Transportation of Aqueous and Alcoholic Solutions through the

May 29, 2009 - Real-time ESR observations have been made on the aqueous solutions of di-tert-butyl nitroxide and 2,2,6,6-tetramethylpiperidine-1-oxyl-...
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J. Phys. Chem. C 2009, 113, 11086–11094

Transportation of Aqueous and Alcoholic Solutions through the Nanochannel of MCM-41: A Spin Probe Electron Spin Resonance Study Masaharu Okazaki,* Ping Jin,† Kazutoku Ohta, and Kazumi Toriyama Research Institute of Instrumentation Frontier and Research Institute of Sustainable Materials, National Institute of AdVanced Industrial Science and Technology (AIST), 2266-98 Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan ReceiVed: March 2, 2009; ReVised Manuscript ReceiVed: May 14, 2009

Real-time ESR observations have been made on the aqueous solutions of di-tert-butyl nitroxide and 2,2,6,6tetramethylpiperidine-1-oxyl-4-ol flowing in a quartz column of 0.81 mm L packed with well-dried MCM41. In both systems, the ESR spectrum is composed of two signals, the major signal (>98% of the total) is a very broad one assigned to the radical in the nanochannel of MCM-41, and the minor signal is a sharp one due to the radical in the bulk space between the MCM-41 particles. Although the spin probes are highly condensed deep in the cylindrical nanospace of MCM-41, they are transported downstream in a rather short time. The analysis of these observations led us to conclude that the aqueous solution is transported through the nanochannel of MCM-41 at a small rate but still much larger than that predicted by the conventional law. The same type of experiment was made with ethanol solutions of the same spin probes, whose ESR spectra also show different shapes in the two spaces at a high concentration but not a distinct adsorption. In this case, the transportation must be smooth for both components, since the time profile of the ESR signal for the flow in the MCM-41-packed column is almost homologous with that observed in the open column. Since the translational diffusion of the individual molecules is quenched in the nanochannel, as being reported in earlier studies, the solute as well as the solvent molecules should move collectively through the nanochannel. The present technique to study the fluid flow in the nanospaces may be called “spin probe nano flowmetry”. Introduction

SCHEME 1: Molecular Models for the Spin Probes

MCM-41 is a new type of mesoporous silica with hexagonally packed nanochannels synthesized by the template method1,2 and draws increasing attention of many researchers as a new nanomaterial having many possibilities in future chemistry.3,4 The molecular dynamics and the chemical reactions of the solution systems in the nanospace of MCM-41 have been studied for years, and many interesting features are known.3-6 For example, nonpolar liquid molecules diffuse rapidly in the nanochannel,7 whereas the diffusion of water molecules is heavily suppressed.8 The slow diffusion of the latter is due to the intensified intermolecular H-bonding in the nanochannel. As for the chemical reactions in the nanochannel, a large cage effect has been reported for the photochemical reaction of a ketone in an alcohol flowing in the column packed with MCM41 and has been interpreted with a model, in which the reaction intermediate radicals are not allowed to diffuse away since the molecules move collectively in the nanochannel.9-11 In previous works, we have also shown that upon condensation in the nanochannel the alcohol molecules form a wide hydrogen bond network after the completion of the first condensation molecular layer on the surface, and thus, the interaction with the silica wall is then weakened.12 At the same time the solute molecules, DTBN, for example, get a considerable rotational mobility in the nanochannel, but the translational diffusion is left severely inhibited.13,14 Therefore, we have concluded that the alcohol molecules form a liquid-crystalline-like structure in the nanochannel and flow with slipping on the surface of the nanochannel * To whom correspondence should be addressed. E-mail: masa-okazaki@ aist.go.jp. † Research Institute of Sustainable Materials.

collectively under much reduced shear.11 In the present work we made real-time ESR observations for the flowing aqueous and ethanol solutions of a few spin probes with different molecular polarities in the column packed with MCM-41. Since transportation of the fluid in the nanospace, especially of the aqueous solution, is important chemically as well as biologically,15,16 we would like to develop a simple model system for its better understanding. As the spin probes di-tert-butyl nitroxide (DTBN) and 2,2,6,6-tetramethylpiperidine-1-oxy-4ol (TEMPOL), as the pair for precise comparison, and 2,2,5,5tetramethylpyrrolidine-1-oxyl-4-carboxylic acid (TEMPOCA) as a reference have been employed (Scheme 1).17 Since the ESR spectra of both the DTBN and the TEMPOL systems show different patterns in the two spaces, the nanochannel and the space between the MCM-41 granules,18,19 we can analyze the flow behaviors of these molecules in these two spaces by the real-time ESR observations of these solutions flowing in the column packed with MCM-41. Comparison of the results to those obtained with the open column and the analysis based on the conservation rule for the spin probe during the flow process give some essential information on the flow behaviors of these systems. As a result, the transportation of the solution

10.1021/jp901908z CCC: $40.75  2009 American Chemical Society Published on Web 05/29/2009

Spin Probe Study on Water Flow in an MCM-41 Channel

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Figure 1. (A) Schematic diagram for the apparatus. (B) Definition of the spaces in the flow column, their cross sections, and the flow velocities of the solute. The volume of the stock loop is 0.4 mL, and the flow rate employed was 0.05 mL/min, which corresponds to a flow velocity of 9.7 cm/min in the column with an inner diameter of 0.81 mm. The actual flow rate was determined by weighting the effluent, and all data were processed with the corrected flow volume. In addition to V1 and V2, another cock, V3, is located between V2 and the column to ease the setup processes. The concentration profile of the spin probe at the inlet of the column is drawn schematically at the bottom of (A).

