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Electronic Structure of Water Molecules Confined in a Micelle Lattice Johan Gråsjo¨,† Egil Andersson,‡ Johan Forsberg,‡ Emad F. Aziz,*,§ Barbara Brena,‡ Christian Johansson,† Joseph Nordgren,‡ Laurent Duda,‡ Joakim Andersson,‡ Franz Hennies,| Jan-Erik Rubensson,*,‡ and Per Hansson*,† Department of Pharmacy, Uppsala UniVersity, Box 580, SE-751 23 Uppsala, Sweden, Department of Physics and Materials Science, Uppsala UniVersity, Box 530, SE-751 21 Uppsala, Sweden, Helmholtz-Zentrum Berlin fu¨r Materialien und Energie (HZB), BESSY, Albert-Einstein-Strasse 15, 12489 Berlin, Germany, and MAX-lab, Box 188, SE-221 00 Lund, Sweden ReceiVed: March 06, 2009; ReVised Manuscript ReceiVed: April 27, 2009
Oxygen K absorption and emission spectra of water molecules confined in dodecyltrimethyl ammonium chloride micelle structures are presented. The local electronic structure of the water molecules is found to be dramatically different from the electronic structure of water molecules in the gas-phase as well as in liquid water. Hybridization with states of the ions in the surrounding ions is directly observed, and evidence for stabilization of the water molecules relative to molecules in bulk water is found. Introduction The water molecule is of paramount importance for many kinds of vital processes in fields ranging from biology to environmental science. A deeper understanding of the role of water in these processes requires information about the microscopic geometry and electronic structure of the molecule. This poses a scientific challenge, partly due to the need for in situ methods able to probe local electronic structure, and partly due to the complexity of the problem: the structure of the water molecule is heavily influenced by its chemical environment, for example, due to hydrogen bonds, interactions with ions, and confinement on the nanometer scale.1 In a micelle system composed of dodecyltrimethyl ammonium (CH3(CH2)11N(CH3)3) chloride (C12TAC) and water (see Figure 1), the water content, and therefore the dimensions of the water domains, can be varied extensively while maintaining a detailed knowledge of the microstructure. This makes the system suitable as a model system for studies of water confined in structures of amphiphilic self-assemblies and charged macromolecules, omnipresent in living cells and their organelles. A better description of the water molecule in such situations may contribute to the understanding of the role of water in, for example, DNA condensation in the chromosome, protein folding, and secretion and may be important for development of biomimetic gene and protein drug delivery systems.2-6 In soft X-ray spectroscopy, transitions involving quasi-atomic core levels are used to gain information about local electronic structure, and in addition the short lifetime of the core hole states gives access to the femtosecond time scale, which is relevant for nuclear rearrangements.7 Here we present fluorescence yield * To whom correspondence should be addressed. E-mail: (E.F.A.) Emad.
[email protected]; (J.-E.R.)
[email protected]; (P.H.)
[email protected]. † Department of Pharmacy, Uppsala University. ‡ Department of Physics and Materials Science, Uppsala University. § Helmholtz-Zentrum Berlin fu¨r Materialien und Energie (HZB). | MAX-lab.
10.1021/jp902058w CCC: $40.75
(FY) soft X-ray absorption (SXA) and soft X-ray emission (SXE) spectra of water molecules confined in C12TAC structures, and interpret the experimental results in terms of density functional theory (DFT) calculations. The influence on the electronic structure of the chemical environment is directly monitored in the spectra, and we demonstrate that the valence orbitals of the water molecule are modified in a complex manner, e.g., new spectral features give evidence for strong hybridization with orbitals of the chlorine ions. Dramatic differences from the spectra of bulk liquid water show that nuclear dynamics is less important in the micelle water, suggesting a stabilizing effect of the confinement. Materials and Methods The experiments were carried out at the I511-38 beamline at MAX-lab. The C12TAC sample was placed behind an ultrathin carbon membrane and was kept under controlled humid atmosphere in a flow-system.9 The equilibrium relation between relative humidity of the atmosphere and the water content of the sample was stated with a symmetric vapor sorption analyzer (SGA 100). In the same setup, bulk liquid water measurements were performed for calibration and comparison. FY spectra were recorded using a microchannel plate detector, and the energy resolution of the monochromator was estimated to be 0.25 eV. SXE spectra were measured in the polarization direction of the incident radiation using a Gammadata Scienta XES-300 Rowland spectrometer,10 equipped with a grating of 5 m radius and a groove density of 1200 L/mm. The structure of the sample is schematically shown in Figure 1. For the higher water concentration (25 wt %), the majority of the C12TA+ ions (>99.9%) are distributed in rodlike micelles with the aliphatic chains forming a water-free core and positively charged head-groups at the micelle surface. The micelles are arranged in a hexagonal periodic lattice with the Cl- ions in the aqueous domains between the micelles. The phase belongs to the class of lyotropic amphiphilic liquid crystals in which 2009 American Chemical Society
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Figure 2. Oxygen K edge spectra of C12TAC with 25 wt % (green) and 10 wt % (red) water. The FY spectra are shown in the upper right, and the excitation energies used for the SXE spectra are indicated. The spectra of bulk liquid water (blue) are shown for comparison.
