Communications to the Editor
1924
Acknowledgments. The author wishes to thank Professor Eigen and his coworkers, especially Professor G. Maass and Dr. Mrs. M. L. Ahrens, for their kind and friendly help during his visit to the M. Planck Institut in Gottingen. He also thanks Professor G. Schwarz for his interest in and discussion of the subject of this note. References and Notes (1) This research has been partially supported by the Italian C.N.R. (2) The author is indebted to the E.M.B.O. organization for a short term fellowship for visiting the Max Planck lnstitut fur physik. Chemie in Gottingen. (3) L. Michaelis, J. Phys. Chem., 54,1 (1950). (4) E. Rabinowitch and L. F. Epstein, J. Amer. Chem. SOC., 63, 69
(I941) . (5) V. Zanker, Z.Phys. Chem., 199,225 (1952). (6) G. Barone. L. Costantino, and V. Vitagiiano, Ric. Sci., 34, 87 (1964). (7) M. E. Lamm and D. M. Neville, J. Phys. Chem., 69,3872 (1965). (8) D. F. Bradley and M. K. Wolf, Proc. Nat. Acad. Sci., U. S., 45,944 (1959). (9) T. L. Hill, "Statistical Mechanics," McGraw-Hill, New York, N. Y., 1956,Chapter 7. ( I O ) S. Lifson, J. Chem. Phys., 40,3705 (1964). (11) D. F. Bradley and S. Lifson in "Molecular Association in Biology," 6. Pullman, Ed., Academic Press, New York, N. Y., 1969,p 261. (12) G. Schwarz, Biopolymers, 6,873 (1968). (13) G. Schwarz, Eur. J. Biochem., 12,442(1970). (14) V. Vitagliano and L. Costantino, Boil. SOC.Natur. in Napoii, 78, 169 (1969). (15) V. Vitagliano and L. Costantino, J. Phys. Chem., 74,197 (1970). (16) In this case P corresponds to the sulfonic groups' equivalent concentration.
(17) V. Vitagliano, L. Costantino, and A. Zagari, J. Phys. Chem., 77,204 (1973). (18) G. Schwarzand W. Balthasar, €ur. J. Biochem., 12,461 (1970). (19) G. Schwarz, Ber. Bunsenges. Phys. Chem., 75,40 (1971). (20) Reference 13,eq 51-55. (21) According to Schwarz' treatment, cr is assumed to be a constant, r~ = l / q ~ :using Lifson's model, as done in ref 15 and 17, and neglecting second and higher order neighbor interactions
where Ft is the fraction of monomer bound dye (cuau/ca) and the terms in square brackets account for the aua sequences
-D -P
1-fl
1
+ (SI - 1)FI lim
CT
(ref 11, ey 13)
= l/q,
F,-1
+
(22) Reactions of the type aaa -t uuu F-' aua uau can be neglected. (23) G. Schwarz, Ber. Bunsenges Phys. Chem., 76,373 (1972). (24) Direct measurements of the free A 0 at P/D values higher than 2-3 were not made. However, from the adsorption isotherms shown in the previous paper, which were taken both in the absence and in the presence of NaCi, one can easily guess the absence of free dye at room temperature for P/D ratios higher than 50-60. (25) R. M. Fuoss, A. Katchalsky, and S. Lifson, Proc. Nat. Acad. Sci. U.
S.,37,579 (1951). (26) S. Lifson and A. Katchalsky, J. Polym. Sci., 13,43 (1954). 127) F. Oosawa. J. Polvm. Sci.. 23. 421 (19571. i28) E. F I Calle'n, "Fast Reactions in Solution," Biackwell Science Publishers, Oxford, 1964. (29) Most of the T-jump experiments were carried out at the Max Planck lnstitut fur physik. Chemie in Gottingen. Some control experiments were made in Professor Schwarz's laboratory in Basel and in Professors Accascina's laboratory in Palermo by using the same standard apparatus used in Gottingen. (30) G. Schwarz, Rev. Mod. Phys., 40,206 (1968). lstituto Chimico Universita di N a p o l i 80734 Napoli, l t a i y
Vincenzo Vitagliano
Received October 30, 1970 Revised manuscript received December 18, 1972 The Journal of Physical Chemistry. Voi. 77,No. 75. 1973
An Electron SDectroscoDv . - for Chemical Analysis Stud,, of Lead'Adsorbed on Montmorillonite' Publication costs assisted by the National Aeronautics and Space Administration
Sir: The bonding state of adsorbed species on solid surfaces is of fundamental interest. The results of using electron spectroscopy for chemical analysis (ESCA) to determine the bonding state of lead adsorbed from aqueous solutions onto montmorillonite are presented in this paper. We have measured recently the adsorption of Pb(I1) from aqueous lead nitrate solutions onto sodium-saturated montmorillonite at room temperature. Changes in lead concentration due to adsorption were determined using a Perkin-Elmer 303 atomic absorption spectrophotometer. Adsorption isotherms were determined at pH 4 and 6 over the concentration range 25-1000 ppm. Significant quantities of lead are adsorbed on montmorillonite over the concentration range studied. The clay sample was equilibrated with an initial lo00 ppm of Pb(II) solution at pH 6. The quantity of Pb(IT) adsorbed per gram of montmorillonite under these conditions