2181
REACTION OF HEXAMETHYLDISILAZANE WITH SILICA The initial concentration of O(3P) atoms (2.1 X M ) was taken equal t o the concentration of ozone produced by photolysis of an oxygenated solution assuming that reaction 12 alone is efficient (eos at 260 nm is 2900 M-I cm-I) . l9 This the ratio IC18/k12 to be determined by computer calculations.
This ratio is equal t o 7.0 & 0.5, which is in good agreement with the results obtained in the gaseous phase, which give kla/k12 for reactions 12 and 13 written in the form of three-body collisions ranging from4 t o 20.18 (19) J. W.Boyle, J. A. Ghormley, and C . J. Hochanadel, Chemistry Division Annual Progress Report, May 20, 1969,ORNL 4437,p 49.
Reaction of Hexamethyldisilazane with Silica by W.Hertl and M. L. Hair*’ Research and Development Laboratories, Corning Glass Works, Corning, New York l48SO (Received November 6, 1060) Publication costs assisted by Corning Glass Works
Infrared spectroscopy has been used to determine the kinetics of the reaction of hexamethyldisilazane (HMDS) with the OH groups on the surface of silica. The reaction with the freely vibrating OH groups follows secondorder kinetics ( i e . , two OH are removed when one HMDS molecule reacts) and has an activation energy of where A is a con18.5 kcal/mol. The reaction rate is given by: rate = A exp(-18,500/RT)[OH]2.0[e]1.’, stant, [OH]is the fraction of OH present at any time, and [e] is the fraction of OH groups covered by physically adsorbed HMDS. Only a small fraction of mutually H-bonded OH groups react with HMDS. Two reaction schemes are considered and the significance of the intermediate physically adsorbed state is discussed. Introduction Hexamethyldisilazane is widely used for deactivating gas chromatographic support materiak2 I n view of its importance for this purpose, a kinetic study w~tsundertaken in order to better understand the mechanism of the reaction and to determine the optimum conditions for the reaction with silica. Previous studiesa-6 with other coupling and deactivating agents such as the chloro- and methoxysilanes have shown that the principal bonding site on silica is the freely vibrating hydroxyl group. These other studies have also helped to better elucidate the nature of the surface hydroxyl groups. The chemical properties of the silica OH groups which are of importance in these surface bonding reations can be summarized as follows. (i) The freely vibrating hydroxyl groups can occur in a geminal [Si-(OH)2] or single [Si-(OH)] configuration on the surface. With the silica used in these studies, about 60% of the OH groups are in a geminal and 40% in a single configuration. (ii) The freely vibrating hydroxyl groups are essentially monoenergetic, Le., the reactivity of all the OH groups is the same. This means that these groups react randomly. No particular preference is shown in the reactivity of the single or geminal groups, and the presence of these groups is shown by the stoichiometry of the reaction rather than by any difference in reactivity.
(iii) Since these OH groups are monoenergetic, physical adsorption results in the same fraction of OH groups being covered for a given pressure and temperature, whether there are many or few OH groups on the surface. (iv) I n general, mutually H-bonded OH groups on the surface are essentially nonreactive toward the various bonding agents. With hexamethyldisilazane (HMDS) the overall reaction taking place with the surface is 2Si,-OH
+ (CH~)SS~-N-S~(CH& = I
H 2Si,-O-Si(CHa)a
+ NH3
(1)
By spectroscopically measuring the disappearance of Si,-OH during the course of the reaction, a reaction curve can be obtained. Since the OH groups react monoenergetically, the kinetic analysis of the reaction is relatively straightforward. This paper describes such an analysis. (1) Xerox Corporation, Research Laboratories, Rochester, N. Y. 14603. (2) J. Bohemen, 9. H. Langer, R. H. Perrett, and J. H. Purnell, J. Chem. Soc., London, 2444 (1960). (3) W.Hertl, J. Phys. Chem., 7 2 , 1248 (1968). (4) W.Hertl, ibid., 7 2 , 3993 (1968). (5) M.L. Hair and W. Hertl, ibid., 73, 2372 (1969). The Journal of Physical Chemistry, Vol. 76, No. 14, 1071
2182 Experimental Section The experimental apparatus and procedures used were similar to those described previously for similar s t ~ d i e s . ~ -Briefly, ~ a silica disk, mounted in a furnace connected to a vacuum rack, was placed in an infrared spectrophotometer. The gaseous reagent was admitted t o the furnace and allowed to react for a given time; the furnace was then evacuated. A spectrum was taken and the procedure was repeated. Spectra were taken a t intervals of a few per cent reaction so that the gas-phase composition was essentially constant during the course of the reaction. Kinetic Analysis of the Reaction Curves Measurement of the intensity of the band due to the freely vibrating hydroxyl group (3745 cm-I) a t various times during the course of the reaction gives the reaction curve. The points from the reaction curves were plotted using the integrated form of various order rate equations. The order of the integrated rate equation m, which gives a linear plot for the greater part of the reaction (>75% reaction) gives the order of the reaction with respect to the number of surface sites (the OH groups) removed when one siloxane molecule reacts with the surface. This interpretation is valid since the OH groups are monoenergetic and react randomly. The generalized integrated rate equation has the form
W. HERTLAND M. L. HAIR
'"p
REACTION)
J
Figure 1. Some typical second-order kinetic plots for reaction of HMDS with silica: A, P = 5 Torr of HMDS, T = 31'; 0, P = 15 Torr of HMDS, T = 31'; 0, P = 10 Torr of HMDS, T = 65'.
