2070
M. L. Hair and W. Hertl
methyl radicals, determined from esr spectra, indicating that not all methyl radicals decay by hydrogen atom abstraction. These results should be carefully reexamined. In analogy with the results from other matrices,5-8 it is reasonable to assume that hydrogen atom abstraction is possible in 3MP a t 77°K because of the operation of quantum mechanical tunneling. In order to test this hypothesis, it would be necessary to measure apparent activation energies and isotope effects a t temperatures around 77°K. Glassy 3MP, however, is a very unfavorable matrix for such experiments. Viscosity m e a ~ u r e m e n t s~4l ~ have shown that 77°K lies within the glass transition range for 3MP. The viscosity changes by more than four orders of magnitude between 77 and 87"K.13,14 A reduction in viscosity results in an increased probability for other decay reactions made possible by diffusion of the methyl radicals. At the same time, raising the temperature above 77°K increases the reaction rate so that it becomes experimentally very difficult to establish the reaction path with certainty. Because of these difficulties, apparent activation energies and isotope effects measured in this temperature range have little value. Future measurements a t temperatures significantly below 77°K may provide reliable information strictly related to the abstraction reaction.
Acknowledgment. The author wishes to express his appreciation to Professor D. Schulte-Frohlinde for encouragement and supbort during the execution of this work. A helpful exchange of letters with Professor John E. Willard is also acknowledged. References and Notes (1) Present address, Department of Chemistry, University of Wisconsin,
Madison, Wis. 53706.
(2) (a) J,; E. Willard in "Fundamental Processes in Radiation Chem(3) (4)
(5) (6) (7) (8)
(9)
(IO) (11) (12) (13) (14)
istry, P. Ausloos; Ed., Interscience, New York, N. Y., 1968, p 599; (b) M . Shirom and J. E. Willard, J. Phys. Chem., 72, 1702 (1968). W. G .French and J. E. Willard, J. Phys. Chem., 72,4604 (1968). C. R. Roy and J. E. Willard, J. Phys. Chem., 76, 1405 (1972). E. D. Sprague and F. Williams, J. Amer. Chem. Soc., 93, 787 (1971). J . -T. Wang and F. Williams, J. Amer. Chem. Soc., 94, 2930 (1972). R. J . LeRoy, E. D. Sprague, and F. Williams, J. Phys. Chem., 76, 546 ( 1 972). A. Campion and F. Williams, J. Amer. Chem. Soc., 94, 7633 (19721 i;E;Wllard, private communication. J. L. Dyeand V. A. Nicely, J. Chem. Educ., 48,443 (1971). D. J. Henderson and J. E. Willard, J. Amer. Chem. SOC., 91, 3014 (1969). D. W. Skelly, R. G. Hayes, and W. H. Hamili, J. Chem. Phys., 43, 2795 (1965). A. C. Ling and J. E. Willard,J. Phys. Chem., 72, 1918 (1968). A. C. Ling and J. E. Willard,J. Phys. Chem., 72,3349 (1968).
Chlorination of Silica Surfaces M. L. Hair and W . Hertl" Research and Deveiopment Laboratories. Corning Glass Works, Corning. New Vork 74830 (Received March 12 1973) Publication costs assisted by Corning Glass Works
A combination of infrared spectroscopic kinetic data and chemical analyses was used to study the reactions of a number of chlorinated compounds with the surface of silica. These compounds included: Pc13, BC13, C C 4 , Cl2, TiC14, SnCl4, GeC14, and COC12. It was expected that the metal chlorides would react analogously to the manner in which Sic14 reacts, uiz., Si,-OH Sic14 = Si,-O-SiCl3 HC1, Although a reaction of this type does take place, there is also a concurrent direct chlorination of the surface hydroxyl groups. Some of the reagents react in a complex manner and show the features of autocatalysis.