molecules through the nanochannel has been confirmed for the aqueous and the ethanol solutions. Experimental Section Materials. MCM-41 was synthesized following the method given in the literature from tetraethyl orthosilicate (TEOS) and dodecyltrimethylammonium bromide (DTAB).20 Template molecules were removed by washing with water and then calcination at 820 K for 5 h. The average granular size is around 5 µm (SEM images, Supporting Information, Figure 1S). A pore diameter of 3.0 nm and a specific volume of the nanochannel of 0.75 mL/g have been estimated from the separation of 3.12 nm ()d100) between the two adjacent layers obtained by the XRD analysis, the surface area of 1000 m2/g obtained by the BET method (Flowsorb II 2300, of Micrometrics Inc.), and the density of silica, which is 2.2521 by a simple calculation based on the model, in which the area of the outer surface of the MCM-41 particle is neglected.22 MCM-41 is stored in a bottle with anhydrous calcium sulfate (Drierite). The nitroxide radicals were purchased from Aldrich Japan (Tokyo), and the other chemicals were from Wako Pure Chemicals (Tokyo). Apparatus and ESR Observation. Figure 1 shows the diagram of the apparatus employed in the present study (A) and the averaged cross sections (R, β, γ) of the three spaces (I, nanochannel; II, space between MCM-41 particles; silica), respectively, and the flow velocities of the solute in the two open spaces (VI*, VII*) for the column packed with MCM-41 (B). The time profile of the spin probe concentration at the inlet of the MCM-41 column is also shown schematically below the apparatus. Column Packed with MCM. A quartz tube with an outer diameter of 4.2 mm was purchased from Eiko-sha (Osaka, Japan) as a handmade ESR cell for aqueous solution and was converted into the flow column by attaching a stainless joint (Supporting Information, Figure 2S). The inner diameter was determined as 0.81 mm from the volume of water that fills a part of the cell. MCM-41 was packed in the column as follows: First, quartz wool was packed at a height of about 1.5 cm, then MCM-41 was filled at a height of about 8 cm after cracking of large coagulated particles, and the quartz wool was again packed to retain the MCM-41 powder in the column. Ethanol was allowed to flow first to compress the packed MCM-41 at a

TABLE 1: Cross Sections of the Column Packed with MCM-41a L/mm

σ0/cm2

R/σ0

β/σ0

γ/σ0

0.81 2.0

0.0052 0.0804

0.349 ( 0.012 0.362 ( 0.017

0.44 ( 0.017 0.423 ( 0.026

0.21 ( 0.006 0.22 ( 0.01

a The sectional area σ0 of a ca. 0.8 mm L tube was carefully determined by filling water in the tubular space by the microsyringe and measuring the length of the filled part eight times. The quantity of MCM-41 in the flow column was weighed after being taken out of the column and drying overnight at 120 °C. Measurement for the 2.0 mm L column was also made in a similar way.

maximum pressure of about 10 MPa. The filling efficiency of MCM-41 particles, which was determined from the weight of MCM-41 and the filled height in the column, was obtained as a density F ) 0.47. The averaged sectional areas (R, β, γ) are (0.349 ( 0.012, 0.44 ( 0.017, 0.21 ( 0.006)σ0, respectively, where σ0 (0.0052 cm2) is the sectional area of the open space of the column, and have been determined on the basis of the weight of MCM-41, the volume of the filled part, and the volume of the nanochannel per unit weight. These are listed in Table 1, together with those of the 2.0 mm L column. The latter column was employed to confirm the results for the ethanol system and check the effect of the column diameter. Flow Experiments. The solutions were deoxygenized by argon gas bubbling for more than 30 min. A pump for liquid chromatography (Tosoh PeekDP-8020) flows the spin probe solution to fill the stock loop (Figure 1A), and then the other parts of the line are washed with the solvent after switching of the two cocks V1 and V2 to the other sides. After completion of the washing process, the flow rate is set at 0.05 mL/min (9.7 cm/min in the open column) and a period of time is allowed until a stable flow is established. Then the two cocks are returned to the original position in a short time, and the watch is started to measure the time of flow. The spin probe solution flows as a thread of about 87 cm toward the MCM-41 column following the pure solvent. The ESR spectrum is monitored repeatedly every 180 s. The observation errors are about (5% or so, and no error bars are added in the diagrams. ESR ObserWations. ESR observations were made with a conventional spectrometer (JEOL FX-1). The microwave power was 1.0 mW, and the amplitude of the field modulation at 100 kHz was 0.02 mT. The signal was stored in an NEC 9801

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Figure 2. First-derivative ESR spectra for the aqueous solutions of the spin probes at 2 mM (a, c-f) or at 30 mM (b) under the various flow conditions. Spin probe solutions flow in the open column (a, b), the column packed with MCM-41 (c-e), or Nucleosil-50 (f). In the open column the spectrum of TEMPOL at 2.0 mM is composed of three sharp lines with equal heights (a) but is heavily broadened at 30 mM (b) by the spin exchange effect. When the solution of TEMPOL (c) or DTBN (d) initially at 2.0 mM flows in the MCM-41-packed column, the main part of the spectrum is broadened in the nanochannel by both condensation (>80 mM) and immobilization. The sharp components in these spectra are due to the spin probe in the bulk space between the MCM-41 particles. When an ionic spin probe, TEMPOCA anion (e), is employed, the relative intensity of the broad component decreases considerably. This drastic condensation of the spin probe, DTBN, for example, is not observed when the packing material is changed to Nucleosil-50 (f), another mesoporous silica without the nanochannel structure. However, the large amount of the spin probe is in the nanospace of Nucleosil-50 as indicated by the modulation of the line heights from the usual triplet lines with equal heights. The vertical scale for each spectrum is arbitrarily chosen to ease the comparison between the spectral shapes. Other parameters are given in the Experimental Section.

