Orientation Correlation of Sulfosuccinate-based Room-Temperature

Oct 20, 2009 - Hyper-Rayleigh scattering (HRS) of sulfosuccinate-based room-temperature ionic liquids (RTILs), tetraalkylammonium salts of bis(2-ethyl...
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Orientation Correlation of Sulfosuccinate-based Room-Temperature Ionic Liquids Studied by Polarization-Resolved Hyper-Rayleigh Scattering Guillaume Revillod, Naoya Nishi, and Takashi Kakiuchi* Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto UniVersity, Kyoto 615-8510, Japan ReceiVed: August 1, 2009; ReVised Manuscript ReceiVed: September 12, 2009

Hyper-Rayleigh scattering (HRS) of sulfosuccinate-based room-temperature ionic liquids (RTILs), tetraalkylammonium salts of bis(2-ethylhexyl)sulfosuccinate (BEHSS-) and di(isobutyl)sulfosuccinate (DIBSS-), shows strong orientation correlation between sulfosuccinate ions for both BEHSS--based and DIBSS--based RTILs. The hyperpolarizability and polarization resolved HRS, giving information at the dipolar electric order and at the quadrupolar electric order, respectively, have been measured as a function of the distance between sulfosuccinates by changing the size of the tetraalkylammonium ions constituting the ionic liquids. The variations obtained for the dipolar electric and quadrupolar electric components of the HRS intensity show the orientation correlation between sulfosuccinate ions. In particular, the weight parameter η introduced for molecular assemblies deviates from a pure dipolar electric order that is generally expected for a homogeneous molecular solution. Nonmonotonic variations of η with increasing the alkyl chain length of tetraalkylammonium ions indicate a transition in the organization of the anions from an homogeneous to an heterogeneous arrangement with increasing the intermolecular distance between sulfosuccinate ions due to the competition between the short- and long-range molecular interactions. 1. Introduction Room-temperature ionic liquids (RTILs) have received much attention because of their potential for a variety of applications.1-3 Their unique properties, notably, negligibly low vapor pressures, make them attractive to both academia and industry. The tunability of their physicochemical properties is achieved by choosing suitable combinations of cations and anions to make up the RTILs.4 A fundamental understanding of the microscopic structures of RTILs is needed to systematically design their macroscopic properties. The microscopic structures of RTILs have been studied through both computer simulation and several experimental techniques. Important microscopic properties of RTILs, such as charge ordering and nanoscale structural heterogeneities, have been reported for various ions on the basis of molecular dynamics studies.5-8 Experimentally, the charge ordering and nanoscale structural heterogeneities of RTILs have been investigated using neutron diffraction,9 small-angle neutron scattering,10 X-ray scattering,11-15 Raman spectroscopy,16 and optical Kerr effect spectroscopy.17-19 In the present study, we propose a way to experimentally investigate the specific orientation correlation between ions in RTILs. The technique of hyper-Rayleigh scattering (HRS), which has been used to measure the hyperpolarizabilities of nanoparticles and molecules, is sensitive to the symmetry of its sources,20-22 for example, the shape of nanoparticles23 or the symmetry of molecules.24 This sensitivity can be used to investigate nanometer-scale molecular assemblies.25 The orientation correlation between dyes in a molecular assembly leads to a specific HRS response.26 In particular, it was shown that a quadrupolar electric order intensity appears with strong orientation correlation in molecular assemblies.27 HRS is a good candidate for revealing correlations between neighboring ions * To whom correspondence should be addressed. Tel.: (81)-75-383-2489. Fax: (81)-75-383-2490. E-mail: [email protected].

