2858
J . Phys. Chem. 1988, 92, 2858-2862
EXAFS: A New Approach to the Structure of Micellar Aggregates E. Giglio, S. Loreti, and N. V. Pavel* Dipartimento di Chimica, Universitli di Roma "La Sapienza", 00185 Roma. Italy (Received: August 18, 1987; In Final Form: December 3, 1987)
A helical model was previously proposed for sodium (NaDC) and rubidium (RbDC) deoxycholate micellar aggregates in aqueous solutions. SAXS, NMR, ESR, and CD meehurements verified the helical model. Here we report an EXAFS study by the sphericalwave formalism on the coordination of the Rb' ions in the crystal, macromolecular fiber, and micellar aqueous solution of RbDC together with measurements carried out on rubidium oxalate (crystal and aqueous solution). The results show that the Rb" ions of the micellar aggregates have the same (or very similar) peculiar coordination as that inside the helices of the crystal and macromolecular fiber. Moreover, their coordination remarkably differs from that observed in the aqueous solutions of rubidium oxalate, where the situation would be similar to that of a classical micelle. Thus, the helical model is unquestionably confirmed.
Introduction factorily confirmed that the helix describes very well the NaDC micelle. The experimental results allowed the rejection of another The study of the aqueous micellar solutions has not provided model proposed by Sma118,9and currently accepted for the bile so far unambiguous structural models owing to the considerable salt micelles. In this model, as in those of the classical micelles, difficulty connected with the interpretation of the experimental the counterions are dipped in water, outside the micelle. data obtained from the liquid state. Therefore, we attempted to Since the helical model is unique in having the counterions inside infer sufficiently precise models from the solid state, identifying the micelle and, what is more, presents a very peculiar coordination surfactants that increase the micellar size by varying parameters of Rb" ions (see Figure 2), surely different from that occurring such as ionic strength, pH, and temperature up to the formation when the cations are outside the micelle, it was obvious to resort of a gel. From this gel a macromolecular fiber can be drawn that to a technique that is able to distinguish the local environment, transforms, sometimes, into crystals by aging. If all these solid formed by the nearest-neighbor atoms surrounding the Rb" ions and liquid phases (crystal, macromolecular fiber, gel, and aqueous to definitively confirm the helical model. This technique was micellar solution) give rise to X-ray diffraction patterns with identified in the extended X-ray absorption fine structure (EXintensity maxima and minima in the same 6 regions, it is probable AFS) spectroscopy. The samples studied are as follows: RbDC that the structural unit occurring in all these phases is the same crystal (RbDCC) and macromolecular fiber (RbDCF) to establish or a very similar one. Thus, the solution of a crystal structure if the Rb' coordinations are similar in the solid phases; RbDC may provide a proper model of well-defined geometry to be in aqueous micellar solution (RbDCS, 0.30 M) to check if the checked in the investigation of the micellar systems. Rb" coordination in the solution is similar to that in the crystal, Sodium and rubidium deoxycholate (NaDC and RbDC, rethus confirming the helix as micelle; rubidium oxalate (RbOXS) spectively) were identified as surfactants satisfying the abovementioned requirement^.'-^ X-ray and calorimetric s t u d i e ~ ' - ~ > ~ in aqueous solution (0.15 M) to determine if the Rb" coordination is different in water from that in the helix; rubidium oxalate crystal carried out on NaDC and RbDC crystals and fibers showed that (RbOXC) to verify the phases calculated for the atomic types the structural unit is a helix (Figure 1) with the lateral surface necessary to investigate the RbDC samples. The rubidium comcovered by nonpolar groups and with its inside filled with cations pounds were analyzed instead of those with sodium since the surrounded by oxygen atoms belonging to water molecules and K-edge absorption of this ion is low and cannot be studied in hydroxylic groups (Ow) and to carboxylic groups (O0.