personal computer equipped with an A/D converter and then transferred to another personal computer for data processing with Excel. The change in the signal intensity due to the quality factor (Q value) of the cavity resonator has been corrected for all data before numerical processing. The relative Q values for the data adjustments are 1.0 for the ethanol solution regardless of the existence of MCM-41 in the column, and those for the aqueous solutions are 0.75 and 0.60 with and without MCM41 in the column, respectively. Results Flow Experiment for the Aqueous System. Figure 2 shows the first-derivative ESR spectra for the aqueous spin probe solutions flowing in the quartz column under various conditions. In the open column all the spin probes show sharp triplet signals with equal heights at a concentration below a few millimolar (a). At a higher concentration (30 mM) the spectrum is broadened mainly by the Heisenberg spin exchange upon collision between the spin probe molecules (b). When the column is packed with silica powder, the ESR spectrum changes widely depending on both the pore structure and the spin probe. Spectrum c is observed for the aqueous solution of TEMPOL at 2 mM flowing in the column packed with MCM-41. The broad component is due to TEMPOL condensed in the nanochannel, and the sharp component is due to TEMPOL flowing in the bulk space between the MCM-41 particles.18 The concentrations of the TEMPOL radical in the nanochannel and that in the space between MCM-41 particles are calculated as about 81 and 1.4 mM, respectively, from the doubly integrated

Okazaki et al. values of each component and the cross sections of the spaces (Table 1) as will be described later. Spectrum d is observed for the aqueous solution of DTBN at 2 mM flowing in the column packed with MCM-41. In this case the concentration of DTBN in the nanochannel is larger, and that in the bulk space making the sharp component is smaller than the respective values for TEMPOL. This is due to the hydrophobic nature of DTBN.18 The unequal heights of the three major lines of spectra c and d have been assigned to the restricted motion of these radicals in the liquid-crystalline-like water in the nanochannel of MCM41 (Supporting Information, Figure 3S).23 Since little effect of spin exchange is observed on the spectrum even at these high concentrations,24 translational diffusion of the individual free radicals (spin probes) must be quenched strictly.12,14 The rather sharp spectrum e is that of an aqueous solution of TEMPOCA anion under the same experimental situation. Although the broad pattern is also overlapped, its intensity is much less than those in the two above systems. The ordered structure of the water molecules formed in the nanochannel may prevent the solvation shell formation around the TEMPOCA anion. The aqueous DTBN solution flowing in the column packed with Nuclesil50, another mesoporous silica without the channel structure, shows spectrum f. The sharp three-line spectrum indicates that the water structure and the dynamics of the solute molecule differ widely in the nanospace of Nucleosil-50 from those in the nanochannel of MCM-41, though the anisotropic rotational diffusion of the spin probe in the nanospace of Nucleosil is clearly shown as the modulation in the line heights of the hyperfine components. The use of TEMPOL and DTBN must be advantageous in studying the transportation process of the aqueous systems in the nanochannel of MCM-41, since the two components in both spectra c and d can be separated easily numerically (Supporting Information, Figure 4S). Figure 3 shows the ESR spectra of TEMPOL in water flowing in the column packed with MCM-41 (A) and the ESR intensities represented as the apparent concentration Cap in the flow column and other related values as functions of the flow volume (B). Cap is the concentration averaged over all the space in the ESRsensitive region of the column. A 0.4 mL portion of the spin probe solution is released at time zero from the storage loop (Figure 1) to the main flow toward the column. The time sequence of the spectra in (A) starts at the bottom and goes upward. The ESR spectra are shown as the absorption type, which is obtained by integrating the observed first-derivative spectrum. In this mode of the spectrum the area of a peak is proportional to the quantity of the corresponding free radical. As mentioned above, the main parts of the ESR spectra are broadened in the nanochannel seriously by both the restriction in the rotational diffusion and the electron-electron dipolar interaction.25 The total area of the spectrum is enlarged drastically since the spin probe is highly condensed in the nanochannel. A very small sharp signal is superimposed, which is due to the spin probe in the bulk space between the MCM41 particles. Labeled as a and b in Figure 3B are the concentrations of TEMPOL derived from the ESR intensities, in the open column and in the column packed with MCM-41, respectively, as functions of the flow volume q(t):

q(t) ) V0t

(1)

where V0 represents the flow rate, which is set at 0.05 mL/min, which corresponds to 9.7 cm/min in the opened column, and t is time (Figure 1). Here, we define S as the integrated value of Cap with respect to the flow volume q. Since the solution flows

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Figure 3. (A) Real-time ESR spectra of the aqueous solutions of TEMPOL at 2.0 mM flowing through the column packed with MCM-41 and (B) the apparent concentration of TEMPOL obtained from the ESR spectra (a, in the open column; b, in the MCM-41-packed column; c, the sharp component in the latter column) as a function of the flow volume. The ratio between the values on curves c and b multiplied by 500 are given as curve d. The data points for the corresponding spectra in (A) are shown as closed circles. The ESR spectra are shown as the absorption type, which are obtained by integrating the usual first-derivative-type spectra. The magnetic field is scanned by 10 mT in 2.5 min. The vertical axis for curves a-c has been graduated as the apparent radical concentration in the column. The ESR intensity has been corrected by the sensitivity factor checked by the standard DPPH sample inserted into the equipped hole. The integration of the spectrum was made with “ax + b”-type baseline correction. Most of the errors in obtaining the ESR intensities are mainly due to ambiguity in the spectral baseline, which could be suppressed within 5%. Data points marked with “$” are employed to calculate the transportation velocity of the TEMPOL downstream.

