2982
J. Phys. Chem. 1994,98, 2982-2990
XAS Study of Solubilization Loci of Brominated Molecules in Aqueous Micellar Solutions I. Ascone,t P. D'Angelo,* and N. V. Pavelo** LURE (Luboratoire CNRS, CEA, MEN) Bat. 209D, Universitb Paris-Sud, 91405 Orsay Cedex, France, and Dipartimento di Chimica, Universitb degli Studi di Roma "La Sapienza", P.le Aldo Mor0 5, 00185 Rome, Italy Received: September 13, 1993; In Final Form: January 2, 1994'
X-ray absorption measurements performed on various systems containing brominated hydrocarbons showed remarkable differences in the bromine K-edge spectra recorded in polar or nonpolar media. For this reason, the brominated hydrocarbons can be used to monitor the polarity of the medium in which they are buried. By determining the coordination of the bromine atom, information on the interactions between the probe molecule and several systems such as micelles, macromolecules, membranes, solvent molecules, and host molecules in inclusion compounds can be obtained. Brominated hydrocarbons in aqueous micellar solutions of sodium and rubidium deoxycholate and sodium dodecyl sulfate have been investigated by means of the XAS technique. Experimental and calculated EXAFS spectra and Fourier transform functions are presented. The results are supported by the XANES experimental spectra. As already found for 2-bromopropane, the polarity of the solubilization loci of bromoethane in sodium and rubidium deoxycholate micellar solutions decreases by increasing the solute concentration. On the contrary, in micellar solutions of sodium dodecyl sulfate, these molecules are embedded in a more apolar environment. The polarity of the spectra of 2-bromopropane changes with the probe concentration, while bromoethane presents a marked apolar coordination which seems to be independent from the probehost ratio. The bromine intermolecular coordination in 1-bromobutane has been found to be apolar in both classes of surfactants. Functionalized surfactants with -ene, -yne, or bromo as the terminal group of the alkyl chain were used by different researchers to investigate micellar structures. In fact, for a micellar aggregate with a hydrocarbon core and an outer region containing the polar heads, the chain terminal group may be buried in the micelle core or placed in the head group region of the micelle, in contact with water. In the latter case, the alkyl chain flexibility may give rise to a chain folded conformation which increases the probability of finding the terminal group a t the micelle-water interface. EXAFS data analysis of aqueous micellar solutions of sodium and rubidium 12-bromo dodecyl sulfate and sodium 1 I-bromoundecanoate at the Br K-edge has been accomplished. The chemical constitution of the locus of solubilization of the terminal bromine has been determined, and it has been found to be apolar for the dodecyl sulfate salts and polar in the case of sodium 1 1-bromoundecanoate.
Introduction The study of the physicochemical propertiesof self-assembled surfactant aggregates is a subject frequently present in the literature. Due to their solubilization properties, the micellar systems have several applications;thus, a careful investigation of these properties is of paramount interest. The locus of solubilization of different compounds within the micellar systems can be correlated with the structural organization of the aggregates. Generally accepted 'picture models" of micellar aggregation suggest globular, rod, or disklike, lamellar and vesicular primary shapes. Probe molecules solubilized in surfactant aggregates are employed to obtain structural information by means of different spectroscopic techniques such as fluorescence, electron spin resonance, nuclear magnetic resonance, and circular dichroism. In most cases, the interpretationof the spectroscopicdata is carried out by assuming a definite shape of the host assembly. In the case of surfactants with a polar head and an alkyl chain a less defined but more generalized pictoral description may be inferred by considering aggregates with hydrophilic or hydrophobicregions with not well-defined boundaries. These vague boundaries may be explained by the presence of both a permeable micelle-water interface, which allows some water molecules to penetrate into the hydrophobic region, and a conformational mobility of the alkyl chains which can give rise to folded conformations. The chain folding can draw the terminal groups at the micellewater UniversitC Paris-Sud. Studi di Roma "La Sapienza". Abstract published in Aduunce ACS Absfructs, February 1, 1994.
t Universita degli
0022-3654f 94f 2098-2982304.50f 0
interface, a region which can be more appropriately defined as a water-rich Stern region.' Extended X-ray absorption fine structure (EXAFS)and X-ray absorption near-edge structure (XANES) spectra are sensitive to the local coordinationof an excited atom or ion. In a previous application, the EXAFS technique was used toverifyif the peculiar Rb+ coordination found in the rubidium deoxycholate (RbDC) crystal were the same as that in aqueous micellar solution.2 The results allowed us to confirm definitely that the helical model observed in the crystal adequately representsthe RbDC micellar aggregate. Subsequently, the RbDC micelle-probe interactionwas studied by investigating the Br K-edge spectra of some brominated hydrocarbons used as probes.) The bromine excited atom can be embedded in a polar medium formed by water molecules or in a nonpolar one, with methylene and methyl groups. These two situations are characterized by different coordination numbers of the bromine as well as by different Bra-0 and Bra-C interatomic distances, since the hydrogen atoms, which present a very low scattering amplitude, can be neglected. We showed that the XANES and EXAFS spectra of a brominated hydrocarbon are characteristic fingerprintsof the polarity of the medium in which these molecules are buried. By solubilizing these hydrocarbons in RbDC micellar solutions it was possible to obtain information on the interaction complexes between the probe molecules and the micellar system. The bile salts represent a peculiar class of detergents with physicochemicalpropertieswhich differ from those of alkyl chain ionic surfactants as, for example, sodium dodecyl sulfate (SDS). The micellar structure of such ionic surfactants is a topic strongly 0 1994 American Chemical Society
Loci of Brominated Molecules in Micellar Solutions debated in the literature. Moreover, end-brominated alkyl surfactants can provide information on the locus of solubilization of the terminal group. In this work we have studied, by means of the X-ray absorption spectroscopy (XAS), both the interaction of different selfassembled systems, as sodium deoxycholate (NaDC), RbDC, and SDS,with some brominated hydrocarbons and the local environment of the chain termini of end-brominated surfactants as sodium and rubidium 12-bromo dodecyl sulfate and sodium 11-bromoundecanoate (BlZSDS, BlZRDS, and BUND, respectively). It is important to outline that the XAS data analysis does not require a priori the knowledge of the micellar structure, but the results could better support one micellar model than another. The standard EXAFS data analysis takes into account only the single scattering contribution, neglecting, in most cases, the multiplc-scattering (MS) one. On the other hand, the intramolecular MS contribution could sometimes lie in the same frequency region of possible intermolecular contacts. When a strong MS contribution is present, the determination of the photoabsorber coordination parameters could be compromised. Therefore, before starting this study, we evaluated the MS contribution to the EXAFS spectra of some simple gaseous brominated hydrocarbons as bromoethane (BE), 2-bromopropane (BPZ), 2-bromo-2-methylpropane, and bromobenzene (BBZ).4 The MS contribution was found to be much higher in the case of BBZ than in the others. The intramolecular signals of BE, BP2, and 2-bromo-2-methylpropanehad differentamplitudesand frequencies from the intermolecular ones and, hence, the Br intermolecular contacts could be identified. On the contrary, the MS and the intermolecular contributions fell in the same frequency region in the case of BBZ, so that the intermolecular coordination could not be unambiguously defined. Therefore, end-brominatedaliphatic hydrocarbonsseem to be more suitable to be used as probe molecules solubilized in different assemblies.
