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Dynamics of Water Confined in Mesopores with Variable Surface Interaction

We have investigated the dynamics of liquid water confined in mesostructured porous silica (MCM-41) and periodic mesoporous organosilicas (PMOs) by incoherent quasielastic neutron scattering experiments. The effect of tuning the water/surface interaction from hydrophilic to more hydrophobic on the water mobility, while keeping the pore size in the range 3.5-4.1 nm, was assessed from the comparative study of three PMOs comprising different organic bridging units and the purely siliceous MCM-41 case. An extended dynamical range was achieved by combining time-of-flight (IN5B) and backscattering (IN16B) quasielastic neutron spectrometers providing complementary energy resolutions. Liquid water was studied at regularly spaced temperatures ranging from 300 K to 243 K. In all systems, the molecular dynamics could be described consistently by the combination of two independent motions resulting from fast local motion around the average molecule position and the confined translational jump diffusion of its center of mass. All the molecules performed local relaxations, whereas the translational motion of a fraction of molecules was frozen on the experimental timescale. This study provides a comprehensive microscopic view on the dynamics of liquid water confined in mesopores, with distinct surface chemistries, in terms of non-mobile/mobile fraction, self-diffusion coefficient, residence time, confining radius, local relaxation time, and their temperature dependence. Importantly, it demonstrates that the strength of the water/surface interaction determines the long-time tail of the dynamics, which we attributed to the translational diffusion of interfacial molecules, while the water dynamics in the pore center is barely affected by the interface hydrophilicity.


INTRODUCTION
Liquid water is ubiquitous in nature and plays a central role in many applications related to energy and environment. It is noteworthy that in many situations, water is not present as a bulk liquid phase, but rather as a thin interfacial film or as a fluid embedded in a micro-/mesoporous medium. Interestingly, the specific properties of this nanoconfined situation determine the role played by water in many different fields, encompassing the biological activity of proteins, 1, 2 transport through membranes (including biological cells or membranes for proton exchange fuel cells or nanofluidics, nanofiltration/desalinization technology) [3][4][5][6] and environments of geological relevance. [7][8][9] Mesoporous silica has been extensively used as a model system to study the dynamics of liquid water in spatially confined geometry. 10 Along with other methods such as nuclear magnetic resonance (NMR), [11][12][13] dielectric spectroscopy, 12,14 or molecular simulation, [15][16][17] quasielastic neutron scattering (QENS) has been widely used because it provides a unique viewpoint on the molecular dynamics at the exact space and time scales, which are relevant to nanoconfined liquids (i.e. a few 0.1-1 nm, 0.1-10 ns). [18][19][20][21][22][23][24][25][26] In general, these studies have revealed a reduction of the water diffusivity compared to the bulk counterpart, which becomes more prominent the smaller the pore size is.
The pioneering studies performed in the mid-90's used Vycor glass. 18 Although the distinction between the surface dynamics and that of the in-pore liquid has been addressed by tuning the filling fraction, additional difficulty stemmed from the interconnected character of the Vycor porosity.
The advent of mesostructured porous silica phases, such as MCM-41 and SBA-15, has allowed studying liquid water in well-defined geometry in terms of straight and parallel nanochannels with 4 tunable diameter. 20,21 These studies made a distinction between two populations of molecules: the water molecules located in the vicinity of the pore walls are immobile or much slower than those located in the inner part of the pores. In addition to this bimodal description, the possible existence of non-exponential relaxations was also discussed and modelled by stretched exponential functions.