TABLE 2: Flow Parameters of the Solute Spin Probesa,b solvent

spin probe

CI(s)

CII(s)

S(I)

S(II)

H 2O H2O ethanol ethanol

DTBN TEMPOL DTBN TEMPOL

93.6 81.3 18.5 21.5

0.39 1.41 43.9 39.8

60.7 16.4 2.6 3.9

0.27 0.31 7.7 7.0

a CI(s) and CII(s) are the concentrations at the peak top values in each space (mM/L). S(I) and S(II) are the integration of the signal intensities of the spin probes in each space with respect to the flow volume. The units of C and S are mM and mM · mL, respectively. The errors due to data processing are within 5%. b S(I) and S(II) for the ethanol system are obtained by assuming that the sharp and the broad components are exactly due to the spin probes flowing in spaces I and II, respectively.

freely in the opened column, S for the open column is equal to the product of the original concentration C0 (mM/L) and the volume of the storage loop Q0 (mL):

S(open) ) C0Q0

(2)

The filled circles of curve b indicate that the corresponding ESR spectra are displayed in (A). Curve c is the concentration of TEMPOL that gives the sharp spectrum, and curve d is the ratio between the values on curves b and c. The highest concentrations Cap for the broad and sharp components are 28.4 and 0.62 mM, respectively. The true concentration is (σ0/σ)Cap where σ is the sectional area of the corresponding space accumulated in the flow column. The values are 81.3 and 1.4 mM for CI and CII, respectively, as referred to above. These peak concentrations, listed in Table 2, coincide with those of the steady-state flows (CI or II(s)) within the error range. It is remarkable that TEMPOL is highly adsorbed in the MCM-41 nanochannel but is transported through the ESR-sensitive region in a relatively short period, even though the translational diffusion must be quenched in the nanochannel. This suggests that the solute TEMPOL flows

Figure 4. (A) Real-time ESR spectra of the aqueous solutions of DTBN at 2.0 mM flowing through the column packed with MCM-41. (B) Intensities of the ESR spectra as converted into the apparent concentration in the column: a, in the open column; b, in the MCM41-packed column; c, in the MCM-41-packed column only for the sharp component. The ratio between the values on curves c and b are given as curve d. All are displayed as functions of the flow volume q. The closed circles in (B) are the integrals of the spectra in (A). The other parameters and procedures for the observations are the same as employed in the previous systems (Figure 3). Data points marked with “$” are employed to calculate the moving velocity of DTBN downstream.

through the nanochannel, though at a much slower velocity than that in the bulk space between the MCM-41 particles. Figure 4 shows the results obtained for the aqueous DTBN solution arranged in the same manner in Figure 3. Shown as column A are the ESR spectra for a series of observations during the flow of a 0.4 mL portion of the DTBN solution at 2.0 mM. Curves a-d of (B) are the apparent concentrations (Cap) for the open column (a), broad and sharp components (b and c, respectively) for the MCM-41-packed column, and the ratio of the values on curves c and b (d) as functions of the flow volume. The filled circles of curve b indicate that the corresponding ESR spectra

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Figure 5. (A) ESR spectra observed for the ethanol solution of DTBN at 30 mM flowing in the open column (a, thin; multiplied by 0.7) or in the MCM-41-packed column (a, thick) and the difference between them (b). The integrated spectra of these spectra are given as (c). The thin part of the duplex spectra at the bottom of (c) is the ESR spectrum observed for the DTBN solution condensed in the nanochannel of MCM-41.12 (B) ESR intensities as functions of the flow volume determined by the same procedures described above (cf. Figures 3 and 4) with the open column (open circles) or the MCM-41-packed column (closed circles). The vertical axis is the apparent concentration averaged over all the column space including the silica region.

are shown in (A). It is quite impressive that only a trace of the sharp component appears even in the period of rapid growth of the signal. The highest apparent concentrations of the broad and sharp components are 32.8 and 0.17 mM, and the real concentrations in their particular spaces are calculated as 94 and 0.39 mM, respectively, with employment of the sectional areas in Table 1. These are listed in Table 2 as CI(s) and CII(s). Since the ratio between the Cap of the two components is almost constant, ca. 0.4-0.5%, throughout the experiment, the solute DTBN that shows the two different signals move forward in parallel. It is also noticed that the appearance of the ESR signals for the two systems shown in Figures 3 and 4 is delayed by about 0.2 and 0.65 mL, respectively, compared with that for the flow in the open column. Flow Experiment for the Ethanol Solutions. Flow of alcohols in the nanochannel of MCM-41 has been proposed for the explanation of the large cage effect in the photoreaction of a carbonyl compound.9-11 Therefore, the flow profiles of the spin probes in alcohols and the differences from those in the water systems are quite interesting. We employed ethanol as the representative in the present study, since the solubilities of these spin probes in ethanol are high, especially for TEMPOL, and the solute adsorption is expected to be small. The original concentration of the spin probe, C0, is made high at 30 mM, to separate the ESR spectrum in the nanochannel from that in the space between the MCM-41 particles. The ESR spectrum of the spin probe in the nanochannel remains sharp even at the peak concentration, since the spin probes do not exchange their spins frequently in the nanochannel as in the bulk solution.18 DTBN Solution. As shown in Figure 5A, the ESR spectrum of DTBN in ethanol at a concentration of 30 mM flowing in the MCM-41-packed column is composed of two signals, the sharp spectrum for DTBN in the nanochannel (space I) and the other broad one for DTBN in the bulk space between the MCM41 particles (space II). It is noticed that the above relation between the line widths for the two regions is apparently opposite that in the aqueous systems at 2.0 mM. When the broad signal for space II (thin line of spectrum a) is subtracted from the observed spectrum (thick line of spectrum a), a sharp spectrum (b) is obtained, which is almost equivalent to that observed for the same solution condensed in space I of MCM41.12 The complex spectra c are composed of the integrated version of the three spectra given as (a) and (b) and the spectrum

Okazaki et al.

Figure 6. (A) ESR spectra observed for the ethanol solution of TEMPOL at 30 mM flowing in the open column (a, thin; multiplied by 0.7) or in the MCM-41-packed column (a, thick) and the difference between them (b). The integrated spectra of these are shown in (c). Spectrum b and the bottom of the complex spectra c is the ESR spectrum and its integrated version for the TEMPOL solution flowing in the nanochannel of MCM-41. (B) ESR intensity represented as the apparent concentration in the flow column observed in the same procedures for the data of Figures 3 and 4 as functions of the flow volume: O, with the open column; b, with the MCM-41-packed column.

