Microphase Separation of Binary Liquids Confined in Cylindrical

Article pubs.acs.org/JPCC

Microphase Separation of Binary Liquids Confined in Cylindrical Pores A. Razzak Abdel Hamid,† Ramona Mhanna,†,§ Ronan Lefort,† Aziz Ghoufi,† Christiane Alba-Simionesco,‡ Bernhard Frick,§ 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: We have investigated the structure of tert-butanol−toluene liquid mixtures when confined in the cylindrical channels of MCM-41 mesoporous silicate, and we can conclude on the existence of a microphase-separated tubular structure of these binary liquids although fully miscible in bulk conditions. Neutron diffraction experiments with selective isotopic compositions have been performed to vary systematically the scattering length density and thus to highlight different mixture components. The observed apparently anomalous variation of the intensity of the Bragg peaks of the MCM-41 after filling demonstrates without further analysis that the liquid mixture must be inhomogeneous at the nanoscale. Our experimental observations can be rationalized when comparing with the predictions of simple core−shell models. Within this core− shell model, we deduce that tert-butanol segregates as a pore surface layer of about one atomic size surrounding a toluene-rich core of two to three atomic layers.

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mixtures.22−25 The physics of the latter systems is driven by the presence of a miscibility gap and large scale critical concentration fluctuations, which would eventually exceed the pore size in the bulk and are controlled by the topology of the porous host in confined geometry. Contrarily, the former microphase separation phenomenon seems specific to the confined state, and the question arises of the driving force involved. Especially, what is the role of specific surface interaction of one of the two components with the pore surface? Existing results about these issues are rather controversial: experiments on alcohol aqueous solutions are interpreted in terms of the formation of a water surface layer,17−19 while molecular simulations show the opposite phenomenon with preferential segregation of alcohols at the pore surface.20,21 These experimental conclusions were derived indirectly from dynamical properties (glass transition temperature, dielectric relaxation, and quasielastic neutron scattering). Therefore, the development of direct structural experiments to characterize the nature of such microphase- separated systems is highly desired. The aim of the present study is to establish the existence of a microphase-separated core−shell order for prototypical tertButanol (TBA)−toluene (TOL) binary mixtures confined in mesostructured porous material silicates (MCM-41). The TBA−TOL binary mixtures and MCM-41 mesostructured porous material silicates are particularly interesting to address

INTRODUCTION Confinement presents an unprecedented opportunity to understand the emergence of new materials properties on the nanometer scale. Over the past decades, fundamental questions arising from systems confined in nanochannels have been addressed by impregnation of molecular fluids within nano/ mesoporous structures. For pore sizes smaller than few tens of nanometers, strong interfacial and finite size effects dominate the static and dynamical properties of the confined phase. Different topics of condensed matter physics have been (re)considered in confined geometry including H-bond interactions, soft-matter physics, phase transitions, critical phenomena, and the glass transition.1−10 Among many fascinating confinement effects, the formation of new liquid structures in nanochannels that differ from their bulk counterparts is remarkable. These original structures are related for instance to anisotropic order, layering or distorted H-bond network. They have been mostly observed for systems that already exhibit supramolecular order in the bulk, such as liquid-crystals11−13 or associated liquids,14,15 while the structure of confined simple liquids could be often rationalized with simple geometric considerations.16 Quite interestingly, there is recent evidence that confinement could induce microphase separation of binary liquids.17−21 Such phenomena have been reported for aqueous solutions of alcohol molecules, such as glycerol17 and propylene glycol derivatives18,19 confined in mesoporous silicates MCM-41, although they are fully miscible in bulk conditions. The situation differs markedly from previous studies performed in the vicinity of phase separation or in the two-phase region of © XXXX American Chemical Society

