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Hydrogen-Bond-Induced Supermolecular Assemblies in a Nanoconfined Tertiary Alcohol Aziz Ghoufi,* Ivanne Hureau, Ronan Lefort, and Denis Morineau* Institut de Physique de Rennes, CNRS-University of Rennes 1, UMR 6251, 35042 Rennes, France ABSTRACT:
Tert-butanol is the molecular archetype of a variety of H-bonded systems that form micelle-like supermolecular clusters in the liquid state. This self-association process is characterized by a prepeak in the static structure factor. Recently, it has been shown by neutron scattering that this prepeak is totally suppressed in a nanoconfined geometry [Morineau, D.; Alba-Simionesco, C. J. Phys. Chem. Lett. 2010, 1, 7155.]. The authors have shown that excluded volume effects was one main ingredient of this suppression, but the question of the survival of H-bonded self-association in nanochannels has remained unresolved experimentally. From molecular dynamics simulations, we prove that self-association survives under confinement despite the absence of the prepeak. Furthermore, we show that its apparent suppression is due to a negative contribution arising from the liquidporous solid correlations, which cannot be disentangled experimentally. Strikingly, the stability of micelle-like clusters surpasses the putative formation of interfacial H-bonds with surface silanols, leading to an unexpected hydrophobic interaction of tert-butanol with surface silica. This work highlights the foremost interest of combining neutron scattering and molecular simulations with a careful account of the complex situation encountered under confinement to better understand the molecular microstructure of H-bonded liquids.
’ INTRODUCTION Self-association in the liquid state is important1 in diverse phenomena from the formation of microemulsions2,3 and nanoparticles4,5 to the assembly of proteins6 and to drug delivery.7,8 Supermolecular assemblies result fom the subtle balance between hydrophobic and hydrophilic interactions.1 During the past 50 years, many efforts have been devoted to understanding the selfassemblies at the microscale in the liquid state from experiments911 and molecular simulations.1,12 Tert-butanol ((CH3)3COH) (Figure 1a) is considered a model system to explore fundamentally the microscopic mechanisms related to self-association in condensed matter. Tert-butanol11,12 spontaneously forms clusters, centered on the hydrophilic groups of the molecules, which are involved in hydrogen bonds and surrounded by a hydrophobic shell, as shown in Figure 2a. This micelle-like structure involving four to six molecules is commonly observed in a variety of H-bonded liquids, including alcohols and amines, where the self-association tendency is balanced by the bulky repulsive part of the molecule.12,13 This generic structure differs from the network-like organization observed in water r 2011 American Chemical Society
and low-molecular-weight n-alcohols, such as methanol. Interestingly, such clusters in bulk liquids can be unambiguously detected by diffraction experiments, in terms of a prepeak in the static structure factor.9,1416 From molecular simulations, this prepeak has been assigned to the mesoscale spatial correlations between the hydrophilic parts of adjacent molecules. More specifically, it shows up as a prominent feature in the partial structure factors related to the hydroxyl or amino groups.13,14,16 Although this type of self-association has aroused considerable attention in the liquid state, its survey in the confined state remains seldom. In nanoconfined systems, many physical properties drastically differ from their bulk counterparts.1720 This issue is of prior interest for H-bonding liquids, given the dominant role of self-association in interfacial fluids in biological systems and in nanotechnology. Recently, it has been reported that the experimental signature of clustering of tert-butanol was Received: June 24, 2011 Revised: August 5, 2011 Published: August 09, 2011 17761
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Figure 1. Snapshot of tert-butanol and labels of atoms (a) and a cylindrical silica nanopore (b).