of the solution condensed in the nanochannel in a vacuum: the thin sharp line at the bottom of (c).12,13 The percentage of the sharp component due to DTBN in space I is about 25%. If the solute concentrations for the two spaces I and II are the same, the intensity ratio between the sharp and the broad components should be a ratio of 0.44:0.56 (R:β), which is far from the observed 0.25 vs 0.75. Two mechanisms are considered for this discrepancy: (1) a part of the DTBN radical located near the inlet of the nanochannel is broadened by the spin exchange mechanism, and the rapid exchange of solution occurs between the two spaces I and II; (2) the solute concentration in space I is more dilute than that in space II. We choose the latter model for the time being, since it is natural to consider that the liquid-crystalline-like ethanol may dissolve a lower amount of spin probe. Figure 5B shows the ESR signal intensity displayed as Cap at the ESR cavity as a function of the flow volume for the ethanol solution of DTBN, a 0.4 mL portion of which was allowed to flow in the open column (open circles) or in the MCM-41-packed column (closed circles). The time profiles of these are almost equal to each other except the absolute values. The maximum Cap for the MCM-41-packed column is reduced to about 86% of that for the open column. Since the experiment is performed in the constant flow mode, the integrated value of S must be proportional to the cross section of the space to flow, since the spin probe flows without delay to the solvent alcohol. Since the value γ/σ0 of 0.21 determined in the Experimental Section is only a little larger than 0.14, which corresponds to the difference between the peak values of the two curves in Figure 5B, and the two curves are homologous to each other, adsorption or retention of the solute spin probe occurs to only a small extent on average. Therefore, the solution must flow in space I (nanochannel) at a rate near that in space II. TEMPOL Solution. The same experiments were made for TEMPOL, and the results are displayed as Figure 6. In this case the intensity ratio between the sharp spectrum and the broad one is 0.3:0.7, which is a little deviated from that for the DTBN system. The area under the curve of Figure 6B for the MCM41 column is about 83% of that for the opened column, which is a little smaller than the corresponding value for the DTBN system. Since the OH group of TEMPOL makes the molecule

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Figure 7. Flow models for the spin probe solutions in the column packed with MCM-41 (a-c) and sensitivity f(x) as a function of x and coordinate of the flowing direction (d). Models a, b, and c are for the ethanol solution, the aqueous TEMPOL solution, and the aqueous DTBN solution, respectively. The central half-toned rectangle represents the MCM-41-packed column, and the two narrower tubes are connected to the flow line. The density of color reflects the condensation of the solute spin probe. The sensitivity at position x was monitored by the ESR signal height of a small DPPH (2,2-diphenyl-1-picrylhydrazyl) crystal suspended down to the position in a narrow sample tube and fitted well with the Gaussian function.

more soluble in alcohols, the adsorption must be reduced a little. The time profiles of the two curves are almost homologous to each other. Since the results obtained with the two spin probes of different polarities are almost the same, the flow profiles reflect well that of the solvent ethanol. At the same time, the flow of ethanol should be smooth in both spaces. Definition of Parameter S. The apparent molar concentration of the spin probe Cap, which is proportional to the ESR intensity, integrated by the flow volume q is defined as S. Therefore, the area under curve a in Figure 3, for example, is S for the aqueous TEMPOL solution at 2.0 mM flowing in the open column. S for the flow in the column packed with MCM-41 can be defined in another way, giving a more distinct physical meaning:

S ) kR )

∫0

kLC V0

∞

CI(t) dt + kβ

∫0

∞

Discussion

CII(t) dt

∫0∞ (RCavI (q) + βCIIav(q)) dq

(3)

Here Ci(t) ) ∫0∞Ci(t) f(x) dx and LC ) ∫0∞f(x) dx ≈ 2.2 cm. LC is the effective length of the ESR cavity resonator, whose relative sensitivity at the coordinate x (the coordinate for the flow direction) is given by f(x) (Figure 7d); the parameter k is the proportional constant for ESR detection, which depends on the microwave power, field modulation amplitude, receiver gain, and Q factor of the ESR cavity. Thus, RCI(t) ) RLCCIav(t) is the molar quantity of the spin probe in space I (nanochannel) of the ESR-sensitive region at time t. Here Cav I (t) is the averaged concentration of the solute in space I in that region. Under the present experimental conditions and with enough solution volume the superscript “av” can be omitted.26 A constant kσ0LC/ V0 can be employed as the unit of S, and k is adjusted to validate eq 2 for the open column. Therefore, the practical parameter S for the flow in the MCM-41-packed column is defined:

S)

∫0∞

(

)

Here (R/σ0)CI(q) and (β/σ0)CII(q) are the apparent concentrations and are directly related to the observed ESR intensities. Therefore, we can obtain S(I) and S(II) without determination of the cross sections R, β, and σ0. S(I) and S(I) + S(II) for the aqueous solution of the spin probe are obtained by integrating curves c and b, respectively, of the ESR intensity diagram of Figures 3 and 4 with respect to q. For the alcoholic systems, S(total) can be divided into S(I) and S(II) by the ratios between the broad and sharp components at the peak of the ESR intensity of Figures 5 and 6. Both S(I) and S(II) are listed in the last two columns of Table 2. From the above definition and eq 2, S(open) is 0.8 mM · mL for the aqueous systems and 12 mM · mL for the alcoholic systems.

R β C (q) + CI(q) dq ) S(I) + S(II) σ0 I σ0

(4)

Flow Models. Figure 7 depicts three models for the transportation of the spin probe in the MCM-41 column (a-c) and the ESR sensitivity f(x), which is a function of the coordinate along the flow direction x (d). In model a, the solute spin probe is not condensed very much and flows smoothly in the column as in the systems of TEMPOL and DTBN in alcohol. Models b and c are for the aqueous solutions of TEMPOL and DTBN, respectively, where the spin probe is highly condensed in the MCM-41 column. In both models b and c the solute spin probe moves in the MCM-41 column as a condensed band whose length in the flow direction is compressed to several centimeters from the original length of about 87 cm.27 This compression in the aqueous system is due to the difference between the transportation rates of the solute before the column and that in the MCM-41-packed column. Since the ratio between the ESR intensity of the sharp component and that of the broad main signal is almost constant or slightly decreases as the flow volume increases (curve d of Figures 3 and 4), the solute is transported slowly downstream, keeping a spatially homogeneous distribution in a macroscopic point of view. In the nanometer scale, however, the transportation proceeds in a very inhomogeneous way. Transportation of the Solute and the ESR Signal Intensity. The flow velocities of the solute in the two spaces I (nanochannel) and II (space between MCM-41 particles) are defined

