Tomas Rego

Sylvie Spagnoli

Marie-Claude Fauré

Clémence Allain

Benoit Coasne

Jérémy Malinge

Chen Shen

Philippe Fontaine

Michel Goldmann

Unexpected order-disorder transition in diacetylene alcohol Langmuir film

Keywords: Diacetylene alcohol, phase-transition, 2D organization, Langmuir monolayer

The phase diagram of Langmuir film of diacetylene alcohol -henicosa-5,7-diyn-1-ol -is investigated by means of surface pressure versus surface area isotherms, Brewster Angle Microscopy (BAM), X-Ray Reflectivity (XRR) and Grazing Incident X-ray Diffraction (GIXD).

Among the usual phases described in the generic phase diagram of small head group molecules, one observes an unexpected reversible transition from an ordered condensed phase to a disordered one upon increasing the surface pressure. We postulate that the origin of this unusual, unprecedented transition results from the competition between the interactions between the diacetylene blocks in the hydrophobic chain and the hydrogen bonds between head-groups and water.

■ INTRODUCTION

Significant research activities are currently devoted to the development of biological and chemical sensors using molecular films made of diacetylene (DA) molecules which general formula is R-C≡C-C≡C-R' where C≡C-C≡C is the diacetylene moiety and R and R' are substituent groups. These materials can polymerize under irradiation in a topochemical way forming, usually, a blue polymer that can turn red using stimuli such as pH, temperature, and solvent [1]. Their potential use in sensing applications was first explored in 1993 by Charych et al [START_REF] Charych | Direct colorimetric detection of a receptor-ligand interaction by a polymerized bilayer assembly[END_REF] where a polydiacetylene monolayer bearing special receptors at the end of side chains change from blue to red when incubated with biomolecules. Since then, increasing efforts have been made to design various sensing systems with polydiacetylene based materials [3], mainly on the synthesis of specific target-receptor couples derived from 10,12 pentacosadiynoic acid (PCDA). However, monolayer appears as more appropriate than bulk crystal to improve the signal/noise ratio when probing color changes. Considering this point, the Langmuir film procedure, made up of insoluble molecules adsorbed at the air/water interface, appears as an adapted study base since one can easily adjust the physico-chemical parameters (temperature, surface pressure) with the aim to understand the color transition.

Langmuir monolayer is an excellent model for studying two-dimensional systems. The study of insoluble fatty alcohols and acids on surface water leads to a generic phase diagram, which can be extended to various molecules by rescaling in function of the number of aliphatic carbon atoms [START_REF] Kaganer | Structure and phase transitions in Langmuir monolayers[END_REF][START_REF] Peterson | Equivalent states of amphiphilic lamellae[END_REF]. However, the rigidity of the diacetylene group prevents the use of such approach for amphiphilic DA molecules. As far as these complex compounds are concerned, only few data concerning the structure of PCDA have been published [START_REF] Lifshitz | Structural Transitions in Polydiacetylene Langmuir Films[END_REF]. Among which an important result is that monolayer of PCDA at the air/water interface appears unstable with respect to the trilayer film, which prevents the formation of polymerized monolayer by UV irradiation [START_REF] Gourier | Collapse of Monolayers of 10,12pentacosadiyonic acide: kinetics and structure[END_REF]. To overcome this drawback, we propose to replace the acid headgroup by an alcohol. We synthetize a new DA molecule, henicosa-5,7-diyn-1-ol (13-4OH), (CH3(CH2)12-C≡C-C≡C-(CH2)3-CH2OH) with 13 saturated carbons followed by the DA group in the hydrophobic chain. We determine the phase diagram of 13-4OH Langmuir monolayers at the air/water interface, by surface pressure versus surface area isotherms, Brewster Angle Microscopy (BAM), X-Ray Reflectivity (XRR) and Grazing Incidence X-ray Diffraction (GIXD) experiments. The structure of the organized phase was also probed by molecular dynamics approach. Among the expected 2D gas (G), liquid expanded (LE) and liquid condensed (LC) phases, one observes that 13-4OH molecules undergo an unexpected reversible phase transition from an ordered LC phase to a disordered phase as the surface pressure increases. Comparison with henicosa-5,7-diynoic acid (13-3COOH), an analogous molecule to 13-4OH suggests that this transition is related to specific intermolecular interactions in 13-4OH monolayer at the air/water interface.

■ MATERIALS AND METHODS

Synthesis of henicosa-5,7-diyn-1-ol (13-4OH

). n-BuNH2 (2.5 mL, 25.4 mmol), distilled water (13 mL) and CH2Cl2 (13 mL) were successively added to a round bottom flask containing CuCl (0.10 g, 1.0 mmol). The resulting mixture was vigorously stirred and cooled to 0°C. A few crystals of hydroxylamine were used to discharge the blue color and pentadecyne (0.7 mL, 2.54 mmol) was slowly added at 0°C. The mixture turned bright yellow. After 5 min at 0°C, a solution of 6bromohex-5-yn-1-ol (prepared as described in [START_REF] Montierth | The Polymer-Supported Cadiot-Chodkiewicz Coupling of Acetylenes to Produce Unsynunetrical Diynes[END_REF]) (0.45 g, 2.54 mmol) in CH2Cl2 (5 mL) was slowly added. Few crystals of hydroxylamine may be needed to prevent the reaction from turning green. Once the addition completed, the reaction mixture was stirred 20 min at 0°C and was warmed up to room temperature over 90 min. The resulting pale green solution was extracted with CH2Cl2. The combined organic layers were washed with a saturated aqueous ammonium chloride solution, dried over MgSO4 and concentrated in vacuo. The residue was then purified over silica (20% EtOAc in petroleum ether) providing a white solid which should be cautiously protected from light because of efficient polymerization in the powder state (0.55 g, 71 % yield). The synthesis has been summarized in Scheme S1. 1 H NMR (400 MHz, CDCl3):  = 3.63 (t, J = 6.0 Hz, 2H), 2.31 (t, J = 6.4 Hz, 2H), 2.22 (t, J = 7.3 Hz, 2H), 1.86 (bs, 1H), 1.63 (m, 4H), 1.48 (m, 2H), 1.23 (bs, 20H), 0.86 (t, J = 6.4 Hz, 3H). 13 C NMR (100 MHz, CDCl3):  = 78.0, 65. . 13-3COOH was synthesized from 5hexynoic acid and 1-bromopentadecyne (prepared by bromination of commercial 1-pentadecyne by N-bromosuccinimide in acetone in the presence of a catalytic amount of silver nitrate) following the procedure described in [START_REF] Aida | Photoluminescent Silicate Microsticks Containing Aligned Nanodomains Conjugated Polymers by Sol-Gel-Based In Situ Polymerization[END_REF].

Synthesis of henicosa-5,7-diynoic acid

1 H-NMR (400 MHz, CDCl3)  2.51 (t, J = 7.6 Hz, 2H), 2.36 (t, J = 7.0 Hz, 2H), 2.24 (t, J = 7.2, 2H), 1.86 (tt, J = 7.0 Hz, J = 7.6 Hz, 2H), 1.52 (tt, J = 7.0 Hz, J = 7.6 Hz, 2H), 1.39-1.32 (m, 2H), 1.26 (m, 18H), 0.88 (t, J = 6.9 Hz, 3H). 13 -A Isotherms. The molecules were dissolved in chloroform to obtain a spreading solution of typical concentration of 1 mmol/L. Monolayers were spread on a Langmuir trough equipped with two movable barriers and the surface pressure () was measured using the Wilhelmy plate method.

The plate was made with filter paper, and the measurement device was a microbalance from Riegler & Kirstein GmbH (Germany). Milli-Q Millipore ultrapure water was used for the subphase (MilliQ R ≥ 18 MΩ.cm). Temperature was regulated at +/-0.5°C using a circulating water bath.