It should be noted that in most studies of this type, the rate dependence on the surface coverage is usually given by a complex function of the pressure, which represents the adsorption isotherm. I n this study this is unnecessary, since the surface coverage is measured directly.
Results and Discussion where k is the experimental rate constant, a is the initial concentration of OH groups, here defined as 1.00, x is the fraction of the OH which reacted a t time t , and rn is the order of the reaction. Comparison of the peak intensity of the OH band with the gas present at any time during the reaction with that under vacuum gives the fraction of OH groups not covered by physically adsorbed molecules (1 - 0). The fraction of unreacted OH groups covered by physically adsorbed HMDS (e), at any given temperature and pressure, remains constant during the course of a reaction. At a given temperature a log-log plot of the rates obtained with various HMDS pressures against the fraction of OH groups covered by physically adsorbed HMDS (0) at that pressure results in a linear plot; the slope (n) of this plot gives the dependence of the rate as a function of 0 (ie., 0.). The quantity, n, is interpreted as the number of sites occupied by one physically adsorbed HBIDS molecule and is in agreement with data obtained for the physical adsorption of ammonia on surface silanol groups. The specific rate constant, then, is k / P . By measuring both 0 and the reaction rates at various temperatures, an Arrhenius plot of k / e n can be obtained, which gives the experimental activation energy of the reaction. The Journal of Physical Chemistry, Vol. 76,No. 14, 1971
After following a number of HMDS-Si02 reactions, it was found that linear plots were obtained when a second-order rate equation was used. Figure 1 shows some typical second-order kinetic plots. It should be noted that no consistent initial fast reaction is revealed by these plots, although some do show a small positive intercept. If this intercept is real, it amounts to 8% or less of the total reaction. (In the case of the chlorosilanes, a large initial fast reaction, approximately 15%, was always observed.) To determine the dependence of the reaction rate on surface coverage, a series of runs were carried out using various pressures of HMDS. Measurements of the surface coverage (0) were also made at each pressure. As the absolute surface reactivities varied from silica sample to silica sample, the first 40% of each reaction was carried out under standardized conditions (in this case 5 Torr of HMDS) and the remainder of the reaction used the desired pressure. By comparing the slope of the latter part of the kinetic plot with the initial part, a relative rate constant (k/k,,f) was obtained. The upper part of Figure 2 gives the values of the relative rate constants measured a t various pressures, and the lower part gives the measured surface coverage (0) a t the same pressures. Figure 3 gives log-log plots of the relative rate constants against the surface coverage
2183
REACTION OF HEXAMETHYLDISILAZANE WITH SILICA
TR)
Figure 4. Plots of relative rate constants, k/k,,f, (solid line) and fraction of OH groups (e) covered by physically adsorbed HMDS (dashed line) at various t'emperatures. All pressures are 5 Torr; kref measured a t 65".
-l2LzLLd 1 \ r
0.I0
5
P (torr)
10
15
2 .o
Figure 2. Top, plots of the relative rate constants, k/k,,f, for various pressures of HMDS at 65". kref is the rate constant with 5 Torr of HMDS. Bottom, surface coverage of the hydroxyl groups (e) for various pressures of HMDS. 2.6
2.4
-LJ-Ll2.7
2.8 IOOOlT
Figure 5. Arrhenius plot of specific rate constants, (k/krer)/O". The data are taken from Figure 4.