Experimental seftion Materials. BE, BP2, l-bromobutane (BBl), l-bromopropane (BPI), 12-bromododecane (BDO) (Fluka purum), and NaDC (Sigma) are commercial products. BE, BP2, and BBl have been distilled before being used. SDS (Fluka) has been crystallized from a water-methanol solution. RbDC has been prepared by adding a little less than the equivalent amount of RbOH solution (Aldrich) to deoxycholic acid (Calbiochem), by filtering the resulting suspension, and by adding acetone until the solution became cloudy. RbDC crystals have been subsequently grown from the solution.2 1l-Bromoundecanoic acid is a commercial product (Sigma), and the corresponding sodium salt has been prepared by adding to the aqueous solution of the acid the equivalent amount of NaOH. Bl2SDS and Bl2RDS have been synthesized as previously described for dodecyl sulfate surfactants containing various counterions.5 Toa stirred solutionof 1OmLofether at 5 OC have been added 1 mL (0.015 mol) of chlorosulfonic acid (Fluka) and 3.9786 g (0.015 mol) of 1Zbromododecanol (Aldrich). The solution volume has been then doubled with ethanol, and a stoichiometric amount of NaOH (or RbOH) aqueous solution has been added to ensure complete neutralization. The white, heterogeneous mixture has been stirred at room temperature for 1 hand then filtered, washed repeatedly with ether, and purified by recrystallization from water at 4 OC. A critical micellar concentration (cmc) of about 6-7 mM has been determined for the Bl2SDS surfactant. The value of 70 mM has been confirmed for BUND.6 The surface tension has been measured by means of a Lauda tensiometer quipped with a platinum Du Noiiy ring. The solutions have been located in a double walled vase1 and thermostated at 30 f 0.05 OC. XAS M e " t s and c.lculntions. The XAS spectra above the bromine K-edge have been recorded in the transmission mode
The Journal of Physical Chemistry, Vol. 98, No. 1I, 1994 2983 at the synchrotronsof Frascati, LURE-Orsay, and HASYLABHamburg. All the measurements have been performed at room temperature with the exception of the spectra of the 50 and 100 mM aqueous solutions of Bl2SDS (27 "C) and Bl2RDS (40 "C). The BX1 and BX2 beam lines of the PWA laboratory (wiggler source, Adone storage ring, Frascati) were equipped with Si(220) and Si( 111) channel cut monochromatorsand ion chambers filled with krypton gas. The ring was operating at 1.5 GeV with decaying currents from 60 to 20 mA. The XAS2 beam line (bending magnet source, DCI storage ring, Orsay), was equipped with Si (31 1) channel cut monochromator, ion chambers filled with air or argon. The DCI ring was operating at 1.85 GeV with a typical current of 300 mA. The ROMO I1 beam line (bending magnet source, Doris storage ring, Hamburg) was equipped with a Si(311) double crystal monochromator controlled by a special feedback system.' The ionization chamberswere filled with argon at atmospheric pressure. The storage ring was running at an energy of 4.45 GeV with an electron current between 40 and 15 mA. The data were calibrated to the Au LII-edge at 13.73 1 keV. In the case of the ROMO I1 beam line a third ionizationchamber allowed possible shifts in energy to be checked by recording a reference substance together with the investigated sample. All the calculations have been performed using the spherical wave formalism"1° with the EXCURV88 program." Resulb and Discussion
In a previous paper3 we showed that the Br K-edge absorption spectra of some brominated hydrocarbons, such as BE or BP2, present different features if they are recorded in a polar or in a nonpolar solvent. Thus, we were able to identify, by a direct inspection of the XANES region, the polarity of the medium in which these molecules are buried. In particular, the feature which distinguishes a 'polar" from a 'nonpolar" spectrum is the marked shift of the first oscillation peak toward the continuum threshold and the corresponding increasing of the oscillation frequency. This can be verified, for example, by inspection of the BE spectra recorded in n-hexane and in water reported in Figure 1. As shown elsewhere,) the polarity of the medium experienced by the photoabsorbercan be correlated with the energy difference between the first two maxima of the spectrum (A,??), excluding the pre-edge peak. By passing from n-hexane to water, A,?? changes from 8 to 16 eV in the cases of both BP2 and BE. The way of the XAS study of the brominated probe-micelle interaction was opened by employing small probe molecules without conformational flexibility and a bile salt as surfactant. In the present work BBl has been used as the probe molecule together with BE and BP2. The use of a linear brominated hydrocarbon isjustified by its similarity to the n-alkanes generally solubilizedby surfactants. In addition,while BE and BP2 dissolve in water up to a concentration of 80 and 60 mM, respectively, BB1 is practically insoluble in water. This compound, as all the linear end-brominated hydrocarbons, gives rise to conformers with a short B r 4 3 intramoleculardistance. In particular Aksnes and StprgHrd calculated the conformational energy of BB1 as a function of two rotationangles, considering steric and electrostatic interactions plus torsional terms.I2 Two energy minima show a B r 4 3 distance of about 3.4 A which contribute to the total population with a molar fraction of 0.66. Also the EXAFS analysisof a 150 mM solution of BB1 in n-hexane showed a Br-C distance of 3.3 A with a coordination number of 0.6 (see ref 3 and Table 1). This Br-C3 intramolecular contact gives rise to an oscillation frequency similar to that of the Br-O first shell in water. For this reason the determination of the intermolecular contactsby means of the EXAES technique becomes more difficult for this class of molecules. In fact, the carbon and oxygen amplitudes and backscattering phase shifts are similar, and it is
2984 The Journal of Physical Chemistry, Vol. 98, No. 11, 1994
LJ 13450
13550
13500
13800
E (4 Figure 1. X-ray absorption spectra (arbitrary units) at the bromine K-edge: (a) BPI in water (20 mM); (b) BE in water (80 mM); (c) BP1 inn-hexane(l50mM);(d) BEinn-hexane(150mM);(e) BBl inn-hexane (1 50 mM).