The surface pressure balance was calibrated with the value of surface tension of pure water for each temperature. The solution was spread at the air/water interface by drop-wise addition and allowed to equilibrate for 20 min to ensure evaporation of organic solvent. Surface pressure vs. area per molecule (-A) isotherms were measured by continuously moving the barrier compressing the layer with a linear speed of 0.23 cm 2 s -1 (1.2 10 -4 nm 2 molecule -1 s -1 ). The stability of the 13-4OH monolayer has been tested at different temperatures during two hours for various surface pressures.

The film is stable as a monolayer, since no pressure drift occurs at any of the pressures used in this study.

The compressibility of the phases is computed using the following definition:

𝜒 = - 1 𝐴 𝑑𝐴 𝑑𝜋
where A and 𝑑𝐴 𝑑𝜋 are determined by numerical derivative of the experimental -A isotherm for the macroscopic compressibility and from the unit cell area deduced by GIXD for the microscopic compressibility.

Brewster Angle Microscopy (BAM). We used a homemade BAM with a laser wavelength at 532 nm and a power of 100 mW (CNI MGL-III-532). The angle between the beam and the normal to the interface was adjusted to 53.1°, the Brewster angle of the air/water interface. Pictures of the monolayer were captured thanks to a CCD camera (Watec: WAT-902H-Ultimate) used in automatic gain control. The image size is 425 µm x 410 μm and due to the tilt between the water surface and the optical axis, only a stripe of about 20 μm is in focus. The surface pressure was measured using a Wilhelmy plate made of calibrated filter paper.

Reflectivity (XRR). X-ray Reflectivity (XRR) measurements were performed using the LISA diffractometer [10] on the P08 beamline [START_REF] Seeck | The high-resolution diffraction beamline P08 at PETRA III[END_REF] at PETRA III. The incident x-ray beam energy was 18 KeV and the Langmuir trough was enclosed in a sealed chamber flushed with helium. The reflected photons were measured by a 2D detector (Lambda 750K GaAs, X-Spectrum, Germany) allowing to subtract the background from the diffuse scattering measured in the plane of incidence.

At each angle, the reflected intensity is normalized to the incident beam intensity. Thereafter, the reflectivity curve is normalized by the intensity value at the critical angle. Spectra analysis was performed with the interdif module of GenX [START_REF] Björck | an extensible X-ray reflectivity refinement program utilizing differential evolution[END_REF].

Grazing Incidence X-ray Diffraction (GIXD). Grazing Incidence X-ray Diffraction (GIXD)

experiments were carried out at the SIRUS beamline at SOLEIL synchrotron. Details of the beamline optics can be found in the literature [START_REF] Fontaine | Soft Interfaces and Resonant Investigation on Undulator Source: A Surface X-ray Scattering Beamline to Study Organic Molecular Films at the SOLEIL Synchrotron[END_REF]. The incident x-ray beam energy was 8 keV (λ = 0.155 nm). Beam size (V  H) was 0.1  0.5 mm 2 . A custom-built Langmuir trough was enclosed in a temperature controlled sealed chamber and flushed with helium during data collection. The water surface was illuminated at an incident angle of 2 mrad below the critical angle (2.7 mrad) of the air-water interface so that the incident wave was almost totally reflected. In this regime the refracted wave is evanescent, probing the interface with a penetration depth of a few nanometers.

The scattered signal was measured by a vertical 1D low noise position sensitive detector of 150 mm height associated with a Soller collimator.

The diffraction peaks were fitted with a Lorentzian shape to extract the in plane peak position (Qhk), Full Width at Half Maximum (FWHM), and intensity (I). The positional correlation length  was obtained from the full width at half maximum FWHM using  = 2/FWHM. The error associated with the lattice parameters is lower than 0.1 nm and estimated at 1 nm for . Along the 𝑧 direction (perpendicular to the air/water interface) spectra were fitted with a squared Cardinal Sinus function to determine the Qz position of the maximum intensity. Details on the analysis of the GIXD data can be found in the literature [START_REF] Kaganer | Structure and phase transitions in Langmuir monolayers[END_REF]14]. temperature is controlled using a Nose-Hoover thermostat with a time constant of 0.05 ps. Water is modeled using the SPC/E water model [START_REF] Berendsen | The missing term in effective pair potentials[END_REF][START_REF] Kusalik | The spatial structure in liquid water[END_REF]. The CH3, CH2, CH, C≡C and OH groups are modeled with the Lennard-Jones interactions from the all-atom OPLS force field [START_REF] Jorgensen | Development and Testing of the OPLS All-Atom Force Fieldon Conformational Energetics and Properties of Organic Liquids[END_REF].

We conducted simulations in the NVT ensemble in which the lattice parameters were imposed (a = 0.57 nm et b = 0.901 nm). To constrain the system to remain in a state compatible with the experimental data, the alcohol head groups (O atom) were fixed at the corresponding crystallographic data. The system was first equilibrated at 285 K in the NVT ensemble prior to a production run that lasted at least a few ns (it was checked that longer production times do not alter the results). The diffraction pattern was determined by calculating the square of the Fourier transform of the electron density (x,y). (x,y) is the integrated electronic density along the z axis taking all the atoms into account and averaged on the last twenty configurations.

■ RESULTS AND DISCUSSION

-A Isotherms. Figure 1 presents the -A isotherms of 13-4OH between 12°C and 28°C. The values (lift-off, surface pressure of phase transitions) measured from these curves are presented in table 1. Below 16°C, a direct gas (G) to liquid condensed (LC1) transition is observed with a collapse pressure at about 30 mNm -1 . The liquid expanded (LE) phase is observed at 16°C and above. The macroscopic compressibility of the LE and LC1 phases are χ = 37 ± 3 mN -1 and χ = 10 ± 5 mN -1 , respectively, in the range of LE and LC phase of fatty acids or alcohols [START_REF] Yu | Characterization of the liquid-expanded to liquid-condensed phase transition of monolayers by means of compressibility[END_REF][START_REF] Fradin | Microscopic measurement of the linear compressibilities of two-dimensional fatty acid mesophases[END_REF].

In the 12-18°C temperatures range, a careful analysis of the -A isotherms (Figure S6b) evidences the presence of a small plateau at high surface pressure indicating a phase transition between the called LC1 phase and a less compressible phase labelled LC2. Its compressibility value was determined between 5 < χ < 8 mN -1 . The surface pressure plateau value, LC1-LC2, remains stable even when the compression barrier speed is doubled or halved. Moreover, it is reproducible along compression-expansion before the collapse of the monolayer (Figure S2).

Upon compressing the film above the collapse surface pressure, a plateau at about 30 mNm -1 is observed which extends up to about 0.08 nm 2 /molecule (Figure S3). The latter could be attributed to a transition from monolayer to a triple layer. Although an hysteresis is observed upon expansion/compression cycles (Figure S3) the compression/expansion cycles are reproducible which indicate that the film re-spreads as a monolayer, in stark contrast to the behavior of a film of PCDA molecules [START_REF] Gourier | Collapse of Monolayers of 10,12pentacosadiyonic acide: kinetics and structure[END_REF].

The phase diagram deduced from the isotherm data is presented on Figure 2. The LE-LC phase transition behaves as expected (LC means a Liquid Condense phase whatever its nature LC1 or LC2). The two-dimensional analogue of the Clausius-Clapeyron equation may be written as:

∆𝐻 = 𝑇(𝐴 𝐿𝐸 -𝐴 𝐿𝐶 ) 𝑑𝜋 𝐿𝐸-𝐿𝐶 𝑑𝑇 = 𝑇𝐴 𝑑𝜋 𝐿𝐸-𝐿𝐶 𝑑𝑇
where πLE-LC is the value of the LE-LC pressure of the phase transition plateau and ALE and ALC the molar areas at the corresponding transition temperature T (Figure S4). The subscripts LE and LC refer to the liquid expanded and liquid condensed states respectively (both LC1and LC2).

Assuming a linear variation of πLE-LC with temperature, one obtains a coefficient dπLE-LC/dT = 1.8 mNm -1 K -1 . The calculated enthalpies versus temperature are given in Table S6. H decreases linearly with temperature leading to a critical temperature of about 27-28°C (Figure S5), in agreement with the isotherms on Figure 1.

