Mixtures of Isobutyric Acid and Water Confined in ... - ACS Publications

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Mixtures of Isobutyric Acid and Water Confined in Cylindrical Silica Nanopores Revisited: A Combined Solid-State NMR and Molecular Dynamics Simulation Study Michael F. Harrach,† Barbara Drossel,*,† Wadim Winschel,‡ Torsten Gutmann,‡ and Gerd Buntkowsky*,‡ †

Institut für Festkörperphysik, Technische Universität Darmstadt, Hochschulstrasse 6, 64289 Darmstadt, Germany Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Alarich-Weiss-Strasse 8, D-64287 Darmstadt, Germany

‡

S Supporting Information *

ABSTRACT: The phase behavior of a mixture of isobutyric acid and water confined in mesoporous SBA-15 silica material is studied by a combination of solid-state NMR spectroscopy and molecular dynamics (MD) simulations. The combination of these two techniques shows that the iBA-rich phase is close to the pore wall and the water-rich phase is in the center of the pores. The MD simulations reveal that this surprising phase behavior is mainly the result of the minimization of the hydrogen-bonding enthalpy. Moreover, the simulations show also that upon increasing temperature the relative water concentration increases at the pore surface, which is attributed to entropic contributions which lead to a more thorough miscibility. These results solve a long-standing problem of the microphase separation inside these materials.

1. INTRODUCTION Water plays an important role in our everyday life as well as in various scientific fields. In many biological, geological, or technological systems, water and water mixtures are spatially confined on the nanoscale. Examples and applications range from environmental studies1 over microfluidics2 to lab-on-achip,3 biological investigations,4,5 and efforts to verify the existence of a liquid−liquid critical point.6−8 Confinement changes the structure and dynamics of the confined fluid. If the fluid is a binary mixture, confinement also changes the phase diagram, especially for very small pores9 with a large surface area. Understanding the effect of confinement on the phase behavior is important for developing new separation technologies and optimizing currently used techniques, such as oil recovery, lubrication, coating technology, and elimination of contamination.10−12 An important example of such binary fluids is the mixture of isobutyric acid and water. Isobutyric acid (iBA), or 2methylpropanoic acid, has both hydrophilic and hydrophobic functional groups, leading to phase separation when mixed with water. iBA belongs to the group of carboxylic acids which play an important role in a number of industrially relevant separation processes. Furthermore, carboxylic acids are used as basic building blocks of soaps and detergents, as raw materials for the production of nylon, biodegradable plastics, and certain pharmaceuticals, and as buffers and acidulents (food preservatives), flavoring agents, fungicides, insecticides, biomass products, and catalysts.13 Carboxylic acid derivatives find © 2015 American Chemical Society

widespread application in various commercial sectors and are often present in aqueous waste streams and as byproducts of industrial operations.14 For a detailed understanding of the fluid behavior in these heterogeneous systems, a detailed modeling and experimental study of the guest−host interactions, the resulting phase behavior, and the changes of the physicochemical properties of the guest molecules in confinement is mandatory. Molecular Dynamics (MD) simulations are a valuable modeling tool since they yield structural and dynamic information at the level of single molecules. In addition, MD simulations can be used to change different parameters such as state of the fluid (confined or bulk), the partial charges, the roughness of the confining wall, and the density of the confined fluid, independently from each other, and to prepare initial configurations which are not easily experimentally accessible. Typical experimental techniques for the investigation of phase transitions at the thermodynamic level are differential scanning calorimetry (DSC) or thermogravimetric analysis (TGA).15,16 A deeper understanding of the interactions necessitates the analysis of the structure−property relations beyond the thermodynamic level. These systems are disordered and often include structural impurities or defects. Therefore, diffraction techniques are not applicable and other techniques, Received: September 30, 2015 Revised: December 4, 2015 Published: December 4, 2015 28961