Figure 2. (a) Cluster of four tert-butanol molecules in the liquid state. (b) Structure factor of tert-butanol in bulk (solid line) and confined phases (dashed line) obtained from ref 15.
suppressed by cylindrical nanoconfinement.15 Indeed, Morineau and Alba-Simionesco15 showed the absence of the characteristic prepeak in the neutron scattering (NS) static structure factor of liquid tert-butanol confined in a mesoporous MCM-41 silicate of diameter D = 3.5 nm (Figure 2b). Details of experimental data is given in the Experimental Section. This first observation raises the question of the survival of supermolecular clusters in nanoconfined tert-butanol and their possible detection by classical diffraction methods. More specifically, the authors discussed three possible origins of this observation: first is the so-called “excluded volume effect”, which is the direct geometrical consequence of spatial restriction on the density correlation functions and which has been largely discussed in the literature.2123 The second possible origin is due to the contribution from porous matrixliquid cross-correlations to the total scattered intensity. The third possible origin is the actual suppression of the H-bonded supermolecular clusters of tert-butanol in the confined state. The computation of the excluded volume effect has clearly shown its predominant role in the modifications of the structure factor observed experimentally.15 It leads to a notable reduction of the intensity of both the main diffraction peak and the prepeak. However, a prominent shoulder is still visible in the region of the prepeak in the computed structure factor. This suggests that the two other origins should be considered additionally to interpret the experimental suppression of the prepeak. However, disentangling matrixliquid cross-correlations from the actual liquidliquid order is hardly possible experimentally,
and this issue has remained unresolved. At variance, molecular simulation allows one to capture the atomistic structure of tertbutanol and can provide a direct insight onto the fluidwall correlations and clustering phenomena in the confined state. We report such a detailed survey of the structure of tert-butanol under confinement in a cylindrical pore of silica.
’ MODELS AND COMPUTATIONAL PROCEDURE A model silica nanopore of diameter D = 2.4 nm (Figure 1b) was obtained by carving a hole in amorphous silica according to the method of Brodka.24 To study the surface effect on selfassociation, we investigated two values of the surface hydration level, corresponding to the highly hydrated (HH: 7.5 SiOH/ nm2) and weakly hydrated (WH: 3.5 SiOH/nm2) systems. Intermolecular interactions are the sum of both electrostatic and dispersiverepulsive Lennard-Jones contributions. Details on the framework building method, framework force field, and overall intermolecular parameters used for tert-butanol have been taken from elsewhere.2527 To get good statistics in our calculations, we considered a porous solid with values of the unit cell of Lz = 4Lx = 142 Å. To ensure the Lennard-Jones parameters and partial charges of the framework, we compared the models of Brodka24 and Zhuo et al.28 Both provide the same tert-butanol structure and adsorbed amount. We finally used the force field developed by Brodka, to be in line with previous works. To model tert-butanol, we opted for the flexible united-atoms force 17762
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Figure 3. (a) Structure factor of tert-butanol in the bulk phase obtained from neutron scattering (solid line), the UA model (dashed line), and the UAh model (dotted line). (b) Computed structure factor of tert-butanol (solid line and left axis) and methanol (dashed line and right axis) in the liquid state.
field (UA) developed by Siepmann29 (Figure 1a). This choice was motivated by the ability of the UA model to reproduce the liquidvapor diagram and the liquid structure of the phase at a weaker computational cost than the all atoms description. The motion of the hydrogen atoms of the surface silanol hydroxyl groups is performed from the constraints algorithm,30 where the distance between the oxygen and hydrogen atoms are kept fixed at 1.09 Å. The computational procedure and force field have been validated from a good agreement between the experimental (785 kg m3) and simulated bulk density (786 kg m3). The density of the confined liquid has been calculated from Grand Canonical Monte Carlo simulations (GCMC).31 The chemical potential of tert-butanol was calculated from the Widom’s insertion method25,31,32 and from the perturbation method.33 Details of GCMC simulations and μ calculation are reported in ref 31. We obtained a value of the chemical potential μ = 2412 K, which is in good agreement with experiment (2406 K).34 Accordingly, we obtained a value of the liquid density in the HH silicate nanopore of 900.1 kg m3, which is 14% higher than the bulk density. As previously reported,35 this stresses the need to compute explicitly the confined density rather than to consider the bulk density. Monte Carlo simulations were performed in the Grand Canonical statistical ensemble (GCMC). Periodic boundary conditions were applied in the three directions. The