However, one notes on Figure 2 that the LC1-LC2 transition presents an unusual behavior as LC1-LC2 decreases upon increasing the temperature. Assuming a linear variation of πLC1-LC2 with temperature, one obtains a coefficient dπLC1-LC2/dT = -2 mNm -1 K -1 . The enthalpy variation for this endothermic transition is calculated as above with ∆𝐻 = 𝑇(𝐴 𝐿𝐶1 -𝐴 𝐿𝐶2 ) 𝑑𝜋 𝐿𝐶1-𝐿𝐶2

𝑑𝑇

. The results are presented Table S7. The enthalpy variation for the LC1-LC2 transition is one order of magnitude lower than the one for the LE-LC transition.

Brewster Angle Microscopy. Figure 3 presents Brewster Angle Microscopy (BAM) images as recorded during compression at T = 13 ± 0.5°C. This temperature was chosen below the triple point so the LE phase is not observed. Up to 15 mNm -1 (Figure 3a-c), one observes LC1 domains coexisting with the G phase. These domains present a curved shape suggesting a rather small line tension value. When the surface pressure reaches 24 mNm -1 , which corresponds to the transition between the two LC phases detected by the isotherms, the large LC1 domains split into smaller domains as shown on Figure 3d. One can see on this figure that the reflected intensity is similar for the two types of domains, which indicates a negligible variation of the optical index of the two domain types and, hence, a constant thickness and density.

X-ray reflectivity. X-ray reflectivity (XRR) of 13-4OH monolayer were performed at 14 ± 0.5°C upon compression and expansion. A single layer model did not allow for a reasonable adjustment of the reflectivity spectra. Consequently, a two-layers model was used, labeled bottom and top layers for the one in contact with the water subphase and the one in contact with air, respectively. The resulting fitted parameters are presented in Table 2 and the variation of the thicknesses versus surface pressure in Figure 4. Attempts in adjusting the spectra with a three layers model lead to unsatisfactory results since the intermediate layer thickness associated with the diacetylene group is always thinner that its roughness. The reflectivity spectra and adjustments with the two-layer model are presented in Figure S8. Even if the total thickness of the film remains constant at about 1.7 nm, one observes a variation of the thickness of each layer for surface pressure varying between 18 and 22 mN.m -1 (Figure 4). This result is thought to be a signature of the transition between the two LC phases. Assuming an all-trans carbon chain that can be described as a rod with a length of 2.45 nm, one obtains a tilt angle of 45°, in the range of the one measured by GIXD (see next paragraph). This suggest that the measured thickness is associated to the chain indicating that the contrast between the average densities of the water subphase and the one of the hydrophilic group region remains weak. These results appear reversible upon expansion.

Reflectivity experiments on polymerized monolayer formed by molecules wearing two diacetylene chains with a bulky headgroup has been fitted with three layers (one representing this hydrophilic head group) [START_REF] Bourdieu | Polymerization of a diacetylenic phospholipid monolayer at the air-water interface[END_REF]. A four-layer model has been used only when the film was polymerized. Our two layers model is in the same category since 13-4OH wears a small hydrophilic headgroup and the monolayer is not polymerized.

Grazing Incidence X-ray Diffraction. 13-4OH Langmuir films were further investigated at the air/water interface using in situ synchrotron grazing incidence x-ray diffraction (GIXD). We notice that 13-4OH monolayer is not reactive under x-ray nor UV irradiations. Figure 5 presents the diffraction profiles obtained at 12°C for  = 1, 5, 10, 15, 20 and 25 mNm -1 . The corresponding integrated scattered intensity along Qz with respect to Qxy are presented in Figure S9. Peak parameters extracted through fits of the experimental curves are gathered in Table 3. At low surface pressure (1 mNm -1 ), 13-4OH monolayer exhibits two diffraction peaks: one at Qxy=12.82 nm -1 and with a maximum intensity at Qz=9.32 nm -1 (out of plane) and one at Qxy=13.86 nm -1 with a maximum intensity in the plane. The FWHM of their rodscan along Qz range between 1,8 nm -1 and 1 nm -1 which indicates that the thickness of the corresponding diffracting plane remains to the order of the one of the monolayer (figure S9). These peaks can be indexed as [-1,1] and [0,2] respectively by using a rectangular unit cell which parameters for the various surface pressure are presented in Table S10. The configuration of the [-1,1] and [0,2] peaks indicates a Next Neighbor (NN) tilt of the chains with a tilt value around 40 degrees as reported in Table S10, of the same order of magnitude to the one deduced from XRR (45 degrees). Upon compression, both peaks shift to higher Qxy as the out-of-plane Qz maximum of the [-1,1] peak decreases, indicating a gentle decrease of the tilt angle. A microscopic compressibility can be determined from the area of the rectangular unit cell and one obtains m =2.4 m/N, in agreement with the value expected for a LC phase [START_REF] Fradin | Microscopic measurement of the linear compressibilities of two-dimensional fatty acid mesophases[END_REF]. One notice (Table S10) that only the a parameter is varying with , evidencing that the compressibility is almost zero along the b direction, which is compatible with a tilt along the a direction. It can be also noticed that the coherence length is 6 times larger along the b direction than along the a direction. Such an anisotropy has been observed in long fatty acids [START_REF] Fradin | Microscopic measurement of the linear compressibilities of two-dimensional fatty acid mesophases[END_REF], long fatty alcohols at zero pressure [START_REF] Majewski | The Structural Properties of Uncompressed Crystalline Monolayers of Alcohols CnH2n+1OH (n = 13-31) on Water and Their Role as Ice Nucleators[END_REF] and in amphiphilic alcohols [START_REF] Popovitz-Biro | Induced Freezing of Supercooled Water into Ice by Self-Assembled Crystalline Monolayers of Amphiphilic Alcohols at the Air-Water Interface[END_REF]. However, the most striking result is that at higher pressures the diffraction peaks vanish when the surface pressure increases as shown on Figure 5.

Contrary to the XRR measurements, the transition between LC1 and LC2 phases is clearly detected by GIXD. Upon compression, one observes a decrease of the intensities at the transition (Figure 5). Simultaneously, the coherence length of the [-1,1] peak remains almost constant up to 15 mNm -1 , and strongly decreases upon further compression as the coherence length of the [0,2] peak decreases continuously when the surface pressure increases (Table 3). This clearly, although surprisingly, indicates a transition to a disordered phase. One cannot link this change to a precollapse kinetic effect as it happens at a surface pressure far below the collapse pressure, 5 mNm - 1 below the collapse at 12°C and up to 20 mNm -1 below the collapse at 18°C. Moreover, XRR indicates that the film remains as a monolayer (constant layer thickness with surface roughness remaining in the same range). In addition, the isotherms (Figure S2) and the diffraction spectra (Figure 6) are reversible, which is not the case when the film collapse. The loss of signal starts by a broadening of the [0,2] peak, but the symmetry of the remaining in plane unit cell remains unchanged. Then, at a surface pressure corresponding to LC1-LC2 (second plateau of the isotherm), the signal vanishes completely indicating that 13-4OH monolayer undergoes a reversible loss of positional order under compression before collapse. This behavior is not usual as one expects a better order when the pressure is increased.

Molecular arrangement. Due to the limited diffraction peaks observed and the various possibilities for the chain configuration in the LC1 phase (induced by the possible rotation of the rigid diacetylene block), the diffraction data do not allow in deducing the conformation of the molecule in this structured phase. To shed some light into the molecular conformation of the 13-4OH chains, we performed a simple molecular dynamics (MD) approach. While an in-depth molecular simulation strategy is out of the scope of the present paper, the approach below aims in providing a simple tool to extract refined structural/conformational information from available experimental data. In practice, we built a molecular configuration consisting of a single monolayer adsorbed on the water subphase while constraining simple parameters (namely, the lattice parameter/symmetry) to their value inferred from the x-ray analysis. The schemes S11 and S12

give the labeled carbon atoms of 13-4OH and its orientation. The simulations are performed at a temperature T = 12°C where the monolayer is in the LC1 phase. To build a system as close as possible to the experimental one, the oxygen atoms of the alcohol headgroup has been settled to fixed positions corresponding to the experimentally observed structure. All the rest of the system (DA chains and water molecules) is free to relax.