DOI: 10.1021/acs.jpcc.5b09537 J. Phys. Chem. C 2015, 119, 28961−28969

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The Journal of Physical Chemistry C such as solid-state NMR (ssNMR) spectroscopy have to be employed. An overview about the current state of the art is given in refs 17−27. A few years ago, some of us28−30 used 1H NMR spectrometry, 1H NMR relaxometry, and 1H-pulsed field gradient diffusion measurements to investigate water, iBA, and a 54 wt % iBA and D2O mixture confined in mesoporous silica materials. For a controlled pore glass of roughly 10.3 nm diameter, at temperatures above 39 °C, an anomalous temperature dependence of the self-diffusion coefficient and a bifurcation of the T2 relaxation occurred, which are monitors of the translational and rotational motion of the iBA molecules. On the basis of these findings, a qualitative model of the phase separation process in the pores was developed, which assumes a temperature-dependent concentric cylinder domain-like structure of the liquid below the phase transition temperature and a continuous breakdown of these domains above the transition temperature. The two cylinders are formed by an iBA phase with small amounts of dissolved water and a water phase with small amounts of dissolved iBA molecules. Their NMR data were not sufficient to make an unequivocal assignment of the two phases to the inner or outer cylinder. Instead, they performed a tentative model of the phase structure, where they attributed the water-rich phase to the outer cylinder and the iBA-rich phase to the inner cylinder. The main argument of this assignment was the observed bifurcation of the T2 relaxation data. The present paper employs a combination of dipolar solidstate NMR spectroscopy and MD simulations to identify the iBA/water structure inside the mesopores. The experimental part of this in-depth investigation has become possible due to recent advances in low-temperature magic-angle spinning (MAS) NMR methodology. In the first step of this study, we performed extensive MD simulations to develop a quantitative modeling of the radial fluid distribution function inside the pores. On the basis of these data we then performed twodimensional 29Si/1H cross-polarization magic-angle spinning heteronuclear correlation spectroscopy (CP-MAS HETCOR) experiments to reveal the neighborhood conditions of probe molecules and pore walls. Such HETCOR experiments originally introduced by van Rossum et al.31 including homonuclear Lee−Goldberg decoupling during the evolution of the chemical shift have been widespread used to monitor the strength of dipolar interactions between protons and heteronuclei such as 29Si, 13C, or 31P.32−44 Depending on the contact time, short- and long-range correlations are distinguishable which may enable one to analyze the orientation and sites of small molecules such as water and iBA containing protons with respect to the surface of the porous silica material containing the 29Si heteronuclei. The rest of this paper is organized as follows. Section 2 gives details concerning materials and methods. Results concerning phase behavior of the confined mixture are given in section 3 and discussed herein. Section 4 presents results and discussions on the dynamic behavior of the confined mixture, and section 5 summarizes and concludes this work.

PSF generation plug-in using ideal coordinates for the iBA taken from HIC-UP48−50 and parameters taken from the CHARMM2251 force field. We compiled a basic blueprint of the molecule by combining elements of the residues LEU and GLUP (Scheme 1). In order to ensure charge neutrality we Scheme 1. Graphic Representation of an iBA Molecule and Defining the Different Atom Positions

slightly increased the partial charge on the connecting atom, Table 1. The silica nanopore has a diameter of roughly 4.2 nm Table 1. Force Field Parametersa atoms charge q ε (LJ) rmin (LJ)

CT1 −0.12 0.02 4.55

CT3 −0.27 0.08 4.12

CD 0.75 0.07 4.0

OB −0.55 0.12 3.4

OH1 −0.61 0.1521 3.54

HA 0.09 0.022 2.64

H 0.44 0.046 0.449

a

Charges are given in units of the elementary charge. LJ parameters in kcal/mol (energy well-depth ε) and angstroms (position of potential minimum rmin), angle potentials, etc. correspond to the CHARMM22 force field.

and a density of roughly seven silanol groups per square nanometer. The manufacturing process of the pore was conducted analogously to the procedure described by Rovere et al.52 and undertaken as part of the master thesis of Janz (Vogel group, TU Darmstadt). However, while we also used the Lennard-Jones (LJ) interaction parameters and partial charges for the silica pore as given by Bródka and Zerda,53 hydrogen atoms were not kept fixed but restricted by the harmonic part of the angle potentials as given by Hill and Sauer.54 Interactions between the various species were governed by the Lorentz−Berthelot mixing rules. Mixtures with iBA weight percentages (wt %) of 40, 50, 60, and 70 both in the pores and in the bulk were simulated. For comparison, for some temperatures pure iBA or water systems were simulated as well. We used temperatures in the range of 300−400 K, which covers the demixing phase transition in the simulated bulk system. The temperature of demixing is higher than for the experimental system;55−57 however, such temperature shifts are not uncommon in MD simulations. We adjusted the number of molecules in the pore such that the density in the pore center agreed with the value expected for a pressure of 1 bar. This density value at the center was obtained from NPT simulations of a pore where the confinement was generated by a cylindrically symmetric external potential (Note: NPT simulations cannot be performed with atomistic pores.) Although employing the NVT ensemble is not equivalent to utilizing the NPT ensemble, it is a useful approximation that preserves the mixing ratio of iBA/water within the pore, which makes the study computationally feasible. Simulation results for an iBA−water mixture in such cylindrically symmetric potential pores are given in ref 58.

