Structural Characterization of Frozen n-Heptane Solutions of Metal

The microstructure of temperature-quenched solutions of reverse micelles ... of water, surfactant counterions, and oxygen atoms of the SO3-head groups...
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Langmuir 2007, 23, 11482-11487

Structural Characterization of Frozen n-Heptane Solutions of Metal-Containing Reverse Micelles Alessandro Longo,*,† Giuseppe Portale,‡ Wim Bras,‡ Francesco Giannici,§ Angela M. Ruggirello,|| and Vincenzo Turco Liveri|| ISMN, Istituto per lo Studio dei Materiali Nanostrutturati, CNR, Via U. La Malfa 153, 90146 Palermo, Italy, Netherlands Organization for Scientific Research (NWO), 6 rue Jules Horowitz, BP220, 38043 Grenoble CEDEX, France, Dipartimento di Chimica Inorganica e Analitica “Stanislao Cannizzaro”, UniVersita` di Palermo, Viale delle Scienze, Parco D’Orleans II, 90128, Palermo, Italy, and Dipartimento di Chimica Fisica “Filippo Accascina”, UniVersita` di Palermo, Viale delle Scienze, Parco D’Orleans II, 90128 Palermo, Italy ReceiVed July 3, 2007. In Final Form: August 21, 2007 The microstructure of temperature-quenched solutions of reverse micelles formed by sodium, cobalt, ytterbium, and cobalt/ytterbium bis(2-ethylhexyl)sulfosuccinate in n-heptane has been investigated by SAXS and EXAFS. Some changes in the X-ray absorption spectra with respect to the same systems at room temperature have been observed. The analysis of the SAXS spectra leads to the hypothesis that at 77 K the closed spherical structure of reverse micelles is retained and that during the temperature quench they undergo a clustering process involving the transition from a quite random dispersion to the formation of more or less large clusters of strongly packed reverse micelles. This behavior is attributed to competitive effects caused by the temperature decrease. The prevalence of intermicellar attractive interactions with respect to Brownian motions leading to a collapse to more compact structure is in competition with the rapid decrease of reverse micelle diffusion rate involving a freezing of the local structures. In the case of cobalt, ytterbium, and cobalt/ytterbium bis(2-ethylhexyl)sulfosuccinate reverse micelles, further information from EXAFS measurements indicates that within the reverse micelle core exists a quite ordered nanosized domain composed of water, surfactant counterions, and oxygen atoms of the SO3- head groups. The conservation of local order and inverse structure during the clustering phenomenon that results from the fast freezing with liquid nitrogen of solutions of reverse micelles could have biological implications, i.e., the preservation of tissue samples at cryogenic temperatures.

1. Introduction Reverse micelles are nanometer-sized molecular aggregates that can form spontaneously when surfactants are dissolved in apolar media. Graphically they are represented as a micellar hydrophilic core consisting of the surfactant polar heads and a surrounding apolar layer formed by the corresponding hydrocarbon tails. However, this picture is incomplete if one does not consider dynamical processes such as conformational motions of both head and tails of single surfactant molecules, lateral diffusion of amphiphile within the aggregate, exchange of surfactant monomers between micelles and bulk solvent, diffusion and rotation of the entire aggregate, intermicellar encounters, and breaking/reforming of adhesive bonds between contacting micelles and intermicellar exchange of material. Each one of these processes is characterized by a time scale and gives its own contribution to the total relaxation spectrum of the system.1-4 Among the few surfactants able to form reverse micelles in apolar solvents without the addition of cosurfactants, the most * Corresponding author. Phone + 39 0916809359. Fax: +390916809399. E-mail [email protected]. † Istituto per lo Studio dei Materiali Nanostrutturati. ‡ Netherlands Organization for Scientific Research. § Dipartimento di Chimica Inorganica e Analitica “Stanislao Cannizzaro”, Universita` di Palermo. || Dipartimento di Chimica Fisica “Filippo Accascina”, Universita ` di Palermo. (1) Fletcher, P. D. I.; Robinson, B. H. Ber. Bunsenges. Phys. Chem. 1981, 85, 863-867. (2) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H J. Chem. Soc. Faraday Trans. 1987, I 83, 985-1006. (3) Lang, J.; Jada, A.; Malliaris, A. J. Phys. Chem. 1988, 92, 1946-1953. (4) D’Arrigo, G.; Paparelli, A.; D’Aprano, A.; Donato, I. D.; Goffredi, M.; Turco Liveri, V. J. Phys. Chem. 1989, 93, 8367.

