Solvation Effects on Self-Association and Segregation Processes in

Article pubs.acs.org/JPCB

Solvation Effects on Self-Association and Segregation Processes in tert-Butanol−Aprotic Solvent Binary Mixtures A. R. Abdel Hamid,† R. Lefort,† Y. Lechaux,† A. Moréac,† A. Ghoufi,† C. Alba-Simionesco,‡ and D. 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

‡

ABSTRACT: The present study reveals that the fully miscible binary mixtures consisting of tert-butanol with aprotic solvents form well-defined ordered supermolecular structures, which have been characterized on different length scales. Three different types of microstructures have been determined. They are separated by distinct crossovers that appear as a function of the dilution rate, going from “correlated clusters” to “diluted clusters” and “diluted monomer” microstructures. These observations have been made possible by the combination of Raman vibration spectroscopy, 1H NMR, and neutron diffraction that probe, respectively, the cluster formation (self-association) and the intercluster correlations (cluster segregation). The solvation effects on both the cluster formation and the intercluster correlations have been assessed by tuning the alcohol− solvent interaction, i.e., changing the chemical nature of the diluting solvent from a purely inert alkane to a weakly interacting aromatic system.

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simulation and neutron diffraction methods.10,12−14,18 This upper level of structuration has a clear signature in the pair correlation functions involving hydroxyl atoms and takes the appearance of a distinct prepeak in the static structure factor at Q = 0.7 Å−1.10,14 Supermolecular structures are furthermore observed in binary mixtures. Especially, the amphiphilic nature of alcohol molecules is demonstrated by their incomplete mixing at the microscopic level in apolar solvent 7,8 or in aqueous solution.18−22 Although the formation of associated multimeric species in diluted H-bonding molecules has been well studied in the literature, especially by vibrational spectroscopies,7,8 and 1 H NMR,23 these studies did not assess the mesoscopic spatial correlations between these supermolecular species. To address this issue, the case of TBA in aprotic solvent is of special interest. A vibrational analysis using FTIR and Raman spectroscopies has revealed that the dominant tetrametric Hbonded clusters of TBA in alkane mixtures are stable to extreme conditions of dilution (down to 0.1 TBA molar fraction).7,8 This remarkable stability raises some appealing questions about the structure of the mixture at a mesoscopic scale, where spatial correlations between these H-bonded units could come out. The question emerges about the degree of segregation and ordering of these H-bonded clusters as a function of composition. For instance, the hypothetical segregation of TBA clusters in mesoscopic regions (“alcohol

INTRODUCTION Monohydric alcohols exhibit a multiplicity of intermolecular orders related to their ability to form H-bonds.1−14 The nature of the microstructures observed in associating liquids is mostly determined by the balance between hydrophobic and hydrophilic interactions,15 which depends on the chemical nature of the molecule. Experimental and computational studies have reported that methanol forms mostly chainlike structures. They have been related to the prevailing hydrophilic interaction of small alcohol molecules.1−3,12,13 Different types of microheterogeneous structures have been observed for larger molecules such as tert-butanol (TBA), which form nanosegregated cyclic clusters.7,8,11−13 These micellar clusters centered about the hydroxyl H-bonded groups are surrounded by the tert-butyl hydrophobic parts of the molecules. The formation of these supermolecular assemblies has been attributed to the frustration of the H-bonding system by steric hindrance due to the large aliphatic group.7,8,11−13 TBA is certainly one of the most studied prototypical molecules among a variety of systems,16,17 which exemplify Hbonded micellar clusters in the pure liquid state. It is worth pointing out that the structure of liquid TBA extends over different length scales ranging from the H-bond itself, to the Hbonded clusters, and up to the mesoscopic arrangements of different interacting clusters. Vibrational spectroscopy is relevant to self-association per se, and it has been used to resolve the size distribution of the H-bonded clusters, which are dominated by tetrameric species at room temperature.7,8 Moreover, spatial correlations between different neighboring H-bonded micellar clusters have been demonstrated by © XXXX American Chemical Society

Received: March 8, 2013 Revised: June 19, 2013

A

dx.doi.org/10.1021/jp402380f | J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

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intermolecular order that exists in the liquid at the short (nearest neighbors) and intermediate distances. In the case of TOL−TBA mixtures, the experiments were combined with measurements obtained on the double-axis spectrometer 7C2, using a shorter incident wavelength of 1.1 Å.28 It allows covering an extended range of momentum transfer Q (up to 10 Å−1). It has been therefore possible to evaluate the intramolecular contribution that is dominant at large Q values, and to perform a comprehensive analysis of the intermolecular correlations through the pair distribution functions obtained by Fourier transformation. Fully deuterated TBA (99.8%), TOL (99.5%), and MeCY (99.5%) were purchased from Eurisotop Saclay and used with no further purification. TBA−solvent binary mixtures were prepared in a glovebox by weighting. The samples were filled in sealed cylindrical vanadium cells with 6 mm diameter. The diffracted intensity measured on G6.1 for the TBA− MeCY mixture is limited to a Q-range encompassing the prepeak of TBA. It was sufficient to correct the diffracted intensity for detector efficiencies, empty cell background, and neutron incident beam count in order to analyze the relative evolution of the prepeak intensity as a function of solvation. In the case of the TBA−TOL mixture, a comprehensive Fourier analysis combining both G6.1 and 7C2 spectrometers has been made possible according to a complete data reduction procedure. The neutron diffraction intensities were first corrected for detector efficiencies by using a high-statistics spectrum of a vanadium rod. Background intensity was obtained from a usual procedure combining the spectra of the empty cryostat and of a neutron absorbing cadmium rod. The cell contribution was subtracted from the spectra according to the Paalman and Pings formalism.29 Multiple scattering was simultaneously evaluated with a program derived from the Blech and Averbach formalism, assuming multiple scattering was isotropic and that contributions of a degree higher than 2 were negligible.30 Intensities were then normalized using the spectrum of a vanadium rod that had the geometry of the sample. Finally, the Placzek corrections were evaluated in a semiempirical way, as it is usually done for molecular liquids, with a polynomial function P(Q) = A + BQ2 + CQ4, where A, B, and C are adjustable parameters.17,31 The neutron scattered intensity measured on 7C2 and G6.1 for TBA and the polynomial evaluation of the Placzek correction are shown in Figure 1. In the case of a binary mixture comprising N = N1 + N2 molecules, the total coherent scattering intensity expressed in barn·sr−1·molec.−1 is

