Spin-Probe ESR Study on the Dynamics of Liquid Molecules in the

XRD pattern together with the surface area obtained by the BET method. Since we .... the nanochannel of me-MCM-41, and the larger rotational anisotrop...
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13180

J. Phys. Chem. B 2005, 109, 13180-13185

Spin-Probe ESR Study on the Dynamics of Liquid Molecules in the MCM-41 Nanochannel: Temperature Dependence on 2-Propanol and Water Masaharu Okazaki* and Kazumi Toriyama Research Institute of Instrumentation Frontier, National Institute of AdVanced Industrial Science and Technology (AIST), 2266-98, Shimoshidami, Moriyama-ku, Nagoya, 463-8560, Japan ReceiVed: March 11, 2005; In Final Form: May 22, 2005

A spin-probe ESR study has been made on the dynamics of 2-propanol and water molecules in the nanochannel of MCM-41 at various temperatures. In the former system, 2-propanol is separated into two phases: one with molecules immobilized in the ESR time scale and the other with mobile ones, even at temperatures more than 40 degrees higher than the bulk melting point. In the case of water, on the other hand, only the “immobilized” water was detected at a temperature as high as 313 K. At higher temperature, spin-probe molecule undergoes anisotropic rotational diffusion to reduce resistance from the solvent molecules in the nanochannel. These results are explained in relation to the intermolecular network intensified in the nanochannel. Static as well as dynamic structures of these solutions have been discussed.

Introduction Since the discovery of MCM-41, a mesoporous material with ordered nanochannels,1-3 the research field of nanochemical processes has been expanding steadily.4-6 Although this is a new field, relating studies were made from older times. For example, the effect of confinement in a nanospace of the reaction systems, which have a pair of radicals as an intermediate, have been studied through the magnetic field effect on their yield.7-9 The effect of spin manipulation on the radical pair intermediates in the photoreactions in SDS micelle has also been studied.10 Extension of these studies to the reactions in the nanospace supplied by MCM-41, a new mesoporous material, is quite natural,11-13 since the nanochannels are regularly ordered and well-defined.1-3 In this new line of studies, it has been shown that a solution can be flown through the nanochannel of MCM41,11,12 which is open at both ends, with a flow apparatus at a medium pressure, and a large magnetic field effect has been observed on the photoreduction of quinones in a solution flowing there.11 These observations are against the classical hydrodynamic theory, that is, according to Hagen-Poiseulle’s law, a solution cannot flow in the nanochannel of MCM-41 packed in the column using a usual flow apparatus.12 In addition, the molecules should flow collectively, since the successful observation of a large magnetic field effect on a reaction indicates that the two intermediate radicals do not diffuse away during the flow in the nanochannel.11,13 These studies show the necessity to accumulate experimental data to clarify the molecular dynamics in the nanochannel. In the present study, we focus on the dynamical nature and its temperature dependence of the liquid molecules in the nanochannel of MCM-41. ESR observation was made for 2-propanol, employed in the previous studies,11,13 and water, the most important solvent in many respects, solutions of a few spin probes encapsulated in the MCM-41 nanochannels at a series of temperatures. As the spin probes,14 di-tert-butylnitroxide (DTBN) and 2,2,5,5-tetramethyl-piperidine-1-oxyl (TEM* Corresponding author. Telephone: +81-52-7367138. Fax: +81-52736-7405. E-mail: [email protected].