widely investigated is sodium bis(2-ethylhexyl) sulfosuccinate (NaDEHSS). A detailed knowledge of the NaDEHSS aggregation behavior in most of the relevant experimental conditions has been obtained nowadays.5 Another feature of NaDEHSS is that by simply exchanging the sodium counterion with other mono-, bi- and trivalent cations an interesting class of surfactants able to form reverse micelles can easily be synthesized. Depending on the counterion nature, specialized functionalities of the resulting supramolecular aggregates can be engineered, such as enhanced luminescence of confined species, improved nanoparticles stability, and formation of orientationally constrained counterion-ligand complexes.6-9 However, the majority of studies concerning the structural properties of these systems is restricted to the liquid phase and ambient temperature/pressure. Recently, we have investigated the structure of reverse micelles formed under extreme conditions such as high vacuum10 or cryogenic temperatures. Our interest in the latter case was inspired by the increasing industrial exploitation of cryogenic techniques in biomedicine for the longterm preservation of biological materials in combination with the ability of reverse micelles to model and/or to mime some (5) Turco Liveri, V. Nano-surface chemistry; M. Dekker: New York, 2001; Chapter 13. (6) Mwalupindi, A. G.; Blyshak, L. A.; Ndou, T. T.; Warner, I. M. Anal. Chem. 1992, 64, 1840. (7) Eastoe, J.; Towey, T. F.; Robinson, B. H.; Williams, J.; Heenan, R. H. J. Phys. Chem. 1993, 97, 1459. (8) Eastoe, J.; Steyler, D. C.; Robinson, B. H.; Heenan, R. H.; North, A. N.; Dore, J. C. J. Chem. Soc. Faraday Trans. 1994, I 90, 2479. (9) Longo, A.; Ruggirello, A.; Turco Liveri, V. Chem. Mater. 2007, 19, 11271133. (10) Bongiorno, D.; Ceraulo, L.; Ruggirello, A.; Turco Liveri, V.; Basso, E.; R Seraglia, R.; Traldi, P. J. Mass Spectrom. 2005, 40, 1618-1625.

10.1021/la701974q CCC: $37.00 © 2007 American Chemical Society Published on Web 10/04/2007

Characterization of Solutions of ReVerse Micelles

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Table 1. Fitting Parameters Derived from SAXS Data Analysis of Surfactant-n-Heptane Solutions at 77 and 298 K and Fixed Surfactant Concentration ([surfactant] ) 0.2 M)

a

system

T (K)

rm (Å)

b

η

NaDEHSS NaDEHSS Co(DEHSS)2 Co(DEHSS)2 Yb(DEHSS)3 Yb(DEHSS)3a CoYb(DEHSS)5 CoYb(DEHSS)5

77 298 77 298 77 298 77 298

10.1 ( 0.2 10.8 ( 0.2 10.2 ( 0.2 9.2 ( 0.2 9.9 ( 0.2 9.4 ( 0.2 10.1 ( 0.2 10.7 ( 0.2

16 ( 1 14 ( 1 6(1 6(1 8(1 4(1 8(1 4(1

0.32 0.10 0.35 0.14 0.36 0.10 0.36 0.10

qmax (Å-1)

d (Å)

∆qmax (Å-1)

D (Å)

0.32

19.9

0.007

897

0.31

20.1

0.020

314

0.31

20.0

0.008

785

Reference 9.