was determined to be 18 mg. The sample was centrifuged and the clay was oven dried a t 105" for 1.5 hr. A small quantity (about 10 mg) of the dried clay was suspended in acetone and spread on the sample probe of the electron spectrometer. In a second sample preparation, a small quantity (about 10 mg) of the dried clay was pressed into indium foil. The ESCA measurements were made using an AEI ES 100 photoelectron spectrometer employing A1 K, radiation (1486.6 eV). Data acquisition was accomplished using the AEI DS-100 data system and a Digital PDP-8/e computer. Specific spectrometer conditions are listed on the subsequent spectra. The binding energies of gold 4f levels at 87.1 and 83.4 eV2 were measured during some of the runs and used to evaluate the work function of the spectrometer. The binding energies of lead were determined in a series of known lead compounds as a basis for comparison of the binding energy of adsorbed lead. The narrow scan ESCA spectrum of Pb in lead foil pressed from lead shot is shown in Figure la. The shoulders a t 141.1 A 0.2 and 136.4 0.3 correspond to the 4f6/2 and 4f712 levels of Pb metal whereas the more intense peaks at 142.4 f 0.3 and 137.5 -+ 0.4 eV correspond to the same levels of P b in a presumed surface oxide layer. The binding energies have been corrected for the work function of the spectrometer. The sample was then argon ion etched for 10 min. The narrow scan ESCA spectrum of Pb after etching is shown in Figure lb. The change is dramatic. The Pb metal peaks have now become the most intense whereas the lead oxide peaks have been reduced in intensity. The ESCA spectrum obtained after an additional 10-min etching is shown in Figure IC.The peaks attributed to lead oxide are barely discernible. There was no observed shift in the binding energy of the predominant peaks. An authentic sample (PbO) was run to support the assignments made in Figure 1. The narrow scan ESCA spectrum of Pb in lead oxide particles pressed into indium foil is shown in Figure 2. The binding energies at 143.0 f 0.2 and 138.0 f 0.2 eV in Figure 2 match those of the more intense peaks in Figure l a thereby supporting the peak
1925
Communications to the Editor
153
133
133
153
B.E. (ev)
B.E.(sv)
Figure 1. ESCA spectrum of lead 4f electrons obtained from lead foil: (a) as pressed, (b) after 10-rnin Ar+ etch, (c) after 20 rnin A r + etch (scanning: 200 channels, 3 sec/channel, 3
Figure 3. ESCA spectrum of lead 4f electrons obtained from lead dioxide (scanning; 200 channels, 3 sec/channel, 5 scans).
scans).
153
153
B,E, (ev)
133
Figure 2. ESCA spectrum of lead 4f electrons obtained from lead oxide (scanning:200 channels, 3 sec/channel, 5 scans).
assignments above. Jorgensen3 reported values of 149.0 and 144.15 eV for PbO which appear too high and may be due to sample charging discussed below. An authentic sample of lead dioxide (PbOz) was also run to support the subsequent assignment of lead adsorbed on montmorillonite. The lead dioxide sample was prepared by heating lead foil in an oven at 110"for 20 hr in an oxygen rich atmosphere.4 The narrow scan ESCA spectrum of Pb in lead dioxide is shown in Figure 3. The binding energies a t 1143.6 and 138.6 eV occur at significantly higher binding energies than Pb in lead oxide. The charging of nonconducting sample is a continuing problem in ESCA studies. For example, charging effects were neglected in a recent ESCA study of Rh adsorbed on
133 B.E.(w)
Figure 4. ESCA spectrum of lead 4f electrons obtained from lead adsorbed on montmorillonite (scanning: 200 channels, 3 sec/channel, 66 scans).
carbones It was anticipated that the clay particles in the present work would charge and therefore the particles were either pressed into indium foil or deposited as a very thin film. The narrow scan ESCA spectrum of clay which had not been equilibrated with lead was obtained over the binding energy range 153-133 eV. No detectable signal due to Pb was observed. The narrow scan ESCA spectrum of clay equilibrated with an aqueous lead nitrate solution is shown in Figure 4 for clay particles pressed into indium. A similar spectrum was obtained for clay particles suspended in acetone. The presence of the adsorbed lead is readily seen. Binding energies at 142.9 and 138.0 eV are very close to the values assigned for lead oxide. Thus, we have established the binding energies of lead adsorbed on montThe Journal of Physical Chemistry. Vol. 77, No. 15. 7973
Communications to the Editor
1926
morillonite and further that the adsorbed lead is in a similar bonding state as the lead in lead oxide.