+
Introduction The reactions of functional silanes with a silica surface have been widely investigated in attempts to elucidate the molecular nature of the silica surface.l-6 Reaction occurs mainly with the surface hydroxyl groups. The boron halides have been widely used in attempts to determine both free and total hydroxyl content.7-9 More recently, reactions of Tic14 and GeCl4 have been reported.lOJ1 In this work we have attempted to determine the kinetics and stoichiometry of some of these reactions. The kinetics are complicated, but the combination of infrared and quantitative analytical data shows that earlier mechanisms proposed for some of these reactions are incorrect: The Journal of Physical Chemistry, Vol. 77, No. 17, 1973
+
considerable chlorination of the surface occurs rather than simple reaction. Experimental Section The experimental technique and apparatus for obtaining the reaction curves have been described previously.1 Essentially, the silica sample (Cab-0-531, 160 m2/g) was pressed into a thin disk and mounted in a controlled atmosphere furnace in the beam of a Perkin-Elmer 421 spectrophotometer. Reactive gases were admitted and the reactions were monitored by measuring the rate of disappearance of the sharp, freely vibrating hydroxyl band (3747 cm-1). In all cases the Si02 was heated to 800"
2071
Chlorination of Silica Surfaces prior to chemical reaction so that only the freely vibrating hydroxyl group is present on the surface. The samples for chemical analysis were allowed to react in a closed end tube, which was sealed off under vacuum at the end of the reaction. For analysis this closed ampoule was broken under liquid; this avoided any possibility of hydrolysis; of the chlorine on the silica surface due to atmospheric water vapor. The samples were then submitted for analysis.
Results and Discussion ( I ) Germanium Tetrachloride. Reactions of GeC14 with silica were carried out at various temperatures in order to obtain the reaction curves. A kinetic analysis of these curves was then made to determine the order of the reaction. The curves all gave good linear plots to greater than 75% reaction when a 1.4- to 1.6-order kinetic equation was used. Some tylpical plots are given in Figure 1. All the following results are based on a 1.6-order rate equation. Due to the variations in reaction rate from silica sample to silica sample, a relative rate method was used to determine the pressure and temperature dependences. The first part of the reaction was carried out under standardized conditions of temperature and pressure and the latter part of the reaction under the desired conditions. A kinetic plot was then made and the slope of the latter part of the plot compared to the slope of the initial part which was carried out under the standardized conditions. The ratio of these slopes gives the value of the relative rate constant. The pressure dependence was measured a t 150 and 350°, using pressures from 5 to 30 Torr. At 350" the reaction rate is independent of the ambient pressure. This was also observed with the chlorosilane reactions in this temperature region.1 At E O " , however, a pressure dependence is noted. In Figure 2 the values of the relative rate constants measured a t 150" are plotted against the pressure of GeC14 used. It is seen that, in general, the rate increases with increasing pressure, and it results in a curve which is more or less concave downward and possibly sigmoid. This probably reflects the shape of the adsorption isotherm of GeC14 on silica. At 350", the same ambient pressure corresponds to a much lower value of p / p o , and it is believed that in this region the isotherm is flat, as with the chlorosilanes.1 The relative rate constants were also determined a t various temperatures, and the results are given on the Arrhenius plot in Figure 3. The curve is concave upward. This is probably due to concurrent reactions taking place, so that the rate constant measured is that due to the sum of two reactions, rather than that of a single reaction with the surface. Evidence for this, on the basis of the chemical analysis, will be discussed below. Table I gives the results of the chemical analyses on the treated silica, in millimoles per gram of silica. With these values and the known surface area of the silica, the values have been calculated for the ratio of chlorine to metal on the surface and the number of metal atoms/100 A2. The silica used here has a surface area of 160 m2/g and, as a result of the thermal pretreatment a t 800°, contains 1.7 OH/100 Az. Comparing the values for GeC14, the most astonishing feature is that the ratio of C1 to Ge on the surface is greater than 3.0 (ie., about 3.6). This means that little chlorine has left the system and that the C1 produced via the bonding reaction of Ge to the surface has
:IT= 400', P.10 ton
IT
t (80.2)
Figure 1. Kinetic plots, 1.6-order, for reaction of GeC14 with sur-
face silanol groups.
0
I /
/
/'
3
si
1''
"Ot
/
/
/
I
I
la
20
I 30
P (torr1
Figure 2. Pressure dependence of t h e reaction of GeC14 with surface silanol groups at 150'. Each rate constant ( k ) is measured with respect to the rate constant determined at the datum pressure (k,,f).
