Encapsulated Water Inside Mo132 Capsules: The ... - ACS Publications

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Encapsulated Water Inside Mo132 Capsules: The Role of Long-Range Correlations of about 1 nm Miquel Garcia-Ratés,† Pere Miró,‡ Achim Müller,¶ Carles Bo,‡,§ and Josep Bonet Avalos†,* †

Departament d’Enginyeria Química, ETSEQ, Universitat Rovira i Virgili, Avinguda dels Països Catalans 26, 4300s7 Tarragona, Spain Institute of Chemical Research of Catalonia, ICIQ, Avinguda dels Països Catalans 16, 43007 Tarragona, Spain § Departament de Química Física i Química Inorgànica, Universitat Rovira i Virgili, Marcel.lí Domingo s/n, 43007 Tarragona, Spain ¶ Fakultät für Chemie der Universität, Anorganische Chemie I, Postfach 100131, 33501 Bielefeld, Germany ‡

ABSTRACT: The dynamics of water encapsulated in the well-known polyoxomolybdate nanocapsules of the type Mo132 is studied through molecular dynamics simulations. Two different ligands, namely, formate and sulfate ligands, are considered as decoration of the inner surface of the capsule. In both cases it is found that 172 water molecules are trapped inside, 72 of which are coordination water molecules, and the remaining 100 form a water nanodrop whose properties are studied. We find that the dynamic behavior of the nanodrop is significantly different between the two types of capsules considered as they have different interiors. We argue that the commensurability of the sulfate ligand nanocapsule imposes a high degree of tetrahedrality that confers rigidity to the threedimensional water network affecting the whole water nanodrop in the cavity. The formate ligand capsule, instead, permits additional degrees of freedom that confer the water nanodrop a more liquid-like behavior. The overall size of the inner cavity is of the order of 1 nm which is comparable to the crossover length observed for effects related to the rigidity of water layers in contact with hydrophobic molecules.1 The results could stimulate the investigation of encapsulated water in the presently used capsule, the interior of which can be widely tuned (even from hydrophilic to hydrophobic) in context with the fact that knowledge about water under confined conditions is of extreme importance.

1. INTRODUCTION Water is perhaps the most fundamental substance regarding life on this planet, and its importance justifies the enormous amount of scientific work done upon it.2,3 Despite the significant advances, there is still an intense continuous activity aiming at a better understanding of many of its anomalous properties, such as the maximum density of liquid water at about 4 °C, the decrease of viscosity under pressure, the minimum of the isothermal compressibility, or the minimum of the specific heat Cp, both at atmospheric pressure,4−6 as well as its role as “biological water” in contact with cell ingredients and in hydrophilic protein pockets.2,7 To explain the properties several hypotheses have been proposed. In particular, the explanation based on the existence of a liquid−liquid phase transition critical point buried below the solid−liquid binodal line has received much attention in recent years. Water would then present a polymorphism between a low-density (LD) and a high-density (HD) phase in the deeply supercooled region, with a liquid−liquid critical point (LLCP) estimated to be at TC ≈ 193 °C and PC ≈ 1350 bar.8 These phases would be reminiscent of two known phases of ice of low and high density. Some simulations seem to suggest the existence of a LLCP,8−10 although conclusive evidence has not been found yet.11,12 According to this picture the enhancement of the anomalies in the supercooled region would correspond to the continuation of the liquid−liquid coexistence line into the onephase region along the so-called Widom line. The Widom line © 2014 American Chemical Society

is generally defined as the locus at which the density−density fluctuations show a maximum in the P−T plane, related to an increase of the correlation length. Moreover, along the interpretation line of the LLCP, the locus in the maxima of the thermodynamic susceptibilities in water, such as κT, is regarded as the Widom line which therefore would separate two regions13 dominated by a high-density and a low-density liquid-like behavior.14−16 It has been argued that X-ray data of water17 indicate that at ambient conditions, far above the temperature at which the Widom line can be clearly identified, water has density fluctuations due to a bimodal distribution of local structures reminiscent of the LD and HD water, although the attempts to identify patches of liquid in one of these two states from molecular simulations have failed in providing any evidence.18 Nevertheless, the simplicity of the molecular models for liquid water investigated may blur such an effect.19 The long structural correlation length, ξ say, displayed by systems like water, able to form tridimensional networks of hydrogen-bonded molecules, is particularly susceptible to confinement. The confinement affects structural as well as dynamic properties and also influences the phase diagram, for instance, hindering spontaneous nucleation upon cooling below the coexistence line. Experimental and simulation studies have Received: November 15, 2013 Revised: February 10, 2014 Published: February 10, 2014 5545