50
I
Acknowledgment. We wish to acknowledge gratefully the assistance of the National Science Foundation under the Capital Equipment Grant Program in the purchase of the AEI electron spectrometer. We thank Dr. Lucian Zelamy, Department of Soils, University of Florida, for supplying the clay sample. References and Notes ( 1 ) This work was supported in part by the Center for Environmental Studies and the College of Arts and Sciences at Virginia Polytechnic (2) (3) (4j (5)
Institute and State University inciuding a graduate research assistantship for one of us (M. E. C.), K. Siegbahn, et a/., "ESCA-Atomic, Molecular and Solid State Structure Studies by Means of Electron Spectroscopy," Almqvist and Wiksells, Uppsala, 1967. C. K. Jorgensen, Chimia, 25, 213 (1971). J. F. Rendina, Amer. Lab., 17 (Feb 1972). J. S. Bririen and A. Melera, J. Phys. Chem., 76, 2525 (1972).
Department of Chemistry Virginia Polytechnic Institute and State University Blacksburg, Virginia 24067
Ellen Counts James S. C. Jen James P. Wightman* Mary
Received April 7 7, 7973
ecay of Hydrated Electrons in Radiolytic Spurs at Picosecond Times' Publication costs assisted by the U.S. Atomic Energy Commission and Carnegie-Me//on University
Sir. A recent report on the radiolytic yield of hydrated electrons as observed a t subnanosecond times by both the stroboscopic pulse radiolysis technique and by direct measurements with single pulses indicates that G(e,, - ) is -4.0 f 0.2 on the 30-1000-psec time scale.2 This yield is somewhat lower than the value of 4.8 required in the calculations of Schwarz3 to explain the net observed yield of water decomposition on the basis of the spur diffusion model. Furthermore very little decay was observed over the period 30-350 psec after irradiation in contrast to the expected -15% drop. The purpose of the present communication is to point out that, to a large extent, these observations can be accommodated by the spur diffusion model provided hydration requires a period 10-12 sec. It has previously been shown4 that the concentration dependence observed for scavenging hydrated electrons by both NzO and CH3C1 is in reasonable agreement with the calculations of Schwarz.3 This concentration dependence has been used4 to obtain a description of the decay of the electron population, which should be accurate for times longer than nanoseconds, as
-
G(e,,-)
= 2.55
4- 2.23eAt erfc(Xt)'"
0)
where X is a constant which can be evaluated from the scavenging data as 8 X 108 sec-1. Exploration of the shorter time scale by scavenging methods requires inordiThe Joilrriei of Physicai Chemistry. V u / . 77. No. 15. 7973
00
L loo
0
, 200
300
t - p sec
Figure 1. Time dependence of the hydrated electron population (-----) as given by eq 1 with h 8 X l o 8 sec-I and (-) as modified by a hydration period of 1 . 7 psec taking into account reaction of the electrons at a 1 00-fold higher rate prior to hydration. nately high solute concentrations ( > 1 M ) so that one can only infer the behavior by extrapolation of the trends observed at experimentally accessible concentrations. Yields for electron scavenging as high as -3.8 have been observed and these yields are obviously still increasing with concentration in the molar region so that a limiting yield considerably in excess of 4 is indicated.4 Making the very reasonable assumption that the limiting yield at very high solute concentrations is in the vicinity of the value of 4.78 used in the calculations of Schwarzs the decay on the time scale of the stroboscopic pulse radiolysis experiment as predicted by eq 1 is given by the dashed curve in Figure 1. Schwarz' predictions are very similar (see Figure 7 in ref 4). The detailed differences between the direct ohservations (which over the region 30-350 psec give yields very similar to the solid curve in Figure 1) and the conclusions from both the scavenging studies and also the model calculations have led a number of investigators to question the applicability of the spur diffusion model. Attention has particularly been focused on the fact that the full initial yield required in the spur diffusion calculations is not observed in the pulse experiments. Also, in the stroboscopic experiment, the yield averaged over a finite pulse of 20 psec should decay from a value of 4.65 to 4.0 whereas the actual data indicate an initial yield of only 4.0 and a decay considerably less than 8%.6 From the experimental facts that the initial yield, which was initially thought to be only 3.3,7 is now observed to be 4.05 or greater and also that appreciable decay occurs in a 30-nsec period, it seems obvious that the general aspects of spur diffusion theory must be incorporated into any detailed model. It can be shown that if the relative rates for reaction of the electrons with an externally added scavenger and with the other intermediates produced within the spur are unaffected by hydration then the concentration dependence for reaction of the electrons with the scavenger will be independent of whether or not hydration occurs.8 Under these conditions one can, from the scavenging studies, obtain a description of the distribution of the lifetimes that the electrons would have if hydration did not occur. By use of mathematical devices similar to those employed in the discussion of hydrocarbon radiolysis,g one can then explore the possible effects of the hydration period on the