2. MATERIALS AND METHODS In the following, the experimental details and the setup of the simulations are briefly summarized and the observables are evaluated. 2.1. Simulation Setup. The MD simulations were carried out using the NAMD45 simulation package and the TIP3P water model.46 The iBA molecule was built using the VMD47 28962


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external standard where 1H was set to 0 ppm and 29Si was set to 0 ppm. To take the chemical shift scaling of the FSLG experiment into account, the strong aliphatic signal referring to the CH3 groups of isobutyric acid was set to 1.2 ppm afterward. For comparison, an additional room-temperature 29Si/1H CP-MAS HETCOR experiment was carried out on a 600 MHz Bruker AVANCE III HD spectrometer operating at a frequency of 600.12 MHz for 1H and 119.22 MHz for 29Si. The sample was packed into a 4 mm ZrO2 rotor. The 29Si/1H CP-MAS HETCOR experiment was performed for the iBA/H2O−SBA15 sample at 8 kHz spinning employing a contact time of 3 ms and tppm1560 decoupling was applied during data acquisition. FSLG homonuclear decoupling31 was utilized with a decoupling field of 100 kHz during the evolution of the chemical shift. The spectrum was referenced employing trimethylsilylpropanoic acid (TSP) (measured at room temperature). To take the chemical shift scaling of the FSLG experiment into account, the aliphatic signal referring to the −CH− group of the isobutyric acid was set to 2.6 ppm afterward. 2.3.2. Sample Preparation. 2.3.2.1. General. The hydrophilic behavior of SBA-15 and the controlled porous glass CPC10-75 are comparable, as was shown by solid-state NMR investigations of the adsorption of water on SBA-1561 and CPC-10-75.29 Since SBA-15 has a ca. 4 times larger specific surface area compared to the controlled porous glass employed in the previous study,28 and thus better sensitivity for the 29 Si/1H CP-MAS HETCOR experiments, we decided to employ SBA-15 as porous host material, instead of the controlled porous glass CPC-10-75. iBA was purchased from Sigma-Aldrich, and D2O (99.90%) was from Eurisotop, and used without further purification. The SBA-15 material was prepared according to the literature37 employing Pluronic F127 as structure-directing template. The obtained material was dried under vacuum at 100 °C for 3 days to remove water. The specific pore volume (BET) of 522 m2/g and the averaged pore diameter (BJH) of 4.75 nm were determined by nitrogen adsorption. Adsorption of the iBA/ water mixtures on the SBA-15 materials was carried out in an argon glovebox to avoid adsorption of water from air. 2.3.2.2. iBA/H2O−SBA-15. To prepare the iBA/H2O mixture, a composition of 56 wt % iBA and 44 wt % H2O, corresponding to 20 mol % iBA and 80 mol % H2O, was used according to the literature.30 After mixing the components, a clear solution was obtained at room temperature indicating complete miscibility of the two components. This mixture was then adsorbed at room temperature on the SBA-15 material. The amount of adsorbed iBA/H2O mixture corresponded to a filling degree of ca. 70% of the pore volume, which ensures that all liquid is inside the pore and no extra liquid is localized on the outer surface. To determine the total pore volume of the mesoporous material, the obtained weight (∼30 mg) and the specific characteristic pore volume were used. The sample was then brayed with a glass rod and packed into a 3.2 mm rotor for NMR measurement. 2.3.2.3. iBA/D2O−SBA-15. To prepare the iBA/D2O mixture, a composition of 54 wt % iBA and 46 wt % D2O, corresponding to 20 mol % iBA and 80 mol % D2O, was used according to the literature.30 After mixing, the components were warmed to obtain a clear solution. This warmed mixture was then adsorbed on the SBA-15 material. The amount of adsorbed iBA/D2O mixture corresponded to a filling degree of ca. 70% of the pore volume, which ensures that all liquid is inside the pore and no extra liquid is localized on the outer