dx.doi.org/10.1021/jp411240u | J. Phys. Chem. C 2014, 118, 5545−5555

The Journal of Physical Chemistry C

Article

“pentagonal+spacers” building blocks results in a cavity with 20 Mo9O9 rings containing a pore of 3.2 Å in diameter, based on the van der Waals radii of opposing O atoms. The interest in the Mo132 capsules as containers of water nanodrops is that their study is experimentally accessible.50 In particular, in ref 50, we analyzed the X-ray data of the water containing the Mo132 anionic capsules with 30 formate ligands HCO2− (capsule 2a in ref 50) at 183 K, with an overall charge of 42−, with formamidinium cations covering the pores of the cavity. The structure of the confined water for this system is compared to that of Mo132 with 20 H2PO2− and 10 SO4−2 ligands (capsule 3a in ref 50) at 188 K, with a charge of 52−; for the improvement of the results of an earlier study see also ref 51. In both capsules 100 water molecules were trapped inside the cavity together with 72 coordinated water molecules covering the inner side. However, the structure of both water nanodrops was revealed as being substantially different. It was thus conjectured that it could constitute a low-density water assembly for the former capsule and a high-density one for the latter.50 For both capsules the large cations effectively seal the cavity, and no material transport is considered to take place between the inner and outer media; uptaken small cations would destroy the water structure. In our simulation analysis we have considered two systems that differ only in the type of ligand used inside the Mo132 capsule, namely, formate HCO2− for the first case and sulfate SO42− for the second. Both capsules thus contain 172 water molecules plus 30 ligands and have the 20 pores sealed by 20 formamidinium cations. The simulations are done at 25 °C. We will refer to these systems as Mo132(HCO2) and Mo132(SO4) for the ease of notation. Experimentally the Mo132 capsule with 30 SO42−, due to its large negative charge (72−), contains some small NH4+ ions inside the cavity after sealing it with larger formamidinium cations.50,52,53 Therefore, our results for pure water in this capsule cannot be directly compared with experimental data. Hence, the simulation results for this particular capsule are compared to the closer experimental system with similar characteristics of the inner medium, namely, the Mo132 capsule with {20 H2PO2− and 10 SO42−} ligands (system 3a (see ref 50)). The simulations have revealed that the structure of the water inside the Mo132(SO4) capsule is similar to that of the experimental Mo132(H2PO2−&SO42−).50,54 In the following we will show the distinctive dynamic behavior of the confined water in the Mo132(HCO2) and Mo132(SO4) cavities. Our results should stimulate further important studies of water under confined conditions, which are optimal in the case of the present capsules as their interior can be tuned widely from hydrophilic to hydrophobic. The article is organized as follows. In Section 2 we give the details of the simulation procedure and parameters, together with a short description of the experimental system given in ref 50. In Section 3 we show the simulation results for the two systems under consideration and provide an interpretation of their meaning. Finally Section 4 is devoted to the main conclusions that can be drawn from our study.

been conducted to address water confined in hydrophobic carbon surfaces like graphite plates20,21 and carbon nanotubes,22,23 in hydrophilic silica pores,16,24−35 and even in softer containers like reverse micelles,36−45 to mention some of them. The confining dimensions of these systems range from 1 to several nanometers. In turn, ξ in bulk water at ambient conditions is estimated as being of the order of 1 nm according, for instance, to fitting scattering data at small wave vector q by means of an Ornstein−Zernike (OZ) fit.46 However, despite this conjectured long correlation length, in the analysis of the results in the confined systems usually only two kinds of water are identified, namely, bound water, strongly affected by the interface, and free water, whose properties are close to those of the bulk. The thickness of the bound water layer depends on the properties of the wall like the total charge, the hydrophobic/hydrophilic character, as well as the geometry of the internal surface. It is usually estimated as being of a few angstroms, involving 1−2 water layers. In the system studied in this article, we have evidence that the tridimensional network can be stabilized when the size of the container is of the order of such correlation lengths under certain conditions. Then the characteristics of bulk water disappear in these cases for the whole cavity. Small changes in the container, in turn, permit the splitting of the system into free and bound water, as observed in other cases of water under confinement. This particular finding, related to commensurability between the tridimensional water network and the container, is the major outcome of this study. Such a dual behavior of confined water may have potential applications related to selectivity of ions47 and specific catalysis,48 among others. In this article we have performed molecular dynamics simulations aiming at the analysis of the dynamics of water confined in Keplerates, particularly in giant quasi-spherical polyoxometalate capsules of the type [{(Mo V I )MoVI5O21(H2O)6}12{MoV2O4}30L30]n−, which we will generally refer to as Mo132(L), where L stands for the different ligands.49 This practically rigid hollow capsule shown in Figure 1 belongs to a class of inorganic molecular metal-oxide clusters formed by 12 negatively charged pentagonal structures {(Mo VI)MoVI5O21(H2O)6}6−, linked by 30 cationic MoV2O42+ spacers. The inner side of the cavity can be functionalized with different ligands bound in a bidentate manner to the two molybdenum(V) centers of the spacers. This particular arrangement of the

2. COMPUTATIONAL DETAILS AND X-RAY CRYSTALLOGRAPHY OF THE NANODROPS MD simulations of water inside Mo 1 3 2 (SO 4 ) and Mo132(HCO2) capsules have been performed under ambient conditions (T = 298.15 K and P = 1 atm) using the DLPOLY v2.19 code.55 Water, the POMs, and the ligands have been treated as rigid molecules in the simulations. Although the

Figure 1. Schematic view of the Mo132 Keplerate with SO4 ligands. Notice the pentagonal structures that are connected by the spacers and the pores created by this arrangement. The SO4 ligands are located under the spacers. 5546



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[{(NH 2 ) 3 C + } 2 0 ⊂ {(Mo)Mo 5 O 2 1 (H 2 O) 6 } 1 2 {Mo 2 O 4 (H2PO2)}20{Mo2O4(SO4)}10]32− contains a structurally welldefined template-driven49,70 {H2O}100-type nanodrop with icosahedral symmetry49,50 (see also earlier publications, refs 51 and 71, in which the second capsulepresent in the extremely complex mixed crystal compound mentioned in ref 50 exhibiting another H2PO2−/SO42− ratio and correspondingly another water structurewas not mentioned). The water molecules are positioned in the three following concentric Platonic or Archimedean solids: two {H2O}20 dodecahedra and a distorted {H2O}60 rhombicosidodecahedron. The {H2O}60 solid can also be described as a {(H2O)5}12; interestingly, pentagons were assumed by some authors to be of importance in liquid water.72 On the other hand, the 20 H2O molecules of the second dodecahedral shell show a tetrahedral environment consisting of three molecules of the third shell and one molecule of the central {H2O}20 dodecahedron, which might be of interest as water was also considered as a type of tetrahedral liquid.6,70,73,74 The encapsulated water refers to the capsule [{(HC(NH2) 2)20 + (H2O)100} ⊂ {(MoVI)Mo5VIO21(H2O) 6}12{Mo2VO4(HCO2)}30]22− where the water molecules are found arranged in a well-defined and unprecedented structure. This can be described by four concentric shells with radii of 3.92− 4.13, 6.72−6.78, 7.59−7.78, and 8.31−8.70 Å.50 In the same sequence the shells have the following composition/structures: {H2O}20/dodecahedron, {H2O}20/2/dodecahedron, {H2O}60/ distorted rhombicosidodecahedron, and {H2O}20/2/dodecahedron; the two {H2O}20/2 shells are not separated enough from each other (related O···O distances on the C3 axes 1 ps system