the aggregates display periodic order but the molecules are free to move as in a liquid. The structure varies with the water content,11 and for the lower water concentration (10 wt %) we expect lamellar micelles to dominate. The scaling in Figure 1 is based on small-angle X-ray scattering measurements on samples with the higher water concentration (25 wt %) from which the volume occupancy of H2O molecules, Cl-, and C12TA+ ions can be estimated.12 The majority of water molecules have C12TA+ or Cl- ions as nearest neighbors also in the case of higher water concentration. Results
Figure 1. (A) The structure formula of the dodecyltrimethyl ammonium ion (CH3(CH2)11N(CH3)3). (B) The dodecyltrimethyl ammonium ions form rodlike micelles with ammonium methyl groups at the surface. These micelles are organized in a hexagonal structure. (C) The space between the micelles is occupied by water molecules and chlorine counterions. (D) An energy optimized model configuration where the water nearest neighbors are a positively charged tetramethyl ammonium ion representing the trimethyl ammonium group and a negatively charged chlorine ion (blue, gray, red, white, and green spheres indicate nitrogen, carbon, oxygen, hydrogen, and chlorine atoms, respectively).
The oxygen K edge FY spectra of C12TAC with 10 and 25 wt % water are compared to the SXA spectrum of liquid water in the upper right part of Figure 2. Because of the low water content distortions by saturation effects in the FY, spectra13 are small and for which they have not been corrected. In the micelle spectra, the peak at 531.2 eV coincides with the 1 s-11πg peak in the SXA spectrum of molecular O2.14 Oxygen molecules are present in the humid atmosphere above the sample surface, and they may also to some extent adsorb in the sample. The FY spectrum of the oxygen molecule does not have any hidden spectral features at higher energies which are strong enough to further influence the discussion of the results. Apart from the O2 signature, the FY spectra of the wet C12TAC samples show striking similarities to the SXA spectrum of liquid water, with a pre-edge structure at around 534.5 eV, and a main-edge structure peaking around 537.2 eV. The spectrum of the sample with the lowest water content is within the experimental accuracy identical to the spectrum of liquid water. Similarities to the pure water spectrum are to be expected as H2O molecules provide the only oxygen atoms in the sample. Different chemical surroundings of the H2O molecules do, however, strongly influence the SXA spectrum, and the interpretation of the spectral differences is heavily debated.15-18 Before discussing the FY results further we turn our attention to resonantly excited SXE spectra of the same systems (see Figure 2). For the higher excitation energy (537.2 eV) we discuss five pertinent features: a broad peak at 520.5 eV, (no. 2 in Figure 2) a rather narrow peak at around 525.5 eV (no. 4), and regions with increased intensity at the low energy flanks of these peaks
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Figure 3. Predicted SXE spectra for (A) a single water molecule, (B) water with a tetramethyl ammonium ion as neighbor, (C) with a chlorine ion as neighbor, and (D) with both ions. The nuclear positions are optimized for the latter configuration. Electron density plots are included to show the orbitals corresponding to the three contributing valence orbitals in the water molecule (above) and to some representative orbitals in the complex with surrounding ions (below).
at 517.5 eV (no. 1) and 523.5 eV (no. 3). In addition we note varying intensity at the high-energy flank of feature no. 4 around 527 eV (no. 5). To analyze how the micelle environment influences the electronic structure of the water molecules, we performed model calculations by means of hybrid DFT implemented in the quantum chemical program DALTON.19 We used the B3LYP functional20 and the 6-311G** basis set for all the atoms. The nonresonant SXE spectra were obtained from the orbital energies and the transition moments calculated in the ground state electronic structure (adiabatic approximation). To facilitate the comparison with the experimental measurements, we convoluted the computed spectra by a Gaussian profile of 0.8 eV full width at half-maximum. We considered four cases and for each of them computed the nonresonant soft X-ray emission spectrum in the O 1s region. The models included are one water molecule (Figure 3A), one water in the vicinity of the headgroup of a C12TA+ (Figure 3B), a water molecule close to a Cl- ion (Figure 3C), and one water molecule close to the headgroup of a C12TA+ and a Cl- (Figure 3D). Test calculations with the inclusion of the whole C12TA+ ion gave the same results as simply considering the headgroup shown in Figure 3. The SXE spectrum of a single water molecule is dominated by transitions from the three outermost valence orbitals: the bonding 1b2 and 3a1 orbitals and the 1b1 “lone-pair” orbital to the 1a1 core orbital at 519.5, 523.6, and 525.7 eV, respectively. The spatial extent of the orbitals is illustrated by electron density maps at the top of Figure 3.
