NOTEB
2570 k:
k,“
+
cyanide water,2 the maximum in a / f 2occurs quite ) ~ zero. In terms close to the composition where ( K ~ ‘ is of the Argand diagram approach outlined above, this means that where the velocity changes rapidly and a / f z remains unaffected, the vector K ~ *is directed along ‘ At the maximum in a / f 2 the , projection on the K ~ axis. the K ~ ” axis has correspondingly reached it maximum value. Here all the nonideal properties of the binary liquid mixture contribute towards the imaginary (outof-phase component) isentropic compressibility.
(total)
I
‘
k,*(ideal)
Figure 1. Relationships between the various isentropic compressibilities.
+’1
Acknowledgments. We thank Professor M. C. R. Symons and Dr. N. J. Hidden for valuable discussion. We acknowledge the award of a maintenance grant to D. W. from S. R. C.
I*
6
’,
?h
0.6 0.7 0.0 0.9 1.0 mole fraction x p
.-N E ( b ) O .w*
0! .
2 0.3 0.4 0.5 0.6 0.7
(9) M. J. Blandamer, D. E. Clarke, N. J. Hidden, and M. C. R. Symons, T m n s . Faraday Soc., 64, 2691 (1968).
0.0 0:9 1!0
The Reaction of Silica Surfaces with Hydrogen Sequestering Agents -1
1-
mole fraction x2
0.4
by J. A. Hockey Chemistry Department, University of Manchester Institute of Science and Technology, Manchester, England (Received December 8, 1968)
0.5 0.6 0.7 0.8 0.9 1.0 mole froction x;!
Figure 2. Variation of excess isentropic compressibility with mixture composition for: (a) ethanol water; (b) n-propyl alcohol water; (c) t-butyl alcohol water; (d) water, all at 298’ K. T h e asterisk methyl cyanide indicates the position of the maximum in the ultrasonic absorption (the PSAC, see text). (“,)E
+ +
+
+
The real (in-phase) component of the isentropic compressibility of an ideal binary mixture, id, can be calculated from the mole fraction of water, xl, and of the cosolvent, x2, by (Ks’)id
=
X1(Ks’)lo
+
Z~(KS’);~~
(3)
where ( K ~ ’ ) ~ Oand ( K ~ ’ ) ~ Oare the isentropic compressibilities of the two pure liquids. The excess isentropic Compressibility is given by (Ks’)‘
=
Ks’
-
(4)
(Ks’)id
The relationship between the various isentropic compressibilities is summarized in Figure 1. Plots of ( K ~ ’ ) ~as, a function of liquid composition, are given in Figure 2 for a number of binary aqueous mixtures. For each system there is a given composition where ( K ~ ’ )is~ zero; in this context these are ideal mixtures, however, on other grounds they are not ideal. Thus they correspond to systems having excess absorptions. A stiking feature, and one which we wish to draw attention to, is that for those mixtures with large excess absorptions, e.g., t-butyl alcohol waterJ9methyl
+
The Journal of Physical Chemistry, Vol. 74,No. I f , 1970
It is possible to obtain information on the concentration and coordination of the surface hydroxyls present on silicas by studying the reactions of these surface groups with hydrogen sequestering a g e n t ~ . l - ~I n a recently published study,* the reactions of alkyl chlorosilanes with the surface hydroxyls of a Cabosil silica have been followed in an elegant manner by a combination of analytical and spectroscopic techniques. The infrared spectrum shown in this paper’ illustrates that with high spectral resolution the absorption band a t about 3750 cm-’, which previous authors6have assigned as corresponding to isolated or single surface hydroxyls, may be resolved into three adjacent sharp peaks at 3751,3747, and 3743 cm-l. Similar results to this have been obtained in the author’s own laboratory. HOWever, it has always been felt that this splitting is artefactual rather than real. The spectra in Figure 1 illustrate this point. Spectrum a corresponds to the absorption spectrum of the residual water vapor in the optical path of a “dry air” flushed PE 125 spectrophotometer on “single (1) M . L. Hair and W. Hertl, J . Phys. Chem., 73,2372 (1969). (2) C. G . Armistead, A. J. Tyler, F. H . Hambleton, 9. A. Mitchell, and J. A. Hockey, ibid., 73, 3947 (1969). (3) J. B. Peri, ibid.,70, 3168 (1966). (4) H. P. Boehm, M. Schneider, and F. Arendt, 2.Anorg. Chem., 66, 800 (1962).