liquid pockets”) could be in favor of a highly heterogeneous description of the binary mixture related to large fluctuations of the local composition. This description could be balanced by a more homogeneous picture consisting of noninteracting Hbonded micellar clusters dispersed in the “bath” of aprotic solvent. This issue is of fundamental interest to better understand the different processes intervening in the incomplete mixing of alcohol solutes, from the early stage of dilution to the complete decay of H-bond association in the limit of infinite dilution. The aim of the present study is to combine a Raman spectroscopy analysis with 1H NMR and neutron diffraction experiments to assess simultaneously the dilution/solvation effects on the size distribution of supermolecular H-bonded clusters (self-association) and the structural correlations between distinct clusters (segregation). Two aprotic solvents with similar molecular size (methylcyclohexane and toluene) have been used in order to tune the interaction between the alcohol solute and the solvent. Alkanes are inert solvents which display purely van der Waals interactions with alcohols.7,8,24 Unlike methylcyclohexane (MeCY), toluene (TOL) is a interacting solvent, which presents weak specific interactions with a hydroxyl moiety due to the presence of an electron-rich unsaturated aromatic ring.24−27 It has allowed identifying three regimes of dilution that are related to different types of microstructures, and which result from the successive effects of solvation on the segregation and the self-association processes.

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EXPERIMENTAL AND DATA REDUCTION Raman Spectroscopy. Raman experiments have been performed on fully hydrogenated solutions at room temperature with a HR800 micro-Raman spectrometer (HORIBA Scientific/Jobin-Yvon) using a He−Ne laser 632.8 nm excitation line. The laser beam was focused on the solution through a 10× Olympus objective, and the scattered light was collected in backscattering configuration. Rayleigh scattering was removed by means of a dielectric edge filter. We used a 600 grooves/mm grating, with a spectral resolution around 0.8 cm−1 per pixel in this 3010−3812 cm−1 spectral range studied. The solutions were filled in a closed glass tube to avoid evaporation. The data reduction and the spectral analysis have been inspired by a comprehensive study of tert-butanol dimethylbutane mixtures by Sassi et al.,7,8 and it will be detailed with the presentation of the results. 1 H Nuclear Magnetic Resonance. 1H NMR measurements were performed on a 300 MHz spectrometer (Bruker Avance 300) using a high resolution Diff50 5 mm probe, for a series of TOL−TBA and MeCY−TBA fully hydrogenated solutions of varying compositions. Seventeen different concentrations of both types of mixtures were prepared, with TBA molar fractions ranging from x = 0.1 to 1 in accordance with the range of compositions studied by Raman spectroscopy. The solutions were prepared by a volumetric method, and also controlled by weighting. The chemical shifts of the hydroxyl proton were referred to the TMS proton signal. Neutron Scattering. Neutron scattering experiments were performed at T = 303 K on the double-axis spectrometer G6.1 of the Laboratoire Léon Brillouin neutron source facility (CEACNRS, Saclay) using a monochromatic incident wavelength of 4.7 Å. It allows measuring the diffracted intensity on a range of momentum transfer Q (from 0.15 to 1.25 Å−1), encompassing the region of the prepeak of TBA (Q = 0.7 Å−1). This region of the structure factor contains essential information about the

2

⎛ dσ ⎞ 1 ⎜ ⎟ = ⟨ ∑ ∑ b b exp(iQ·rαiαj)⟩ ⎝ dΩ ⎠ N i , j = 1 α , α = 1 αi αj i j 2

=

∑ xi i=1

1 Ni

+ 2 x1x 2

∑ bα b β exp(iQ·rα β ) i

αi , βi

i i

i

1 N1N2

∑ bα b β 1

α1, β2

2

exp(iQ·rα1β2) (1)

where αi (respectively αj) spans over all the atoms of each molecule species of type i (respectively j), with xi, Ni, ni, and bαi being, respectively, the molar fraction, number of molecules of type i, number of atoms in one molecule of type i, and coherent B



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Table 1. Geometrical Parameters Used to Calculate the Intramolecular Cross Section Toluene

scattering length of atom αi. raiaj is the vector connecting the two atoms αi and αj. The brackets stand for a volume and isotropic powder average. The expression can be split into inter- and intramolecular contributions according to ni , nj

2

∑ xi i=1

∑ αi , βi = 1

bαib βi

sin(Qrαiβi) Qrαiβi

⎛ ⟨u 2⟩ ⎞ αiβi exp⎜⎜ − Q 2⎟⎟ 2 ⎝ ⎠

(2)

where ⟨uαiβi2⟩ is the mean square displacement (MSD) of atoms i and j in the direction rαiβi. The intermolecular pair correlation function gL(r) and radial distribution function dL(r) can be obtained by Fourier transform of the intermolecular scattering differential cross section according to

1 2π

1.51

dC−D (Å)

⟨uC−C2⟩ (Å2)

⟨uC−D2⟩ (Å2)

0.01

0.02

1.08 tert-Butanol

dC−C (Å)

dC−O (Å)

dC−D (Å)

dO−D (Å)

⟨u2⟩ (Å2)

⟨Δθ2⟩1/2 (deg)

1.57

1.396

1.08

1.02

0.01

15

NB

∑ nαβk ⟨uk 2⟩ + k

NR

∑

drαβ

2

dθl

l

⟨Δθl 2⟩

(4)

In the case of TOL, we assumed a free rotation of the methyl group, verified for the working temperature. Considering two different types of chemical bonds (C−C and C−D) was sufficient to obtain good fits. The expression simplifies to eq 5.