in RTILs on the nanometer scale. In this study, we used RTILs consisting of bis(2-ethylhexyl)sulfosuccinate (BEHSS-) or di(isobutyl)sulfosuccinate (DIBSS-) and tetraalkylammonium ions. We chose these two anions for their similar HRS responses expected from the identical structures around the SO3- group, but with a possible difference in orientation correlation due to the difference in size. We changed the average distance between the anions by changing the size of the cations and evaluated its effect on the orientation correlation of the anions. All of the studied RTILs were purified to reduce background fluorescence that hampered the detection of HRS intensity. Polarizationresolved HRS was also measured to detect quadrupolar electric order intensity. The weight parameter introduced for molecular assemblies was then evaluated. A comparative study of this quadrupolar order parameter showed a correlation between the anions, as well as a transition from a more homogeneous to a more heterogeneous organization of ions depending on the size of the cations. 2. Experimental Section Chemicals. Sodium salts of BEHSS- and DIBSS- were purchased from Sigma and Fluka, respectively, and were used as received. Tetraalkylammonium halides used in this study were purchased from the indicated companies: tetrabutylammonium chloride [Tokyo Chemical Industry (TCI)], tetrabutylammonium hydroxide [Wako Pure Chemical Industries (Wako)], tetrapentylammonium bromide (TCI), tetrahexylammonium bromide (TCI), tetradecylammonium bromide (TCI), tetraheptylammonium bromide (Acros Organics), tetraoctylammonium chloride (Fluka), and tetraoctylammonium bromide (Wako). Tetranonylammonium chloride and tetraundecylammonium chloride were synthesized according to the procedures in the literature.28 For this purpose, trinonylamine, iodononane, triundecylamine, and iodoundecane were obtained from TCI.

10.1021/jp907416m CCC: $40.75  2009 American Chemical Society Published on Web 10/20/2009

Orientation Correlation of Sulfosuccinate-based RTILs For the synthesis of tetranonylammonium chloride, trinonylamine, iodononane, and potassium carbonate were mixed in a round flask, and acetonitrile was added to the reactants. The solution was stirred at 90 °C under reflux for 48 h, after which a solid was obtained. The presence of tetranonylammonium iodide ([TNA+][I-]) was checked by 1H NMR spectroscopy. The solid was dissolved in chloroform. A 10% ammonia solution and then pure water were added to wash out residual reactants; this washing procedure was repeated several times, after which chloroform and water were evaporated with an evaporator. [TNA+][I-] was recrystallized with acetonitrile, and its purity was estimated to be higher than 98% by 1H NMR measurements. Iodide ions were exchanged to chloride ions to diminish the fluorescence background in HRS measurements. To do this, [TNA+][I-] was dissolved in methanol, and the solution was passed through a column made of a polymer resin containing chloride ions (Oregano, IRA900JCL). Methanol was evaporated by an evaporator and then with an oil pump. The exchange efficiency was estimated to be better than 98% by X-ray fluorescence (Horiba, XGT-1000WR) measurements. Preparation and Purification of RTILs. Equimolar amounts of a tetraalkylammonium halide [(CnH2n+1)4NX, where n ) 4-11; X ) Br, Cl] and the sodium salt of either BEHSS- or DIBSS- were dissolved in methanol. The methanol was then evaporated with an evaporator, and the precipitate was dissolved in dichloromethane. Water was added to wash out NaX, and residual NaX was checked with a AgNO3 solution. This washing treatment was repeated until no precipitation of AgX was observed. Water and dichloromethane were evaporated with an evaporator and then with an oil pump. Viscous liquids were obtained. Most of them were colored yellow and had a high fluorescence background in HRS measurements. Even colorless RTILs from bromide salts of alkylammonium ions exhibited a high fluorescence background. This background hampered HRS intensity measurements. To suppress the background fluorescence, all RTILs were purified with the method proposed by Earle et al.29 According to this method, each RTIL dissolved in dichloromethane was passed through a column with two layers. Dichloromethane was then added to the column after the RTIL had passed to collect RTIL. Then, the dichloromethane was evaporated with an evaporator and an oil pump. Colorless RTILs were obtained, and their fluorescence was then significantly decreased. After this decolorization, the RTILs were used for HRS intensity measurements. HRS Measurements. The HRS setup was made of a Tisapphire oscillator laser source (Tsunami, Spectra-Physics) pumped by a continuous laser of 5-W power (Millennia Vs, Spectra-Physics). It delivers a pulse with a duration of 150 fs at a 76-MHz repetition rate. The laser was tuned at 800 nm with an average output power of 850 mW. The spectral shape and power of the pulse were measured before each measurement. The laser beam was focused into a standard quartz spectrophotometric cell, and the second-harmonic light, which was generated in a process whereby two photons at the fundamental frequency were converted into one photon at the secondharmonic frequency, was collected at right angles with respect to the beam with a microscope objective (04 OAS 010, NA ) 0.25, Melles Griot). The polarization of the incident beam was controlled with a half-wave plate. The vertically polarized HRS intensity was analyzed with a sheet polarizer. The secondharmonic collected light was focused with a 120-mm-focallength lens onto a monochromator and was detected with a photomultiplier tube using a photon counting mode. Color filters were used to eliminate parasite second-harmonic light before

