Multiple Glass Transitions of Microphase Separed Binary Liquids

May 11, 2016 - bulk liquid structure.2,10 The absence of additional Bragg peaks in the region of the main diffraction peak confirms that the confined ...
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Multiple Glass Transitions of Microphase Separed Binary Liquids Confined in MCM-41 Abdel Razzak Abdel Hamid,†,‡ Ramona Mhanna,†,§ Pierre Catrou,† Yann Bulteau,† Ronan Lefort,† and Denis Morineau*,† †

Institute of Physics of Rennes, CNRSUniversity of Rennes 1, UMR 6251, F-35042 Rennes, France Laboratoire Léon Brillouin, UMR 12, CEA-CNRS, F-91191 Gif-sur-Yvette, France § Institut Laue-Langevin, 71 avenue des Martyrs, F-38000 Grenoble, France ‡

ABSTRACT: The confinement of fluids in channels of nanometer size presents unprecedented opportunities to reveal emergent physicochemical properties that have no equivalent in the corresponding bulk system. We present an experimental study of fully miscible binary low-molecular weight liquids confined in nanopores. Under these conditions, the mixtures form a microphase-separated state with two regions of different compositions forming the core and the shell of a concentric tubular nanostructure. We combine neutron diffraction and DSC measurements to assess the thermal behavior of this unusual phase. These results show that crystallization is suppressed, leading to glassy behaviors characterized by anomalies in the density and the heat capacity. We reveal the coexistence of two different glass transitions, providing the direct proof for two different enthalpic relaxation times. It leads to the conclusion that the subcomponents of the microphase-separated mixture each obey distinct dynamical behaviors. This phenomenon is all the more unusual considering that the thicknesses of the core and the shell are only one to three molecular sizes. These results have important implications for processes involving interfacial multicomponents fluids, including heterogeneous catalysis, microfluidic devices, nanofiltration, or oil industry.

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INTRODUCTION The microphase separation of binary liquids confined in nanoporous channels has been recently reported.1 This fascinating confinement effect was observed for low molecular weight mixtures,1 which are fully miscible at the macroscopic scale under bulk conditions.2,3 The structure of this unusual fluid phase has been determined by carefully designed neutron scattering experiments with hydrogen/deuterium isotopic substitution for tert-Butanol (TBA)−Toluene (TOL) mixtures imbibed in the cylindrical nanochannels of MCM-41 mesoporous silicates D = 3.65 nm. After impregnation of the porous matrix, these liquids develop a core−shell cylindrical organization that consists in two concentric regions with different compositions. The measured inhomogeneous radial concentration profile shows that TBA molecules segregate at the pore surface, forming an interfacial region comprising about one molecular layer that surrounds a TOL-rich core of about two to three molecular layers. However, despite this welldefined segregation of the two compounds at the molecular scale, there exists no indication of any classical phase separation in the thermodynamic sense on the macroscopic scale. Therefore, the studied system goes beyond the hypothetical and straightforward case of conventional biphasic systems. The term microphase separated system is used in the manuscript to identify this specific structure induced by confinement. According to the recent literature, the microphase separation phenomenon could be a more general feature of confined © XXXX American Chemical Society

mixtures. It has been observed in molecular dynamics simulations4,5 and also invoked in experimental studies for aqueous solutions of alcohol molecules, such as glycerol6 and propylene glycol derivatives7,8 confined in MCM-41. This conclusion was derived indirectly from the dynamical properties of the alcohol molecules. It shows that the dynamics of the alcohol molecules in confinement would conform to the bulk counterpart mixture with a lower water fraction. This result was interpreted as resulting from a partial mixing of the two liquids, induced by a preferential adsorption of water at the pore surface. However, the dynamics of the two regions could not be distinguished from each other so far. The direct experimental evidence of the core−shell organization of confined binary liquids achieved for the prototypical TBA−TOL mixtures offers a unique opportunity to examine fundamental questions emerging from the peculiar properties of such systems: How can we derive the concept of microphase-separated-system induced by nanoscale confinement in terms of dynamics? Can we observe separate dynamical properties arising from the two distinct regions? For that purpose, we have studied the temperature variation of the density and the heat capacity obtained from neutron diffraction and DSC measurements in order to assess the phase behavior and the glassy dynamics. Received: May 10, 2016