⎛ dσ ⎞inter ⎜ ⎟ sin Qr dQ n x ∑αi = 1 bαi)2 ⎝ dΩ ⎠ i=1 i

∫ (∑2

1.39

⟨uαβ 2⟩ =

dL(r ) = 4πρr(gL (r ) − 1) =

dC−C(Me) (Å)

the oscillations of the resulting gL(r) in the small-r region (r < 1.6 Å) where it is supposed to be exactly zero. We chose to reduce the number of free and possibly correlated parameters. Indeed, the assignment of one MSD to each different couple of atoms would lead to an increasing number of free parameters and unreliable results. This limitation can be circumvented by exploring an extended Q-range with isotopic exchange. The other robust method that we used consists of taking account of the constraints that may exist between the different parameters. MSD factors mostly reflect distance fluctuations arising from molecular vibrational and librational modes. With a good level of approximation, one can assume that these interatomic distance fluctuations arise from independent chemical bond length vibrations and rotations. If the molecules consist of NB different types of chemical bonds (e.g., C−C, C−O, C−D) and show NR different internal rotational degrees of freedom, then each MSD can be reduced to a combination of NB and NR independent squared bond length fluctuation ⟨uk2⟩ and librational mean squared angle rotation ⟨Δθl2⟩.33 This is expressed in eq 4, where the coefficient nkαβ is the number of chemical bonds of type k counted along the shortest path connecting atom α to β, and drαβ/dθl is the derivative of the distance with respect to the angular internal degree of freedom θ. Both quantities can be easily computed for known molecular geometry.

Figure 1. Experimental neutron scattered intensity of the pure liquid tert-butanol (99.8% D) at T = 303 K measured on the 7C2 spectrometer (open symbols) and on the G6.1 spectrometer (solid line). Polynomial function for the Placzek correction of the selfscattering (dashed line).

⎛ dσ ⎞ ⎛ dσ ⎞inter ⎜ ⎟ = ⎜ ⎟ + ⎝ dΩ ⎠ ⎝ dΩ ⎠

dC−C (Å)

Q

i

⟨uαβ 2⟩toluene = nαC,−β Cbonds⟨uC − C 2⟩ + nαC,−β Dbonds⟨uC − D2⟩

(3)

where ρ is the number density obtained from the literature. In order to get the intermolecular differential cross section, one has to evaluate the intramolecular contribution. This latter term in eq 2 is the major contribution to the total scattered intensity at high Q-values (typically above 3 Å−1). The high-Q part of the spectra obtained on 7C2 allows us to evaluate simultaneously P(Q) and the intramolecular contribution.4,10,17 We assumed that the molecular structure and the corresponding intramolecular form factor of each constituent of the binary system remain the same for all the mixtures. Therefore, the intramolecular parameters (cf. Table 1) were obtained from an analysis of the differential cross section of the two pure components and introduced in terms of a composite function, weighted by the molar fraction, for the binary systems according to eq 2, with no further refinement of the molecular parameters. For the pure components, P(Q) and (dσ/dΩ)intra were simultaneously obtained from a fit in order to minimize 32

(5)

For TBA, the libration of the three methyl groups was described more accurately with a mean square angular amplitude ⟨Δθ2⟩, and only one average bond length amplitude vibration mode was required. 2 tert ‐ butanol

⟨uαβ ⟩

=

bonds 2 nαβ ⟨u ⟩

+

drαβ dθ

2

⟨Δθ 2⟩ (6)

This method is based on reasonable assumptions, at least for “not-too-flexible” molecules, and provides an extraordinary reduction of the number of free parameters with a robust and good quality fit of the data. As an illustration, we present in Figure 2 the coherent cross section of TBA and the intramolecular cross section obtained from this fitting procedure. The two curves fairly coincide for Q-values above 3 Å−1, which supports the choice of a simplified description of C



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dilution. This is a general feature already observed for many types of alcohols during dilution by an inert solvent. The spectral analysis of the OH-bands of TBA−MeCY mixtures and the quantification of the size distribution of the different H-bonded species described in the following part are in very good agreement with the results obtained for another noninteracting alkyl solvent.7,8 A qualitative difference is observed for the OH vibrational spectra of TBA diluted with TOL. The free-OH band exhibits an additional sub-band at 3588 cm−1, which is absent for the inert solvents MeCY and dimethylbutane.8 A comparison between the free-OH bands of TBA−MeCY and TBA−TOL mixtures is shown in Figure 4 for

Figure 2. Experimental neutron coherent cross section of the pure liquid tert-butanol (99.8% D) at T = 303 K (open circles) and the intramolecular cross section (solid line).

the molecular form factor and confirms the predominance of intramolecular correlations in this Q-range.

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RESULTS AND DISCUSSION Raman Profiles. The Raman OH-stretching bands of TBA−solvent mixtures are shown in Figure 3. The spectra were

Figure 3. Raman OH-stretching bands of tert-butanol methylcyclohexane mixtures as a function of the tert-butanol molar fraction (for x = 1 to x = 0 from top to bottom) at T = 294 K. Inset: Raman OHstretching bands of tert-butanol toluene mixtures for x = 1 and x = 0.11.

Figure 4. Raman spectra of (a) tert-butanol−methylcyclohexane and (b) tert-butanol−toluene mixtures with molar fractions x = 0.67 and 0.63, respectively.

acquired for different compositions and the pure components at 294 K. All spectra were subtracted for CH-stretching fundamental and combination bands.8,34 The OH-stretching profile of TBA presents a prominent band that extends in the frequency range from 3150 to 3550 cm−1. It has been assigned to the different H-bonded self-associated species. Their vibrational OH-stretching frequencies present a marked shift with respect to the free-OH band at ca. 3620 cm−1. The freeOH band is composed of two narrower lines, at 3625 and 3617 cm−1 , which comprise OH belonging, respectively, to monomeric molecules and terminal molecules, which accept but do not donate a hydrogen bond.7,8 The ratio between the intensity of the H-bonded component and the free-OH band decreases continuously on decreasing the TBA molar fraction. This is a direct signature of the destabilization of the Hbonding species and the prevalence of monomers at high

a similar molar fraction. The intensity of the new sub-band increases continuously on increasing the dilution rate of TBA in TOL and becomes the dominant one in the high dilution (monomeric) regime (cf. inset in Figure 3). This suggests that this new band is assigned to monomeric TBA species, which are in interaction with TOL. Theoretical calculations have predicted the formation of TBA−benzene complexes in the gas phase, which are stabilized by a weak hydrogen bond with π electrons.26 An experimental infrared spectroscopy study of monomeric TBA in benzene and cyclohexane liquid mixtures is also in agreement with the frequency shift we measured by Raman spectroscopy for TBA−TOL and TBA−MeCY mixtures.27 The infrared free-OH band of TBA is 3588 and 3621 cm−1 in interacting benzene and inert cyclohexane liquid mixtures, respectively. These values agree with our Raman band assignments (ca. 3588 and 3625 cm−1 for TBA in interacting TOL and inert MeCY). This assignment is also in agreement D