POL) were selected, since these spin probes are freely soluble in both solvents and do not have strong interactions with the channel wall of MCM-41. The ESR spectra are analyzed by taking the interaction between the spin-probe molecule and the solvent molecules into account; next, the physical states of the solvents are discussed on the basis of the assumption that the dynamics of probe molecule well reflects the dynamics of solvent molecules. Experimental Section MCM-41 was synthesized following the method in a reference by using TEOS, tetraethyl-orthosilicate, as silica source and CTAB, cetyltrimethylammonium-bromide, as the template micelle molecules.15 The pore size was estimated as 3.4 nm from the lattice constant for (100) planes obtained from the powder XRD pattern together with the surface area obtained by the BET method. Since we only employed CTAB as the template source, we need not use the carbon number of the surfactant molecule to specify the MCM-41 in this paper. Trimethylsilylation of the silicate surface was performed using trimethyl-silyl chloride following the procedure described in a reference.16 This modified MCM-41 is referred to hereafter as me-MCM-41. All the chemicals were purchased from Wako Pure Chemicals (Tokyo, Japan), and quartz beads with diameter of 100 micron (Q-beads) are from TOSOH, Japan. Samples for ESR observation were prepared by the following procedure, using DTBN and TEMPOL as the spin probes. Onetenth gram of MCM-41 powder was put into a spherical Pyrex vessel, to which an ESR sample tube was attached, and was dehydrated at 400 K for more than 10 h in atmosphere and then was degassed at the same temperature for more than 1 h under the vacuum ( 25) upon lowering the temperature in parallel with strengthening of the H-bonding. In this system, rotation along its x-axis does not break the H-bond, that is, the x-axis is the virtual longest axis for the complex with TEMPOL and the solvent molecules. To see the effect of surface structure, the system in the nanochannel of me-MCM-41 was also observed. By comparing spectra a and c, it is found that the rotation of the DTBN molecule is inhibited to a little larger extent and that the rotational anisotropy is also larger in the nanochannel of me-MCM-41 than in that of MCM-41. Restriction in rotational motion may be due to the smaller diameter of the nanochannel of me-MCM-41, and the larger rotational anisotropy may be related with the fact that 2-propanol is less miscible with the methylated channel surface. The solvent network must be stronger in the nanochannel whose wall expels the solvent molecules. 2. Temperature Dependence of the ESR Spectrum of DTBN in 2-Propanol. ESR spectra at various temperatures for a 2-propanol solution of DTBN encapsulated in the nanochannel are shown in Figure 2. Upon lowering the temperature, the heights of the sharp lines decrease and a broad signal, which is characteristic to the immobilized nitroxide radical,14 appears at around 250 K. This temperature is more than 60 K higher than the freezing point of the bulk solvent. The sharp three-line spectrum (mobile nitroxide radical) and the broad spectrum (almost immobilized radical) coexist in the temperature range from 253 to 193 K, and the former completely disappears at around the freezing point of 2-propanol in the bulk. These spectral changes are reversible. Such phenomena were not observed in the control system, where quartz beads were used to give the solution a narrow space, as shown in Figure 3. The main features are (1) the threeline spectrum at higher temperatures are those for the radical undergoing isotropic rotational diffusion, (2) the three-line component becomes broader gradually as the temperature of the system is lowered, and (3) the spectrum changes abruptly into that of a rigid system at a temperature (158 K) a little lower than the freezing point (185 K) of the solvent. The last observation is due to supercooling followed by rapid solidification of the medium in a cooperative way. Simultaneously, with this rapid spectral change, the loaded Q-value of the ESR cavity

13182 J. Phys. Chem. B, Vol. 109, No. 27, 2005

Okazaki and Toriyama

TABLE 1: Rotational Anisotropy and the Correlation Times of Spin Probes in 2-Propanol Estimated from the Spectrual Simulationa spectrum

τav/s averaged correlation time

τ (//)/τ (⊥)

preferredaxis

∆H/mT

aN & aH/mTb

DTBN/MCM-41

Figure 1a

9.0 × 10-11

13

Y

0.020

DTBN TEMPOL/MCM-41

Figure 1b Figure 1d

5.0 × 10-12 8.5 × 10-11

aN ) 1.59 aH ) 0.01

1 3

Y

0.020 0.020

Figure 1e Figure 1f Figure 1c

4.0 × 1.4 × 10-10 2.0 × 10-10

10 25 25

X X Y

0.020 0.020 0.020

system

TEMPOL TEMPOL/MCM-41 at 253 K DTBN/me-MCM-41

10-11

aN ) 1.605 aH ) 0.04 aN ) 1.591 aH ) 0.01

a A g-tensor (2.0089, 2.0061, 2.0027) and a hyperfine coupling tensor (Axx, Ayy, Azz) ) (0.76, 0.60, 3.18 mT) were employed for simulation in all cases. ∆H is the residual width for the component lines. b aN and aH are the isotropic hyperfine coupling constants for nitrogen and β-hydrogen nuclei, respectively. aH values were taken from literature19 and ∆H for simulation was adjusted to get the best fit in simulation of the TEMPOL system.

