Solvation Dynamics of Formamide and N,N-Dimethylformamide in

formamide in the reverse micelle solution depends strongly on the molar ratio between ... dynamics andwdependence in formamide andN,N-dimethylformamid...
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Langmuir 2004, 20, 329-335

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Solvation Dynamics of Formamide and N,N-Dimethylformamide in Aerosol OT Reverse Micelles Hideaki Shirota*,† and Hiroshi Segawa Department of General Systems Sciences, Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan Received April 15, 2003. In Final Form: October 14, 2003 The solvation dynamics of formamide and N,N-dimethylformamide in Aerosol OT reverse micelles has been investigated in this work. The solvation dynamics of formamide and N,N-dimethylformamide in the reverse micelles is more than 100 times slower than that of the pure solvents. The solvation dynamics of formamide in the reverse micelle solution depends strongly on the molar ratio between formamide and Aerosol OT (w ) [polar solvent]/[Aerosol OT]), but that of N,N-dimethylformamide in the reverse micelle solution shows a tiny w dependence. We have estimated the interaction energies of the geometry-optimized clusters of a simple model of the Aerosol OT polar headgroup (CH3SO3-) and formamide or N,Ndimethylformamide by ab initio calculations (the second-order Møller-Plesset perturbation theory) to find their interactions. The interaction energies of the mimic clusters estimated by the ab initio calculations and the features of the slow solvation dynamics and w dependence in formamide and N,N-dimethylformamide reverse micelles are discussed.

Introduction Reverse micelles are the aggregates of surfactants formed in a nonpolar solvent, in which the polar headgroups of the surfactants point inward and the hydrocarbon chains point toward to the nonpolar medium.1,2 One of the most interesting features of the reverse micelles is their ability to encapsulate water to form a microemulsion. Some polar organic solvents, such as formamide (FA), ethylene glycol, and methanol, are also encapsulated in reverse micelles, as well as water.3-9 The polar solvent molecules in reverse micelles are confined to the nanometer-scale polar-solvent pools of the reverse micelles. It is suggested that such polar solvent molecules behave differently from the pure solutions as a result of the specific interactions and confined geometries. One of the useful methods to study the dynamical feature of the polar solvents in reverse micelles is the dynamic fluorescence Stokes shift measurement, so-called the solvation dynamics,10,11 of a solvatochromic probe molecule in the polar solvent pool.12-16 Among polar solvents in reverse micelles, water has been most exten* Author to whom correspondence should be addressed. E-mail: [email protected]. † Present Address: Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854. (1) De, T. K.; Maitra, A. Adv. Colloid Interface Sci. 1995, 59, 95. (2) Moulik, S. P.; Paul, B. K. Adv. Colloid Interface Sci. 1998, 78, 99. (3) Rico, I.; Lattes, A. J. Colloid Interface Sci. 1984, 102, 285. (4) Gautier, M.; Rico, I.; Samii, A.-Z.; De Savignac, A.; Lattes, A. J. Colloid Interface Sci. 1986, 112, 484. (5) Mukherjee, K.; Moulik, S. P.; Mukherjee, D. C. Langmuir 1993, 9, 1727. (6) Mukherjee, K.; Mukherjee, D. C.; Moulik, S. P. J. Phys. Chem. 1994, 98, 4713. (7) Ray, S.; Moulik, S. P. Langmuir 1994, 10, 2511. (8) Hayes, D. G.; Gulari, E. Langmuir 1995, 11, 4695. (9) Riter, R. E.; Kimmel, J. R.; Undiks, E. P.; Levinger, N. E. J. Phys. Chem. B 1997, 101, 8292. (10) Jarzeba, W.; Walker, G. C.; Johnson, A. E.; Barbara, P. F. Chem. Phys. 1991, 152, 57. (11) Maroncelli, M. J. Mol. Liq. 1993, 57, 1. (12) Nandi, N.; Bhattacharyya, K.; Bagchi, B. Chem. Rev. 2000, 100, 2013. (13) Bhattacharyya, K.; Bagchi, B. J. Phys. Chem. A 2000, 104, 10603. (14) Bhattacharyya, K. J. Fluoresc. 2001, 11, 167. (15) Bhattacharyya, K. Acc. Chem. Res. 2003, 36, 95. (16) Levinger, N. E. Curr. Opin. Colloid Interface Sci. 2000, 5, 118.