fundamental aspects of biomembranes.11,12 Moreover, investigations of the processes occurring during fast freezing of microheterogeneous systems could also be of theoretical and technological interest. Information in this field is mostly obtained by freeze-fracture TEM13 and more recently by cryoTEM14 techniques. However, in these studies the preservation of the reversed micellar structure upon quenching has been dogmatically assumed and not tested empirically. We are aiming to unveil molecular processes occurring during fast freezing of surfactant-based systems to cryogenic temperatures. In this preliminary investigation, we compare the structure of temperature-quenched solutions of reverse micelles with those at room temperature. In particular, the comparison will concern the microstructure of solutions of reverse micelles formed by sodium, cobalt, ytterbium, and cobalt/ytterbium bis(2-ethylhexyl)sulfosuccinate in n-heptane. 2. Methods and Materials Sodium bis(2-ethylhexyl) sulfosuccinate (NaDEHSS, Sigma 99%) was vacuum-dried for several days. n-Heptane (Aldrich, 99% spectrophotometric grade) and cobalt(II) and ytterbium(III) nitrate (Aldrich, 99.9%) were used as received. Co(DEHSS)2 and Yb(DEHSS)3 were prepared by mixing appropriate amounts of an aqueous solution of Co(NO3)2 or Yb(NO3)3 with a 10-2 M aqueous solution of NaDEHSS. After aging for at least 2 days, the precipitate was filtered, washed several times, and vacuum-dried at room temperature.15 CoYb(DEHSS)5 was obtained by mixing equimolar amounts of Co(DEHSS)2 and Yb(DEHSS)3. The residual water content of NaDEHSS, Co(DEHSS)2, Yb(DEHSS)3, and CoYb(DEHSS)5 was estimated spectrophotometrically9 and expressed as W (W ) [moles of water]/[moles of surfactant]). The contents were 0.5, 2.8, 3.0, and 3.0, respectively. Sodium, cobalt, ytterbium, and cobalt/ytterbium (1:1) bis(2-ethylhexyl)sulfosuccinate-n-heptane solutions were prepared at fixed surfactant concentration (0.2 M). The procedure followed to quench biological materials to liquid nitrogen temperature (77 K) was mimicked by plunging each sample, contained in a cell (cell total thickness of about 5 mm) with thin Kapton walls (25 µm thickness), in a liquid nitrogen bath. On the basis of literature data, we estimate that the samples are vitrified in about 0.1 s.16 The X-ray measurements were performed on beam line BM26A at the European Synchrotron Radiation Facility (ESRF) in Grenoble. A particular beamline configuration was recently developed in order (11) Zhai, S.; Hansen, R. K.; Taylor, R.; Skepper, J. N.; Sanches, R.; Nigel, K. H.; Slater, N. K. H. Biotechnol. Prog. 2004, 20, 1113-1120. (12) Crans, D. C.; Rithner, C. D.; Baruah, B.; Gourley, B. L.; Levinger, N. E. J. Am. Chem. Soc. 2006, 128, 4437-4445. (13) Gao, H.; Li, J.; Han, B.; Chen, W.; Zhang, J.; Zhang, R.; Yan, D. Phys. Chem. Chem. Phys. 2004, 6, 2914-2916. (14) Goto, A.; Kuwahara, Y.; Suzuki, A.; Yoshioka, H.; Goto, R.; Iwamoto, T.; Imae, T. J. Mol. Liq. 1997, 72, 137-144. (15) Mwalupindi, A. G.; Blyshak, L. A.; Ndou, T. T.; Warner, I. M. Anal. Chem. 1991, 63, 1326. (16) Portale, G.; Longo, A.; Bras, W. Unpublished data.

