Water Behavior in MCM-41 As a Function of Pore Filling and

Sep 25, 2014 - C , 2014, 118 (41), pp 23701–23710 ... The behavior of water confined in MCM-41 mesopores has been investigated by means of 1H NMR ...
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Water Behavior in MCM-41 As a Function of Pore Filling and Temperature Studied by NMR and Molecular Dynamics Simulations A. Pajzderska,*,† M. A. Gonzalez,‡ J. Mielcarek,§ and J. Wąsicki† †

Faculty of Physics, A. Mickiewicz University, ul. Umultowska 85, 61-614 Poznań, Poland Institute Laue Langevin, B.P. 156x, 38042 Grenoble Cedex 9, France § Department of Inorganic and Analytical Chemistry, Poznan University of Medical Sciences, Grunwaldzka 6, 60-780 Poznan, Poland ‡

ABSTRACT: The behavior of water confined in MCM-41 mesopores has been investigated by means of 1H NMR methods as a function of pore filling and temperature. The translational and rotational dynamics of water have been explored in the frame of molecular dynamics simulations providing a good understanding of the solid-state NMR results for three samples with different pore fillings: 10% (MCM-A), 45% (MCM-B), and completely filled (MCM-C). In MCM-B and MCM-C samples one can distinguish the presence of core and surface water, while all water molecules in MCM-A are attached to the surface and do not retain any translational degree of freedom. The activation energies for translational motion (MCM-B and MCM-C) obtained from NMR and MD simulations are in good agreement. For MCM-B and MCM-C samples we relate the decrease in the NMR intensity signal with decreasing temperature mainly with the loss of translational mobility of water molecules, while in the case of MCM-A the decrease is related to the freezing of rotational degrees of freedom.

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INTRODUCTION Mesoporous silica materials and the properties of confined liquids are a current topic of intense research in physics, biophysics, and pharmacy. They can exhibit large surface areas (up to about 1000 m2/g), large pore volumes (close to 1 cm3/ g), small dispersion of pore diameters, and high thermal stability. The best-known examples are MCM-41 and SBA-15, which have an ordered hexagonal structure with mesopores of sizes ranging from 2 to 10 nm.1 For the above-mentioned reasons, porous silica materials are very good adsorbents and can be used as hosts for the adsorption of guest molecules. However, the properties of the guest molecules will differ from those in the bulk state and the changes in the dynamics of confined liquids (e.g., water, benzene, toluene, acetonitrile, or liquid crystals) relative to the dynamics in the bulk have been extensively studied.2−9 Apart from the fundamental relevance, one of the main reasons for the large attention devoted to the properties of confined water is the fact that in many real life systems water does not appear in its bulk form, but located in small cavities (for instance in rocks, in polymer gels, and in biological membranes) or near surfaces. Therefore, the study of water confined in nanopores is essential for understanding the effects observed in systems of interest in biology, chemistry, geophysics, or biophysics and has been a subject of intense research work both experimentally (calorimetric, thermoporometry, infrared absorption, Raman scattering, nuclear magnetic resonance, X-ray and neutron diffraction, neutron scattering)10−12 and by means of simulations (Monte Carlo, molecular dynamics).13−18 It is well-known that the structural © 2014 American Chemical Society

and dynamical properties of water confined in pores differ from those in the bulk state and that confinement results in changes in the character of phase transitions, density, specific heat, dynamics, and structure. In 2001 it was shown for the first time that MCM-41 can be applied as a drug delivery system.19 Since then, many experimental studies have shown the possibilities of adsorption of different medical therapeutic substances.20 But besides the pharmacologically active substances absorbed by the host, the later usually contains water molecules as well. The presence of confined water can influence how effectively the therapeutic substance can be incorporated in the mesopores and later released, so it is also of great importance to understand the behavior of water when it fills partially or completely the pores. For this reason, and taking into account that the behavior of water in partially filled pores has not been so intensively studied21−23 as for completely filled pores we have performed a nuclear magnetic resonance (NMR) study of water confined in MCM-41 with three different hydration levels (corresponding to completely filled and partially filled pores), together with the results of molecular dynamics simulations (MD). NMR permits the characterization of porous systems and it is particularly suitable for the investigation of water in limited volumes (pores). Thanks to a great difference in the spin−spin relaxation time T2 between water in liquid and solid phases, Received: June 3, 2014 Revised: September 24, 2014 Published: September 25, 2014 23701