TABLE 1: Parameters Used in the Calculationsa BE, 80 mM in water
BE, 150mM inn-hex BP2,40 mM in water
C* C* 0 0
C* C*
c
C* C* 0 0
BP2, 150mMinn-hex
C* C* C
BBl, 150mM inn-hex
C* C* C* C
DWAE
N
R
1.0 1.0 3.8 6.2 1.0 1.0
1.90 2.80 3.32 3.58 1.91 2.83 3.94
1 7 26 50 1 11 80 3
2.83 3.38 3.68 1.92 2.83 3.87 1.89 2.82
7 27 60
4.9 1.0 2.0 3.9
4.7
1.0
2.0 5.9 1.0
1.0 0.6 3.9
1.93
3.29 3.90
1 6
80 1 12 27 80
FI
16
0.001
8
0.002
16
0.003
8
0.003
9
0.002
" T and N indicate the atomic type and the number of atoms, respectively,R is the shell radius (A), DW is the Debyc-Waller factor ( ~ 1 0 3 )(A2) (2.9, AE is the energy difference between the first two maxima of the absorption spectrum, excluding the pre-edge pcak (eV), and FI represents the fit index parameter (!?weighting). C*represents a carbon atom of the brominated hydrocarbon. very difficult to discriminate between these two atoms when they are located at the same distance. Nevertheless it is possible to understand if the first intermolecular coordination shell is due to water or to the methyl or methylene groups of the micelle since the coordination distance is different in these two cases. In accordance with the sum of the van der Waals radii a Br-methyl (or methylene) distance of about 3.9 A and a Br-0 distance of about 3.3 A are expected. In the case of end-brominated hydrocarbon chains with more than three C atoms, owing to the Br-C3 intramolecular contribution at 3.3-3.4 A, an accurate determination of the Br-O coordination number at that distance cannot be obtained. For this reason, it is important to verify if the XANES region of the spectrum is sensitive to the polarity of the solvent. To this end, we have recorded the XAS spectra
Ascone et al. of BP1 in water and n-hexane, since it is a compound slightly soluble in water (about 20 mM). As shown in Figure 1, the first oscillation peak after the white line of the spectra of BP1 and BE in water falls quite at the same energy. On the contrary, in a nonpolar solvent such as n-hexane, the BP1 first peak lies at lower energy and the trend of the spectrum is similar to those of BB1 and BE in n-hexane. The spectra of BP1 in water and n-hexane are characterized by AE values of 16 and 9 eV, respectively. As a result, we can assess that also for brominated linear hydrocarbons,theXANES region is sensitivetoshort (3.33.4 A) Br-O contacts (and therefore to a polar environment), in spite of the presence of the Br-C intramolecular contribution at about 3.3 A. It is important to stress that the Br coordination in the systems analyzed in this work presents two kinds of contributions: a known intramolecular ordered contribution and an intermolecular disordered one. The main parameters describing the intermolecular contacts in Gaussian approximationare the shelldistances, the coordination numbers, and the Debye-Waller (DW) factors. In particular, the last two parameters are strongly correlated, and, therefore, they can lead to significant errors when the pair correlation function is analyzed in terms of Gaussian13 instead of asymmetric peaks. For this reason the parameters obtained for the disordered contribution must be consideredonly as a coarse description of the local disordered environment around the excited atom.14 In these cases, supplementary structural information can be useful to individuate the numerical range of such parameters. Recently, it was shown that the use of a new method, which takes into account the MS and the multiple excitations, allows a better accuracy on the XAS results to be obtained.'J5 Moreover, a proper analysis of the EXAFS spectra was done for molecules in solution by performing the simulation of the disordered contribution in terms of the radial distribution function g(r), which is the only meaningful quantity describing disordered systems. This kind of analysis was accomplished for RbBr, BE, and BP2 diluted aqueous solutions, on the basis of Br-O radial distributionfunctionscalculated by means of Molecular Dynamics (MD) simulations. The Br-O signal was calculated both from the g(r) itself and by means of asymmetric peaks obtained from thefirstdr) peakdeconvolution.16 In thelatter casethesimulated EXAFS spectrum was fitted to the experimental one and the parameters describing the asymmetric peaks were refined during the minimization procedure. Finally, a new g(r)was reconstructed on the basis of the EXAFS information by convolutingthe refined peaks. This work allowed us to check the Br-O coordination numbers obtained for BE in a previous work3 by means of the EXCURV88 program, where the MS and multiple excitation contributions were not considered.' The BE intermolecular contribution was fitted by means of two water shells at 3.32 and 3.59 A with 6.8 oxygen atoms altogether, whereas the average number of water molecules obtained from the g(r) first peak was about 9. The g(r) deconvolutionhelped us to have an approximate idea on the DW values of the two or three peaks which have to be used to satisfactorily represent the g(r) first peak region. The number of peaks, as lower as possible for limiting the fitting parameters, can be chosen on the basis of the g(r) first peak sharpness and other structural results, when available. The g(r)deconvolution for BEand BP2 indicatesan order of magnitude within the range 0.05-0.08 A2 (2.2) for the DW factors. Since supplementary MD information on the Br-O and Br-C pair distribution functions are not yet available for the other systems analyzed in this work, the same range of DW values has been adopted for shell distancesgreater than 3.5 A. As a consquence, the coordination of BE in water and of BE, BP2, and BB1 in n-hexane3 has been revised. The new data, reported in Table 1, seem more coherent with the close packing principle, the van der Waals distances, and the steric hindrance of the water molecules
Loci of Brominated Molecules in Micellar Solutions
The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 2985 oa 0.1
i
h
3
0.0
-0.1
t 'h I
0.16
.