to enable us to simultaneously perform both small-angle X-ray scattering (SAXS) and extended X-ray absorption fine structure (EXAFS) measurements.17 The beamline optics consists of a first slit, defining the incoming white beam impinging on the monochromator. The monochromator used is a Si(111) double-crystal with an energy resolution ∆E/E = 10-4. The monochromatic beam is then reflected by a vertical focusing mirror with both Si as well as Pt reflecting strips, thus enabling the removal of the higher harmonic contamination. The useful energy range of the beamline is 5-30 keV. The experimental setup will be described in detail in a forthcoming paper and will only be briefly discussed here. The primary beam intensity is measured with an Oxford Instrument ionization chamber (filled with 1 bar of 50:50 N2:He mixture). A vacuum flight tube is placed between the sample and the position-sensitive detector used for the collection of SAXS pattern. A beam stop with an integrated PIN photodiode is placed close to the exit window. The PIN photodiode is used to measure the transmitted X-rays. A 1D quadrant gas-filled multiwire detector with 200 µm spatial resolution is used to record the SAXS data.18 EXAFS data were collected at the cobalt (K-edge at 7709 eV) and at the ytterbium (LIII-edge at 8944 eV) absorption edge, respectively. The resolution of the EXAFS measurements was about 1.5 eV. The energy calibration was performed by using a standard thin Co foil for the Co-containing samples and with a Cu thin foil for the Ybcontaining samples. The SAXS data were recorded at 14 KeV using the quadrant detector and a sample-to-detector distance of around 2 m. This configuration allowed us to record SAXS data in a q range from 0.071 to 0.51 Å-1, where q is the modulus of the scattering vector defined as 4π sin θ/λ. The four observed diffraction maxima of a standard silver behenate sample were used for the scattering vector calibration.19 The samples, contained in a cell for liquids with two 25 µm Kapton windows, were maintained at 77 or 298 K using an Oxford Instrument cryostat equipped with two 100 µm Kapton windows. Acquisition time ranging from 1 to 10 min was used depending on the samples nature. Standard procedures were applied to correct the SAXS data for the air and the scattering contributions due to the sample holder windows.

3. Results and Discussion 3.1. SAXS Analysis. The comparison between the SAXS profiles of samples at 77 K with those at 298 K is shown in Figure 1. In the curves obtained from samples at 77 K, a reduction of the scattering intensity at low wave vector values and a broad maximum at about q ) 0.25 Å-1 are observed. Moreover, apart from the cobalt/ytterbium bis(2-ethylhexyl)sulfosuccinate-nheptane system, the frozen samples show a more or less prominent interference Bragg peak at about q ) 0.31 Å-1. The appearance (17) Beale, A. M.; Van der Eerden, A. M. J.; Jacques, S. D. M.; Leynaud, O.; O’Brien, M. G.; Meneau, F.; Nikitenko, S.; Bras, W.; Weckhuysen, B. M. J. Am. Chem. Soc. 2006, 128, 12386-12387. (18) Gabriel, A.; Dauvergne, F. Nucl. Instrum. Meth. 1982, 201, 223-230. (19) Huang, T. C.; Toraya, H.; Blanton, T. N.; Wu, Y. J. Appl. Crystallogr. 1993, 26 (2), 180-184.

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Figure 2. (a) k3-Weighted EXAFS data at Co K-edge of samples shown at 77 K. (b) Fourier transform of the EXAFS data at Co K-edge of samples shown at 77 K, uncorrected for phase shift.

of interference features suggests a preferred interparticle correlation distance imposed by intermicellar interactions. In spite of these differences, we have found that all SAXS spectra are well-described by a model of interacting polydisperse homogeneous scattering spheres. The sharp Bragg peak at q ) 0.31 Å-1 was fitted by mean of a Gaussian curve. The derived parameters, obtained following the fitting procedure previously described,20 are the micellar core mean radius (rm); the local volume fraction of scattering objects (η), whose deviation from the mean volume fraction is a measure of the intermicellar interactions;21 and the parameter b, which is a quantitative descriptor of the size polydispersity. Larger b values indicate lower size polydispersion.22,23 Concerning the Gaussian curve, the parameters of interest are the peak position (qmax) and its full-width at half-maximum (∆qmax). All these quantities are collected in Table 1. Let us consider first the parameters rm, b, and η. It must be stressed that, because small-angle X-ray scattering arises from the contrast between different adjacent domains, taking into account the peculiar structure of reverse micelles, the hydrophilic