. 1.2
1.0
Figure 3. Log-log plots of k/k,,r against 0 for three different temperatures ( n = slope). The data for 65" are taken from Figure 2.
for three different temperatures. The average of the three slopes is 1.72, so that the rate of the reaction varies as The rapidity of the reaction makes it difficult
to obtain the precise values of the surface coverages at the various temperatures and pressures, resulting in a rather large amount of scatter in the data. I n Figure 4 both the relative rate constants and the surface coverages are plotted as a function of temperature at a constant pressure. It is seen that the reaction rate goes through a maximum due to the fact that the rate of the bonding reaction increases continuously with increasing temperature, but the surface coverage decreases with increasing temperature. In order to obtain the specific rate constant, the calculated values of the relative rate constant must be divided by the surface coverage dependence (ie., by These are given on the Arrhenius plot in Figure 5. The slope of this plot gives the experimental activation energy of 18.5kcal/mol. The experimental reaction rate at 31" is independent of HMDS pressure (from 4 to 15 Torr). Figure 4 shows that with 5 Torr of HRTDS the surface coverage is The Journal of Physical Chemistpy, Vol. 76, N o . l d t 1971
2184
W. HERTLAND M. L. HAIR
about 97%. Thus, any increase in pressure could only make the surface coverage approach 100%) the difference between these two values being too small in which to detect a difference in rate. HMDS is considerably more reactive than any of the other silanes previously studied. The HRIDS reaction proceeds at a rapid rate at 30", compared with 200" for the methoxysilanes and 350" for the chlorosilanes.
Role of H-Bonded OH Groups With other coupling agents (methoxysilanes, chlorosilanes) it was found that the H-bonded OH groups, if present on the silica surface, reacted only slightly. Experiments with HMDS on a silica surface containing H-bonded OH groups showed that this reagent, also, reacts only slightly with these groups. Even at 200" no further reaction with the H-bonded OH groups is observed. The HJIDS reacts only with those Hbonded OH groups which give rise to the high frequency end of the OH band (above 3650 cm-l) and this slight amount of reaction takes place only initially during the reaction. Snyder and Ward6 observed a similar effect in the reaction of methylchlorosilanes with silica. As with the other systems which have been studied, the reactivity of the silica varied from sample to sample (hence the need to measure relative rate constants), but no correlation was found between the reactivity and the presence or absence of H-bonded OH groups. Such a correlation had been found with the methoxysilanes. Reaction with Adsorbed Water With polyfunctional coupling agents, it is possible to obtain a polymerization reaction when the surface contains adsorbed water. This reaction can be differentiated from simple interaction with the surface hydroxyl groups by spectroscopically measuring the amount of organic material (the intensity of the C-H stretching bands) which is present on the sample after complete reaction with a number of wet and dry silica samples. Results are given in Table I. These results show that only a slightly larger amount of organic material is present on the wet samples than on the dry samples. This slightly larger amount is probably due to the concurrent presence of the H-bonded OH groups, which have been shown to react slightly, rather than to chemical reaction with the water that is present. Spectra taken at the end of the reaction show no evidence of perturbed hydroxyl groups, thus indicating that all the freely vibrating OH groups are removed by chemical reaction and that none of them are tied up via hydrogen bonding with the other groups which are present on the surface (i.e., with the -Si(CH3)$groups). Reaction with Other Amines Since diethylamine, Et2KH, has a somewhat similar structure to HMDS, some experiments were carried out t o see if this compound reacted with silica. Although it did not react with the silica surface, it did The Journal
of
Phvsical Chemistry, Vola76,No. 14, 1971
Table I : Normalized Intensity of C-H Bands a t End of Reaction Reaction conditions
C-H intensity
31 " 5 Torr of HMDS 31" 10 Torr of HMDS 31 " 5 Torr of HMDS looo 5 Torr of HMDS 65" 5 Torr of HMDS 65 O 5 Torr of HMDS 65 " 5 Torr of HMDS 31" 15 Torr of HMDS 31 O liquid reagent
0.456 0.451 0.483 0.473 0.514 0.447 0.454 0.565 0.558
Silica treatment
800" 800" 800"
800' 800"
800 800" Wet Wet O
physically adsorb at 400", to the extent of about e = 0.04. At 400" this corresponds to p/pO 'v Triethylamine did react, however, and a t 400" about 30% of the hydroxyl groups were removed after 7 hr. None of the observed bands occur when silica reacts with KH3 a t high temperatures (800') and thus the reaction taking place must be of the type Et3N
+ %,OH = Si,O-Et + Etz"
(3)
Et3K
+ Si,OH = Si,-Et + EtzNOH
(4)
or
Interpretation of Kinetic Parameters The second-order kinetics observed in these experiments can be accounted for by two reaction schemes. The first scheme is simply that shown in eq 1 where a simple, one-step, three-centered reaction is proposed. This scheme implies that all the hydroxyl groups on the surface are sufficiently close together that they can react in pairs. Alternatively, the reaction might be considered t o consist of the following reactions. HRIDS A
+ OH
kl
B
(intermediate) C
(intermediate)
+ OH J_ products
(5)
kr
C
(6)
B (7)
The steady-state approximation dC/dt = 0 gives C = k1/k2A and -dB/dt = 2klAB, which is first order in B . However, it is experimentally known that, for given conditions, the fraction of B covered by A remains constant, thus A = k'B and -dB/dt = 2klk'B2, thus accounting for the second-order kinetics. (Alternatively in these surface reactions it seems reasonable to assume that the intermediate C is adsorbed on B. I n (6) L. R. Snyder and J. W. Ward, J. Phys. Chem., 70, 3941 (1966).
