3188
W I L ~ ~ D. R DBASCOM
ectroseopy Study of Water Adsorption at the etraehloride Interface
by ~
i l D.~Bascow ~ r ~
Surface Chemistry Branch, Naval Research Laboratory, Washington, D. C. E’zib/,&xtbn costs assisted by the O&e
80.390 (Received April 6,1972)
of Naval Research
Suspensions of amorphous silica (Cabosil) in CCL were studied by infrared spectroscopy in the 4000-2500om--1region. The silica was predrisd at 750” so that the only major spectral band was at 3700 cm-1, attributable to isolated surface hydroxyls interacting with the CCId media. Rehydration of the silica in suspension produced bands at 3400 cm-’ for adsorbed molecular water and at 3660 cm-l for adjacent, hydrogenbonded surface hydroxyls. Rehydration did not alter the intensity of the free hydroxyl band at 3700 cm-1, even though the water is adsorbed by hydrogen bonding to the hydroxyl groups. An estimate of the molar absorptivity of the molecular water band (3400 cm-l) gave a value of 53 M-’ cm-l, close to that of bulk water,
A large body of literature has developed on the infrared spectra of water chemisorbed and physically adsorbed on silica surfaces. Much of the work stems from the technical and fundamental interest in the silica-wal,er interaction. Also, water adsorption on silica is particularly amenable t o infrared studies since silica is $,ransparent in the 0-H stretching vibration region ~ 4 0 0 ~ cm-l) - ~ ~ ~ There 0 is reasonable agreement as to the band assignments for surface silanols ( P Si-OH) and adsorbed molecular water; nonetheless, there are still details and ambiguities about these bands that deserve attention. The literature dealing with this problem has been reviewed by Little, Kiselev,2 and Hair. Most of the infrared spectroscopy of Si02 surfaces has been done using thin porous plates or disks of pressed powder. ‘These samples are fragile and because of their particulate structure there is considerable beam scattering which reduces band intensity and resolution. In the work reported here the silica was suspended rn carbon tetrachloride. These suspensions can be conveiiientXy handled without contamination by atmospheric water and because of the close match between the refractive indices of CC1, and SiOz there is negligible beam scattering. I
was a nonporous aerosil (Cabosil, having a surface area of 202 m2 gm-I after outgassing at 80” for 20 hr to a residual gas evolution 0,” cO.1 pmol hr-l m-2. The SiOz was dried at mm pressure for 24 hr in the drying cell illustrated in Figure 1. The drying temperature was usually ‘950°, although the effect of drying at other temperatur*ls betwcen 25 and 800” was also studied. All vacuum fittings wcre copper gasket flanges and all Ths Journal o,f Ph2jsicnl Chemistry, Vol. Y 6 , No. 22, 1072
valves were stainless steel with Kel-F seats. Back diffusion of oil from the pumps to the cell was suppressed by a molecular sieve-Dry Ice trap. After the Si02 had been dried the cell was allowed to cool and brought to atmospheric pressure by introducing dried He gas. A Teflon stirring bar, held out of the furnace by a magnet during the heating, was allowed t o fall into the silica powder. An appropriate amount of CCl, was added through a long, wide-bore (1.5 mm) hypodermic needle that extended from the septum inlet, e, (Figure 1)t o the powder, a? (Figure 1). The CC1, had been dried by percolation through aetivated Florisil. Stirring for about 2 hr gave a clear suspension containing 2.00 f 0.04% silica, The suspension was transparent due to the close match between the refractive indices of Si02 (1.46) and CC14 (1.463). Because the particle size was about 0.015 p the rate of settling of the SiQ2 was very slow. The hypodermic needle was left in the cell and used for adding or withdrawing material t o or from the suspension. To rehydrate the Si02, wet He gas was bubbled through the suspension using the hypodermic needle as a gas inlet tube. Infrared spectra of the SiO2--CCLsuspensions in the 4000-2500-cm-1 region were determined by withdrawing 2.5-mI aliquots with a gas-tight syringe and injecting them into an optical cell having a 1-cm path length. The cell was constructed of Lnfrasil, a silica with low absorption in the 4000-2500-cm-’ region (Amersil Inc., Hillside, N.J.). The spectrometer was a double-beam Perkin-Elmer 457 Xodel operated at (1) L.H. Little, “Infrared Spectra of Adsorbed Species,” Academic Press, New York, N. Y . ,1966,p 228. (2) A. V. Kiselev and V. I. Lygin, ref 1, pp 213 and 352. (3) M. L. Hair, “Infrared Spectroscopy in Surface Chemistry,” Marcel Dekker, New York, N. Y., 1967, p 79.
h"RARED
STUDY O F
ATER
ADSORPTION
4 WAVENUMBER (CM")
Figure 2. Infrared spectra of SiO2-CC14 suspensions. Si02 was dried for 20 hr and 10-3 mm pressure at the indicated temperatures.