In the bulk, we generally used a minimal equilibration time of 10 ns, starting from a configuration with separated components. In the pore, the slowdown of dynamics at the boundary made additional equilibration times between 7.5 and 17.5 ns necessary, depending on temperature and weight percentage of the mixture. For simulations within the pore a time step of 1 fs was utilized; simulations in the bulk were carried out with a time step of 2 fs. The Coulomb interactions were calculated using the particle mesh Ewald sum. Periodic boundary conditions were applied. We used a cutoff at 15 Å and a switching distance of 12 Å. In the NPT ensemble pressure was kept constant using the Langevin−Piston59 method. NVT simulations were done using the Langevin thermostat, with a coupling coefficient of 1.0 ps−1 and with the hydrogen atoms included in the Langevin dynamics. Temperature values were spaced out in intervals of 10 K, with simulations generally in the 300−400 K region. 2.2. Measures of Structure and Dynamics. Except for the density profiles, which are shown as a function of the distance to the pore center, the properties of iBA and water were analyzed as a function of the distance to the pore boundary, which is defined as the distance to the nearest nonhydrogen wall atom. 2.2.1. Intermediate Scattering Function. We calculate the incoherent intermediate scattering function Fq,incoh(t) as ⃗ | Fq ,incoh(t ) = ⟨e−iq | ri (⃗ t ) − ri (0) ⟩

In the case of iBA ri⃗ (t) gives the position of the center of mass of the ith molecule at time t. The scattering vector q is set to 1.05 Å−1 for the iBA. This roughly corresponds to 2π divided by the nearest-neighbor distance between molecules in the bulk. The correlation time τ is defined as the time at which the incoherent intermediate scattering function has decayed to a value of e−1. 2.2.2. Orientational Autocorrelation Function. We calculate the orientational autocorrelation function Foa(t) as Foa(t ) =

1 ⟨3( ei⃗ (t ) ei⃗ (0))2 − 1⟩ 2

where ei⃗ (t) denotes the normalized unit vector describing the particular orientation of the chosen vector of the ith molecule at time t. We denote the relaxation time as the time at which the function reaches a value of e−1. 2.2.3. Susceptibility. We calculate the susceptibility χ″(t) based on the correlation function CAA(t) as ∞

χ ″ (t ) ∼ ω

∫−∞ CAA(t ) cos(ωt ) dt

2.3. Experimental Section. 2.3.1. Solid-State NMR Spectroscopy. Low-temperature measurements were carried out on a Bruker Ascend 400DNP AVANCE III spectrometer operating at a frequency of 400.02 MHz for 1H and 79.47 MHz for 29Si at a temperature of nominally 100 K. At this temperature 3.2 mm rotors equipped with ZrO2 caps were used. The 29Si/1H CP-MAS HETCOR experiments were performed for iBA/H2O−SBA-15 and iBA/D2O−SBA-15 at 8 kHz spinning employing contact times of 3 and 0.5 ms, respectively, and tppm1560 decoupling was utilized during data acquisition. FSLG homonuclear decoupling31 was applied with a decoupling field of 89 kHz during the evolution of the chemical shift. Spectra were referenced employing trimethylsilylpropanoic acid (TSP) (measured at nominally 100 K) as 28963


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Figure 1. Low-temperature 1H/29Si CP-MAS HETCOR experiments at 3 ms contact time. Left panel: 56 wt % iBA/H2O−SBA-15 sample. Right panel: 54 wt % iBA/D2O−SBA-15 sample.

Table 2. Relative Peak Volumes in Relation to the Aliphatic Signal for the Cross Peaks in the HETCOR Spectraa iBA/H2O−SBA-15 iBA/D2O−SBA-15 iBA/H2O−SBA-15 iBA/H2O−SBA-15 a

(3 ms) (3 ms) (0.5 ms) (3 ms)

temperature

aliphatic

hydroxyl

carboxyl

100 K 100 K 100 K RT

1 1 1 1

0.73 0.29 1 9.13

0.24 0 0 0

Note: these volume integrals are only semiquantitative, as the measurements were performed by cross-polarization (CP).