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of the FID signal, the so-called apparent spin−spin relaxation time T2* was found. Measurements were made for temperatures in the range 296−100 K. d. Molecular Dynamics. MD simulations were performed on a single cylindrical pore with diameter 24 Å, corresponding to the dimensions of the pore calculated from the adsorption− desorption method. We applied the procedure proposed by Brodka29 to obtain a realistic pore. Using graphical tools built in the Materials Studio 6.0 package30 we generated a block (42 Å × 42 Å × 24 Å) of amorphous silica and all atoms within a cylinder of diameter 24 Å along the z-axis were removed. Then all silica atoms which did not have a complete tetrahedral environment were also removed. Finally, nonbridging oxygen atoms (i.e., bonded to only one silicon atom) were saturated with hydrogen atoms, giving a concentration of hydroxyl groups equal to 3.5 per nm2, which is close to the density of silanol groups measured for silica surfaces (4.9 ± 0.5 per nm2).31 The surface obtained in such a way is irregular and rough. The Adsorption Locator module in the Materials Studio 6.0 package30 was used to add three different amounts of water molecules to the simulated pore corresponding approximately to the fillings of MCM-A, MCM-B, and MCM-C samples. Using the generated configurations as the starting points, MD simulations were then performed with use of the DL_POLY package.32 NVT simulations were done at eight different temperatures in the range 100−300 K, using Berendsen’s thermostat33 with a relaxation constant of 1 ps to maintain the temperature. Periodic boundary conditions were applied in all directions. The silica frame was kept rigid by freezing the positions of O and Si atoms, while rotations of the H atoms around Si−O bonds were allowed and the O−H distance was kept fixed at 0.95 Å with use of the SHAKE algorithm. For silica we used the Lennard-Jones potential parameters proposed by ref 29 and for water the TIP4P/2005 potential model.34 In this model Lennard-Jones interaction sites are located on the oxygen atom, positive charges are located on both hydrogen atoms, and a negative charge is located on the H−O−H bisector at a distance of 0.1546 Å from the oxygen atom. A cutoff distance of 11 Å was applied for the van der Waals forces and the electrostatic interactions were treated by using the Ewald summation method with the same cutoff in real space. In all cases a time step of 1 fs was used and the systems were equilibrated over 1 ns. Then the trajectory was saved every 1 ps for a total simulation time of 10 ns. The analysis of the trajectories was performed with nMoldyn.35

it is possible to monitor the process of water freezing in pores by measuring the amplitude of the NMR signal (free induction decay, FID). On the other hand, the temperature dependence of the T2 relaxation time provides information on the activation enthalpy for reorientation/translation in these systems.24−27 Finally, MD simulations can provide detailed information about translational and reorientational dynamics of water molecules and are very useful for understanding the behavior of confined water.

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METHODS a. Sample Preparation and Characterization. Silica mesostructured MCM-41 (hexagonal) was purchased from Sigma-Aldrich, and is used without further purification. The porosity of the MCM-41 sample was characterized by gas adsorption. The nitrogen adsorption isotherm at 77 K was measured with a Quantachrome Autosorb iQ gas sorption analyzer. The BET surface area (S) obtained is 815.2 m2/g, the specific pore volume (V, single point total pore volume) is 0.78 cm3/g, and the pore size determined from the adsorption curve by the BJH method gives a diameter of 24 Å. Three samples with very different water contents were then prepared. They are named MCM-A, MCM-B, and MCM-C, and their water content corresponds to pore fillings of 10%, 45%, and 125%, respectively, as determined by thermogravimetry (see below). To obtain the MCM-A sample a conical flask of 150 cm3 capacity was charged with 0.5 g of the mesoporous MCM-41 and flooded with 50 cm3 of deionized water obtained by filtration through a Seradest USF 800 filter. The flask was placed on a magnetic stirrer REAX-top 541 and stirred for 48 h at room temperature, then stored at 4 °C for 24 h. After the content was filtered by a filter of mesh size 0.4 μm, the precipitate obtained was dried for 24 h at 70 °C. To prepare the MCM-B sample a 0.5-g portion of the MCM-41 was weighted and placed in a flat cuvette, forming a layer not more than 2 mm thick. The cuvette was stored for 7 days in a climatic chamber KKLLAB-WMD-080, at 90 °C and relative humidity of 70%. Sample MCM-C was prepared by the method described in ref 28. To prepare all the samples we used deionized water obtained by filtration through a Seradest USF 800 filter. b. Thermogravimetry. Accurately weighed quantities (∼10−15 mg) of the prepared samples were placed in platinum pans and heated in air atmosphere at a rate of 5 deg/min to 1000 °C. Thermal gravimetric analyses were performed with a thermogravimeter Setsys 1200 from Setaram, and the temperature dependence of the mass of the decomposing substances was recorded. c. Nuclear Magnetic Resonance. NMR measurements on protons (1H NMR) were performed with a pulse spectrometer working at 58.9 MHz and a continuous wave spectrometer working at 27 MHz. The temperature of the sample was controlled by means of a gas-flow cryostat and monitored with a Pt resistor with an accuracy of 1 K. The two spectrometers were constructed at the Radiospectroscopy Laboratory, Faculty of Physics, AMU. The samples for 1H NMR measurements were sealed off in glass ampules. After a π/2 pulse of 3.6 μs duration was applied, the free induction decay FID signal was recorded. The FID signal intensity was measured at 20 μs after the end of the π/2 pulse. The dead time of the measuring head was 10 μs, while the FID signal coming from ice was no longer than 20 μs. Thus, only the signal coming from liquid water is recorded. On the basis of the measured width at half-maximum