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EXPERIMENTAL SECTION Hydrogenated solvents TBA and TOL (>99%) were purchased from Sigma-Aldrich and fully deuterated solvents TBA (C4D10O, 99.8%) and TOL (C7D8, 99.5%) were from Eurisotop and used directly without further purification. The mesoporous materials MCM-41 silicates were prepared in our laboratory according to a procedure similar to that described elsewhere9 and already used in previous works.10−13 Hexadecyl-ammonium bromide was used as template to get a mesostructured triangular array of aligned channels with pore diameter D = 3.65 nm, as confirmed by nitrogen adsorption, transmission electron microscopy, and neutron diffraction. For DSC experiments, MCM-41 samples were filled by liquid imbibition with the appropriate weighted amount of fully hydrogenated TBA−TOL mixtures to allow complete loading of the porous volume VP = 0.665 cm3 g−1, measured by nitrogen adsorption, and sealed hermetically in aluminum pans. The measurements were performed on a TA Instrument Q20 DSC. The thermograms were recorded at a heating rate of 10 K min−1 after the sample was quenched from 313 to 100 K. For neutron diffraction experiments, the inner surface of the MCM-41 (silanol groups) was deuterated by isotopic substitution with heavy water, then dried, and outgassed under vacuum at 170 °C for 24h. This treatment reduces the incoherent scattering induced by the hydrogen of the surface silanols. The empty MCM-41 was transferred in a glovebox under an atmosphere of Helium into in a cylindrical vanadium cell of internal diameter of 6 mm a height of 80 mm, and then filled by liquid imbibition with fully deuterated TBA−TOL mixtures injected from a syringe. The loaded cells were sealed with an indium joint. The neutron diffraction experiments were performed on the cold neutron double-axis instrument G6.1 (λ = 4.7 Å) at the Laboratoire Léon Brillouin (CEA-CNRS Saclay), covering a momentum transfer q range from 0.13 to 1.8 Å−1. The sample temperature was controlled with a Helium cryostat and the spectra were acquired at regularly spaced temperatures, typically 15 to 20 values for each sample, covering the range from 310 to 60 K for a total acquisition time of 12h. The spectra were corrected for detector efficiency and empty cell contribution and then normalized to the amount of MCM-41 using standard procedures.12,13 Both DSC and low-temperature neutron diffraction experiments have concluded an absence of crystallization. They indicate that no bulk excess liquid is present out of the matrix and that the porosity is therefore completely filled, in agreement with previous studies using the same filling method.11−13

Figure 1. Neutron diffraction patterns for MCM-41 filled with a deuterated TBA−TOL mixture (volume fraction xTBA = 0.90) as a function of the temperature.

during the entire cooling process. It shows that confinement in MCM-41 inhibits the crystallization of the TBA−TOL mixtures, a result that extends preceding observations made for pure liquids, including Toluene, Benzene, and Methanol.11−13 We focus now on the temperature dependence of the MCM41 (10) Bragg peak. Its position remains constant, q10 = 0.16 Å−1, which means that the porous lattice does not change notably with temperature, reflecting the rigidity of silica units. Contrariwise, the intensity increases significantly on cooling and exhibits two different regimes: a rapid increase at high temperature and a weaker increase at lower temperature. This phenomenon was observed for all studied compositions and resembles the observations previously reported for pure liquids.11−13 In the case of a pure liquid, which fills the pores completely and homogeneously, it was shown that the Bragg peak intensity provides a direct measurement of the liquid density and that the crossover temperature provides the location of the glass transition. Indeed, the Bragg peak intensity Empty IFilled (10) of the filled MCM-41 is related to the intensity I(10) of the empty MCM-41 by the following: Filled I(10)

⎛ ρ − ρ ⎞2 Liq SiO2 ⎟ I Empty = ⎜⎜ ⎟ (10) ρ ⎝ ⎠ SiO2

(1)

where ρLiq and ρSiO2 are the average neutron scattering length densities of the liquid and the silica.12 The situation is more complex for mixtures that exhibit microphase separation, and the formation of a cylindrical core−shell structure with a nonuniform scattering length density profile across the pore diameter. The more sophisticated models that apply in this case have been described in detail for H−D isotopic mixtures by Abdel Hamid et al.1 Isotopic labeling was applied to amplify the difference of scattering length density of the two liquid components and hence the sensitivity to the core−shell structure. In the present study, the two liquid components are fully deuterated and thus have comparable scattering length ρ −ρ densities ( TBAρ TOL ≈ 10%). In order to locate the glass