J. Phys. Chem. B, Vol. 113, No. 46, 2009 15323 the cell and the fundamental light after the cell. Photon counting was performed with an FOG-100 system (Tokyo Instruments). 3. Results and Discussion Preparative Measurements for the Evaluation of Hyperpolarizability. To change the distance between the anions, we used a set of tetraalkylammonium ions with which we could systematically change the size of the cations from tetrabutylammonium to tetraundecylammonium. Because the structure of these cations gives rise to an unmeasurable HRS response, the anions were the primary source of HRS in the RTILs that we studied. For all measurements, the fluorescence intensity at the HRS wavelength was subtracted from the total light intensity by performing a polynomial fit to the fluorescence background outside the HRS peak range. The first step was to evaluate the hyperpolarizabilities of the anions and cations individually. Toward this end, methanol solutions of 0.3 M tetrabutylammonium hydroxide and sodium salt of BEHSS- were prepared. The HRS intensity is given by21

IHRS ) G〈NSβS2 + Nβ2〉I02

(1)

where βS and NS are the quadratic hyperpolarizability and the number density of the solvent, respectively; β and N are the corresponding quantities for the ion; G is a geometric constant; and I0 is the intensity of the fundamental light. 〈NSβS2 + Nβ2〉 is the average over the orientations of independent HRS sources. For hyperpolarizability measurements, the incoming polarization was fixed vertically, so that only the dipolar electric intensity was measured.24,27 The hyperpolarizability in the laboratory frame XYZ is then (〈βXXX2〉)1/2, where Z is the direction of propagation of the incoming wave and X is the vertical polarization axis. Using the internal reference method with βMeOH ) 0.69 × 10-30 esu,21 the quadratic hyperpolarizabilities of TBA+ and BEHSS- were estimated to be 0.0 and 2.1 × 10-30 esu for βTBA+ and βBEHSS-, respectively. Calculations30 by Gaussian 0331 gave βTBA+ ) 0 esu and βBEHSS- ) 2.2 × 10-30 esu at 800 nm. Not only TBA+ but tetraalkylammonium ions with longer chains were then considered to have no significant HRS intensity in the investigated RTILs. Because of the similar structures of BEHSS- and DIBSS- ions around the SO3- group, the hyperpolarizability of DIBSS- should be on the same order of magnitude as that of BEHSS-. The HRS intensities for RTILs [(CnH2n+1)4N+][BEHSS-] and [(CnH2n+1)4N+][DIBSS-] at 400 nm extracted from the fluorescence background were recorded for a series of tetraalkylammonium ions (n ) 4-11). Densities at 25 °C were measured with a pycnometer and are listed in Table 1. The van der Waals diameters of the tetraalkylammonium ions were calculated from the literature values of partial molar ionic volumes.32 Water was used as the external reference of HRS intensity, where βwater ) 0.56 × 10-30 esu.33 The background fluorescence is compared in Figure 1 for three samples of the IL [TOA+] [BEHSS-], where TOA+ stands for tetraoctylammonium from different sources: two from a yellowish bromide salt of TOA+ with and without the decolorization of the RTIL and the third from a chloride salt of TOA+ without yellow color after decolorization. The HRS peak was localized at the second-harmonic wavelength, 400 nm. For the bromide salts, the maximum fluorescence at 470 nm decreased by about 60% after decolorization. The decolorization was effective and sufficiently decreased fluorescence to allow measurement of the HRS intensity (Figure 2). Before decolorization, the peak at 400 nm was not discernible even after the

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TABLE 1: Densities of RTILs at 25 °C and van der Waals Diameters of Tetraalkylammonium Ions