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Okazaki et al.

respectively as VI* and VII*, and the corresponding solution velocities are VI and VII. The transportation rate of the solute spin probe in the MCM-41 column (τMCM(t)) is expressed as the sum of those in the two spaces (τI and τΙI) as follows:

τMCM ) τI + τII ) CI(t)RVI* + CII(t)βVII*

(5)

where CI(t) and CII(t) are the concentrations in spaces I and II, respectively. The total transportation of the solute spin probe (TMCM), which is the integral of τMCM, should coincide with the product of the original concentration C0 and the quantity Q0. Thus

TMCM ≡

∫0∞ dt (CI(t)RVI* + CII(t)βVII*) ) C0Q0

(6)

The solution flow, which is 0.05 mL/min in the present study, is described with VI and VII:

V0 ) RVI + βVII

(7)

Equations 3 and 6 yield

C0Q0V0 ) S(I)VI* + S(II)VII* σ0

(8)

If we postulate the flow rates of the solute spin probe are equal to the flow rates of the solution

VI* ) VI

VII* ) VII

(9)

Equation 7 is modified as

V0 ) RVI* + βVII*

(10)

With eqs 8 and 10 we can calculate VI* and VII* under the assumption given above. If the steady-state flow is achieved in the column, the flow velocities of the solute can also be obtained from eqs 5 and 10 under the steady-state condition. Thus

V0C0 ) CI(t)RVI* + CII(t)βVII*

(11)

Another very useful criterion to check the flow in the nanochannel is obtained from eqs 10 and 11. If VI ) VI* ) 0 and the solute flows with the solvent, CII(t) must be equal to C0. If some delay occurs for the solute relative to the solvent, then CII(t) must be larger than C0. Therefore, if the steady-state flow is established and CII(t) < C0, the solution should flow through the nanochannel. Aqueous Solution. With the above approximation the flow velocities in space I (nanochannel) and in space II (the space between MCM-41 particles) of 0.54 mm/min and 21.8 cm/min are obtained respectively for the TEMPOL system. In the case of DTBN, they are 0.29 mm/min and 21.8 cm/min, respectively. The larger flow rate of TEMPOL in the nanochannel is reasonable, since DTBN is more hydrophobic and trapped more efficiently in the nanochannel of MCM-41 compared with TEMPOL. The solute does not flow faster than the solvent in space II; thus, the above flow velocity in space I is the minimum

Figure 8. Sensitivity curve of the ESR cavity resonator (f(x)) along the flow direction and the relative positions of the aqueous spin probe solutions flowing in the MCM-41-packed column for the observed points marked with “$” in Figures 3 and 4 for TEMPOL (upper) and DTBN (lower). The integral of f(x) from x ) 0 to the value pointed to by the arrow would be proportional to the ESR intensity. The rectangular concentration profile along the x axis is postulated.

estimate. Transportation of the spin probe through the nanochannel can also be tested from analyzing the slope of curve b in the right graphs of Figures 3 and 4. Figure 8 demonstrates the process of signal growth for the two aqueous systems in relation to the ESR sensitivity function f(x) on the basis of the model of Figure 7. A step function is assumed as the solute concentration in the column. According to the diagram of Figure 4, the apparent DTBN concentration in the region where f(x) is defined increases from 3.8 to 23 mM (marked with “$”) during the flow of 0.44 mL. Since the apparent concentration in the ESRsensitive region becomes a maximum value of 32.8 mM (Figure 4B) when all the space defined by f(x) is filled by the condensed band of DTBN, those of 3.8 and 23 mM correspond to the situations in which the head of the condensed DTBN band proceeds at x ) 3.47 and 4.93 cm, respectively. Therefore, during this period, the total transportation of DTBN is 32.8 mM × σ0 (0.0052 cm2) × 1.46 cm ) 0.249 µM. On the other hand, the maximum transportation of DTBN in space II during this period is 0.39 mM × 0.44 mL ) 0.172 µM, which is less than 69% of the former expected transportation. Therefore, at least 31% of the solute should be transported through the nanochannel. In the case of the TEMPOL system,28 the same type of discussion for the two points marked with “$” in the diagram of Figure 3B results in the same conclusion. Even if the concentrations of both spaces I and II are modeled by convex functions, the conclusion may be the same. Since the real concentration of the spin probe in the nanochannel is σ0/R (ca. 2.9) times the averaged concentration given as C(s) in Table 2, the ESR line would be a single broad line at around the peak top,29 if the translational diffusion occurs. The resolved three-line spectrum should ensure that the spin exchange mechanism does not make a contribution to the line width. The observed line width of 0.4 mT for the central line can be explained solely with the electron-electron dipolar interaction, which is calculated as 0.39 mT for a concentration of 80 mM.25 As has been reported, quenching of translational diffusion for the solute is a characteristic feature for the solvent that forms intermolecular H-bonding in the MCM-41 nanochan-