The simulation leads to conformations in which the DA molecules are tilted by 44° along the 𝑎 direction. Table S13 summarizes the values of the different orientational angles calculated. Such configuration is compatible with the experimental results, suggesting that our simple molecular simulations capture this important structural feature. The thickness of the two distinct slices formed by (1) C≡C-C≡C-(CH2)3-CH2OH DA+head group) and ( 2) the saturated alkyl chain (CH3(CH2)12-) were analyzed. Thicknesses of 0.827 nm and 1.14 nm were obtained for the DA+head group and the alkyl chain, respectively. These values are reasonably close to the ones obtained by means of XRR experiments (0.742 nm and 1.12 nm, respectively). The diffraction pattern calculated from the simulated configurations is presented in Figure S14. There is also a reasonable agreement for both the [0,2] and [-1,1] peaks which are localized at Qxy = 14 nm -1 and Qxy = 13.3 nm -1 from the simulations as they are experimentally measured at Qxy = 13.94 nm -1 and Qxy = 13.1 nm -1 at 15 mNm -1 . The poorer agreement for the [-1,1] peak can be justified by the fact that, despite the presence of the di-acetylene groups the alkyl-alkyl interactions are considered as correctly treated by the OPLS force field (similar simulations performed on fatty alcohols are in good agreement with experimental data [START_REF] Hoang | Investigating molecular mechanism for the stability of ternary systems containing cetrimide, fatty alcohol and water by using computer simulation[END_REF]). Therefore, considering that the molecular mechanics approach reported in the present paper correctly describes the tilt angle and direction, we conclude that the small discrepancy between the simulated and experimental diffraction spectrum results from a less accurate force field description of the interactions between the DA group of the molecules.

Analogous DA acid 13-3COOH. In order to determine the influence of the head group interaction with respect to the DA interactions, we synthesized the fatty acid CH3-(CH2)11-CH2-C≡C-C≡C-(CH2)3-COOH (13-3COOH) which has the same hydrophobic chain with 13 saturated carbons. It appears then as analog to 13-4OH, but with a carboxylic acid head group. We probe the monolayer phase diagram by compression isotherms at temperatures between 10°C and 25°C (Figure S15), as well by BAM (Figure S16) and GIXD (Figure S17) experiments. As expected, the collapse occurs at a lower surface-pressure than the one observed for 13-4OH. More importantly, no signature of a transition between two LC phases can be detected, neither by isotherms nor by BAM experiments. GIXD measurements indicate a continuous growth of the peaks when the surface pressure increases with only a slight increase of the FWHM of the [-1,1] peak leading to coherence length ranging between 40 nm to 30nm. Such a result compared to the ones obtained with 13-4OH suggests that both the DA-DA interactions and the alcohol head group interactions are involved in this order-disorder phase transition. This evidences the importance of the alcoholic head group in the LC phases, although it is considered a "small head".

Discussion. The study of fatty alcohol and fatty acid monolayers at the air/water interface leads to a generic phase diagram which is normalized by the number of aliphatic carbon atoms [START_REF] Kaganer | Structure and phase transitions in Langmuir monolayers[END_REF][START_REF] Peterson | Equivalent states of amphiphilic lamellae[END_REF]. This generic phase diagram is mainly based on the chain interactions as the hydrophilic head interactions appear as negligible (small hydrophilic head approximation). This was expected for 13-4OH but the presence of the rigid 6 carbons of the DA group CH2-C≡C-C≡C-CH2 in the hydrophobic chain modifies the entropy of the chain configuration which is one of the parameters determining the LE-LC transition. Considering the thermodynamic behavior of the LE-LC transition, we deduced that 13-4OH is equivalent to a C15 aliphatic chain if one considers the Figure 3b of reference [START_REF] Kaganer | Structure and phase transitions in Langmuir monolayers[END_REF]). In the LE phase, the interactions between DA groups are rather weak and the transition to the LC phase appears at least mainly driven by the alkyl chains interactions with respect to their possible configurations.

This entropy term almost disappears in the LC phase in which it is however reasonable to consider that the DA-DA interactions are no longer negligible and probably also those between the hydrophilic head groups. We believe that these interactions are related to the origin of the unexpected order-disorder transition observed in the 13-4OH monolayer when the surface pressure increases. Indeed, an hypothesis based on the breaking of an H-bond network in the polar head region competing with the DA-DA interactions cannot be excluded to explain this transition. It is, to our knowledge, the first observation of such a transition in Langmuir monolayers. However, a partial loss of positional correlation has been observed under pressure increase in mono and bipolar docosanol derivatives [START_REF] Yue | Weak First-Order Tilting Transition in Monolayers of Mono-and Bipolar Docosanol Derivatives[END_REF] or in octadecylurea [26]. Majewski et al [START_REF] Majewski | The Structural Properties of Uncompressed Crystalline Monolayers of Alcohols CnH2n+1OH (n = 13-31) on Water and Their Role as Ice Nucleators[END_REF] have shown that alcohols monolayer can induce the nucleation of ice due to H-bonds between water molecules and OH group of the amphiphilic molecule. Moreover, the presence of an ice-like structure at the water surface covered by octadecanol monolayer was observed via IR spectroscopy of the OH group [27]. An area contraction upon temperature increase at constant pressure has been observed on monolayer with urea headgroups and was explained by an hydrogen-bond network formed between the urea headgroups [START_REF] Yue | Weak First-Order Tilting Transition in Monolayers of Mono-and Bipolar Docosanol Derivatives[END_REF]. In 13-4OH monolayer, the smallest distance between two oxygen atoms is 0.538 nm at  = 5 mNm -1 and 0.530 nm at  = 20 mNm -1 . Despite this rather large distance the range of the enthalpy variation, H = -1 kJ/mol, is of the same order of magnitude of the endothermic transition in docosanol H = 0.428 kJ/mol at 30°C and 0.844 kJ/mol at 35°C [START_REF] Yue | Weak First-Order Tilting Transition in Monolayers of Mono-and Bipolar Docosanol Derivatives[END_REF].

The properties of 13-4OH can be summarized as follows. The experimental results show that under compression, the monolayer undergoes a LE-ordered LC1 then an ordered LC1-disordered LC2 phase transitions. However, the LC1-LC2 phase transition is not observed in the analogous fatty acid 13-3COOH, which indicates that the OH head group is involved in this transition. The disorder could then result from a competition between DA-DA interactions and OH-OH and/or OH-H2O interactions when the pressure increases. At least two hypotheses can be proposed. In the first one, an hydrogen bond network appears between OH head group and water in the LC1 phase.

This hydrogen bond network should be weak since the unit cell parameters are large compared to those involving the usual H-bond length. The transition LC1-LC2 would then result from the breaking of this network. In the second one, on the contrary, the probability to form H-bond increases when the pressure increases leading to the disordered structure due to the competition with the LC1 structure resulting from aliphatic chains and DA interactions. The knowledge of the atomic conformation of the molecule would be of great help to understand this transition.

■ CONCLUSION

We synthesized a DA-OH molecule, 13-4OH, whose hydrophilic headgroup leads to stable monolayer in contrast to DA-COOH in which the trilayer structure appears stable with respect to the monolayer. This is an important feature for the future use of these systems as key ingredient in sensors. The thermodynamic analysis indicates that the DA group can be considered as equivalent to one CH2 in the LE phase. However, one observes that, besides the expected LE-LC transition, 13-4OH monolayer exhibits an unusual order-disorder transition between two LC phases as the surface pressure increases. It is reasonable to consider that the driving force of the order-disorder transition results from the competition between this DA-DA interaction and the interactions involved in the hydrophilic layer. 2). Bottom layer and top layer are the one in contact with the water subphase and the one in contact with the air respectively. (nm -1 ) 1

Molecular simulation.