RTILa

density at 25 °C (g dm-3)

van der Waals diameter of cations dvdW (Å)

[TBA+][BEHSS-] [TBA+][DIBSS-] [TPnA+][BEHSS-] [TPnA+][DIBSS-] [THxA+][BEHSS-] [THxA+][[DIBSS-] [THpA+][BEHSS-] [[THpA+][DIBSS-] [TOA+][BEHSS-] [TOA+][DIBSS-] [TNA+][BEHSS-] [TNA+][DIBSS-] [TDA+][[BEHSS-] [TDA+][[DIBSS-] [TUdA+][[BEHSS-] [TUdA+][DIBSS-]

993 1030 978 997 968 981 961 974 952 965 941 944 931 935 921 924

8.3 8.3 8.9 8.9 9.4 9.4 9.8 9.8 10.3 10.3 10.8 10.8 11.3 11.3 11.8 11.8

TBA+, tetrabutylammonium; TPnA+, tetrapentylammonium; THxA+, tetrahexylammonium; THpA+, tetraheptylammonium; TOA+, tetraoctylammonium; TNA+, tetranonylammonium; TDA+, tetradecylammonium; and TUdA+, tetraundecylammonium. a

Figure 1. Plots of light intensity measured at right angles to the beam for [TOA+][BEHSS-] versus wavelength: (0) [TOA+][BEHSS-] made from a bromide salt of TOA+ before decolorization, (O) [TOA+][BEHSS-] made from a bromide salt of TOA+ after decolorization, (∆) [TOA+][BEHSS-] made from a chloride salt of TOA+ after decolorization. The vertical dashed line shows the location of the HRS peak maximum (400 nm).

subtraction of the background fluorescence (0). After decolorization, a Gaussian-shaped HRS peak emerged (∆). The RTIL made from a chloride salt of TOA+ that was colorless before decolorization was much less fluorescent than those from the bromide salts. In this case, the fluorescence maximum decreased by about 15% after decolorization. RTIL Hyperpolarizability. The hyperpolarizability of watersaturated RTILs was also measured for [TBA+][BEHSS-] and [THA+][BEHSS-], where TBA+ and THA+ stand for tetrabutylammonium and tetrahexylammonium, respectively. The solubility of water in these sulfosuccinate-based RTILs was measured to be around 3 wt %.4 The values of the hyperpolarizability agreed within the experimental error with those obtained at ambient humidity.

Figure 2. Plots of light intensity measured at right angles to the beam for [TOA+][BEHSS-] versus wavelength and fitted curves as fluorescence background: (0) [TOA+][BEHSS-] made with a bromide salt of [TOA+][BEHSS-] before decolorization, (∆) [TOA+][BEHSS-] made from a chloride salt of TOA+ after decolorization. Inset: Magnified view of the baseline-corrected HRS peak from [TOA+][BEHSS-] made from a chloride salt of TOA+ after decolorization.

Figure 3. Quadratic hyperpolarizabilities of(4) [BEHSS-] and (0) [DIBSS-] RTILs as a function of the van der Waals diameter of the tetraalkylammonium ions (Table 1).

Figure 3 shows the hyperpolarizabilities of the RTILs obtained from the HRS intensity using eq 1 as a function of the van der Waals diameter of the cations, dvdW (Table 1). The hyperpolarizabilities of BEHSS- and DIBSS- showed very similar dependences on the size of the cations. For both sets of RTILs, the hyperpolarizabilities are distributed around βXXX ) 2.4 × 10-30 esu. The small variation of βXXX with dvdW reflects the weak change in the environment around the -SO3- group of BEHSS- and DIBSS- with increasing the cation size. Because this measure of the hyperpolarizability is only of the dipolar order, it does not reflect the orientation correlation effect. To investigate the orientation correlation between anions in the RTILs, we measured the polarization-resolved HRS, which is more sensitive to the molecular organization.26 Polarization-Resolved HRS of RTILs. In the case of molecular assemblies such as micelles, it has been shown that the HRS intensity is not only of dipolar electric order,27 but rather, a quadrupolar electric component of the HRS intensity appears in a molecular assembly depending on the spatial

