J . Phys. Chem. 1988, 92. 6357-6359
through liberation of phosphorus oxide. Ratios of the PO3+and PO+ transient features from Figure 9 to the DMMP+ transient feature in Figure 9 were then measured and compared with the ratios obtained from the cracking pattern of DMMP (Figure 2). The ratio of P03+/DMMP+ from Figure 9 was 2.1 times greater than the ratio obtained from the DMMP mass spectral cracking pattern, while the PO+/DMMP+ ratio was 1.9 times greater than the ratio obtained from the DMMP mass spectral cracking patterns. Thus, the PO3+ and PO+ transient features must contain contributions from both the DMMP cracking pattern in the mass spectrometer and the phosphorus oxidation products. In the last experiment performed, steady conditions were obtained at 898 K with the 1:2 DMMP/oxygen reactant beam. Next, the temperature of the crystal was reduced to 279 K while the crystal was still being exposed to the DMMP/oxygen gas mixture. It was observed that the scattered DMMP signal increased to a steady value 2 times greater than that obtained when the temperature of the crystal was 898 K. This experiment confirms that activated oxidation of DMMP is being observed in the studies of the DMMP scattering from the crystal at elevated temperatures in the presence of oxygen.
IV. Summary and Conclusion The oxidation of DMMP by a Mo(l10) surface has been shown to occur catalytically, producing C O and phosphorus oxide gasphase species at -900 K. Auger spectroscopy indicates that an
6357
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oxide overlayer ( 2 monolayers) exists on the Mo( 110) surface under steady conditions at 900 K following exposure to a 1:2 DMMPloxygen gas mixture. Beam dosing studies of the DMMP scattering from the oxidized Mo( 110) surface suggest that maximum catalytic oxidative activity may be achieved on an oxide overlayer which contains little surface phosphorus and which possibly exposes Mo sites. In the absence of oxygen, DMMP decomposes on Mo( 110) above 700 K leaving an overlayer consisting mostly of phosphorus, possibly as a molybdenum phosphide. It is likely that catalytic oxidation of DMMP will occur on a number of transition metals and that a major factor leading to sustained catalytic chemistry is the desorption of surface carbon as C O and the desorption of surface phosphorus as phosphorus oxide(s).
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Acknowledgment. We thank Brett J. Stanley for preparing the crystal for mounting as well as for assisting in the construction of the UHV chamber and preliminary experiments. We thank L. Colaianni and J. G. Chen for assistance in studies of O2 adsorption on Mo(l10). We also thank Professor W. H. Weinberg for useful comments on the work. We also thank Dr. Noel H. Turner of the Naval Research Laboratory for helpful discussions regarding the P(LVV) Auger spectra. We gratefully thank the Army Research Office for support of this work through Contract NO. DAAL03-86-K-0005. Registry No. Mo, 7439-98-7; P,7723-14-0; DMMP, 756-79-6.
Probing Water Pools in Aerosol OT Reversed Micelles by the ESR Spin-Probe Method Yashige Kotake* and Edward G. Janzen* Guelph- Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario, Canada N1 G 2 W l (Received: February 1 1 , 1988)
ESR spectra of Fremy's salt (sodium peroxylaminedisulfonate) have been obtained in the water pools of Aerosol OT reversed micelles in heptane. ESR line widths measured at various water pool sizes were analyzed to give rotational correlation times for the nitroxide. As the water pool size decreases, the radical experiences a more hindered environment for rotational diffusion. It is shown that the rotational correlation time of Fremy's salt is shorter than that reported for water in similar water pools.
Introduction
Reversed micelles can form surfactant solubilized water pools in nonpolar solvents. The size of the water pool can be varied by changing the molar ratio of water and The properties of the water in the water pool have been investigated by various techniques. Direct observation of water by proton N M R in the water pool of sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT or A0T)-heptane solutions has shown that water in small water pools has a drastically different character from that of the bulk water.4 The dynamic behavior of fluorescence probes or the hydrated electron in water pools as well as the N M R spectra of the AOT molecule itself suggests a considerable change in the property of water in small water p00ls.~3~9~ Recently water pools in reversed micelles have been shown to give favorable conditions for ENDOR s p e c t r o ~ c o p y . ~ ~ ~
Since the water pool in a reversed micelle has been used as a unique medium for reaction chemistry and for spectroscopy, it is of importance to study the state of solutes in the water pool. Probing techniques can be useful for this purpose. Previous ESR spin-probing studies using organic nitroxide spin probes have monitored both the organic and aqueous phases in a reversed micelle9-13because the probes are soluble in both. The hyperfine splitting (hfs) of the nitrogen nucleus in a nitroxide probes shows that when the water pool size decreases, the probe moves into the surfactant layer from the water pool. Thus the observed ESR spectrum is usually a dynamic average of probe spectra from both sites. In fact Yoshioka observed13the presence of two species when the temperature was lowered in a large water pool in AOTheptane and calculated the positional exchange rate. Fremy's salt (Na2(S03-)2NO)is a unique inorganic nitroxide which is apparently only soluble in aqueous solutions. Rotational
( I ) Menger, F. M.; Donohue, J. A.; Williams, R. A. J . A m . Chem. Sac. 1973, 95, 286.
(2) Fendler, J. H. Acc. Chem. Res. 1976, 9, 153. (3) Eicke, H.-F., Kvita, P. Reverse Micelles; Plenum: New York, 1984; p 21. (4) Wong, M.; Thomas, J. K.; Nowak, T. J . A m . Chem. SOC.1977, 99, 477n