Figure 3. Temperature dependence of the ESR spectrum of DTBN in 2-propanol in the presence of quartz beads, whose diameter is 100 µm. Figure 2. Temperature dependence of the ESR spectrum of DTBN in 2-propanol encapsulated in the nanochannel of MCM-41.

resonator becomes much higher.21 These results indicate that the “immobilized pattern” in the control system is observed only for the frozen solution. Appearance of the ESR pattern for the immobilized spin probe at a temperature much higher than the melting point of the solvent must be one of the nanospace effects that can be explained by the intermolecular network intensified in the nanochannel. Because of the small volume of liquid in the nanochannel, whose diameter and length are about 3.4 nm and 5 µm, respectively, and the stiff intermolecular network, the liquid molecules do not diffuse freely as in the bulk. If we consider molecular diffusion and molecular-cluster diffusion as the elementary processes, the rate of the latter in the nanochannel may be much less than that in the bulk. Since local density fluctuations may be small in amplitude in the nanospace compared with that in the bulk, the excess space for diffusion (which corresponds to the vacancy for diffusion in a molecular crystal) may not be supplied enough in the nanochannel as in the bulk. Therefore, translational diffusion must be reduced considerably in the nanochannel as has been observed experimentally.11,13 When the intergranular space is also filled with the solution, however, this model does not mean that the solvent molecules stay for a long time in the nanochannel.22 Instead,

the solution goes in and comes out frequently18,23 as a group or a cluster. In fact, molecules flow collectively in the nanochannel at a usual pressure given to the column packed with MCM41.12 By the same mechanism of this collective flow, rapid “collective diffusion” in to and out of the nanochannel must be possible by the instantaneous pressure difference between the two ends of the nanochannel induced by thermal aggitation.20 Another interesting result is the observation of an ESR spectrum with two components even at a temperature 60 K above the freezing point. This phenomenon can be explained with the phase-separation model:18,23,24 one is the ordered phase in the central portion and the other is the random phase near the channel wall. We also presented the phase-separation model for the following two systems: (1) long-chained n-alkanes solidified in the MCM-41 nanochannel,25 in which amorphous phase is created near the channel wall and the crystalline phase at the central part; (2) a mixture solution, where the hydrophilic component is rich in one phase near the channel wall and the hydrophobic component is rich in the other phase near the center.18 We consider that the same kind of phase separation also occurs in the present liquid system in the temperature range between the melting point and several tens of degrees higher than that. Since the guest molecule can get a larger free volume in the random phase, the spin probe there may give a sharp signal because of rapid rotation. On the other hand, spin-probe

Spin-Probe ESR Study on 2-Propanol and Water

J. Phys. Chem. B, Vol. 109, No. 27, 2005 13183

Figure 4. ESR spectra of DTBN (a and b) and TEMPOL (c and d) at 253 K and at 233 K, respectively, in 2-propanol entrapped in the nanochannel of MCM-41. Integrated spectra are overlaid on the firstderivative spectra.

molecule in the ordered phase may be immobilized, since it is difficult for the large guest molecule either to get rid of the solvent molecules from the rotational orbit or to get enough free space. One might consider that the broad signal is due to adsorption of the spin-probe molecule on the nanochannel wall. To check this possibility, the spectra for both the DTBN and TEMPOL systems were integrated as shown in Figure 4, where usual derivative ESR spectra were integrated once and overlaid to the originals to ease estimation by eye of the relative concentrations in the two phases. The percentages of the mobile part obtained by spectral simulation are 10% and 18% for the DTBN system and 15% and 25% for the TEMPOL system, at 233 K and 253 K, respectively. Differences of these percentages between the two systems are too small, if H-bonding between the SidO or Si-OH group on the nanochannel surface and the OH group of TEMPOL plays the main role. At the same time, it is difficult to consider that both nitroxide radicals are adsorped by hydrophobic interaction on the hydrophilic silica surface. So, the simultaneous appearance of two ESR patterns may be due to the partitioning of the free radicals into two liquid phases in the MCM-41 nanochannel. 3. ESR Spectra for Aqueous Solutions at Higher Temperatures. Figure 5 shows the ESR spectra of DTBN (a-c) and TEMPOL (d) in water observed in the bulk (a), in the nanochannel of MCM-41 (b, d), or in that of me-MCM-41(c) at the ambient temperature (296 K). We are surprised at the large difference between the ESR spectra observed in bulk water (a) and in water encapsulated in the nanochannel of MCM-41 (b). This reflects a large difference between the dynamics of water molecules in the two states, that is, mobility of water molecules is restricted severely in the MCM nanochannel. We have to pay attention to the characteristic shape of spectrum b, whose middle line is as sharp as that usually observed in the bulk solution sample but the rest appears broad enough which indicates that the motion of the spin probe is in the “slow tumbling region”.26 Although the details of the motion will be given after spectral simulation by the method specifically devised for this time domain, the qualitative feature will be given in a straightforward way by using the spin Hamiltonian averaged by the anisotropic rapid rotation. It is understood quite easily that the spin-probe DTBN rotates most easily along its y-axis, since only the central line is very sharp. The spin Hamiltonian H that characterizes the ESR spectrum is

