J. Phys. Chem. 1988, 92, 4432-4436
4432
of about 130 Rh atoms and have an average diameter of
-
15
motionally averaged species and a rigid species whose line shape is indistinguishable from that of linearly bonded CO. Of course, any combination of these descriptions is also consistent with the experimental data.
A. Assuming instead that the particles are raftlike, the rafts would
be 1.5 layers thick on average. The ultradispersed catalysts, which were prepared by amine exchange and have CO-to-metal ratios greater than 1, are inconsistent with metal surface topologies and/or C O coverages based on bulk metals. For example, Rh/silica sample (D) has an overall C 0 : R h ratio of about 1.12, with 19% of the C O as dicarbonyls, which indicates that the C0:Rh ratio for the agglomerates (8) is 1.01. The situation is more extreme for the Ru catalysts. Based on the N M R intensity, the CO:Ru ratio of sample (A) is 1.29, in which 23% of the C O is adsorbed at multicarbonyl sites. Assuming these multicarbonyls are tricarbonyls, as expected for Ru” or RuIV in octahedral bonding, the CO:Ru ratio of the particles is 1.1. Similarly for Ru/silica (B), with an overall CO:Ru ratio of 1.40, 8 = 1.2. If the Ru multicarbonyls are dicarbonyls or one uses the CO:Ru ratios measured by volumetric uptake, the local coverages are yet higher. These coverages greater than 0.75 (the measured saturation coverage) are not due to increased packing on the metal surface; the T2’sof samples B, C, and D indicate that the average CO-CO separation is similar to that of C O on a Rh[ 11 11 surface at 8 = 0.75. We propose three scenarios consistent with the Tz’s and the high coverages. The amine-prepared agglomerates may resemble small clusters of 10-30 metal atoms that resemble carbonyl clusters in which the C0:metal ratio commonly exceeds 1. In small metal clusters, the meta-metal distance is 10-20% longer than the bulk metals. However, EXAFS studies of Rh particle on alumina show that the Rh-Rh interatomic distance is the same as that of bulk Rh, within experimental confidence limits, even in the presence of CO.’* A second possibility is that the agglomerates may be small raftlike particles, with a significant fraction of edge atoms (30-50%) that are capable of adsorbing more than one CO. A third option is that at least two types of multicarbonyls exist: a (37) Wang, T.;
V. Summary On the basis of the I3C N M R spectra of transition-metal carbonyl clusters, we interpret the I3C N M R spectra of C O adsorbed on silica-supported Ru and Rh to be the superposition of three adsorbed states: linear- and bridge-bonded C O on metal particles and multicarbonyls on isolated metal atoms. The line shapes indicate the motional characteristics of each adsorbed state. The species adsorbed on the metal particles are immobile at 295 K (on the N M R time scale of a few milliseconds) whereas the multicarbonyls are motionally averaged. The absolute population of each type of adsorbed C O is directly obtained from the integrated intensity. The morphology of the metal particles can be inferred from the C O site distributions, the overall CO-to-metal ratio, and the local density of CO. The particles on a Rh/silica sample prepared by incipient wetness have a local coverage of 0.75, consistent with the saturation coverage measured on Rh[ll 11, and are either small spherical (or cuboctahedral) clusters of diameter 15 A, rafts approximately 1.5 layers thick, or a combination. Three catalysts prepared by amine exchange had CO-to-metal ratios greater than 0.75, but the T2’sindicate CO-CO densities comparable to the less-dispersed catalyst. We hypothesize that these particles are smaller (5-8 A) and resemble metal carbonyl clusters or small rafts with multiple absorption at the edge metal atoms. This study demonstrates the potential of I3C NMR spectroscopy to characterize a catalytic surface by quantitative analysis of the site distribution of probe molecules. Moreover, the quantitative analysis is not compromised by changes in absorption cross sections (e.g., with electron or vibrational spectroscopies), which may vary nonlinearly with coverage, site, and surface properties. Registry No. CO, 630-08-0; Ru, 7440-18-8; Rh, 7440-16-6.
Lee, C.; Schmidt, L. D. Surf. Sci. 1985, 163, 181.