sively studied by the time-resolved fluorescence measurements17-30 and molecular dynamics simulations.31-35 The slow components of the water dynamics appear in aqueous reverse micelles, and the dynamical feature depends strongly on the size of the nanocavity. In contrast to water, the other polar solvents have not been much extensively studied. Levinger and co-workers studied the solvation dynamics in FA/Aerosol (AOT)/ isooctane reverse micelles.36 They found that FA in a reverse micelle is nearly completely immobilized in the subpicosecond to hundreds of picoseconds time scale. We investigated the subnanosecond to nanosecond solvation dynamics of methanol and acetonitrile in AOT reverse micelles using coumarin 343 as a fluorescence probe.37 Interestingly, the solvation dynamics of methanol and acetonitrile in AOT reverse micelles is about 1000 times (17) Sarkar, N.; Das, K.; Datta, A.; Das, S.; Bhattacharyya, K. J. Phys. Chem. 1996, 100, 10523. (18) Das, S.; Datta, A.; Bhattacharyya, K. J. Phys. Chem. A 1997, 101, 3299. (19) Mandal, D.; Datta, A.; Pal, S. K.; Bhattacharyya, K. J. Phys. Chem. B 1998, 102, 9070. (20) Pal, S. K.; Mandal, D.; Sukul, D.; Bhattacharyya, K. Chem. Phys. Lett. 1999, 312, 178. (21) Bhattacharyya, K.; Hara, K.; Kometani, N.; Uozu, Y.; Kajimoto, O. Chem. Phys. Lett. 2002, 361, 136. (22) Riter, R. E.; Willard, D. M.; Levinger, N. E. J. Phys. Chem. B 1998, 102, 2705. (23) Pant, D.; Riter, R. E.; Levinger, N. E. J. Chem. Phys. 1998, 109, 9995. (24) Riter, R. E.; Undiks, E. P.; Levinger, N. E. J. Am. Chem. Soc. 1998, 120, 6062. (25) Willard, D. M.; Riter, R. E.; Levinger, N. E. J. Am. Chem. Soc. 1998, 120, 4151. (26) Willard, D. M.; Levinger, N. E. J. Phys. Chem. B 2000, 104, 11075. (27) Pant, D.; Levinger, N. E. Langmuir 2000, 16, 10123. (28) Lundgren, J. S.; Heitz, M. P.; Bright, F. V. Anal. Chem. 1995, 67, 3775. (29) Raju, B. B.; Costa, S. M. B. Phys. Chem. Chem. Phys. 1999, 1, 5029. (30) Hazra, P.; Sarkar, N. Chem. Phys. Lett. 2001, 342, 303. (31) Faeder, J.; Ladanyi, B. M. J. Phys. Chem. B 2000, 104, 1033. (32) Faeder, J.; Ladanyi, B. M. J. Phys. Chem. B 2001, 105, 11148. (33) Faeder, J.; Albert, M. V.; Ladanyi, B. M. Langmuir 2003, 19, 2514. (34) Senapati, S.; Chandra, A. J. Phys. Chem. B 2001, 105, 5106. (35) Senapati, S.; Chandra, A. J. Chem. Phys. 2001, 111, 1223. (36) Riter, R. E.; Undiks, E. P.; Kimmel, J. R.; Levinger, N. E. J. Phys. Chem. B 1998, 102, 7931. (37) Shirota, H.; Horie, K. J. Phys. Chem. B 1999, 103, 1437.

10.1021/la030161r CCC: $27.50 © 2004 American Chemical Society Published on Web 12/20/2003

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Figure 1. Chemical structure of coumarin 102 (C102).

slower than that of pure methanol and acetonitrile,10,38-43 and the solvation time constants of methanol reverse micelles depend strongly on w but those of acetonitrile reverse micelles do not much. Recently, Sarkar and coworkers observed the similar w dependence on the solvation dynamics of methanol and acetonitrile in reverse micelles using coumarin 480,44 coumarin 152A,45,46 and coumarin 153.47 This difference between methanol and acetonitrile could be due to the presence and absence of the intermolecular hydrogen-bonding network. They also studied the nanosecond solvation dynamics of FA in reverse micelles very recently.46 In this study, we have investigated the solvation dynamics of FA and N,N-dimethylformamide (DMF) in AOT reverse micelles. The solvation dynamics of FA in reverse micelles with w e 1 has been observed by Riter et al.36 and Hazra et al.46 However, the solvation dynamics of FA in reverse micelles with the larger w has not been measured, and the solvation dynamics of DMF in reverse micelles is still unknown. It is interesting to see the comparison of the solvation dynamics between FA and DMF in reverse micelles to find out the effect of the amide hydrogen bond on the dynamical features of these liquids in the solvent-pool nanocavities in the solutions. Further, the comparison with the hydroxyl hydrogen bond (methanol reverse micelles)37,44-47 gives a more detailed understanding of the effect of the hydrogen bond on the solvation dynamics in reverse micelles. We have also examined the interaction energies for the optimized geometry clusters of a simple model of the AOT polar headgroup (CH3SO3-) and FA or DMF by ab initio calculations to find their specific interactions. Experimental Section Laser-grade coumarin 102 (C102, Figure 1, Exciton), purestgrade FA (Wako Pure Chemical), DMF (Aldrich), and dehydrated n-heptane (Wako Pure Chemical) were used without further purification. AOT (Wako Pure Chemical) was purified by the standard procedure and then dried at 403 K under a vacuum for more than 15 h.37,48 The concentration of AOT was kept at 0.09 M. The amount of the polar solvent was kept at each value of the ratio of the polar solvent to the surfactant, w ) [polar solvent]/ [AOT]: w ) 1 and 2 for FA and w ) 1, 2, and 4 for DMF. Although we prepared the FA reverse micelle solution with w ) 4, the solution separated into two phases. The concentration of C102 was about 1.5 × 10-5 M. The steady-state absorption and fluorescence spectra of C102 in nonaqueous AOT reverse micelle (38) Horng, M. L.; Gardecki, J. A.; Frankland, S. J. V.; Maroncelli, M. J. Phys. Chem. 1995, 99, 17311. (39) Tominaga, K.; Walker, G. C. J. Photochem. Photobiol., A 1995, 87, 127. (40) Shirota, H.; Pal, H.; Tominaga, K.; Yoshihara, K. J. Phys. Chem. 1996, 100, 14575. (41) Nishiyama, K.; Okadata, T. J. Phys. Chem. A 1997, 101, 5729. (42) Gustovsson, T.; Gulbinas, V.; Gurzadyan, G.; Mialocq, J.-C.; Pommeret, S.; Sorginus, M.; van der Meulen, P. J. Phys. Chem. A 1998, 102, 4229. (43) Kovalenko, S. A.; Ruthmann, J.; Ernsting, N. P. J. Chem. Phys. 1998, 109, 1894. (44) Hazra, P.; Sarkar, N. Phys. Chem. Chem. Phys. 2002, 4, 1040. (45) Hazra, P.; Chakrabarty, D.; Sarkar, N. Chem. Phys. Lett. 2002, 358, 523. (46) Hazra, P.; Chakrabarty, D.; Sarkar, N. Langmuir 2002, 18, 7872. (47) Hazra, P.; Chakrabarty, D.; Sarkar, N. Chem. Phys. Lett. 2003, 371, 553. (48) Maitra, A. N.; Eicke, H.-F. J. Phys. Chem. 1981, 85, 2687.