Figure 3. Arrhenius plot of the reaction of 10 Torr of GeC14 with surface silanol groups. Each rate constant ( k ) is measured with
respect to the rate constant determined at the datum temperature (kFer). reacted further with the surface. Also, the concentration of Ge on the surface (0.7-0.8 Ge/100 .A2) corresponds only to about 40% of the OH groups having reacted with Ge (on a 1:l basis), whereas all the OH groups were removed by the reaction. Thus, by difference, about 60% of the OH groups have reacted with the product C1 (or with the The Journal of Physical Chemistry, Vol. 77, No. 17, 1973
2072
M. L. Hair and W. Hertl
TABLE I: Metal and Chlorine Analyses of Reacted Silica Calculatlon of number of OH chlorinated directly
Ge Ti
Snf
CI, mmol/g
M, mmol/g
CI/M
0.75 f 0.13 0.405 f 0.005 0.28 f 0.02
0.21 f 0.02 1.37 f 0.04 0.087 f 0.001
3.6 3.4 3.2
CI, M, atoms4 atoms/ IOOA I O O A 2.82 5.15
1.05
0.79 1.52 0.33
Assume M-C12.57n Cl(hllb
2.03 3.91 0.85
OH(M)= x'sCld 1.13 2.17 0.47
0.79 1.24 0.20
Assume M-CI3 x'sOHe
0.57 -0.47
f
C I ( M ) ~ OH(M)= x'sCld 2.37 4.56 0.99
Assume M - C I I . ~ ~
0.79 1.52 0.33
0.45 0.59 0.06
x'sOHe
0.91 0.18 f
Assume M-Cl2
l _ _ _ _ _ l l
P
0.395
E?
*
0.65
f 0.03
CC14
0.422
0.04
-
0.105 f 0.005 0.215 f 0.03
3.8 3.0
-
1.48 2.44 1.59
0.39 0.81
0.61 1.27
0.56 1.16
0.87 1.17
1.14 0.54
0.78 1.62
0.39 0.81
0.70 0.82
1.31 0.89
a Based on 40% OH MC13 or M'C12 and 60% O t i MCI2 or M'CI Number of chlorine atoms associated with each metal atom = cI[M). Number of OH consumed by each metal atom = OH,,MJ. Total chlorine - b. e Total QH(1.7/100 A2) - c (In b, c, d, and e units are atoms/100 Az) 7 Thls reaction did not go to completion.
GeC14 to chlorinate the OH group and allow the product, a Ge-containing compound, to leave the system). When the silica had completely reacted, water vapor was added to the system a t reaction temperature so as to hydrolyze the metal-chlorine bond. In all cases, between 61 and 64% of the free OH groups reappeared. In no case did any bands appear which could be ascribed to the germanium hydroxyl group. This is in agreement with the recent spectroscopic data of Low and Shimizull and apparently contradicts those of Fink, et al.12 It should be noted, however, that Fink, et al., after reaction with GeC14 and hydrolysis, dehydrated their samples a t 600-700", a much higher temperaturc than that used here. Low and Shimizu also find considerable quantities of Si-OH produced on hydrolysis of their GeC14 product and assign the appearance of these groups to the hydrolysis of SSi-Cl. It is instructive to consider the mass balance as indicated by the quantitative analytical data (Table I). The kinetic experiments show that the reaction order is 1.6, as with SiC14, and this is indicative that 1.6 OH groups are removed on average as each GeC14 reacts. We know that the surface contains 1.7 OH groups/100 A2, 40% of which are single and 60% of which are bifunctional. Thus, if we can assume that the germanium species on the surface contains, on the average, 2.57 chlorine atoms, then the excess OH (assuming 10:7 reaction with Ge) is approximately equal to the number of excess C1 atoms (those that cannot be accounted for by the Gee12 57 species). It should be pointed out that there is a f 1 0 % scatter in all the analytical data and thus this good agreement may be a coincidence. Moreover, it does not preclude some small interaction with siloxane bridges. (2) Tin Tetrachloride. The reaction of SnC14 with the silica was carried out a t various temperatures up to 400". The reaction only proceeded halfway to complete removal of OH groups, after which very little further reaction took place. With only about 50% of the reaction having taken place it was not possible to determine accurately the order of the reaction. Chemical analyses (Table I), however, showed that the amount of tin on the surface corresponds to about 0.33 Sn/100 Az.As with the GeC14 reaction, the ratio of chlorine to metal is greater than 3. Since these chlorides can, in all probability, only react either monoor di-functionally, this means that some direct chlorination of the OH groups is again taking place. The Journal of Physicai Chemistry, Voi. 77, No. 17, 1973