silica nanopore was considered as rigid, and only the hydrogen of silanol can rotate from an angular move, implying SiOH groups.24 Each cycle consisted of N randomly selected moves with fixed probabilities: translation of the center of mass of a randomly chosen tert-butanol molecule, rotation of a randomly selected tertbutanol molecule around its center of mass, and the change of the internal conformation by using the configurational bias regrowth move.29,32 The frequencies of each type of move are 0.20 for translation, 0.20 for rotation, 0.20 for the change of the conformation, and 0.4 for insertion/deletion. GCMC simulations consisted of 700 000 cycles. Molecular dynamics (MD) simulations were performed using a time step of 0.002 ps to sample 2 ns (acquisition phase). The equilibration time corresponds to 10 ns. All MD simulations have been carried out with the DL_POLY package36 using the combination of the velocityVerlet algorithm31 and the NoseHoover thermostat.31,37
’ NEUTRON SCATTERING EXPERIMENTS The neutron scattering experiments were performed at T = 300 K on two different double-axis spectrometers G6.1 and 7C2
of the Laboratoire Leon Brillouin neutron source facility (CEACNRS, Saclay) using a monochromatic incident wavelength of 4.7 and 0.7 Å, respectively. The static structure factor of the confined tert-butanol was derived from a difference between the experimental differential cross sections of the confined system and an empty MCM-41.15,38 The scattered intensity arising from the empty matrix corresponds to less than one-third of the total intensity like in previous studies. We combined the normalized spectra obtained from these complementary spectrometers to cover an extended range of momentum transfer Q (from 0.1 to 16 Å1) with an improved resolution at low Q (Q < 1.8 Å1). This region of the structure factor contains essential information about the intermolecular order that exists in the liquid at the short (nearest neighbors) and intermediate distances.
’ BULK PHASE The structure factor (SLiq(Q)) of the bulk phase was computed from eq 1, where Q is the momentum transfer vector, NL the number of molecules of liquid, nL the number of atoms belonging to one liquid molecule, rkm is the vector position rkm = rm rk, and bi the coherent diffusion length of atom i. The brackets stand for time and isotropic average carried out over the angles of Q in spherical coordinates of a sum of the scattering intensity arising from every couple of atoms (θ, ϕ). Thus, the structure factor is expressed by steradians corresponding to the cross section measured by neutron scattering. We normalize SLiq(Q) by the number of molecules (NL) to compare the bulk and confined phases. The unit of eq 1 is molecules by steradian (molec. sr1). For more of clarity, we will omit the average on θ, ϕ. NL nL NL nL ∑ ∑ ∑ ∑ bk bm expðiQ rkm Þ SLiq ðQ Þ ¼
j¼1 k¼1 l¼1 m¼1
NL
nL
2
θ, ϕ
ð1Þ
∑ bi
i¼1
We compare in Figure 3a the structure factor obtained from the UA model with published NS experiments. Because the methyl hydrogen atoms are not taken into account explicitly in the UA model, it leads to a different normalization factor in eq 1. Therefore, the computed SUA(Q) was rescaled to NS data at maximum intensity for a better comparison. A good agreement between SUA(Q) and NS is obtained for the location of the main peak and prepeak (1.3 and 0.700.75 Å1, respectively). The intensity of the prepeak is weaker in simulation than in NS, and it 17763
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rather appears as a distinct shoulder on the left-side of the main diffraction peak. Discrepancies between UA simulations and experiment start to show up above 2 Å1 and are related to the nonexplicit account for methyl hydrogen atoms in the UA model. This can be demonstrated by computing the structure factor after having rebuilt the entire molecule structure on the basis of the UA trajectories (UAh model). In this case, no ad-hoc scaling factor was required and a much better concordance with experiment was obtained, as shown in Figure 3a, though the intensity in the prepeak region remains weaker than in the experiment. In the following, we restrict our discussion to the evolution of the prepeak for which the results obtained from the UA model are satisfactory. In Figure 3b, we compared the structure factors computed from molecular simulation of tertbutanol and methanol. The latter system is a network-forming H-bonded liquid, which does not present any prepeak. This is actually confirmed by our simulation results. Moreover, this comparison allows us to identify the shoulder at 0.700.75 Å1 for tert-butanol as the most probable evidence of intermediaterange correlations associated with clustering phenomena. This confirms the ability of the UA model to reproduce the main structural features observed by NS in liquid tert-butanol. To get a definite attribution of the origin of the prepeak in the total structure factor, we show in Figure 4 the partial structure factors of oxygen and hydrogen atoms of the hydroxyl groups computed according to * +
Figure 4. SOh(Q) (solid line) and SHo(Q) (dashed line) in the bulk phase from the UA model. The dotted line corresponds to SOh(Q) of methanol in the bulk phase.