.
.
.
I @
.
.
k
.
.
I
.
(1-l)
.
.
b 0.16
t 0.10 0.06 0.00
E
'
R (A)
0.w @
Figure 2. BE (80 mM) in RbDC (200 mM): (a) x(k)k2 vs k filtered (b) FT vs R experimental spectrum (-) and calculated spectrum filtered experimental (-) and calculated or methyl and methylene groups of n-hexane. However, it must bestressed that thestrong correlationamongthe fitting parameters does not allow a unique unambiguous set of data to be inferred; this indetermination can be partly removed on the basis of physical considerations. The greater coordination number of the shells at about 3.3-3.4and 3.6-3.7AforBEandBP2inwater,ascompared with that reported in Table I of ref 3, is in better agreement with that obtained from MD calculations (10.0 and 8.6 against 9.1 and 9.7 for BE and BP2, respectively), even though the shells used in the two methods are described by different functions and the distribution function and the g(r) have a slightly different upper limit of integration. The assumption of similar DW factors for the carbon atoms of n-hexane increases the low bromine coordination numbers, previously obtained for the BE, BP2, and BB 1 solutions, giving more realistic values. Aqueous micellar solutions of NaDC and RbDC were previously studied employing BP2 as probe m ~ l e c u l e .The ~ structures of the RbDC and NaDC micellar aggregates are known and were proved by small-angle X-ray scattering17and EXAFS measurements.2 The micelles of these two salts are very similar helices with the lateral surfaces covered by nonpolar groups and the interior part filled with cations surrounded by water molecules.1&20 Ion-ion or ion-dipole interactions between alkali metal and carboxylate ions or water molecules and a close net of hydrogen bonds give rise to a verystable helix. Nuclear magnetic resonance and circular dichroism studies supported the idea that some probe molecules are solubilized in aqueous solutions of RbDC and NaDC by means of apolar interactions mainly with the two angular methyl groups of the steroid m o l e ~ u l e Potential . ~ ~ ~ ~energy ~~~~ calculations of the interaction model between the RbDC micellar aggregate and the BP2 molecule were consistent with this result, which was confirmed by EXAFS measurements above the Br K-edge of RbDC and NaDC solutions containing BP2,3 and showed that the bromine is surrounded by water molecules. In particular it was found that the polarity of the medium experienced by the Br atom decreases by increasing the BP2 concentration. This could be explained by taking into account that BP2 is soluble in water, and hence, only a fraction of the probe molecules interacted with the micelles. By increasing the BP2 concentration, the probe molecules interacting with the helices increased, promoting a progressive aggregation of the RbDC micelles. Thus, the BP2 molecules were sandwiched between nonpolar lateral surfaces. The interaction of BE and BB1 with NaDC and RbDC in aqueous micellar solutions has now been investigated. The XAS (-e);
(a-).
Figure 3. BE (120 mM) in RbDC (200 mM): (a) x(k)kz vs k filtered experimental spectrum (-) and calculated spectrum (b) FT vs R filtered experimental (-) and calculated (.-). (-e);
TABLE 2 Parameters Used in the Calculations' T
N
R
DWAE
FI
BE in RbDC (200 mM), mM
80
C* 1.0 1.90 4 C* 1.0 2.78 8 0 3.9 3.29 30 0 6.1 3.51 45 120 C* 1.0 1.91 1 C* 1.0 2.82 6 0 1.5 3.44 30 C 5.0 3.81 65 150 C* 1.0 1.89 1 C* 1.0 2.79 7 0 1.5 3.40 30 C 5.0 3.80 65 BE,80mMinNaDC(200mM) C* 1.0 1.90 1 C* 1.0 2.80 12 0 3.5 3.31 27 0 5.1 3.57 50 BBl,60mMinNaDC(200mM) C* 1.0 1.93 1 C* 1.0 2.87 12 C* 0.8 3.30 27 0 0.8 3.40 30 C 3.5 3.80 65 The symbols have the same meaning as in Table 1.
16 0.001
11 0.003
10 0.002
16 0,001
10 0.002
spectra of three different solutions (80,120, and 150 mM) of BE in RbDC (200 mM) have been recorded. Such concentrations have been chosen, bearing in mind that BE dissolves in water to an extent of 80 mM. The observed and calculated EXAFS and FT spectra of the 80 and 120 mM samples are shown in Figures 2 and 3 (the 150 mM sample spectrum is similar to the 120 mM one). The agreement between experiment and theory is generally satisfactory. All the relevant data obtained from the fitting procedure are listed in Table 2 and can be compared with those of BE in water and in n-hexane reported in Table 1. The data in Table 2 prove that, in the case of the 80 mM sample, all the BE molecules experience a completely polar environment. Notice that the Br-0 coordination of the 80 mM sample both with RbDC and NaDC is nearly equal to that of BE in water, within the uncertainty of the EXAFS technique. By passing from the 80 to the 120 mM sample, the polarity of the spectra drastically decreases. Moreover, the spectra of the 120 and 150 mM samples practically coincide. This behavior is confirmed by the AE values reported in Table 2. The nonpolar behavior of BE a t higher concentration could be explained, as in the case of BP2, by an induced aggregation of the RbDC micelles.
2986
The Journal of Physical Chemistry, Vol. 98, No. 11, 1994
Ascone et al.
a 0.1 Y A
Y
'Ei
0.0
-0.1
0.18
b: 0.10
Figure 4. BBl (60 mM) in NaDC (200 mM): (a) x(k)kz vs k filtered experimental spectrum (-) and calculated spectrum (.-); (b) FT vs R filtered experimental (-) and calculated (e-).