>,hereafter aqueous solution. indicated as Oc). The structure of the helix, which is strongly stabilized by ion-ion, ion-dipole, and hydrogen bonding interExperimental Section actions, is surprisingly the opposite, like in the case of an inverted Materials. RbDC was prepared by adding to deoxycholic acid micelle, of that proposed for a classical micelle (inside nonpolar (Calbiochem) a little less than the equivalent amount of RbOH and outside polar). Furthermore, the model could hardly explain (Ventron) solution and by filtering the resulting su~pension.~ the solubility of this bile salt in water. However, a careful inAcetone was then added until the solution became cloudy. Crystals spection of the helix shows that large empty regions occur between of 3RbDC.10H20 were obtained by cooling. The fibers were adjacent deoxycholate anions, so that the polar groups can be prepared by adding H I (0.03 M) to a RbDC aqueous solution approached by the water molecules of the solvent toward the lateral (0.3 M) until a gel was formed.' The gel was drawn out into glassy surface in addition to the bases of the helix. and brittle fibers. I- was oxidized to I2 and removed from the In order to verify the validity of this model in aqueous micellar solution by use of oxygen to prevent the presence of RbI in the solutions, we performed circular dichroism5 (CD), nuclear magfibers. RbDCS was prepared by dissolving RbDC crystals in netic r e s ~ n a n c e(NMR), ~.~ small-angle X-ray scattering (SAXS), doubly distilled water. Rubidium oxalate monohydrate was and electron spin resonance7 (ESR) measurements, which satisprepared by adding oxalic acid to an aqueous solution of Rb2C0, (Fluka) at 60 OC. The crystals grew by evaporation after cooling. (1) Campanelli, A. R.; Ferro, D.; Giglio, E.; Imperatori, P.; Piacente, V. The crystal structure'" was confirmed by an X-ray diffraction Thermochim. Acta 1983, 67, 223. powder pattern. A sample of rubidium metal (Fluka) was used (2) Campanelli, A. R.; Candeloro De Sanctis, S.; Giglio, E.; Petriconi, S . for the calibration of the rubidium K-edge energy in the meaActa Crystallogr., Sect. C Cryst. Struct. Commun. 1984, 40, 631. (3) Conte, G.; Di Blasi, R.; Giglio, E.; Parretta, A,; Pavel, N. V. J . Phys. surements. Since it decomposes rapidly in the presence of moisture Chem. 1984.88, 5720. and ignites spontaneously in oxygen, it was encased in eicosane (4) Candeloro De Sanctis, S.; Imperatori, P.; Campanelli, A. R.; Piacente, and handled in drybox containing argon. V. Thermochim. Acta 1984, 77, 7 7 . ( 5 ) D'AIagni, M.; Forcellese, M. L.; Giglio, E. Colloid Polym. Sci. 1985, 263, 160. (6) Esposito, G.; Zanobi, A,; Giglio, E.; Pavel, N. V.; Campbell, I. D. J . Phys. Chem. 1987, 91, 8 3 . (7) Esposito, G.; Giglio, E.; Pavel, N. V.; Zanobi, A. J . Phys. Chem. 1987, 91. 356.
0022-3654188 , ,12092-2858SO1SO10 I
( 8 ) Small, D. M. Adu. Chem. Ser. 1968, 84, 31. (9) Small, D. M.; Penkett, S. A,; Chapman, D. Biochim. Biophys. Acta 1969, 176, 178. (10) Pedersen, B. F. Acta Chem. Scand. 1965, 19, 1815.
0 1988 American Chemical Societv -
The Journal of Physical Chemistry, Vol. 92, No. 10, 1988 2859
Structure of Micellar Aggregates
1
A
0.4
I " " " ' ~ ' ' '
a
0.2 N
24
s
0.0
Y -0.2
o Oxygen of water
0.1a
Figure 1. View of the RbDC helix along the helical axis. Full and broken
lines represent the strongest ion-ion interactions and the hydrogen bonds, respectively. The ion-dipole interactions are omitted.
0.100 Eo.075
0.050 0.025
0.000
2
R(1)
*
Figure 3. RbOXC x(k)k2versus k: (a) experimental spectrum (-) and calculated points (b) as in a but filtered (-); (c) FT versus R (-e.);
experimental (-), filtered (---), and calculated
Figure 2. View along the helical axis of the coordination spheres of the three Rb' ions belonging to the asymmetric unit in the RbDC crystal structurc2 O1-OI1are Ow; 025 and 0 z 6 are oxygen atoms of hydroxylic are oxygen atoms of carboxylic groups. The nugroups; Oz7and 028 merical values represent the Rb'. e 0 distances in angstroms.
.
EXAFS Measurements. The EXAFS spectra were recorded in transmission mode on X-ray beam line BX1 by using a synchrotron radiation produced at the Adone wiggler facility" (PWA Laboratory, Frascati) with a ring energy of 1.5 GeV and ring currents between 10 and 85 mA. The radiation was monochromatized with a Si( 11 1) and Si(220) channel-cut crystals.