Surface Extended X-ray Absorption Flne Structure of Underpotentially Deposited Silver on Au( 111) Electrodes J. H. White, M. J. Albarelli, H. D. Abruiia,* Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, New York I4853
L. Blum,* Department of Physics, College of Natural Sciences, University of Puerto Rico, P.O. Box AT, Rio Piedras, Puerto Rico 00931
0. R. Melroy,* M. G. Samant, G. L. Borges, and J. G. Gordon, I1 IBM Almaden Research Laboratories, 650 Harry Road, San Jose, California 95120 (Received: September 29, 1987; In Final Form: February 8, 1988) The fluorescence-detected surface EXAFS of silver, underpotentially deposited from aqueous solutions of silver ion (HC104 electrolyte) on Au(ll1) electrodes was used to investigate the local structure of the adsorbate. Analysis of the data indicates that the silver atoms are bonded to three surface gold atoms at a distance of 2.75 A 0.05 A and to an oxygen atom (from solvent of electrolyte) at a distance of 2.42 f 0.05 A. These data suggest that the silver atoms sit at 3-fold sites on the gold surface with water or perchlorate anion (from the electrolyte) bonded at a well-defined distance. Introduction The electrode/electrolyte interface has heretofore been the object of much interest. The elucidation of interfacial structure has, however, been hampered by the lack of probes sensitive to microscopic structural features, particularly those on the atomic scale. The in situ use of spectroscopies based on electromagnetic radiation of wavelengths from infrared to ultraviolet has provided 0022-3654/88/2092-4432$01 .50/0
TABLE I bond distance. 8, near neighbors Debve-Waller factor Ag-0 2.42 f 0.05 1 & 0.3 0.002 f 0.001 0.02 f 0.02 3 f 0.8 Ag-Au 2.75 f 0.05
(uz)
interesting and extremely valuable information concerning vibrational modes and electronic energy levels at electrode surfaces.’ 0 1988 American Chemical Society
Surface EXAFS of Silver on A u ( l l 1 ) Electrodes
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4433
The major limitation of these techniques is that the information extracted relates only indirectly to microscopic structural detail, and hence, the accuracy of the conclusions rests on the appropriateness of the assumptions made in the models of particular interest. The ex situ use of electron spectroscopies and diffraction techniques has provided a wealth of information concerning structural detaiL2 However, the lack of control of electrode potential and the need to employ ultrahigh vacuum (UHV) introduce questions about the relevance of those data to the structure actually present at the interface. Because of their short wavelengths and significant penetration depths, X-rays represent a unique tool with which to study atomic and molecular details of electrochemical interfaces. The applicability of X-ray diffraction to the in situ study of electrochemical interfaces has been recently demonstrated in a variety of s t ~ d i e s . ~ However, this technique is relatively insensitive to low-z (atomic number) adsorbates because of their small scattering cross sections. The advent of synchrotron sources of X-rays of high intensity has provided an alternative to electron spectroscopy in structural studies of electrodes. In addition to the possibility of obtaining photoelectron spectra of high quality, the extended X-ray absorption fine structure is obtainable under a wide variety of condition^.^ The method of detection, however, determines the applicability of X-ray absorption spectroscopy to the in situ characterization of electrode s ~ r f a c e s . ~ The fact that the fluorescence cross section is proportional to X-ray absorption cross section allows the detection of photons emitted upon relaxation of a core hole to be used as a measure of the post-edge X-ray absorption coefficient (EXAFS).4 The application of fluorescence-detected surface EXAFS to the study of the electrosorbed metal ions is of great interest, since (1) underpotential deposition (UPD) of metals on foreign metal substrates is clearly a process that would involve definite structures in various potential regimes, (2) some of these structures should be discernible by a short-range-order technique such as EXAFS, and (3) UPD is an example of an important group of electrochemical processes involving specific adsorption of ions with subsequent and/or concomitant discharge of the ion. Reviews of UPD of metals have appeared.6 We have also previously reported on surface EXAFS studies of Cu UPD on gold (1 11) as well as other systems.'