This approach was intended either to reflect the intrinsic distribution of relaxation times in the liquid dynamics or to reveal the spatially heterogeneous nature of confined fluids. 19,21,22 While MCM-41 and SBA-15 provided adjustable pore size, 20,26 the evaluation of the effect of surface interaction on the water dynamics was limited due to the unchanged chemical composition of hydrophilic silicas. This issue was addressed by studies, which aimed at disentangling the respective roles of surface interaction and purely geometric effects. Hydrophobic nanopores formed by carbon pores, 27,28 hydrophobically modified MCM-41, 23 and organosilica phases 25 were used to weaken the surface/water interaction. The other limiting case of ultra-strong interaction was illustrated by the ability of Zr−OH and Al−OH terminated MCM-41 surfaces to immobilize water molecules. 24 A recent study focused on the surface dynamics of water measured at very low coverage fraction of SBA-15 revealed that the non-diffusing fraction of water molecules was involved in slow jump processes between sites that are spatially separated by up to 0.4 nm. 29 From the above literature, it seems that the chemical nature of the surface of nanoporous materials can be envisioned as a parameter affecting the water dynamics. However, clear-cut conclusions from a comparison of the existing studies are limited by the diversity of instrumental conditions and fitted models. To better address this issue, the present study considers the interfacial mobility of nanoconfined water in the presence of a spatial modulation of the surface/water interaction. This confining condition is realized by using a series of periodic mesoporous silicas (MCM-41) 30, 31 and organosilicas (PMOs). [32][33][34] These ideal host matrices are formed by a regular 5 triangular lattice of parallel and cylindrical channels, conforming a honeycomb structure . The pore   walls of MCM-41 are uniformly formed by silica units with surface H-bond forming silanol groups, serving as a reference hydrophilic matrix. PMOs consist of hydrophobic and (weakly or highly) hydrophilic units, which are alternating along the main pore axis. This is achieved by the introduction of bridging organic units within the silica framework. The molecular-scale periodicity of the arrangement of organic/silica units along the pore axis is demonstrated by (00l) Bragg reflections. Together with the hydrophilic reference pure silica MCM-41, three PMOs comprising different organic bridging units were studied, as illustrated in Table 1. They contain biphenyl (BP-PMO), divinyl-benzene (DVB-PMO), and divinyl-aniline (DVA-PMO) organic bridges, respectively. The four different matrices present comparable pore diameters with values between 3.5 and 4.1 nm as detailed in the materials and methods part (Table 1). However, one expects variations in their surface polarity and tendency to form H-bonds, as indicated by water physisorption and multidimensional solid-state NMR spectroscopy studies. 33 Therefore, we can study the effect of tuning the water/surface interaction from hydrophilic to more hydrophobic by comparing results obtained for DVA-PMO and MCM-41 with respect to DVB-PMO and BP-PMO.
It was recently demonstrated in a pulsed field gradient (PFG) NMR study that the water transport in PMOs actually depends on their chemistry. 34 This method probes the molecule displacements in the m-range. As a result, the diffusivities were found to be primarily determined by the macroscopic textural properties of the mesoporous particles. Although significant for technological applications, the determination of long-distance transport leaves unanswered questions about the dynamics of water within the pores.
Quasielastic neutron scattering (QENS) is a unique method to resolve the spatio-temporal correlations of water molecule on a timescale ranging from a few picoseconds to a few 6 nanoseconds, and at the nanometer length scale; that is to say a few times smaller than the characteristic distances defined by the pore diameter and by the period of modulation of the surface interaction. We performed incoherent QENS experiments combining two spectrometers: the high flux time-of-flight spectrometer IN5B (ILL, Grenoble) and the high-resolution backscattering spectrometer IN16B (ILL, Grenoble), with respective energy resolution (FWHM) 22 eV and 0.75 eV. In the explored temperature range (243-300 K), our study demonstrates that the dynamics of confined liquid water can be interpreted by the coexistence of two populations, comprising frozen and mobile molecules on the instrument timescale. We modelled the dynamics of the mobile fraction with two independent relaxations, corresponding to local fast motions and translational jump diffusion. As such, this model conforms to the latest description adopted for bulk water, 35 that was yet to be applied for confined systems. Thanks to the fine control of the surface chemistry of the MCM-41 silica and the three different PMOs, the present work provides a rigorous assessment of the relative role of spatial confinement and surface interaction on the dynamics of confined water. On the one hand, the translational diffusion of interfacial molecules, which is detected in the QENS intensity in the region of small energy transfer, demonstrates a dependence on the surface hydrophilicity. On the other hand, despite clear evidence of confinement effects, this study also concludes on the limited impact of surface chemistry on the dynamics of water molecules located in the center of pores with sizes of 3.5 nm and larger. This conclusion can be reached after a fine and systematic comparison of the dynamic parameters of comparable systems studied under the same conditions. 7