Figure 1. Drying cell: (a) sample area, (b) movable heater, (c) copper gasket flange, (d) vacuum outlet, (e) inlet septum.
a spectral slit width of about 4 cm-l- Compensation for solvent absorption was achieved by placing in the reference bean1 a matched Infrasil cell containing only CCL The amount of molecular water in a SiOrCC14 suspension was determined by titration with the Karl Fisher reagent. The applicability of this method t o water on silica has been demonstrated by Kellum and Smith.4 A 5-mX sample of suspension was injected into approximately 50 ml of a methanol-chloroform solvent (1:3 by volume) and titrated to a minimum, constant conductivity. The major difficulty with this technique is that the Karl Fisher reagent is adsorbed and may react with the Si02 surface. To correct for this error the volume of reagent required for a. suspension of SiOz dried at 450" was subtracted from the volume required for an equivalent sample of rehydrated
SiOz.
Results The infrared spectra obtained for Cabosil dried at temperatures from 25 Lo 750" and suspended in CC14 are given in Figure 2. The silica that had been dricd at 750" (and suspended in CCl,) was rehydrated by passing met He gas through the suspension. The resulting spectrum is given in Figure 3. Note that the major spectral change is the appearance of the broad and at 3400 cm-I# The intensity of this band was varied by varying the time of treatment with wet helium
I
4000
I
3500
3000
WAVENUMBER (CM-')
Figure 3. Spectra of Si02 suspended in C C 4 : (R) predried at 750"; (b) rehydrated.
and its absorbance ( A = log 1/T) was plotted against the moles of adsorbed water, as determined by Karl Fisher titration. The results given in Figure 4 indicate the Beer-Lambert relation ( A = alc) was followed over most of the data. I n these expressions T is the transmittance, a is the molar absorptivity (g nio1-I cm-l), I the cell length (cm), and c the concentration (mol gm-l). The values for T were obtained from the ratio, Trehydrated/Tdried, Le., the transmittance of the silica at 3400 cm-' before and after rehydration. The silica dried at 750" was also treated with DgQ. First, before adding CCl,, the chamber w a s opened to a side tube of liquid D,O until the volume taken up was slightly more than required for a monolayer coverage of the powder (0.1 ml of liquid). This procedure of drying followed by DzO exposure was repeated ten (4) G. E. Kellum and
R.C . Smith, Anal. Chem., 39, 341 (196%).
The Journal of Physical Chemistry, Vol. 76, no^ fB, 1978
3198
ILLARD B). BASCOM
I
3900
1
I
3000
2500
I
2000
WAVENUMBER EM-':
Figure 6. Spectrum of a Si02-CCll suspension after treatment with DaO-saturated He gas. Figure 4. Beers-Lambert plot €or the adsorbed water band at 3400 c11z-l.
100
80
60
-E 40 2
2
t
..e
3 40-
7:
20
-
L----,
040008
;.i
A
3500
3000
2500
2000
WAVENUMBER (CM-')
Figure 5 . Spectrum of Xi02 (in CCId) that had been repeatedly dried (750":)and exposed to DIO vapor. 4000
3500
3000
2 00
WAVENUMBER E M - ' )
times, then the sample was given a final drying at 750" and suspended in (X1, (2% solution). The spectrum of this suspension is given in Figure 5. The silica was then further treated by bubbling D20-saturated He gas through the suspension and the resulting spectrum is shown in Figure 6. A, more detailed examination was made of the spectral changes occurring in the rehydration of the silica. Differential spectra were obtained by placing matched samples of Si02 (dried at 750") in CC1, in both the sample and reference beams and wet He was passed through the suspension in the sample beam. Spectra were taken after various time intervals over a 2-hr period. Figure 7b is typical of the spectra. 111 addition to the principle band at 3370 cm-l (attributed t o H-bonded molecular water) the three other bands included two (3710 :znd 3620 em-l) found t o be due t o monomeric (non H-bonded) water in the CC1, (Figure 7 c ) . Dissolved n ~ i t e rin the CC14 was present in these experiments because of incomplete stirring of the susThe Journal of Physicai! Chemistry, 1701. 76, N o . $3, 1078
Figure 7. Differential spectra (a) before wet He treatment of sample, (b) 30 min after treatment, (c) filtrate of b, (d) manual subtraction of c from b.
pension in the optical cells. Stirring was not a problem when treating the suspension in the larger chamber (Le., Figure 1). The infrared bands observed in all these experiments are listed in Table I along with the corresponding bands observed for Cabosil in oucuo, the molecular assignments that have been given these bands, and the appropriate references.