intensities are only a rough qualitative measure of the surface-near concentration of the different types of protons. In the 1H dimension of the iBA/H2O−SBA-15 different proton types are visible. The strong signal at ca. 6.5 ppm assigns to hydroxyl groups from H2O molecules, which are in fast exchange with the carboxyl protons of COOH in iBA and the silanol protons61 of the silica. This chemical shift is in agreement with an assumed iBA-enriched phase (see liquidstate spectra in the Supporting Information Figure S2 for comparison). The second strong signal at 1.2 ppm is assigned to aliphatic protons of the iBA. In addition, there is a weak acidic signal at 11.5 ppm, which assigns to nonexchanging protons from the carboxyl group of the iBA. A similar result is obtained for the iBA/D2O−SBA-15 sample, however, with reduced intensity of the hydroxyl signal and vanished carboxyl signal, which relates to an H/D exchange. The still observable hydroxyl at 6.5 ppm may refer to nonexchanged hydroxyl groups or H2O present in the D2O solution. Looking closer at the peaks, it is evident that the lines of the aliphatic protons are shifted high field in the 29Si dimension, relative to the hydroxyl signals and the carboxyl signal. Since the line positions in the 29Si spectra are characteristic for different types of silica sites (Q2, Q3, Q4), this is an indication that there are preferential positions for both the iBA and the water molecules on the surfaces. To shed further light on this observation, we performed a second 1H/29Si CP-MAS HETCOR experiment with a shorter contact time of only 0.5 ms, shown in Figure 2. Owing to the shorter contact time, this experiment is more sensitive toward smaller distances. This spectrum shows that the hydroxyl protons are mainly correlated to the Q2 and Q3 groups and the main correlation of the aliphatic protons is with the Q4 groups (note that the ratio of Qn groups is nonstoichiometric, since only groups near protons are visible). Finally, for comparison a 1H/29Si CP-MAS HETCOR experiment for iBA/H2O−SBA-15 was recorded at room temperature (Figure 3). Compared to the low-temperature spectra this spectrum displays a strongly reduced intensity of a

surface. To determine the total pore volume of the mesoporous material, the obtained weight (∼30 mg) and the specific characteristic pore volume were used. The sample was then brayed with a glass rod and packed into a 3.2 mm rotor for NMR measurement.

3. RESULTS AND DISCUSSION A: PHASE BEHAVIOR OF THE CONFINED MIXTURE In a previous study, some of us28 found the existence of a concentric water-rich and an acid-rich phase in controlled porous glasses and tentatively assigned the water-rich phase as the outer one, due to the interactions of the water molecules with the pore wall which is preferentially wetted by water. Similar results were reported by Frisken and Cannel,62 who performed light scattering experiments on iBA and water in a 4 wt % silica network with a crossover length of 300 Å. As the critical region was approached, they found that the scattered intensity initially decreased, indicating that the lower refractive index component, namely, water, is preferentially adsorbed by silica. To probe this assignment, we performed combined solidstate HETCOR NMR experiments and MD simulations. 3.1. Solid-State NMR Results. In the bulk mixture, the phase-separated state is the thermodynamically stable state. For this reason, it is reasonable to assume that, if a phase-separated state is present in the pores at room temperature, it will be stable upon decrease of the temperature and thus observable at low temperatures. There, the molecular motions are frozen and details of the dipolar interactions between the surface and the iBA protons are visible. To probe the calculated iBA distribution inside the pores, we performed low-temperature 1 H/29Si CP-MAS HETCOR experiments on an iBA/H2O− SBA-15 and an iBA/D2O−SBA-15 sample. The resulting spectra for a contact time of 3 ms are shown in Figure 1. All types of protons exhibit a cross peak with the 29Si signal. The relative peak volumes of the cross peaks are given in Table 2. Since CP-MAS intensities are nonstoichiometric, these 28964


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Figure 2. Low-temperature 1H/29Si CP-MAS HETCOR experiment performed at the 56 wt % iBA/H2O−SBA-15 sample employing a contact time of 0.5 ms.