  Molecular dynamics (MD) simulations of a 13-4OH monolayer on water were performed with LAMMPS [15]. The simulation box contains a slab of about 11000 water molecules in contact with vacuum on one side and with the hydroxyl end of a monolayer with 162 alcohol molecules on the other side (corresponding to a rectangular box with 9 × 9 unit cells as determined in the experiments). The ends of the alkyl chains of the alcohols are also exposed to vacuum. The equations of motion are integrated with the velocity Verlet algorithm using a time step of 5 fs. Periodic boundary conditions (PBC) were used in the three directions of space. The

Figure 1 :

 1 Figure 1: 13-4OH -A isotherms at different temperatures between 12°C and 28°C

Figure 2 .

 2 Figure 2. -T phase diagram of 13-4OH film at the air-water interface. Dot, square and inverted triangle are the surface pressure values of the collapse, LC1-LC2 and LE-LC transitions respectively (deduced from the isotherms). The nature of the LC phases is determined by GIXD. Straight lines are guides for the eyes.

Figure 3 .

 3 Figure 3. BAM images when compressing 13-4OH monolayer at T = 13°C. a) π = 2.4 mNm -1 , b) π = 4.7 mNm -1 , c) π = 15 mNm -1 and d) π = 24 mNm -1 . Figures a, b and c correspond to the LC1 phase and d to the LC1-LC2 transition. The image size is 425 x 350 μm 2 . All images are obtained on the same sample.

Figure 4 :

 4 Figure 4: Thickness of each layer and total thickness versus surface pressure π at T=14°C, deduced from the adjustment of the reflectivity spectra with a two layers' model (data reported from Table

Figure 5 .

 5 Figure 5. Grazing incidence X-ray diffraction spectra: Qxy-Qz intensity maps for 13-4OH monolayer at T = 12°C at different surface pressures  = 1, 5, 10, 15, 20 and 25 mNm -1 .

Figure 6 .

 6 Figure 6. Qz integrated GIXD spectra of 13-4OH monolayer at 12°C: up to bottom:  = 1 mNm -1 in compression,  = 25 mNm -1 and  = 1 mNm -1 in decompression.

Table 1 .

 1 Values of molecular area of lift-off and surface pressures of LE-LC1 and LC1-LC2 transition plateau measured on the -A isotherms.

	Temperature (°C)	12	14	16	18	20	22	24	28
	Lift-off		0.35	0.38	0.44	0.46	0.47	0.48	0.51	0.5
	(nm 2 /molecule)
	πLE-LC1 (mNm -1 )			1.5	4	7	10.5	14
	πLC1-LC2 (mNm -1 )	26	22	18.5	14
	Collapse	pressure	30	29	30	31	30	30	29	32
	(mNm -1 )

Table 2 :

 2 Reflectivity parameters at T = 14°C

	surface	bottom	bottom layer	bottom layer	top layer	top layer	top layer
	pressure	layer	thickness	roughness	density	thickness	roughness
	(mNm	-1 )	density	(nm)	(nm)	(unit/Ǻ 3 )	(nm)	(nm)
			(unit/Ǻ 3 )
	3 (LC1)	0.0044	0.678	0.225	0.0030	0.9035	0.304
	14 (LC1)	0.0048	0.581	0.609	0.0030	1.119	0.303
	18 (LC1)	0.0048	0.34	0.397	0.0030	1.311	0.323
	22(LC2)	0.0048	0.346	0.238	0.0030	1.264	0.36
	26(LC2)	0.0044	0.742	0.302	0.0030	1.123	0.343

Table 3 :

 3 Peak positions Qxy (nm -1 ) and coherence lengths  (nm) for pressures below the LC1-LC2 order-disorder transition at T = 12°C.

■ ASSOCIATED CONTENT

Supporting Information.

The unique file contains 6 items: 1. 13-4OH isotherms, 2. Enthalpy, 3. Reflectivity at T = 14°C, Email : michel.goldmann@insp.jussieu.fr

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.


Mario Santoro

Marta Morana

Demetrio Scelta

Jérôme Rouquette

Kamil Dziubek

Federico A Gorelli

Roberto Bini

Gaston Garbarino

Arie Van Der Lee

Francesco Di Renzo

Roberto Bini §▲

Benoît Coasne

Julien Haines

Insertion of Oxygen and Nitrogen in the Siliceous Zeolite TON at High Pressure

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Zeolites are natural or synthetic aluminosilicates built up of corner-sharing TO4 (T = Si, Al…) tetrahedra, which form a network of pores with typical diameters between 3 and 8 Å. These microporous materials are used at a large scale for applications in, for example, catalysis, adsorption, separation and ion exchange. In the recent years, we conducted several foundational studies on the high pressure (GPa) insertion of simple molecular systems into inert, purely siliceous zeolites, with an at least twofold purpose 1- 11 . On one hand, we aimed to obtain sub-nanophases of densely confined guest simple systems in the host zeolites and to compare the intermolecular interactions and distances in the confined matter to those of the corresponding bulk systems at the same pressures. These interactions were indeed found to be very similar in the two cases, thereby providing a rationale and also a guide for tailoring potential pressure induced chemical reactions in this sort of confined molecular matter [START_REF] Santoro | Intermolecular Interactions in Highly Disordered, Confined Dense N2[END_REF] . On the other hand, we aimed to obtain purely pressure induced chemical reactions such as the polymerization, in the case of simple Cbased molecules as the guest species. In this case, novel hybrid organic/inorganic, polymer/zeolites subnanocomposites were synthesized and recovered at ambient conditions [START_REF] Santoro | High-Pressure Synthesis of a Polyethylene/Zeolite Nano-Composite Material[END_REF][START_REF] Scelta | High Pressure Polymerization in a Confined Space: Conjugated Chain/Zeolite Nanocomposites[END_REF][START_REF] Santoro | High Pressure Synthesis of All-Transoid Polycarbonyl [-(C═O)-]n in a Zeolite[END_REF][START_REF] Santoro | Synthesis of 1d Polymer/Zeolite Nanocomposites under High Pressure[END_REF] , a class of materials which may exhibit a rich diversity of physical properties suitable for potential applications in modern micro-electronics/photonics/mechanics and energy storage. In this context, we note that these studies are connected to high-pressure effects that have been proposed to rationalize strong confinement effects on catalytic reactions and/or phase transitions [START_REF] Coasne | Effect of Pressure on the Freezing of Pure Fluids and Mixtures Confined in Nanopores[END_REF][START_REF] Long | Under Pressure: Quasi-High Pressure Effects in Nanopores[END_REF] .

One crucial, fundamental aspect in this research is the understanding of how the degree of filling of the inserted molecules depends on pressure and also how the structure of the host zeolite is affected. These two aspects were quantitatively investigated by means of X-ray diffraction on the purely SiO2 zeolite ZSM-22 (TON) filled by Ne and Ar [START_REF] Thibaud | Saturation, of the Siliceous Zeolite TON with Neon at High Pressure[END_REF] . TON is a 1D micro-channel system based on 10-membered rings of SiO4 tetrahedra with elliptical cross sections of 5.7 Å and 4.6 Å, respectively [START_REF] Barri | Structure of Theta-1, the First Unidimensional Medium-Pore High-Silica Zeolite[END_REF] . The insertion of the rare gas systems was found to strongly depend on the van der Waals (vdW) radius, equal to 1.5 Å and 1.8 Å in Ne and Ar, respectively, thereby differing by about 18%. Ne was found to completely fill the pores of TON at 4 GPa, achieving the saturation value of 12 Ne atoms per unit cell of the host zeolite, whereas the degree of filling with Ar was almost two times lower. Also, the compressibility and structural distortions, such as pore ellipticity, are considerably reduced in the case of Ne as compared to the argon-filled and the empty pore material [START_REF] Thibaud | High-Pressure Phase Transition, Pore Collapse, and Amorphization in the Siliceous 1d Zeolite, TON[END_REF] .