Received: February 11, 2016 Revised: April 14, 2016

A

DOI: 10.1021/acs.jpcc.6b01446 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

structures in MCM-41 by two approaches. First we investigate two different ways to contrast match the mixtures to the silica host and second we choose different types of mixtures that fulfill a selective contrast matching condition. In the following, we describe the two approaches. Contrast Matching Liquids. The diffraction pattern of empty MCM-41 (Figure 1) exhibits Bragg peaks reflecting the

this issue, because the two liquid components exhibit dissimilar types of interactions (TBA is a H-bonded system, and TOL is aprotic). A preferential interaction of alcohol with silicate was indeed demonstrated by the formation of interfacial H-bonded clusters in alcohol-alkane binary mixtures in contact with hydrophilic surfaces.26 Moreover, the TBA−TOL mixtures are among the simplest systems that already exemplify various microstructures in the bulk, resulting from the balance between hydrophobic and hydrophilic interactions. In the pure liquid, TBA molecules spontaneously form mesoscopic supramolecular H-bonded clusters, comprising two to six molecules with intermediate range order. Many studies were devoted to pure TBA27−30 and binary mixtures.31−37 They have shown the striking existence of stable supermolecular clusters at the nanometer scale, although these binary liquids remain fully miscible and homogeneous at the macroscopic scale.31−34,38,39 In the confined state, structural studies were limited to pure TBA so far.7,15,40 The present study goes much further by showing a novel surface-induced nanosegregation (core−shell structure) of the two components, based on an original application of neutron diffraction methods with isotopic substitution to characterize the binary liquid structure. The determination of the concentration profile across the nanochannels is achieved from the derivation of a quantitative model based on neutron contrast variation.

Figure 1. Neutron diffraction patterns of empty MCM-41 (black solid line) and MCM-41 filled with contrast matching isotopic D/Hmixtures comprising either only tert-butanol (red solid line) or only toluene (red dashed line).

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EXPERIMENTAL DETAILS 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 elsewhere41 and already used in previous works.5,42−44 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. The calcined matrix was dried at 120 °C under primary vacuum for 12 h prior to the neutron experiments and transferred in a glovebox in an atmosphere of Helium. The empty MCM-41 was packed in a cylindrical vanadium cell of internal diameter of 6 mm, and then filled by liquid imbibition with the appropriate weighted amount of TBA-TOL mixtures injected from a syringe to allow complete loading of the porous volume VP = 0.665 cm3 g−1, measured by nitrogen adsorption. Moreover, DSC and low-temperature neutron diffraction experiments have concluded to the 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.5,43,44 The neutron diffraction experiments were performed at T = 300 K 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 spectra were corrected for detector efficiency, empty cell contribution and normalized to the amount of MCM-41 using standard procedures.

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 parallel nanochannels. The wall of the MCM-41 is formed by amorphous silicate with homogeneous density. Under the conditions of a complete filling of the pores with a liquid of homogeneous composition (number density and concentration), the intensity of the different Bragg peaks of the filled samples should simply be scaled with respect to the Bragg peak intensities of the empty MCM-41 by the density factor ⎛ ⎞2 (n b − nliq b liq )2 ⎜ Δρ ⎟ = SiO2 SiO2 ⎜ρ ⎟ (nSiO2bSiO2)2 ⎝ SiO2 ⎠

(1)

where nx is the number density, bx the average coherent scattering length, and ρx = nxbx is the scattering length density of the constituents.45 Δρ is the difference of scattering length density between the filling liquid and the porous host. This formalism has been applied successfully in order to assess the density of confined liquids in MCM-41 and related materials.43,46 A well-known implication of this formalism is the prediction of a complete extinction of the Bragg peaks intensity if the matrix is filled with a compound that has the same average scattering length density as the host, in our case as silica (i.e., nSiO2bSiO2 = nliqbliq). Such compounds are called contrast matching (CM) liquids and they are easily obtained by varying the D/H isotopic composition of the confined liquid. We have prepared two such liquids with isotopic compositions 54% D/H for TOL and 55% D/H for TBA. These mixtures fulfill an almost perfect contrast matching condition

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2

( ) Δρ ρ

RESULTS In this paper, we will derive at the conclusion that nanoconfined TBA-TOL mixtures do form core−shell

