33 research outputs found
Cavity ring down spectroscopy on solid C<sub>60</sub>
The light absorption of a solid sample in the 8.5 μm region is measured via cavity ring down (CRD) absorption spectroscopy, using a free electron laser (FEL) as a source of widely tunable infrared (IR) radiation. A 3 mm thick zinc-selenide (ZnSe) window is used as a substrate for a 20–30 nm thick C60 film. On top of the structureless absorption due to ZnSe (60 is measured with monolayer sensitivity
Isotropic-nematic transition in liquid crystals confined between rough walls
The effect of rough walls on the phase behaviour of a confined liquid crystal
(LC) fluid is studied using constant pressure Monte Carlo simulations. The LC
is modelled as a fluid of soft ellipsoidal molecules and the rough walls are
represented as a hard wall with a number of molecules randomly embedded in
them. It is found that the isotropic-nematic (IN) transition is shifted to
higher pressures for rougher walls.Comment: 4 pages, 4 figures Accepted in Chemical Physics Letter
Many-body interactions and melting of colloidal crystals
We study the melting behavior of charged colloidal crystals, using a
simulation technique that combines a continuous mean-field Poisson-Boltzmann
description for the microscopic electrolyte ions with a Brownian-dynamics
simulation for the mesoscopic colloids. This technique ensures that many-body
interactions between the colloids are fully taken into account, and thus allows
us to investigate how many-body interactions affect the solid-liquid phase
behavior of charged colloids. Using the Lindemann criterion, we determine the
melting line in a phase-diagram spanned by the colloidal charge and the salt
concentration. We compare our results to predictions based on the established
description of colloidal suspensions in terms of pairwise additive Yukawa
potentials, and find good agreement at high-salt, but not at low-salt
concentration. Analyzing the effective pair-interaction between two colloids in
a crystalline environment, we demonstrate that the difference in the melting
behavior observed at low salt is due to many-body interactions
On the fluid-fluid phase separation in charged-stabilized colloidal suspensions
We develop a thermodynamic description of particles held at a fixed surface
potential. This system is of particular interest in view of the continuing
controversy over the possibility of a fluid-fluid phase separation in aqueous
colloidal suspensions with monovalent counterions. The condition of fixed
surface potential allows in a natural way to account for the colloidal charge
renormalization. In a first approach, we assess the importance of the so called
``volume terms'', and find that in the absence of salt, charge renormalization
is sufficient to stabilize suspension against a fluid-fluid phase separation.
Presence of salt, on the other hand, is found to lead to an instability. A very
strong dependence on the approximations used, however, puts the reality of this
phase transition in a serious doubt. To further understand the nature of the
instability we next study a Jellium-like approximation, which does not lead to
a phase separation and produces a relatively accurate analytical equation of
state for a deionized suspensions of highly charged colloidal spheres. A
critical analysis of various theories of strongly asymmetric electrolytes is
presented to asses their reliability as compared to the Monte Carlo
simulations
Testing the relevance of effective interaction potentials between highly charged colloids in suspension
Combining cell and Jellium model mean-field approaches, Monte Carlo together
with integral equation techniques, and finally more demanding many-colloid
mean-field computations, we investigate the thermodynamic behavior, pressure
and compressibility of highly charged colloidal dispersions, and at a more
microscopic level, the force distribution acting on the colloids. The
Kirkwood-Buff identity provides a useful probe to challenge the
self-consistency of an approximate effective screened Coulomb (Yukawa)
potential between colloids. Two effective parameter models are put to the test:
cell against renormalized Jellium models
Poisson-Boltzmann Theory of Charged Colloids: Limits of the Cell Model for Salty Suspensions
Thermodynamic properties of charge-stabilised colloidal suspensions are
commonly modeled by implementing the mean-field Poisson-Boltzmann (PB) theory
within a cell model. This approach models a bulk system by a single macroion,
together with counterions and salt ions, confined to a symmetrically shaped,
electroneutral cell. While easing solution of the nonlinear PB equation, the
cell model neglects microion-induced correlations between macroions, precluding
modeling of macroion ordering phenomena. An alternative approach, avoiding
artificial constraints of cell geometry, maps a macroion-microion mixture onto
a one-component model of pseudo-macroions governed by effective interactions.
In practice, effective-interaction models are usually based on linear screening
approximations, which can accurately describe nonlinear screening only by
incorporating an effective (renormalized) macroion charge. Combining charge
renormalization and linearized PB theories, in both the cell model and an
effective-interaction (cell-free) model, we compute osmotic pressures of highly
charged colloids and monovalent microions over a range of concentrations. By
comparing predictions with primitive model simulation data for salt-free
suspensions, and with predictions of nonlinear PB theory for salty suspensions,
we chart the limits of both the cell model and linear-screening approximations
in modeling bulk thermodynamic properties. Up to moderately strong
electrostatic couplings, the cell model proves accurate in predicting osmotic
pressures of deionized suspensions. With increasing salt concentration,
however, the relative contribution of macroion interactions grows, leading
predictions of the cell and effective-interaction models to deviate. No
evidence is found for a liquid-vapour phase instability driven by monovalent
microions. These results may guide applications of PB theory to soft materials.Comment: 27 pages, 5 figures, special issue of Journal of Physics: Condensed
Matter on "Classical density functional theory methods in soft and hard
matter
The osmotic pressure of charged colloidal suspensions: A unified approach to linearized Poisson-Boltzmann theory
We study theoretically the osmotic pressure of a suspension of charged
objects (e.g., colloids, polyelectrolytes, clay platelets, etc.) dialyzed
against an electrolyte solution using the cell model and linear
Poisson-Boltzmann (PB) theory. From the volume derivative of the grand
potential functional of linear theory we obtain two novel expressions for the
osmotic pressure in terms of the potential- or ion-profiles, neither of which
coincides with the expression known from nonlinear PB theory, namely, the
density of microions at the cell boundary. We show that the range of validity
of linearization depends strongly on the linearization point and proof that
expansion about the selfconsistently determined average potential is optimal in
several respects. For instance, screening inside the suspension is
automatically described by the actual ionic strength, resulting in the correct
asymptotics at high colloid concentration. Together with the analytical
solution of the linear PB equation for cell models of arbitrary dimension and
electrolyte composition explicit and very general formulas for the osmotic
pressure ensue. A comparison with nonlinear PB theory is provided. Our analysis
also shows that whether or not linear theory predicts a phase separation
depends crucially on the precise definition of the pressure, showing that an
improper choice could predict an artificial phase separation in systems as
important as DNA in physiological salt solution.Comment: 16 pages, 5 figures, REVTeX4 styl
Cavity ring down spectroscopy on solid c-60
Contains fulltext :
98965.pdf (publisher's version ) (Open Access
Gas-phase infrared photodissociation spectroscopy of cationic polyaromatic hydrocarbons
Contains fulltext :
98956.pdf (publisher's version ) (Open Access
Shedding new light on thermionic electron emission of fullerenes
Contains fulltext :
98985.pdf (publisher's version ) (Open Access