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Figure 4. Typical plot of HRS intensity at 400 nm for [THpA+][BEHSS-] as a function of incident beam polarization. A polarization angle of 0° corresponds to vertical polarization.

arrangement of the anions. A parameter, η (defined in eq 3), that measures the weights of the dipolar electric and quadrupolar electric components can be obtained from polarization-resolved HRS measurements. The vertically polarized HRS intensity depends on the incoming wave polarization angle, γ, through the relation24

IHRS ) a cos4 γ + b cos2 γ sin2 γ + c sin4 γ

(2)

where a, b, and c are coefficients whose magnitudes depend on the hyperpolarizability tensor elements. The coefficient a is proportional to βXXX, the hyperpolarizability reported in Figure 3. The parameter η is defined as

η)

a+c b

(3)

This parameter is unity for a pure dipolar electric intensity because it corresponds to the particular case when a + c ) b, and it is 0 for a pure quadrupolar intensity.26,27 To obtain this parameter, the light intensity at 400 nm was recorded as a function of the incident polarization angle. The incident polarization dependence of the fluorescence is, by definition, identical at 416 nm and at the HRS peak maximum, 400 nm. Then, to extract the HRS intensity as a function of the incident polarization from the total light intensity, the fluorescence background as a function of the incident polarization angle was measured with a monochromator at 416 nm. After being renormalized to the fluorescence intensity at 400 nm, this background was subtracted from the total light intensity.25 A typical result for the extracted HRS intensity is shown in Figure 4. For a pure dipolar electric order intensity, a bell-shaped curve should be obtained, with a maximum at vertical polarization (0°).24,27 For pure quadrupolar electric order, two peaks should appear at -45° and 45°. Figure 4 shows a mix between these two extreme cases. A theoretical curve of eq 2 was fitted to experimental points to extract the parameters a, b and c. Values of η were calculated from the parameters a, b, and c for the RTILs and are shown in Figure 5 as a function of the van der Waals diameter of the cations. The most important point in Figure 5 is that η is always lower than unity for both sets of RTILs. Previously, η has been found to be unity for orientationally noncorrelated molecules in diluted solutions,24,26 which corresponds to the pure dipolar contribution of the molecules to HRS. The η values lower than unity in the present study clearly indicate the presence of the strong orientation correlation

Figure 5. Quadrupolar/dipolar weight parameters η of (4) [BEHSS-] and (0) [DIBSS-] RTILs as a function of the van der Waals diameter of the tetraalkylammonium ions.

between the anions in the bulk RTILs, as was seen for molecular assemblies in solutions.26,27 For both sets of RTILs, η was higher for intermediate-sized cations. This corresponds to a smaller contribution of the quadrupolar electric component, that is, a weaker correlation between the anions. The maximum (η ) 0.8) was located around 10 Å for both sets of RTILs, but a clear difference in the variation of η appears between RTILs of BEHSS- and DIBSS-. An increase in η, that is, a decorrelation of the anions, occurs for smaller cations in the case of DIBSSRTILs. A smaller size of the anion makes geometrical rearrangement easier, and hence, the decorrelation proceeds with increasing cation size. The lower values of η for the smallest and largest cations studied indicate that the anions are more correlated for these cation sizes. BEHSS- salts of tetrapropyammonium and tetrahexadecylammonium ions were found to be solids at room temperature. This corroborates the supposition that the correlations are stronger for cations with very small and very large diameters. What is remarkable is the increase of η at intermediate sizes, showing a transition of the correlations between the anions. The hyperpolarizability in Figure 5 exhibits an inflection point at dvdW ) 10.3 Å. This fact indicates that the change in the anion environment deduced from the hyperpolarizability measurements corresponds to a change in the orientation correlation measured by the polarization-resolved HRS. The change in the distance between the anions introduces a competition between the interaction forces. Particularly, the increase of the cation size decreases the electrostatic force from the cations and increases the volume occupied by the anions. Such a competitive effect between electrostatic forces and van der Waals forces in RTILs has already been discussed in regard to the tail aggregation mechanism of RTILs.34 The present results specifically show that this competitive effect can induce spatial heterogeneity in RTILs. This heterogeneity should be of nanometer scale, typically around a few nanometers, corresponding to the average range of cation size that induces the reorganization of anions. Such structural heterogeneity of ILs has recently been studied for several cases by molecular dynamics simulation35-41 and several experimental methods, as described in the Introduction. Such nanoscale heterogeneous structures can be formed for many different reasons. For example, the addition of water induces such a mesoscopic structure.42 At the interfaces, such heterogeneity due to tail aggregation was observed in the layering of RTIL ions.39 The simplest model used to describe RTILs is an organized charge network.7,8,43 This simple view does not take into account the interactions between the alkyl