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RESULTS a. Thermogravimetric (TG) Measurements. Figure 1 shows the TG curves for the three samples. The rapid mass loss corresponding to the release of physisorbed water is clearly visible above room temperature and extends up to about 405 K. The mass loss observed represents 7%, 24%, and 50% of the total mass for MCM-A, MCM-B, and MCM-C, respectively. With increasing temperature an additional small mass loss is noted, interpreted as corresponding to the removal of the socalled chemisorbed water.36 As the size of the pores is determined from the nitrogen adsorption and desorption measurements, it is possible to make an approximate estimation of the pore filling in the three samples by using the known pore volume and assuming a water density similar to that of bulk water. The pore fillings obtained are 10% in MCM-A, 45% in MCM-B, and 125% in MCM-C. The last result implies that in our third sample there is an excess of water outside the pores. 23702

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intensity taking place for MCM-C at 266 K can be attributed to the freezing of the excess water located outside the pores. The additional decrease in the signal intensity with decreasing temperature observed below ∼240 K for MCM-B and ∼266 K for MCM-C can be attributed to a gradual freezing of water molecules localized inside the pores. Disregarding the hightemperature range (above ∼240 K), the temperature dependence of the intensity of the NMR signal for samples MCM-B and MCM-C is very similar. But the behavior of sample MCMA is quite different. As this system has the lower water content and the number of water molecules is smaller than the number of OH groups present on the pore walls, it seems justified to assume that here all the water molecules are on the pore surface. Therefore, the decrease in the signal intensity can be related to progressive slowing down of proton reorientations. Figure 3 presents a plot of the spin−spin relaxation time T2* versus reciprocal temperature for the three samples. For MCM-

Figure 1. Thermogravimetry curves for samples MCM-A, MCM-B, and MCM-C.

b. 1H Nuclear Magnetic Resonance Study. The temperature dependence of the FID signal intensity for all samples, obtained after applying the correction related to the Curie law and normalizing by the mass of dehydrated MCM samples, is shown in Figure 2. The maximum intensity is

Figure 3. Spin−spin relaxation time T2* versus reciprocal temperature for MCM-A (■), MCM-B (blue ●), and MCM-C (red ▲).

Figure 2. Temperature dependence of the intensity of the NMR signal for MCM-A (■), MCM-B (blue ●), and MCM-C (red ▲) samples and MD simulations for MCM-Bs (○) and MCM-Cs (□) systems. The inset shows a detail of the low-temperature behavior.

A, T2* decreases with decreasing temperature in the whole temperature range, falling from 250 μs at room temperature to 7 μs at 107 K. For MCM-C and MCM-B, we can be distinguish three ranges: a high-temperature range (from RT to ∼240 K), an intermediate one (between ∼240 and ∼180 K for MCM-C and between ∼240 and ∼166 K for MCM-B), and a lowtemperature range below that. At room temperature, T2* is ∼1000 μs for both samples and in the high-temperature range the variation of T2* with temperature is quite smooth. In the intermediate range, the change in T2* is much stronger and it is accompanied by a considerable change in the FID signal intensity. Finally, in the low-temperature range, T2* does remain constant at a value of 2.5 μs for both MCM-B and MCM-C. Assuming that below 240 K the decrease in the NMR signal intensity for samples MCM-C and MCM-B is due to the freezing of the water molecules inside the pores, it is possible to estimate the activation enthalpy corresponding to this process of those water molecules from the slope of the linear sections of T2*(1/T). Our data give 37.2 ± 0.9 kJ/mol for MCM-C in the range 245−196 K, 32.8 ± 1.0 kJ/mol for MCM-B in the range 240−185 K, and 4.9 ± 0.3 kJ/mol for MCM-A in the range 280−107 K. As the FID signal originates from liquid water, it disappears below 180 K for samples MCM-C and MCM-B, as water freezes. Our data do not allow us to tell which is the final state of such water (amorphous ice or a disordered crystal) and

proportional to the total number of mobile protons in the sample and the ratios of these intensities for the three samples agree reasonably well with the pore fillings estimated from the thermogravimetric measurements. The intensity of the NMR signal of sample MCM-C is constant in the temperature range between 296 and 266 K and shows a jumpwise decay at this temperature, similar to those observed in refs 25 and 26. On further cooling, the signal decreases in a continuous way, disappearing below 180 K. The intensity of the signal of sample MCM-B is also constant down to ∼240 K. Between 240 and 180 K the signal decreases and then again it disappears below 180 K. Finally, for MCM-A the signal remains roughly constant down to ∼190 K where it starts to decrease slowly to 130 K. It should be noted that below ∼130 K this signal has a small but constant intensity. Detailed differential scanning calorimetry (DSC)37,38 studies performed for MCM-41, SBA-15, and SBA-16 revealed for overfilled pores (when external water is present) exothermic peaks (corresponding to freezing of bulk-like external water) at around 265 K. Therefore, the jumpwise decrease in the signal 23703