J. Phys. Chem. B, Vol. 113, No. 24, 2009 8203 In the experimental spectrum, vibronic coupling leads to extensive vibrational progressions that determine the line shapes and may also introduce shifts of the peak positions.21 Although the present model disregards nuclear wave functions, the energy spacing between the peaks correspond rather well to peak spacing in experimental gas-phase spectra,22,23 especially the predicted 1b2-1b1 split (6.2 eV) that coincides with the experimental value. The influence on the SXE spectrum due to interaction between the water molecule and the C12TA+ headgroups, represented by a tetramethyl ammonium ion, is investigated in Figure 3B. Test calculations with full C12TA+ ions (not shown) demonstrate that this replacement only has a minute influence on the spectrum. In Figure 3C, we show the predicted influence due to the interaction with a Cl- ion, and in Figure 3D the simultaneous influence of both ions is shown. The configurations in Figure 3 are all energy optimized for the latter case. As demonstrated, the interactions with the ions are predicted to change the SXE spectrum substantially. The experimental features nos. 2, 3, and 4 in the experimental spectrum (Figure 2) can be unambiguously assigned to states derived from the 1b2, 3a1, and 1b1 states of the free molecule, respectively. It is predicted that the energy spacing between the 1b2 and 1b1 peaks is reduced by 0.7 eV due to the interaction with the ions to compare with the experimental micelle spectra where this spacing is reduced by 1.2 eV from the gas-phase value. Feature no. 1 has no counterpart in the SXE spectrum of the single water molecule. The result of the simple model shows an additional faint structure at 517.5 eV due to states of primarily Cl- 3s character forming bonding combinations with the 1b2 orbital of H2O, and we assign feature no. 1 in the micelle spectra accordingly. Note that the 517.5 eV peak is absent in the spectrum where the tetramethyl ammonium ion has been removed and there is only one interacting Cl- (Figure 3C), demonstrating that the appearance of this peak must be due to complex interactions. Although we do not expect the simple model to take all the interactions in the sample into account, it allows for a general assignment of the most significant features in the experimental spectrum. However, there are notable discrepancies between the model predictions and the observations, for example, the 1b2-1b1 split reduction is larger in the experimental spectrum, feature no. 1 is much more intense in the experimental spectrum than predicted, and the predicted high-energy peak due to interaction with the Cl- 3p states is not observed in the experimental spectrum. The remaining differences between the theoretical predictions and the experimental results we attribute to the complex interactions in the confining micelle structure beyond the simple two-ion-neighbor model. Feature no. 5 in the experimental spectrum shows the only significant difference between the two water contents. The intensity of this high-energy feature increases with water concentration, a change in the direction of the spectrum of bulk liquid water. This trend suggests that the higher intensity is due an increasing number of water neighbors. Apart from this small difference the spectra of the samples with the two water concentrations are very similar, showing that although the electronic structure is strongly modified in the micelle, this modification is surprisingly well-defined, and not overly dependent on the detailed micelle structure. There are substantial differences between the micelle spectra and the SXE spectrum of liquid water. Peak no. 4 is much narrower than the corresponding peak in liquid water spectra (Figure 2). The features no. 2 and no. 3 coincide in energy with