βγ

βδA

βδB

βδC

Mo132(SO4) Mo132(HCO2)

0.39 0.68

0.44 0.77

0.26 0.78

0.72

2 MSD(t ) = ⟨| rCM ⃗ (t ) − rCM ⃗ (0)| ⟩

(3)

where rC⃗ M(t) stands for the position of the center of mass of a given tagged molecule, and the brackets indicate the equilibrium average. For water molecules confined in both Mo132 capsules the obtained dynamics of the MSD(t) is illustrated in Figure 7. In this calculation, we have taken the average value over all molecules of the cavity, and no distinction between layers is made. In both capsules, three well-defined regions are observed. At short times, water molecules follow a ballistic motion, and the MSD increases proportional to t2. After the ballistic regime, for intermediate times, 0.1−1 ps, the MSD flattens due to the rattling of the water particles trapped in their nearest-neighbor cage. Finally, the cage itself follows its own relaxation dynamics, and the MSD increases until reaching a second plateau due to the limited accessible volume inside the POMs. This separation of time scales is typical for glassy systems as described by the mode coupling theory (MCT)76 and is more evident for the case of the Mo132(SO4) capsule than for the Mo132(HCO2). For the former, the first plateau has a lower height and a larger extension in time as compared to what is observed for the latter. Notice that the time scale of this plateau is very close to that of the plateau observed in the Cii, which is a signature of the cage effect. Furthermore, from the height of the plateau, an average local cage radius can be extracted. We find ⟨rc⟩ = 0.52 and 0.72 Å for the Mo132(SO4) and Mo132(HCO2) capsules, respectively, also in agreement with the already mentioned extra freedom in water confined in the latter. The exchange of formate by sulfate ligands has, therefore, a similar effect in the MSD of confined water as lowering the temperature. At longer times, the difference in the diffusive behavior between both systems is still quite remarkable, the MSD being around 4−5 times larger when formate is present in the POM instead of sulfate. When sulfates instead are placed inside the Mo132 capsule, diffusion between external layers occurs only via a simultaneous switch of two water molecules,54 causing an important rearrangement of the HB network. Therefore, these switches seldom occur in a nanosecond time scale, yielding the slow diffusive process observed for this capsule. In Figure 8 we have plotted the distance to the center of one tagged molecule in both capsules. We can see the different dynamics, in particular, the constraint of the molecules to move preferably

general, the exponents are smaller than in the bulk (β ≈ 0.85 around 300 K78) but also smaller than in silica pores (β ≈ 0.6 around 300 K16). The different values of the exponents are due to the different dynamics of each layer. It is known that these exponents are determined by the dynamics of the cage, and lower values indicate that the relaxation processes are more hampered. Although layer γ lies far from the inner wall of the cavity, its dynamic behavior is still far from what would correspond to bulk water at this temperature. Therefore, the structural order existing in this system prevents the bulk behavior from appearing even at this high temperature and far from the walls. In comparison with the sulfate ligand case, the relaxation of the layers shown in Figure 6b for the formate ligand capsule has significant differences. In the first place, we observe that the plateau region at the limit of the ballistic behavior is not present in layer γF, except as a change in the slope. Such a relaxation corresponds to that of the density in bulk water at the same temperature.78 Thus, we can state here that at the center of the cavity water behaves bulk-like, unlike in the previous capsule for the corresponding layer. Second, the α-relaxation of layer εF takes place 2 orders of magnitude faster than in the sulfate ligand capsule. This indicates that the behavior of the so-called bound water is not completely determined by the interaction with the surface but that the sulfate ligand system has strong correlations along the radial direction that are not present in the formate ligand capsule. The relaxation for the three layers corresponding to the δF structure displays the plateau region preceding the α-relaxation. However, the long-time decay of the α-relaxation of the δF layers is at least 1 order of magnitude faster than in the corresponding δS layers in the Mo132(SO4) capsule. Furthermore, the plateau itself also displays significant differences between both capsules. In the formate ligand case we observe that the plateau has become a mere change of slope in the relaxation of both δAF and δCF layers, while it is still identified in the layer δBF. The latter layer corresponds to the water molecules forming pentagons lying under the pentagonal 5550



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MSD(t ) = 2⟨ r ⃗ 2⟩[1 − Γ(t )]