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c. Molecular Dynamics Simulations. The MD simulations presented here were motivated by these NMR results. They were performed for pores filled with different amounts of water. The number of water molecules in each sample was estimated on the basis of TG results, and for MCM-C also from the NMR measurements and the jump in the signal intensity at 260 K. Figure 5 presents three snapshots from the simulations performed at 300 K for the three systems corresponding to MCM-A, MCM-B, and MCM-C samples and labeled as MCMAs, MCM-Bs, and MCM-Cs. The number of water molecules in each system and the corresponding theoretical pore filling ϕ, calculated for a perfectly regular pore of length 24 Å and diameter 24 Å, are 66 and ϕ = 0.18 for MCM-As, 200 and ϕ = 0.55 for MCM-Bs, and 389 and ϕ = 1.07 for MCM-Cs. For the three systems we have performed simulations at 300, 275, 250, 225, 200, 175, 150, and 100 K and analyzed the temperature dependence of the density profiles, the translational and reorientational degrees of freedom and the diffusion coefficients, along with the correlation functions and times characterizing the reorientations of water molecules. c.1. Density Profiles. The first stage was to determine the distribution of water inside the pore. Figure 6 shows the density

it is worthy to mention that even structural measurements cannot give a completely unambiguous answer.14 To be able to observe and analyze the low-temperature signal, we recorded the first derivative of the 1H NMR line at the second modulation amplitude of 1.6 G, in the range 200− 103 K. They are shown in Figure 4 for the three samples

Figure 4. Shape of the 1H NMR line for samples MCM-A, MCM-B, and MCM-C recorded at 140 K. For samples MCM-B and MCM-C the figure also shows the fitting using three different components that can be identified in the NMR signal.

measured at 140 K. For sample MCM-C the line is composed of three components: a broad one (slope width of 20 G), an intermediate one (slope width of 12 G), and a narrow one (slope width of 2.6 G). In the temperature range studied the widths of the component lines practically did not change. A similar situation is found for sample MCM-B and Figure 4 shows the 1H NMR lines recorded and the fitting of the NMR signal for MCM-B and MCM-C with the three different components. The same measurements were also made for sample MCM-A, for which only a single narrow line was observed, assigned to OH groups and/or water attached to pore walls. Thus, the other two components observed for MCM-B and MCM-C are assigned to core water molecules that are frozen. The NMR signal of a sample containing several groups of molecules differing significantly in mobility (in frequencies of reorientation/translation jumps) is composed of several components of different widths and the broader one corresponds to the less mobile molecules. As the widths of the broad and intermediate lines recorded for MCM-B and MCMC are comparable and do not depend on temperature, it is reasonable to assume that below 180 K we have a rigid lattice (in the NMR scale) in both cases. Therefore, the differences in the widths of the two components are possibly not related to the existence of water molecules having different mobilities, but to the difference in the distances between protons, i.e. in the packing of water molecules, as the NMR line width is proportional to 1/rij3 (where rij is the interproton distance).39

Figure 6. Density profiles for samples MCM-As (black), MCM-Bs (blue), and MCM-Cs (red) obtained at 300 (left) and 100 K (right). The abscissa represents the distance from the center of the pore.

profiles ρ(r) as a function of the distance from the center of the pore along the radius, averaged over the full trajectory. In the completely filled pore, water density is close to 0.85 g/cm3 at the center of the pore (0−8.5 Å) and has a maximum of about 1 g/cm3 near the pore wall. It is interesting to note the low density of water all along the pore, much smaller than the value of 1.07 that could be expected from the pore filling given above. The reason is that a significant number of water molecules are located beyond the expected pore limit of 12 Å, as shown in Figure 6. This is due on one side to the roughness of the pore surface and on the other to the fact that some water molecules are able to penetrate slightly into the silica matrix. At room temperature the density profile shows three small maxima that are not very pronounced and a fourth one close to the pore wall. This is a well-known effect related to the

Figure 5. Snapshots from the simulations performed at 300 K for the three systems (MCM-As, MCM-Bs, MCM-Cs). Yellow points represent Si atoms, red points O atoms, and white points H atoms. 23704