Results and Discussion The K-edge absorption spectra of rubidium compounds were affected by multielectron excitation,I2 giving rise to a peak at k (11) Burattini, E.; Reale, A,; Bernieri, E.; Cavallo, N.; Morone, A.; Massullo, M. R.; Rinzivillo, R.; Dalba, G.; Fornasini, P.; Mencuccini, C. Nucl. Instrum. Methods 1983, 208, 91. (12) Bernieri, E.; Balerna, A,; Mobilio, S.; Burattini, E. In Synchrotron Radiation at Frascati: 1986 Users Meeting; Conference Proceedings of Italian Physical Society; Mobilio S . , Patella F.; Stipcich S., Eds.; Italian Physical Society: Bologna, 1986; Vol. 5, p 65.
(-.a).
about 6 A-' followed by a change of slope in the curve after the peak. A less evident change of slope occurs at k about 8.6 A-'. These effects in the EXAFS region introduce anomalous frequencies that affect the process of extraction of x ( k ) . For this reason, the absorption spectrum of the gas krypton was resorted to, which is isoelectronic with Rb' and presents a similar structure above the K-edge, interpreted as due to multielectron transition^.'^ By use of a program kindly supplied by Dr. S. Mobilio, the atomic background of Rb' was simulated by means of spline functions by taking into account the Kr spectrum for comparison. This method has permitted the elimination of, in part, the anomalous frequencies and, in consequence, the reduction of the low-distances "nonstructural" peaks which slightly convolute with the first shell peak in the Fourier transform (FT). The calculation of the phase shifts 6l (lmx = 12) for the excited Rb' and for the scattering atoms Rb+, Ow, Oc, and C was performed in the RbOXC by using the MUFPOT program and the SRS Clementi atomic wave functions data baseI4 kindly allowed by S E R C Daresbury Laboratory. In addition, the 6 , of the excited Rb' was compared for safety's sake with those obtained from the known crystal structures of R b 2 0 , Rbz02, and RbI.IS All the 6]'s were nearly equal. This analysis showed also that the backscattering phase shifts move toward higher k , increasing the charge on the oxygen atom, and that the difference between Ow and Oc, although slight, is meaningful. All these phase shifts were satisfactorily checked in the known RbOXC crystal structurelo by computing X ( k ) (see Figure 3) with the version of the exact curved-wave single-scattering theory for EXAFS calculations16valid for amorphous and (13) Bernieri, E.; Burattini, E. Phys. Reu. A 1987, 3.5, 3322. (14) Pantos, E.; Firth, G. D. The EXAFS Database: Proposed Specifcation; Daresbury Internal Report DL/CSE/TM21, Oct 1982. (15) Wyckoff, R. W. G. In CrystaIStructures; Interscience: New York, 1968; Vol. 1. (16) Lee, P. A.; Pendry, J. B. Phys. Rev. B 1975, 11, 2195.
2860 The Journal of Physical Chemistry, Vol. 92, No. IO, 1988
Giglio et al. 0.4k'
"
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'
I
'
'
'
'
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0.1
-0.1
-0.2
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-o.2 0 IO
0.06
0.04
0.02 0.00
Figure 4. RbDCC x(k)k2 versus k: (a) experimental spectrum (-) and calculated points (b) as in a but filtered (-); (c) FT versus R experimental (-), filtered (-- -), and calculated
Figure 5. RbDCS x(k)k2versus k: (a) experimental spectrum (-) and calculated points (b) as in a but filtered (-); (c) FT versus R experimental (-), filtered (- - -), and calculated (. .),
polycrystalline systems." The corresponding calculations were carried out by means of the EXCURVE86 program package developed by N . Binsted and adapted for VAX computer by J. w. Campbell at the Daresbury Laboratory. The parameters used in the calculation are reported in Table I together with those for all the samples analyzed in this work. RbDCC, RbDCF, and RbDCS have the same parameters. A peculiar feature is the high number of shells considered in the case of RbOXC, RbDCC, RbDCF, and RbDCS. In fact, several different distances in a broad distribution are present in these systems, and, unfortunately, the grouping of some of them in order to reduce N did not allow us to obtain a satisfactory fit of the experimental spectrum and of its FT. This situation is complicated still more by the occurrence of different atomic species a t approximately the same distance. Moreover, the calculations carried out on RbDCC, RbDCF, and RbDCS were accomplished by giving occupation factors of 1 and 0 to Ol0and OI1,respectively, instead of 0.5, since these two