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RESULTS AND DISCUSSION Neutron Diffraction. Figure 1 shows the typical neutron diffraction spectra of MCM-41 filled with a deuterated TBA− TOL mixture with volume fraction xTBA = 0.90 as a function of the temperature. The spectra exhibit a series of Bragg peaks that reflect the crystalline arrangements of the pores. They can be indexed in the frame of a triangular 2D lattice, which is in agreement with the well-known honeycomb organization of the MCM-41 parallel nanochannels. A broader main diffraction peak located at about qMP = 1.3 Å−1 corresponds to the liquid− liquid correlations, which are in qualitative agreement with the bulk liquid structure.2,10 The absence of additional Bragg peaks in the region of the main diffraction peak confirms that the confined mixtures remain amorphous (liquid or vitreous)

TBA

transition temperature, we simply computed an effective liquid mixture density scaled to its value measured at T = 300 K, according to the following: B

DOI: 10.1021/acs.jpcc.6b04596 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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=

Filled I(10) (T ) Filled I(10) (300 K)

(2)

Figure 2 shows the temperature dependence of the rescaled density of the confined mixtures. In all cases, an obvious

Figure 2. Density of TBA-TOL mixtures confined in MCM-41 rescaled to unity at T = 300 K obtained from neutron diffraction experiments as a function of the temperature and the volume fraction.

crossover exists between two regions where the density is almost linearly dependent on temperature. For each composition, we have obtained a value of the glass transition temperature by fitting with a piecewise function consisting of two linear segments. A monotonic variation of Tg from about 130 K for pure TOL to about 200 K for pure TBA was observed. The obtained values will be discussed later in relation with the DSC measurements. Differential Scanning Calorimetry. Figure 3a and b shows the DSC heating curves of pure TBA and TOL, respectively. In both parts, the bulk system (black line) is compared to the confined one (red line). Interestingly, the two confined systems exhibit a heat capacity jump, which is typical for a glass transition (TTOL = 125 K, and TTBA = 188 K). The g g absence of any other thermal anomaly confirms that confinement inhibits the crystallization of the liquids and opens a unique opportunity to study their glassforming properties. Although this result was awaited for TOL, it is worth mentioning that to our knowledge, it is the first successful demonstration of the possible vitrification of TBA. Indeed, it is noteworthy that bulk TBA cannot resist crystallizing under supercooling conditions, as illustrated in Figure 3a by the two endothermic peaks. The phase behavior of bulk TBA was studied by DSC, ab initio calculation, and X-ray diffraction by McGregor et al.14 According to this study, we can attribute the first endothermic peak to the solid−solid transition between the trigonal P3̅ phase (phase II) and the triclinic P1̅ (phase IV) at TII−IV = 268 K. The second peak corresponds to the subsequent melting of the triclinic P1̅ phase (phase IV) at Tm = 290 K. It is also well-known that TOL can be vitrified in the bulk, as illustrated in Figure 3b by a glass transition at about Tg = 120 K, but on further heating it is followed by crystallization (exothermic peak at TCr = 144 K) and melting (endothermic peak at Tm = 180 K). We now consider the thermal behavior of the confined mixtures. Figure 4a shows the DSC heating curves of a series of mixtures confined in MCM-41 as a function of the TBA volume fraction. Again, there is no indication of crystallization, but a jump that is characteristic of the glass transition. It is indicated

Figure 3. DSC thermograms on heating of bulk (black line) and confined (red line) (a) pure TBA and (b) pure TOL. The bulk data of TBA are scaled by a factor 0.25 for clarity.

Figure 4. (a) DSC thermograms and (b) temperature derivative of the DSC thermograms on heating of TBA−TOL mixtures confined in MCM-41. The heat capacity jumps related to the glass transition are indicated by two arrows. The volume fraction of TBA in each mixture is given. The curves haves been shifted for clarity.

