agree in Table I. HiSil contains a minor amount of micropores. While water would enter the micropores, of course the H M D S ligands would not. Isosteric heats for water adsorption at three different H M D S coverages on HiSil are presented in Fig. 4. These curves overlap and only approach the heat of liquefaction at 0 = 2. The interaction of water with the underlying OH's is independent of the H M D S coverages. These values are higher than have been reported at 0.5 to 1.0 0 for partially dehydrated HiSil (1, 2). Micropores may explain the high heats and these may be eliminated by the heat treatments used to produce the partially dehydrated samples. Water adsorption isotherms for H M D S TABLE I HMDS + H~O on HMDS-HiSil HMDS Groups/ 100 .~2
0
HMDS %
H~O's/ 100 . ~
HMDS + H20/ 100 .~2
0
15.8
15.8
14.8 14.1 13.0
15.6 15.4 15.5
0.73
29.6
1.28 2.45
52.2 100
Journal of Colloid and Interface Science, Vol. 55, No. 3, J u n e 1976
treated Cab-O-Sil are compared with that for the bare Cab-O-Sil in Fig. 1. At the highest H M D S coverage (unpumpable at 25°C), there are 1.75 ligands/100/~2 and the remaining OH's yield a water monolayer value of 0.58 by the B E T model. The latter value has been corrected to about 0.7 from the N I R studies reported below. The total of 2.45/100/~ gives about 40/~2 per adsorbate unit as expected. This total is somewhat smaller, however, than the N I R monolayer value of 2.9 for the bare Cab-O-Sil reported below.
Near Infrared Reflectance Spectroscopy H M D S interaction with the silica surfaces. Examples of the spectra obtained for HiSil at several increasing H M D S pressures are presented in Fig. 5. The peak intensities of the several bands are plotted against H M D S pressure in Fig. 6. The C - H stretch frequencies at 5730 and 5880 cm-1 produce curves s'milar to the adsorption isotherms as might be expected. The Si~OH band (2v) is shifted slightly frem 7289 to 7241 cm 1 apparently due to perturbation by the adsorbed ligands; this band diminishes with pressure and then levels off
WATER ON SILICA HEATS:
ISOSTERIC B
13
WATER
O
Q
ov~v~
641
ON
HMDS--H~Sil
v I •
v~ v~
oD ~ o
t~
lZ
7 &HL
,
9-
8 FIC. 4. Isosteric heats of adsorption on HMDS-treated HiSil: 0% O, 29.6% V, 52.2% [~, showing almost no effect of the presence of ligands on the energetics of water adsorption. Band assignments are indicated here and in Table IL above 2 Torr in accord with the H M D S isotherms. The band at 6530 cm -~ is assigned to the N - H strecth frequency. The development of this band parallels the diminution in the Si,OH band. This evidence for the ammonia adsorption is, we believe, the first that has been provided. The shoulder which develops at 6610 cm -~ is ascribed to the build up of the second layer of physically adsorbed HMDS. Thus, the complete picture of chemisorbed and physisorbed H M D S on HiSil is established by this spectral analysis. The HMDS-treated HiSil was further 9.9 HMDS
ON
HMDS ( t o r r )
HISIL
8.8
+
0
7.7
m
1.33
6.6 X
studied by obtaining spectra after evacuating to 10 ~ Tort at room temperature and then at a sequence of elevated temperatures. Evacuation at room temperatures reduces all the bands except for a slight increase in the Si,OH (2v) band. The N - H (NH3) band at 6530 cm -~ decreases only slightly indicating the strong adsorption of NH8 to the surface OH's. Apparently, the main event upon evacuation at room temperature is the loss of physically adsorbed HMDS. Furthermore, little change in band intensities occurs up to 50°C. Large changes then occur up to 140°C which plateau at 170°C. The complete elimination of the
(9
1.72
A
3.54
X
5.45
O-H
5.5
%
4,4
t C.
3.3
c-H
4
1
2.2 1.1 0.0 4SO0 ENERGY {WAVE NOS.)