micellar cores are considered to be the scattering centers.24 The data in Table 1 suggest that the mean micellar core radius and the polydispersity index of samples at 298 K are in agreement with literature data.9,25,26,27 They change little after the fast freezing, indicating that the micellar structure is only slightly affected by the process. The small changes of the micellar core radius could result from thermal shrinkage and a minor structural rearrangement of counterions and surfactant head groups within the micellar core. The size polydispersity, b, of Co(DEHSS)2, Yb(DEHSS)3, and CoYb(DEHSS)5 reversed micelles are comparable but larger than that of NaDEHSS reverse micelles, implying that the number of the surfactant alkyl chains accompanying each counterion plays a major role in determining the stability of the different populations of reverse micelles. This is most likely due to sterical interactions. On the other hand, the marked variation of the η parameter caused by the temperature decrease indicates an increase of local volume fraction of reverse micelles. This finding strongly suggests that, as a consequence of the temperature reduction, a decrease of the local intermicellar distance occurs. Taking into account that at the used surfactant concentration (0.2 M) the mean distance among the micelles is about 100 Å, the hypothesis of micelle-clustering seems plausible if one considers that during the time interval (about 0.1 s) required for the sample vitrification the mean distance travelled by aggregates

(20) Calandra, P.; Longo, A.; Ruggirello, A.; Turco Liveri, V. J. Phys. Chem. B 2004, 108, 8260-8268. (21) Persello, J.; Boisvert, J. P.; Guyard, A.; Cabane, B. J. Phys. Chem. B 2004, 108, 9678-9684. (22) Calandra, P.; Giordano, C.; Longo, A.; Turco Liveri, V. Mater. Chem. Phys. 2006, 98, 494-499. (23) Aliotta, F.; Fontanella, M. E.; Sacchi, M.; Vasi, C.; La Manna, G.; Turco Liveri, V. J. Mol. Struct. 1996, 383, 99-106.

(24) North, A. N.; Dore, J. C.; McDonald, J. A.; Robinson, B. H.; Heenan, R. K.; Howe, A. M. Colloids Surf. 1986, 19, 21-29. (25) Kotlatchyk, M.; Huang, J. S.; Chen, S. H. J. Phys. Chem. 1985, 89, 4382-4286. (26) Eastoe, J.; Steyler, D. C.; Robinson, B. H.; Heenan, R. K.; North, A. N.; Dore, J. C. J. Chem. Soc. Faraday Trans. 1994, 90, 2497-2504. (27) Eastoe, J.; Stebbing, S.; Dalton, J.; Heenan, R. K. Colloids Surf. A 1996, 119, 123-131.

Figure 1. Scattering profiles of surfactant-n-heptane solutions: (a) NaDEHSS, (b) Co(DEHSS)2, (c) Yb(DEHSS)3, (d) CoYb(DEHSS)5 (O, 77 K; b, 298 K).

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Figure 5. Fourier transform, uncorrected for phase shift, of EXAFS data at the Co K-edge (left panel) and Yb LIII-edge (right panel) of CoYb(DEHSS)5 collected at 77 and 298 K.

Figure 3. (a) k2-Weighted EXAFS data at Yb LIII-edge of the samples shown at 77 K. (b) Fourier transform of EXAFS data at the Yb LIII-edge of the samples shown at 77 K, uncorrected for phase shift.

Figure 4. Fourier transform, uncorrected for phase shift, of EXAFS data at the Co K-edge (left panel) of Co(DEHSS)2 and Yb LIII-edge (right panel) of Yb(DEHSS)3 collected at 77 and 298 K.

with a diffusion coefficient of about 10-10 m2s-1 is on the order of 1000 micelle diameters.28 Let us now consider the peak position (qmax) and full-width at half-maximum (∆qmax) values. From the qmax, the Bragg diffraction distance (d) of 20 ( 0.1 Å for frozen micellar solutions can be calculated, which is practically two times the mean micellar core radius value, while from the ∆qmax values the average domain size, D, in the range 300-900 Å, can be estimated with the equation D ) 2π/∆qmax.29 These data can confirm the hypothesis (28) Fletcher, P. D. I.; Robinson, B. H.; Tabony, J. J. Chem. Soc. Faraday Trans. I 1986, 82, 2311-2321. (29) Imae, X.; Li, T.; Leisher, D.; Lopez-Quintela, M. A. J. Phys. Chem. B 2002, 106, 12170-12177.