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comparison with the existing literature assuming the same level of approximations. In this model, the contribution of each component to the Raman spectra is weighted by the fraction of each n-mer and the Raman scattering activity of the stretching mode considered. The value of the Raman activity of each mode was derived from ab initio calculations6 in accordance with the previous line shape analysis.8 In the case of TBA− TOL mixtures, we considered an additional band (denoted G1′) assigned to the monomeric TBA molecules solvated by TOL. In this case, a Gaussian form was adopted to account for the different interactions with TOL, the hydroxyls being a proton donor to the π-electrons of aromatic rings. We assigned to this mode the value of the Raman activity determined for monomeric TBA.6 This can be considered as a rough approximation, which neglects the effect of the weak TBA− TOL interaction on the value of the Raman scattering activity. In order to assess a possible overestimation of the monomer fraction induced by this approximation, we have compared the obtained results with the limiting case when the value of the Raman activity of the G1′ is taken as high as the G2 one. This issue will be addressed more specifically during the discussion of Figure 5b. The equation fitted to the Raman spectra is as follows:

with an infrared study of ternary mixtures, comprising one alcohol diluted in a binary solvent composed by toluene and nhexane as interacting and inert liquids, respectively. In this study, a splitting of the free-OH band has been observed and the two sub-bands have been assigned to the coexistence of noninteracting and solvated monomeric alcohol species.24 Size Distribution of H-Bonded Clusters. The size distribution of the different H-bonded species can be inferred by a quantitative shape analysis of the OH stretching bands, although different assignments of the OH vibrational bands have been considered in the literature. Spectral analyses are usually based on models that consider the possibility to decompose the spectra into a limited number of sub-bands. The situation encountered in a liquid phase is obviously more complex, with the presence of a large number of different local environments, interactions, and molecular configurations/ conformations that contribute differently to the total vibration spectrum. At best, one can expect that such a simple decomposition could provide a general view on the most salient properties of self-association, that could help for understanding its evolution as a function of control parameters (concentration, temperature, solvent interaction) and should be compared to other experimental methods, molecular simulation, and/or theoretical models. There exist different types of approximations in the models found in the literature. A usual approximation assigns the features observed in the vibrational spectra to different kinds of OH species like free OH, acceptor only (free terminal OH), donor only (weak hydrogen bonding), and both donor and acceptor (strong hydrogen bonding). Alternatively, it has also been assumed that the sub-bands arise from a limited number of fixed size species. Again, different approximations can be made with respect to the number, the size, and the nature of the different multimeric species (both linear and cyclic). In a recent study, Sassi et al. derived a systematic procedure, which has been successfully applied to the spectral analysis of the IR and Raman OH stretching modes of TBA− dimethylbutane mixtures.7,8 A detailed justification of the parameters that have been retained in the model is provided by the authors.7 A principal approximation concerns the selection of n-mers used to describe the population of H-bonded clusters. It has been limited to monomer, linear dimer, cyclic tetramer, and cyclic hexamer. This choice is supported by molecular simulation results showing that H-bonded clusters in liquid TBA are essentially cyclic, the tetrameric form is the dominant one, and the size distribution of multimers decays rapidly beyond six-membered species.12,35 In the spectral analysis, the Raman scattering consists of a linear combination of sub-bands arising from the different Hbonded species. In the fitting procedure, Gaussian forms are adopted for the three components of the broad H-bonded band, while Lorentzian forms are adopted for the two components of the free-OH bands. Indeed, Gaussian distribution of spectral lines usually better accounts for the distribution of local environments, which is the expected situation for H-bonded components in the liquid state. This is unlike the free-OH components, which are expected to be less involved in intermolecular interactions. More general fitting functions could have been used, combining combination (sum or product) of Gaussian and Lorentzian functions.36 We have decided to conform to the initially proposed model,7 in order to be consistent with the underlying ab initio calculations,6 to limit the number of free parameters, and to allow for a direct

I( v ̅ ) = n1IL1L1( v ̅ ) + n2IL2L 2( v ̅ ) + n2IG2G2( v ̅ ) + n4IG4G4( v ̅ ) + n6IG6G6( v ̅ ) + n1 ′IL1G1 ′( v ̅ )

Figure 5. Population fractions of H-bonded clusters as a function of composition: (a) tert-butanol−methylcyclohexane mixtures; (b) tertbutanol−toluene mixtures. Solid lines are only guides for the eyes. Dashed lines represent the limit case obtained when the Raman activity of the monomeric TBA in interaction with TOL is assumed to be as large as dimeric TBA (see text for details). E