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can be concluded that Tg1 and Tg2 are respectively the glass transition temperatures of the TOL-rich and TBA-rich domains. The decrease of Tg2 with the addition of TOL could be attributed to the disruption of the H-bond interactions involving TBA molecules.10,15,16 Indeed, the segregation of TBA molecules in continuously smaller volumes could hinder molecular self-association, alongside with the plasticizing effect of TOL.2 By comparison with Tg2, it is striking that Tg1 remains virtually constant. A different behavior that could be related to the weaker van der Waals interaction between TOL and both silica and TBA, that makes the dynamics of TOL-rich domains less sensitive to the local environment.

by dashed lines for the two pure components. For intermediate compositions, the signal is smeared out, suggesting that the glass transition rather occurs on a broadened region of temperatures. In order to emphasize the transition, we show in Figure 4b the temperature derivative of the heat flow. In this representation, a glass transition appears as a peak, and it is obvious that two glass transitions really occur for the binary mixtures. The first peak (Tg1) remains located close to the glass transition of pure TOL (x = 0) and its intensity eventually vanishes when the amount of TOL in the mixture decreases. The second peak (Tg2) emerges from the glass transition of pure TBA (x = 1) and it shifts to lower temperature as x decreases to eventually merges with the first peak for x ≈ 0. In order to disentangle the two phenomena, we fitted the derivative of the heat flow by two Gaussian functions (thin solid lines in Figure 4b), and extracted the mean glass transition temperature (maximum peak position) and the transition broadening (fwhm). Figure 5 shows the two glass transition

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CONCLUSIONS

Recent diffraction experiments have demonstrated that the TBA−TOL binary liquids confined in nanoporous channels exist in an unusual microphase separated state, where the TBArich phase forms a cylindrical shell surrounding a TOL-rich core.1 The present observation provides the direct proof that this microphase-separated binary liquid exhibits two distinct dynamics, leading to two different glass transitions even though the overall diameter of this concentric tubular structure is as small as D = 3.65 nm. According to the structural characterization, the thickness of the TBA interfacial layer is one molecular size (only 0.5 nm).1 It is of fundamental interest, and not predictable a priori, that such a thin shell could exhibit a distinct calorimetric glass transition, and so a distinct structural relaxation. The concept of multiple glass transitions arising from nanophase separated domains has been intensively discussed for macromolecules.17−19 Usually, the formation of nanodomains is promoted by incompatible interactions between segments while the macroscopic demixing is avoided by chemical bonding (sidechain or block copolymers). The situation differs for lowmolecular weight mixtures. Some of them are macroscopically miscible but exhibit supramolecular aggregates or partial mixing at the molecular level. Studies on such glass forming binary liquids have reported the observation of secondary relaxation processes, which were especially obvious from the dielectric permittivity.20−23 However, the relation between these additional processes and the liquid structural relaxation, assessed in terms of calorimetric glass transition or viscosity, remains an open question.20,23 As a matter of fact, it was shown for a series of binary liquids the existence of a single calorimetric glass transition located at the temperature predicted from standard mixing rules despite the presence of sub-Tg relaxation processes due to the incomplete mixing.24 The observation made in the present study on the glassy dynamics of confinement-induced nanophase-separated systems calls for further investigations. Beyond the necessity to extent the study to different liquids and pore sizes, it would be valuable to assess the specific interfacial interactions that control the core−shell structure. The study of the adsorption mechanisms that pave the way to this original structure could be carried out along a coadsorption isotherm starting from the adsorbed layers toward the capillary condensation. Moreover, the study of the molecular dynamics of the different components on different time scales (using for instance quasielastic neutron scattering, NMR, and dielectric spectroscopy) would be beneficial, a goal that we are now pursuing.

Figure 5. Glass transition temperatures of TBA−TOL mixtures confined in MCM-41, determined from the two heat capacity jumps observed by DSC (red circle and black square) and from the thermal expansion jump observed by neutron diffraction (green triangle). The underlying contour plot is the temperature derivative of the DSC thermograms as a function of the volume fraction of TBA. Solid lines are guides for the eyes.