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interaction of water with the hydrophilic pore wall and called layering effect, as noted already by other authors who have obtained similar density profiles for water confined inside silica nanopores.7,16 With decreasing temperature the positions of maxima or minima on the density profiles do not change, but they become more pronounced, suggesting that inside the pore there are regions with different concentrations. As mentioned above, the NMR lines measured at low temperature contain three components of different widths indicating different water packings. The intermediate component (of smaller width) corresponds to frozen water of less dense packing (greater distances between protons) than the broad component. The density profile obtained from the MD simulations confirms the presence of regions characterized by different water concentration (and therefore having different intermolecular distances), thus explaining the different widths of the NMR line observed. In partly filled pores the overall density is naturally much smaller and the density profiles show that water molecules remain close to the pore wall. For MCM-Bs there is only one broad maximum, but interestingly two maxima are observed for MCM-As, particularly pronounced at low temperature. They are attributed to water molecules directly joined to the silanol groups of the matrix and to a second less extended layer of water molecules hydrogen bonded to the first one. As in MCMAs the number of silanols is roughly equivalent to the number of water molecules; this result implies that water molecules are not distributed homogeneously all around the pore surface, as clearly appears in Figure 8. c.2. Translational Dynamics at 300 K. The translational dynamics of water has been analyzed by following the trajectories of the oxygen atoms. It is well-known that the motion of water molecules close to the pore surface is strongly constrained. Therefore, we use the mobility of each individual molecule as a parameter to determine its dynamical nature and assign it to one of two groups: surface (slow) or core (fast) water molecules. This is done in the following way. For each water oxygen we determine its mean-square displacement (m.s.d.) during the full trajectory and we use then this value to sort the water molecules in order of increasing mobility. Figure 7 shows the mobility of the water molecules contained in MCM-As, MCM-Bs, and MCM-Cs at different temperatures. For MCM-Cs at 300 K we observe a clear step (corresponding to a m.s.d of 1 Å2) indicating a well-defined distinction between water molecules having low mobilities and water molecules diffusing rapidly inside the pore. When the temperature is lowered, the mobility decreases as expected, but additionally the distinction between slow and fast water is blurred. Similarly, for MCM-Bs at higher temperature we can distinguish between fast and slow water (where again the change appears when the m.s.d. is approximately 1 Å2), while all water molecules in sample MCM-As can be considered “slow”. Taking this into account, we adopted the value of 1 Å2 as a useful threshold in order to class water molecules in samples MCM-Bs and MCMCs into two different categories. Thus, all the molecules with m.s.d below 1 Å2 were considered “slow”, while the remaining were classed as “fast”. It is worth noting that with decreasing temperature the amount of slow water molecules increases. The relation between mobility and location in the pore is clearly apparent in Figure 8, showing the projection on the xy and xz planes of the trajectory of all slow and fast water oxygens. It is immediately visible that in the three systems the less mobile waters are close to the pore surface. For MCM-As

Figure 7. Mobility of each individual molecule in samples MCM-As (top), MCM-Bs (middle), and MCM-Cs (bottom) at several temperatures.

all the waters are in this state, while in the case of MCM-Cs and MCM-Bs there are additional molecules further away from the pore wall that retain some translational freedom. In the case of MCM-Cs, as such core water completely fills the pore we see that a single molecule can explore the full pore (cyan trajectory in Figure 8c) and show a behavior close to that expected for bulk water. On the other hand, in the partially filled pore (MCM-Bs) water does not enter into the pore center and therefore also the mobility of core water is strongly restricted. A final remark about Figure 8 is that along the long simulations that we have performed we observe that some water molecules can penetrate deeply into the matrix. This is particularly evident in the case of the MCM-Bs simulation. Naturally the dynamics of those water molecules that penetrate into the matrix is strongly diminished, as most of them become trapped (as for surface water). It should be noted that in all the analysis shown here slow (surface) and fast (core) water have been defined from the average deviation defined above, implying that a fast water that is trapped by the pore will still be counted as “fast” all along the trajectory. To quantify the translational dynamics as a function of pore filling and temperature we have computed the average meansquare displacement (m.s.d.) along the axis of the pore, u2z(t), and on the plane perpendicular to the pore axis, u2xy(t). Figure 9 shows u2z(t) and u2xy(t) at 300 K for both core and surface water and the three systems studied. For surface water, the m.s.d. in the xy plane and along the z axis is strongly restricted 23705

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Figure 8. Top: Projection (red and blue dots) in the xy plane of the positions of all the oxygen atoms of the water molecules during the full trajectory obtained at 300 K. The gray points represent the atoms of the fixed silica matrix, the red dots correspond to the water molecules having the slower dynamics (surface water), and the blue dots correspond to the fast water molecules (core water). The green and cyan points show the individual trajectories of some representative water molecules corresponding to the group of surface water (green) or core water (cyan). Bottom: Same as in the top panels, but projected on the xz plane.