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where ni is the population factor of each type of H-bonded species of i molecules and Ii is the Raman scattering activity of the ith mode described by a Lorentzian (Li) or Gaussian (Gi) function. No significant variation of the band widths and positions was observed as a function of composition if all of these parameters were allowed to vary during the fitting procedure. Therefore, the intensity of the different lines was eventually obtained for all the different spectra of TBA−MeCY and TBA−TOL samples with a same set of nine fixed band width and position parameters, this constraint being only slightly relaxed for three band positions, as specified in Table 2. In general, the obtained values of the band widths and positions are compatible with the study by Sassi et al., which underlines the consistency of the band shape analysis.8

modest dilution rates (xTBA < 0.5). Note that this behavior is not qualitatively altered by the choice of the value of the Raman scattering activity for the monomeric units interacting with the aromatic π-electron. This is illustrated by the dashed lines, which have been computed assuming that the monomer Raman activity due to the TBA−TOL interaction was as high as that for proton donor H-bonded TBA in dimers (i.e., G1′ = G2 = 130.7 Å4 amu−1). Comparison of the Raman Analysis with 1H NMR Measurements. The relevance of the size distribution of Hbonded clusters derived from the analysis of the Raman vibration spectroscopy has been appraised by a comparison with 1H NMR measurements. It has been shown in the literature that different values of the hydroxyl proton chemical shift can be assigned to the different types of H-bonded species.23 The measured value of the chemical shift δOH is a linear combination of the different chemical shifts δOH,i, which are assigned to each type of H-bonded unit i, weighted by the fraction pi of proton present in an environment of type i. Following a former NMR study on different butanol mixtures by Bich et al.,23 it has been further assumed that δOH,i is determined by two values, δOH,b and δOH,f, that depend on the H-bond donor (or not) character of the proton (bonded or free). Interestingly, it is possible to establish a direct relation between the Raman study and the NMR hydroxyl proton chemical shift according to

Table 2. Frequency, Band Width, and Scattering Activity of Each OH Stretching Mode Obtained from the Spectral Lineshape Analysis of the Raman Spectraa H-bonded species

νOH (cm−1)

ΓOH (cm−1)

IRaman (Å4 amu−1)

cyclic hexamer (G6) cyclic tetramer (G4) linear dimer (G2) linear dimer (L2) monomer (L1) solvated monomer (G1′)

3260 3395 (±5) 3510 3617 (±2) 3625 (±2) 3588

160 180 130 13 7 40

646.9 446.9 130.7 54.5 71.4 71.4 (also 130.7)

a

A single average value is presented, since no composition dependence of these quantities was observed.

δOH =

δOH,f (f1 + f2 ) + δOH,b(f2 + 4f4 + 6f6 ) ∑ ifi

(8)

where the values of f i are the population fractions of cyclic Hbonded clusters of size i, as derived by the Raman vibrational spectroscopy analysis. The only two unknown parameters, δOH,b and δOH,f, have been obtained from a previous study in the literature, that combined NMR measurements with a thermodynamic model of H-bond association (δOH,b = 5.151 and δOH,f = −0.89).23 These parameters have been used as direct inputs to compute the values of δOH predicted by the analysis of the Raman spectra, without any numerical fit. The quality of the agreement between the measured 1H NMR chemical shift and the prediction from the Raman population fractions of H-bonded clusters is manifest, as shown in Figure 6. More specifically, the NMR results confirm the H-bond destabilizing effect induced by the TOL−TBA interaction and provide plain support to the Raman band-shape analysis. Neutron Structure Factor and the Additivity Approximation. The experimental neutron coherent cross section of tert-butanol/toluene liquid mixtures as a function of the molar fraction of tert-butanol x is shown in Figure 7. A systematic variation of the coherent cross section is observed over the entire Q-range. More specifically, it corresponds to a decrease in the intensity of the main diffraction peak (QMP = 1.3 Å−1) and of the prepeak (QPP = 0.7 Å−1). Moreover, the position of the prepeak maximum is slightly shifted toward smaller Qvalues by mixing. In the higher Q-region of the structure factor, noticeable modifications are also observed around 1.8−2.2 and 5.8−8 Å−1 with a damping of oscillating features. The latter changes have a rather trivial interpretation, which is mostly related to the intramolecular correlations. The contribution from intramolecular correlations is shown in Figure 8 for the different compositions. An excellent agreement between the observed changes in the coherent cross section and the computed intramolecular form factor is obtained. On

The result from the band shape analysis is shown in Figure 4 for two different mixtures. Equally good agreements with the experimental profiles were obtained for the different compositions studied. We then deduced the fraction of the different H-bonded clusters as a function of their size f i = ni/ ∑ni, as shown in Figure 5. In the case of TBA−MeCY, the tetrameric species are dominant over an extended range of concentration (0.2 < xTBA < 1). The fraction of the other Hbonded species (dimers and hexamers) remains small and weakly dependent on dilution over the entire range of concentrations studied. The exact values of these minority species should probably be considered with more caution than for monomers and tetramers, given the assumption involved in the band shape analysis and the large statistical error bars at low concentration. The destabilization of the H-bonded tetrameric clusters occurs for high dilution rates (xTBA < 0.2). It goes together with an abrupt rise of the fraction of monomeric TBA. This highlights the exceptional stability of micellar clusters in TBA toward dilution in a noninteracting solvent. The values of the different cluster fractions of TBA−MeCY shown in Figure 5 are in perfect agreement with the study of TBA−dimethylbutane.8 This demonstrates the stability of the band shape analysis used in the two different studies. In addition, it shows that selfassociation of TBA is not affected by the chemical differences between MeCY and dimethylbutane. This observation is consistent with the fact that alkanes can be essentially considered as inert diluting solvents. A different situation is encountered for the TBA−TOL mixture, which we interpret as the direct illustration of the solvation effect on self-association. The fraction of tetrameric clusters rapidly drops with the addition of TOL. The destabilization of clusters leads to an increase of the fraction of monomeric units, which become the dominant species for F



Article

⎛ dσ ⎞ ⎜ ⎟ = ⎝ dΩ ⎠ideal

2

⎛ dσ ⎞ ⎟ dΩ ⎠i

∑ xi⎝⎜ i=1

(9)

The comparison between the additivity approximation and the experimental results has been discussed for a variety of benzene and fluoro-benzene derivatives.37 It has provided information on the tendency of each component to retain its local structure upon mixing, while in some cases it has revealed enhanced ordering and the formation of heterodimers. In the case of the TBA−TOL mixture, the comparison with the additivity approximation provides illuminating information about the interaction between TBA clusters and the interacting TOL. Indeed, in the absence of correlation between the two liquids at the mesoscopic scale, which would correspond to the situation where the TBA clusters remain correlated in “mesoscopic liquid pockets”, a good agreement with the additivity approximation should be observed. On the contrary, solvent interactions should induce large deviations with respect to this approximation. The coherent cross sections deduced from the ideality approximation for the different compositions are displayed in Figure 9 for a direct comparison with the experimental one

Figure 6. 1H NMR chemical shifts predicted from the Raman population fractions of H-bonded clusters as a function of the molar fraction of tert-butanol: tert-butanol methylcyclohexane (triangles up) and tert-butanol−toluene (triangles down). Experimental 1H NMR chemical shifts (solid lines with open circles).