Samples
PMO powders were prepared according to the following procedure. NaOH and the alkyltrimethylammonium bromide surfactant were dissolved in deionized water. The bis-silylated precursors of the form (EtO)3Si− R−Si(OEt)3 (R = organic bridging unit) were added at room temperature and the mixtures were stirred for 20 hours. The mixtures were transferred into a Teflon-lined steel autoclave and statically heated to 95 °C or 100 °C for 24 h. The resultant precipitate was collected by filtration and washed with 200 mL deionized water. After drying at 60 °C, the powder was extracted with a mixture of ethanol and hydrochloric acid (EtOH:HCl (37 %), 97:3, v/v) using a Soxhlet extractor. The porosity and the pore structure of the dried materials were characterized by powder X-ray diffraction (PXRD) and nitrogen physisorption. 34 In this work, we used three different PMO materials with different bridging groups: biphenyl BP-PMO, divinylbenzene DVB-PMO and divinylaniline DVA-PMO (cf. Table 1). The mesoporous materials MCM-41 silicates were prepared in our laboratory according to a procedure similar to that described elsewhere 31 and already used in previous works. [36][37][38][39][40] Hexadecyl ammonium bromide was used as template to get a mesostructured triangular array of aligned channels with pore diameter D = 3.65 nm, as confirmed by nitrogen physisorption, transmission electron microscopy (TEM) and neutron diffraction. The structural parameters of all the matrices are summarized in Table 1. All matrices were dried at 120 °C under a primary vacuum. To prepare hydrated matrices, a constant amount of mesoporous materials was charged in a flat alumina rectangular cell (1 mm of thickness) and placed in a desiccator in the presence of a beaker containing a saturated aqueous solution of KCl. The resulting relative humidity (RH) of 86% at 25 °C was above the partial pressure of the capillary condensation 33 and allowed the complete filling of the pore system. The

QENS experiments
Quasielastic neutron scattering experiments were carried out using two spectrometers with different energy resolutions at the Institut Laue Langevin (ILL, Grenoble, France). 41

Data modelling
The QENS spectra of confined water in DVB-PMO and MCM-41 are shown in Fig. 1  Thus, it provides information on the single particle dynamics of water molecules. A continuous sharpening of the quasielastic peak was observed on decreasing the temperature from 300 K to 243 K. On a qualitative level, the width of the quasielastic scattering is inversely proportional to the typical timescale of motion of particles. Therefore, this sharpening indicates that the water dynamics slows down gradually on cooling. The absence of a sharp increase of the elastic signal confirms that water remained liquid on the entire studied temperature range. It allows excluding the hypothetical presence of water outside the pores and the crystallization of in-pore water, which 11 is in agreement with the DSC results. The Q-dependence of the scattering intensity is illustrated in All spectra were fitted individually at each Q with a model comprising one elastic component, i.e. a Dirac function ( ), and a quasielastic contribution, which was approximated by a sum of Lorentzian functions. Importantly, we did not assume any specific Q-dependence of the parameters, and therefore no implicit reference to a theoretical model was made. This reduced the possible introduction of bias in the data analysis, as demonstrated in the recent QENS study of bulk water. 35 The contribution arising from the pore wall atoms was accounted for by adding scattered We assumed that the two Lorentzian functions observed on IN5B reflected the presence of two independent dynamics. In the pioneering QENS studies of bulk water, two motions were identified and attributed to continuous rotational diffusion and jump-like translation. 46,47 QENS studies on bulk 48,49 and confined water have later adopted this model. 18,20,25 In some studies, this model was also invoked, but because a simplified version of the fitted functional form was considered, the reference to rotational diffusion was actually lost. 22,24 The continuous nature of the rotational motion of water molecules has been questioned by experiments and simulations, 50,51 and it was also argued that the assumptions underlying this QENS model might not be strictly verified. 35 To limit any interpretational bias, we first performed a model-free fit of the data. Both the intensity and the width of the two Lorentzians were determined independently at each Q. On IN5B, the first fit revealed that the linewidth of the second (broadest) Lorentzian Γ 2 was barely varying with Q.