Spin Probe Study on Water Flow in an MCM-41 Channel nel.13 It is very hard to consider that the solute molecules get deep into the nanochannel by a few micrometers and get out of the nanochannel from the same side in a short time, say 1 ms or so by diffusion.30 Thus, the transportation of the solution through the nanochannel must proceed by the collective movement of the solution molecules.13 Here the solution molecules form a network structure and move forward by slipping on the surface of the nanochannel, which becomes possible since the nanochannel has a constant radius throughout and the surface is smooth in the atomic level. SiOH groups on the surface of MCM-41 are randomly distributed in a medium surface density;31 thus, they do not strongly disturb the networking of the solvent molecules with the H-bonding.12 Under these conditions, the collective diffusion and collective flow model work more efficiently when the length of the nanochannel becomes shorter. When we remember the plug flow near the inlet of a pipe in the bulk dimension, the mechanism of the collective molecular flow can be understood easily. Although the usual plug flow is transformed into the Hagen-Poiseuille flow in a short distance, the smooth surface of the nanochannel, no directional interaction such as H-bonding between the solution and the wall, and the intensified collective nature of the solution molecules make the plug flow appear in our nanosystem extended until the end of the channel. Ethanol Solution. S(I) + S(II) in the ethanol solution, 10.3 and 10.0 mM · mL for the DTBN and the TEMPOL systems, respectively, is a little larger than (R + β)/σ0 × S(open), which is 12 mM · mL (30 mM × 0.4 mL). Therefore, a small extent of adsorption occurs for both solutes. Since the solubility of the spin probes in ethanol is much larger than those in water, an extensive retention of the solute compared with that in the aqueous system must not occur. Therefore, eqs 8 and 10 yield the velocities of the solution flow in the two spaces. For the TEMPOL system, they are VI* ) 14.7 cm/min and VII* ) 10.2 cm/min, and for DTBN, they are VI* ) 15.1 cm/min and VII* ) 9.9 cm/min. Although there is a large difference in the two spin probes, i.e., TEMPOL has a OH group but DTBN does not, similar values are obtained for both VI* and VII*. This result ensures the validity of the assumption of eq 9. If a part of the broad signal is assigned to space I, the difference between VI* and VII* must be considerably reduced. One may consider that if the solute rapidly diffuses into and out of the nanochannel, a model without the flow in the nanochannel may explain the result, where all the solution flows in space II at 21.9 cm/min. This does not occur, since the time profiles of the two IESR vs q diagrams of Figures 5 and 6 are homologous to each other. The diffusion distance of the spin probe molecule in ethanol may be calculated by the equation (2Dt)1/2 with a diffusion rate constant D ) kT/6πaη of about 5 × 10-10 m2/s, where a radius of 0.4 nm is employed for the spin probe molecule. This gives 1.0 µm for the diffusion distance during 1.0 ms, which is much smaller than the flow velocity of 3.6 µm/ms when the solution flows in space II. When 25% of the DTBN molecules were in the nanochannel and if they diffused in by about 0.5 µm and out of the nanochannel from the same side, this process reduced the averaged traveling distance in unit time to 75% in the MCM-41-packed column. The time profile of the flow for the spin probe in Figures 5 and 6 should have been extended to the larger q value by about 25%. Almost the same discussion is possible by using eqs 10 and 11. Effects of the Surface Condition and Pore Dimensions. For the observation of the present effect most clearly, we have to stock MCM-41 in a well-dried state. When silica gel is employed as the drying agent instead of Drierite, the hydro-

J. Phys. Chem. C, Vol. 113, No. 25, 2009 11093 phobic character of the nanochannel becomes weakened during the storage and the adsorption of the hydrophobic guest molecule is reduced in water. Therefore, the condensation ratio of the solute spin probe in the nanochannel is reduced considerably (Supporting Information, Figure 5S). When a column with an internal diameter of 2.0 mm is employed, the packing efficiency increases a little as referred to above and in Table 1. Although the ESR observation is impossible for the aqueous system, since the microwave loss is so large, the flow experiment for the ethanol system is possible and the same result given above is obtained (Supporting Information, Figure 6S). In addition to the surface conditions, the physical dimensions of the nanochannel must also affect the phenomenon. The collective feature of ethanol molecules disappears partially in the nanochannel of SBA-15 with a channel diameter of 20 nm, another mesoporous silica, but is almost retained in that of SBA-15 with a channel diameter of 10 nm.32 Therefore, the small radius of the MCM41 nanochannel is an essential condition for the present observations, but a small distribution in the channel diameter of MCM-41 may not affect the essential feature of the system. Distribution in the channel length of MCM-41 must be another important factor for the flow behavior as mentioned above. Since most of the MCM-41 particles have sharp edges, and the nanochannel can be elongated even in the millimeter scale,33 we consider that the length of the nanochannel is basically one of the three dimensions of the particle. Therefore, the distribution is rather wide, submicrometer to several micrometers, compared with that of the diameter. Since the surface of well-dried silica is not rich in OH groups, the nanochannel wall of MCM-41 employed here is rather hydrophobic and the collective nature of both water and ethanol is reinforced. The intermolecular H-bonding must be the origin of the collective nature of both ethanol and water molecules in the nanochannel. Since the water molecule has two OH bonds, the H-bonding network has a threedimensional structure. Therefore, water molecules can interact with the surface of silica, which also have OH groups, in addition to the strong networking among themselves. The networking between the liquid molecules and weak interaction between the liquid and the wall of a pipe cause the large slip effect,34 which is essential to the persistent collective molecular flow as mentioned above. Therefore, the flow rate of water may become lowered greatly in the longer nanochannels, where the collective molecular flow may be quenched. Conclusion DTBN and TEMPOL in water are highly condensed in the nanochannel but rather smoothly transported in the column packed with MCM-41. Thus, both the spaces between the MCM-41 particles and the nanochannels contribute to the transportation of the solute molecule. Since the translational diffusion of the individual solute molecule is quenched severely, we propose that the “collective flow mechanism” 11 is effective also in the aqueous system. The cage effect in a chemical reaction must be observed also in the aqueous systems as has been observed in the alcoholic systems.9-11 This is very important for the understanding of biological functions in a system having this kind of nanostructure. Ethanol solutions of the above spin probes at high concentrations show also two ESR components for the two spaces: a broad one in the space between the MCM-41 particles and a sharp one in the nanochannel. Since the time profile of the ESR signal intensity is homologous to that for the solution flowing in the open column but the absolute ESR intensity is considerably smaller, we can conclude that the ethanol solution flows freely in the space between the MCM41 particles.