The BE molecules could be entrapped in hydrophobic channels formed by the nonpolar lateral surfaces of the interacting helices. The low coordination with oxygen atoms at about 3.4 A accounts for the BE molecules dissolved in water. The higher concentration of BE, with respect to BP2, needed to promote the aggregation of the RbDC micelles is likely due to its greater solubility in water. The NaDC micelle behaves similarly when interacting with BE, and in this connection, the coordination of a 80 mM sample of BE in NaDC (200 mM) has been reported in Table 2. Later on we have investigated the interaction of NaDC (200 mM) with BB1 (60 mM) in aqueous solution. The observed and calculated EXAFS and FT spectra are reported in Figure 4. The fitting parameters are listed in Table 2. By considering the uncertainty on the coordination numbers for this type of analysis, the existence of an oxygen coordination shell a t 3.3 8, is supported by the AE value of this spectrum, which is slightly greater than that of BB1 in n-hexane (see Table 1). Nevertheless, the trend of the spectrum and the low intermolecular coordination numbers at 3.3-3.4 A found from the EXAFS analysis prove that the Br atom is mainly embedded in a nonpolar medium. For these reasons one of the two shells at 3.3 or 3.4 A with the same coordination number of 0.8 can be reasonably ascribed to the Br-C3 intramolecular contact on the basis of the results obtained for BBl in n-hexane (see Table 1). Van der Waals energy calculations have been performed to model the possible interactions between BBl and NaDC. The interactions have been investigated in a system formed by one RbDC helix and by a BB1 molecule kept fixed in gauche c~nformation,~ without considering the aqueous medium. The RbDC helix has been chosen instead of the NaDC one since they are very similar,lg but its structure has been determined with a better accuracy than that of NaDC.18 It has been generated by taking the atomic coordinates of the anion B observed in the crystal structure1*and applying a 65 screw axis, with the hydrogen atoms at the expected positions (C-H = 1.08 A). The deoxycholate anions have been kept fixed, whereas the BB1 molecule has been moved in a right-handed rectangular frameworkOXYZ, having OY and OZ coinciding with the b and c crystal axes of RbDC, respectively. Its degrees of freedom have been the Eulerian angles, $1, $2, and $3, defined as counterclockwiserotations around OZ, OX, and OZ, respectively, and the translations tx, t y , and tz, accomplished along OX, OY, and OZ, respectively. The BB1 starting position $1 = $2 = $3
Figure!% StereoscopicviewoftheBB1-RbDCinteraction. Thediameter of the circles increases with the atomic number. The methyl group is represented by one circle.
= Oo and t X = t y = tz = 0 A corresponds to the bromine atom a t the origin, the C1 carbon atom on the OZ axis at 1.92 A, and the C3 atom at 0.000,2.493, and 2.250 A. Semiempirical atomatom potentials have been used for the hydrogen, carbon, bromine, and methyl group, which has been treated as one atom.23 For the interatomic interactions, a cutoff distance of 7 A has been assumed together with final angular and translational increments of So and 0.1 A. The six-dimensional parametric space has been explored without resorting to minimization procedures. The lowest energy minimum (-6.6 kcal/mol) has been located at $1 = -15O, $2 = 45', $3 = So, tx = 11.5 %I, t y = -2.1 A, and tz = 4.6 A. A stereoscopic view, drawn with the MolDraw is shown in Figure 5. For the sake of clarity, only the two most involved RbDC anions, related by a translation along c, are reported. In this minimum the bromine atom fills the empty room delimited by the rings A and DZ5 belonging to the two anions visible in Figure 5. The bromine nearest-neighbors non-hydrogen atoms of the anions are C1, C 15, C 16, and C 18 at distances ranging from 4.0 to 4.4 A. Since the potentials used in the calculations are hard and their minima are located at distances much greater than the sum of the generally accepted van der Waals radii of the two anions involved, the bromine coordination satisfactorily compares with the EXAFS data of Table 2 ( N = 3.5, R = 3.80 A). Moreover, a not occupied region around bromine is visible in Figure 5. Therefore, the approaching of one water molecule to bromine is justified, thus supporting the oxygen shell at 3.3 A ( N = 0.8). The next energy values of the other minima are included within the range -6.0 to -5.8 kcal/mol and are remarkably higher than the value of the deepest minimum, taking into account the low number of significant interactions between the small BB1 molecule and RbDC. These minima are characterized by the bromine atom very weakly interacting with the RbDC anions and oriented toward the solvent region, whereas the methylene and methyl groups give rise to strong attractive interactions. This suggests a bromine coordination with oxygen atoms of water molecules at about 3.3 A much greater than that observed ( N = 0.8), and hence these minima are scarcely populated. Furthermore, the AE value of 10 eV (see Table 2) is strongly in favor of an apolar environment for bromine. The BBl-RbDC interaction model provides an alternative explanation to the remarkable decreasing of the BE polarity in the 120 and 150 mM samples, because the bromine of BE could be coordinated as that of BB1. Unfortunately, potential energy calculations cannot discriminate, in our opinion, among interaction models similar to that of BP2 or BB1. In fact, the similarity between the Br...Br and CH3-CH3 potentials
Loci of Brominated Molecules in Micellar Solutions
The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 2987
OP
3
::
0.1
'Li
0.0
0.0
-0.1
-0.1
a
Y
0.1
Y
-oa
-0) O S
0.16 0.16
k 0.10
0.06 0.00
0 R (A) Figure 6. BE (80 mM) in SDS (200 mM): (a) x(k)kz vs k filtered (b) FT vs R experimental spectrum (-) and calculated spectrum filtered experimental (-) and calculated (-). (-e);
TABLE 3: Parameters Used in the Calculationsa SDS200mM
T
N
R
DW
AE
FI
C* C*
1.0 1.0 0.4 6.1 1.0 1.0 0.6 5.9 1.0 2.0 3.3 5.9 1.0 2.0 1.6 5.6 1.0 2.0 0.7 5.0 1.0 1.0 0.6 0.3
1.94 2.86 3.51 3.89 1.93 2.84 3.41 3.85 1.92 2.81 3.31 3.78 1.94 2.83 3.34 3.80 1.93 2.83 3.43 3.83 1.93 2.87 3.30 3.40 3.89
1 4 45 80 1 I 45 80 2 5 40
10
0.003
10
0.001
12
0.002
10
0.002
10
0.002
10
0.003
~~
BE, 80 mM
0 C BE, 120mM
C* C*
0 C BP2, 20 mM
C* C*
0 C BP2,60mM
C* C*
0 C BP2, 100mM
C* C*
0 C BB1,60mM
C* C* C*
0 C 0
4.1
IO 5 9 40 70 3 9 40 70 1 4 27 40 80
The symbols have the same meaning as in Table 1.