REACTION O F HEXAMETHYLD~SILAZANE WlTH SILICA
2185
this case its concentration will vary as the concentrat i o n o f B a n d C = k"B. Then
-dB - klk'B2 + kzk"B2 dt
= (klk'
+ kzk")B2
(9)
and the reaction is second order in B.) The rate of the surface reaction can be given by an equation of the form rate = A exp( -E/RT) [OH]"[8]"
(10)
or, a t constant temperature rate = r = k[OHl"[Ol"
(11)
Two possibilities can be envisaged. If m = n, then eq 11 can be written rate = IC{ [OH]. [e]}"
(12)
The product [OH]. [e] is just the total number of OH available a t any time which are actually covered with an adsorbed reactive molecule. This quantity is the actual "active mass" which can enter into reaction a t any given time, and the value of OH which must be considered is not the total number which remain unreacted a t any time, but the number which remain unreacted and which are also covered with a physically adsorbed molecule. This implies that the rate-determining step involves a rearrangement of a "(reacting molecule)-(OH group)" complex. I n this case m must always be equal to n and is determined by the stoichiometry of the reaction. If m = n, this indicates that the rate-determining step is the reaction between the physically adsorbed molecule and the OH group. I n this case, m is controlled by the stoichiometry of the reaction and n is controlled by the stoichiometry of the adsorption process. Experimentally, eq 2 is found to be
r
=
A exp(-18,500/RT)[OH]2~0[~]'~72 (13)
The dependence of the reaction rate on the surface coverage is and one interpretation of this is that 1.72 surface sites, on average, are covered by an adsorbed HMDS molecule (since the molecule must be physically adsorbed before it can react). This value of 1.72, on average, corresponds to about 70% of the
molecules adsorbed on two sites and 30% adsorbed on a single site. This is in agreement with previous data for silica surfaces which indicated that about 60% of the surface OH groups are in a geminal configuration and 40% are ~ i n g l e . ~It appears, then, that the H N D S molecules adsorb randomly on the surface OH groups (which is consistent with their being monoenergetic) ; those adsorbed on a single OH group, of course, occupy one site, and those adsorbed on geminal OH groups occupy both OH groups. The value of f91.72 is proportional to the effective surface concentration of HMDS, [HMDS], and thus eq 4 reduces to
r = k[OH]2.0[HR/IDS] (14) This indicates that the rate-determining step is the reaction between the physically adsorbed molecules and the OH groups. These points have been considered in some detail as in most surface kinetic studies they are either ignored or avoided by adjusting an isotherm equation to fit the data. Previously obtained values for m and n are collected in Table 11. It is seen that for the methoxysilane systems, where it is possible to compare the values of m and n, the evidence indicates that m = n and that the first explanation could be correct. Table 11: Rate Parameters for Reactions of Various Silanes with Silica" Silane
Monomethoxy trimethyl Dimethoxy dimethyl Trimethoxy monomethyl Monochlorotrimethyl Dichlorodimethyl Trichlorome thy1 Tetrachloro HMDS
112
1.6 2.2 3.0 1.0 1.6 1.6 1.6 2.0
n
1.7 2.2 3.0
ca. 1 . 7
E, koal/mol
22 i 3 32 f 1 . 5 30.6 22.0 22.6 21.0 22.0 15.5
The parameters m, n, and E are defined by eq 5 .
Acknowledgment. The authors thank Miss E. R. Herritt for assistance in the experimental work. (7) M. L. Hair and W . Hertl, J . Phys. Chem., 70, 4269 (1969).
The Journal of Physical Chemistry, Vol. 76, No. 14, 1971