In this work, we aim to provide a more general view on pressure driven filling of TON by extending our investigation to the case of the insertion of the simplest possible molecular systems possessing orientational degrees of freedom: the homonuclear diatomic molecules. Specifically, we chose N2 and O2 as the test systems, since the high pressure phase diagram of these two bulk substances, which we use as a reference here, is well known in the literature [START_REF] Gregoryanz | High P-T Transformations of Nitrogen to 170 GPa[END_REF][START_REF] Freiman | Solid Oxygen[END_REF][START_REF] Freiman | Solid Oxygen Revisited[END_REF][START_REF] Reichlin | Optical Studies of Nitrogen to 130-GPa[END_REF][START_REF] Bini | Infrared and Raman Studies on High Pressure Phases of Solid N2: An Intermediate Structural Modification between Epsilon and Delta Phases[END_REF][START_REF] Olijnyk | Vibrational Dynamics of Isotopically Dilute Nitrogen to 104 GPa[END_REF][START_REF] Bini | High-Pressure Phases of Solid Nitrogen by Raman and Infrared Spectroscopy[END_REF][START_REF] Gregoryanz | Infrared, and X-Ray Evidence for New Phases of Nitrogen at High Pressures and Temperatures[END_REF][START_REF] Frost | Novel High-Pressure Nitrogen Phase Formed by Compression at Low Temperature[END_REF][START_REF] Goncharov | Triple Point on the Melting Curve and Polymorphism of Nitrogen at High Pressure[END_REF][START_REF] Goncharov | Optical Evidence for a Nonmolecular Phase of Nitrogen above 150 GPa[END_REF][START_REF] Gregoryanz | High-Pressure Amorphous Nitrogen[END_REF][START_REF] Eremets | Semiconducting Non-Molecular Nitrogen up to 240 GPa and Its Low-Pressure Stability[END_REF][START_REF] Eremets | Single-Bonded Cubic Form of Nitrogen[END_REF][START_REF] Tomasino | Pressure-Induced Symmetry-Lowering Transition in Dense Nitrogen to Layered Polymeric Nitrogen (Lp-N) with Colossal Raman Intensity[END_REF] . The present study on confined N2 and O2 in TON is based on Raman spectroscopy, X-ray diffraction, and Monte Carlo (MC) molecular modeling in the 0-20 GPa pressure range.

Experimental and theoretical methods

The TON sample was prepared by sol-gel techniques using triethylenetetramine as the structure directing agent followed by crystallization at 170°C under hydrothermal conditions and then calcination at 600°C [START_REF] Thibaud | High-Pressure Phase Transition, Pore Collapse, and Amorphization in the Siliceous 1d Zeolite, TON[END_REF][START_REF] Direnzo | Influence of Diffusional Barriers on the Thermal-Analysis of the Zeolite TON[END_REF][START_REF] Di Renzo | Crystallization Kinetics of Zeolite TON[END_REF] . Samples were prepared in which either oxygen or nitrogen was loaded with micron/submicron-size TON powder in stainless steel gasketed diamond anvil cells (DACs), with typical initial sample dimensions of 150-320 m in diameter and 40-50 m in thickness. We first homogeneously filled the gasket hole by TON powder and the powder was just gently compacted in order not to prevent the subsequent loading of the condensed gases. Either O2 or N2 was then loaded cryogenically at 77 K in its liquid phase at about 1 bar, and the liquid filled both the pores of the zeolite (confined O2 and N2) and residual empty space between the grains of TON (bulk O2 and N2). These samples were used for in situ Raman spectroscopy and X-ray diffraction (XRD) investigations under pressure. Ruby chips were inserted in the samples and pressure was measured based on the shift of the ruby R1 fluorescence line [START_REF] Mao | Calibration of the Ruby Pressure Gauge to 800 Kbar under Quasi-Hydrostatic Conditions[END_REF] . Raman signals were excited by the 647.1 nm line of a Kr + laser. Backscattering geometry was used through a 20× Mitutoyo micro-objective providing a beam spot of 2-3 m, and the scattered light was detected by a single monochromator (Acton/SpectraPro 2500i) equipped with super notch filters and a CCD detector (Princeton Instruments Spec-10:100BR) [START_REF] Ceppatelli | Probing High-Pressure Reactions in Heterogeneous Materials by Raman Spectroscopy[END_REF] with typical spectral resolution of 0.6 cm -1 .

X-ray diffraction (=0.41121 Å) was performed in a diamond anvil cell (DAC) on beamline ID15B at the ESRF synchrotron using a MAR555 flat panel detector. The nominal size of the focal spot was 30 μm.

The diffraction patterns were analyzed and integrated using the FIT2D program [START_REF] Hammersley | Two-Dimensional Detector Software: From Real Detector to Idealised Image or Two-Theta Scan[END_REF] . Rietveld refinements were performed using the program Fullprof [START_REF] Rodriguez-Carvajal | Magnetic Structure Determination from Powder Diffraction Using the Program Fullprof[END_REF] . Soft constraints were applied to the Si-O distances, O-O distances and/or the O-Si-O angles. Crystal structures were plotted using the program VESTA [START_REF] Momma | Vesta 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data[END_REF] .

Monte Carlo simulations in the Grand Canonical ensemble (GCMC) were performed to determine the number, the positions and orientations of O2 and N2 confined at room temperature in TON. Such atomscale simulations at constant chemical potential μ, volume V, and temperature T allow determining the number of adsorbed oxygen, nO2(), and nitrogen, nN2(), molecules per zeolite unit cell as a function of chemical potential   . For a given  and T, the corresponding pressure P was obtained according to the following strategy [START_REF] Desbiens | Water Condensation in Hydrophobic Silicalite-1 Zeolite: A Molecular Simulation Study[END_REF][START_REF] Coasne | Poroelastic Theory Applied to the Adsorption-Induced Deformation of Vitreous Silica[END_REF] . We perform GCMC simulations of bulk O2 and bulk N2 to obtain the relationship between chemical potential  and density, (μ). On the other hand, the relationship (P) between pressure P and density  is determined using Monte Carlo simulations in the isobaric-isothermal ensemble (NPT).

From (μ) and (P), μ(P) can be determined readily. For each gas, the pressures (or chemical potential) and unit cell parameters obtained from the experiments were used to perform these molecular simulations.

A TON structure made of 2 × 2 × 5 rigid unit cells was considered and periodic boundary conditions were used along the x, y, z directions to avoid finite size effects. O2 and N2 were described in these atom-scale simulations using all-atom potentials from the TRAPPE force field [START_REF] Potoff | Vapor-Liquid Equilibria of Mixtures Containing Alkanes, Carbon Dioxide, and Nitrogen[END_REF][START_REF] Zhang | Direct Calculation of Henry's Law Constants from Gibbs Ensemble Monte Carlo Simulations: Nitrogen, Oxygen, Carbon Dioxide and Methane in Ethanol[END_REF] . In more detail, for these two molecules, each atomic site is a Lennard-Jones center with the following parameters:  = 0.302 nm, /kB = 49 K for the O atom in O2 and  = 0.331 nm, /kB = 36 K for the N atom in N2. In addition, to describe the electric quadrupole moment, each of these atomic sites carries a partial charge; qO = -0.113e and qN = charge neutrality, the center of mass carries a charge -2q. Interactions with the O and Si atoms of zeolite TON were also described using the Lennard -Jones potential and the Coulomb electrostatic potential.

The cross parameters  and  were determined by combining the like-atom parameters using the Lorentz -Berthelot rules (the like-atom parameters for zeolite TON are  = 0.3795 nm, /kB = 64.2 K for Si and  = 0.3154 nm, /kB = 78.0 K for O). As for the electrostatic charges on the Si and O atoms of the zeolite, there are taken from the force field for silica by Carre et al. [START_REF] Carré | New Fitting Scheme to Obtain Effective Potential from Car-Parrinello Molecular-Dynamics Simulations: Application to Silica[END_REF]

Results and discussion

Raman spectra of the molecular stretching were measured up to about 20 GPa on O2/TON and N2/TON mixtures, respectively (figure 1). In all cases, close to the peaks for the bulk systems, other much weaker yet clearly visible peaks appear, which we easily assign to confined O2 and N2. In the O2/TON mixture at 0.5 GPa, we observe one single peak for confined O2, and the two peaks of confined and bulk O2 are rather sharp and well separated with the peak for confined O2 positioned at slightly lower frequencies.