(4)

with the crossover function Γ(t) described by the functional form Γ(t ) =

a1e−b1t + a 2e−b2t a1 + a 2

(5)

as seen in Figure 7. The fits agree very well with the long-time behavior of the simulation data after the local cage effect has faded away. The long-time plateau is hence estimated for both systems, obtaining a value of MSD(∞) ≈ 128 and 126 Å2 for Mo132(SO4) and Mo132(HCO2), respectively. These values correspond to a MSD of the order of the square of the accessible cavity radius, as expected. Although the fits yield analogous MSD(∞) for both capsules, the time scale needed to reach the long-time plateau is very different and contains the relevant information. According to eq 4, we have proposed only two relaxation times to characterize the dynamics of the MSD. The larger of the two time constants, namely, τdiff = 1/b1 from now on, corresponds to the slowest translational process which is a reasonable estimate of the time scale for water to diffuse across the overall POM cavity. The values found for this parameter are τSdiff ≈ 5630 ps and τFdiff ≈ 740 ps for the Mo132(SO4) and Mo132(HCO2) capsules, respectively. We see that the ratio τSdiff/τFdiff is then approximately of the order of 10. Our analysis also yields a value for the fastest relaxation time, namely, 1/b2, still referring to long time behavior, i.e., diffusive motion. We find that 1/b2 = 190 ps for water in the sulfate case and 410 ps for water in the formate capsule. We observe that there is a large difference between the short (1/b2) and long relaxation times (1/b1) for the sulfate capsule, while these two times are very close in the case of the formate capsule. Although it is difficult to provide an unequivocal picture about the kind of process underlying the fastest relaxation, we suggest that 1/b2 stands for diffusion of molecules inside a given layer. That interpretation would agree with the disparate characteristic times found in the sulfate capsule as well as the similitude of the times found for the formate case. 3.2.3. Reorientational Dynamics. We have also studied the relaxation of the dynamics of the water OH bond reorientation through the decay of the C2,OH(t) correlation function defined as

Figure 7. Log−log plot of the mean-squared displacement of confined water inside the Mo132(SO4) capsule (a) and inside the Mo132(HCO2) capsule (b). The green curve corresponds to the fit to the Gaussian confinement model. To evidence the quality of the fits, the comparison between simulated data and the fits is also shown in real scale in each figure.

C2,OH(t ) = ⟨P2( rOH ⃗ (t ) · rOH ⃗ (0))⟩

(6)

where P2 is the second Legendre polynomial, and rO⃗ H stands for a unit vector in the direction of the OH bond. Figure 9 illustrates the simulation results for this quantity for water in both Mo132 nanocapsules, as well as for bulk water. At short times (∼1 ps), C2,OH(t) decays rapidly for the three systems. This time domain corresponds to fast motions, that is, inertial rotation and librational motion of water. Again, the strong interaction with the neighbors (the cage) is responsible for the librational motion that we can identify in the bump around 0.1 ps, the same scale as the caging effect observed in the plateau of the Cii functions. After this time region, the decay of C2,OH(t) for water in both POMs is substantially slower than relaxation in the bulk. However, as a general trend, the decay for the formate case is similar to that of bulk water and only differs in the long time relaxation governed by the collective motion of the system.

Figure 8. Distance d(t) of two tagged molecules from the center of the Mo132 cavities.

inside the layer for the sulfate ligand case. The populations of the layers remain basically constant. A more quantitative analysis of the different translational mobility in the Mo132 cavities has been obtained from the fits of the MSD data with the Gaussian confinement model79 5551



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Figure 9. Reorientational correlation functions, C2,OH(t), for water inside Mo132 cavities and for bulk water. We have also added the fits using eq 7.

Figure 10. Comparison of calculated reorientational correlation functions, C2,OH(t), for the Mo132(SO4) and Mo132(HCO2) capsules with simulated data for a RM, w0 = 4, and experimental data for a RM, w0 = 5.

To extract the time scales associated to the most relevant water reorientational processes in both POMs, we have fitted C2,OH(t) with the multiexponential sum of the form

Mo132(HCO2) capsules is practically the same as that in a w0 = 4 RM. This result evidences again the similitude between the disordered water medium in the RM and the weakly ordered water inside the formate capsule. The experimental result for w0 = 5 is qualitatively similar, although it shows a slower decay than that obtained in simulations.

5

C2,OH(t ) =

∑ A i e −t / τ

i

(7)

i=1

Here, each of the five τi should be enough to characterize the different subensembles of water molecules in terms of reorientation. The results of the fits for each capsule are given in Table 3 and are plotted in Figure 9. With regard to the analysis of the fits, the time scale given by τ1 corresponds to the residual (note that A1 is the smallest) relaxation at long times, that is, to the slowest reorientational motions. We have verified that this kind of reorientational motion corresponds to molecules belonging to the pentagons in the outermost layer ε which are bound to the cavity. In general, the relaxation times and the amplitudes are of the same order of magnitude for both capsules and slightly larger for the sulfate case, except for τ5 which is almost identical. When compared to bulk water, the longest relaxation time is more than 1 order of magnitude slower than τ1 for the capsules. Our simulation results for C2,OH(t) are compared to those obtained in reverse micelles (RMs) of comparable size. In particular, simulation data for a RM, w0 = 4, with 140 water molecules and experimental data for a RM, w0 = 5, with about 300 molecules are given in ref 41. Here, w0 stands for the ratio between the total water and the total surfactant concentrations, w0 = [H2O]/[surf.], whose value conditions the overall size of the water cluster trapped inside the micelle. The comparison to our systems containing 172 water molecules is given in Figure 10. Although POMs are inorganic capsules much more rigid than the soft protecting surfactant layer in the RMs, the time correlation function for the water reorientation in the

4. DISCUSSION AND CONCLUSIONS In this paper we have compared the different structure and dynamics of water confined in Mo132 polyoxometalates distinguished by the ligand decorating the inner part of the cavity. In particular, we have studied the case that this ligand is either sulfate or the smaller formate, but both cases contain the same number (172) of water molecules trapped, according to the interpretation of the X-ray data for analogous systems as described in the Introduction.50 Our analysis reveals that despite the similitude of the systems compared the behavior of the water confined is significantly different in both capsules. In particular, the slight increase in the accessible volume for the formate ligand capsule allows the confined water in this system to display a more bulky behavior in both the structure and the dynamics, of course, when disregarding the bound water in contact with the inner side of the cavity. Water in the Mo132(HCO2) displays a similar behavior as in silica pores, according to Gallo et al.,32 where the authors observe a bulky behavior for water confined in MCM-41 cylindrical pores whose radius is similar to that of our cavity. Furthermore, the same similarities are also observed in water confined in RMs (see Figure 10). In all those systems, one can basically distinguish between bound water and free water with the properties of the latter quite close to what is expected for bulk water.