temperatures measured by DSC, along with the average transition obtained from the scattering length density measured by neutron diffraction. The underlying contour plot is the temperature derivative of the DSC thermograms shown in Figure 4b. With the sensitivity of our DSC experiments, Tg1 was measurable only for x < 0.7, indicating that a minimum volume fraction of 30% of TOL is required, while Tg2 was measurable only for x > 0.2, indicating a minimum volume fraction of 20% of TBA on the opposite side of composition. Only a single average Tg could be measured by neutron diffraction. Indeed, only DSC proved to have the sensitivity to detect and separate the transition from the minority phase. Moreover, neutron diffraction experiments suffer from a broad temperature resolution (temperature step between two data points is about ΔT = 10 to 20 K). Interestingly enough, the obtained value of Tg deduced from neutron diffraction agrees well with the most prominent peak position measured by DSC. The most exciting situation is encountered for intermediate compositions, (0.2 < x < 0.7) were the two glass transitions measured by DSC really coexist. It corresponds to the situation where a microphase separation of the two components has been observed.1 According to their concentration variation, it D

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(14) McGregor, P. A.; Allan, D. R.; Parsons, S.; Clark, S. J. Hexamer Formation in Tertiary Butyl Alcohol (2-Methyl-2-Propanol, C4H10O). Acta Crystallogr., Sect. B: Struct. Sci. 2006, 62, 599−605. (15) Ghoufi, A.; Hureau, I.; Lefort, R.; Morineau, D. HydrogenBond-Induced Supermolecular Assemblies in a Nanoconfined Tertiary Alcohol. J. Phys. Chem. C 2011, 115, 17761−17767. (16) Ghoufi, A.; Hureau, I.; Morineau, D.; Renou, R.; Szymczyk, A. Confinement of tert-Butanol Nanoclusters in Hydrophilic and Hydrophobic Silica Nanopores. J. Phys. Chem. C 2013, 117, 15203− 15212. (17) Beiner, M.; Schroter, K.; Hempel, E.; Reissig, S.; Donth, E. Multiple Glass Transition and Nanophase Separation in Poly(N-Alkyl Methacrylate) Homopolymers. Macromolecules 1999, 32, 6278−6282. (18) Beiner, M.; Huth, H. Nanophase Separation and Hindered Glass Transition in Side-Chain Polymers. Nat. Mater. 2003, 2, 595−599. (19) Arbe, A.; Genix, A. C.; Arrese-Igor, S.; Colmenero, J.; Richter, D. Dynamics in Poly(n-alkyl methacrylates): A Neutron Scattering, Calorimetric, and Dielectric Study. Macromolecules 2010, 43, 3107− 3119. (20) Power, G.; Vij, J. K.; Johari, G. P. Relaxations and Nano-PhaseSeparation in Ultraviscous Heptanol-Alkyl Halide Mixture. J. Chem. Phys. 2007, 126, 034512. (21) El Goresy, T.; Bohmer, R. Diluting the Hydrogen Bonds in Viscous Solutions of N-Butanol with N-Bromobutane: A Dielectric Study. J. Chem. Phys. 2008, 128, 154520. (22) Lederle, C.; Hiller, W.; Gainaru, C.; Bohmer, R. Diluting the Hydrogen Bonds in Viscous Solutions of N-Butanol with nBromobutane: II. A Comparison of Rotational and Translational Motions. J. Chem. Phys. 2011, 134, 064512. (23) Preuss, M.; Gainaru, C.; Hecksher, T.; Bauer, S.; Dyre, J. C.; Richert, R.; Bohmer, R. Experimental Studies of Debye-Like Process and Structural Relaxation in Mixtures of 2-Ethyl-1-Hexanol and 2Ethyl-1-Hexyl Bromide. J. Chem. Phys. 2012, 137, 144502. (24) Shahin, M.; Murthy, S. S. N. Sub-T-g Relaxations Due to Dipolar Solutes in Nonpolar Glass-Forming Solvents. J. Chem. Phys. 2005, 122, 014507.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The experiments were performed in the frame of the PhD project of A.R.A.H., supported by the Brittany Region (ARED 5453/NanoFlu). R.M. acknowledges funding of her PhD by the Institute Laue-Langevin and the Brittany Region (ARED 7784/ NanoBina). Support from Europe (FEDER) and Rennes Metropole is expressly acknowledged. We thank I. Mirebeau and F. Porcher (LLB-Saclay) for their assistance with the neutron diffraction and O. Merdrignac-Conanec (ISCRRennes) for nitrogen adsorption experiments. This study has benefitted from valuable scientific discussions with B. Frick (ILL-Grenoble) and C. Alba-Simionesco (LLB-Saclay).

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REFERENCES

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