causes situations rather indistinguishablefrom an energetic point of view when bromine and methyl are exchanged. The probablelocation of BE, BP2, and BB1 within SDS, which can be considered the most representative of the vast number of anionic surfactants with a molecular structure formed by a flexible aliphatic chain a a polar head group, has then been analyzed. XAS spectra of 20, 80, and 120 mM samples of BE in SDS (200 mM) have been recorded. The behavior of the 80 and 120 mM EXAFS spectra are very similar. For the sake of brevity, the observed and calculated EXAFS and FT spectra of only the 80 mM sample are reported in Figure 6. By inspection of Table 3, where the relevant data obtained from the fitting procedure are reported for the 80 and 120 mM samples, one can see the presence of a Br-0 intermolecular shell with a very low coordinationnumber with respect to those of BE in water, NaDC, and RbDC. The low signal to noise ratio of the 20 mM sample has not allowed the EXAFS analysis to be performed. Nevertheless, a reliable A E value of 10.5 eV has been obtained in the
Figure 7. BP2 (60 mM) in SDS (200 mM): (a) x(k)kz vs k filtered (b) FT vs R experimental spectrum (-) and calculated spectrum filtered experimental (-) and calculated (.e-);
(-e).
XANES region. By comparing this value with those of the 80 and 120 mM samples reported in Table 3, it comes out that the bromine coordination does not change by increasing the concentration from 20 to 120 mM. Three different concentrations (20,60, and 100 mM) of BP2 in SDS (200 mM) have been used in order to investigate the possible locus of solubilization of this probe within SDS micellar aggregates. Figure 7 shows, as an example, the observed and calculated EXAFS and FT function of the 60 mM sample. The number of oxygen atoms slightly decreases by increasing the BP2 concentration (see Table 3). This trend is confirmed by the AE values reported in Table 3. It is relevant to underline that these values fall within the PE interval determined for BP2 in n-hexane and water. The behavior of BP2 and especiallyof BE in SDS is unexpected, and before an attempt to interpret it, some remarks on the nature of the micellar systems have to be made. When a probe molecule is solubilized in an aqueous micellar solution, it is involved in several phenomena. In particular, if the probe is solublein water, a distribution of molecules between the micelles and the solvent occurs. In addition, the probe is able to migrate among the micellar aggregates on a time scale of 1W s and senses the solvent during this migration.26 On the other hand, when interacting with the micelle, the probe may be bound to several micellar sites with different hydrophilic-hydrophobic character and its distribution can change as the ratio solubilizate/host aggregate increases. Also the structural parameters of the micelle and the intermicellar interactions are a function of the solubilizate/host aggregate ratio. The time scale of an observed single XAS event is about s, as it is correlated with the core hole lifetime of the bromine atom (2.52 eV).27 Therefore, the probe molecule senses phenomena occurring in a very short time as compared with the time scaleof thedynamicevolution of the micellar system. As a result the Br coordination determined by the EXAFS technique is an average which takes account of the intermolecular contacts originated from all the possible locations of the probe within the micellar solution. Bearing these considerationsin mind, let us consider the coordination of BP2 and BE determined by our analysis. The BP2 behavior in SDS is similar to that found in RbDC and NaDC. In particular, the variation of the spectra polarity with the BP2 concentration can be explained by a different partition of the molecules both between the micelles and the solvent, and/or between the apolar and the Stern region of the
2988
The Journal of Physical Chemistry, Vol. 98, No. 11, 1994
Ascone et al. i
t
oa
*
oa
0.1
*
a
0.1
Q
Q
A
A
0.0
-0.1
0.0
-0.1
-01 010 0.18
t
t: OJ8
0.10
0.10 0.08
0.08
0.00
R (A) e Figure 8. BBl (60 mM) in SDS (200 mM): (a) x(k)ki vs k filtered (b) FT vs R experimental spectrum (-) and calculated spectrum filtered experimental (-) and calculated I
0.00
4
(..e);
(e-).
micelles. On the contrary, the BE molecule is preferentially located within theapolar region of the SDS micelle. This situation occurs also for very low concentration of BE in spite of its solubility in water, greater than that of BP2. The low bulkiness of BE could play an important role in the easier location of this molecule among the aliphatic chains of the surfactant. The small oxygen coordination numbers prove that the bromine atoms are not preferentially adsorbed a t the micellewater interface. It is reasonable that the fraction of molecules dissolved in water is mainly responsible of the coordination with the oxygen atoms. The behavior of BB1 in SDS has been considered by studying a 60 mM sample of BBl in SDS (200 mM). The EXAFS and FT spectra of this sample are reported in Figure 8. As it is evident from the fitting results reported in Table 3, this molecule mainly experiences a nonpolar medium when solubilized in a SDS micellar solution. As in the case of BB1 in NaDC, an oxygen shell a t about 3.4 A has been included in the fitting on the basis of the AE value. The low coordination with oxygen atoms allows one to assess that the bromine atom spends a considerable amount of time in a hydrophobic micellar environment and it does not soak in water a t the micelle-water interface. The XAS data analysis of BB1 in NaDC and SDS helped us to approach the study of terminally brominated surfactants as B12SDS and B12RDS. Terminally functionalized surfactants have been previously employed as probes to study water penetration inside micellar aggregates.28.29 Terminal groups as -ene, -yne, and bromo were used to understand whether such groups were buried in the micellar core or placed near the head group region of the micelles (see ref 6 (Shobha and Balasubramanian)). Menger showed that olefinic and acetylenic chain termini reside for a great percentage of time in an aqueous region. Shobha and Balasubramanian compared the chemical shifts of terminal segment protons of sodium 10-undecenoate and sodium 10undecynoatesolubilized in micelles of surfactants with and without aromatic head groups, i.e., the pair sodium dodecylbenzenesulfonate (SDBS) and SDS or the pair cetylpiridinium bromide and cetyltrymethylammonium bromide. For the two pairs of host micelles, differences in the chemical shifts of terminal segment protons were observed. An end-brominated carboxylate surfactant (BUND) in SDBS and SDS was also studied. Also the multiplet of the -CH2Br protons was upfield shifted in SDBS as compared with SDS. On the basis of these data, Shobha and Balasubramanian deduced that these probe molecules fold and are located largely in the head group region of the micelle. In
Figure 9. BlZSDS in water (50 mM): (a) x(k)k2 vs k filtered experimental spectrum (-) and calculated spectrum (-); (b) FT vs R
filtered experimental (-) and calculated
oa
(e-).
