169 research outputs found
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
angstrom and D=35 angstrom. 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 an 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.Comment: version March 12 200
Microphase separation of a miscible binary liquid mixture under confinement at the nanoscale
International audienc
Structure and relaxation processes of an anisotropic molecular fluid confined into 1D nanochannels
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
First 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 \textit{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 nano-channels offers new perspectives on quenched
disorder effects, which observation has been restricted to homogeneous random
porous materials so far.Comment: Submitted to Phys. Rev.
Criticality of an isotropic-to-smectic transition induced by anisotropic quenched disorder
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
Computation of the hindrance factor for the diffusion for nanoconfined ions: molecular dynamics simulations versus continuum-based models
Special Issue: Thermodynamics 2011 ConferenceInternational audienceWe report the self-diffusion coefficients and hindrance factor for the diffusion of ions into cylindrical hydrophilic silica nanopores (hydrated silica) determined from molecular dynamics (MD) simulations. We make a comparison with the hindered diffusion coefficients used in continuum-based models of nanofiltration (NF). Hindrance factors for diffusion estimated from the macroscopic hydrodynamic theory were found to be in fair quantitative agreement with MD simulations for a protonated pore, but they strongly overestimate diffusion inside a deprotonated pore
Zhu et al. Reply
International audienceA Reply to the Comment by S. Gekle and A. Arnold. Original Article: Stephan Gekle and Axel Arnold, Comment on "Anomalous Dielectric Behavior of Nanoconfined Electrolytic Solutions", Phys. Rev. Lett. 111, 089801 (2013)
Relation between static short-range order and dynamic heterogeneities in a nanoconfined liquid crystal
International audienceWe 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 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
Sliding and translational diffusion of molecular phases confined into nanotubes
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.Comment: 17 pages, 8 fig
High-resolution dielectric study reveals pore-size-dependent orientational order of a discotic liquid crystal confined in tubular nanopores
International audienceWe report a high-resolution dielectric study on a pyrene-based discotic liquid crystal (DLC) in the bulk state and confined in parallel tubular nanopores of monolithic silica and alumina membranes. The positive dielectric anisotropy of the DLC molecule at low frequencies (in the quasistatic case) allows us to explore the thermotropic collective orientational order. A face-on arrangement of the molecular discs on the pore walls and a corresponding radial arrangement of the molecules is found. In contrast to the bulk, the isotropic-to-columnar transition of the confined DLC is continuous, shifts with decreasing pore diameter to lower temperatures, and exhibits a pronounced hysteresis between cooling and heating. These findings corroborate conclusions from previous neutron and x-ray-scattering experiments as well as optical birefringence measurements. Our study also indicates that the relative simple dielectric technique presented here is a quite efficient method in order to study the thermotropic orientational order of DLC-based nanocomposites
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