(1) (a) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 173. (b) Jenamaire, D. J.; Van Duyne, R. P. J. Electroanal. Chem. 1977,84, 1. (c) Pons, C. J. Electroanal. Chem. 1983,150,495. (d) Bewick, A. J . Electroanal. Chem. 1983, 150, 481. (e) Gordon, J. G.; Ernst, S.Surf.Sci. 1980,101,499. (f) Chen, C. K.; Heinz, T.F.; Ricard, D.; Shen, Y. R. Phys. Rev. Lett. 1981,46,1010. (g) Com,R. M.; Philpott, M. J. Chem. Phys. 1984,80, 5245. (2) (a) Yeager, E. J . Electroanal. Chem. 1981,128, 1600. (b) Hubbard, A. T. Acc. Chem. Res. 1980, 13, 177. (c) Ross, P. N. Surf. Sci. 1981, 102, 463. (d) Bange, K.; Grider, D. E.; Madney, T. E.; Sass, J. K. Surf.Sci. 1984, 136, 381. (3) (a) Machida, K.; Enyo, M. Chem. Lett. 1986, 1437. (b) Nazri, G.; Muller, R. H. J. Electrochem. SOC.1985, 132, 1385. (c) Fleischmann, M.; Hendra, P. J.; Robinson, J. Nature (London) 1980, 288, 152. (d) Fleischmann, M.; Oliver, A.; Robinson, J. Electrochim. Acta 1986, 31, 899. (e)
Fleischmann, M.; Graves, P.; Hill, I.; Oliver, A.; Robinson, J. J . Electroanal. Chem. 1983,150,33. (f) Fleischmann, M.; Mao, B. W. J. Electroanal. Chem.
1987, 229, 125. (4) (a) Sayers, D. E.; Lytle, F. W.; Stern, A. Adu. X-ray Anal. 1970, 13, 248. (b) Eisenberger, P.;Kincaid, B. M. Science (Washington, D.C.)1978, 200, 1441. (c) Shulman, R. G.; Yafet, Y.; Eisenberger, P.; Blumberg, W. E. Proc. Natl. Acad. Sci. U.S.A. 1976, 73, 1384. ( 5 ) (a) Kordesch, M. E.; Hoffman, R. W. lyucl. Inrtrum. Methods Phys. Res. 1984,222,347. (b) Bosio, L.; Cortes, R. Froment, M. EXAFS and Near
Edge Structure; Hodgson, K. O., Hedman, B., Penner-Hahn, J. E., Eds.; Springer: Berlin, 1984. (6) Kolb, D. M. Electrochemistry and Electrochemical Engineering, Gerischer, H., Tobias, C., Eds.; Pergamon: New York, 1978; Vol. 11, p 125. (b) Kolb, D. M.; Przanyski, M.; Gerischer, H. J . Electroanal. Chem. 1974, 54, 25. (c) Astley, D. J.; Harrison, J. A.; Thirsk, H. R. J. Electroanal. Chem. 1968, 19, 325. (d) Lorenz, W. J.; Hermann, H. D.; Wuthrich, N.; Hilbert, F. J. Electrochem. SOC.1974, 12, 1167.
In this paper we present an in situ surface EXAFS study of silver underpotentially deposited onto gold (1 1 1) electrodes. The underpotential deposition of silver on gold substrates has been studied electrochemically* and by radiotracer techniquesg However, in these cases, definitive structural assignments were not made. The observation of the UPD was attributed to the variation of the activity coefficient of the metal deposit with coverage below a monolayer. An expression for UPD current, similar to that expected from calculations by Anson and Hubbardlo for the reversible deposition of metals, was found to fit these equations. Experimental Section Gold (1 11) electrodes were prepared by epitaxially depositing 2500 A of Au onto cleaved (in air) ruby mica surfaces (1 X 3 in.) which were maintained at 300 "C during the deposition." Epitaxy was confirmed by Laue X-ray diffraction backscattering. The films were stored in an inert atmosphere prior to use in the electrochemical cell. The electrochemical cell employed has been previously de~cribed.'