LA
Figure10. BUNDin water (50mM): (a) x(k)k2vsk filtered experimental spectrum (-) and calculated spectrum (b) FT vs R filtered (..a);
experimental (-) and calculated
(e-).
a successive ESR and fluorescence study it was shown that the chain folding of several probes is promoted by the micelles, while liposomes do not appear to do ~ 0 . 3 0 The XAS study of a brominated surfactant can give complementary information on the chain termini interactions, based on the determination of the polarity of the locus of solubilization of the excited atom. XAS spectra of aqueous solutions of Bl2SDS (50 and 100 mM), Bl2RDS (100 mM), and BUND (50 and 100 mM) have been recorded. In addition B12RDS (50 mM) and BUND (30 mM) have been studied in SDS micellar solutions 150 and 120 mM, respectively. In Figures 9 and 10 we report, as an example, the EXAFS and FT spectra of the 50 mM samples of Bl2SDS and BUND. The XAS spectrum of BDO (1 50 mM) in n-hexane has also been recorded. By means of the EXAFS analysis of this sample, the intra- and intermolecular coordination of a longchain brominated hydrocarbon embedded in a nonpolar medium has been defined within the uncertainties previously discussed. The parameters obtained from the analysis of all the samples mentioned above are listed in Table 4. By inspection of these data one result seems tostand out aboveall theothers: the behavior
The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 2989
Loci of Brominated Molecules in Micellar Solutions
TABLE 4: Parameters Used in the C a l ~ u h t i ~ n ~ . T N R D W A E FI BDO, 150 mM in n-hex C* 1.0 1.93 1 8 0.001 C* 1.0 2.88 7 C* 0.8 3.32 30 C BlZSDS, 50 mM in water
4.0 3.98
0 0.8 3.44 C 4.8 3.85 BlZSDS, 100 mM in water
C* 1.0 1.92 C* 1.0 2.86 C* 0.9 3.34 0 0.9 3.52 C 5.0 3.96
Bl2RDS. 100 mM in water
C* 1.0 1.92 C* 1.0 2.86 C* 0.9 3.36 0 C
BlZRDS,5OmMinSDS 150mM C*
C* C* 0 C BUND, 50 mM in water
C* C* C* 0 0
BUND, 100 mM in water
C* C* C* 0 0
BUND,30mMinSDS 120mM
C* C* C* 0 0
a The
80
C* 1.0 1.92 1 C* 1.0 2.87 10 C* 0.8 3.30 27
0.8 5.8 1.0 1.0 0.4 0.7 6.5 1.0 1.0 0.6 3.0 5.6 1.0 1.0 0.6 2.5 3.4 1.0 1.0 0.8 2.6 3.4
3.57 3.91 1.94 2.87 3.30 3.42 3.79 1.90 2.82 3.26 3.43 3.79 1.89 2.83 3.24 3.40 3.76 1.89 2.83 3.25 3.42 3.78
30 65 1 9 27 35 80 1 14 27 40 80 1 5 27 30 65 1 7 27 45 70 1 6 27 45 70 1 5 27 45 70
10 0.001
10 0.001
10 0.002
10 0.0003
14 0.001
13 0.002
13 0.002
symbols have the same meaning as in Table 1.
of the brominated dodecyl sulfate and BUND solutionsisdifferent. BlZSDS and BlZRDS show essentially a nonpolar coordination. An Oxygen at about 3'4-3*6 A to the spectrum with low coordination numbers in all these spectra. This finding is by the hE listed in which are 'lightly greater than that Of BDo in The coordination determined from the EXAFS analysis undoubtly indicatesthat the brominated chain termini are buried in an apolar region of the micelles, not being directly in contact with the polar heads. The scarce Br-O contacts are probably due to the low fraction of molecules dissolved in water ad monomers. An aqueous solution of BlZRDS in SDS with a molar ratio of 1:3 shows a behavior similar to that of the samples containing only the brominated surfactants. By considering that the molecular structure of SDS and BlZRDS are very similar, the formation of mixed micelles seems to be probable. Also in this case, the brominated chain termini prevalently sense the apolar region of the SDS micelle. The high cmc value of BUND has allowed the acquisition of transmission EXAFS spectra both below and above the cmc. As evident from the results reported in Table 4, the medium experienced by the bromine atom is essentially polar. The intermolecular coordination has been analyzed by considering twooxygen shells. Nevertheless,the second shell could be partially due to Br-C contacts also if the main contribution to the crosssection is likely due to Br-O interactions. This is confirmed by the BUND AE values which are much greater than that found for BDO in n-hexane (and that of the BlZSDS and BlZRDS solutions), but a little smaller than that of BP1 in water. A small decrease of the hE value occurs by exceeding the cmc value. This 49
indicates only a slight variation of the polarity of the spectra induced by the formation of micelles. While a strong Br-O interaction seems quite reasonable for the 50 mM solution (which is below the cmc), from the polar coordination of the 100 mM sample some conjectures on the behavior of the BUND micelles can be made. The direct information given by the EXAFS technique indicates that the brominated chain termini of BUND spend a considerableamount of time in a hydrophilic region. The behavior of BUND does not change when it is mixed with SDS in a molar ratio of 1:4 (see the parameters in Table 4). The formation of mixed micellescan be accepted on the basis of the NMR results on mixed BUNDSDBS micelles, obtained by Shobha and Balasubramanian. The EXAFS experiments alone cannot establish directly if the alkyl chains of the BUND molecules are folded or not. The brominated terminus can be located in the same space region of its own polar head group as well as in the region of another head group or in another polar region of the space. These last two possibilities do not imply the presence of a folded chain, if a disordered micellar structure is accepted. The EXAFS experiments alone cannot distinguish among different micellar models. However, we can assert that the BUND EXAFS data are in agreement with the NMR data of Shobha and Balasubramanian and with Menger's micellar picture.'J8 From the results presented here it is evident that the chain termini of BUND and BlZSDS (and BlZRDS) are buried in regions with different polarity. This difference implies an unlike micellar organization for the two surfactants. BlZSDS and B1 ZRDS may be organized in lamellar or cylindrical micelles, with the brominated termini far from the Stern region. The challenge comes in planning the employment of other techniques which can give more information on the structure of these micellar systems. Generally it is accepted that probe molecules (especially the small ones) containing polarizable groups or atoms are located in the Stern region. The behavior of a small molecule as BE in SDS seems to contradict this assertion.