1.8 Å1. This corresponds to the region of the main diffraction peak of the total structure factor. To summarize, SLiq X (Q) (X = Oh, Ho) reflects the correlations between the atoms involved in the H-bond formation. In the case of bulk tert-butanol, they provide an unambiguous and better defined evidence of supermolecular cluster correlations than the total structure factor.
’ CONFINED PHASE The situation encountered in NS experiments for a liquid confined in a solid porous material is more tricky than in the bulk state. The scattered intensity arises from liquidliquid, liquid solid, and solidsolid correlations.23 After subtraction of the scattered intensity by the empty porous material, one eventually gets the composite structure factor S(Q), which is the sum of the liquidliquid term and the liquidsolid cross correlation. It is expressed in eq 3, where NS is the number of solid units (e.g., SiO2 for pure silica) and nS the number of atoms belonging to one solid unit.
NL nX NL nX
Liq
SX ðQ Þ ¼
∑ ∑ ∑ ∑ expðiQ rm ÞexpðiQ rk Þ j¼1 m¼1 l¼1 k¼1 NL nX
ð2Þ
where X is the label of the considered atom (hydroxyl oxygen (Oh) and hydrogen (Ho), respectively) and nX is the number of X atoms. These two partial structure factors present a prominent diffraction peak, whose location is in fair accordance with the experimental position of the prepeak, respectively, 0.79 and 0.73 Å1. At variance for methanol, which does not present any prepeak, the maximum of SLiq Oh (Q) is located at about *
NL
nL
NL
nL
∑∑∑ ∑
j¼1 k¼1 l¼1 m¼1
SðQ Þ ¼
bk bm expðiQ rkm Þ þ 2 NL
It can be simplified to the following expression rffiffiffiffiffi~ XS bS LiqSol Liq S ðQ Þ SðQ Þ ¼ S ðQ Þ þ 2 XL~bL
ð4Þ
with Ni ~ Xi ¼ ; bi ¼ NS þ NL and
* LiqSol
S
ðQ Þ ¼
NL
nL
ni
∑ bj
! ð5Þ
j¼1
NS
nS
∑∑∑ ∑
j¼1 k¼1 l¼1 m¼1
+ bk bm expðiQ rkm Þ
pffiffiffiffiffiffiffiffiffiffiffi~ ~ NS NL bS bL
ð6Þ
nL
NL
nL
NS
nS
∑∑∑ ∑
j¼1 k¼1 l¼1 m¼1 2
+ bk bm expðiQ rkm Þ ð3Þ
∑ bi
i¼1
In Figure 5, we report the measured (Figure 5a) and calculated (Figure 5b) total structure factor in the bulk and confined geometry. The effect of confinement on the simulated structure factor is in fair agreement with the experimental observations. First, we reproduce the significant decrease of intensity of the main diffraction peak (reduction of about 30% of its intensity). It has been shown that this decrease can be essentially attributed to the excluded volume effect,15 that is, the presence of regions where the fluid is not allowed. Moreover, Figure 5b reveals that the prepeak is essentially suppressed, which is also in agreement with the NS experiments. However, this second feature was only partly attributed to the excluded volume effect.15 It suggests either some fundamental changes in the liquid structure or significant contributions from the cross-correlation terms or both. Molecular simulation has allowed us to consider these two possible interpretations independently. 17764
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Figure 5. Experimental15 (a) and simulated (b) structure factors of tert-butanol in confined (dashed line) and bulk (solid line) phases.