Fla. 5. Typical NIR reflectance spectra at a variety of pressures of HMDS adsorbed on HiSil at 25°C. Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976
642
ZETTLEMOYER
AZ;D
HSING
0.8,
HMDS
- HiSil 25’C
10
8 15 % x f ‘L
6 IQ a .x 5
4 05
2
0
00 0
2
6
4
8
10
12
14
P norr1
FIG. 6. Intensity of bands (Schuster-Kubelka-Monk function) Fig. 5: 0 Si,OH(2 Y) band at 7289 cm-l; A and V C-H bands at 6530 cm-l and 0 N-H (HMDS) at 6610 cm-l.
N-H (NHB) band at 170°C is of interest because the same temperature is required at 1OV Torr to remove physically adsorbed water from HMDS
ON
versus HMDS pressure at 25’C from at 5730 and 5880 cm-l; 0 N-H(NHJ
HiSil. The shift of some 80 cm-’ (6610 to 6530 cm-l) suggests a strong perturbation in the decrease of the N-H vibration frequency
CABOSIL
HMDS
O-H
i
(tow-)
+
bare
0
0.22
A
2.17
0
4.13
4
5.54
Y
0 (pumped
out)
2.4
FIG. Journal
of
7. Typical
Colloid
and
NIR
reflectance
I?zlrvfacc Science,
spectra
at a variety
Vol. 55, No. 3, June
1976
of HMDS
pressures
on Cab-O-%1
at 25°C.
643
WATER ON SILICA TABLE II NIR Bands HMDS Silicas Freq. cm-I
Surface
Group A s s i g n m e n t s ~ v e r t o n e s
4975 5730 & 5880 6530 6580 6610 7246 7289 7331
HiSil & Cab-O-Sil HiSil & Cab-O-Sil HiSil Cab-O-Sil HiSil Cab-O-Sil HiSil Cab-O-Sil
Adsorbed NH3--bending 4- assymetric stretch C-H stretch N-H stretch--chemi. NH3 N-H stretch--chemi. NH3 N-H stretch--physi. NH3 O-H stretch--hydrogen bonded O-H stretch (2p)--free OH's O-H stretch--free OH's
from HMDS to NH3 Adsorption. Two N - H bands, for symmetric and asymmetric vibrations, were not found in the overtone region as has been reported for the fundamental region using the transmission method. The N I R reflectance technique was also applied to the bare and HMDS-treated Cab-OSil surfaces. The spectra are displayed in Fig. 7. The assignments made for the bands are tabu-
0.3
HMDS
--
Cab-
lated in Table II. The most striking discovery is that two distinct bands due to Si-OH are found at 7246 and 7331 cm -1. I n the transmission mode, only a mixed (3665 and 3749 cm -1) band has been reported [ 8 ( a ) ] with a wider shoulder on the low frequency side attributed to hydrogen bonded OH's or bulk OH groups. The results presented here indicate that hydrogen bonded OH's either do not react with
o-
Sil
25°C
%
0.2~ r r l ol
-1
~\%"\\\\ x k\
0.0
2.2" 2"0"~S ].5"
i-ON (2v)
53~*'~""-_
'~.,~
1
2
5880
3 4 P(Torr)
5
06
~o.4
L
6 evacuati at25°C on
FIG. 8. Band intensities for HMDS adsorption on Cab-O-Silfrom Fig. 7 : O free OH band at 7331 cm-1; /k and V C-H bands at 5730 and 5880 cm-t; [] N-H (NHs) band at 6580 cm-z. Dashed fines show changes caused by evacuation at room temperature to 10-6 Torr. Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976
644
ZETTLEMOYER AND HSING
W, mg/g
7I I
10
20
t
I
SiOH ( 2 u ) Intensity vs H20 Adsorbed
61 5 ¢q
O x4 O
HMDS- HiSII
0- 3 h_
• k
OQ \
•
\ \
Cabos~l
•
\
•
N o
I0
•
L
2O
3u
~0'\
5'0
6'0
73
V, cc/g
FIG. 9. Band intensities (S-K-M function) for water on 100% HMDS-treated HiSil O and on bare Cab-OSil. The tailing off is due to clustering of water molecules around those first adsorbed.