Contrariwise, the sharpest Lorentzian Γ 1 was dispersive for both spectrometers. In order to get a more robust fit, Γ 2 was fixed to its averaged value at each temperature, and the values of all the other parameters were refined again. Following the assignment made in recent studies of bulk water, we related the broad and sharp components to two distinct dynamics: local motion (L) and translational diffusion (T), respectively. 35,52 It is likely that only the slowest one, attributed to translation, was actually detected on IN16B. In fact, the broad quasielastic contribution arising from local dynamics likely appeared as a vanishing flat background, due to the reduced energy range covered by the high-resolution instrument. Therefore, the incoherent dynamic structure factor of water writes as:

Local water dynamics
The experimental EISFs ( ) obtained from the fitting of Eq. (3) are illustrated in Fig. 4. They present an important elastic contribution, which is above 60% at 300 K and 95% at 243 K on the studied Q-range. It demonstrates the non-diffusive (i.e. localized) character of this motion. In the classical approach of Teixeira et al. 47  water. 35 This motion was attributed to a local dynamical "intra-basin" relaxation, also observed in the water molecules trajectories of Molecular Dynamics (MD) simulations. A comparison is made in Fig. 4(a) with the polynomial fit of the bulk water data. 35 It is similar to the EISFs that we measured at 258 K. A univocal determination of the geometry of this local relaxation is elusive, as also discussed for bulk water by Arbe et al. 52 However, based on a Debye-Waller factor type of analysis, we can conclude on the gradual shrinkage of the mean-squared-amplitude of the local motion on decreasing the temperature for confined water. On the entire temperature range studied, the relaxation time L attributed to the local motion of water varied from 1 to 3 ps. These values are within the range of relaxation times reported for bulk water (1 to 3.5 ps) on a slightly reduced temperature range. 35 The effect of the matrix can be estimated from a comparison of the EISFs measured for water confined in MCM-41 (cf. Fig. 4(a)) and the three PMOs (cf. Figs. 4(b-d)). If present, the difference between the EISFs is very small The radius R T of the sphere confining the translational diffusion of the fraction (1-p) of mobile molecules decreases from 9 to 5 Å, during cooling from 300 to 243 K (cf. Fig. 6(b)). The spatial restriction of the translational displacements on cooling is similar to the observation made for the local dynamics in the previous part in that it is decreasing. The values of RT spanned from one quarter to one half of the pore radius. Hence, this confining sphere cannot be strictly identified as the pore itself. However, a correlation between RT and Rpore is suggested by the systematically larger values of RT for DVB-PMO, which pore size is 15% larger than for the three other matrices (cf. Table 1). This indicates a possible link between RT and geometrical aspects of confinement.
Moreover, the role of surface effects can be invoked. Indeed, the fraction p of non-mobile 23 molecules could be also taken into account as a secondary source of confinement. For simplicity, it can be assumed that these molecules are forming a homogeneous layer located at the interface with the pore wall. In this case, the radius of the core region of the pore, where the mobile water molecules are located, is = √(1 − ). Using the experimental values of p and from Table 1 The measured linewidths are in fair agreement with this prediction, as illustrated in Fig. 8 by the shaded areas. At large Qs, T bends and tends asymptotically towards a constant value denoted 1/0 (Fig. 7). In fact, the Fickian diffusion model assumes a continuous motion process.
