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Journal of Physical Chemistry C. 01/2012; 116(35):18990-18998.
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ABSTRACT: It has been previously reported that α-lactose could be totally amorphized by ball milling. In this paper we report a detailed investigation of the structural and microstructural changes by which this solid state amorphization takes place. The investigations have been performed by Powder X-ray Diffraction, Solid State Nuclear Magnetic Resonance ((13)C CP-MAS) and Differential Scanning Calorimetry. The results reveal the structural complexity of the material in the course of its amorphization so that it cannot be considered as a simple mixture made of a decreasing crystalline fraction and an increasing amorphous fraction. Heating this complexity can give rise to a fully nano-crystalline material. The results also show that chemical degradations upon heating are strongly connected to the melting process.
Carbohydrate research 11/2011; 346(16):2622-8. · 2.03 Impact Factor
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ABSTRACT: Tert-butanol is the molecular archetype of a variety of H-bonded systems that form micelle-like supermolecular clusters in the liquid state. This self-association process is characterized by a prepeak in the static structure factor. Recently, it has been shown by neutron scattering that this prepeak is totally suppressed in a nanoconfined geometry [Morineau, D.; Alba-Simionesco, C. J. Phys. Chem. Lett. 2010, 1, 7155.]. The authors have shown that excluded volume effects was one main ingredient of this suppression, but the question of the survival of H-bonded self-association in nanochannels has remained unresolved experimentally. From molecular dynamics simulations, we prove that self-association survives under confinement despite the absence of the prepeak. Furthermore, we show that its apparent suppression is due to a negative contribution arising from the liquid–porous solid correlations, which cannot be disentangled experimentally. Strikingly, the stability of micelle-like clusters surpasses the putative formation of interfacial H-bonds with surface silanols, leading to an unexpected hydrophobic interaction of tert-butanol with surface silica. This work highlights the foremost interest of combining neutron scattering and molecular simulations with a careful account of the complex situation encountered under confinement to better understand the molecular microstructure of H-bonded liquids.
08/2011;
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ABSTRACT: Commonly, the confinement effects are studied from the grand canonical Monte Carlo (GCMC) simulations from the computation of the density of liquid in the confined phase. The GCMC modeling and chemical potential (μ) calculations are based on the insertion/deletion of the real and ghost particle, respectively. At high density, i.e., at high pressure or low temperature, the insertions fail from the Widom insertions while the performing methods as expanded method or perturbation approach are not efficient to treat the large and complex molecules. To overcome this problem we use a simple and efficient method to compute the liquid's density in the confined medium. This method does not require the precalculation of μ and is an alternative to the GCMC simulations. From the isothermal-isosurface-isobaric statistical ensemble we consider the explicit framework/liquid external interface to model an explicit liquid's reservoir. In this procedure only the liquid molecules undergo the volume changes while the volume of the framework is kept constant. Therefore, this method is described in the Np(n)AV(f)T statistical ensemble, where N is the number of particles, p(n) is the normal pressure, V(f) is the volume of framework, A is the surface of the solid/fluid interface, and T is the temperature. This approach is applied and validated from the computation of the density of the methanol and water confined in the mesoporous cylindrical silica nanopores and the MIL-53(Cr) metal organic framework type, respectively.
The Journal of chemical physics 02/2011; 134(7):074104. · 3.09 Impact Factor
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ABSTRACT: Molecular simulations in the isothermal statistical ensembles require that the macroscopic thermal and mechanical equilibriums are respected and that the local values of these properties are constant at every point in the system. The thermal equilibrium in Monte Carlo simulations can be checked through the calculation of the configurational temperature, k(B)T(conf)=<|∇(r)U(r(N))|(2)>/<∇(r) (2)U(r(N))>, where ∇(r) is the nabla operator of position vector r. As far as we know, T(conf) was never calculated with the anisotropic Gay-Berne potential, whereas the calculation of T(conf) is much more widespread with more common potentials (Lennard Jones, electrostatic, ...). We establish here an operational expression of the macroscopic and local configurational temperatures, and we investigate locally the isotropic liquid phase, the liquid / vapor interface, and the isotropic-nematic transition by Monte Carlo simulations.