that fast freezing of samples can lead to the formation of more or less extended clusters of closely packed reverse micelles.30 The absence of the peak in the case of frozen CoYb(DEHSS)5n-heptane sample indicates that its fast freezing should involve the formation of too small and/or too disordered clusters. It is also worth noting that the average domain size (D, see Table 1) depends on the surfactant nature following the same trend observed in the micellar size polydispersion. In fact, an increase in size polydispersion should involve a parallel disorder increase of the resulting reverse micelle clusters and consequently a smaller domain size. 3.2. EXAFS Analysis. From the X-ray absorption spectra it is possible to obtain quantitative information on the number and distance of atomic species surrounding the absorbing atom in a limited range, up to a few angstroms. Due to this atomic species selectivity, EXAFS is a useful tool to characterize the internal structure of nanosized aggregates containing metal atoms. In this work, we use the X-ray absorbing features of cobalt and ytterbium atoms to get information on the internal structure of cobalt, ytterbium, and cobalt/ytterbium bis(2-ethylhexyl)sulfosuccinate reverse micelles. EXAFS data analysis was carried out using Viper.31 All the data were extracted using a Bayesian smoothing algorithm. The data fitting was performed in R space, windowing data from 1 to 4.3 Å. For Co K-edge, the data were weighted by k3,while for Yb LIII-edge, by k2. Considering that the Co2+ and Yb3+ counterions as well as some water molecules are inside the micellar core, we adopted the structure of CoO and Yb2O3 as reference model. On the basis of these structures, FEFF8.2 ab initio code32 was used to generate amplitudes and phase shift for the scattering paths used later during the minimization procedure. The phase shift and the backscattering amplitude were calibrated by using CoO and Yb2O3 as reference samples, with S02 and ∆E0 set to fixed values. The EXAFS analysis and the corresponding Fourier transforms performed on the Co K-edge of Co(DEHSS)2 and CoYb(DEHSS)5 solutions frozen at 77 K are shown in parts a and b of Figure 2, respectively, whereas those on the Yb LIIIedge of Yb(DEHSS)3 and CoYb(DEHSS)5 solutions at the same temperature are shown in Figure 3a,b. The structural parameters resulting from the fitting procedure of Co K- and Yb LIII-edges of the frozen samples are shown in Tables 2 and 3, respectively. (30) Gupta, R.; Muralidhara, H. S.; Davis, H. T. Langmuir 2001, 17, 51765183. (31) Klementiev, K. V. VIPER for Windows, freeware: www.desy.de/∼klmn/ viper.html; Klementiev, K. V. J. Phys. D: Appl. Phys. 2001, 34, 209-17. (32) Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D. Phys. ReV. B 1998, 58, 7565.

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Table 2. Fitting Parameters Derived from EXAFS Data Analysis at the Co K-Edge of Surfactant-n-Heptane Solutions at 77 and 298 K and Fixed Surfactant Concentration ([surfactant] ) 0.2 M)] system

T (K)

N1

R1 (Å)

σ1 (10-2 Å2)

N2

R2 (Å)

σ2 (10-2 Å2)

Co(DEHSS)2 Co(DEHSS)2 CoYb(DEHSS)5 CoYb(DEHSS)5

77 298 77 298

6.0 ( 0.5 5.7 ( 0.5 6.0 ( 0.5 5.6 ( 0.5

2.11 ( 0.01 2.08 ( 0.01 2.08 ( 0.01 2.10 ( 0.01

1.0 ( 0.1 1.0 ( 0.1 1.0 ( 0.1 1.6 ( 0.1

6.0 ( 0.5 5.7 ( 0.5 6.0 ( 0.5 6.0 ( 0.5

4.02 ( 0.02 4.07 ( 0.02 4.04 ( 0.02 3.98 ( 0.02

3.0 ( 0.1 3.0 ( 0.1 2.0 ( 0.1 2.4 ( 0.1

Table 3. Fitting Parameters Derived from EXAFS Data Analysis at the Yb LIII-Edge of Surfactant-n-Heptane Solutions at 77 and 298 K and Fixed Surfactant Concentration ([surfactant] ) 0.2 M)] system

T (K)

N1

R1 (Å)

σ1 (10-2 Å2)

N2

R2 (Å)