Figure 7. Experimental neutron coherent cross section tert-butanol− toluene liquid mixtures at T = 310 K as a function of the molar fraction of tert-butanol x. Inset: region of the prepeak. Figure 9. Ideal mixture approximation of the neutron coherent cross section tert-butanol−toluene liquid mixtures at T = 303 K as a function of the molar fraction of tert-butanol x.

shown in Figure 7. A good agreement is obtained in the high Qpart (Q > 2 Å−1), which is consistent with the large contribution arising from intramolecular correlation in this region, as discussed previously. Large deviations from the additivity approximation are observed in the region of the prepeak and main diffraction peak, which are significantly less intense experimentally. The difference can be better exemplified by the excess cross section ⎛ dσ ⎞ ⎛ dσ ⎞ ⎛ dσ ⎞ ⎜ ⎟ ⎟ − ⎜ ⎟ =⎜ ⎝ dΩ ⎠exc ⎝ dΩ ⎠ ⎝ dΩ ⎠ideal

Figure 8. Intramolecular coherent cross section tert-butanol−toluene liquid mixtures at T = 303 K as a function of the molar fraction of tertbutanol x.

(10)

Excess thermodynamic quantities, such as the excess volume, are widely used to quantify the nonideal character of binary mixtures.38 Figure 10 shows the excess coherent cross section, which has been normalized to the experimental coherent cross section of the mixtures in order to highlight the region of largest relative variations. Two excess peaks observed at QPP = 0.7 Å−1 and QMP = 1.3 Å−1 indicate that mixing induces a significant suppression of the prepeak and the main diffraction peak with respect to the additivity approximation.

the contrary, the modifications of the prepeak and the main diffraction peak are most probably related to intermolecular correlations, which is a central issue of this study. To address this point, we have first calculated the coherent cross section of each mixture assuming the additivity of the contributions from the pure components in the mixture. G



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nonzero excess function dexc(r). The first peak at 5.2 Å of dexc(r) stands on the left side of the first peak of dL(r), i.e., at distances corresponding to the first coordination shell. At larger distances, dexc(r) exhibits oscillation features, which are in perfect antiphase with the peaks observed in dL(r). This indicates a decay of molecular correlations with respect to the ideality approximation that concerns the region that extends beyond the first coordination shell, i.e., intermediate range correlations, and corresponds to intercluster distances. Correlation between Clusters. The prepeak in the structure factor of liquid tert-butanol is a signature of intermediate range correlations, and more specifically spatial correlations between the H-bonded micellar clusters. At variance, the vibrational analysis of the OH stretching mode provides direct insight into H-bond formation, and it is a signature of self-association per se. The prepeak has been observed in bulky alcohols and amines that form H-bonded supramolecular cyclic clusters centered about H-bonded OH groups with the apolar groups pointing outside.1,10−14,16,17 This gives rise to a depletion in the radial distribution functions of hydroxyl atoms in the region from 4 to 7 Å and a second maximum at dclus ≈ 9 Å. This typical distance corresponds to the correlation between the hydroxyl atoms at the center of two neighboring clusters. Therefore, it has been established that the prepeak mostly arises from this intercluster correlation that shows up in the partial structure factors involving OH atoms at QPP ≈ 2π/dclus ≈ 0.7 Å−1. The value of the intercluster distance dclus is controlled by the tail-to-tail interaction between the surrounding tert-butyl groups and corresponds to two molecular diameters. The prepeak is not simply the signature of self-association through H-bonded hydroxyl groups, but it is evidence of the existence of an intermediate range ordering between adjacent cyclic clusters. A comprehensive view of the dilution and solvation effects on the segregation and self-association processes is provided in Figure 12. It shows the integrated intensity of the prepeak as a function of the composition, which is compared with the mean cluster size derived from the Raman spectra analysis (i.e., Nclus = ∑if i). In the case of the inert diluent MeCY (Figure 12a), the mean cluster size of TBA barely changes as of function of composition; i.e., Nclus only varies from 3.5 to 4 over an extended range of dilution (0.2 < xtert‑butanol < 1). This confirms the exceptional stability of the TBA micellar clusters with respect to drastic dilution conditions in an inert solvent. The intensity of the prepeak presents a linear decrease as a function of dilution fraction for the composition range (0.55 < xtert‑butanol < 1). It eventually decays more gradually in the region where MeCY is the dominant component and vanishes in the limit of infinite dilution. The observation of a prepeak of significant intensity for 0.55 < xtert‑butanol < 1 implies that the H-bonded micellar clusters maintain a certain degree of intermediate range spatial correlations despite dilution (“correlated clusters”). It is noteworthy that the prepeak decay observed in this concentration range is only slightly steeper than what would be predicted by the additivity approximation, i.e., when the two systems do not mix at all. This observation reveals that the addition of MeCY leads to an incomplete mixing of the two components. Is also suggests that the structure of TBA in the concentration range (0.55 < xTBA < 1) consists of neighboring H-bonded clusters, which essentially retain the intermediate range correlations of the pure component (“correlated clusters”) and are segregated in alcohol liquid pockets.

Figure 10. Excess neutron coherent cross section tert-butanol−toluene liquid mixtures with respect to the ideal mixture at T = 303 K as a function of the molar fraction of tert-butanol x.