H ) βB0gS + INAS

(1)

where the first term expresses electron Zeeman interaction and

Figure 5. ESR spectra of DTBN (a-c) and TEMPOL (d) in water: in clear solution (a), in the nanochannel of MCM-41 (b, d) or in that of me-MCM-41 (c) at 296 K.

the second term hyperfine interaction of the electron spin with the nitrogen nucleus. The tensor components have been obtained as (gXX, gYY, gZZ) ) (2.0087, 2.0062, 2.0027) and (AXX, AYY, AZZ)/mT ) (0.76, 0.6, 3.18).27 By using these values, the tensor components partially averaged by rapid y-axis rotation are obtained as

(gXX, gYY, gZZ) ) (2.0057, 2.0062, 2.0057) (AXX, AYY, AZZ)/mT ) (1.97, 0.6, 1.97)

(2)

Because of the small difference between the g// (gYY) and g⊥ (gXX, gZZ) and the fact that the secular terms of the hyperfine interaction do not contribute to the width of the MN ) 0 line, the central line becomes very sharp. The observation that the widths for both the MN ) +1 and -1 components of spectrum b are also much smaller than those of the powder spectrum (e.g., a spectrum in Figure 3 observed at a temperature lower than 158 K) can also be explained with the above partially averaged spin Hamiltonian. Comparison between the spectrum b of Figure 5 and the spectrum of Figure 3 observed at 173 K, both of which have similar line widths for the central line, may demonstrate nicely the effect of rapid rotation along the y-axis to the ESR spectrum. The broader lines of spectrum c of Figure 5 compared with that of spectrum b may be due to either a smaller diameter of the nanochannel of me-MCM-41 or a bumpy surface (or both) of the nanochannel caused by trimethylsilylation. In the nanochannel of me-MCM-41, thus, it may be difficult to make a hollow for the guest molecule by the close intermolecular network. The cause of the broadening for spectrum d (TEMPOL) is easily assigned to both the H-bonding between the OH group of TEMPOL and the water molecules and the absence of an axis along which rotation occurs easily. As mentioned above for the smooth rotational diffusion of the large guest molecule, a spin probe, translational motion of the small solvent molecules must be allowed. Thus, the translational diffusion of water molecules here must be severely restricted. However, other motions such as low-frequency rotation and wagging motion, which modulate the dipolar vector of water molecules, are allowed at this temperature, since the

13184 J. Phys. Chem. B, Vol. 109, No. 27, 2005

Okazaki and Toriyama molecule does not necessarily indicate the true solidification of the medium. The solidification temperature as a function of channel radius is given by the Gibbs-Thompson equation:32

∆Tm/Tm ) S(Γlw - Γcw)/Qm

Figure 6. Temperature dependence of the ESR spectrum of DTBN in water encapsulated in the nanochannel of MCM-41