Shirota and Segawa solutions were measured with a JASCO V-570 UV/visible/nearinfrared spectrometer and a JASCO FP-777 spectrofluorometer, respectively. The diameters of the reverse micelles were estimated by dynamic light scattering measurements (Otsuka Electronics, DLS-70): 5.4 ( 1 nm for w ) 1 (FA), 15.7 ( 2 nm for w ) 2 (FA), 4.0 ( 2 nm for w ) 1 (DMF), 3.3 ( 2 nm for w ) 2 (DMF), and 3.6 ( 2 nm for w ) 4 (DMF). The diameters of the reverse micelles are quite similar to those reported by Riter et al.9 As well as the results reported by Riter et al.,9 the diameter for the FA reverse micelles at w ) 2 measured in this study is quite large in comparison with the other polar solvent reverse micelles. This could be due to the strong intermolecular hydrogen-bonding network49-51 and the high polarity.52 Details of the picosecond laser apparatus were reported elsewhere,53 except for the light source. The fundamental light of an oscillator in Spectra Physics Hurricane at about 795 nm with an average power of about 770 mW was used as the light source. A combined type doubler and pulse picker (Spectra Physics, model 3980) was used to reduce the repetition frequency (from 80 to 4 MHz) and to produce the second harmonic light at about 397.5 nm. The sample was excited by the second harmonic light after passing though a Glan-Laser polarizer to set the vertical polarization angle. The fluorescence of the sample was passed though a 2-mm slit attached with a 1-cm cell, a GlanLaser polarizer set at the magic angle to the polarization of the pump beam and a polychromator (Jobin Yvon CP-200), and was detected by a streak camera (Hamamatsu Photonics, C4334). The instrument’s responses were estimated by the excitation light scattering from an aqueous powdered milk solution in the sample cell. The full width at half-maximum of the instrument’s responses were 20-30 ps for the 2-ns full-scale detection and 200-300 ps for the 20-ns full-scale detection. All the measurements were made at ambient temperature (293 ( 2 K). Calculations for geometry optimizations were carried out using the Gaussian 98 program package (Revision A.11.2).54 Calculations were done by the second-order Møller-Plesset perturbation theory (MP2)55-57 with the basis set of 6-31G(d,p). Prior to the energy calculations based on the self-consistent reaction field theory (the isodensity surface polarized continuum model, IPCM),58 the calculations for geometry optimization were made at the gas-phase condition. Because AOT is a large molecule, CH3SO3- was used as a simple model of the polar group of AOT in this study. FA, DMF, and CH3SO3- monomers and FA-CH3SO3-, DMF-CH3SO3-, and 2FA-CH3SO3- clusters were calculated to see the interactions of the polar solvent molecules and the polar group of AOT.