Table 3. Parameters for the Fit of C2,OH(t) with Equation 7a

a

system

A1

τ1

A2

τ2

A3

τ3

A4

τ4

A5

τ5

Mo132(SO4) Mo132(HCO2) bulk water

0.031 0.013 0.002

1633.70 326.50 39.68

0.299 0.179 0.550

72.60 22.00 2.72

0.183 0.338 0.17

14.50 6.30 1.14

0.194 0.213 0.14

2.60 1.5 0.21

0.224 0.186 0.15

0.08 0.09 0.01

Relaxation times are given in picosecond units. 5552



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The main thesis of this article is that the long-range correlations characteristic for the HB network entangle with the characteristic dimensions of the container to the point that they significantly alter the picture described above. We have seen that the matching between the preferred structure of the δ layer with the inner part of the cavity + the solvation layer (ε) in the Mo132(SO4) capsule stabilizes a stack of concentric water layers practically affecting the whole cavity. The high degree of the tetrahedral order parameter for the layer δB for this capsule (see Figure 5) is particularly significant for the interpretation of the differences between the behavior of both capsules. Hence, in the latter system we can identify no bulky behavior. This strong difference in the behavior in apparently similar systems has been seen in all properties ranging from g(r), the relaxation times of properties such as Cii(t), C2,OH, to the characteristic time scales of the MSD. For some of these properties the difference observed is of 1 order of magnitude with respect to the formate ligand case. Therefore, our analysis suggests that these kinds of effects, related to the commensurability between the HB network and the characteristic length scales of the container, may play an important role for some applications. For example, if a given substrate would induce the formation of such a commensurate water structure in the cavity, its diffusion would be strongly hampered. Conversely, the randomization of the HB correlations inside the cavity by size mismatch would favor the transport of species in and out of the cavity with open pores. This aspect could be particularly relevant for possible catalytic applications of the Mo132 along the lines of those discussed by Kopilevich et al.48 These effects and similar ones could be expected for cavities whose size would be significantly smaller than 1 nm, as is believed for the size of the correlated HB network at ambient conditions.17,46,80 Although we have done this study in silico, we have strong evidence that our simulations using the TIP4P/2005 water model describe the relevant part of the physicochemical processes behind the properties analyzed in this paper, as seen, for instance, in the simulated and measured enhancement of the structure factor S(q) at very low q vector,18 among others.57 Another interesting feature is related to the α-relaxation in the cavities. We have seen that the ⟨Δni(0)Δni(t)⟩ is coupled to density−density fluctuations in the system.32 Centering our attention in the γ layer (the more bulky layer), our results for this quantity in the formate ligand case are analogous to FS(Q0,t) at the same temperature for bulk water.78 However, for the sulfate ligand case, the relaxation of ⟨Δni(0)Δni(t)⟩ for the γ layer would roughly correspond to the FS(Q0,t) at T = 260 K (supercooled water) despite the fact that the temperature in our simulations is 298 K,78 with a lower KWW exponent for the α-relaxation. This is another evidence that for this particular system one cannot make the simple distinction between free and bound water as in the case for the formate ligand capsule. We believe that this analysis sheds some light on the understanding of the collective properties of water. Aqueous solutions containing the easily available Mo132 capsules are excellent candidates to investigate experimentally many of the theses raised in this paper and in the current literature, while the present results could stimulate further investigations on the important behavior of water under different confined conditions.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +34 (0)977 559645. Fax: +34 (0)977 559621. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the ICIQ Foundation, Ministerio de Economia y Competitividad (CTQ2011-29054-C02-02), Generalitat de Catalunya (2009SGR-00259 and 2009SGR-00882), and EU COST Action CM1203 PoCheMoN. A.M. thanks the Deutsche Forschungsgemeinschaft for continuous support and the ERC (Brussels) for an Advanced Grant.