< 0.002 , assuming that the density of the matrix wall

is the same as that of bulk vitreous silica at ambient conditions (i.e., 2.2 g.cm−3), which corresponds to a scattering length density of ρSiO2 = nSiO2 bSiO2 = 34.6 × 109 cm−2 for silica. B


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liquids should be exactly the same. Hypothetical isotopic effects on the strength of the H-bond have insignificant effects on the liquid structure of alcohol with such a high molecular mass and can be safely neglected.28 Any observed difference in the total neutron scattering diffraction pattern can therefore be attributed solely to different weightings of the constituting partial structure factors. (2) The second condition implies that the twin binary liquids should exhibit exactly the same neutron scattering diffraction patterns if their constituents form a homogeneous mixture inside the pores. It results that any difference between their two spectra could be related unambiguously to the formation of nanophase separated regions. (3) In the case of complete separation of the two components, the third condition implies that the diffraction pattern would reflect solely the structure of the deuterated component TBA-(D) or TOL-(D), the other contrast-matched component being “invisible” to neutrons. It should be pointed out that a unique value of the chemical composition can fulfill all the conditions of twin symmetric mixtures. It is fully determined by the liquid density39 and the molecular scattering lengths, and corresponds to a TBA/TOL volume fraction of

The scattering intensity of the contrast matching TOL and TBA liquids confined in MCM-41 is shown in Figure 1. In both cases, one observes an almost perfect extinction of the Bragg peaks as one would expect for a homogeneous scattering length density in the case of contrast matching. One also finds an increase of the flat background, which is simply due to the incoherent scattering from hydrogens. Thus, these experiments are directly proving the complete and homogeneous filling of the two pure liquids within the channels and confirm previously published studies.43 If we apply now contrast matching to a binary liquid mixture of TOL and TBA imbibed in MCM-41, we observe the most striking and very original result shown in Figure 2.

43/57 with the same contrast factor

Perdeuterated TBA and fully hydrogenated TOL, mixed in a volume composition of 51/49, should nearly fulfill the contrast 2

( ) Δρ ρ

Δρ ρ

= 0.12 .

The neutron diffraction patterns measured for the two twin binary liquids are quantitatively different and exhibit very different features (Figure 3). The scattered intensity of the TBA-(CM)/TOL-(D) mixture presents a unique intense (10) Bragg peak. On the contrary, its twin system, TBA-(D)/TOL(CM), presents a weaker (10) Bragg peak (almost four-times less intense) but relatively intense (11) and (21) Bragg peaks. The (21) reflection is even more intense than for the empty MCM-41. This result confirms further that the liquid mixtures form a novel structure in the confined state that induces a static inhomogeneous distribution of concentration. This observation supports the conclusions drawn from contrast matching mixtures described in the previous chapter. In addition, because the scattering intensity will be sensitive to only one of the two components, the twin binary mixtures can provide a unique insight into the nature of the nanophase separation.

Figure 2. Neutron diffraction patterns for empty MCM-41 (black solid line) and MCM-41 filled with a contrast matching binary mixture of deuterated tert-butanol with hydrogenated toluene (volume fraction 51/49%).

matching condition with

2

( )

= 0.004 . However, we find

that the first Bragg peaks remain very intense, in contradiction to eq 1. Moreover, the confinement does not affect the intensities of the different Bragg peaks in the same way. Whereas the intensity of the first (10) peak is about half as intense for the confined mixture compared to the empty MCM41, the two subsequent peaks, (11) and (20), are weakened by nearly a factor of 10, while the fourth peak, (21), becomes even stronger. This demonstrates that the assumption underlying eq 1 that the concentration of TOL/TBA molecules is homogeneous within the porous volume, does not apply in the present case. This might be a first hint to the microstructuring of the mixture in the pores. Twin Binary Liquids. In order to learn more about the nature of the microstructure formed by the confined binary liquids, we have prepared two samples with very specific compositions. We call these two samples which fulfill the following three peculiar conditions “twin binary liquids”: (1) They both have the same chemical composition, i.e., the same TBA/TOL volume fraction but different isotopic composition. (2) They both have the same average total neutron scattering length density. (3) Each binary mixture comprises a fully deuterated compound, TOL-(D) or TBA-(D), which is then mixed with its contrast matching counterpart of either TBA(CM) or TOL-(CM). The implications of the above conditions are very important for the structure determination: (1) The first condition implies that the structure of the two twin binary