σ2 (10-2 Å2)

N3

R3 (Å)

σ3 (10-2 Å2)

Yb(DEHSS)3 Yb(DEHSS)3 CoYb(DEHSS)5 CoYb(DEHSS)5

77 29 8 77 29 8

9.2 ( 0.5 9.1 ( 0.5 10.0 ( 0.5 9.2 ( 0.5

2.32 ( 0.01 2.32 ( 0.02 2.33 ( 0.02 2.32 ( 0.01

1.0 ( 0.1 1.2 ( 0.2 1.3 ( 0.1 1.0 ( 0.1

6.8 ( 0.7 7.2 ( 0.7 6.6 ( 0.5 6.0 ( 0.7

3.53 ( 0.02 3.55 ( 0.02 3.54 ( 0.02 3.53 ( 0.02

1.4 ( 0.1 1.1 ( 0.1 1.0 ( 0.1 1.3 ( 0.1

9.2 ( 0.5 9.0 ( 0.5 10 ( 0.5 7.5 ( 0.5

4.35 ( 0.8 4.35 ( 0.8 4.40 ( 0.8 4.28 ( 0.7

1.4 ( 0.1 1.1 ( 0.1 1.0 ( 0.1 1.3 ( 0.1

From Figure 2b and Table 2 it is evident that the cobalt(II) local environment in Co(DEHSS)2 and CoYb(DEHSS)5 solutions at 77 K is characterized by a first shell of 6 ( 0.5 octahedrally arranged oxygen atoms at 2.1 Å and a second one at 4.0 Å constituted by Co(II) and/or Yb(III) ions, while three coordination shells are clearly detectable around the Yb(III) ion (see Figure 3b and Table 3). The first shell consists of 9 ( 0.5 oxygen atoms at 2.3 Å in a tricapped trigonal prism geometry, while the second and third shells are formed by Co(II) and/or Yb(III) ions at 3.5 and 4.3 Å, respectively. We note that the third shell is observed due to multiple scattering effects involving the Y-O-M configuration, and in order to reproduce the experimental data, it was required to include this contribution in the fitting procedure.31,32 A comparison of the fitting results reported in Table 2 with literature data shows that the Co-O distance and coordination number (N1) of the first shell are similar to the corresponding values of bulk CoO (2.13 Å, N1 ) 6), whereas the second shell is characterized by a bond length that is about 1.0 Å longer than the Co-Co distance (3.02 Å) of the reference sample and a significantly lower coordination number (6 instead of 12).33 Similar considerations hold for the samples collected at the Yb LIII-edge (see Table 3). Also in this case the structural parameters of the first shell are in agreement with those of bulk Yb2O3, but the higher shells of the samples are at longer distances and different coordination number with respect to the reference ones (Yb-O ) 2.235 Å; in this case we have considered the mean distance of the three possible Yb sites of the bixbyite structure, Yb-O ) 2.2 Å, Yb-Yb ) 3.449 Å, Yb-Yb ) 3.944 Å).34 These findings suggest that the local structure generated by Co and Yb counterions within the reverse micellar core is different from the bulk oxides, whereas the occurrence of two or three shells around them indicates that, despite their small size, the micellar cores exhibit well-defined structural order. We find that the nearest species to the Co or Yb ions are oxygen atoms, which could belong to the surfactant SO3- head group at the surface of the micellar core and/or to the water traces present in the core of reverse micelles. Considering that the amount of water present in our samples is not enough to complete the inner coordination sphere of Co and Yb ions, it can be stated that at least one of the oxygen atoms in their first shell belongs to the surfactant SO3- head group. Then, the second (33) Choi, H. C.; Lee, S. Y.; Kim, S. B.; Kim, M. G.; Lee, M. K.; Shin, H. J.; Lee, J. S. J. Phys. Chem. B 2002, 106, 9252-9260. (34) Allen, P. G.; Bucher, J. J.; Shuh, D. K.; Edelstein, N. M.; Craig, I. Inorg. Chem. 2000, 39, 595-601.