Discussion The results in Table I indicate that 1Tit.h the exception of the isolated 3SiOH and 3SSiOD bands the correspondence between the SiOz-CC1* and the SiOzvacuum bands was nearly exact>. The frequency of the H-bonded molecular water bond obtained here is given as a range between 3370 and 3400 em-l because
INFRARED STUDYOF WATERADSORPTION
3191
Table I : Infrared Band Frequencies and Assignments --Frequency,
sior car
3'700 3660 3370-3400 2725
2660 25110
cm "I----, 8102-
Group assignment
Ref
Isolated 3SiOH Adjacent H-bonded 3 SiOH H-bonded H20 Isolated 4SiOD Adjacent 4 SiOD H-bonded DzO
9, 10 10
vacmm
3750 3660 3400 2760 -A2690 25350
10 10, 11 11 11
__I(___^_
it appears at 3400 cm-l in the spectra of the suspensions (Figure 3) but at 3370 cm-' in the differential spectrum (Figure 7 ) . The 3400-em-' value may reflect overlap with the 3700-cm-1 band and so the lower value of 3370 cm-l is probably closer t o the true frequency. The shift of the +Si011 (or 3SiOD) bands from 3750 t o 3700 em-' (or 2760 to 2725 cm-l) is undoubtedly due to a relatively weak interaction of these surface groups with CC14. Gas-phase adsorption of CC14 on silica produieed a comparable shift.5 Low and Hasegawd observed the isolated 3 SiOH band a t 3686 em-' for pressed Cabosil plates immersed in CCl,. The fact that the band at 3660 cm-l, which corresponds to adjacent hydroxyl interaction O--frI..
I --si-
,
. .O-H I
'\ 0
7si-
and the band at 3370 cm-l for H-bonded water are noh displaced in CCla compared to vacuum indicates that these interactions are stronger than the hydroxylCC1, interaction Indeed, the shift of the 3750-cm-' band for the latter is only 50 cm-l whereas the shift due to adjacent hydroxyl H-bonding and the water Hbonding are 98 and 380 cm-', respectively. The good correlation between the band frequencies of the SiO2-CCI4 and the SiOz-vacuum systems demonstrates that the infrared spectroscopy of water adsorption on silica can be conveniently studied using Si02-CC14 suspensions. The handling of these suspensions is relatively easy and free of the encumbrances of working with pressed silica disks in gas adsorption systems. Initially there was some concern about water pickup from the ailnosphere during manipulation of the samples. However, the RzO band (3400 cm-I) in the spectrum of the deuterated SiO, suspension (Figure 6) was very weal; but would not have been so had there been any significant water uptake. The spectral changes that occurred when the dried silica was rehydraked allow an estimate of the moiar absorptivity of adsorbed water and also give some insight into how the water is held on the silica.
The molar absorptivity (a)of the 3400-cm-' band of the adsorbed water was computed from the Lambert plot (Figure 4). A value of 53 M-' cm-I was obtained if the following assupptions are made: (a) the water film thickness was 3 A, ie., a monolaycr of water; and (b) the cell thickness was 0.8 X em. This latter value was obtained by taking the thickness of Cabosil as 2% of the cell thickness 6p em), the path length through the water film t o be 6 A, and the SiOz particle size as 150 A. All considered, the value for Q of 53 M-' cm-* is surprisingly close t o the values of 55-95 M-I cm-l reported for It was somewhat surprising to find in the differential spectra that there had been no change in the intensity of the 9SiOH band at 3700 cm-I. There i s general agreement that adsorbed molecular water is held on the surface of silica by H bonding t o the sudace bydroxyls. For example, Kiselev has shown that the heat of adsorption of water on silica i s lowered by dehydroxylation of the silica surface.8 I n fact, he argued that the intensity of the free 0-H is progressively reduced with increased adsorption of molecular water. This would seem t o be the case in Figure 3 where the intensity of the 3700-cm-' band in the spectrum of the rehydrated sample is reduced relative t o the dricd silica by subtraction of the background due t o the broad 3400-cm-' water band. However, this subtraction involves assuming a shape for the h ~ g ~ ~ f r e ~ re-u e ~ ~ y gion of the 3400-cm-' band. Closer examination of the problem reveals that failure to observe a major decrease in intensity at 3700 em-l by differential spectroscopy does not mean the water is not H bonded to the surface hydroxyls. I n fact, it is difficult to visualize a H-bonded configuration which does not leave a,t least one free 8-€3 bond. For example, on a sparsely hydroxylated surface
I1
there would be two free hydroxyls for each silanojwater pair. If the silanols are close enough (