Figure 4. (a) Density profiles for iBA and water. In contrast to the other figures, the pore boundary is on the right-hand side. (b) Distribution of hydrogen atoms as a function of the distance to the closest surface silanol group.

carboxylic hydrogen atoms beyond a distance of 3 Å, due to their larger number. The reasons why water accumulates at the pore center rather than at the hydrophilic pore boundary are further discussed in section 3.2.3. 3.2.1.2. IBA Orientation. Figure 5 shows the orientation of the iBA molecules for a 60 wt % mixture, based on the vector

Figure 3. Room-temperature 1H/29Si CP-MAS HETCOR experiment at 3 ms contact time.

correlation peak in the aliphatic region at 2.6 ppm compared to the signal of hydroxyl at ca. 6.6 ppm (Table 2). The weak intensity of the signal at 2.6 ppm and the absence of a signal at 1.2 ppm suggest that in the room-temperature case the iBA is oriented in a way that only the −CH− group interacts with the silica surface and the CH3 groups are oriented to the center of the pore. Furthermore, the reorientational motions of the isopropyl group of iBA are faster at room temperature than at 100 K, which may reduce the dipolar coupling between the methyl protons and the surface. 3.2. Simulations. 3.2.1. Structural Properties of the Confined Mixture. 3.2.1.1. Radial Density Distribution. Figure 4 shows the density profile of water and iBA as a function of the distance from the pore center in a mixture with 60 wt % iBA (left) as well as the distribution of hydrogen atoms around surface Si atoms (right). The curves for 40, 50, and 70 wt % mixtures are qualitatively similar and therefore not shown here. It is clearly obtained that iBA accumulates close to the pore wall. With increasing temperature the density peaks become smaller. Despite the dominance of iBA near the pore wall, there is also a density peak of water close to the wall. In fact, up to distances of 4.5 Å from silanol groups, water molecules dominate over iBA, as is shown in the right graph of Figure 4. The number of water molecules bound by hydrogen bonds to a silanol group is roughly between 0.71 (70 wt %) and 0.85 (40 wt %), and thus is larger by a factor 2.0 (70 wt %) to 3.6 (40 wt %) than the number of hydrogen bonds from silanol to iBA. Interestingly, non-carboxylic iBA hydrogen atoms dominate

Figure 5. Angular orientation of the vector between CG and CB atoms in the iBA molecules. Vectors are considered parallel or antiparallel if they maximally deviate by 30° from ideal orientations. The numbers give the ratio of the proportion of suitably oriented vectors to the proportion of the uniform distribution. The data shown is from a 60 wt % mixture. The pore boundary is situated on the left-hand side.

between the CG and CB atoms. Vectors are considered parallel or antiparallel if they maximally deviate by 30° from ideal parallel or antiparallel orientation. The distance to the pore is defined as the distance to the closest non-hydrogen wall atom. We observe a slight preference for the molecules to orient themselves radially with the carboxylic group pointing toward the pore wall. This can be seen on the left-hand side of Figure 5, where the number of molecules oriented with the carboxylic group toward the pore boundary is up to 5 times higher than for a uniform distribution when close to the pore wall. However, the methyl groups are preferentially not pointed toward the pore boundary when in close proximity to it, as evidenced by the extremely low ratio, down to 1.0 × 10−3, as seen on the right-hand side of Figure 5. This first layer is 28965