The Journal of chemical physics 01/2011; 134(3):034116. · 3.09 Impact Factor
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ABSTRACT: We study the smectic director structure of the rodlike liquid crystal 4-n-dodecyl-4'-cyanobiphenyl (12CB) confined in cylindrical cavities of 200 nm diameter in porous alumina templates by means of combined broadband dielectric spectroscopy, optical birefringence, and neutron scattering measurements. We show that the collective molecular orientation differs between entering the smectic A phase upon cooling from the isotropic state and entering the same phase upon heating while melting the confined crystal. We discuss this collective molecular realignment in terms of a competition between weak planar anchoring at the p-Al2O3/12CB interface and a preferred texture typical of the crystallization of rodlike molecules in nanochannels (Bridgman growth).
Physical Review E 07/2010; 82(1 Pt 1):011706. · 2.26 Impact Factor
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ABSTRACT: We report combined optical birefringence and neutron scattering measurements on the liquid crystal 12CB nanoconfined in mesoporous silicon layers. This liquid crystal exhibits strong nematic-smectic coupling responsible for a discontinuous isotropic-to-smectic phase transition in the bulk state. Confined in porous silicon, 12CB is subjected to strong anisotropic quenched disorder: a short-ranged smectic state evolves out of a paranematic phase. This transformation appears continuous, losing its bulk first-order character. This contrasts with previously reported observations on liquid crystals under isotropic quenched disorder. In the low temperature phase, both orientational and translational order parameters obey the same power law.
Physical Review E 03/2010; 81(3 Pt 1):031703. · 2.26 Impact Factor
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ABSTRACT: 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.
The Journal of chemical physics 07/2009; 130(23):234501. · 3.09 Impact Factor
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ABSTRACT: Glycerol and trehalose-glycerol binary solutions are glass-forming liquids with remarkable bioprotectant properties. In this paper, we address the effects of confining of these solutions in straight channels of diameter D=8 nm formed by porous silicon. Neutron diffraction and incoherent quasielastic neutron scattering are used to reveal the different effects of nanoconfinement and addition of trehalose on the intermolecular structure and molecular dynamics of the liquid and glassy phases, on a nanosecond timescale.
02/2009;
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ABSTRACT: We analyze the molecular dynamics heterogeneity of the liquid crystal 4-n-octyl-4'-cyanobiphenyl nanoconfined in porous silicon. We show that the temperature dependence of the dynamic correlation length xi_(wall) , which measures the distance over which a memory of the interfacial slowing down of the molecular dynamics persists, is closely related to the growth of the short-range static order arising from quenched random fields. More generally, this result may also shed some light on the connection between static and dynamic heterogeneities in a wide class of condensed and soft matter systems.
Physical Review E 11/2008; 78(4 Pt 1):040701. · 2.26 Impact Factor
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ABSTRACT: 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.
Physical Review Letters 11/2008; 101(18):187801. · 7.37 Impact Factor
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ABSTRACT: We report incoherent quasielastic neutron scattering experiments on the thermotropic liquid crystal 4-n-octyl-4'-cyanobiphenyl. The combination of time-of-flight and backscattering data allows analysis of the intermediate scattering function over about three decades of relaxation times. Translational diffusion and uniaxial molecular rotations are clearly identified as the major relaxation processes in, respectively, the nanosecond and picosecond time scales.
Physical Chemistry Chemical Physics 06/2008; 10(20):2993-9. · 3.57 Impact Factor
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ABSTRACT: We report incoherent quasielastic neutron scattering experiments on the
thermotropic liquid crystal 4-n-cyano-4'-octylbiphenyl. The combination of
time-of-flight and backscattering data allows analyzing the intermediate
scattering function over about three decades of relaxation times. Translational
diffusion and uniaxial molecular rotations are clearly identified as the major
relaxation processes in respectively the nanosecond and picosecond time scales.
The comparison with literature data obtained by other techniques is discussed.
04/2008;
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ABSTRACT: We present a neutron and X-rays scattering study of the phase transitions of
4-n-octyl-4'-cyanobiphenyl (8CB) confined in unidirectional nanopores of porous
alumina and porous silicon (PSi) membranes with an average diameter of 30 nm.