Figure 9. Mean-square displacement for core (top) and surface water (bottom) in the xy plane (left) and the z direction (right) for samples MCMAs, MCM-Bs, and MCM-Cs at 300 K.

in all cases so even after 5 ns the average m.s.d. is less than ∼0.3 Å2. For core water (present only in MCM-Bs and MCM-Cs), 2 u xy(t) is bounded by the limited size of the pore. However, while for MCM-Cs this limit is almost achieved after 1 or 2 ns, this is not the case for MCM-Bs, where as mentioned before

the dynamics is much slower. This is even more evident for the m.s.d. along the z axis, where the different diffusivity of the two systems is clearly apparent. To determine the diffusion coefficient in the plane of the pore and along the pore direction, both u2xy(t) and u2z(t) were fitted by using the two expressions described in ref 40. The resulting values are given 23706

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Table 1. Self-Diffusion Coefficient of Core Water for MCM-Bs and MCM-Cs [10−10 m2/s] Bs Dxy 300 275 250 225 200

K K K K K

0.50 0.24 0.18 0.06 0.02

± ± ± ± ±

Cs Dz

0.08 0.04 0.02 0.01 0.01

2.49 0.90 0.51 0.11 0.04

± ± ± ± ±

Dz/Dxy 0.33 0.11 0.07 0.01 0.01

4.9 3.8 2.8 2.2 2.0

Dxy 4.70 1.90 0.61 0.07 0.02

± ± ± ± ±

Dz 0.23 0.09 0.07 0.08 0.01

13.91 5.46 1.37 0.14 0.04

± ± ± ± ±

Dz/Dxy 1.67 0.66 0.14 0.01 0.01

2.9 2.9 2.2 2.0 2.0

vector r1 (perpendicular to the molecular plane) at 300 K is shown in Figure 10a. In the case of surface water, reorientations

in Table 1. For core water, we observe a clear anisotropy in the self-diffusion coefficients Dxy and Dz. Thus, for MCM-Cs at 300 K we obtain Dxy = 4.7 × 10−10 m2/s and Dz = 13.9 × 10−10 m2/ s. This is in contrast to the results found by Milischuk and Ladanyi, who obtained larger Dxy values and no anisotropy.16 A possible reason for such difference is the effect of water molecules that due to the diffusion inside the pore approach the surface and become trapped, as shown in Figure 7, as well as the use of longer trajectories here. For MCM-Bs we have Dxy = 0.50 × 10−10 m2/s and Dz = 4.7 × 10−10 m2/s, confirming the reduced diffusivity in the partially filled pores. In the case of MCM-Cs, Dz is about 2 times smaller than the self-diffusion coefficient of bulk water.41 This is in qualitative agreement with earlier results of MD simulations for water confined in pores of different sizes,15−18 although as said above our results indicate a slightly lower diffusivity. They also agree qualitatively with experimental values available in the literature, although the latter show a large dispersion. For example, Hansen et al.12 found D = 0.49 × 10−10 m2/s from 1H NMR spin−echo measurements on water confined in pores 26 Å in diameter, Faraone et al. give D = 3.24 × 10−10 m2/s at 280 K from QENS experiments on water confined in MCM-41-S with 25 Å diameter pores,11 and also from QENS measurements Takahara et al.10 obtain D = 10.7 × 10−10 and 14.5 × 10−10 m2/s at 298 K for water inside pores of diameter 21.4 and 28.4 Å, respectively. c.3. Reorientational Dynamics at 300 K. Water reorientations were analyzed by monitoring the behavior as a function of time of a vector r1(t) perpendicular to the molecular plane and analyzing the time dependence of the angle Θ1(t) swept by this vector between times t = 0 and t for each water molecule. As expected from the behavior of the translational dynamics, core water molecules exhibit isotropic rotations. This is not the case for surface water. For these molecules we observe two types of behavior. Some water molecules appear to be completely frozen in a given orientation and remain librating around the initial orientation even after 10 ns. The number of those molecules is very similar for the three samples: 27 for MCM-As (41% of the total number of water molecules), 30 for MCM-Bs (15% and 37% of the total and surface water, respectively), and 34 for MCM-Cs (9% and 34% of the total and surface water, respectively), suggesting that these are the molecules that link strongly to the pore surface and have a very limited mobility, possibly due to the rough nature of the surface. The remaining surface water exhibits different degrees of rotational freedom, but without achieving the isotropic regime observed for core water. The existence of preferred orientations close to the pore walls has also been described in refs 15 and 16. The characteristic reorientational correlation times can be extracted from the analysis of the corresponding autocorrelation function. We have computed the autocorrelation function corresponding to the second Legendre polynomial, C1(t) = ⟨0.5[3 cos2(Θ1(t)) − 1]⟩ corresponding to the angle previously defined. The autocorrelation function for the rotation of the

Figure 10. Angular correlation function C1(t) for (a) MCM-As, MCM-Bs, and MCM-Cs at 300 K calculated for surface and core water, for (b) MCM-Cs, and for (c) MCM-Bs for core water as a function of temperature.

are extremely slow and the autocorrelation function does not decay to zero. This is in agreement with the finding that between 35% and 40% of the surface water is orientationally frozen. An interesting finding is that, even if the reorientation is very slow (so that an accurate correlation time cannot be extracted), molecular rotations in MCM-As are less impeded than in the case of surface water for the other two samples. This is possibly due to the additional constraints that core water imposes in surface water for both samples. The formation of long-living Hbonds with the pore walls together with the standard H-bonds with adjacent water molecules results in slower dynamics for surface water in MCM-Bs and MCM-Cs than in MCM-As. This would explain the unexpected feature observed in the NMR 23707