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REFERENCES

(1) Davis, J. G.; Gierszal, K. P.; Wang, P.; Ben-Amotz, D. Water Structural Transformation at Molecular Hydrophobic Interfaces. Nature 2012, 491, 582−585. (2) Ball, P. Water as an Active Constituent in Cell Biology. Chem. Rev. 2008, 108, 74−108. (3) Zhong, D.; Pal, S. K.; Zewail, A. H. Biological Water: A Critique. Chem. Phys. Lett. 2011, 503, 1−11. (4) Franks, F. Water, a Comprehensive Treatise; Plenum Press: New York, 1972. (5) Speedy, R. J.; Angell, C. A. Isothermal Compressibility of Supercooled Water and Evidence for a Thermodynamic Singularity at −45 °C. J. Chem. Phys. 1976, 65, 851−858. (6) Stillinger, F. H. Water Revisited. Science 1980, 209, 451−457. (7) Ball, P. Water: Water - An Enduring Mystery. Nature 2008, 452, 291−292. (8) Abascal, J. L. F.; Vega, C. Widom Line and the Liquid-Liquid Critical Point for the TIP4P/2005 Water Model. J. Chem. Phys. 2010, 133, 234502(8). (9) Mishima, O.; Stanley, H. E. Decompression-Induced Melting of Ice IV and the Liquid-Liquid Transition in Water. Nature 1998, 392, 164−166. (10) Holten, V.; Bertrand, C. E.; Anisimov, M. A.; Sengers, J. V. Thermodynamics of Supercooled Water. J. Chem. Phys. 2012, 136, 094507(18). (11) Limmer, D. T.; Chandler, D. The Putative Liquid-Liquid Transition is a Liquid-Solid Transition in Atomistic Models of Water. J. Chem. Phys. 2011, 135, 134503(10). (12) Limmer, D. T.; Chandler, D. The Putative Liquid-Liquid Transition is a Liquid-Solid Transition in Atomistic Models of Water. II. J. Chem. Phys. 2013, 138, 214504(15). (13) Simeoni, G. G.; Bryk, T.; Gorelli, F. A.; Krisch, M.; Ruocco, G.; Santoro, M.; Scopigno, T. The Widom Line as the Crossover between Liquid-Like and Gas-Like Behaviour in Supercritical Fluids. Nat. Phys. 2010, 6, 503−507. (14) Xu, L.; Kumar, P.; Buldyrev, S. V.; Chen, S.-H.; Poole, P. H.; Sciortino, F.; Stanley, H. E. Relation between the Widom Line and the Dynamic Crossover in Systems with a Liquid-Liquid Phase Transition. Proc. Natl. Acad. Sci. 2005, 102, 16558−16562. (15) Faraone, A.; Liu, L.; Yen, C.-W.; Chen, S.-H. Fragile-to-Strong Liquid Transition in Deeply Supercooled Confined Water. J. Chem. Phys. 2004, 121, 10843−10846. (16) Gallo, P.; Rovere, M.; Chen, S.-H. Dynamic Crossover in Supercooled Confined Water: Understanding Bulk Properties through Confinement. J. Phys. Chem. Lett. 2010, 1, 729−733. (17) Nilsson, A.; Pettersson, L. G. M. Perspective on the Structure of Liquid Water. Chem. Phys. 2011, 389, 1−34. (18) Sedlmeier, F.; Horinek, D.; Netz, R. R. Spatial Correlations of Density and Structural Fluctuations in Liquid Water: A Comparative Simulation Study. J. Am. Chem. Soc. 2011, 133, 1391−1398. (19) Nilsson, A.; Huang, C.; Petterson, L. G. M. Fluctuations in Ambient Water. J. Mol. Liq. 2012, 176, 2−16. 5553



Article

Confined inside Reverse Micelles. J. Phys. Chem. B 2009, 113, 15017− 15028. (42) Faeder, J.; Ladanyi, B. M. Molecular Dynamics Simulations of the Interior of Aqueous Reverse Micelles. J. Phys. Chem. B 2000, 104, 1033−1046. (43) Chowdhary, J.; Ladanyi, B. M. Molecular Dynamics Simulation of Aerosol-OT Reverse Micelles. J. Phys. Chem. B 2009, 113, 15029− 15039. (44) Rosenfeld, D. E.; Schmuttenmaer, C. A. Dynamics of the Water Hydrogen Bond Network at Ionic, Nonionic, and Hydrophobic Interfaces in Nanopores and Reverse Micelles. J. Phys. Chem. B 2011, 115, 1021−1031. (45) Chowdhary, J.; Ladanyi, B. M. Molecular Simulation Study of Water Mobility in Aerosol-OT Reverse Micelles. J. Phys. Chem. A 2011, 115, 6306−6316. (46) Huang, C.; Wikfeldt, K. T.; Tokushima, T.; Nordlund, D.; Harada, Y.; Bergmann, U.; Niebuhr, M.; Weiss, T. M.; Horikawa, Y.; Leetmaa, M.; et al. The Inhomogeneous Structure of Water at Ambient Conditions. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 15214− 15218. (47) Gilles, A.; Mihai, S.; Nasr, G.; Mahon, E.; Garai, S.; Müller, A.; Barboiu, M. Highly Selective Li+ Ion Transport by Porous Molybdenum-Oxide Keplerate-Type Nanocapsules Integrated in a Supported Liquid Membrane. Isr. J. Chem. 2013, 53, 102−107. (48) Kopilevich, S.; Gil, A.; Garcia-Ratés, M.; Avalos, J. B.; Bo, C.; Müller, A.; Weinstock, I. A. Catalysis in a Porous Molecular Capsule: Activation by Regulated Access to Sixty Metal Centers Spanning a Truncated Icosahedron. J. Am. Chem. Soc. 2012, 134, 13082−13088. (49) Müller, A.; Gouzerh, P. From Linking of Metal-Oxide Building Blocks in a Dynamic Library to Giant Clusters with Unique Properties and towards Adaptive Chemistry. Chem. Soc. Rev. 2012, 41, 7431− 7463. (50) Mitra, T.; Miró, P.; Tomsa, A. R.; Merca, A.; Bögge, H.; Avalos, J. B.; Poblet, J. M.; Bo, C.; Müller, A. Gated and Differently Functionalized (New) Porous Capsules Direct Encapsulates’ Structures: Higher and Lower Density Water. Chem.Eur. J. 2009, 15, 1844−1852. (51) Müller, A.; Bögge, H.; Diemann, E. Structure of a CavityEncapsulated Nanodrop of Water. Inorg. Chem. Commun. 2003, 6, 52− 53; Corrigendum: Structure of a Cavity-Encapsulated Nanodrop of Water Inorg. Chem. Commun. 2003, 6, 52; Inorg. Chem. Commun. 2003, 6, 329. (52) Müller, A.; Krickemeyer, E.; Bögge, H.; Schmidtmann, M.; Botar, B.; Talismanova, M. O. Drawing Small Cations into Highly Charged Porous Nanocontainers Reveals “Water” Assembly and Related Interaction Problems. Angew. Chem., Int. Ed. 2003, 42, 2085− 2090. (53) Müller, A.; Zhou, Y.; Bögge, H.; Schmidtmann, M.; Mitra, T.; Haupt, E. T. K.; Berkle, A. Gating” the Pores of a Metal Oxide Based Capsule: After Initial Cation Uptake Subsequent Cations Are Found Hydrated and Suparmolecularly Fixed above the Pores. Angew. Chem., Int. Ed. 2006, 45, 460−465. (54) Garcia-Ratés, M.; Miró, P.; Poblet, J. M.; Bo, C.; Avalos, J. B. Dynamics of Encapsulated Water inside Mo132 Cavities. J. Phys. Chem. B 2011, 115, 5980−5992. Notice that in this article the reference to the experimental Mo132(SO4) is incorrect. It should be the Mo132(H2PO2−&SO42−), as defined in the Introduction. (55) Smith, W.; Forester, T. R. DL-POLY-2.0: A General-Purpose Parallel Molecular Dynamics Simulation Package. J. Mol. Graph. 1996, 14, 136−141. (56) Ziv, A.; Grego, A.; Kopilevich, S.; Zeiri, L.; Bo, C.; Müller, A.; Weinstock, I. A. Flexible Pores of a Metal Oxide-Based Capsule Permit Entry of Comparatively Larger Organic Guests. J. Am. Chem. Soc. 2009, 131, 6380−6382. (57) Abascal, J. L. F.; Vega, C. A General Purpose Model for the Condensed Phases of Water: TIP4P/2005. J. Chem. Phys. 2005, 123, 234505(12). (58) López, X.; Nieto-Draghi, C.; Bo, C.; Avalos, J. B.; Poblet, J. M. Polyoxometalates in Solution: Molecular Dynamics Simulations on the