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DISCUSSION Different features about the nanophase separated system can be obtained from the diffraction patterns. It should be first pointed out that the spectra present two contributions: a continuous background and Bragg peaks located as the same qhk positions as the empty MCM-41, where h and k correspond to the Miller indices of the triangular crystalline lattice. The background is featureless with an overall intensity that increases with the filling of the porosity. It is mostly related to incoherent scattering and a small low-q diffuse scattering. It could be fitted with a constant term and a small Gaussian function centered at q = 0, which agrees with the Guinier approximation.47 The essential structural information about the confined mixture could be extracted from the Bragg peaks intensity. The Bragg peaks were simultaneously fitted as a sum of Gaussian peaks and their integrated intensity derived as I (̃ qhk ) = ∫ I(qhk )q2 dq (Figure 4). In order to interpret the data, we derived a quantitative analysis based on a model used for the adsorption of fluids in similar materials.48−54 The neutron diffraction intensity of the MCM-41 materials, considered as a periodic triangular array of cylindrical pores is given by C


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Figure 4. Neutron diffraction intensity of empty MCM-41 after subtraction of the background IBg. The five Bragg peaks labeled by Miller indexes have been fitted by a sum of Gaussian functions (solid line).

term was introduced in eq 3 by the Gaussian function, which contains ⟨u2⟩, the mean square displacement of the cylinders from the regular lattice site. The form factor of a single pore can be calculated from the scattering amplitude of a cylindrical channel and is F empty(q) =

ρSiO R pore 2Z(qR pore) 2

ρSiO R pore 2 2

where ρSiO2 is the coherent scattering length density of silica and Rpore is the pore radius. The function Z is given by Z(qRpore) = 2J1(qRpore)/qRpore, with J1 being the first-order Bessel function of the first kind. The value ρSiO2 = 34.6 × 109 cm−2 was used in agreement with contrast matching experiments discussed in the previous part. The pore radius was also fixed to the value Rpore = 1.83 nm, as determined from nitrogen adsorption based on the improved Kruk−Jaroniec−Sayari (KJS) description.55 Thus, the scaling factor K and the mean square displacement (MSD) ⟨u2⟩ are the only two free parameters, which were obtained from a fit of the form factor to the experimental total intensity of the Bragg peaks. The obtained value of the MSD is ⟨u2⟩ = 0.12 nm2, that corresponds to small displacements with respect to the lattice parameter,

Figure 3. Neutron diffraction patterns of MCM-41 filled with twin binary liquids having the same total average scattering length density, the same chemical composition (except isotopic differences) with a TBA/TOL volume fraction composition 43/57% and comprising a fully deuterated component mixed with its contrast matching counterpart. (a) Contrast matching TBA mixed with deuterated TOL and (b) deuterated TBA mixed with contrast matching TOL. The black solid line corresponds to a fit of the baseline, comprising incoherent background and a slight coherent increase at low q due to the sharp interfaces of the MCM nanoporous structure. It was approximated by the sum of a constant term and a Gaussian function centered at q = 0. Inset: sketch of contrast matching for a core−shell structure corresponding to the TBA@Surface model with TOL-(D) in blue, TBA-(D) in red, and silica, TOL-(CM), and TBA-(CM) in gray (see Discussion for details). 2