On hydrolysis with water vapor some of the silanol groups reappear, but no bands ascribable to Sn-OH were observed. (3) Titanium Tetrachloride. The reaction of Tic14 with silica takes place too rapidly, even a t room temperature, to be able to obtain a reaction curve. This was also true a t higher temperatures. Chemical analyses of the reacted silica again show that the ratio of C1 to T i is greater than 3, as with the GeC14 and SnC14 reactions. The amount of T i on the surface, however, is much greater than with germanium (1.52 Ti/100 A2). Comparing the surface concentration of Ti, C1, and OH, it seems reasonable to conclude that all the Tic14 reacts monofunctionally with about 90% of the OH groups, and that the remainder of the OH groups are chlorinated directly. The chemical analyses also indicate that there is much more C1 on the surface than can be accounted for on the basis of this mechanism. This means that not only is there direct chlorination taking place, but also that some of the siloxane bonds are being broken and reacting with TiC14. Whether there is direct chlorination of the siloxane bonds or a concerted reaction with Tic14 occurs (as proposed by Kunawic, et a l . I 0 ) cannot be identified by this work. On hydrolysis, the silanol band reappeared in part, but no bands attributable to Ti-OH were observed. (4) Boron Trichloride. The reaction of BC13 with silica was very rapid a t all the temperatures studied. In order to obtain the reaction curves it was necessary for the silica to react a t 30" using only 0.1 to 1.0 Torr of BC13. Some typical reaction curves are given in Figure 4. These reaction curves are sigmoid in shape, i e . , the initial part of the reaction is very slow; it then increases in rate until the bulk of the hydroxyl groups have reacted and then slows down as the reaction approaches completion. A sigmoidal curve is typical of an autocatalytic-type reaction. Since the gas phase in this system is pumped out between readings and fresh BC13 is then added, this means that the autocatalysis must be due to changes on the surface, rather than only to gas-phase constituents which are produced by the reaction. It is possible to fit an autocatalytic rate equation to the data. Such an equation has the form where (a - x ) is the fraction of OH groups remaining a t
2073
Chlorination of Silica Surfaces
4 $ 0.2 0.1
0
-
IIO -0
Do-x)(xI l o q
t (set)
Reaction curves for reactioon of various pressures of BCla with surface silanol groups, at 31 .
Figure 4.
any time, ( x ) is the fraction of OH groups which have reacted at that, time, and "slope" is also measured a t that time. The exponent n is the number of OH groups which are removed when one molecule reacts. A plot of this type is given in Figure 5, using a value of n = 2.0 with the data taken from Figure 4. A function of this type is very sensitive to the accuracy of the reaction curve, so that too much significance must not be placed on the fact that a linear plot resulted when using a value of n = 2. The chemical analyses for this system show that (cf. Table I) the ratio of C1 to B is about 3. Thus, in this reaction also, there seems to be direct chlorination of the OH groups, and about half of the originally available OH groups are occupied by boron. The stoichiometry can be accounted for if it is assumed that all the BC13 reacts monofunctionally and the remainder of the OH groups are chlorinated directly. Out of 1.7 OH/100 A2, 0.81 OH forms Si-0-BC12 and the remaining 0.89 OH is directly chlorinated, giving a value of 2.51 C1/100 A2. This is in good agreement with the analytical value of about 2.44 C1/100
A=. On hydrolysis, the SiOH band is re-formed with approximately one-half of its original intensity. In this case bands due to B-OH groups also appear, but initially they are weak and several BC13/H20 reactions are needed to achieve SiOH and BOH bands of comparable intensity.13 The autocata1,ytic reaction is in itself not surprising. In a previous study we reported the kinetics for the reactions of several silanifzing agents (chlorosilanes and hexamethyldisilazane) with the SiOH and BOH groups on a surface formed by such repeated BC13/H20 treatments.13 It was clearly shown that the effect of the boron was to increase the reactivity (and change the kinetic order) of the SiOH groups on silica/boria as compared to those present on the original silica. Such an effect would automatically ensure an autocatalytic effect for the BC13-SiOH reaction. ( 5 ) Phosphorous Trichloride. The reaction of PC13 with silica proceeds a t a conveniently rapid rate a t temperatures of 200" and above. Some typical reaction curves are given in Figure 6. Comparison of these curves with those given for the lBC13 reaction shows that they have the same general features, i.e., initially the reaction rate is quite slow; it then increases and finally falls off as the number of available hlydroxyl groups becomes small. Attempts to fit the autocatalytic rate equation to the data did not yield very good plots, although the curves do show qualitatively the same features. Hydrolysis of the treated samples results in the reappearance of some of the SiOH groups and also of two small bands which are ascribed to P-OH groups. The
zo
of differential rate ( = slope of reaction curve1 against [(a - X ) ( X ) ] ~ , O for reaction of 0.1 Torr of BC13 at 31 with surface siianol groups. Data taken from Figure 4. Figure 5. Plot
loor
10000
20000
30000
40000
t (sec)
Reaction curves for reaction of 50 Torr of PC13 with surface siianol groups, measured at various temperatures.