Deviation from this assumption is observed when considering small displacements (i.e. for Q larger than the inverse particle distance), where a discontinuous mechanism is related to the finite molecular size and the local order in the liquid. The linewidth was modelled by the well-known jump-diffusion model, which assumes that the translation motion proceeds by successive elementary jumps (thin dashed lines in Fig. 7). 53,58 Between two jumps, the particle remains localized for a typical residence time 0 on a molecular site, with a spatial extension limited to the amplitude of vibrational modes. Applying the usual assumptions that the jump can be regarded as instantaneous with respect to the residence time 0 spent by the particle on a site, and that the jump length l is much larger than the spatial extension of each site, the linewidth of the Lorentzian was fitted with where DT is the diffusion coefficient and 0 the mean residence time. temperature could indicate that the activation energy is reduced in confinement, although the difference remained in the limit of the data accuracy, especially at 243 K as T approaches the instrumental resolution limit of IN5B. Significant differences between the confined and bulk water were obtained for the residence time 0. It is systematically larger in confinement than in the bulk as illustrated in Fig. 9(b). At 300 K, the residence time is three to four times longer in the porous matrices. Moreover, the activation energy is smaller in confinement, so that the residence times of the different systems apparently approach the bulk system at lower temperature (243 K). It is worth noting that both DT and 0 exhibit a curvature, which indicates a super-Arrhenius character as usually observed in supercooled glass-forming liquids. 60,61 This is illustrated in Fig. 9 A comparison with other QENS data obtained from the literature is given in Table 2. These experiments have been analyzed differently, and often the EISFs related to the fast local dynamics, the confined translation diffusion and the fraction of non-mobile molecules were not available simultaneously. Also, the temperature range was sometimes limited, which prevented a thorough comparison with our results. Bearing in mind these possible limitations, it is, however, possible to compare the values of DT and 0 that were obtained from the quasielastic linewidth. Similar conclusions can be made from these previous evaluations of DT, which values remain similar or marginally smaller than that of the bulk water whenever the pore diameter is larger or equal to 3 nm. An enhanced slowdown of the translational dynamics is observed for a pore size smaller than 3 nm (Table 2).

Interfacial translational water dynamics -high resolution neutron backscattering study
In the previous section, the translational dynamics of confined water has been discussed in terms of two populations: an interfacial layer of thickness − in the range 1-5 Å, with a highly reduced dynamics due to the interaction with the pore surface, and a liquid region at the pore center with a 'bulk-like' dynamics with moderate slowdown and marginal effect of the pore chemistry. It is noteworthy that the interfacial dynamics is likely not frozen on a longer time scale. The fraction of molecules named 'non-mobile' rather corresponds to the long-time tail of the broad distribution of dynamics. 61 This simplified bimodal description is widely used to account for the dynamics that  The linewidth of the quasielastic intensity measured on IN16B was fitted with a jump-diffusion model, as illustrated in Fig. 10. The diffusion coefficients and residences times derived from these fits are illustrated in Fig. 11, and also given in Table 3. The diffusion coefficient and the residence time of bulk water are added in Fig. 11, which recalls the presentation made in Fig. 9   It is within 10% for DT but it is more marked for 0 that is up to four times longer than for bulk water. The EISFs demonstrate that the translational diffusion is spatially restricted on the instrumental timescale, which differs from bulk water. The radius RT of the sphere confining the translational diffusion of the mobile molecules decreases from 9 to 5 Å, during cooling from 300 to 243 K. It is not strictly related to the pore size itself. We consider that it rather emerges from the hindrance due to the surrounding molecules that are influenced by the pore interface. This is supported by the increase on cooling of the fraction p of molecules that are dynamically frozen (i.e. not diffusing on the timescale of IN5B), and that are most likely located at the interface. It is about 10-20% at 300 K, and reaches values as large as 40-50% at 243 K, which corresponds to a thickness of the 'non-mobile' layer in the range of 1-5 Å. This fraction also varies as a function of the nature of the matrix, illustrating the effect of the surface interaction on the dynamics of the interfacial layer. Finally, our study using the high-resolution spectrometer (IN16B) demonstrates that interfacial molecules are dynamically active on a longer timescale. Their motion could be described by a jump-diffusion model as well. However, the corresponding characteristic parameters DT and 0 demonstrate a significantly slower process than that of the liquid located in the pore center.
Moreover, the diffusivity of interfacial water molecules shows dependence on the nature of the confining matrix. The largest slowdown is obtained for hydrophilic matrices (MCM-41 and DVA-PMO) with respect to the more hydrophobic one (DVB-PMO). As a whole, this study indicates that, on the ps-to-ns timescale and for a pore size of about 3.5 nm, the details of the water/surface interaction (i.e. matrix hydrophilicity) hardly affect the generic confinement effects that involve the water molecules located in the core of the pore but they are important in determining the translational motion of the water molecules located at the interface.