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the peaks in the liquid water spectrum, although the peak no. 2 is narrower and feature no. 3 more intense in the micelle spectrum. Although the interpretation of the SXE spectrum of liquid water is debated,24-27 there seems to be a general consensus that the smearing out of the intensity related to the 3a1 orbital in particular is due to nuclear dynamics during the scattering process.28 In the micelle spectra, the vibrational broadening to a larger extent simulates the gas-phase case, suggesting that the confined water molecules are less subject to destabilization, which occurs in the hydrogen bond network in liquid water. This is in line with the notion that hydrogen bond dynamics is slower in confined than in bulk liquid water.1,29 We note that the energy spacing between the 1b2 and 1b1 peaks found in gas-phase SXE is essentially the same as in photoemission spectra of gas-phase water, of liquid water,30 as well as of ice.31 In a large survey of photoelectron spectra of adsorbed water molecules,32 it is found that this 1b2-1b1 splitting scatters around the gas-phase value for most substrates, and that a split as small as 5 eV is very unusual. In SXE spectra of pure liquid water24,25 and water as a solvent for ions,33 the sharp 1b1 peak has internal structure and at somewhat higher energy resolution than in the present experiment a double feature appears. The peak no. 4 in the micelle spectrum largely coincides with the low-energy partner of this double peak. We believe that the present results may help resolving the controversy over the interpretation of the spectra of bulk liquid water, but this discussion is beyond the scope of this paper. For excitations on the prepeak in the absorption spectrum at 533.7 and 534.3 eV, the (Figure 2) 1b1 derived peak broadens, mostly toward the low-energy side, and also the 1b2 and 3a1 features become smeared-out. Note that the spectra are normalized to the highest intensity, so that the apparent relative intensity enhancement in the 1b2 and 3a1 region may be partially explained by the broadening of 1b1 peak. Compared to the spectra of liquid water, the broadening of the 1b1 peak is the most apparent difference, a difference that increases with decreasing water content. To interpret this observation, we first briefly discuss the SXA spectra in the following. Because of the similarity of the SXA (Figure 2) spectra of liquid water and water confined in the micelle structure, it is tempting to assign them it in a similar way. There seems to be a consensus that the pre-edge peak originates from transitions from the core level into local 4a1-like states of the water molecule. That the excited electron for these excitation energies with a large probability stays at the core hole site during the process is obvious by inspection of the SXE spectra (Figure 2); the intensity plateau starting at the incident energy and present all the way down to the 1b1 peak can be assigned to recombination back to the electronic ground state with the energy loss due to vibronic coupling. This long tail (almost 10 eV) demonstrates that substantial energy goes into the nuclear motion, which is in accordance with the observation that the 1a1 f 4a1 excitation in the free molecule leads to ultrafast dissociation.34 The general smearing out of spectral features when exciting on the prepeak is partially a consequence of vibronic coupling, also for the water molecules in the micelle. Note that no corresponding broadening of the 1b1 peak is observed at resonant excitation, neither in gas-phase23 nor in liquid water.24,25 We believe that the additional 1b1 broadening in the micelle structure is due to selective excitation of water molecules at various inequivalent sites. Improved data quality is needed before far-reaching conclusions can be drawn on the electronic structure at such specific sites. Note that there