Figure 6.
properties of the treated surface are thus similar to the surfaces treated with BC13. The chemical analyses for the samples treated with PC13 (Table I) show rather a large amount of scatter. It is clear, however, that the amount of P on the surface is rather small and that the ratio of C1 to P is greater than 3. The number of P atoms on the surface accounts for only about 20-2570 of the number of originally available OH groups. This means that with this system the direct chlorination reaction takes place to a greater extent than does the bonding reaction between the Si-OH groups and the P.
Other Chlorides ( 1 ) Carbon Tetrachloride. The reaction of Cc14 with the surface silanol groups was carried out a t 400". This reaction has been briefly reported in the literaturel4Jb as producing a chlorinated surface with COCl2 and HC1 as gaseous products. A chemical analysis of the reacted silica (cf. Table I) gave a value of 1.59 C1/100 Az. This is in good agreement with the value determined previously (1.7 OH/100 A2) by means of the methanol reaction with silica. Two typical reaction curves are given in Figure 7 . These show the same general features as do the metal trichloride reaction curves; i.e., they are sigmoid in shape. Although some of the reaction curves gave a good plot using the autocatalytic rate equation, not all of them did, so that, clearly, the surface reaction is quite complex. Several reactions were carried out in a closed system; i . e . , the gas was not changed periodically. About halfway through some of the reactions, the system was pumped out and The Journal of Physical Chemistry, Vol. 77, No. 17, 1973
2074
M. L.
lOOr
1
lsec)
Reaction curves for reaction of 24 Torr of CCI4 with surface silanol groups at 400". Figure 7.
* O r
I 0
20000
I
I
40000
60000
t (sec)
Figure 8. Reaction curve for reaction of 24 Torr of CI:, with surface silanol groups at 400'.
fresh cc14 was then added. Interestingly, after the addition of the fresh cc14 the reaction rate was very slow for a short period of time, after which the reaction proceeded faster. These experiments show that one of the gas-phase products must also have a n effect on the rate of the reaction. This reaction is thus autocatalytic with respect to both the amount of surface reaction which has taken place and with respect to the amount of gaseous product present. At the end of several reactions water vapor was admitted to the cell in order to hydrolyze the chlorine. In all cases more than 95%of the silanol groups reappeared. The autocatalytic nature of the gas-phase product shows that one of products is a more reactive chlorinating agent than is cc14. An experiment was carried out with COC12, one of the reported14 reaction products of the CC14/Si-OH reaction. Although COClz does chlorinate the surface, the reaction rate is very slow. The observed rate was slower than that observed using Clz (see below). It is not possible a t this time to say which gaseous product is responsible for the increased chlorination rate. (2) Chlorine. Only a few reactions were carried out with chlorine, due to the exceedingly slow reaction rate. One of the reaction curves is given in Figure 8. After 70,000 sec a t 400" only about 60% of the surface was chlorinated. Since the reaction was only carried to about 60% completion, it is not possible to say precisely what order the reaction follows, except that it falls between 1.0 and 2.0 order. Summary Discussion The data presented in this paper are a combination of kinetic data and chemical analyses. Although some detail is still lacking, these results lend new interpretation to previous data in the literature and suggest reinvestigation in certain cases. The Journal of Physical Chemistry, Vol. 77, No. 17, 1973
Hair and W. Hertl
The reactions of Cab-0-Si1 that has been heat-treated to 800", the nature of the hydroxyl groups, and their subsequent activity, have been well examined. The results may be summarized. After heating to 800" in uacuo, infrared spectra show that the Cab-0-Si1 surface contains a single sharp absorption at 3747 cm-1 which is attributed to a freely vibrating hydroxyl group on the silica surface. There is some discussion in the literature as to the exact nature of this hydroxyl group, but adsorption,5 kinetic,l and analyticaP3 data all tend to indicate that this band is composed of two types of grouping: (i) a group which is truly a free hydroxyl group and reacts monofunctionally with chlorosilanes such as silicon tetrachloride and (ii) hydroxyl groups which are sufficiently close together to react bifunctionally with the same chlorosilanes although they do not hydrogen bond. These groups are present to the extent of approximately 40% as free hydroxyl groups