Denis Morineau

Finite-size and surface effects on the glass transition of liquid toluene confined in cylindrical mesopores

Some of the most regular porous silicates (MCM-41 and SBA-15), with several different pore diameters from 2.4 to 8.7 nm, are used to study the van der Waals fragile liquid toluene in confined geometry. We measure two major macroscopic signatures of a glass transition, i.e., a discontinuous change in the heat capacity and in the thermal expansion, by adiabatic calorimetry and neutron scattering experiments. A nontrivial size dependence of the glass transition features, most notably a nonmonotonic variation of the mean glass transition temperature, is observed. The range of the glass transition is found extremely broad. This supports the notion of competition between surface boundary conditions and cutoff or finite-size effects.

Continuous Paranematic-to-Nematic Ordering Transitions of Liquid Crystals in Tubular Silica Nanochannels

The optical birefringence of rodlike nematogens (7CB, 8CB), imbibed in parallel silica channels with 10 nm diameter and 300 microm length, is measured and compared to the thermotropic bulk behavior. The orientational order of the confined liquid crystals, quantified by the uniaxial nematic ordering parameter, evolves continuously between paranematic and nematic states, in contrast to the discontinuous isotropic-to-nematic bulk phase transitions. A Landau-de Gennes model reveals that the strength of the orientational ordering fields, imposed by the silica walls, is beyond a critical threshold, that separates discontinuous from continuous paranematic-to-nematic behavior. Quenched disorder effects, attributable to wall irregularities, leave the transition temperatures affected only marginally, despite the strong ordering fields in the channels.

Hydrogen-bond-induced clustering in the fragile glass-forming liquid m-toluidine: Experiments and simulations

We study the “prepeak” appearing in the static structure factor of the molecular glass-former m-toluidine by means of neutron scattering experiments and Monte Carlo simulations. The occurrence of this prepeak is interpreted as resulting from spatial organization of the molecules that goes beyond the usual short range liquid order and has a typical length scale of several molecular diameters. The origin of this phenomenon, as well as its specific temperature and density dependence, is explained by the competition between hydrogen-bonding interactions that tend to favor clustering and steric hindrance between aromatic rings that limits the extension of the H-bond network. Finally, effects of such clustering on the relaxational properties of the liquid and on the glass transition are discussed.

Liquids in confined geometry: How to connect changes in the structure factor to modifications of local order

The recent advances in the syntheses of mesostructured porous silicates (MCM-41 and SBA-15) allow us to study liquids confined in highly regular geometry. Hence, one might get to a better understanding of the structure and the dynamics of confined fluids. In this paper, we address the problem of the interpretation of the structure factor of a confined phase. Distortions due to geometric effects—so-called “excluded volume effects” and “cross-correlation terms”—may dominate the observed features and cannot be ignored. We present a generalization of the formalism introduced by Soper et al. It is applied in the case of a honeycomb-type lattice of parallel cylindrical pores, which corresponds to the topology of these novel porous materials. It shows that the large variations of the experimental structure factor of confined liquid benzene at room temperature are essentially attributed to an “excluded volume effect” that does not reflect different local ordering of the confined phase.

Structure of liquid and glassy methanol confined in cylindrical pores

We present a neutron scattering analysis of the density and the static structure factor of confined methanol at various temperatures. Confinement is performed in the cylindrical pores of MCM-41 silicates with pore diameters D=24 and 35 A. A change of the thermal expansivity of confined methanol at low temperature is the signature of a glass transition, which occurs at higher temperature for the smallest pore. This is evidence of a surface induced slowing down of the dynamics of the fluid. The structure factor presents a systematic evolution with the pore diameter, which has been analyzed in terms of excluded volume effects and fluid-matrix cross correlation. Conversely to the case of Van der Waals fluids, it shows that stronger fluid-matrix correlations must be invoked most probably in relation with the H-bonding character of both methanol and silicate surface.

Evidence of anisotropic quenched disorder effects on a smectic liquid crystal confined in porous silicon.