Figure 6. (a) SOh(Q) in the bulk phase (solid line) and in HH (dashed line) and WH (dotted line) confined phases. (b) Distribution of the cluster size in bulk (solid line) and confined (dashed line) phases.
Figure 7. Structure factors SLiq-Liq(Q) + SLiq-Sol(Q) (dashed line), SLiq-Liq(Q) (solid line), and SLiq-Sol(Q) (dotted line) in the confined HH phase.
To clarify about the existence of supermolecular clusters in confined tert-butanol, we computed in Figure 6a the partial structure factor between Oh atoms calculated from eq 2. It exhibits a prominent peak at 0.79 Å1, which is the location of the prepeak. This result indicates the presence of intermediaterange correlations between hydroxyl groups and confirms the persistence of clusters under confinement. Its intensity is relatively reduced with respect to the bulk state. This reduction is comparable with the decrease of intensity observed for the main diffraction peak, so it seems compatible with the effects of excluded volume on the partial correlation functions. In addition, we carried out this calculation for two different rates of surface silanols (WH and HH) in Figure 6a. It does not reveal any significant variation of this intermediate-range order with respect to the nature of the surface interaction. This could
Figure 8. (a) Radial profiles of the density of methyl groups (solid line and right axis) and oxygen atoms (dotted line and left axis) of tertbutanol in the HH silicate nanopore.
underline the stability of supermolecular clustering with regard to geometrical constraints and interfacial interactions. This is further corroborated by the calculation of the profile of cluster size31 based on the hydrogen bonding25 shown in Figure 6b. For this calculation, we adapted the algorithm of Stoddard39 to the hydrogen-bonding network. Thus, we found aggregates formed by four molecules linked by hydrogen bonding in both the bulk and the confined phases. The persistence of clusters in nanoconfined tert-butanol would suggest that the excluded volume effects and the matrixliquid cross-correlations are the two dominant origins of the suppression of the prepeak in the NS structure factor. To validate this hypothesis, we computed the different terms involved in the composite structure factor S(Q) of eq 4, that is, SLiq(Q) and SLiqSol(Q), as shown in 17765
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The Journal of Physical Chemistry C Figure 7. We observe a prepeak in SLiq(Q), which is absent in the composite function S(Q). Strikingly, SLiqSol(Q) presents a negative contribution in the same Q range. The amplitude of this anticorrelation between tert-butanol and the porous solid roughly compensates the LiqLiq correlation, which results in the apparent suppression of the prepeak in S(Q). To the best of our knowledge, it is the first observation where the crosscorrelations qualitatively change some important features of the experimental structure factor. The LiqSol cross-correlations were shown to play a minor role in the modification of the structure factor of confined benzene, which is a van der Waals weakly interacting fluid.23 At variance, the relevance of negative cross-correlations were reported from a combination of simulation and NS for the H-bond network forming methanol.38 In the latter case, LiqSol anticorrelations essentially lie above 1 Å1 and change modestly the shape of the main diffraction peak. In the case of tert-butanol, the suppression of the prepeak illustrates more convincingly than ever the foremost interest to combine NS and molecular simulations. A careful account for the complex situation encountered under confinement is required to avoid possible misunderstanding of the molecular microstructure of H-bonded liquids. To get a microscopic view of the confined liquid and liquid solid orders, we provide in Figure 8 the radial density profile across the pore radius calculated from a histogram of r = (xi2 + yi2)1/2. We considered the methyl carbons and the oxygen atoms, which are characteristics of the hydrophobic core and H-bonding group of the molecule, respectively. Both density profiles exhibit layering, which is usual in confined states, but with a very different structure. The methyl density profile presents three concentric layers across the pore diameter. The period of oscillation is about 5 Å, which basically corresponds to the diameter of one molecule. At variance, the oxygen
Figure 9. Radial profiles of the density (solid line and right axis) and hydrogen bonds (dotted line and left axis) in the HH silicate nanopore.