~The reference electrode was a Ag/AgCl (3 M KCl) microelectrode, against which all potentials are reported. A Pt coil was used as the counter electrode. Solutions were deoxygenated with nitrogen and added (and removed) from the cell through Teflon tubing connected to Teflon syringes. The electrolyte was contained between a thin (0.5 mil) polypropylene film and the electrode. The electrochemical cell was continuously flushed with nitrogen to eliminate any problems from diffusion of oxygen through the thin polypropylene film. The electrolyte was 0.1 M perchloric acid (G. F. Smith double distilled) containing 5X M silver ions and was prepared from Aldrich Gold Label reagents in pyrolytically distilled water.12 Prior to deposition, solution was added to the cell so that the polypropylene film distended somewhat, allowing the UPD layer to be deposited from bulk electrolyte. The monolayer was deposited from bulk electrolyte because of the low silver concentration. At this concentration approximately 10 min were required to form the monolayer, which was deposited at +0.60 V. All measurements were made at full monolayer coverage. After deposition, solution was removed, leaving only a thin layer of electrolyte between the electrode and polypropylene window. We estimate that the thickness of the electrolyte layer is of the order of 30 pm. Well-developed voltammetric scans could be obtained in the thin-layer configuration, but very low scan rates (1 mV/s) had to be employed. In the thin-layer configuration (at full monolayer coverage), we calculate that less than 5% of the absorbing silver atoms are present as ions in the electrolyte. As a result, the interference from silver ions in solution can be ignored. All experiments were conducted at room temperature. Data were collected at the Cornel1 High Energy Synchrotron Source (CHESS) on beam line A-3. A Si(220) double-crystal monochromator was used to select the incident wavelength. The absorption spectrum was measured about the Ag K edge, at 25.5 keV, by monitoring the Ag Ka fluorescence line at 22.1 keV. This was detected with a 36-mm-diameter high-purity germanium solid-state detector (Ortec GLP-36360/ 13-S) in conjunction with an Ortec Model 673 spectroscopy amplifier and a single-channel analyzer. The X-ray beam was incident on the sample at near (7) (a) Blum, L.; Abrufia, H. D.; White, J. H.; Albarelli, M. J.; Gordon, J. G.; Borges, G. L.; Samant, M.; Melroy, 0. R. J . Chem. Phys. 1986, 85, 6732. (b) Gordon, J. G.; Melroy, 0. R.; Borges, G. L.; Reisner, D. L.; Abruiia, H. D.; Chandrasekhar, P.; Albarelli, M. J.; Blum, L. J . Electroanal. Chem. 1984, 210, 31 1. (c) Samant, M.; Borges, G.;Gordon, J.; Melroy, 0.;Blum, L. J. Am. Chem. SOC.1987, 109, 5970. (8) Sandoz, D. P.; Peekema, R. M.; Freund, H.; Morrison, C. F. J . Electroanal. Chem. 1970, 24, 165. (9) Rogers, A. T.; Krause, D. P.; Griess, J. C.; Ehrlinger, D. B. J . Electrochem. Soc. 1949, 95, 33. (10) Hubbard, A. T.; Anson, F. C. Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1971; Vol. 4, p 129. (11) (a) Pashley, D. W. Philos. Mag. 1959, 4 , 316. (b) Grunbaum, E. Vacuum 1973, 24, 153. (c) Reichelt, K.; Lutz, H. 0.J . Cryst. Growth 1971, 10, 103. (12) Conway, B. E.; Angerstein-Kozlowska,H.; Sharp, B. A. Anal. Chem. 1973, 45, 1331.
4434
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988
White et al.