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chains of anions when the electrostatic forces are decreased. In this case, the alkyl chains agglomerate and induce the formation of nonpolar domains between the polar parts of the molecules. This heterogeneity corresponds to the reorganization in the present study seen for longer alkyl chains of cations (Figure 5). When the alkyl chains interact more strongly with each other and the degree of heterogeneity becomes stronger, BEHSS- and DIBSS- should become more correlated. A model for the hyperpolarizability of agglomerated molecules has been developed but only for simple structures such as spheres.27 An evolution of this model to fit the nanoscale structures in RTILs should be able to show the real organization of the anions with their orientation and allow the organization of the heterogeneities in RTILs to be studied more precisely. 4. Conclusions Nonmonotonic variations of the weight parameter between dipolar electric and quadrupolar electric HRS intensities of RTILs as a function of the size of the cation have been illustrated for a series of BEHSS- and DIBSS- salts of tetraalkylammonium ions. These nonmonotonic variations occurred in both sets of RTILs containing BEHSS- and DIBSS-. The environment around the anions and the correlations between the anions are altered by changes in the size of the cations. These measurements show a link between the strong correlations between the anions and the change in the hyperpolarizability of the anions. A change of interaction forces is caused by a change of the size of the cations. Particularly, the geometrical rearrangements due to van der Waals forces and electrostatic forces are in competition. The nanoscale heterogeneity formed for cations with longer alkyl chains causes nonmonotonic variations in the hyperpolarizability and the weight parameter with the size of the cations. The HRS technique exhibited a high sensitivity to organization in the structure of the RTILs. Further HRS experiments, for example, at different temperatures, with different water contents, or for different RTILs with surfactantlike anions that have a more predictable aggregation process, would elucidate more detailed properties of RTILs. Acknowledgment. This work was made possible thanks to the financing of a postdoctoral fellowship by the Japan Society for Promotion of Science (JSPS). References and Notes (1) Ranke, J.; Stolte, S.; Störmann, R.; Arning, J.; Jastorff, B. Chem. ReV. 2007, 107, 2183–2206. (2) MacFarlane, D. R.; Seddon, K. R. Aust. J. Chem. 2007, 60, 3–5. (3) Plechkova, N. V.; Seddon, K. R. Chem. Soc. ReV. 2008, 37, 123– 150. (4) Nishi, N.; Kawakami, T.; Shigematsu, F.; Yamamoto, M.; Kakiuchi, T. Green Chem. 2006, 8, 349–355. (5) Lynden-Bell, R. M.; Del Pópolo, M. G.; Youngs, T. G. A.; Kohanoff, J.; Hanke, C. G.; Harper, J. B.; Pinilla, C. C. Acc. Chem. Res. 2007, 40, 1138–1145. (6) Castner, E. W., Jr.; Wishart, J. F.; Shirota, H. Acc. Chem. Res. 2007, 40, 1217–1227. (7) Del Pópolo, M. G.; Mullan, C. L.; Holbrey, J. D.; Hardacre, C.; Ballone, P. J. Am. Chem. Soc. 2008, 130, 7032–7041. (8) Schröder, C.; Steinhauser, O. J. Chem. Phys. 2008, 128, 224503. (9) Hardacre, C.; Holbrey, J. D.; Mullan, C. L.; Nieuwenhuyzen, M.; Youngs, T. G. A.; Bowron, D. T. J. Phys. Chem. B 2008, 112, 8049–8056. (10) Atkin, R.; Warr, G. G. J. Phys. Chem. B 2008, 112, 4164–4166. (11) Triolo, A.; Russina, O.; Bleif, H. J.; Di Cola, E. J. Phys. Chem. B 2007, 111, 4641–4644.

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