I(q) = KS(q)|F(q)|

u2 a

= 8%. The comparison of the model predictions with the Bragg peak intensity is shown in Figure 5. In the case of a filled matrix, the structure of the confined liquids can be characterized by the analysis of the square of form factor |F(q)|2. Indeed, the MCM-41 being a 2D crystal with a translational invariance symmetry along the direction of the channel axis, only the averaged structure of the confined liquids within a plane perpendicular to the channel axis is accessible. In the absence of new diffraction peaks at off-Bragg positions no information could be inferred about a hypothetical liquid order along the channel axis. Moreover, the cylindrical symmetry of the pores implies that the form factor can be related solely to the average radial concentration profile. We have considered the three simplest different cases. The first is the homogeneous filling of the pore by the mixture with a constant concentration profile. The resulting diffraction pattern is simply proportional to the one of the empty MCM41, with a different contrast in the form factor:

(2)

where K is a constant normalization term, S(q) the powder averaged structure factor of the MCM-41 lattice and F(q) the scattering amplitude of a single pore. The structure factor can be written as S(q) =

1 q2

⎛ u 2 2⎞ q⎟ ⎝ 2 ⎠

∑ M(hk)exp⎜− hk

(4)

(3)

where M(hk) is the line multiplicity, with M(hk) = 6 for (h0) or (hh) and 12 otherwise. The Bragg peak locations are related to 4π

the Miller indices with qhk = a 3 h2 + k 2 + hk and a is the lattice parameter. The term 1/q2 represents the Lorenz factor powder average. In order to account for static structural disorder in the crystalline lattice, a Debye−Waller-factor like D


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Table 1. Geometrical Parameters of the Two Core−Shell Models for the Studied Mixtures Compositionsa model composition: xTBA TBA@surface TBA@core composition: xTBA TBA@surface TBA@core

Rcore (nm) = 0.51 1.28 1.31 = 0.43 1.38 1.20

shell thickness, Rpore − Rcore (nm)

Rpore (nm)

⟨u2⟩ (nm2)

0.55 0.52

1.83 1.83

0.12 0.12

0.45 0.63

1.83 1.83

0.12 0.12

Note that these parameters were not fitted specifically to the confined liquids, but fixed by the composition of the mixture (Rcore) or retrieved from the study of the empty MCM-41 (Rpore and ⟨u2⟩). a

and molecular simulation studies of confined TBA.40 The core radius corresponds to about 2−3 molecular layers. The predictions from these three different models are now compared with the experimental Bragg peak intensities for the TBA-(D)/TOL-(H) mixture (Figure 6). The radial profiles of

Figure 5. Experimental integrated intensities of five Bragg peaks of the empty MCM-41. The solid line is the result from the model according to eqs 3 and 4 after fitting the scaling factor K and the mean square displacement ⟨u2⟩ The shaded area is an estimate of the average experimental statistical fluctuations of intensity at off-Bragg positions after subtraction of the baseline.

F

homo

(q) =

(ρliq − ρSiO )R pore 2Z(qR pore) 2

ρSiO R pore 2 2

(5)

Comparing to eqs 2 and 4, this leads to the intensity ⎛ Δρ ⎞2 empty I homo(q) = ⎜ (q) ⎟I ⎝ ρ ⎠

(6)

The two other cases correspond to the complete separation of the two components of the binary mixtures, forming a cylindrical core−shell structure, where either TBA or TOL is segregated at the surface of the pore, and the counterpart component is located in the core of the pore. The form factor for this core−shell structure is

(7)

Figure 6. Experimental integrated intensities of five Bragg peaks of the MCM-41 filled with a contrast matching binary mixture of deuterated tert-butanol and hydrogenated toluene in volume fraction 51/49%. The solid lines are the predicted results from the model according to eqs 2 and 3 for three different cases: full mixing (homogeneous), microphase separated core−shell structure with TBA at the surface (TBA@surface), and microphase separated core−shell structure with TOL at the surface (TBA@core). Inset: Sketch of the scattering length density profile.