(20) Martí, J.; Nagy, G.; Guàrdia, E.; Gordillo, M. C. Molecular Dynamics Simulation of Liquid Water Confined inside Graphite Channels: Dielectric and Dynamical Properties. J. Phys. Chem. B 2006, 110, 23987−23994. (21) Nagy, G.; Gordillo, M. C.; Guàrdia, E.; Martí, J. Liquid Water Confined in Carbon Nanochannels at High Temperatures. J. Phys. Chem. B 2007, 111, 12524−12530. (22) Martí, J.; Gordillo, M. C. Time-Dependent Properties of Liquid Water Isotopes Adsorbed in Carbon Nanotubes. J. Chem. Phys. 2001, 114, 10486−10492. (23) Martí, J.; Guàrdia, E.; Gordillo, M. C. Reorientational Motions in Sub- and Supercritical Water under Extreme Confinement. Chem. Phys. Lett. 2002, 365, 536−541. (24) Bellissent-Funel, M.-C.; Chen, S. H.; Zanotti, J.-M. SingleParticle Dynamics of Water Molecules in Confined Space. Phys. Rev. E 1995, 51, 4558−4569. (25) (a) Bruni, F.; Ricci, M. A.; Soper, A. K. Water Confined in Vycor Glass. I. A Neutron Diffraction Study. J. Chem. Phys. 1998, 109, 1478−1485. (b) Soper, A. K.; Bruni, F.; Ricci, M. A. Water Confined in Vycor Glass. II. Excluded Volume Effects on the Radial Distribution Functions. J. Chem. Phys. 1998, 109, 1486−1494. (26) Gallo, P. Single Particle Slow Dynamics of Confined Water. Phys. Chem. Chem. Phys. 2000, 2, 1607−1611. (27) Ricci, M. A.; Bruni, F.; Giuliani, A. Similarities” between Confined and Supercooled Water. Faraday Discuss. 2009, 141, 347− 358. (28) Rovere, M.; Ricci, M. A.; Vellati, D.; Bruni, F. A Molecular Dynamics Simulation of Water Confined in a Cylindrical SiO2 Pore. J. Chem. Phys. 1998, 108, 9859−9867. (29) Gallo, P.; Rovere, M.; Spohr, E. Glass Transition and Layering Effects in Confined Water: A Computer Simulation Study. J. Chem. Phys. 2000, 113, 11324−11335. (30) Hartnig, C.; Witschel, W.; Spohr, E.; Gallo, P.; Ricci, M. A.; Rovere, M. Modifications of the Hydrogen Bond Network of Liquid Water in a Cylindrical SiO2 Pore. J. Mol. Liq. 2000, 85, 127−137. (31) Malani, A.; Ayappa, K. G. Relaxation and Jump Dynamics of Water at the Mica Interface. J. Chem. Phys. 2012, 136, 194701(11). (32) Gallo, P.; Rovere, M.; Chen, S.-H. Anomalous Dynamics of Water Confined in MCM-41 at Different Hydrations. J. Phys.: Condens. Matter 2010, 22, 284102(8). (33) Lerbret, A.; Lelong, G.; Mason, P. E.; Saboungi, M.-L.; Brady, J. W. Water Confined in Cylindrical Pores: A Molecular Dynamics Study. Food Biophys. 2011, 6, 233−240. (34) Milischuk, A. A.; Ladanyi, B. M. Structure and Dynamics of Water Confined in Silica Nanopores. J. Chem. Phys. 2011, 135, 174709(11). (35) Bourg, I. C.; Steefel, C. I. Molecular Dynamics Simulations of Water Structure and Diffusion in Silica Nanopores. J. Phys. Chem. C 2012, 116, 11556−11564. (36) Onori, G.; Santucci, A. IR Investigations of Water Structure in Aerosol OT Reverse Micellar Aggregates. J. Phys. Chem. 1993, 97, 5430−5434. (37) Venables, D. S.; Huang, K.; Schuttenmaer, C. A. Effect of Reverse Micelle Size on the Librational Band of Confined Water and Methanol. J. Phys. Chem. B 2001, 105, 9132−9138. (38) Harpham, M. R.; Ladanyi, B. M.; Levinger, N. E.; Herwig, K. W. Water Motion in Reverse Micelles Studied by Quasielastic Neutron Scattering and Molecular Dynamics Simulations. J. Chem. Phys. 2004, 121, 7855−7868. (39) Dokter, A. M.; Woutersen, S.; Bakker, H. J. Inhomogeneous Dynamics in Confined Water Nanodroplets. Proc. Natl. Acad. Sci. 2006, 103, 15355−15358. (40) Piletic, I. R.; Moilanen, D. E.; Spry, D. B.; Levinger, N. E.; Fayer, M. D. Testing the Core/Shell Model of Nanoconfined Water in Reverse Micelles Using Linear and Nonlinear IR Spectroscopy. J. Phys. Chem. A 2006, 110, 4985−4999. (41) Pieniazek, P. A.; Lin, Y.-S.; Chowdhary, J.; Ladanyi, B. M.; Skinner, J. L. Vibrational Spectroscopy and Dynamics of Water 5554