and 60% as hydroxyl pairs. It is well established that these hydroxyl groups may be replaced by chlorine atoms to give a surface Si-C1 bond which is readily hydrolyzed back to SiOH.14J6 In studies of the interactions of the chlorosilanes with the hydroxyl groups both kinetic and analytic data suggest that about 10% of the hydroxyl groups are removed by an initial fast reaction which may well be due to direct chlorine substitution into the surface SiOH group. The present results show that carbon tetrachloride reacts quantitatively with the surface hydroxyl groups, presumably to give a surface Sic1 species, which is almost quantitatively hydrolyzed back to the original spectrum. Of the other tetrachlorides, GeC14 appears to react with less than half of the free hydroxyl groups to give either a GeC13 or a GeC12.57 species which is not immediately hydrolyzed by water. The other OH groups apparently react to give chlorine substitution, the Sic1 bond being hydrolyzed back to SiOH. Tin tetrachloride reacts to a lesser extent with the surface hydroxyl groups. Only about half of the surface grouping can be removed and the reactions seem equally divided between formation of an SnC13 species and direct chlorination. Titanium tetrachloride reacts almost completely with the SiOH groups, probably giving either a Tic13 or a TiC12.5, species. Some of the SiOH groups may be directly chlorinated and there is probably reaction with siloxane bridges. Phosphorus trichloride reacts with only about 25% of the hydroxyl groups, the major reaction being direct chlorination. The most interesting reaction is probably that with the boron trichloride. This reaction has been studied by several other workers in an endeavor to determine relative proportions of single and adjacent hydroxyl groups. Thus, Boehm, et al.,? used this reaction on silicas that had been heated to lower temperatures, analyzed for boron and chlorine, assumed reactions of type 1 or type 2, and suggested that this could be used to determine the number of H-bonded hydroxyls on the surface. Si-OH Si-OH
Si-OH
-
Si-0-BCl2
(1)
Si, --+ ,0-BCl Si
Increasing the temperatures of silica gel is known to diminish the number of adjacent hydroxyl groups, reducing
2075
Chlorination of Sillica Surfaces potential type 2 reaction and eventually yielding a Cl/B ratio which approaches 2/1 (assuming the above mechanism is correct). Boehm and coworkers7 obtained a value of 1.8 for this ratio for a silica which had been evacuated at 550". Armistead and Hockey6 have since used this BC13 reaction and subsequent C1 analysis (not boron) to determine total OH on silica surfacles heated to 300 and 500". Bermudez8 has pointed out the discrepancy between the numbers obtained by that method and the methanol reaction method and suggested that the former assumes only a type 1 reaction, whereas the 3747 cm-1 band is in fact due to chemically distinguishable species which react as a combination of type 1 and type 2, but are not H bonded. In neither study was any concern given to the possibility of direct chlorination. It is of some interest to note that Hambleton and Hockey1? were able to show that many of the hydrogen-bonded groups on the silica surface (type 2 reaction) were able to undergo exchange with DzO, but were unable to react a t room temperature with the boron trichloride (cf. ref 17, Figure 1). Our own cibservations are that on silicas containing hydrogen-bonded hydroxyl groups the boron trichloride a t room temperature reacts little with hydrogen-bonded groups, but that at 300" BC13 reacts completely with these groupings. The present data show that the reaction of the boron trichloride with the free surface hydroxyl groups is more complicated than the reaction of the other tetrahalides in that it is autocatalytic-this in itself implies a complex reaction. The analytical data clearly demonstrate that the Cl/B ratio is approximately 3, that the amount of boron on the surface (corresponds to reaction with about 60% of the hydroxyl groups, and thus direct chlorination of the remaining 40% must occur. It is proposed that the addition of water to this system causes hydrolysis primarily of the Sic1 bond and not rupture of the SiOB bonds. It is difficult to correlate these results with spectra .obtained by othei? workers on different samples that have been given different treatments. Reference to Figure 4, reference 17, however, shows the spectrum of an Aerosil silica which had been heated to 450" prior to the BC13/ hydrolysis cycle. The hydrolyzed sample quite clearly shows the return of much of the original Si-OH. The nature of the volatile species which must be formed during the direct chlorination reaction is open to speculation. The chlorination reaction be