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density profile presents only two concentric shells, with an interlayer distance of about 9 Å. This intermediate-range correlation between oxygens is obviously at the origin of the prepeak in the LiqLiq structure factor at Q = 2π/9 ≈ 0.7 Å1. It is noteworthy that these two radial density profiles provide a direct insight into supermolecular clusters that are formed in a confined geometry. The LiqSol contact layer (at 10.2 Å from the pore center) is essentially occupied by the CH3 groups. The first peak in the Oh radial profile appears at 8.4 Å from the pore center. It shows that the hydroxyl groups are clustered preferentially in the confined space located between the two CH3 peripheral layers and also at the pore center. This suggests that these two peripheral methyl layers are linked one to the other by H-bond bridges. That is further confirmed by the calculation of the number of hydrogen bonds per unit of volume along the radial axis. Hydrogen bonds were calculated by considering the quantum criteria distance,25 and the profile was computed from the position of the center of a hydrogen bond. Indeed, in Figure 9, we show that the hydrogen bonds are preferentially located in the confined space between the two CH3 layers. For completeness, we checked that there was no azimuthal and axial organization along the principal axis of the pore. Further, as shown in Figure 10, the typical structure of aggregates was similar to the bulk counterpart phase. Indeed, the radial distribution functions (not corrected from the excluded volume) of the center of mass, as well as between Oh and Ho in the confined phase, display the same features as in the bulk phase. The decrease of the intensity reflects the excluded volume and a slight restriction of the number of hydrogen bonds. The stability of bulklike short-range correlations is consistent with the survival of clusters under confinement. However, characterization of the hydrogenbonding network requires extensive analysis and is not in the current scope of this work. This peculiar micelle-like configuration highlights the amphiphilic character of tert-butanol. It leads to a predominant hydrophobic interaction between tert-butanol and the silica surface. This result was not expected, because of the hydrophilic nature of silica and its ability to form H-bonds with alcohols. Despite the high rate of interfacial silanol (HH), tertbutanol forms an average number of only 25 hydrogen bonds out of the 408 possible sites (204 silanol groups with 204 acceptors and 204 donors). Obviously, hydrophobic anchoring is preferred because the hydrogen-bonding bridges between the first and second shells are energetically more favorable than the interfacial liquidsilica hydrophilic interaction. This feature nicely illustrates the competition between the formation of bulklike supermolecular clusters in a confined geometry and the most favorable interfacial interactions.
Figure 10. Radial distribution functions of the center of mass (a) and between Oh and Ho (b) in bulk (solid line) and confined (dashed line) phases. 17766
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’ CONCLUDING REMARKS In this work, we used molecular simulations to complete the neutron scattering study of the microstructure induced by selfassembly of tert-butanol in a confined geometry. Tert-butanol is a prototypical globular alcohol that spontaneously forms supermolecular assemblies in the liquid state. In the bulk, the formation of micelle-like clusters has been assigned to the existence of a prepeak in the static structure factor. In confinement, we show that supermolecular assemblies survive despite the suppression of this characteristic signature in S(Q). Molecular simulation has allowed us to study independently the liquidliquid and solid liquid correlations, which are entangled experimentally. The apparent suppression of the prepeak in experiments is attributed to both the excluded volume effects and the negative hostguest correlations. The formation of clusters most probably competes with the most favorable structure of the liquidsolid contact layer. In the present system, the stability of micelle-like clusters remains very high in nanochannels. It surpasses the putative formation of interfacial H-bonds with surface silanols, leading to an unexpected hydrophobic interfacial interaction of tert-butanol with silica. We expect that the persistence of aggregates in the nanoconfined phase may depend on the typical cluster size with respect to the pore diameter, the nature of the porous framework, and the strength of the H-bond interaction. Further studies, combining neutron scattering and molecular simulation with different types of H-bonded liquids and porous materials, are planned to address this issue. ’ AUTHOR INFORMATION Corresponding Author
*E-mail: aziz.ghoufi@univ-rennes1.fr (A.G.), denis.morineau@ univ-rennes1.fr (D.M.).
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