h
01
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ENERGY
Figure 1. In situ, fluorescence-detected absorption spectrum around the silver K edge for an underpotentiallydeposited monolayer of silver on a gold (1 11) electrode in contact with a 0.1 M perchloric acid solution and at an applied potential of +0.60 V. grazing incidence (Le., angle of incidence greater than 89’) although the actual angle was not measured. This geometry enhances the signal from the surface as the incident X-ray beam undergoes total external reflection,I3 thus increasing the local intensity of X-rays at the surface at least by a factor of 2 . Furthermore, since the penetration depth of the X-rays in the sample is small in such a configuration, the scattered radiation (Compton and elastic), which represents the most significant source of background noise, is greatly reduced. The angle of incidence was adjusted experimentally to minimize background scattering while maximizing the fluorescence peak. Soller slits were used to further reduce the background scattering incident on the detector. Data were collected in scans of 20-30 min, and typically about 35 scans were averaged to obtain a reasonable signal to noise ratio. These data were analyzed by employing a modified version of the program of B. M. Kincaid (AT&T Bell Labs). The major inflection point in the edge jump was taken to be the position of the edge. Bond distances were obtained by fitting the oscillatory part of the EXAFS equation14 to the experimental oscillations with the phase shift for the pair of interest being obtained experimentally from reference compounds. These were silver foil, silver oxide, and a gold/silver alloy. Discussion The raw X-ray spectrum at the silver K edge for a silver UPD layer on a gold (1 11) electrode in contact with a 0.1 M perchloric acid electrolyte solution and at an applied potential of +0.60 V is shown in Figure 1. A very well defined edge is present at about 25.5 keV. Pronounced edge features (e.g., XANESI5) are not readily visible, and the EXAFS is somewhat weak. Such results (13) Heald, S.; Keller, E.; Stern, E. Phys. Lett. A 1984, ZOJA, 155. ( 1 4 ) Teo, B. K. EXAFS Basic Principles and Data Analysis; Springer Verlag: Berlin, 1986. (15) (a) Bianconi, A,; Incoccia, L.; Stipchich, S.,Eds. EXAFS and Near-Edge Structure; Springer Verlag: Berlin, 1983. (b) Hodgson, K. 0.; Hedman, B.; Penner-Hahn, J. E., Eds. EXAFS and Near-Edge Srructure III; Springer Verlag: Berlin, 1984.
e
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k
Figure 2. k-weighted raw (A) and Fourier filtered (B) EXAFS for a silver UPD layer on a gold (1 11) electrode and fitting (C; open circles) when considering only gold as a backscatterer. are expected for adsorbed polarizable metals, since excitation of a bound electron to a conduction band would not be a process that would be resonant, as is the case with semiconductors and certain metal ions that possess unfilled subshells at the valence level. The relatively weak EXAFS can be attributed to substantial inelastic scattering and large-amplitude thermal vibrations and static disorder (i.e., large Debye-Waller factors).I4 Such would be the case for a model consisting of a silver atom adsorbed on the gold substrate and possessing a large amplitude of vibration as would be expected for a relatively low melting metal alloy. While one would anticipate the adsorbed silver layer to exhibit a certain degree or order, this would not necessarily be the case for adsorbed water or electrolyte (perchlorate) ions in the compact layer. However, as we previously reported, there appears to be a considerable degree of order in the layer of sulfate ions adsorbed on UPD copper on gold (1 11) in sulfuric acid.7a Upon stripping of the monolayer and flushing the thin-layer cell volume, only background scattering was observed, indicating that the signal originates from the silver UPD layer. As in our previous study of Cu UPD on Au,’* we considered scattering by surface gold atoms and solvent or electrolyte. These were anticipated to represent the largest contributions to the scattering since the plane of polarization of the X-ray was normal to the electrode surface, thus minimizing the effects of scattering by silver since in this case the polarization of the X-ray beam would be normal to the silver-silver bond vector. Figure 2 shows the k-weighted EXAFS (raw (A) and Fourier filtered (B)) as well as the fitting (C; open circles) obtained when considering only gold as a backscatterer. Although the fit is reasonable at low k values, there are gross deviations for k values larger than 5 . Figure 3 shows the results obtained when considering only oxygen as a backscatterer. Clearly in this case the fit is greatly improved, but again there are significant deviations, especially at intermediate values of k. Finally, Figure 4 shows the results obtained when backscattering by both oxygen and gold is taken into account. In this case an excellent fit of the data is obtained over the entire range of k values. Values obtained from the fitting procedures for the distances, near-neighbor numbers, and Debye-Waller factors are presented
The Journal of Physical Chemistry, Val. 92, No. I S . 1988 4435
s.
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Figure 3. Same as Figure 2 but fitting with only oxygen as a back-
scatterer.