Both peaks then suddenly broaden upon increasing pressure up to about 5.5 GPa, and the peak for confined O2 appears as a not resolved, low frequency shoulder of the much sharper bulk O2 peak. Above 5.5 GPa, bulk oxygen solidifies ( phase) and the corresponding peak sharpens significantly, while the peak for confined O2 remains quite broad and much better distinguishable from the peak of the bulk phase. Above 10 GPa, at the - phase transition for pure O2, the bulk O2 peak jumps down to lower frequencies, due the formation of weakly bonded (O2)4 tetramers, which are the molecular units of the  phase. Correspondingly, confined oxygen displays two distinct peaks, slightly sharper than those observed below 10 GPa, now positioned at higher frequencies with respect to the peak of pure oxygen, which originate from weakly bonded (O2)2 dimers mixed with monomeric units both forming the confined material, as shown by us recently [START_REF] Santoro | Dense, Subnano Phase of Clustered O2[END_REF] . In N2/TON mixtures, we observe two distinct peaks for confined nitrogen at 2.8 GPa, at slightly lower and higher frequencies, respectively, with respect to the much stronger single peak of solid bulk nitrogen indicating that we have at least two symmetry independent molecules per unit cell. Upon increasing pressure, the two peaks for confined nitrogen become more separated, but also increasingly broader, while solid nitrogen displays the sharp 1 and 2 peaks, in both  and  phase. Our spectroscopic investigation allows us to draw a few important observations on the inserted molecular systems. Firstly, pressure shift of frequency for the confined and bulk systems are roughly similar. Also, at least for nitrogen, where two crystalline components are observed above 5.0 GPa in the pure system with a frequency spread that increases with pressure, we observe that the pressure behavior of the frequency spread for the confined and bulk systems are very similar to one another. These facts suggest that intermolecular interactions and, as a consequence, intermolecular distances are very similar in the two cases, the confined and the bulk system, as already observed by us in the O2/TON and the N2/silicalite systems [START_REF] Santoro | Intermolecular Interactions in Highly Disordered, Confined Dense N2[END_REF][START_REF] Santoro | Dense, Subnano Phase of Clustered O2[END_REF] . A second important observation deals with the confined systems displaying two Raman peaks, clearly visible for oxygen above 10 GPa and for nitrogen starting from low pressure. This is a first hint that at least two crystallographically independent, inserted molecules should sit within the primitive cell of TON. This finding is quantitatively assessed below, by the XRD investigation. A third very important outcome from the Raman studies stems from the pressure behavior of the integrated, total intensity ratio for confined O2 and N2, normalized to the integrated intensity for the bulk peak (in the case of N2, only the 1 peak was considered above 5.0 GPa). These intensities have been obtained after a fitting procedure of the measured peaks to pseudo-Voigt functions. In figure 2, we report the normalized intensity for confined O2 and N2 vs. pressure up to 10 GPa, which is the range of pressures where Rietveld refinements of our XRD data are also available for a quantitative comparison. Incidentally, in the case of oxygen, we note that although the Raman intensity can change upon pressure increase because of changes in the molecular electronic structure [ref. 17-18], the normalized quantity reported in figure 3 should not be affected by these changes, which indeed reasonably occur for confined and bulk oxygen to the same extent. The normalized intensity for the confined matter clearly increases upon increasing pressure, reaching a sort of saturation above 3-4 GPa. Fitting of data points to an empirical increasing exponential function: A[1-exp(-P/Pr)], leads to a typical pressure grow rate of Pr=1.8 GPa. It is very straightforward to assign this type of growing mainly to the increasing number of inserted molecules with pressure in the host zeolite, a result which is quantitatively confirmed below by XRD. Powder XRD patterns were measured up to 20 GPa on TON in oxygen and in nitrogen. The data obtained above 10 GPa for oxygen were subject to extremely strong contamination from the diffraction due to bulk -oxygen. In both cases changes in relative intensity were observed due to pore filling and TON remained crystalline up to the highest pressure reached, Figure 3. Rietveld refinements were performed up to 10 GPa for both systems, which except for the lowest pressure point at 0.45 GPa in the TON-O2 experiment, correspond to the high pressure Pbn21 phase of TON. Fourier difference maps were calculated. Strong peaks were found in the pores and these peaks were used to place the guest molecules in the unit cell, which were modelled using isoelectronic silicon and sulfur atoms for N2 and O2 molecules, respectively. The results indicate that the N2 and O2 molecules enter the pores, but not the smaller cages and rings of the framework. The structure was then refined using the same model as in previous work using soft constraints on intratetrahedral Si-O and O-O distances and including some carbon atoms to account for the residue arising from the calcination of the structure directing agent [START_REF] Thibaud | High-Pressure Phase Transition, Pore Collapse, and Amorphization in the Siliceous 1d Zeolite, TON[END_REF] . Reasonable agreement factors were obtained and the refined framework was used as the starting point for the MC molecular modelling to obtain a more meaningful structural model. The result from MC models were then used as starting points for the refinements at various pressures. It became clear that no improvement to the fits were obtained using individual diatomic molecules and thus the isoelectronic atoms were used in the refinements, Figure 4, to determine the centers of gravity of the diatomic molecules. The diatomic molecules in both systems partially occupy two sets of 4a Wyckoff sites (x,y,z) (Figures 5 and CIF files in SI).

The exception is the low pressure point at 0.45 GPa in Cmc21 phase of O2-filled TON, in which a set of 4a Wyckoff sites (0,y,z) is occupied. These results are consistent with the Fourier difference maps.

A strong increase in the number of molecules per unit cell occurs up to 3 GPa for O2 and N2, pressure above which the occupancy saturates in both cases at 6 molecules per unit cell and thus 3 molecules per pore distributed with 75 % probability on four sites (Figure 6). The filling curve is slightly shifted to higher pressure for N2. Less than full occupation of these sites could be partially related to the presence in the pores of a residue from the structure directing agent remaining after pyrolysis of the zeolite [START_REF] Thibaud | High-Pressure Phase Transition, Pore Collapse, and Amorphization in the Siliceous 1d Zeolite, TON[END_REF] . The higher intrusion pressure for nitrogen can be linked to its larger kinetic diameter and is in good agreement with the results of the simulations discussed below. The degree of filling is much greater than the saturation at 3.7 nitrogen molecules/UC [START_REF] Direnzo | Influence of Diffusional Barriers on the Thermal-Analysis of the Zeolite TON[END_REF] found by adsorption isotherm data. The filling data are similar to previous results for argon in terms of number of guest species per unit cell and much lower than the corresponding value of 12 for Ne, which has a significantly smaller kinetic diameter.

Figure 6 shows the simulated adsorption isotherms obtained for N2 and O2 at room temperature in TON.