Results and Discussion Time-Integrated Spectroscopy. Figure 2 shows the steady-state absorption and fluorescence spectra of C102 (49) Ohtaki, H.; Funaki, A.; Rode, B. M.; Reibnegger, G. J. Bull. Chem. Soc. Jpn. 1983, 56, 2116. (50) Ohtaki, H.; Itoh, S.; Yamaguchi, T.; Ishiguro, S.; Rode, B. M. Bull. Chem. Soc. Jpn. 1983, 56, 3406. (51) Ohtaki, H.; Itoh, S.; Rode, B. M. Bull. Chem. Soc. Jpn. 1986, 59, 271. (52) CRC Handbook of Chemistry and Physics, 77th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1996. (53) Shirota, H.; Segawa, H. J. Phys. Chem. A 2003, 107, 3719. (54) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A. J.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Rega, N.; Salvador, P.; Dannenberg, J. J.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.11.2; Gaussian, Inc.: Pittsburgh, PA, 2001. (55) Moller, C.; Plesset, M. S. Phys. Rev. 1934, 46, 618. (56) Brinkley, J. S.; Pople, J. A. Int. J. Quantum Chem. 1975, 9, 229. (57) Head-Gordon, M.; Pople, J. A.; Frisch, M. J. Chem. Phys. Lett. 1988, 153, 503. (58) Foresman, J. B.; Keith, T. A.; Wiberg, K. B.; Snoonian, J.; Frisch, M. J. J. Phys. Chem. 1996, 100, 16098.

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Figure 3. Steady-state excitation spectra of C102 in (a) FA/ AOT/n-heptane (solid line for w ) 1 and dotted line for w ) 2) and (b) DMF/AOT/n-heptane (solid line for w ) 1, dotted line for w ) 2, and broken line for w ) 4). Table 1. Characteristic Wavelengths of Absorption, Fluorescence, and Excitation Spectra for C102 in Nonaqueous Reverse Micelles and Pure Solvents (a) Reverse Micelles

Figure 2. Steady-state absorption and fluorescence spectra of C102 in (a) FA/AOT/n-heptane (solid line for w ) 1 and dotted line for w ) 2), (b) DMF/AOT/n-heptane (solid line for w ) 1, dotted line for w ) 2, and broken line for w ) 4), and (c) pure solvents (solid line for n-heptane, dotted line for DMF, and broken line for FA).

in (a) FA/AOT/n-heptane reverse micelle solutions (solid line is for w ) 1 and dotted line is for w ) 2) and (b) DMF/AOT/n-heptane reverse micelle solutions (solid line is for w ) 1, dotted line is for w ) 2, and broken line is for w ) 4). For a comparison, the steady-state absorption and fluorescence spectra of C102 in pure FA (dotted lines), DMF (broken lines), and n-heptane (solid lines) are shown in Figure 2c. The steady-state fluorescence spectra of C102 in the reverse micelles are measured by the 397-nm wavelength excitation because the wavelength of the light source for the time-resolved experiment is 397.5 nm and the peak wavelength of the absorption spectrum includes the fluorescence of C102 in nonpolar media (n-heptane). We have also measured the excitation spectra of C102 in the reverse micelle solutions (the monitoring wavelength is 480 nm) to see the contribution of the absorbent to the fluorescence, as shown in Figure 3. Because the steadystate absorption and excitation spectra of the reverse micelles are structured, the characteristic frequencies of the steady-state absorption, fluorescence, and excitation spectra are defined as the midpoint frequencies.

ν ) (ν- + ν+)/2

(1)

where ν- and ν+ are the low and high frequencies on the

solvent

w

νabs, 103 cm-1 (λabs, nm)

FA FA DMF DMF DMF

1 2 1 2 4

27.68 (361.3) 27.69 (361.1) 27.66 (361.6) 27.61 (362.2) 27.51 (363.5)

νfl, 103 cm-1 (λfl, nm)

νex, 103 cm-1 (λex, nm)

22.62 (442.1) 22.26 (449.2) 22.97 (435.3) 22.79 (438.8) 22.53 (443.9)

27.06 (369.5) 26.85 (372.5) 27.49 (363.8) 27.46 (364.2) 27.32 (366.0)

(b) Pure Solvents 103

solvent

νabs, cm-1 (λabs, nm)

νfl, 103 cm-1 (λfl, nm)

∆ν, 103 cm-1

FA DMF n-heptane

25.74 (388.5) 26.48 (377.6) 27.75 (360.4)

21.04 (475.3) 22.03 (453.9) 24.62 (406.2)

4.70 4.45 3.13

half-height points of the spectrum. The absorption, fluorescence, and excitation spectral characteristic frequencies and wavelengths (1/ν) of C102 in the nonaqueous reverse micelles and the pure solvents are listed in Table 1. The fluorescence spectra of C102 in the reverse micelles indicate that the probing C102 locates in a rather polar microenvironment. This is because the fluorescence spectra are in the long wavelength in comparison with the nonpolar solvent, though the wavelength is not as long as pure FA and DMF. On the other hand, the excitation spectra of C102 in the reverse micelles are slightly shouldered at about 360 nm. Therefore, the probing C102 could locate near the wall of the polar solvent pool. The retardation of the reorientation for coumarins in nonaqueous reverse micelles has also been reported by several groups.37,47 Another interesting point is the w dependence on the fluorescence and excitation spectra of C102 in the reverse micelles: the larger w reverse micelle shifts to the longer wavelength. The similar w dependence of the steady-state

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Shirota and Segawa

Figure 4. Time-resolved fluorescence spectra of C102 in the DMF/AOT/n-heptane solution with w ) 1. From the top spectrum, t ) 0.05, 0.50, 1.50, and 5.00 ns. The log-normal line shape function fit curves are also shown.