Letters is a faint foot structure just above feature no. 4 position, especially evident in the low-energy excited spectrum. We speculate that this intensity is due to hybridization with Cl- 3p states, predicted in the model calculation. Conclusions The electronic structure of water molecules changes dramatically when introduced into the C12TAC micelle structure. The 1b2-1b1 splitting in the SXE spectrum decreases from 6.2 eV for the free molecule to around 5 eV, and new states are formed primarily due to the strong interaction with the Cl 3s state. Furthermore, we find evidence that the molecule stabilizes relative to molecules in the hydrogen bond network of bulk liquid water. These results may be relevant, not only when considering the structure of the water molecule in the specific micelle structures studied here, but also for the structure of water confined in other complex structures of biological and/or environmental importance. Acknowledgment. This work was supported by Swedish Research Council. We are grateful for the support by the MAXlab staff. References and Notes (1) Park, S.; Moilanen, D. E.; Fayer, M. D. J. Phys. Chem. B 2008, 112, 5279. (2) Nilsson, P.; Hansson, P. J. Phys. Chem. B 2005, 109, 23843. (3) Forsman, J. Curr. Opin. Colloid Interface Sci. 2006, 11, 290. (4) Hansson, P. Curr. Opin. Colloid Interface Sci. 2006, 11, 351. (5) Hansson, P.; Schneider, S.; Lindman, B. J. Phys. Chem. B 2002, 106, 9777. (6) Leal, C.; Moniri, E.; Pedago, L.; Wennerstro¨m, H. J. Phys. Chem B 2007, 111, 5999. (7) Aziz, E. F.; Ottosson, N.; Faubel, M.; Hertel, I. V.; Winter, B. Nature 2008, 544, 89. (8) Denecke, R.; Va¨terlein, P.; Ba¨ssler, M.; Wassdahl, N.; Butorin, S.; Nilsson, A.; Rubensson, J.-E.; Nordgren, J.; Mårtensson, N.; Nyholm, R. J. Electron Spectrosc. Relat. Phenom., 101- 1999, 103, 971. (9) Forsberg, J.; Duda, L.-C.; Olsson, A.; Schmitt, T.; Andersson, J.; Nordgren, J.; Hedberg, J.; Leygraf, C.; Aastrup, T.; Wallinder, D.; Guo, J.-H. ReV. Sci. Instrum. 2007, 78, 083110. (10) Nordgren, J.; Bray, G.; Cramm, S.; Nyholm, R.; Rubensson, J.-E.; Wassdahl, N. ReV. Sci. Instrum. 1989, 60, 1690. (11) Balmbra, R.; Clunie, J. Nature 1969, 222, 1159. (12) Kang, C.; So¨derman, O.; Eriksson, P. O.; Stael von Holstein, J. Liquid Crystals 1992, 12, 71. (13) Eisebitt, S.; Bo¨ske, T.; Rubensson, J.-E.; Eberhardt, W. Phys. ReV. B 1993, 47, 14103. (14) Glans, P.; Gunnelin, K.; Skytt, P.; Guo, J.-H.; Wassdahl, N.; Nordgren, J.; Ågren, H.; Kh, F.; Warwick, T.; Rotenberg, E. Phys. ReV. Lett. 1996, 76, 2448. (15) Na¨slund, L.-Å; Lu¨ning, J.; Ufuktepe, Y.; Ogasawara, H.; Wernet, Ph.; Bergmann, U.; Pettersson, L. G. M.; Nilsson, A. J. Phys. Chem. B 2005, 109 (2005), 13835. (16) Smith, J. D.; Cappa, C. D.; Messer, B. M.; Drisdell, W. S.; Cohen, R. C.; Saykally, R. J. J. Phys. Chem. B 2006, 110, 20038. (17) Wernet, P.; Nordlund, D.; Bergmann, U.; Cavalleri, M.; Odelius, M.; Ogasawara, H.; Å, L.; Hirsch, T. K.; Ojama¨e, L.; Glatzel, P.; Pettersson, L. G. M.; Nilsson, A. Science 2004, 304, 995. (18) Wang, R. L. C.; Kreuzer, H. J.; Grunze, M. Phys. Chem. Chem. Phys. 2006, 8, 4744. (19) DALTON, a molecular electronic structure program, release 2.0 (2005); see http://www.kjemi.uio.no/software/dalton/dalton.html. (20) Becke, A. D. J. Chem. Phys. 1993, 98, 548. (21) Cesar, A.; Ågren, H.; Carravaetta, V. Phys. ReV. A. 1989, 40, 187. (22) Rubensson, J.-E.; Petersson, L.; Wassdahl, N.; Ba¨ckstro¨m, M.; Nordgren, J.; Kvalheim, O. M.; Manne, R. J. Chem. Phys. 1985, 82, 4486. (23) Kashtanov, S.; Augustsson, A.; Luo, Y.; Guo, J.-H.; Såthe, C.; Rubensson, J.-E.; Siegbahn, H.; Nordgren, J.; Ågren, H. Phys. ReV. B 2004, 69, 024201. (24) Fuchs, O.; Zharnikov, M.; Weinhardt, L.; Blum, M.; Weigand, M.; Zubavichus, Y.; Ba¨r, M.; Maier, F.; Denlinger, J. D.; Heske, C.; Grunze, M.; Umbach, E. Phys. ReV. Lett. 2008, 100, 027801. (25) Tokushima, T.; Harada, Y.; Takahashi, O.; Senba, Y.; Ohashi, H.; Nilsson, A.; Shin, S. Chem. Phys. Lett. 2008) , 460, 387.
Letters (26) Pettersson, L. G. M.; Tokushima, T.; Harada, Y.; Takahashi, O.; Shin, S.; Nilsson, A. Phys. ReV. Lett. 2008, 100, 249801. (27) Fuchs, O.; Zharnikov, M.; Weinhardt, L.; Blum, M.; Weigand, M.; Zubavichus, Y.; Ba¨r, M.; Maier, F.; Denlinger, J. D.; Heske, C.; Grunze, M.; Umbach, E. Phys. ReV. Lett. 2008, 100, 249802. (28) Odelius, M.; Ogasawara, H.; Nordlund, D.; Fuchs, O.; Weinhardt, L.; Maier, F.; Umbach, E.; Heske, C.; Zubavichus, Y.; Grunze, M.; Denlinger, J. D.; Pettersson, L. G. M.; Nilsson, A. Phys. ReV. Lett. 2005, 94, 227401. (29) Musat, R.; Renault, J. P.; Candelaresi, M.; Palmer, D. J.; Le Car, S.; Righini, R.; Pommeret, S. Angew. Chem., Int. Ed. 2008, 47, 8033.
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