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Acknowledgment. An inner grant of the Institute of Instrumentation Frontier is acknowledged. We also acknowledge Dr. Saburo Sano of the Institute of Sustainable Materials for SEM observations. Sincere thanks also go to Dr. Yukihiko Yamauchi for encouragement throughout this work. Supporting Information Available: (Figures 1S) SEM images of MCM-41 employed, (Figure 2S) photographs of the flow column packed with MCM-41, (Figures 3S) ESR spectra of the aqueous solution of DTBN and TEMPOL at 0.2 mM condensed in the nanochannel of MCM-41 in a vacuum, (Figure 4S) method to separate the sharp component from the complex ESR spectrum for the ethanol solution of DTBN flowing in the MCM-41column, (Figure 5S) ESR spectra of an aqueous solution of DTBN (2 mM) flowing in the column packed with MCM-41, which was stored in the bottle with silica gel for more than one month, and (Figure 6S) ESR intensity as a function of the flow volume (A) and the real-time ESR spectra (B) for the ethanol solution of DTBN flowing in the column of 2.0 mm f packed with MCM-41. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (3) Zhao, X. S.; Lu, G. Q.; Millar, G. J. Ind. Eng. Chem. Res. 1996, 35, 2075. (4) Coma, A. Chem. ReV. 1997, 97, 2373. (5) Anandan, S.; Okazaki, M. Microporous Meoporous Mater. 2005, 87, 77. (6) Sayari, A. Chem. Mater. 1996, 8, 1840. (7) Stallmach, F.; Graeser, A.; Kaerger, J. Micoporous Mesoporous Mater. 2001, 745, 44–45. (8) Faraone, A.; Liu, L.; Mou, C.-Y.; Shih, P.-C.; Copley, J. R. D.; Chen, S.-H. J. Chem. Phys. 2003, 119, 3963. (9) Okazaki, M.; Konishi, Y.; Toriyama, K. Chem. Phys. Lett. 2000, 328, 251. (10) Konishi, Y.; Okazaki, M.; Toriyama, K. J. Phys. Chem. B 2001, 105, 9101. (11) Okazaki, M.; Toriyama, K.; Oda, T.; Kasai, T. Phys. Chem. Chem. Phys. 2002, 4, 1201. (12) Okazaki, M.; Iwamoto, S.; Sueishi, Y.; Toriyama, K. J. Phys. Chem. C 2008, 112, 786. (13) Okazaki, M.; Toriyama, K. J. Phys. Chem. C 2007, 111, 9122.

Okazaki et al. (14) Okazaki, M.; Toriyama, K. J. Phys. Chem. B 2003, 107, 7654. (15) Sui, H.; Han, B.-G.; Lee, J. K.; Wilian, P.; Jap, B. K. Nature 2001, 414, 872. (16) Zhu, F.; Schulten, K. Biophys. J. 2003, 85, 236. (17) Berliner, L. J., Ed. Spin Labeling; Academic Press: New York, 1976. (18) Okazaki, M.; Toriyama, K. J. Phys. Chem. B 2005, 109, 20068. (19) Okazaki, M.; Anandan, S.; Seelan, S.; Nishida, M.; Toriyama, K. Langmuir 2007, 23, 1215. (20) Sayari, A.; Yang, Y. J. Phys. Chem. B 2000, 104, 4835. (21) Handbook of Chemistry, Volume for Applied Chemistry; Chemical Society of Japan, Maruzen: Tokyo, 1984; Vol. I, p 257. (22) Supporting Information 1 for ref 13. (23) Figure 5 in the following reference: Okazaki, M.; Toriyama, K. J. Phys. Chem. B 2005, 109, 13180. (24) In an ordinary solvent, the ESR spectrum of a spin probe at several tens millimolar is broadened severely and the three hyperfine lines start to merge into a single line by the frequent spin exchange. However, the separation between the two outer lines of spectra c and d of Figure 1 is not narrowed at a concentration of more than 80 mM. (25) Dipolar broadening 4.9 mT/M: Berner, B.; Kivelson, D. J. Phys. Chem. 1979, 83, 1406. (26) Since the amount of solution is much larger than the volume of the ESR-sensitive region and the change in the concentration is gradual, the concentration gradient in the ESR cavity can be neglected in the calculation. (27) A 0.4 mL volume corresponds to a length of 77 cm in a tube of 0.81 mm L. (28) The same discussion for the aqueous solution of TEMPOL results in the same conclusion but in a less exquisite way: That is, the average concentration at q ) 0.432 mL is 1.586 mM and increases to 28.4 mM at q ) 0.72 mL (two points marked by “$” in Figure 3B). The transportation of TEMPOL during this period is 28.3 mM × 0.0052 cm2 × 3 cm ) 0.441 µM, which is a little larger than the maximum transportation in space II during the flow of 0.288 mL: 0.288 mL × 1.41 mM ) 0.406 µM. (29) Wertz, J. E.; Bolton, J. R. Electron Spin ResonancesElementary Theory and Practical Applications; McGraw-Hill: New York, 1972; Chapter 9. (30) The diffusion distance of the spin probe molecule in water in 1 ms is around 1 µm, when calculated by (2Dt)1/2 with a diffusion rate constant D ) kT/6πaη of about 5 × 10-10 m2/s. This is much smaller than the flow velocity of 3.6 µm in 1 ms when the solution flows only in space II. The time span of 1 ms is employed here since the flow distance in this period is near the scale of the MCM-41 particle. (31) Zhao, X. S.; Lu, G. O.; Whittaker, A. K.; Millar, G. J.; Zhu, H. Y. J. Phys. Chem. B 1997, 101, 6525. (32) Okazaki, M.; Seelan, S.; Toriyama, K. Appl. Magn. Reson. 2009, 53, 363. (33) Yu-Xiang, Y.; Hai-Ping, Y.; Jian-Guo, S.; Zheng, H.; Xiang-Nong, L.; Ya-Ru, C. J. Am. Ceram. Soc. 2007, 90, 3460. (34) Baudry, J.; Charlaix, E. Langmuir 2001, 17, 5232.

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