HMDS, as suggested previously (5), or, as seems more likely to us, that bulk OH's are responsible for the low frequency peak. Additionally, the N - H (NH3) is both weaker on the Cab-O-Sil surface and is totally removed by evacuation at room temperature; its appearance at a higher frequency (6580 cm-1) also suggest weaker interaction. Furthermore, the Si,OH band (7331 cm-1) is almost entirely quenched again indicating the rather equal spacing of the OH groups on the Cab-O-Sil. The band intensities versus the pressure are plotted in Fig. 8. In accord with the isotherms, these curves are not as sharp as for HiSil. The dashed lines refer to the changes on evacuation at room temperature. Both the C-H bands and the N - H band diminish on evacuation indicating weaker interaction with the Cab-OSil surface. Adsorption of water. N I R reflectance spectra were also measured in this work on the bare and HMDS-treated silica surfaces. Since similar spectra were reported on the bare surfaces heretofore (4), these spectra are not displayed here. As noted before, water adsorption on the HMDS-treated HiSil is little affected by the HMDS treatment except for a reduction in the multilayer region. The intensity curve (SKM function) for
the 100% HMDS-treated HiSil is given in Fig. 9. The tailing off suggests clustering of the water molecules around molecules adsorbed at lower pressures (4). And the extrapolated value is believed to be a more appropriate measure of the residual surface OH concentration than is given by the B E T model or B point value from the isotherm. Of course, the diminution of the Si,OH(2v) peak indicates that adsorption occurs on the OH's below the silazane ligands; apparently about 50% of the OH's are titrated before clustering becomes important. Even at high water vapor relative pressures, the spectra indicate considerable difference of the adsorbed water from bulk water. Water adsorption is too low on 100% HMDS-treated Cab-O-Sil to produce very useful results from the spectra. The intensity results on bare Cab-O-Sil are plotted in Fig. 9 for comparison. The extrapolated monolayer value is 15.9 mg/g. (2.9 molecules/100 A~). This 18% difference is not too surprising; the clustering effect takes place here as well, REFERENCES 1. BASSETT, D. R., BOUCtIER, E. A., AND ZETTLE~ MOYER, A. C., J. Colloid Interface Sci. 27, 649 (1968). 2. BASSETT, D. R., BOUCHER~ E. A., AND ZETTLEMOYER, A. C., J. Colloid Interface Sci. 34, 436 (1970). 3. ZETXLEMOYER, A. C., J. Colloid Interface Sci. 28, 4 (1968). 4. KLIER, K., SI:IEN,J. H., AND ZETTLEMOYER, A. C., J. Phys. Chem. 77, 1458 (1973). 5. HERTL, W. AND HAIR, M. L., J. Phys. Chem. 75,
Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976
181 (1971). 6. (a) KISELEV, A. V., (1958), in "Structure and Properties of Porous Materials," (Everett, D. H. and Stone, F. S., Eds.), p. 210; (b) DAVYDOV, V. Y., KISELEV, A. V., AND ZHURAVLEV, L. T., Trans. Faraday Soc. 60, 2254 (1964). 7. HSING, H. H. AND ZETTLEMOYER, A. C., "Water on Silica and Silicate Surfaces. IV. Silane Treated Silicas," International Conference on Colloid and Surface Science, Budapest, Sept. 15-20, 1975, IUPAC sponsored. 8. (a) HAIR, M. L. "Infrared Spectroscopy in Surface Chemistry," Dekker, New York, 1967; (b) HAIR, M. L. AND HERTL, W., J. Phys. Chem. 73, 2372 (1969). 9. MICALE, F. J., Paper R6, 49th Nat. Colloid Sym., Potsdam, New York, June 16-18, 1975, to appear.