Article

α-PW12O403− Keggin Anion in Aqueous Media. J. Phys. Chem. A 2005, 109, 1216−1222. (59) Leroy, F.; Miró, P.; Poblet, J. M.; Bo, C.; Avalos, J. B. Keggin Polyoxoanions in Aqueous Solution: Ion Pairing and its Effect on Dynamic Properties by Molecular Dynamics Simulations. J. Phys. Chem. B 2008, 112, 8591−8599. (60) Miró, P.; Poblet, J. M.; Avalos, J. B.; Bo, C. Towards a Computational Treatment of Polyoxometalates in Solution Using QM Methods and Explicit Solvent Molecules. Can. J. Chem. 2009, 87, 1296−1301. (61) Chaumont, A.; Wipff, G. Ion Aggregation in Concentrated Aqueous and Methanol Solutions of Polyoxometallates Keggin Anions: The Effect of Counterions Investigated by Molecular Dynamics Simulations. Phys. Chem. Chem. Phys. 2008, 10, 6940−6953. (62) Chaumont, A.; Wipff, G. Polyoxometalate Keggin Anions at Aqueous Interfaces with Organic Solvents, Ionic Liquids, and Graphite: A Molecular Dynamics Study. J. Phys. Chem. C 2009, 113, 18233−18243. (63) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. Development and Testing of the OPLS All-Atom Force Field on Conformational Energetics and Properties of Organic Liquids. J. Am. Chem. Soc. 1996, 118, 11225−11236. (64) Weerasinghe, S.; Smith, P. E. A Kirkwood-Buff Derived Force Field for the Simulation of Aqueous Guanidinium Chloride Solutions. J. Chem. Phys. 2004, 121, 2180−2186. (65) Rappe, A. K.; Goddard, W. A. Charge Equilibration for Molecular Dynamics Simulations. J. Phys. Chem. 1991, 95, 3358−3363. (66) Frenkel, D.; Smith, B. Understanding Molecular Simulation, from Algorithms to Applications; Academic: San Diego, 1996. (67) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Clarendon: Oxford, 1989. (68) Fincham, D. Leapfrog Rotational Algorithms. Mol. Simul. 1992, 8, 165−178. (69) Hoover, W. G. Canonical Dynamics: Equilibrium Phase-Space Distributions. Phys. Rev. A 1985, 31, 1695−1697. (70) Oleinikova, A.; Weingärtner, H.; Chaplin, M.; Diemann, E.; Bögge, H.; Müller, A. Self-Association Based on Interfacial Structured Water Leads to {Mo154}≈1165 Super Clusters: A Dielectric Study. ChemPhysChem. 2007, 8, 646−649. (71) Müller, A.; Krickemeyer, E.; Bögge, H.; Schmidtmann, M.; Roy, S.; Berkle, A. Changeable Pore Sizes Allowing Effective and Specific Recognition by a Molybdenum-Oxide Based “Nanosponge”: En Route to Sphere-Surface and Nanoporous-Cluster Chemistry. Angew. Chem., Int. Ed. 2002, 41, 3604−3609. (72) Keutsch, F.; Saykally, R. J. Water Clusters: Untangling the Mysteries of the Liquid, One Molecule at a Time. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 10533−10540. (73) Frank, H. S.; Quist, A. S. Pauling’s Model and the Thermodynamic Properties of Water. J. Chem. Phys. 1961, 34, 604− 611. (74) Pauling, L. The Nature of the Chemical Bond; Cornell University Press: New York, 1960. (75) Chau, P.-L.; Hardwick, A. J. A New order Parameter for Tetrahedral Configurations. Mol. Phys. 1998, 93, 511−518. (76) Götze, W.; Sjögren, L. Relaxation Processes in Supercooled Liquids. Rep. Prog. Phys. 1992, 55, 241−376. (77) Götze, W. The Essentials of the Mode-Coupling Theory for Glassy Dynamics. Condens. Matter Phys. 1998, 1, 873−904. (78) Gallo, P.; Sciortino, F.; Tartaglia, P.; Chen, S.-H. Slow Dynamics of Water Molecules in Supercooled States. Phys. Rev. Lett. 1996, 76, 2730−2733. (79) Volino, F.; Perrin, J. C.; Lyonnard, S. Gaussian Model for Localized Translational Motion: Application to Incoherent Neutron Scattering. J. Phys. Chem. B 2006, 110, 11217−11223. (80) De los Santos, F.; Franzese, G. Relations between the Diffusion Anomaly and Cooperative Rearranging Regions in a Hydrophobically Nanoconfined Water Monolayer. Phys. Rev. E 2012, 85, 010602(4).

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