To get a complementary viewpoint on the structure, we have computed the excess pair correlation function gexc(r) = gL(r) − gideal(r) according to eq 11. dexc(r ) = 4πρr(gexc(r ) − 1) =

1 2π

∫ (∑2

Q

⎛ dσ ⎞ ⎜

⎟

N x ∑α i= 1 bαi)2 ⎝ dΩ ⎠exc i=1 i i

sin(Qr ) dQ (11)

The experimental radial distribution functions of the different mixtures are shown in Figure 11, for a comparison with the

Figure 11. Radial distribution function of tert-butanol−toluene liquid mixtures at T = 303 K as a function of the molar fraction of tertbutanol x (solid lines, left axis). Excess radial distribution function for the molar fraction of tert-butanol x = 0.707 (filled circles, left axis). Pair correlation function between centers of mass of pure tert-butanol from molecular simulation (shaded area, right axis) of ref 14.

excess function computed for the mixture incorporating 30% of toluene. For all the studied mixtures, the experimental radial distribution functions dL(r) are qualitatively similar to the pure TBA with four broad peaks centered at 6, 10.7, 15.6, and 19.6 Å. According to the literature, the peak at about 6 Å corresponds to interatomic correlations in the first coordination shell, comprising both H-bonded molecules and slightly more distant direct correlations.9 The further oscillations in the experimental dL(r) conform the next neighbor molecular center distances, as computed by molecular simulation.14 Deviations from the additivity approximation can be quantified by a H



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TBA molar fractions smaller than xTBA = 0.8. The crossover from “correlated clusters” to “diluted clusters” is therefore significantly shifted toward higher TBA fraction when increasing the interaction between the two components, going from MeCY to TOL solvent. The segregation process of TBA molecules related to incomplete mixing of the two components is reduced likewise. This is also consistent with the deviation from the additivity approximation discussed above for TOL−TBA. The second crossover from “diluted clusters” to “diluted monomers” is blurred compared to the MeCY−TBA system, and it appears in the range of much higher TBA concentration. It has been located at xTBA = 0.45 in Figure 12b instead of xTBA = 0.15 for MeCY−TBA, with this concentration being also defined as the point where the monomer becomes the dominant form of TBA (see Figure 5). The shift of this second crossover expresses how the self-association process itself (i.e., the stability of H-bonded species) is strongly affected, when tuning from an inert to an interacting solvent.

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CONCLUSIONS Monohydric alcohols exhibit a multiplicity of mesoscopic ordersusually called microstructures, such as chainlike or cyclic clusters. tert-Butanol is of special interest because it counts among the simplest amphiphilic molecules that form supermolecular clusters in the pure liquid state. These clusters are stabilized by self-association of two to six molecules into cyclic “reverse micelle” structures, centered about the Hbonded hydroxyl groups and surrounded by the hydrophobic tert-butyl tails of the molecules. The remarkable stability of tetrameric clusters to extreme conditions of dilution with an inert alkane solvent (down to 0.1 TBA molar fraction) has been recently reported by vibrational spectroscopy. This observation has raised novel questions about the nature of the mesoscopic order that may exist in the wide concentration range where micellar clusters exist. The present study reveals that TBA−aprotic solvent binary mixtures, though macroscopically homogeneous, are indeed ordered on different length scales as a function of concentration. This survey has been made possible by an original combination of Raman vibration spectroscopy, 1H NMR, and neutron diffraction, in order to probe selectively the cluster formation (selfassociation) and the intercluster correlations (cluster segregation). Neutron scattering provides a direct signature of the structural correlations between adjacent clusters, as illustrated by a prepeak at QPP = 0.7 Å−1 in the static structure factor. Raman spectroscopy and 1H NMR allow studying the selfassociation per se, through the line shape analysis of the free and H-bonded OH stretching modes and OH proton chemical shift. We combined the three methods to investigate the structural modifications induced by the addition of an inert solvent (MeCY) and a weakly interacting solvent (TOL). Two distinct crossovers have been identified as a function of the dilution rate. In the alcohol-rich regime, the addition of a diluting solvent induces a segregation of the TBA micellar clusters in “mesoscopic alcohol liquid pockets”, which is consistent with the nonideal character of the binary mixture. This type of microstructures is determined by the existence of “correlated clusters”, as established by the existence of a correlation prepeak in the static structure factor. At further dilution rate, the interaction between micellar clusters vanishes and these independent clusters are increasingly dispersed in the “bath” of aprotic solvent. For the TBA−MeCY mixture, the ultimate dilution of nonassociating alcohol molecules is

Figure 12. Cluster size (filled diamonds) and prepeak intensity (open circles) of (a) tert-butanol−methylcyclohexane and (b) tert-butanol− toluene mixtures as a function of the molar fraction of tert-butanol x. The three different types of microstructures are located by shaded areas. They are separated by two distinct crossovers in the variation of, respectively, the cluster size and the prepeak intensity as a function of the dilution rate.

On reducing TBA molar fraction, one observes a first crossover (at about xTBA = 0.55) in the intensity of the prepeak, which indicates the vanishing of the intercluster correlations, though the self-association process itself remains as in the pure TBA. This intermediate region is denoted “diluted clusters” in Figure 12a, and persists down to about xTBA = 0.15. It is remarkable that there is no visible signature of this crossover from correlated to diluted clusters at xTBA = 0.55 in the OH vibrational spectroscopy, which implies that self-association and cluster segregation are rather independent processes. There exists a second crossover at xTBA = 0.15, which corresponds to the destabilization of the H-bonded n-mers and related to the self-association process. It is illustrated by a rapid drop of the mean cluster size and the formation of “dispersed monomers”. With the TBA−TOL system shown in Figure 12b, one can assess the influence of the solvent−solute interaction on the position of the two crossovers and therefore have a combined view of the solvation and dilution effects on the self-association of TBA molecules and the segregation of clusters in liquid pockets. At variance to the TBA−MeCY, H-bonded clusters induced by self-association are not very stable upon dilution. Mean cluster size values larger than three molecules are only observed when TBA is the dominant component (xTBA > 0.45). Also, the prepeak is extremely sensitive to a slight addition of TOL into TBA. The addition of only 5% TOL into TBA induces a reduction of 20% of the prepeak intensity. The prepeak essentially disappears from the diffraction pattern for I