Figure 5. Fluorescence transients of C102 in the DMF/AOT/ n-heptane solution with w ) 1 at 410, 440, and 470 nm. The instrument’s response is also shown by the dotted curve.

absorption and fluorescence spectra of solvatochromic coumarin dyes has also been reported in both aqueous and nonaqueous reverse micelle solutions.17,23,25-27,30,37,44,45 Because the steady-state fluorescence spectrum for coumarin dye in a more polar solvent shifts to the longer wavelength, the steady-state fluorescence spectrum for coumarin dye in the reverse micelle solution is influenced by the w of the reverse micelle solution. Time-Resolved Spectroscopy. Figure 4 shows the time-resolved fluorescence spectra of C102 in the DMF/ AOT/n-heptane solution (w ) 1) at t ) 0.05, 0.50, 1.50, and 5.00 ns. The fluorescence maximum shifts to the longer wavelength with the time evolution in the nanosecond time region. Similar time scale dynamic fluorescence Stokes shifts are also observed in DMF/AOT/n-heptane solutions with w ) 2 and 4 and FA/AOT/n-heptane solutions with w ) 1 and 2. The fluorescence Stokes shift in the nanosecond time scale has also been reported in the other reverse micelles.17-20,29,30,37,44-46 To estimate the fluorescence maximum at each time, the time-resolved fluorescence spectra are fitted with a log-normal line shape function,59

[

If(ν) ) If0 exp -ln(2)

(

)]

ln[1 + 2b(ν - νp)/∆] b

2

(2)

where If0, νp, b, and ∆ are the peak height, peak frequency, asymmetric parameter, and width parameter, respectively. When 2b(ν - νp)/∆ is less than -1, If(ν) is taken as 0. The log-normal function fit curves to the measured timeresolved fluorescence spectra are also shown in Figure 4. To see the wavelength dependence of the fluorescence transients, Figure 5 shows the fluorescence transients of C102 in the DMF/AOT/n-heptane solution (w ) 1) at 410, (59) Siano, D. B.; Metzler, D. E. J. Chem. Phys. 1969, 51, 1856.

Figure 6. Spectral shift correlation functions S(t) of C102 in (a) FA/AOT/n-heptane (open circles for w ) 1 and filled circles for w ) 2) and (b) DMF/AOT/n-heptane (open circles for w ) 1, filled circles for w ) 2, and crosses for w ) 4). Triexponential fit curves are also shown in the figure.

440, and 470 nm. The data points at t ) -0.5 to 1.5 ns are the 2-ns full-scale detection data, and the data points at t > 1.5 ns are the 20-ns full-scale detection data. The response function of the 2-ns full-scale detection is also in Figure 5 (dotted curve). The features of the solvation dynamics in the reverse micelle solutions are estimated by the following equation.

S(t) )

ν(t) - ν(∞) ν(0) - ν(∞)

(3)

The ν(0) is defined as the frequency of the fluorescence maximum at the time when the sample is excited by a laser pulse. The ν(∞) is estimated from the value for S(∞) ) 0 by fitting the 15-ns time-range experimental data to a triexponential function because the slowest solvation time constant (τ3) is larger than the fluorescence lifetime of C102 in the nonaqueous reverse micelle solutions (3-4 ns). Because a single exponential function or a biexponential function cannot give a good fit, we tentatively use a triexponential function. Figure 6 shows the decays of the S(t) for C102 in (a) FA/AOT/n-heptane reverse micelle solutions and (b) DMF/AOT/n-heptane reverse micelle solutions. The 0-1.5 ns data points are used for the 2-ns full-scale detection data, and the 1.6-15 ns data points are used for the 20-ns full-scale detection data. The triexponential function fit curves are also in Figure 6. The fit parameters are summarized in Table 2. The average solvation time constants 〈τ〉 defined as a1τ1 + a2τ2 + a3τ3 are also listed in Table 2. Because the temporal response in the present laser apparatus is 20-30 ps, we estimate the missing solvation component to find how much solvation process we have observed in the present systems. The fluorescence maximum prior to the solvent relaxation can be estimated by Fee and Maroncelli’s method.60 The Stokes shift of a (60) Fee, R. S.; Maroncelli, M. Chem. Phys. 1994, 183, 235.

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Table 2. Parameters for the Solvation Dynamics in (a) FA and DMF Reverse Micelles and (b) Pure FA and DMF (A) FA and DMF Reverse Micelles. solvent

w

a1a

τ1a, ns

a2b

τ2b, ns

a3b

τ3b, ns

〈τ〉c, ns

FA FA DMF DMF DMF

1 2 1 2 4

0.10 0.17 0.13 0.13 0.10

0.051 0.020 0.057 0.038 0.030

0.19 0.28 0.23 0.21 0.21

0.96 0.94 1.03 0.40 0.40

0.71 0.56 0.64 0.66 0.69

13.05 10.52 13.44 11.73 11.10

9.45 6.16 8.85 7.83 7.75

(B) Pure FA and DMF (Ref 38)d solvent FA DMF

a1e

τ1, ps

a2e

τ2, ps

0.86 0.92

2.94 1.70

0.14 0.08

57.9 29.1

a Experimental error is (30%. b Experimental error is (10%. 〈τ〉 ) a1τ1 + a2τ2 + a3τ3. d Diffusive solvation data only. e Normalized in the diffusive solvation components.

c

fluorescence probe in a nonpolar solvent is assumed as the Stokes shift without any dipolar solvent reorganization around the fluorescence probe in this model. Accordingly, the characteristic frequency of the time-zero fluorescence spectrum can be estimated by the following equation.