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The Journal of Physical Chemistry C followed by a second one with reverse conditions. From roughly 7 Å onward we can see a slight preference to orient methyl groups toward the pore boundary, thus maximizing the number of hydrogen bonds with water. Similar results are obtained for all mixture ratios. This is not surprising, considering the qualitative behavior of the iBA is strongly influenced by energetic considerations with respect to the pore−mixture interaction. 3.2.2. Discussion. In the previous study28 a combination of NMR relaxometry and NMR diffusometry was applied to a mixture of iBA/D2O, confined in controlled pore glass with a diameter of roughly 10.3 nm. From variable-temperature selfdiffusion measurements, they found clear deviation from isotropic single-phase diffusion, with two components visible in the data. Consequently, they employed a two-phase diffusion model, consisting of a water-rich and an acid-rich phase, which is in good agreement with the experimental data. They also investigated 1H spin−lattice T1 and 1H spin−spin T2 relaxation times as a function of temperature. The 1H spin−spin relaxation T2 shows strong deviations from monoexponentiality at temperatures above 39 °C, revealing the presence of two components with different spin−spin relaxation times. From these data, they had developed a tentative assignment, where the water-rich phase is preferentially at the surface and the iBArich phase is preferentially in the center of the pore. Our HETCOR experiments show that both water and iBA molecules are present near the surface and display very strong cross-peaks between the hydroxyl and the aliphatic protons of the iBA/water mixtures and the silica of the pore walls. This result is similar to the MD simulations, which show both a strong iBA and water concentration at the pore wall. Moreover, the simulations reveal that the acid-rich phase is close to the pore surface and the water-rich phase is inside the pore. For bulk water at room temperature a solubility of ca. 210 g/L is found for isobutyric acid.63 This value corresponds to a mass ratio of ca. 1:5, which is in good agreement with the MD simulations of the water-rich central phase and the phase separation concentrations taken from the iBA/D2O phase diagram published in ref 28. In the iBA-rich phase on the pore wall, the calculations estimate a water−iBA ratio of ca. 3:10. This value is again close to the values where the phase separation of water dissolved in iBA starts. Combining these results, we obtain a coherent picture of the water/iBA behavior inside the pores of SBA-15 material: First our study corroborates the results by Vyalikh et al.28 about a phase separation into a water- and an iBA-rich phase inside pores. Next, we can assign the iBA-rich phase to the surface and the water-rich phase to the pore inner. We can show that the composition of the two phases is close to a saturated iBA in water, respectively, a saturated water in iBA solution. Finally, we can show that H2O molecules are interacting with the free Si−OH groups on the surface of the SBA-15 material, while the aliphatic part of the iBA seems close to the Si−O−Si in the case of lower temperatures. 3.2.3. Distribution of Water and iBA Inside the Pores. In thermal equilibrium, a system takes the configuration that minimizes the free energy. A decrease in free energy can be achieved by lowering the energy or by increasing the entropy, or by a combination of both. By evaluating the hydrogen bonds of iBA and water with the pore wall, we found that the energy decrease per hydrogen bond with the wall is larger for iBA than for water, see Figure 6. However, if we take into account the smaller number of hydrogen bonds per unit volume, the energy

Figure 6. Average change in energy between fluid component and pore per hydrogen bond (with pore). Left-hand side: hydrogen bonds between water and pore. Right-hand side: hydrogen bonds between iBA and pore. Middle: hydrogen bonds between iBA and pore, normed by ratio of volume.

decrease due to hydrogen bonds with the wall is weaker for iBA than for water. We consider two water molecules to be hydrogen-bonded when the angle between the intramolecular OH vector and the intermolecular O···O vector is less than 30°, provided that the O···O separation is less than 3.35 Å.64 The same criterion was used for water and iBA. To understand the energy changes in detail, we ran simulations with an initial configuration that had no iBA at the wall, and we tracked the different types of energy contributions as a function of time, see Figure 7. The data is

Figure 7. Simulation of a 60 wt % mixture with iBA at 300 K confined to the pore center as starting configuration. Upper panel: proportion of water molecules beyond the distance r from the pore center (full lines) and equilibrium values (dashed lines). Middle panel: average number of hydrogen bonds between water molecules per water molecule. Lower panel: change in potential energy of the total system and with regard to electrostatic water−water interaction.

shown from 0.85 ns onward, since the artificial separation of iBA and water initially left a void between the fluid components. The upper panel shows the decrease of the proportion of water molecules in the outer regions of the pore, indicating that after 7.5 ns the simulation is close to the equilibrium values, which are indicated with dashed lines. The middle panel shows the number of hydrogen bonds per water molecule formed with another water molecule, which increases by approximately 7% (roughly between 5% and 9% for the different weight percentages) as the water moves toward 28966


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translational and reorientational susceptibility of iBA in the whole pore, calculated from the incoherent intermediate scattering function and the orientational autocorrelation function (see Supporting Information Figure S4). Indeed, the translational motion of iBA shows a temperature-dependent main peak, and a second peak or shoulder for small frequencies. For lower temperatures, the simulation time is insufficient for a complete decline of the intermediate scattering function, resulting in oscillations in the Fourier transform for small frequencies. For the orientational autocorrelation function, we evaluated the orientation of the vector from CG to CB. We can clearly distinguish two processes, with a major peak and a minor peak, which devolves into a shoulder for higher temperatures. In both cases, two dynamic processes can be seen on all temperature scales, albeit only weakly for higher temperatures. In the previous study,28 T2 relaxation was employed to monitor the rotational dynamics, and at temperatures above 39 °C, a bifurcation of the T2 curves was reported, while below this temperature only a single T2 time is observed, where the very short T2 time is caused by partially averaged homonuclear dipolar proton−proton interaction. From the present simulation data, we can conclude that at temperatures below 39 °C the rotational motion is too slow to average the homonuclear dipolar proton−proton interaction, making these spins invisible on the time-scale of the T2 experiment. The second peak refers to the slowing down of the iBA dynamics at the wall. The correlation times of both translational and rotational dynamics of iBA and water (not shown) increase by several orders of magnitude when approaching the pore wall. This slowing down is expected and can be explained by interfacial hydrogen bonds, which often amplify this effect.15,22,65,66 More details on the dynamics of iBA and water in the pore are given in ref 58.