with ρ0 = ρSiO2, ρ1 = ρshell, ρ2 = ρcore, R1 = Rpore, and R2 = Rcore. It is noteworthy that these models do not require any fitting parameter specific to the adsorbed liquids, and thus provide predictive results that can be compared directly to the experiments. Indeed, the radius of the core Rcore is simply determined from the composition of the mixture, and ρshell and ρcore are the scattering length densities of the pure liquids. The values of the scaling factor K and the MSD u2 were also extracted from the analysis of the empty MCM-41. In the case where TBA is forming the shell (thereafter referred to as the TBA@surface model) the core size is R core = (1 − x TBA ) R pore. In the case where TOL is forming the shell and TBA is forming the core, then the core size is R core = x TBA R pore (TBA@core model). The values of the core radius Rcore and the thickness of the shell eshell are given for all the studied compositions and models (Table 1). They are very similar for all the studied systems: Rcore = 1.3 ± 0.1 nm and eshell = 0.55 ± 0.1 nm. The shell thickness corresponds to about one molecular diameter according to structural studies on bulk TBA and TOL29,56,57

the scattering length density across the pore are also sketched for the three models. It is obvious that the model with homogeneous concentration profile fails to reproduce the experiments, predicting an almost total extinction of the Bragg peaks intensity for this nearly contrast matching composition. In contrast, the predictions of the two core−shell models are in good agreement with experiments. They provide both a consistent explanation for the striking observation of high Bragg peak intensities for this contrast matching composition. The predictions of the two core−shell models are almost quantitatively equivalent, which makes it impossible to decide if the TBA compound is segregated at the pore surface or at the core. This can be simply understood by observing that the form factors Fcore−shell(q) of the TBA@surface and TBA@core models (cf. eq 7) would have indeed similar values with opposite sign for a 50/50 contrast matching mixture. Thus, by the scattered intensity, related to the square value of the form factor, one cannot distinguish between the two symmetric models.

F core−shell(q) =

1 ρSiO R pore 2 2

i=1

∑ (ρi − ρ(i− 1))R i 2Z(qR i) 2

E


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Still, retaining the level of simplicity of this model, it is important to make a clear point about the nature of the interfacial and core phases, i.e., TBA@surface vs TBA@core. It is worth recalling that the two twin systems have the same average scattering length density, so that their Bragg peaks intensities should be identical if the radial composition profile was constant. To emphasize the actual deviations from this situation, we show in Figure 8 the ratio between the

This equivalence between the TBA@surface and TBA@core models can be overcome with the selectively deuterated twin binary mixtures, described in the previous section, that break the equivalence between the two components. Now we compare the prediction of the two models to the experimental results for the two twin binary mixtures (Figure 7). For the

Figure 8. Ratio between the experimental integrated intensities of five Bragg peaks of the MCM-41 filled with the two symmetrical twin binary liquids shown in Figure 7. The solid lines are the predicted results from the model for two different microphase separated core− shell structures with TBA at the surface (TBA@surface) and TOL at the surface (TBA@core). The constant line is the expectation for a homogeneous radial concentration profile.

experimental integrated intensities of five Bragg peaks of the MCM-41 filled with the two symmetrical twin binary liquids: I (̃ qhk )TBA−(CM)/TOL−(D)

Figure 7. Experimental integrated intensities of five Bragg peaks of the MCM-41 filled with symmetrical twin binary liquids having the same total average scattering length density, the same chemical composition (except isotopic differences) with a TBA/TOL volume fraction composition 43/57% and comprising a fully deuterated component mixed with its contrast matching counterpart. (a) Contrast matching TBA mixed with deuterated TOL and (b) deuterated TBA mixed with contrast matching TOL. The solid lines are the predicted results from the model according to eqs 2 and 3 for two different microphase separated core−shell structures with TBA at the surface (TBA@ surface), and TOL at the surface (TBA@core). Inset: Sketch of the scattering length density profile.