νfl,p,md(t ) 0) ≈ νex,p,md - [νabs,np,md - νfl,np,md] (4) where the subscripts “p” and “np” refer to the polar and nonpolar spectra, respectively, and the frequencies here are not the values at the maxima but correspond to the midpoint frequencies (eq 1). In this study, we use the excitation spectra in reverse micelles instead of the absorption spectra to eliminate the contribution of the absorption in the nonpolar medium. n-Heptane is used as the nonpolar solvent in the present study. The maximum frequency at time zero, νfl,p(t ) 0), is estimated as the sum of νfl,p,md(t ) 0) and the difference between the midpoint and maximum frequencies in the steady-state fluorescence spectrum of the same system. The missing solvation components for FA/AOT/n-heptane and DMF/AOT/nheptane are about 30 and 35%, respectively. This is slightly increasing with the larger-w micelles (24% for FA w ) 1, 33% for FA w ) 2, 30% for DMF w ) 1, 32% for DMF w ) 2, and 37% for DMF w ) 4). The w dependence on the missing solvation component might be due to the size of polar solvent pool in the reverse micelle because the component of the bulklike polar solvent is changeable by the size of the micelle nanocavity. The solvation dynamics of pure FA and DMF were measured by Maroncelli and co-workers.38 The diffusive solvation time constants are 2.94 and 57.9 ps for FA and 1.7 and 29.1 ps for DMF (Table 2B). Chang and Castner reported that the relaxation time constants of the diffusive polarizability anisotropy decay are 1.45 and 11.6 ps for pure FA and 0.82 and 4.72 ps for pure DMF.61 It is clear from the comparison between their results and the present experimental results (Table 2) that the solvation dynamics of FA and DMF in AOT reverse micelles is extremely slower than that of pure FA and DMF. The slowest solvation time constants (τ3) of FA and DMF in the reverse micelles are about 200-400 times slower than those of pure FA and DMF. The slow nanosecond time scale solvation dynamics in reverse micelles has also been observed in not only water17-20,29,30 but also nonaqueous solvent such as methanol and acetonitrile.37,44-47 As shown in Figure 6 and Table 2, the solvation dynamics of FA and DMF becomes faster with the larger (61) Chang, Y. J.; Castner, E. W., Jr. J. Phys. Chem. 1994, 98, 9712.

w reverse micelles. However, if we closely see Figure 5 and Table 2, the w dependence on the whole solvation dynamics (〈τ〉) of DMF in reverse micelles is rather small but that of FA in reverse micelles is quite large. Specifically, the solvation time constants of DMF in the reverse micelle solution with w ) 2 are quite similar to those in the reverse micelle solution with w ) 4. Interestingly, these features of the w dependence on the solvation dynamics are similar to those of acetonitrile and methanol in AOT reverse micelle solutions.37 The solvation dynamics in the acetonitrile/AOT/n-heptane reverse micelle solution with w ) 2 is similar to that in the w ) 4 acetonitrile/ AOT/n-heptane reverse micelle solution. In contrast to that of acetonitrile, the solvation dynamics of methanol in reverse micelle solutions depends strongly on w. Such a large w dependence on the solvation dynamics in reverse micelles is well-recognized in water reverse micelles in both the experimental and simulation results.16-20,22-32 Ab Initio Calculation of Interaction Energy. It has been proposed that the intermolecular hydrogen-bond network of the protic solvent in reverse micelles affects the solvation dynamics.37,44-46 FA is a protic molecule, and DMF is an aprotic molecule. It can be expected that the interaction between FA and the polar group of AOT and the other FA molecules in polar solvent pool should be different from that between DMF and the polar group of AOT and the other DMF molecules in the polar solvent pool of the reverse micelle solutions. Namely, a FA molecule can directly interact with AOT and the other FA molecules in the polar solvent pool via intermolecular hydrogen bonds. To find the interaction strengths between the polar group of AOT and the solvent molecules, we estimate the geometry-optimization energies of clusters of a simple model of the polar group of AOT (CH3SO3-) and FA or DMF on the basis of the level of MP2/6-31G(d,p) theory. Although we have used the other basis sets, such as 6-31+G(d,p), 6-311+G(d,p), and 6-311++G(d,p), for optimizations of FA and DMF, the dipole moments of FA and DMF at the gas-phase condition estimated using 6-31G(d,p) show quite similar values to the experimental data (3.73 D for FA and 3.82 D for DMF in the gas phase): 52 6G-31(d,p), 3.76 D for FA and 3.84 D for DMF; 6-31+G(d,p), 4.04 D for FA and 4.30 D for DMF; 6-311+G(d,p), 3.78 D for FA and 4.13 D for DMF; and 6-311++G(d,p), 3.78 D for FA and 4.11 D for DMF. Therefore, we tentatively chose 6-31G(d,p) as a basis set in this study. In the IPCM calculation for a solvent medium contribution,58 the dielectric constant s is chosen as 20. This is because the fluorescence Stokes shift of C102 in the nonaqueous reverse micelles is similar to that in 1-propanol and acetone (s ≈ 20).52 Although the microenvironment of reverse micelles is not simple, the micropolarity around the fluorescence probe depends on w of a reverse micelle solution, and the counterions are absent in the calculation, it would discuss a qualitative feature in the solvent pool of the reverse micelle solutions. Figure 7 shows the optimization geometries of FA, DMF, and CH3SO3- monomers and FA-CH3SO3-, DMF-CH3SO3-, and two different 2FA-CH3SO3- clusters. The calculated optimization energies, interaction energies, hydrogen-bond distances (NH‚‚‚O), and hydrogen-bond angles (N-H‚‚‚O) are listed in Table 3. The interaction energy Ei of the clusters is simply defined as the difference between the energy of the complex Ec and the sum of the energies of the monomers Em.