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diffusivity below 225 K, which indicates that in the nanosecond time scale the molecules do not move enough to feel the constraints imposed by the pore walls. The temperature dependence of the diffusion coefficient in the temperature range 200−300 K could be described by the following Arrhenius relation: D = D0 exp(Ea/RT), giving us the activation energies. For Dz we obtain activation energies Ea = 22 ± 1 and 35 ± 1 kJ/mol for MCM-Bs and MCM-Cs, respectively. For the diffusion in the xy plane the activation energies are slightly smaller: 16 ± 1 and 32 ± 1 kJ/mol, respectively. These values are much higher than the activation energies for the reorientational motion and are comparable with those obtained from the experimental temperature dependence of the FID line (Figure 3). Therefore, this leads us to assume that the process manifested in the NMR spectra of samples MCM-B and MCM-C is related to the translational diffusion of water. With decreasing temperature an increasing number of molecules lose all translational freedom. d. Discussion. We can use the MD results to reinterpret the NMR data, specially the decay of the FID signal as a function of temperature observed for samples MCM-B and MCM-C. As we said above it seems reasonable to assume that the NMR signal originates from liquid water, which is supported by the shape and width of the FID signal and the activation energies. And on the basis of the individual mean-squared displacements calculated for each water molecule we can discriminate between core (fast) and surface (slow) water (all molecules whose m.s.d. was less than 1 Å2). Analyzing Figure 7 we can see that decreasing the temperature causes a decrease in the number of fast (core) water molecules, i.e. molecules that show translational diffusion. We can use the data shown in Figure 7 to estimate the number of molecules that are able to diffuse translationally, as a function of temperature. For MCM-Cs we find that there are 305, 290, 266, 208, 100, 28, and 0 such molecules at 300, 275, 250, 225, 200, 175, and 150 K, respectively. They represent 78%, 75%, 69%, 53%, 26%, 7%, and 0% of all molecules. A similar analysis was done for the sample MCM-Bs. As y-scale at Figure 2 is direcly proportional to pore filling we can compare the simulation results with NMR as shown in Figure 2. As shown in this figure, the agreement between MD and NMR is reasonably good. An interesting observation is that a similar mechanism of freezing is observed in the fully filled pore and in the partially filled pore. The interpretation of the temperature dependence of the NMR signal intensity for the pore containing small amounts of water (MCM-A, MCM-As) is also interesting. The character of the signal recorded for this sample is different than those for MCM-B and MCM-C, as it does not decrease to zero even at the lowest temperature measured (Figure 2), and its width does not change with temperature and it is significantly narrower than those obtained for the other two samples (Figure 3). Moreover, the shape of the signal suggests that water in MCMA is not a typical liquid. Indeed, the results of MD simulations show that in this system water does not show any translational freedom that could give a broadening of the FID signal. However, in contrast to the other two samples (MCM-Bs and MCM-Cs), we observe that they retain some rotational mobility even at low temperatures. This correlates nicely with the decreasing of the NMR signal observed in this sample and is related to the slowing down of reorientational motion with decreasing temperature.

intensity below 190 K, where the FID signal for MCM-As remains non-null and higher than in MCM-Bs and MCM-Cs. Regarding the reorientation of core water molecules in MCM-Bs and MCM-Cs we find that the reorientational correlation function can be fitted by a biexponential function with two very different correlation times. We attribute the fastest one (τ1 = 0.16 ns for MCM-Bs and 0.02 ns for MCMCs) to the standard reorientational motion of core water molecules. The value for the completely filled pore can be compared with rotational correlation times obtained for bulk water.42,43 The existence of a second much larger correlation time (τ2 = 2.8 and 1.5 ns for MCM-Bs and MCM-Cs, respectively) is attributed to the slowing down effect in the average correlation function due to the original core water molecules that become trapped by the surface at a later time. Both correlation times are significantly larger for MCM-Bs than for MCM-Cs, indicating that both the translational and the reorientational dynamics are severely constrained in the partially filled pore. c.4. Temperature Dependence of Translational and Rotational Motions. Analyses of the translational and reorientational motions were performed for all three systems as a function of temperature. As follows from the behavior of the r1 vector defined above, there are no qualitative changes in the character of reorientations, and they simply slow down (Figure 10b,c). To evaluate the degree of this slowing down, the correlation functions were determined for systems MCMAs, MCM-Bs, and MCM-Cs. On the basis of the fit of the exponential function to the experimental results we could determine the reorientational correlation time as a function of temperature. These times showed an Arrhenius dependence, allowing us to extract the corresponding activation energies, which are equal for core water Ea= 12 ± 1 and 18 ± 1 kJ/mol for MCM-Bs and MCM-Cs, respectively. As we mentioned above, the rotational dynamics for surface water is very slow and strong orientational correlations remain at 10 ns, so we cannot extract meaningful correlation times from our simulations. Therefore, we performed a different kind of analysis, determining the temperature dependence of the number of orientationally frozen molecules (in the time scale of the simulation). This quantity was approximated as C1(t = 5 ns), i.e. the value of the reorientational autocorrelation function for the vector r1 at half the total length of the simulation. As expected, this number increases with decreasing temperature. However, for samples MCM-C and MCM-B it increases from about 35% (of the surface water) up to 95%, while for sample MCM-A it increases only to 80%. We can supposed that such behavior reflects the fact that most water molecules are simply attached to a silanol group in this sample and they do not have additional constraints related to the H-bond network with other waters. Of particular concern was the analysis of the long-range translational diffusion of water. As we said above, surface water does not exhibit any measurable (on the ns time scale) diffusion, so we present self-diffusion coefficients only for core water. They are given in Table 1 and, as expected, with decreasing temperature the diffusion coefficient decreases. But we note that the decay is faster for water in MCM-Cs than in MCM-Bs, so that at 225 K the dynamics of water in both samples becomes equivalent. It is also noticeable that the ratio Dz/Dxy decreases with decreasing T, so that again at 225 K we cannot observe any anisotropy in the translational diffusion. This can be interpreted simply as a consequence of the low 23708