the center of the pore and iBA goes to the wall. Clearly, as water moves to the pore center, its structure is less perturbed by surfaces, and this represents an energetic advantage. The lower panel shows the change of different contributions to the energy arising during the simulation. As expected, the electrostatic energy between the pore wall and the liquid becomes less negative as iBA replaces part of the water at the wall. However, at the same time this loss of binding energy is more than compensated by the lowering of the vdW energy between pore and mixture. This means that the interaction energy between the pore and the fluid becomes more negative as iBA goes to the wall, and we ascribe this to the fact that there is still also water at the wall. At the same time the electrostatic energy of the water−water interaction decreases, in agreement with the observed increase in the number of water−water hydrogen bonds. When the hydrogen-bonding energies of iBA are also taken into account, the black curve results, showing that in total the energy decreases as water moves away from the wall. These findings show that energetic considerations are sufficient to explain the tendency of the iBA to wet the pore wall. In addition, the radial density distribution (Figure 4, left panel) and the decreased intensity of the aliphatic correlation peak in the room-temperature spectrum (Figure 3) compared to the low-temperature spectrum (Figure 1, left panel) clearly demonstrate that increasing temperature leads to a relative increase of water at the pore surface, such that entropic contributions lead to a more thorough miscibility. We also performed simulations where the water was initially completely in the pore center and the iBA at the wall. Now an increase in the electrostatic water−water energy is observed, but again a decrease in the fluid−pore electrostatic interaction, as some water molecules moves to the pore boundary. The total energy of hydrogen bonds thus decreases. As a final test, we increased the hydroaffinity of the wall by increasing the partial charges of the pore matrix by 15%. This slightly increases the amount of water at the pore wall but does not change the fact that water accumulates at the pore center. This shows the robustness of the mechanism explained above.

5. SUMMARY AND CONCLUSION The phase behavior of an iBA−water mixture inside mesoporous SBA-15 material was studied by a combination of solid-state NMR spectroscopy and MD simulations. The combination of these two techniques was able to solve the longstanding problem of the microphase separation inside these materials and show that the iBA-rich phase is close to the pore wall and the water-rich phase is in the center of the pores, in contrast to previous assumptions. The MD simulations reveal that this surprising phase behavior is mainly the result of the minimization of the hydrogen-bonding enthalpy. Moreover, the simulations show also that upon increasing temperature the relative water concentration increases at the pore surface, which is attributed to entropic contributions, which lead to a more thorough miscibility. These results not only solve a long-standing problem about the phase behavior of a binary fluid mixture inside porous silica materials, but also clearly demonstrate the unique applicability of modern solid-state NMR methods combined with MD simulations for the characterization of amorphous composites on an atomic level.

4. RESULTS AND DISCUSSION B: DYNAMIC BEHAVIOR OF THE CONFINED MIXTURE To test whether the MD simulations also show two different time scales, as in the study of Vyalikh et al.,28 we evaluated the translational and rotational motion. Figure 8 shows the

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ASSOCIATED CONTENT

S Supporting Information *

Figure 8. Susceptibility χ″(t), calculated from the intermediate scattering function and orientational autocorrelation function of the iBA.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09537. 28967


Article

The Journal of Physical Chemistry C 1

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H/29Si CP-MAS HETCOR control measurements of neat SBA-15 and water inside SBA-15, as well as liquidstate NMR measurements to interpret the −OH signals in the solid-state NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the DFG research unit FOR1983 under Grant Nos. Dr 300-11-1 and Bu 911-18-2. Furthermore, the DFG Grant Bu-911-20-1 is acknowledged that gave us the opportunity to setup a DNP spectrometer equipped with the necessary MAS probe to perform the low-temperature solidstate NMR experiments. Finally, we would like to thank Professor Michael Vogel for helpful discussions and his master student Alexander Janz for the modelling of the silica pores and Dr. Mayke Werner for the synthesis of the SBA-15 host material.

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