We present a neutron scattering analysis of the structure of the smectic liquid crystal octylcyanobiphenyl (8CB) confined in one-dimensional nanopores of porous silicon films (PS). The smectic transition is completely suppressed, leading to the extension of a short-range ordered smectic phase aligned along the pore axis. It evolves reversibly over an extended temperature range, down to 50 K below the N-SmA transition in pure 8CB. This behavior strongly differs from previous observations of smectics in different one-dimensional porous materials. A coherent picture of this striking behavior requires that quenched disorder effects are invoked. The strongly disordered nature of the inner surface of PS acts as random fields coupling to the smectic order. The one-dimensionality of PS nanochannels offers perspectives on quenched disorder effects, of which observation has been restricted to homogeneous random porous materials so far.

Spin transition with a large thermal hysteresis near room temperature in a water solvate of an iron(III) thiosemicarbazone complex

The magnetic properties of the monohydrated ferric complex Li[Fe(5Brthsa)2]·H2O (H2-5Brthsa = 5-bromosalicylaldehyde thiosemicarbazone) have been investigated by SQUID and Mossbauer measurements. The S = 1/2 ↔ S = 5/2 spin transition of the ferric ion is accompanied by a quite broad hysteresis (ΔT = 39 K) centred around 313 K. The spin states involved are characterised by the quadrupole splittings ΔEQ(2T2) = 2.584 ± 0.002 mm s−1 and ΔEQ(6A1) = 0.338 ± 0.006 mm s−1 and the isomer shifts δIS (2T2) =+0.262 ± 0.001 mm s−1 and δIS (6A1) =+0.294 ± 0.002 mm s−1 at 77 and 360 K, respectively. A powder X-ray diffraction study at various temperatures demonstrates the occurrence of a crystallographic first-order phase transition of the lattice coupled to the spin conversion. The enthalpy and entropy variations associated with the transition have been estimated from DSC measurements at ΔH = 5.7 ± 0.5 kJ mol−1 and ΔS = 18 ± 2 J mol−1 K−1. The existence of a crystallographic first-order phase transition associated to the spin crossover is consistent with the cooperative character of the process. This phase transformation might originate from the modification of the extended hydrogen-bond network.

Molecular dynamics simulation of nanoconfined glycerol.

We present results from molecular dynamics simulations of liquid glycerol confined in a realistic model of a cylindrical silica nanopore. The influence of the hydrophilic surface and the geometrical confinement on the structure, hydrogen-bond lifetime, rotational and translational molecular dynamics are analysed. Layering and dynamical heterogeneities are induced by confinement. These features share some similarities with previous observations in simpler van der Waals glass-forming liquids. In addition, the specificity of glycerol as an associated liquid shows up in confinement by the formation of interfacial hydrogen bonds and some modifications of the in-pore hydrogen-bonding network. Confinement is also seen to influence the relaxation dynamics and the glassy behaviour in the supercooled state. These phenomena revealed by molecular simulation are important inputs for a better understanding of the many recent experimental results on confined glycerol and more generally for the possible manipulation of associated liquids in porous or fluidic devices.

Structure and dynamics of a Gay-Berne liquid crystal confined in cylindrical nanopores.

Gay-Berne liquid crystals confined in two cylindrical nanopores with different pore sizes were studied by molecular dynamics simulation. Their structure and dynamics properties were obtained and compared with those of the bulk. Our data show that confinement changes the bulk isotropic-to-nematic transition to a continuous ordering from a paranematic to a nematic phase. Moreover, confinement strongly hinders the smectic translational order. The molecular dynamics is characterized by the translational diffusion coefficients and the first-rank reorientational correlation times. Very different characteristic times and temperature variations in the dynamics are observed in confinement. Spatially resolved quantities illustrate that confinement induces predominant structural and dynamical heterogeneities.

Molecular dynamics of a short-range ordered smectic phase nanoconfined in porous silicon

4-n-octyl-4-cyanobiphenyl has been recently shown to display an unusual sequence of phases when confined into porous silicon (PSi). The gradual increase of oriented short-range smectic (SRS) correlations in place of a phase transition has been interpreted as a consequence of the anisotropic quenched disorder induced by confinement in PSi. Combining two quasielastic neutron scattering experiments with complementary energy resolutions, the authors present the first investigation of the individual molecular dynamics of this system. A large reduction of the molecular dynamics is observed in the confined liquid phase, as a direct consequence of the boundary conditions imposed by the confinement. Temperature fixed window scans reveal a continuous glasslike reduction of the molecular dynamics of the confined liquid and SRS phases on cooling down to 250 K, where a solidlike behavior is finally reached by a two-step crystallization process.