Ei ) Ec -

∑n Em,n

(5)

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Shirota and Segawa

Table 3. Calculated Energies, Interaction Energies, and Hydrogen-Bonding Geometry Parameters of FA, DMF, CH3SO3-, and Their Clusters at MP2/6-31G(d,p) Optimized Geometry (a) Monomers Em, kJ/mol

dipole moment (D)

monomer

gas

s ) 20

gas

s ) 20

FA DMF CH3SO3-

-444 815.18 -650 529.29 -1 739 474.21

-444 848.25 -650 556.39 -1 739 752.67

3.761 3.838 3.749

4.783 5.172 5.125

(b) Clusters Ec, kJ/mol

Ei, kJ/mol

cluster

gas

s ) 20

gas

s ) 20

HB distance (Å)

HB angle (deg)

FA-CH3SO3DMF-CH3SO32FA-CH3SO3-(I)

-2 184 383.47 -2 390 065.86 -2 629 265.85

-2 184 625.74 -2 390 330.15 -2 629 512.44

-94.08 -62.36 -161.28

-24.82 -21.09 -63.27

2FA-CH3SO3-(II)

-2 629 278.73

-2 629 510.02

-174.16

-60.85

1.856 N/A FA-FA, 1.824; FA-CH3SO3-, 1.780 1.894

164.78 N/A FA-FA, 177.23; FA-CH3SO3-, 176.34 166.09

Figure 7. FA, DMF, and CH3SO3- monomers and their clusters at MP2/6-31G(d,p) optimized geometry. White, gray, red, blue, and yellow denote hydrogen, carbon, oxygen, nitrogen, and sulfur, respectively.

Although the geometries of the monomers in a complex should be slightly different from those of the isolated ones, the purpose of this study is a simple estimation of the interaction energies of the polar headgroup of AOT and solvent molecules. The hydrogen-bond distances and angles in the complexes estimated by the MP2/6G-31(d,p) calculations are in the typical range of hydrogen bonds.62-64 The values of the calculated Ei in the hydrogen-bonding complexes are in the range of the typical hydrogen-bonding energies.62-64 The interesting points to be noted from the calculation results are as follows. (i) The Ei of DMFCH3SO3- at s ) 20 is as large as that of FA-CH3SO3- at s ) 20. (ii) The hydrogen bonds become stronger by the extended hydrogen-bonding network of the FA molecules [2FA-CH3SO3-(I)], and the hydrogen bond of FA and (62) Jeffrey, G. A.; Saenger, W. Hydrogen Bonding in Biological Structures, 1st ed.; Springer-Verlag: Berlin, 1991. (63) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997. (64) Stone, A. J. The Theory of Intermolecular Forces; Oxford University Press: Oxford, 1996.