microwave loss in the ESR cavity resonator remains rather high until the water solidifies at 233 K.28 4. Temperature Dependence of the ESR Spectrum of DTBN in Water. Figure 6 shows temperature dependence of the ESR spectrum of DTBN in water included in the nanochannel of MCM-41. At 333 K, the spectrum, which is a “solution ESR”,29 shows that the spin probe undergoes very anisotropic rotational diffusion with preferred rotation along its y-axis. The water molecules must have strong intermolecular network even at this temperature and the DTBN molecule rotates most smoothly along its y-axis in the cave made by the water network. The spectrum at 273 K indicates that the DTBN molecule still rotates almost freely along the y-axis, but the rotation is highly restricted along both the x- and z-axes. Upon cooling the system further, the largest change appears at around 233 K, where the rotational diffusion along all axes becomes prohibited. We know this, since the peak positions of the Mn ) (1 sublevel are almost the same as those observed at 77 K. This temperature must be a transition point, since the quality factor of the ESR cavity resonator changes largely at this temperature. This may be the true melting point in the nanochannel.28 In this system, coexistence of two different ESR signals showing phase separation were not observed at any temperature.30 Since the spin probe is much bigger than the solvent water molecule, the thickness of the random phase in the nanochannel, which may be around 0.4 nm in ice,31 is not enough to accommodate the spin-probe molecule (see Scheme 1). In addition, the interaction between the nanochannel wall and the water molecule is much stronger than that between the wall and the probe molecule; thus, the spin probe is blocked from the phase near the channel wall. 5. On the Restriction of Rotational Diffusion of Spin Probes at High Temperatures. Immobilization of a spin-probe molecule as detected by its ESR spectrum at temperatures much higher than the bulk melting point of the solvent must be an intrinsic phenomenon for a solution encapsulated in the nanochannel. This is because, as mentioned above, a smooth rotational motion of the guest molecule is allowed only when a rapid translational diffusion of solvent molecules occurs in a cooperative way in a nanopore. The observed immobilization of a guest

(3)

Here, Tm and ∆Tm are the melting temperature and its shift (Tm (bulk) -Tm (pore)) due to nanopore effect, respectively; S is the surface area of the nanochannel, Γlw and Γcw are the surface energy for liquid-phase/channel wall and that for crystallinephase/channel wall, respectively; and Qm is the heat of melting. Since Γlw is considered to be larger than Γcw,32 some depression is predicted on the melting temperature. Therefore, we may not expect solidification of the solution employed here at a temperature higher than the melting point. In fact, the quality factor of the ESR cavity remains high until the water in the nanochannel becomes ice at around 233 K. Although elevation of freezing temperature has been observed in the system where the wall-molecular interaction energy is larger than the interaction energy between the molecules,33,34 this is not the case in the present systems. 6. Collective Rapid Flow of Solution in the Nanochannel of MCM-41. In relation to the above nanopore effect, we already presented another nanopore effect: collective flow of liquid molecules in the nanochannel. That is, liquid molecules flow in the nanochannel without rapid translational diffusion as in the bulk.11 The collective behavior of the molecules indicates that molecules tend to be trapped in the intermolecular potential well. Thus, some part of their individual translational energies is converted into the translational energy of the cluster. Therefore, collision frequency as well as collision speed of the molecules to the channel wall decrease, and the transfer rate of momentum (along the flow direction) to the wall must also decrease. Since the friction force is proportional to the rate of momentum transfer, we can expect an increase in the flow rate from that predicted by the Hagen-Poiseulle law.20,23 After all, we can explain all the phenomena presented here and our previous observations by the collective nature of the liquid molecules in the nanochannel. Conclusion Spin-probe molecules in both 2-propanol and water encapsulated in the nanochannel of MCM-41 undergo anisotropic rotational diffusion at high temperatures. In addition, they are immobilized in the ESR time scale even at temperatures several tens degree higher than the melting points. These phenomena have been explained with (1) the collective behavior of the solvent molecules in the nanochannel because of the reinforced intermolecular network, which is especially strong in the water system; (2) structuring of the solution molecules into two regions in the nanochannel, one is ordered in the central part and the other is randomly oriented near the channel wall; and (3) the mechanism of slip flow in the nanochannel at a rate much larger than that predicted by the classical Hagen-Poiseulle law has been suggested. Acknowledgment. Financial support of Japan Science and Technology Corp. through the Cooperative System for Supporting Priority Research is acknowledged. This work was partially assisted by a Grant-in-Aid for Scientific Research on Priority Area Innovative utilization of strong magnetic fields (767, 5085208) from MEXT of Japan. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710.