dx.doi.org/10.1021/jp505490c | J. Phys. Chem. C 2014, 118, 23701−23710

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SUMMARY AND CONCLUSIONS

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AUTHOR INFORMATION

Corresponding Author

Summarizing, the NMR results combined with the output obtained from MD simulations lead to the following conclusions: 1. On the basis of the individual mean-squared displacements calculated for each water molecule we can roughly distinguish between core (fast) and surface (slow) water. 2. At very small fillings (0.1 experimentally and 0.18 in the simulation), we find that all the water molecules remain close to the surface and do not exhibit any long-range translational diffusion. On the other hand, rotational degrees of freedom are not completely inhibited, but they are very slow and the correlation times exceed the dynamical window of our simulations (10 ns). 3. For larger fillings (either on partially filled pores, ϕ = 0.45−0.55, or fully filled pores, ϕ = 0.85−1.07), we observe than in addition to the surface water there are other water molecules showing clear translational and rotational motions. However, in the case of the partially filled pores, water only explores the outer part of the pore and the self-diffusion coefficient and correlation times indicate a much slower dynamics than in bulk water. For the fully filled pore, confinement effects also cause a decrease in the self-diffusion coefficient, but the difference with respect to the bulk is reduced: D(confined)/D(bulk) ≈ 0.6 for the fully filled pore, and ≈0.1 for the partially filled pore. 4. The analysis of the temperature dependence of the MD simulations gives an activation energy for the rotational motion of core water of 12 ± 1 kJ/mol for the partially filled pore and 18 ± 1 kJ/mol for the fully filled pore. The activation energy for the translational motion is larger. Thus, for the diffusion along the pore we obtain Ea = 22 ± 1 and 35 ± 1 kJ/mol for partially and fully filled pores, respectively. And for the diffusion in the plane of the pore the activation energies are slightly smaller: 16 ± 1 and 32 ± 1 kJ/mol, respectively. These activation energies are in reasonable agreement with those obtained from the NMR data in the intermediate range (240−170 K): 32.8 ± 1.0 kJ/mol for ϕ = 0.45 and 37.2 ± 0.9 kJ/mol for ϕ = 0.85. 5. Water inside the pores in samples MCM-B and MCM-C begins to freeze at 240 K and the freezing is completed at about 180 K. MD simulations reveal that the decrease of the NMR intensity signal is related to the loss of translational degrees of freedom of the water molecules and the increasing number of arrested molecules. 6. Interestingly, at very low temperatures only the sample with the lowest filling (MCM-A) shows a remaining molecular motion visible in the small but non-null NMR signal. For the other two samples, instead, the NMR intensity decays completely to zero, indicating that all molecular degrees of freedom (either translational or rotational) have been frozen. The time scale of the simulations does not allow us to explore such low temperatures. However, the comparison of the rotational autocorrelation function for surface water in the three samples is consistent with a larger mobility of surface water in this sample. To conclude, we can say that this study is concerned with a detailed analysis of the behavior of water confined in MCM. The investigations were performed as a function of the pore filling and the combination of an experimental approach (proton NMR) with numerical modeling (molecular dynamics) allowed us to understand better the behavior of water confined in mesopores.

*E-mail: [email protected]. Tel: +48-61-8295208. Fax: +48-61-8295-155. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported in part by PL-Grid Infrastructure. The work has been partially financed by the National Science Centre of Poland, Grant No. 2012/05/B/ST3/03176, and by the operating program POKL 4.1.1. Dr. E. Pellegrini and Dr. G. Goret are gratefully acknowledged for help and advice in using nMoldyn.

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REFERENCES

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