I (̃ qhk )TBA−(D)/TOL−(CM)

. The experimental values vary by almost a

factor of 100 as a function of the Miller indices, which is very different from the hypothetical case of constant composition indicated by the horizontal solid line. Interestingly, the predictions from the two core−shell models are systematically in opposite direction with each other. On the one hand, the TBA@surface model predicts depression or magnification of the different Bragg peaks, which all agrees with the experimental facts. On the contrary, the TBA@core model always predicts the opposite trends and thus can definitively be disregarded. Finally, it is important to stress the very specific character of the microphase separated structure observed in confinement with respect to the structure of their bulk counterparts. Indeed, as discussed in the Introduction, it is well-known that these binary mixtures are not homogeneous at the molecular level in the bulk, although they are homogeneous at the macroscopic scale. However, bulk and confined mixture structural inhomogeneities are very different in nature: In the bulk, the average concentration is the same everywhere in the sample (the liquid mixture maintains it translational invariance symmetry). In the confined state, the “microphase separation” leads to a peculiar core−shell structure with a concentration radial profile that remains inhomogeneous, even after spatial average upon different pores and along the pore axis.

TBA@surface model the general experimental trend is predicted: for the TBA-(CM)/TOL-(D) system, it predicts an intense (10) Bragg peak, while the other Bragg peaks would be too weak to be detectable as already mentioned in the discussion of the experimental results shown in Figure 3a. For the TBA-(D)/TOL-(CM) system, it predicts a smaller (10) Bragg peak, but relatively intense other Bragg peaks. The (21) and (30) reflections are expected to be even larger than those for the empty MCM-41, in agreement with the observation made in the previous section about the experimental data shown in Figure 3b. However, it should be pointed out that a fully quantitative agreement is not achieved, which is conceivable given the simple and predictive nature of the model. F


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CONCLUDING REMARKS Up to now, the structural evidence for a microphase separation of mixtures, which are under normal conditions fully miscible, under confinement was missing. Nevertheless, such phenomena were recently indirectly inferred from dynamical properties for alcohol aqueous solutions.17−19 We note that, to our knowledge, the available structural studies result only from molecular simulations and showed the opposite preferential segregation of alcohols at the pore surface.21,20 The experimental methods developed in this article have proven very effective to reveal the nature of such microphase-separated systems. Neutron diffraction experiments of confined TBA−TOL binary liquids with selective isotopic compositions demonstrate clearly the existence of segregated domains with different compositions. This conclusion is derived from the uncommon variation of the MCM-41 Bragg peak intensities as a function of the average scattering length density of the confined mixture. This original phenomenon was first exemplified with full contrast matching binary mixtures and extended to the case of two symmetrical binary liquids. The Bragg peak intensity is directly related to the radial concentration profile within the channel. We confirmed this approach with a simple core−shell model that has the extreme advantage of being fully predictive, since the different parameters are determined from the study of the empty matrix. Despite its simplicity, the predictions from the core−shell model do reproduce all the experimentally observed trends, and rationalize the apparently anomalous variation of the Bragg peaks intensity with isotopic composition. It provides an unambiguous evidence of the systematic segregation of TBA at the pore surface, forming a layer of about 0.5 nm that surrounds a TOL-rich tubular core. The three different mixtures studied are mainly different in terms of isotopic compositions, but have similar TBA/TOL compositions. It would be certainly worthwhile to further vary as well the composition, the surface interaction and the pore size in order to assess the evolution of the core and shell dimensions. Ongoing studies will address these issues. For this purpose, the unique structural regularity of the mesostructured porous materials and the sensitivity of neutron diffraction methods to isotopic composition will be obvious assets to achieve a direct experimental study of the structure. Finally, the manipulation of complex solvents in nanostructured porous hosts being at the heart of contemporary activities in the fields of chemistry and materials science, we anticipate that the observed phenomena on fully miscible mixtures confined in porous hosts are also relevant to different situations involving fluidic or biotechnological devices.

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and F. Porcher (LLB-Saclay) for their assistance with the neutron diffraction, and O. Merdrignac-Conanec (ISCRRennes) for nitrogen adsorption experiments.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +33 2 23 23 69 84. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The experiments were performed in the frame of the Ph.D. project of A.R.A.H., supported by the Brittany Region (ARED 5453/NanoFlu). R.M. acknowledges funding of her Ph.D. 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 G


Article

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Microphase Separation of Binary Liquids Confined in Cylindrical

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