Spin-Probe ESR Study on 2-Propanol and Water (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, W. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (4) Zhao, X. S.; Lu, G. Q.; Miller, G. J. Ind. Eng. Chem. Res. 1996, 35, 5, 2075. (5) Rouhi, A. M. Chem. Eng. News 2000, 78 (Aug 21), 40. (6) Coma, A. Chem. ReV. 1997, 97, 2373. (7) Sagdeev, R. Z.; Molin, Yu, N.; Salikhov, K. M. Org. Magn. Reson. 1973, 5, 603. (8) Atkins, P. W.; Lambert, T. P. Annu. Rep. Prog. Chem. 1975, 72A, 67. (9) Tanimoto, Y.; Hayashi, H.; Nagakura, S.; Sakurai, H.; Tokumaru, K. Chem. Phys. Lett. 1976, 41, 267. (10) Okazaki, M. In Dynamic Spin Chemistry; Nagakura, S., Hayashi, H., Azumi, T., Eds.; Kodansha and Wiley: Tokyo and New York, 1998; Chapter 8. (11) (a) Okazaki, M.; Konishi, Y.; Toriyama, K. Chem. Phys. Lett. 2000, 328, 251. (b) Konishi, Y.; Okazaki, M.; Toriyama, K.; Kasai, T. J. Phys. Chem., B 2001, 105, 9101. (12) Okazaki, M.; Toriyama, K.; Sawaguchi, N.; Oda, K. Appl. Magn. Reson. 2003, 23, 435. (13) Okazaki, M.; Toriyama, K.; Oda, K.; Kasai, T. Phys. Chem. Chem. Phys. 2002, 4, 1201. (14) Berliner, L. J. Spin Labeling II; Academic Press: New York, 1979. (15) Burkett, S. L.; Sims, S. D.; Mann, S. Chem. Commun. 1996, 1367. (16) Zhao, X. S.; Lu, G. Q. J. Phys. Chem. B 1998, 102, 1556. (17) Freed, J. H. J. Chem. Phys. 1964, 41, 2077. (18) (a) Okazaki, M.; Kuwata, K. J. Phys. Chem. 1984, 88, 4181. (b) Okazaki, M.; Toriyama, K.; Sawaguchi, N.; Oda, K. Bull. Chem. Soc. Jpn. 2004, 77, 87. (19) (a) Windle, J. J. J. Magn. Reson. 1981, 45, 432. (b) Kotake, Y.; Kuwata, K. Chem. Lett. 1984, 83.

J. Phys. Chem. B, Vol. 109, No. 27, 2005 13185 (20) Okazaki, M.; Toriyama, K. J. Phys. Chem. B 2003, 107, 7654. (21) Alger, R. S. Electron Paramagnetic Resonance-Techniques and Applications; Interscience: New York, 1968. (22) The ESR spectrum for the system where the solution also fills the intergranular space is a single-component spectrum and not the overlapped spectra with two components. This means that the rate of site exchange is larger than 1/γT2, where T2 is the spin-spin correlation time of the system. (23) Stallmach, F.; Graeser, A.; Kaerger, J.; Krause, C.; Jeschke, M.; Oberhagemann, U.; Spange, S. Microporous Mesoporous Mater. 2001, 4445, 745. (24) Gelb, L. D.; Gubbins, K. E.; Radhakrishnan, R.; Bartkowiak, M.S. Rep. Prog. Phys. 1999, 62, 1573. (25) (a) Toriyama, K.; Okazaki, M. J. Phys. Chem. B 2004, 108, 12917. (b) Okazaki, M.; Toriyama, K.; Anandan, S. Chem. Phys. Lett. 2005, 401, 363. (26) Goldman, S. A.; Bruno, G. V.; Polnaszek, C. F.; Freed, J. H. J. Chem. Phys. 1972, 56, 716. (27) Libertini, L. J.; Griffith, O. H. J. Chem. Phys. 1970, 53, 1359. (28) Schreiber, A.; Ketelsen, I.; Findenegg, G. H. Phys. Chem. Chem. Phys. 2001, 3, 1185. (29) Solution ESR is defined by the inequality γH1′τ , 1, where γ is the gyromagnetic ratio of electron, H1′ is the perturbation Hamiltonian which gives the line width, and τ is the correlation time. (30) Weak sharp signals at 323 K and at 293 K of Figure 5 and spectrum c of Figure 4 may be due to the DTBN molecule in water on the outer surface of MCM-41 particles. (31) Christenson, H. K. J. Phys.: Condens. Matter 2001, 13, R95. (32) Dullien, F. A. L. Porous Media-Fluid Transport and Pore Structure; Academic Press: New York, 1992. (33) Watanabe, A.; Iiyama, T.; Kaneko, K. Chem. Phys. Lett. 1999, 305, 71. (34) Radhakrishnan, R.; Gubbins, K. E.; Sliwinska-Bartkowskiak, M. J. Chem. Phys. 2002, 116, 1147.