As mentioned above, the pressures selected for each gas in the GCMC simulations correspond to the experimental data points. The adsorbed amount of N2 increases up to 5 N2 molecules per unit cell upon increasing the pressure to ~3 GPa and then remains constant at about 5.5 N2 per unit cell. In contrast, for O2 in a similar pressure range, the adsorbed amount increases from about 4 O2 molecules per unit cell to about 8 O2 molecules per unit cell for increasing the pressure to P < 2 GPa and then decreases to about 6 O2 molecules upon further increasing the pressure beyond P > 2 GPa. Even if O2 has a slightly longer bond length than N2, the larger maximum adsorbed amount for O2 compared to N2 can be explained by the slightly smaller Lennard-Jones size parameter for the former ( = 0.302 for O2 and  = 0.331 nm for N2). Overall, the behavior observed for these two gases can be rationalized as follows. At low pressures, upon compression, despite the observed unit cell volume decrease, the adsorbed amount increases as a result of the increasing gas chemical potential (we recall that at constant pressure the gas pressure and chemical potential are directly related). However, for large pressures, the unit cell compression prevails so that the volume accessible for adsorption -and hence the adsorbed amount -decreases. The strong increase followed by a decrease is not observed experimentally for the case of O2-TON. This is probably due to kinetic factors as the filling curves are shifted to higher pressure in the experiments as compared to the simulations. The large increase followed by a decrease in the case of oxygen below 5 GPa in the MC simulations corresponds to the pressure range over which the oxygen is in the fluid state. Instead, at higher pressures good agreement is observed, and the number of molecules is constant or only decreases slightly at which the bulk N2 and O2 are solid. As a consequence, the relative volume remains systematically much higher and the compressibility lower than that of TON in non-penetrating pressure medium DAPHNE7474. As in the simulations, the filling curve of nitrogen is shifted to higher pressures with respect to that of oxygen due to its larger size. Pore filling has a major effect on the compressibility of TON (Figure 7). The compressibility of TON is significantly lower in N2 and O2 than for empty TON [START_REF] Thibaud | High-Pressure Phase Transition, Pore Collapse, and Amorphization in the Siliceous 1d Zeolite, TON[END_REF] . The values for the cell parameters and volume are slightly lower in N2 as compared to O2 below 3 GPa due to the lower degree of filling of N2 up to this pressure. A similar effect is seen in argon. The compressibility is greater than in the case of neon, which fills TON to a much greater extent. In all cases filled TON compresses to a much lesser degree than empty TON as could be expected. Compression is highly anisotropic with the a direction being particularly stiff (Figure 7) in all filled systems including neon. Compressibility along c is also similar to that observed in Ne. The compressibility along b is much greater in N2 and O2. This can be linked to the high distortion in the xy plane due to pore collapse, which is shifted to significantly higher pressure in neon due to the higher degree of filling. Pore collapse measured by changes in pore ellipticity is similar in all systems except neon, including empty TON (Figure 8). MC simulations can provide further insight concerning the orientations of the inserted guest molecules.

Following previous studies on nanoconfined fluids [START_REF] Coasne | Adsorption, Structure and Dynamics of Benzene in Ordered and Disordered Porous Carbons[END_REF][START_REF] Coasne | Structure and Dynamics of Benzene Confined in Silica Nanopores[END_REF] , we calculated the angular distribution P(cos )

for O2 and N2 confined in TON at room temperature where  is the angle between the molecule axis and the z-axis (this axis corresponds to the channel axis in TON). As shown in Fig. 9, for O2, a non-monotonous behavior is observed. O2 does not show any preferential orientation at the lowest pressure, suggesting that the cavities are large enough to allow for large orientational diffusion. Upon increasing the pressure (from ~2 to ~5 GPa), the confined O2 exhibits a preferred orientation, cos  ~ 0, with their axis that tends to be perpendicular to the channel axis in TON (z-axis). As the pressure keeps increasing (> 6 GPa), the cavities become smaller and smaller and a bimodal angular distribution is observed. In this regime, more and more molecules tend to be aligned with the channel axis (cos  ~ 0.8-0.9), the other molecules exhibit a second preferential orientation with cos  ~ 0 (perpendicular to the pore axis). Interestingly, the data shown in Fig. 9 suggest that a similar scenario is observed for N2 confined in TON, but orientational features are less pronounced compared to O2. At the lowest pressure, like for O2, the N2 molecules do not display a significantly marked angular distribution (even if a slightly preferred orientation with their axis perpendicular to the channel axis seems to be observed, i.e. cos  ~ 0). Upon increasing the pressure from 2 to 6 GPa, the confined N2 exhibits a preferred orientation with cos  ~ 0.8 GPa. If the pressure is further increased (> 6 GPa), as observed for O2, N2 exhibits a bimodal angular distribution with a large number of molecules aligned perpendicular to the channel axis (cos  ~ 0) and a non-negligible number of molecules with cos  ~ 0.8.  is the angle between the molecule axis and the z-axis which corresponds to the channel axis in TON. For O2, the color code is P = 1.05 GPa (black), 1.95 GPa (purple), 3.5 GPa (blue), 5.09 GPa (orange), 8.17 GPa (green), and 9.75 GPa (red). For N2, the color code is 1.5 GPa (black), 2.25 GPa (purple), 2.6 GPa (blue), 4.3 GPa (orange), 6.3 GPa (green), and 8.7 GPa (red). For each gas, we also show a typical molecular configuration with the same color code as in Fig. 5 (the red and blue spheres are the O and N atoms in O2 and N2, respectively).

Conclusions

Strong agreement was observed between the results of Raman spectroscopy and X-ray diffraction in terms of pore filling and the presence of two sites for the diatomic molecules, which exhibit a significant degree of orientational disorder. The filling curve in nitrogen is shifted to slightly higher pressure due to its larger kinetic diameter. MC modelling indicates that as a function of pressure, the orientational distribution of the O2 molecules exhibit the following sequence -isotropic-perpendicular to the channel axisbimodal, whereas for N2 it is isotropic-parallel to the channel axis-bimodal, but less pronounced. TON is an ideal model system to study confined O2 and N2 due to its 1D pore system and its relatively small unit cell for a zeolite. The observed filling behavior of O2 and N2 could be expected to be somewhat similar in more complex 3D systems such as MFI [START_REF] Santoro | Intermolecular Interactions in Highly Disordered, Confined Dense N2[END_REF] , although the larger pore diameter, the presence of two types of pores and channel intersections in this 3D system could favor a higher degree of disorder.

Figure 1 .

 1 Figure 1. Selected Raman spectra of O2 /TON and N2 /TON mixtures measured upon increasing pressure. Loga-

Figure 2 .

 2 Figure 2. Red and blue circles: integrated Raman intensity for confined O2 (red) and N2 (blue) normalized to the

Figure 3 .

 3 Figure 3. X-ray diffraction patterns of TON in O2 and N2 as a function of pressure (=0.41121 Å). The diffraction

Figure 4 .

 4 Figure 4. Experimental (black), calculated (red) and difference (blue) profiles for the the Pbn21 structure of TON-

Figure 5 .

 5 Figure 5. Density map (left) and projection of several unit cells along z (center) from MC modelling

Figure 6 .

 6 Figure 6. Experimental (solid symbols) and simulated (open symbols) adsorption isotherms for O2 (red circles) and

Figure 7 .

 7 Figure 7. Relative cell parameters and volume (V/V0) of TON in N2, O2 and Ar (V/V0) 10 as a function of pressure (solid symbols). Dotted and dashed lines represents the behavior of TON in Ne 10 and DAPHNE7474 15 , respectively. Error bars are smaller than the symbol size.

Figure 8 .

 8 Figure 8. Pore ellipticity (difference between maximum and minimum diameters of the elliptical pores in TON) as a function of pressure of TON in N2 and O2, Ne 10 , Ar 10 and DAPHNE7474 15 . Error bars are

Figure 9 .

 9 Figure 9. Simulated angular distributions P(cos ) for O2 (left) and N2 (right) confined in TON at room temperature

ACKNOWLEDGMENT

We acknowledge funding from the Italian Ministry of Education, Universities and Research, MIUR PRIN project ZAPPING, number 2015HK93L7. We also thank the Deep Carbon Observatory (DCO) initiative under the project Physics and Chemistry of Carbon at Extreme Conditions and the Ente Cassa di Risparmio di Firenze under the project Firenze Hydrolab 2. K.D. acknowledges the Polish Ministry of Science and Higher Education for financial support through the "Mobilność Plus" program. Some of the computations in this paper were performed using the Froggy platform of the GRICAD infrastructure (https://gricad.univ-grenoble-alpes.fr), which is supported by the Rhône-Alpes region (GRANT CPER07-13 CIRA) and the Equip@Meso project (reference ANR-10-EQPX-29-01) of the programme Investissements d'Avenir supervised by the Agence Nationale de la Recherche. ⁋ Present address: Department of Chemistry and INSTM, University of Pavia, Viale Taramelli 16, Pavia 27100, Italy

ASSOCIATED CONTENT

Supporting Information. Crystallographic information files (CIF) file containing the structural data for the N2-TON system at 4.3 and the O2-TON system at 4.5 have been supplied as supporting information.

This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*Julien Haines

Julien.Haines@umontpellier.fr

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interests.

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