CH3SO3- becomes weaker by the additional hydrogen bond with CH3SO3- [2FA-CH3SO3-(II)]. Interestingly, the calculated Ei of the nonhydrogenbonded DMF-CH3SO3- cluster at s ) 20 is quite large. This value is quite similar to the calculated Ei of the hydrogen-bonded FA-CH3SO3- cluster at s ) 20. It indicates that even aprotic DMF interacts strongly with the polar headgroup of AOT as well as protic FA. We can expect that the mobility of DMF near the reverse micelle cavity surface is suppressed by the strong interaction. The solvation result shows that both FA and DMF in reverse micelles have very slow solvation components of about 10 ns. The slowest solvation time constants of FA and DMF in reverse micelles are almost the same: about 13 ns for w ) 1 and about 11 ns for w ) 2. The retardation of the solvation dynamics in reverse micelles could be due to the specific strong interaction between the polar solvent molecule and the polar headgroup of the surfactant. Recently, Fourkas and co-workers investigated the dynamics of liquid molecules in nanoporous glasses using femtosecond optical Kerr effect spectroscopy.65-71 The relaxation time constant of both weakly and strongly interacting solvent molecules with the surface of porous glass is much slower than that for the neat solvent molecules and depends on the pore size. They indicated that the retardation of the relaxation of liquid molecules in nanoporous glass is due to the geometry constraints of the nanocavity. The retardation of the solvation dynamics of FA and DMF near the polar headgroups of the surfactants might be due to the geometry constraints of the polar solvent pool, as well as nanoporous glasses. This is because the polar solvent pool size of the FA reverse micelle becomes larger with the larger w and that of the DMF reverse micelle is independent of w. However, the slowest solvation time constant of the methanol in reverse micelles strongly depends on w, though the diameter of the methanol pool in the reverse micelle solution has less dependence on w.37 If we assume that the origin of the (65) Loughnane, B. J.; Farrer, R. A.; Scodinu, A.; Reilly, T.; Fourkas, J. T. J. Phys. Chem. B 2000, 104, 5421. (66) Farrer, R. A.; Loughnane, B. J.; Fourkas, J. T. J. Phys. Chem. A 1997, 101, 4005. (67) Loughnane, B. J.; Fourkas, J. T. J. Phys. Chem. B 1998, 102, 5409. (68) Loughnane, B. J.; Fourkas, J. T. J. Phys. Chem. B 1998, 102, 10288. (69) Loughnane, B. J.; Farrer, R. A.; Scodinu, A.; Fourkas, J. T. J. Chem. Phys. 1999, 111, 5116. (70) Loughnane, B. J.; Scodinu, A.; Fourkas, J. T. J. Phys. Chem. B 1999, 103, 6061. (71) Loughnane, B. J.; Scodinu, A.; Fourkas, J. T. Chem. Phys. 2000, 253, 323.

Formamide and N,N-Dimethylformamide

retardation of the solvation dynamics of FA and DMF in reverse micelles is similar to that of the methanol one, the effect of geometry constraints could be relatively small in comparison with the effect of the specific strong interaction between the polar solvents and the polar headgroup of the surfactant. Another interesting point of the MP2/6-31G(d,p) calculation results of the mimic clusters is the hydrogenbond strength of the clusters. The 2FA-CH3SO3- clusters show that the hydrogen-bond strength is rather changeable by the hydrogen-bonding network and the number of hydrogen bonds of the polar headgroup of the surfactant with the protic solvent molecules. It can expect that the w dependence on the solvation dynamics of FA in reverse micelles observed in the present study could be due to the complex hydrogen-bonding interactions between FA and AOT or FA and FA in the polar solvent pool of the reverse micelle solution. In contrast to protic FA, only a tiny w dependence on the solvation dynamics has been observed in aprotic DMF in reverse micelles. Because neat DMF has a more ambiguous structure than neat FA,49-51 the interaction between DMF and DMF in reverse micelles should also be weak. Therefore, the feature of the tiny w dependence on the solvation dynamics of DMF in reverse micelles might arise from the distinct regions in the reverse micelles: the bulk region and the nanocavity surface in the simplest case. Interestingly, Fourkas and co-workers suggested that the feature of the diffusive dynamics of the weakly wetting liquid in nanoporous glass can be wellexplained by a two-state model,65-71 and Schmuttenmaer and co-workers also explained the effect of the reverse micelle size on the hydrogen-bonding librational band of confined water and methanol on the basis of a two-state model.72 (72) Venables, D. S.; Huang, K.; Schmuttenmaer, C. A. J. Phys. Chem. B 2001, 105, 9132.

Langmuir, Vol. 20, No. 2, 2004 335

Conclusion The solvation dynamics of FA and DMF in AOT reverse micelles is extremely slow in comparison with that of pure FA and DMF. The solvation dynamics of DMF in reverse micelles shows a tiny w dependence of the reverse micelle solution. In contrast to aprotic DMF, protic FA in reverse micelles depends strongly on the w on the solvation dynamics. From the MP2/6-31G(d,p) optimized calculations, we have found that the interaction energy of FA and the polar headgroup of AOT is as large as that of DMF and the polar headgroup of AOT. Further, the MP2/ 6-31G(d,p) calculation result of the 2FA-CH3SO3- clusters has indicated that the hydrogen-bond strengths of FACH3SO3- and FA-FA are strongly influenced by the hydrogen-bonding network and the number of hydrogen bonds of the polar headgroup of the surfactant with the protic solvent molecules. The different features of the w dependence on the solvation dynamics of FA and DMF in reverse micelles would arise from the different specific interactions between the polar solvent molecule and polar headgroup of the surfactant and the polar solvent molecule and polar solvent molecule. Acknowledgment. We thank Dr. Hiroshi Ushiyama and Dr. Michio Iwaoka (both of University of Tokyo) for helpful discussions regarding the ab initio calculations. The work is supported in part by the Mitsubishi Chemical Corporation Fund and the Shiseido Fund for Science and Technology (H.Shirota). We also thank the Ministry of Education, Culture, Sports, Science and Technology of Japan (Gant-in-Aid for Scientific Research on Priority Areas, 417). LA030161R