Fiyre 5. Model for the structure of an underpotentially deposited monolayer of silver on a gold ( I 11) electrode with either water (A) or perchlorate (B) bonded to the silver adatoms through the oxygen.
claimed, since inelastic scattering lengths and Debye-Waller factors are not known to a high degree of accuracy (Table I). However, it is sufficient to say that gold is the dominant backscatterer in the compact layer in terms of number. The bond lengths obtained from least-squares minimization were 2.75 0.05 and 2.42 + 0.05 A for gold and oxygen backscatterers, respectively. The first value is consistent with, although somewhat shorter than, the bond length anticipated for Ag-Au alloy (2.88 &.I6 This might be due to partial charge transfer (i.e., electrosorption valency less that 1) which would result in a smaller bond length than anticipated on the basis of the bulk alloy. In addition, a contraction of the bond length a t the surface would not necessarily be unexpected. The silveroxygen distance of 2.42 A is significantly longer than the anticipated value of about 2.30 A. This was a rather surprising finding, and we are not yet certain as to its origin. The unambiguous assignment of structure in the system is contingent upon the availability of information related to the polarization dependence of the EXAFS signal. However, it may be said here that a glimpse into the composition of the compact layer may be inferred by the data presented here. Such information is valuable with regard to the system studied here since it is revealing in terms of the kind of interaction to he expected between substrate, adsorbate, and solvent. The results presented here were analyzed on the basis of a single scattering plane wave formalism. It has been shown that atoms arranged in a collinear array should show rather pronounced multiple scattering effe~ts.'~The structural model proposed would probably not give rise to pronounced multiple scattering effects although the structures inferred should be accessible using the single scattering formalism. A multiple scattering formalism analysis would, of course, provide detailed information about bond angles. However, the low signal to noise ratio obtained here would create considerable difficulty in the analysis of this data, since the analysis of higher coordination shells by Fourier filtering is
*
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Figure 4. Same as Figure 2 but fitting with both gold and oxygen as backscatterers.
in Table 1. The fitted values of the uncorrected wordination numbers for oxygen and gold occur in a ratio of approximately one oxygen atom per three gold atoms. On the basis of this, we propose a model, shown in Figure 5 , in which a silver atom sits on a 3-fold site on the gold substrate and is bonded to an oxygen atom from water (Figure 5A) or perchlorate ion (Figure 58). The accurate determination of coordination numbers here is not
(16) Pearson, W.P. Hondbwk of Lorrice Spocings ond Slruerures of Melds ond Alloy$; Pergamon: New York, 1958
J. Phys. Chem. 1988, 92, 4436-4439
4436
quite difficult. It has been shown that the width of the filter window is inversely proportional to the degrees of freedom allowable in the subsequent xz minimization routine. Practically, then, one must perform the minimization on the unfiltered spectrum leading to the undesirable consequence of minimization in many parameters of a maximum likelihood estimator that is large and, hence, relatively insensitive to the small changes in parameters that would differentiate between models.
Conclusions The surface EXAFS of underpotentially deposited silver on gold (1 11) films on mica was studied with the electrode under potential control. It was revealed that both gold and oxygen are present as backscatterers in the first coordination shell of silver. It was
estimated that the bond lengths to gold and oxygen were 2.75 -+ 0.05 and 2.42 f 0.05 A, respectively, and the respective coordination numbers were 3 and 1. A model consistent with these findings is one where the silver atoms sit on 3-fold sites on the gold surface and have oxygen from water or electrolyte bound at a well-defined distance. This model is qualitatively the same as that proposed for the Cu UPD layer on gold (1 11) electrodes.
Acknowledgment. This work was supported by the Office of Naval Research, the Materials Science Center at Cornel1 University, the National Science Foundation, and the Army Research Office. H.D.A. is a recipient of a Presidential Young Investigator Award and an Alfred P. Sloan Fellowship. Registry No. Ag, 7440-22-4; Au, 7440-57-5.