Acknowledgment. We are grateful to Professor Edoardo Giglio for stimulating discussions during the work and critical reading of the manuscript and to Dr. Noemi Proietti for the assistance during the surface tension measurements. We would like to acknowledge the hospitality and technical assistance of the HASYLAB (Hamburg) and in particular Dr. Peter azler for his personal support and invaluable technical help of our research project. This work was sponsored by the Italian Nazionale delle Ricerche and by the Italian Ministero per l,Universita e per la Ricerca Scientifica e Tecnologica. References and Notes (1) Menger, F. M. Acc. Chem. Res. 1979, 12, 111. (2) Giglio, E.; Loreti, S.;Pavel, N. V. J. Phys. Chem. 1988, 92, 2858. (3) Burattini, E.; D'Angelo, P.; Giglio, E.; Pavel, N. V. J. Phys. Chem. 1991, 95, 7880. (4) Burattini, E.; D'Angelo, P.; Di Cicco, A.; Filipponi, A.; Pavel, N. V. J . Phys. Chem. 1993,97, 5486. (5) McIntire, G. L.; Chiappardi, D. M.;Casselberry, R. L.; Blount, H. N. J. Phys. Chem. 1982,86,2632. (6) Shobha, J.; Balasubramanian, D. J. Phys. Chem. 1986, 90, 2800. (7) Krolzig, A.; Materlik, G.; Swars, M.; Zegenhagen, J. Nucl. Instrum. Methods Phys. Res. 1984, 219, 430. (8) Lee, P. A.; Pendry, J. B. Phys. Rev. B 1975, 1 1 , 2795. (9) Gurman, S.J.; Binsted, N.; Ross, 1. J . Phys. C 1984, 17, 143. (10) Gurman, S.J.; Binsted, N.; Ross, I. J . Phys. C 1986, 19, 1845. (11) Binsted, N.; Gurman, S. J.; Campbell, J. W. SERC Daresbury Laboratory EXCURV88 program, 1988. (12) Aksnes, D.W.; Stagiird, J. Acta Chem. Scand. 1973, 27, 3277. (13) Hayes, T. M.; Boyce, J. B. In Exafs Spectroscopy Techniques and Applications;Teo,B. K.,Joy, D. C., Eds.;Plenum: New York, 1981;Chapter 5, p 81. (14) Crozier,E. D.; Rehr, J. J.; Ingalls, R. InX-ray absorption: principles, applications, techniques of EXAFS, SEXAFS and XANES Koningsberger, D. C., Prins, R., Eds.; Wiley: New York, 1988; Chapter 9, p 373.
2990 The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 (IS) D'Angelo, P.; Di Ciao, A,; Filipponi, A,; Pavel, N. V.Phys. Rcv. A 1993. 47. 2055. (16) DAngelo, P.; Di Nola, A,; Filipponi, A,; Pavel, N. V.; Roccatano, D. J. Chem. Phys. 1994,100,985. (17) & p i t o , G.; Giglio, E.;Pavel, N. V.;Zanobi, A. J. Phys. Chem. 1981. -- ., -91. - , -356. - -. (18) Campanelli, A. R.; Candeloro De Sanctis, S.;Giglio, E.; Petriconi, S.Acta Ctystallogr.Sect. C 1984, 40, 631. (19) Conte, G.; Di Blasi, R.; Giglio, E.; Parretta, A,; Pavel, N. V. J. Phys. Chem. 1984,88, 5720. (20) Campanelli, A. R.; Candeloro De Sanctis, S.;Giglio, E.; Pavel, N. V.;Quagliata, C. J. Xncluslon Pheom. Mol. Recognit. Chem. 1989,7,391. (21) B p i t o , 0.;Giglio, E.; Pavel, N. V.;Zanobi, A.; Campbell, I. D. J. Phys. Chem. 1981, 91, 83.
Ascone et al. (22) Campanelli, A. R.;Candeloro De Sanctu, S:, Chimi, E.;D'Ahgni, M.;Giglio, E.;Scaramuua, L.J . PhyJ. Chem. 1989, 93, 1536. (23) Pavel, N. V.;Quaghta, C.; Scaroelli, N. 2. Krlstallogr. 1976,114, 64, and refercnosr cited therein. (24) C e m , J. M. Tetrahedron Comput. Methodol. 1989, 2,65. (25) IUPAC Co"iuioa on Nomenclatureof Organic Chemirtry. Pure
Appl. Chem. 1965,Il, 1. (26) Grimer, F.; Drummond, C. J. J. Phys. Chem. 1988,92,5580. (27) h a w , M.0.;Oliver, J. H. J. Phys. Chem. Re/. Data 1979,8,329. (28) Monger, F. M.;Doll,D. W.J. Am. Chem. Soc. 19M, 106, 1109. (29) Menger, F. M.;Chow, J. F. J. Am. Chem. SIX.1983, 105, 5501. (30) Shobha, J.; Srinivor, V.;Balasubramanian, D. J. Phys. Chem. 1989, 93, 17.
