750
Communications to the Editor
On the Mechanism of the Hydrolysis of Triethylethoxysilaneat the Silica-Carbon Tetrachloride Interface. A Reply
tensive bridging and the formation of a network is not expected until the hydroxyl-water molecule ratio is nearly unity, which is consistent with the model proposed in the hydrolysis study.
Pubiicat/on costs assisted by the Office of Naval Research
References and Notes
Sir: We wish to respond to Prigogine's criticisms of our proposed mechanism of the surface hydrolysis of triethylethoxysilanel and also to comment on the model which she proposes as an alternative.2 Her first objection is that the water coverage was only 15% of the surface area and finds it difficult to imagine that this low a coverage can form a continuous H-bonded network. This objection ignores the more pertinent point that this amount of water represents complete coverage of the surface hydroxyls which themselves covered only 15% of the surface. If we make the reasonable assumption that the hydroxyls are randomly distributed over the surface, a scale drawing will show that even a t as low a coverage of 15% the hydroxyls are close enough to be bridged by water molecules in the fashion we proposed. A much higher water coverage was needed in the work of Fripiat than in our study because their silica had been less intensely dried and therefore had a higher surface hydroxyl ~ o n t e n t . ~ The most surprising result of the hydrolysis study was the, go-no go, aspect of the kinetics. Prigogine does not accept our hypothesis that this is the result of the sudden formation of a H-bonded water-hydroxyl network. Instead, she suggests that the triethylethoxysilane (TEES) is so strongly H bonded t o nonhydrated surface hydroxyls that until the latter are all hydrated, there is no adsorption of TEES on the hydrated hydroxyls and, thus, no reaction. We find it difficult to understand why the bonding of the T E E S should be so different to the two types of sites when H bonding is undoubtedly involved in both cases. We would agree that there might be a distribution that favors T E E S -..HO-Si= adsorption but that in equilibrium with the TEES adsorbed to nonhydrated hydroxyls there would be some adsorption on the hydrated sites. Therefore, there should have been some hydrolysis, albeit small, prior to complete water coverage of the hydroxyls. None was observed. One of the difficulties we encountered in justifying our proposed network formation was that it required a particular orientation of the adsorbed water molecule, i.e., a SiOH donon, H z 0 acceptor configuration (structure I) .4
(1) W. D. Bascom and R. B. Timrnons, J. Phys Chem., 76,3192(1972). (2) M . Prigogine, J. Phys. Chem., 78,757 (1974). (3)J. J. Fripiat, A*. Jeili, G . Poncelet, and J. Andri, J. Phys. Chem., 69,
I
I1
Recently, a study of the infrared bands of adsorbed water in the 4500-9000-~m-~region indicated that the configuration that we assumed is in fact the only orientation consistent with the observed spectra.5 Furthermore, if the preferred H-bonding role of the ZSi-OH is that of a donor, then the bridging of water molecules between hydroxyls, which involves the =Si-OH as an acceptor (structure 11), will not occur until nearly all of the hydroxyls are covered by a water molecule. In other words, exThe Journal of Physical Chemistry, Vol. 78, No. 7, 1974
2185 (1965). (4)W. D. Bascorn, J. Phys. Chem., 76,3188 (1972). (5) K. Kiier, J. H. Shen, and A. C. Zettlemoyer, J. Phys. Chem., 77,
1458 (1973),
Code 6 170 Naval Research Laboratory Washington, D. C. 20375
Willard D. Bascom*
Chemistry Department Catholic University of America Washington, D. C. 20017
Richard B. Timmons
Received September 14, 1973
Interaction of Molecular Hydrogen with Magnesium Oxide Defect Surface
Sir: We have studied the interaction of molecular Hz with high defect concentration MgO surfaces, freshly prepared by vacuum decomposition of high-purity Mg(0H)Z pbwders. It was found that molecular Hz at 1-6 bars is capable of reacting with such MgO surfaces giving rise to a pronounced conductivity maximum a t about 130". The methods employed were dc conductivity and dielectric loss factor measurements in the frequency range from 100 Hz t o 10 kHz, using a cell with circular A1 electrodes, 40 mm diameter with a 0.1-mm gap. The results obtained with dc and ac were essentially the same, which indicates that no polarization effects occur, The higher pressure range, up to 200 bars, is presently under investigation. A typical experiment was carried out as follows, and the results are shown in Figure 1. The cell loaded with about 0.1 g of Mg(0H)z and contained in the pressure recipient was first evacuated to Torr and heated a t 3"/min to near 400". Prior to the dehydration of Mg(OH)z which starts around 250" a conductivity maximum is observed, shown by curve 1. This maximum, already reported elsewhere,] occurs under vacuum as well as under Hz or inert gas pressure. I t is not due to a gas-solid reaction, but rather due to a proton tunnelling mechanism operative in Mg(0H)z prior to decomposition.2 After partial dehydration the sample was slowly cooled under vacuum to room temperature. High-purity Hz gas was then introduced to 6 bars pressure, and the sample was heated again at 3"/min to not more than 250". Curve 2 shows an increased conductivity but no maximum. The sample was then again allowed to cool slowly, first under Hz pressure to about l50", then under vacuum to room temperature. After thorough evacuation a t room temperature 6 bars HS pressure were again applied and