Aggregation States of Water in Reversed AOT Micelles: Raman Evidence A. D’Aprano, A. Lizzio, V. Turco Liveri, Istituto di Chimica Fisica, Uniuersita’ di Palermo, Via Archirafi 26, 90123 Palermo, Italy
F. Aliotta,* C. Vasi, Istituto di Tecniche Spettroscopiche del C.N.R., Via dei Verdi, 98100 Messina, Italy
and P. Migliardo Istituto di Fisica dell’Universita’ and G.N.S.M.-C.I.S.M.,Via dei Verdi, 98100 Messina, Italy (Received: October 6, 1987; In Final Form: January 25, 1988)
Polarized Raman spectra in the 0-H stretching region in the water/sodium bis(2-ethylhexyl) sulfasuccinate (AOT)/n-heptane system as a function of the molar ratio R = [H,O]/[AOT] have been measured at 25 OC. By the isotopic substitution ( D 2 0 H 2 0 ) method, the unwanted C-H contributions have been eliminated, and a careful analysis of the 0-H stretching band was carried out. The results show that the Raman spectra can be partitioned into two contributions linked to the “bonded” and “bulk” water that renresent the two “water domains” within the water pool. The percentage, a(R),of the water tightly bonded to the surfactant sheath was evaluated, and its dependence on R gives information about the filling mechanism of the AOT reversed micelles.
-
Introduction The behavior of the water close to ions, interfaces, biological membranes, or biopolymers is markedly different than that of bulk The study of the water properties in these systems allows to understand the nature of the interactions responsible for important phenomena such as interface curvature, ionic solvation, protein folding, and micellization. In particular, the study of the polarization effects on the water entrapped in sodium bis(2ethylhexyl) sulfosuccinate (AOT) reversed micelles allows to investigate the variation of the local structure of water pools as a function of the size of the reversed micelle c ~ r e . ~ , ~ In relation to the size of the micellar core, different water domainsScan be identified. For smaller size droplets water exists mainly as bonded H 2 0 molecule^^^^ whose static and dynamic properties are determined by the local interactions with the (1) Clifford, J. In Water: a Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1975; Vol. V, p 75. (2) Luisi, P. L.; Magid, L. J. CRC Crit. Reu. Biochem. 1986, 20, 409, and references therein. (3) Kotlarchyk, M.; Chen, S. H.; Huang, J. S.; Kim, M. W. Phys. Rev. Lett. 1984, 53, 941. (4) Kotlarchyk, M.; Chen, S. H.; Huang, J. S.; Kim, M. W. Phys. Rev. A 1984, 29, 2054. (5) Nicholson, J. D.; Clarke, J. H. R. In Surfactant in Solutions; Mittal, K., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 3, p 1671. (6) Zinsli, P. J. J . Phys. Chem. 1979, 83, 3223. (7) Boned, C.; Peyrelasse, J.; Moha-Onchange, N . J . Phys. Chem. 1986, 90, 634.
0022-3654/88/2092-4436$01.50/0
negative head groups and the corresponding counterions of the surfactant. As the droplet’s size increases, bulk water domains, in dynamic equilibrium with the bonded ones, have been postulated.8 The evolution of the droplets size as a function of the molar ,~ ratio R = [HzO]/[AOT] as well as the t r a n ~ p o r t thermodyproperties of the water pools namic,’~’~ and spectros~opic~~~~~*’~-’~ has been studied by a large variety of experimental techniques. Even if these studies give a general picture of the micellar core, the different local structures of the water, their relative amounts, and the filling mechanism^^*'^ are still not fully understood. In order to give more insight into these problems, we present in this paper a Raman scattering investigation on the 0-H stretching dynamics of water in the water/AOT/n-heptane system. We will show that at any R value the spectrum of the 0-H region can be rationalized in terms of the two different contributions associated with bonded and bulk water. The relative amount of (8) Maitra, A. J . Phys. Chem. 1984, 88, 5122. (9) Dreher, K. D.; Hogarty, W. B.; Sydansk, R. D. J . Colloid Interface Sci. 1978, 57, 379. (10) D’Aprano, A.; Lizzio, A.; Turco Liveri, V. J . Phys. 1988, 92, 1985. (1 1) Fletcher, P. D. I.; Robinson, B. H.; Tabony, J. J. Chem. Soc.,Faraday Trans. 1 1986, 82, 2311. (12) Clarcke, J. H. R.; Nicholson, J. D.; Regan, K. N. J . Chem. SOC., Faraday Trans. 2 1985, 81, 1173. (1 3 ) Mallamace, F.; Migliardo, P.; Vasi, C.; Wanderlingh, F. Phys. Chem. Liq. 1981, 11, 47, and references therein. (14) Thompson, K. F.; Gierasch, L. M. J . Am. Chem. SOC.1984, J06, 3648.
0 1988 American Chemical Society