(5) L. H. Little, “Infrared Spectra of Adsorbed Species,” Academic Press, Inc., New York, N.Y., 1966.
2571
NOTES 1
27'50
1
FREOUENCY (cm-')-sptctrum
I
2830
c
Fig. I
3725
3775
Figure 1. Absorption spectra.
beam" operation with a spectral slit width of 0.4 cm-l. It can be seen that the water vapor spectrum exhibits three strong absorption bands a t 3752, 3749, and 3744 cm-1. The intensity of these bands is of course dependent on the efficiency of the system used to dry the flushing air. Spectrum b corresponds to the absorption spectrum, recorded under double-beam conditions with identical resolution, of an Aerosil silica pressed disk after evacuation a t 700" and subsequent cooling in vacuo to room temperature. Three absorption bands similar to those recorded by the authors of ref 1 are clearly apparent. Spectrum c is the absorption spectrum-again recorded under identical spectroscopic conditions to spectrum b-of a similarly treated disk but one in which the surface hydroxyls have been converted by isotopic exchange to their deuterium analogs.2 I n this last spectrum the fine structure is no longer apparent. It has therefore been always concluded that the fine structure which is observed in spectrum b is the result of the combined absorption spectra of the surface hydroxyls and any residual water vapor within the spectrophotometer. The point that the fine structure in spectrum b is due to the water vapor in the instrument is further reinforced by the fact that the intensities of the three bands a t 3751, 3747, and 3744 cm-l in spectrum b may be enhanced by reducing the efficiency of the flushing system used for drying the air within the spectrometer. One other general point: when analyzing the results obtained from studies of the reactions of hydroxylated silicas and compounds such as alkyl chlorosilanes, infrared spectroscopic data are extremely useful in following the reaction of the surface hydroxyls with the organic
reagent. The spectroscopic technique is, however, in this case perhaps too useful, since it leads one to analyze the data in a manner which assumes that the organic reagent reacts solely with the surface hydroxyl groups. As our own recent studies of such systems have shown,'? this is not always the case. The reaction a t room temperature of "fully" hydroxylated silicas with reagents such as BC18 and TiC14 may almost certainly be analyzed correctly on this basis. However, with reagents such as the alkyl chlorosilanes in which the reaction between the solid and the vapor phase silane is carried out at temperatures of 300-400", this assumption is not necessarily valid. In particular, it may be expected to be most incorrect for those silicas which have been evacuated at temperatures 1500" prior to reaction with the organic reagent vapor at 300-400". Such high-temperature evacuation removes the hydrogenbonded hydroxyls from the surface and leaves a corresponding number of strained-surface siloxane bridges. It follows that, since the hydrogen sequestering agents used in such work are of necessity susceptible to nucleophilic attack at the metal or metalloid atom (e.g., Ti, All Si, B) [in those reactions involving the surface hydroxyls the nucleophile is the oxygen atom of the hydroxyl group], it is unjustified to implicitly exclude the possibility that the oxygen atoms of the surface siloxane bridges do not themselves act as nucleophilic centers of sufficient strength to react directly with the organic reagent under the high-temperature conditions used. I n this context, an example of this latter siloxane reactivity has recently been reported by Yates and his coworkers7 in their account of the reaction of A1J!IeJ with silica surfaces. (6) R. J. Peglar, F. H. Hambleton, and J. A. Hockey, to be published.
(7) D. J. C. Ytttes, G. W. Demblinski, W. R. Kroll, and J. J. Elliot,
J. Phys. Chein., 73,911 (1969).
The Absorption Spectra of Triarylborons
by D. S. Miller and J. E. Leffler Department of Chemistry, Florida State University, Tallahassee, Florida 96806 (Received December 17, 1060)
The spectra of triarylborons and triarylcarbonium ions are similar in their response to temperature changes as well as in other respects. Tris(p-N,N-dimethylaminopheny1)boron has a long wavelength absorption band at 357 nm with a shoulder a t 340 nm in methylcyclohexane-isopentane at room temperature. On cooling to 77°K the shoulder decreases in relative intensity. The temperature effect resembles that reported for crystal violet and a similar explanation is suggested. Spectroscopic data are also given for The Journal of Phgsical Chemistry, Vol. 74, N o . 12, 1970