Spatial confinement reveals a rich polymorphism, with at least four different
low temperature phases in addition to the smectic A phase. The structural study
as a function of thermal treatments and conditions of spatial confinement
allows us to get insights into the formation of these phases and their relative
stability. It gives the first description of the complete phase behavior of 8CB
confined in PSi and provides a direct comparison with results obtained in bulk
conditions and in similar geometric conditions of confinement but with reduced
quenched disorder effects using alumina anopore membranes
The European Physical Journal E 04/2008; 26(3). · 1.94 Impact Factor
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ABSTRACT: The remaining dynamical degrees of freedom of molecular fluids confined into
capillaries of nano to sub-nanometer diameter are of fundamental relevance for
future developments in the field of nanofluidics. These properties cannot be
simply deduced from the bulk one since the derivation of macroscopic
hydrodynamics most usually breaks down in nanoporous channels and additional
effects have to be considered. In the present contribution, we review some
general phenomena, which are expected to occur when manipulating fluids under
confinement and ultraconfinement conditions.
03/2007;
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ABSTRACT: This article demonstrates the ability of chemical shift surfaces to provide information on distributions of various conformations of disaccharides in the glassy, solid state. The validity of the general method leading to a simulation of inhomogeneous (13)C chemical shift distributions is discussed in detail. In particular, a proper consideration of extrema and saddle points of the chemical shift map correctly accounts for the observed discontinuities in the experimental cross polarization magic angle spinning spectra. Provided that these basic requirements are met, density functional theory/gauge-independent atomic orbital chemical shift maps calculated on relaxed conformations lead to a very satisfactory description of the experimental line shapes. Using amorphous trehalose as a model disaccharide, the simulation unambiguously defines the range of most populated conformations in the glass.
The Journal of Chemical Physics 02/2007; 126(1):014510. · 3.33 Impact Factor
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ABSTRACT: This paper uses chemical shift surfaces to simulate experimental (13)C cross polarization magic angle spinning spectra for amorphous solid state disaccharides, paying particular attention to the glycosidic linkage atoms in trehalose, sucrose, and lactose. The combination of molecular mechanics with density functional theory/gauge invariant atomic orbital ab initio methods provides reliable structural information on the conformational distribution in the glass. The results are interpreted in terms of an enhanced flexibility that trehalose possesses in the amorphous solid state, at least on the time scale of (13)C nuclear magnetic resonance measurements. Implications of these findings for the fragility of trehalose glass and bioprotectant action are discussed.
The Journal of Chemical Physics 02/2007; 126(1):014511. · 3.33 Impact Factor
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ABSTRACT: This paper deals with the comparative use of the chemical shift surfaces to simulate experimental 13C CPMAS data on amorphous solid state disaccharides, paying particular attention to -1-1 linkage of trehalose, to -1,4 linkage between pyranose rings (lactose) and to linkage implying a furanose ring (sucrose). The combination of molecular mechanics with DFT/GIAO ab-initio methods provides reliable structural information on the conformational distribution in the glass. The results are interpreted in terms of an enhanced flexibility that trehalose experiences in amorphous solid state compared to the other sugars. An attempt to relate this property to the balance between intra- and inter-molecular hydrogen bonding network in the glass is presented.
07/2006;
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ABSTRACT: The aim of this article is to assess the ability of chemical shift surfaces to provide structural information on conformational distributions of disaccharides in glassy solid state. The validity of the general method leading to a simulation of inhomogeneous 13C chemical shift distributions is discussed in detail. In particular, a proper consideration of extrema and saddle points of the chemical shift map correctly accounts for the observed discontinuities in the experimental CPMAS spectra. Provided that these basic requirements are met, DFT/GIAO chemical shift maps calculated on relaxed conformations lead to a very satisfactory description of the experimental lineshapes. On solid-state trehalose as a model of amorphous disaccharide, this simulation approach defines unambiguously the most populated sugar conformation in the glass, and can help in discriminating the validity of different models of intramolecular energy landscape. Application to other molecular systems with broad conformational populations is foreseen to produce a larger dependence of the calculated chemical shift distribution on the conformational map.
07/2006;
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ABSTRACT: Structural order parameters of a smectic liquid crystal confined into the
columnar form of porous silicon are studied using neutron scattering and
optical spectroscopic techniques. It is shown that both the translational and
orientational anisotropic properties of the confined phase strongly couple to
the one-dimensional character of the porous silicon matrix. The influence of
this confinement induced anisotropic local structure on the molecular
reorientations occuring in the picosecond timescale is discussed.
05/2006;