759
Communications to the Editor 0-
,
. . H-H . , . 0 - -OH- . . . . . . OH-
A quite similar description has been proposed by Boudart, et ~ l . to , ~explain the Hz/Dz isotope exchange reaction taking place a t temperatures as low as 77 K on VI centers on MgO surfaces. Between 77 K and room temperature, the range studied by Boudart and coworkers, Hz and Dz seem to form adsorption complexes permitting the isotope exchange but not annihilating the VI center. The present conductivity study indicates that above room temperature a true reaction seems to take place between Hz and some 0 - centers which slowly destroys the active sites. The V I center is thought to consist of a cluster of 0 ions, probably three in a triangular array, and closely connected with an oxygen vacancy and an OH- group.5 Der~ found that the VI center is active for ouane, e t u L , have the Hz/Dz exchange only, if traces of HzO are present. We also found that the conductivity maximum appears strongly only, if traces of HzO are present during the heating cycle. Too much HzO, however, veils the effect by the formation of a tightly bound H20 film on the MgO defect ~ u r f a c e . ~ A full discussion and detailed description of the experimental results, including electron spin resonance data, will be published shortly. References and Notes Figure 1. Results of conductivity vs. temperature measurements.
the sample was once more heated to about 250". Curve 3 shows that the conductivity has again markedly increased and now goes through a pronounced maximum a t about
130". Repeating the same cycling does not further increase the conductivity nor does it increase the intensity of the maximum. On the contrary, the maximum decreases and eventually vanishes after eight-ten cycles. The following features of the conductivity maximum were established. First, no maximum was observed, if in between the heating was performed under either Nz or 0 2 , b u t the maximum reappeared nearly as strong as before during the next run under Hz pressure. Second, after the maximum had disappeared, due to repeated cycling, the activity could largely be restored by heating the sample again above 300", causing new dehydration to occur. We think t h a t the conductivity effects reported here can be connected with the presence of 0 - centers on the MgO defect surface. Mass spectroscopic studies have shown that, when high-purity Mg(0H)Z is heated under vacuum, molecular hydrogen is evolved between 300 and 550", followed by an oxygen evolution around 700O.3 As discussed briefly elsewhere in connection with preliminary work done on partially deuterioxylated MgO samples,4 the hydrogen evolution can be explained by a thermal dissociation of pairs of OH- groups neighboring an oxygen vacancy, schematically OH- . . . . . . OH0 - . . . . . , 0 - + Hz Concomitantly 0 - ions are formed on the MgO defect +
surface, which become unstable a t high temperatures, giving off oxygen. The present conductivity measurements seem to indicate that above reaction can be reversed by applying Ha a t higher pressure
(1) W. Gieseke, H. NBgerl, and F. Freund, Naturwissenschaften, 57, 493 (1970). (2) To be submitted for publication. (3) F. Freund, N. Scheik 01 Esiami, and H. Gentsch, to be submitted for publication. (4) See discussion foiiowing the paper cited in ref 6. (5) M. Boudart, A. Delbouille, E. G. Derouane, V. indovina, and A. B. Walters, J. Amer. Chem. SOC., 94, 6622 (1972). (6) E. G. Derouane, V. Indovina, A. B. Walters, and M. Boudart in "Reactivity of Solids," J. s. Anderson, M. W. Roberts and F. s. Stone, Ed., Butterworths, London, 1972, p 703. (7) R. Martens, H. Nagerl, and F. Freund, lnd. Chim. Beig., 38, 519 (1973).
I V . Physikalisches lnstitut der Universitdt Gbttingen 0-3400Gbttingen, Germany Mineralogisch-Petrographisches l n s t i t u t der Universitdt Koln 0-5000 Koln-41, Germany
W. Gieseke H. Nagerl F. Freund*
Received July 6, 1973
Effect of Pressure on the Thermodynamically Reversible Gelation of 12-Hydroxystearic Acid in Carbon Tetrachloride' Pubiication costs assisted by the Research institute of Science and Engiheering, Ritsumeikan University
Sir: In a thermodynamically reversible gel, the cross links are known to be caused by secondary forces, such as hydrogen bonds ( H bonds), hydrophobic, and electrostatic interactions, rather than by covalent bonds. The gel of 12hydroxystearic acid (CH3(CHZ)&H( OH)(CH2)loCOOH) in carbon tetrachloride (CC14) has been investigated by The Journal of Physical Chemistry, Vol. 78, No. 7, 1974