shell is constituted by Co or Yb ions bridged to the central ion by oxygen atoms belonging to SO3- head groups and water molecules. Comparing the structural parameters of samples at 77 and 298 K (see Tables 2 and 3), we can observe that, in spite of the large temperature difference, the local structure within micellar core is practically unchanged. This quite unexpected finding is emphasized in Figures 4 and 5, where a comparison between the Fourier transforms of samples at 77 and 298 K is shown. It can be noted that, apart from minor changes, the main features are maintained for all the investigated samples, and local order up to 4 Å exists also in room-temperature samples. Furthermore, for each sample, the Debye-Waller factors (σ2) at 77 and 298 K are comparable (see Tables 2 and 3). Taking into account that this parameter is a descriptor of the disorder around the absorbing atom, this peculiar result can be rationalized if one considers that the σ2 value arises from two contributions: (i) structural or static disorder due to unsymmetrical or distorted arrangement of neighboring species and (ii) thermal disorder related to the lattice vibrations and ligand exchange rate.35-37 Within the error margin, the differences in thermal disorder in samples at 77 and 298 K are negligible; therefore, we can conclude that the observed local disorder is primarily due to a structural contribution. This behavior implies that the interactions stabilizing the micellar core are stronger than the kT-dependent term and the fast freezing process preserves in the samples at 77 K the static disorder locally present at room temperature. These conclusions are consistent with the results of molecular dynamics investigations showing that in the restricted domain of the micellar core the mobility of entrapped species is strongly hindered.38

3. Conclusion The analysis of SAXS and EXAFS spectra of quickly frozen and room temperature solutions of reverse micelles provided detailed knowledge of the structure of the micellar core and of the phenomena accompanying the fast freezing of solutions of reverse micelles. While, spectral changes have been observed, the analysis of the SAXS spectra leads to us hypothesize that at 77 K the closed spherical structure of reverse micelles is retained, (35) Benfatto, M.; Natoli, C. R.; Filipponi, A. Phys. ReV. B 1996, 40, 14, 9626-9635. (36) O’Day, P. A.; Newville, M.; Neuhoff, P. S.; Sahai, N.; Carroll, S. A. J. Colloid Interface Sci. 2000, 222, 184-197. (37) O’Day, P. A.; Rehr, J. J.; Zabinsky, S. I.; Brown, G. E., Jr. J. Am. Chem. Soc. 1994, 116, 2938-2949. (38) Harpham, M. R.; Ladanyi, B. M.; Levinger, N. E. J. Phys. Chem. B 2005, 109, 16891-16900.

Characterization of Solutions of ReVerse Micelles

but during the fast freezing at liquid nitrogen temperature, reverse micelles undergo a marked clustering involving the transition from a quite random dispersion to the formation of large clusters formed by closely packed reverse micelles. It has been also found that the counterion nature has a large effect on the size polydispersity but not on the mean micellar core of reverse micelles (about 10 Å). In the case of cobalt, ytterbium, and cobalt/ytterbium bis(2ethylhexyl)sulfosuccinate, the analysis of EXAFS spectra indicates that within the reverse micellar core a nanosized quasicrystalline structure is formed composed of the surfactant counterions, water traces, and the oxygen atoms of the SO3head groups. Surprisingly, our results indicate that despite the large temperature variation, no significant change of the size and of the internal structure of the micelles occurs. Since reverse micelles can mimic some fundamental features of biomembranes, it is very useful to recognize that the local

Langmuir, Vol. 23, No. 23, 2007 11487

structure is retained during the clustering phenomena that accompany fast freezing with liquid nitrogen. This result may demonstrate the feasibility of this type of treatment for evaluating biological samples at cryogenic temperatures. Moreover, being cryo-SAXS and cryo-EXAFS appropriate techniques to study biomolecules with reduced X-ray radiation damage, the observed phenomenology could give some insight for proper data analysis. Acknowledgment. The authors thank Dr. Serge Nikitenko (BM26-ESRF) for the EXAFS measurements, Dr. Harald Mu¨ller (ESRF) for assistance in the sample preparation, and the ESRF for the use of the synchrotron facilities. The authors also gratefully acknowledge the University of Palermo and MIUR (PRIN 2006) for financial support. LA701974Q