atoms are about 1.3 8, apart and, hence, cannot be present simultaneously in one asymmetric unit of the RbDC crystale2Ol0 was preferred since it has a lower thermal parameter and a more plausible coordination than those of OIL. The results obtained for RbDCC are shown in Figure 4. These are very similar to those of RbDCF, which are not reported, therefore, for the sake of simplicity. Of course, the spectrum and the FT calculated by assuming the Rbf coordination found in the crystal2 satisfactorily agree with the experimental ones. However, by inspection of Table I it appears that, with a few exceptions, only the distances among Rb+ ions and the first-neighbor atoms are considered, the range being 2.1-3.7 A. This can be accounted for by the very irregular coordination of the three independent Rb+ ions, that must be taken into account in the calculations,
resembling an amorphous structure. For this reason the observed spectrum shows a meaningless signal-to-noise ratio at high k values, and, therefore, it is not possible to increase the number of parameters used in the fit of the spectrum by including those corresponding to the furthest atoms. However, it must be stressed that the high sensitivity of EXAFS to short-range disorder constitutes a good advantage in the study of the amorphous state and, in particular, in checking a very peculiar model. RbDCS practically behaves as RbDCC (see Figure 5 ) . The fits of the experimental spectrum and FT were performed by using the same parameters as for RbDCC, supposing that the same (or a very similar one) structural unit, the helix, is present both in the crystal and in the aqueous micellar solution. This assumption is strongly supported by the close similarity of the FT of the RbDCC and RbDCS experimental spectra (see Figures 4 and 5 ) in the most meaningful range, which is about 1-4 A. The agreement is satisfactory as in the case of RbDCC. Furthermore, the refinement procedure accomplished with EX CURVE^^, starting with the RbDCC parameters of Table I ( R and DW), give rise to insensitive and meaningless changes for RbDCS. Contemporaneously, a run was carried out introducing a 15th shell of Ow and refining n, R, and DW of this shell, with the aim of verifying a sensitive contribution of Rb' ions outside the micelles in the bulk aqueous medium. The best fit was achieved with n = 0.1, and, therefore, the contribution of Rb+ ions outside the helices is negligible. The spectrum of RbOXS was recorded to reinforce this point. The Rb+ ions are surely dipped in the aqueous medium and surrounded by water molecules, even though some few short interactions with the oxygen atoms of the carboxylic groups cannot be excluded. This system mimics that formed by the micelles of which protrude the carboxylic groups toward the solvent molecules. The observed spectrum and that computed by refining n, R , and DW are shown in Figure 6 together with their FT. The
(..e);
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~
(17) Gurman S . 1984, 17, 143.
J.; Binsted, N.; Ross, I . J . Phys. C: Solid State Phys.
(*.a);
Structure of Micellar Aggregates
The Journal of Physical Chemistry, Vol. 92, No. I O , 1988 2861
TABLE I: Parameters Used in the Calculations" RbDCC, RbDCF, RbDCS N T(n) R DW 2.70 (2.66) 1 2 2.80 (2.85) 2.92 (2.94) 3 2.98 (2.91) 4 5 3.05 (3.03) 3.06 (3.00) 6 7 3.10 (3.06) 8 3.10 (3.10) 3.16 (3.11) 9 3.31 (3.32) 10 11 3.42 (3.36) 3.54 (3.53) 12 3.68 (3.72) 13 3.77 (3.74) 14 15 16 17 18 19 20 21
RbOXC
T(n)
0.014, 0.014, 0.022 0.001, 0.005, 0.002
FIC FId
RbOXS DW 29 12 27 14 20 32 20 34 33 28 27 33 31 29 48 33 22 16 17 22 43
R
2.90 (2.97) 3.04 (3.03) 3.17 (3.isj 3.39 (3.30) 3.50 (3.50) 3.58 (3.56) 3.92 (3.90) 4.14 (4.27) 4.35 (4.35) 4.37 (4.38) 4.57 (4.56) 4.71 (4.68) 5.13 (5.08) 5.13 (5.02) 5.14 (5.18) 5.40 (5.42) 5.69 (5.57) 5.69 (5.71) 5.88 (5.84) 5.94 (5.99) 6.14 (6.03)
T(n) .. Ow(2.3) Ow(3.9) oWi4.0j Ow(4.0)
0.020 0.006
DW 33 35 36 42
R
2.71 2.84 3.72 4.05
0.014 0.005
a N is the shell number, T and n indicate the atomic type and the number of atoms, respectively, R is the shell radius in angstroms, whereas the value in parentheses represents the observed one; DW is the Debye-Waller factor in 103A2(2a2). FI represents the fit-index parameter (k2 weighting). bThis shell contains one Ow treated as Oc in order to decrease the number of shells. 'Experimental spectrum. dFiltered spectrum.
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