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(21) Bowron, D. T.; Finney, J. L.; Soper, A. K. J. Phys. Chem. B 1998, 102, 3551. (22) Dougan, L.; Crain, J.; Finney, J. L.; Soper, A. K. Phys. Chem. Chem. Phys. 2010, 12, 10221. (23) Bich, E.; Hensen, U.; Michalik, M.; Wandschneider, D.; Heintz, A. Phys. Chem. Chem. Phys. 2002, 4, 5827. (24) Asprion, N.; Hasse, H.; Maurer, G. Fluid Phase Equilib. 2003, 208, 23. (25) Singh, K. C.; Kalra, K. C.; Maken, S.; Gupta, V. Fluid Phase Equilib. 1996, 119, 175. (26) Larsen, G.; Ismail, Z. K.; Herreros, B.; Parra, R. D. J. Phys. Chem. A 1998, 102, 4734. (27) Larsen, G.; Ismail, Z. K. J. Solution Chem. 1998, 27, 901. (28) Ambroise, J. P.; Bellissent-Funel, M. C.; Bellissent, R. Rev. Phys. Appl. 1984, 19, 731. (29) Paalman, H. H.; Pings, C. J. J. Appl. Phys. 1962, 33, 2635. (30) Blech, I. A.; Averbach, B. L. Phys. Rev. 1965, 137, 1113. (31) Placzek, G. Phys. Rev. 1952, 86, 377. (32) Nikam, P. S.; Jagdale, B. S.; Sawant, A. B.; Hasan, B. J. Chem. Eng. Data 2000, 45, 559. (33) Leclercq, F.; Damay, P.; Foukani, M.; Chieux, P.; Bellissentfunel, M. C.; Rassat, A.; Fabre, C. Phys. Rev. B 1993, 48, 2748. (34) Korppitommola, J. Spectrochim. Acta, Part A 1978, 34, 1077. (35) Yonker, C. R.; Wallen, S. L.; Palmer, B. J.; Garret, B. C. J. Phys. Chem. A 1997, 101, 9564. (36) Wandschneider, D.; Michalik, M.; Heintz, A. J. Mol. Liq. 2006, 1252, 13. (37) Cabaco, M. I.; Danten, Y.; Besnard, M.; Guissani, Y.; Guillot, B. J. Phys. Chem. B 1998, 102, 10712. (38) Pena, M. P.; Martinez-Soria, V.; Monton, J. B. Fluid Phase Equilib. 1999, 166, 53.

achieved only for TBA molar fraction lower than 0.1, as revealed by Raman spectroscopy. Tuning the strength of the alcohol−solvent interaction from TBA−MeCY to TBA−TOL leads to a significant shift of the two crossovers toward lower solvent dilution rate. This highlights the role of solvatation in determining the microstructure of the binary liquid in addition to the simple dilution effect, which is addressed with an inert solvent. We expect to gain a better insight onto the microstructure of the binary liquid and the role of the balance between H-bonding and solvent− alcohol solvating interaction from studies on other types of alcohols and ongoing molecular simulation.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Support from Région Bretagne (ARED 5453 NanoFLu) and Europe (FEDER) is expressly acknowledged. We thank B. Beuneu, I. Mirebeau, and F. Porcher for their assistance with the neutron diffraction experiments and T. Lé for helping with the 1H NMR measurements.

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REFERENCES

(1) Adya, A. K.; Bianchi, L.; Wormald, C. J. J. Chem. Phys. 2000, 112, 4231. (2) Yamaguchi, T.; Hidaka, K.; Soper, A. K. Mol. Phys. 1999, 96, 1159; 1999, 97, 603. (3) Kosztolanyi, T.; Bako, I.; Palinkas, G. J. Chem. Phys. 2003, 118, 4546. (4) Morineau, D.; Guégan, R.; Xia, Y.; Alba-Simionesco, C. J. Chem. Phys. 2004, 121, 1466. (5) Guégan, R.; Morineau, D.; Alba-Simionesco, C. Chem. Phys. 2005, 317, 236. (6) Andanson, J. M.; Soetens, J. C.; Tassaing, T.; Besnard, M. J. Chem. Phys. 2005, 122, 174512. (7) Palombo, F.; Paolantoni, M.; Sassi, P.; Morresi, A.; Cataliotti, R. S. J. Mol. Liq. 2006, 125, 139. (8) Sassi, P.; Palombo, F.; Cataliotti, R. S.; Paolantoni, M.; Morresi, A. J. Phys. Chem. A 2007, 111, 6020. (9) Bowron, D. T.; Finney, J. L.; Soper, A. K. Mol. Phys. 1998, 93, 531. (10) Morineau, D.; Alba-Simionesco, C. J. Phys. Chem. Lett. 2010, 1, 1155. (11) Kusalik, P. G.; Lyubartsev, A. P.; Bergman, D. L.; Laaksonen, A. J. Phys. Chem. B 2000, 104, 9526. (12) Perera, A.; Sokolic, F.; Zoranic, L. Phys. Rev. E 2007, 75, 060502(R). (13) Zoranic, L.; Sokolic, F.; Perera, A. J. Chem. Phys. 2007, 127, 024502. (14) Ghoufi, A.; Hureau, I.; Lefort, R.; Morineau, D. J. Phys. Chem. C 2011, 115, 17761. (15) Chandler, D. Nature 2005, 437, 640. (16) Morineau, D.; Alba-Simionesco, C.; Bellissent-Funel, M.; Lauthie, M. Europhys. Lett. 1998, 43, 195. (17) Morineau, D.; Alba-Simionesco, C. J. Chem. Phys. 1998, 109, 8494. (18) Artola, P. A.; Raihane, A.; Crauste-Thibierge, C.; Emo M.; Rousseau, B.; Alba-Simionesco, C. J. Phys. Chem. B, submitted for publication. (19) Guo, J. H.; Luo, Y.; Augustsson, A.; Kashtanov, S.; Rubensson, J. E.; Shuh, D. K.; Agren, H.; Nordgren, J. Phys. Rev. Lett. 2003, 91, 157401. (20) Dixit, S.; Crain, J.; Poon, W. C. K.; Finney, J. L.; Soper, A. K. Nature 2002, 416, 829. J


Solvation Effects on Self-Association and Segregation Processes in

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